The 23rd edition of COSMO was held at RWTH Aachen University, Germany, from Monday 2nd of September to Friday 6th of September. As in previous editions, the conference addressed hot topics in cosmology and astroparticle physics, including:
There were plenary sessions every morning, parallel sessions every afternoon, and two poster sessions in the evenings. Click here for the full timetable and slides.
We proposed several activities after the main sessions: on Monday, a welcome reception; on Tuesday, a choice between several social and cultural activities (visits, excursions, games); on Wednesday, a banquet; and on Thursday, a public outreach lecture by Eiichiro Komatsu.
For people coming with children, we also set up a free childcare service during lecturing times.
Regardless of the precise nature of dark matter (DM), its distribution in the central regions of galaxies remains poorly constrained at present. In particular, DM halos may be significantly affected by the presence of central supermassive black holes, leading to the possible formation of high density spikes. Two objects are of particular interest in this context: Sgr A at the center of the Milky Way, where precision astrometry and spectroscopy provide a direct probe of the gravitational potential, and M87 at the center of the M87, which is a prime target of the Event Horizon Telescope, being the first ever black hole observed directly.
I will discuss different avenues that can shed light on the characteristics of the DM distribution in the cores of galaxies and the underlying properties of DM candidates. I will focus in particular on the kinematics of the S2 star at the Galactic center––which constrains the amount of dark mass around Sgr A––and electromagnetic signatures of DM annihilation on the shadow of M87.
Longstanding anomalies in astrophysical observations on small scales suggest that dark matter might not be collisionless, as is commonly assumed, but could have sizable self-interactions. For the first time, we probe the hypothesis of self-interacting dark matter (SIDM) at intermediate scales between galaxies and galaxy clusters. To model the SIDM halo density profiles, we employ an observation-driven approach, the so-called Jeans model. We validate our method by comparing with predictions from SIDM-plus-baryons simulations. So far, the limit on the self-interaction cross section from the Bullet Cluster is often cited as the strongest constraint on dark matter self-interactions. We show that the halo density profiles of relaxed systems like groups and clusters lead to much stronger bounds on the self-interaction cross section.
Self-interacting Dark Matter (SIDM) could have a number of striking observable effects, including modifications to the dark matter density on galactic and sub-galactic scales. Recent studies have revealed both ultra-compact and ultra-diffuse satellite dwarf galaxies within the Milky Way; this degree of diversity seems challenging to explain if the dark matter is collisionless and cold. I will show that tidal stripping of SIDM satellite halos naturally leads to a wider range of halo density profiles, potentially explaining these observations.
The observed Lyman-α flux power spectrum (FPS) is suppressed on scales below ∼ 30 km s−1. This cutoff could be due to the high temperature, T0, and pressure, p0, of
the absorbing gas or, alternatively, it could reflect the free streaming of dark matter particles in the early universe. We perform a set of very high resolution cosmological hydrodynamic simulations in which we vary T0, p0 and the amplitude of the dark matter free streaming, and compare the FPS of mock spectra to the data. We show that the location of the dark matter free-streaming cutoff scales differently with redshift than the cutoff produced by thermal effects and is more pronounced at higher redshift. We, therefore, focus on a comparison to the observed FPS at z > 5. We demonstrate that the FPS cutoff can be fit assuming cold dark matter, but it can be equally well fit assuming that the dark matter consists of ∼ 7 keV sterile neutrinos in which case the cutoff is due primarily to the dark matter free streaming.
Despite its remarkable success, the standard LCDM paradigm has been challenged lately by potential tensions in the Hubble Constant measurements, as well as a slight mismatch between simulations and observations on smaller scales. This has reinvigorated interest in beyond-LCDM models, such as Dark Matter interacting with an additional dark sector. These interactions result in a suppression of the matter power spectrum on small scales, making them an ideal target to be constrained with Lyman-alpha data. In this talk I will discuss a novel parameterisation of this small-scale power suppression, which allows these models to be constrained with Lyman-alpha data without needing new, computationally-expensive hydrodynamical simulations for each set of model parameters. I will also present up-to-date constraints on these interactions and their ability to alleviate the cosmological tensions, obtained with our new method.
The number of extra relativistic degrees of freedom, $\Delta N_{\rm eff}$ , has recently received attention as a possible way to alleviate the Hubble tension. Non-standard values, i.e. $\Delta N_{\rm eff} \neq 0$, can arise from different physical origins, such as the presence of additional ultra-relativistic species or non-standard values of the temperature ratio between photons and standard model neutrinos. In this talk I will show how these distinct origins of $\Delta N_{\rm eff}$ yield different observable features by altering predictions for the CMB and matter power spectra as well as primordial element abundances from BBN in different ways. To obtain meaningful results a consistent treatment and implementation of BSM likelihoods and models is essential. I will discuss how these issues can be addressed with the new CosmoBit module of the Global and Modular Beyond-the-StandardModel Inference Tool (GAMBIT).
I will describe an approach to compute correlation functions of primordial fluctuations that is based on symmetries and singularities. It borrows tools from other areas of theoretical physics, like the S-matrix program of particle physics, as well as the conformal bootstrap. Using this approach, I will present some new results for inflationary three and four-point functions of scalars and tensors. These shapes do not depend on the specific inflationary model, as long as the fluctuations minimally break de Sitter symmetry. I will also comment on further roads in pursuit of this ``cosmological bootstrap”, with a goal towards classifying a large set of shapes of primordial non-gaussianity.
We study the geometrical instability arising in multi-field models of inflation with negatively-curved field space. We analyse how the homogeneous background evolves in presence of geometrical destabilisation, and show that, in simple models, a kinematical backreaction effect takes place that shuts off the instability. We also follow the evolution of the unstable scalar fluctuations. We show that they assist the kinematical backreaction while remaining in the perturbative regime. We conclude that, in the simplest models of geometrical destabilisation, inflation does not end prematurely, but rather proceeds along a modified, sidetracked, field-space trajectory
I will begin by introducing the stochastic formalism for inflation. I will give some motivation for using the stochastic approach and then explain why we may want to consider situations that violate slow roll. I will explain the requirements for stochastic inflation to be a valid approach and discuss whether they are still appropriate when we are away from slow roll. The two requirements that I will consider in detail are the separate universe approach and the choice of gauge that our stochastic equations are written in. I will show that the stochastic formalism is safe to be used, even when slow roll is violated.
Inflation provides a dynamical mechanism to produce the primordial density perturbations that seed the formation of structure in the Universe through gravitational collapse. However, during the inflationary phase, the Universe is in a nearly homogeneous and low-entropy state, which must eventually give rise to the dense thermal plasma of the standard hot big bang. This transition, known as (p)reheating, is a necessary ingredient in inflationary theory. Microscopic models of this transition typically lead to strong instabilities and the eventual onset of highly complex, nonlinear mode-mode coupled behavior.
I will introduce a novel viewpoint on preheating dynamics — the ballistic approximation — where the derivatives coupling nearby points in spacetime (separated by less than Hubble scales) are ignored and individual points in spacetime evolve independently. Remarkably, this approximation captures the relevant dynamics obtained in full lattice simulations, at a tiny fraction of the compuational cost. We define a nonlinear generalization of the comoving curvature perturbation that applies on subhorizon as well as superhorizon scales. Absent couplings between trajectories, this quantity is conserved, and the production of curvature fluctuations can be identified with entropy generation. I will argue that the production of curvature perturbations from end-of-inflation dynamics is ubiquitous, rather than occurring in only a few highly specialized models. Furthermore, our formalism can be extended to study the effects of particle production and evolution on nontrivial potential surfaces during inflation, thus providing a unifying description of both the inflationary and early post-inflationary Universe.
I will discuss the post-inflation dynamics of multi-field models involving nonminimal couplings. In particular I will describe the results of lattice simulations used to capture significant nonlinear effects like backreaction and rescattering. I will show how we can we extract the effective equation of state and typical time-scales for the onset of thermalization, quantities that could affect the usual mapping between predictions for primordial perturbation spectra and measurements of anisotropies in the cosmic microwave background radiation. For large values of the nonminimal coupling constants, efficient particle production gives rise to nearly instantaneous preheating. Moreover, the strong single-field attractor behavior that was identified for these models in linearized analyses remains robust in the full theory, and in all cases considered the attractor persists until the end of preheating. The persistence of the single-field attractor is significant because it suppresses typical signatures of multifield models. I will therefore show that even taking into account the violent preheating phase after inflation, predictions for primordial observables in this class of models retain a close match to the latest observations.
As our advanced telescopes produce ever larger and deeper maps of our Universe we need to consider that observations are taken on our past light cone and on a spacetime geometry that is pervaded by small distortions. A precise understanding of the weak-field regime of General Relativity allows one to model these aspects consistently within N-body simulations of cosmic structure formation. The subtle relativistic effects in cosmic structure can tell us how gravity operates on the largest scales that we observe and may hold the key to unravelling the mystery of dark energy.
Our current perturbation theory techniques for predicting the power spectrum break down when we enter the non-linear regime (k~0.1 h/Mpc). Simulations allow us to probe this regime, however one must be run for every cosmology and gravity model one wishes to constrain. A perturbation theory technique that could push further into the non-linear scales than current methods and match the accuracy of a simulation would be more economical. I will present a trajectories approach to modelling the non-linear power spectrum (Lane et al. 2019, in prep) building on previous results. This technique can be described as performing a simulation on paper and could match current predictions of the non-linear power spectrum from simulations. I will discuss how this method performs versus current perturbation theory techniques and how previous results obtained with this method can be improved upon. I will also show how an analytic expression for the power spectrum can be obtained and use this to compare the technique to simulations.
Cosmic voids gravitationally lens the cosmic microwave background (CMB) radiation, resulting in a distinct imprint on degree scales. We aimed to probe the consistency of simulated ΛCDM estimates and observed imprints of voids identified in the first year data set of the Dark Energy Survey (DES Y1) by cross correlating with CMB . In particular, we intended to explore other aspects of the previously reported excess integrated Sachs-Wolfe (ISW) signal associated with cosmic voids in DES Y1 as lensing is sourced by the gravitational potential, whereas ISW depends on its time derivative. We used a simulated CMB lensing convergence map to find the optimal strategy to extract the lensing imprints given different void types and galaxy tracer density. We then stacked the Planck lensing convergence map on locations of voids identified in DES Y1 data and found a negative signal associated with DES voids that is consistent with simulations. In this presentation, I will discuss the most important aspects of our measurements and provide some prospects for the constraining power of future DES data.
Large scale structure simulations are a fundamental tool to interpret data from large volume galaxy surveys. In fact, modelling the observables and their covariance is affected by the non-linear regime of gravitational collapse, a phenomenon that is best captured through simulations. In this talk I will present some applications of simulations that are relevant to present and upcoming galaxy surveys, focusing on techniques to reduce the computational cost of such simulations.
The large-scale structure of the universe is mostly the consequence of the gravitational clustering of cold dark matter (CDM). Eventually the CDM trajectories begin to intersect ("shell-crossing"), which marks the starting point of elaborate computations in the phase-space. I show that, due to the collisionless nature of CDM, phase-space trajectories exhibit weakly singular behaviour such as local non-differentiability of the particle acceleration. Conventional N-body simulations should be able to handle these intrinsic features of perfectly cold CDM. Alternatively, singular features may be regulated by adding a finite temperature, or by employing semiclassical descriptions for the large-scale structure which I will briefly discuss.
Primordial black holes (PBH) comprising some fraction of the Universe's dark matter is a potentially interesting alternative to the more standard particle based dark matter. If the fraction is large, PBHs can significantly alter how and when nonlinear structures develop. If it is small, they could provide potentially interesting constraints on WIMPs and/or seed the super massive black holes known to exist by redshift ~7. We have run LPBH cosmological simulations of structure formation starting from deep in the radiation era and ending at z=100. We analyze the clustering, structure and mass function of halos in the simulation, as well as typical PBH velocities relevant for CMB constraints. Future use cases include tidal perturbations on primordial PBH binaries and whether the formation of stars differs in this scenario.
It has been shown that the longitudinal mode of a massive vector boson can be p\
roduced by inflationary fluctuations and account for the dark matter content of\
the Universe. In this work we examine the possibility of instead producing the\
transverse mode via the coupling ϕFF̃ between the inflaton and the vector field\
strength. Such a coupling leads to a tachyonic instability and exponential pro\
duction of one transverse polarization of the vector field, reaching its maximu\
m near the end of inflation. We show that these polarized transverse vectors ca\
n account for the observed dark matter relic density in the mass range μeV to h\
undreds of GeV. We also find that the tachyonic production mechanism of the tra\
nsverse mode can accommodate larger vector masses and lower Hubble scales of in\
flation compared to the production mechanism for the longitudinal mode via infl\
ationary fluctuations.
I will discuss the preferred initial conditions that scalar field fluctuations during cosmic inflation generically place for post-inflationary, non-thermal dark matter (DM) production. I will show that accumulation of quantum fluctuations during inflation can account for all or part of DM. I will also discuss the DM isocurvature perturbations that are unavoidably generated in such scenarios and the circumstances under which they are not problematic for viability of non-thermal DM models. I will also discuss implications of DM isocurvature for structure formation, showing that interesting consequences can be expected.
In my talk I will discuss the dependence of the dark matter production mechanism in the early universe on its coupling to the Standard Model and mediator. For illustration, I will focus on the case of compressed mass spectrum dark matter scenario and show that we can continuously go from freeze-in to freeze-out with an intermediate stage of conversion driven freeze-out. In the latter case, the feeble couplings involve give rise to the possibility to exploit the macroscopic decay length of charged mediators to study the resulting long-lived-particle signatures at collider. I will discuss the experimental reach of such searches on the viable portion of the parameter space.
Many well-motivated extensions of the standard model contain a gauge singlet scalar field which mixes with the Higgs boson. The new scalar naturally mediates the interactions between dark and visible matter. I will focus one the GeV mass window, which features exciting signatures of dark matter and the light mediator at upcoming direct detection and accelerator experiments. This mass range bears theoretical challenges since the mediator decay is affected by non-perturbative QCD processes. I will strongly reduce long-standing uncertainties in the the decay rates. This will allow me to revise present and future experimental sensitivities for dark matter coupled through the Higgs portal.
We consider possible detection of nonclassicality of primordial gravitational waves (PGWs) by applying Hanbury Brown - Twiss (HBT) interferometry to cosmology. We characterize the nonclassicality of PGWs in terms of sub-Poissonian statistics that can be measured by the HBT interferometry. We show that the presence of matter fields during inflation makes us possible to detect nonclassical PGWs with the HBT interferometry. We present two examples that realize the classical sources during inflation. It turns out that PGWs with frequencies higher than 10 kHz enable us to detect their nonclassicality.
We study the formation of primordial black holes (PBH) in a single field inflection point model of inflation wherein the effective potential is expanded up to the sextic order and the inversion symmetry is imposed such that only even power terms are retained in the potential. By working with a near inflection point in the potential, we find that PBHs can be produced in our scenario in a very relevant mass range with a nearly monochromatic mass function which can account for a sizeable fraction of the cold dark matter in the universe. By change various parameters in our model, we do generate the PBHs mass fraction in the higher mass range but the primordial spectrum of curvature perturbations becomes strongly tilted at the CMB scales. We also briefly discuss already existing difficulties and uncertainties associated with the PBHs mass fraction for a given inflationary model. Moreover, we study the effects of a reheating epoch after the end of inflation on the PBHs mass fraction and find that an epoch of a prolonged reheating can shift the mass fraction to larger mass ranges as well as increase the fractional contribution of PBHs to the total dark matter. Finally, we summarise our findings and discuss the implications of our results for future directions.
In a first order electroweak phase transition bubbles of Higgs phase
expand into the symmetric phase. Particles hitting a bubble wall
cause friction and slow down the expansion. In some models this can be
insufficient to compensate the pressure difference between the two
phases. Then the bubble wall would accelerate indefinitely, it would 'run
away'. However, particles crossing the bubble wall can emit transition
radiation, causing additional friction which prevents runaway.
~
We will discuss new probes of non-Gaussianity in the distribution of CMB temperature anisotropies as well as that of collapsed objects such as galaxies and clusters. Going beyond polyspectra, we will focus on CMB/LSS probability distribution functions and discuss how the structure of the pre-Big Bang era can be partially encoded in it.
The Kilo-Degree Survey is providing a weak gravitational lensing tomography map of the large-scale structure back to redshift ~1. We recently completed observations. First results, based on 1/3 of the final data set, indicate that the large-scale structure is slightly smoother than predicted in the best-fit Planck 2018 cosmology. I will discuss the methodology that led to this result, in particular in relation to shape measurements and photometric redshifts, as well as the prospects from the ongoing final KiDS analyses and other surveys.
We present a realistic Markov-Chain Monte-Carlo (MCMC) forecast for the precision of neutrino mass and cosmological parameter measurements with a Euclid-like galaxy clustering survey. We use the most general perturbation theory model for the one-loop power galaxy spectrum and tree-level bispectrum. This model is based on cosmological perturbation theory and includes non-linear bias, redshhift space distortions, IR resummation for baryon acoustic oscillations and so-called UV counterterms. The latter encapsulate various effects of short-scale dynamics which cannot be modeled within perturbation theory, e.g. baryonic feedback and fingers-of-God. Our MCMC procedure computes the theoretical power spectra and bispectra for each set of sampled cosmological parameters and nuisance coefficients describing the non-linear effects. The second ingredient of our approach is the theoretical error covariance which captures uncertainties due to higher-order non-linearities omitted in our model. Having specified characteristics of the spectroscopic survey based on the latest models of the Euclid-like galaxy luminosity function, we generate and fit mock galaxy power spectrum and bispectrum datasets. Our results suggest that even under the most agnostic assumptions about non-linear bias and short-scale physics the Euclid alone will be able to measure the sum of neutrino masses with $1\text{-}\sigma$ error of 25 meV. When combined with the most recent Planck likelihood, this uncertainty decreases to 18 meV. Reducing the theoretical error on the bispectrum down to the two-loop level improves the bound to 14 meV.
The statistical analysis of lensed galaxies is a powerful tool to study the dark matter distribution of the Universe. For instance, the distortion of galaxy shapes induced by the large scale structure of the Universe can be used to reconstruct the projected matter density along the line-of-sight (mass maps). Mass maps are useful as they provide a wealth of information that goes beyond and complements the more traditional two-point statistics used in Cosmology. The data from the first three years of observations of the Dark Energy Survey (DES Y3) will allow to construct the largest curved-sky galaxy weak lensing mass map to date, covering about 5000 sq. deg of the southern sky. During this talk, we will show preliminary DES Y3 mass maps and explore the possibility of constraining cosmological parameters using the second and third moments of the mass maps.
The large scale structure bispectrum in the squeezed limit couples large with small scales. Since relativity is important at large scales and non-linear loop corrections are important at small scales, the proper calculation of the observed bispectrum in this limit requires a non-linear relativistic calculation. We compute the matter bispectrum in general relativity in the weak field approximation. The calculation is as involved as existing second-order results. We find several differences with the Newtonian calculation such as the non-cancellation of IR divergences, the need to renormalize the background, and the fact that initial conditions must be set at second order in perturbation theory. For the bispectrum, we find relativistic corrections to be as large as the newtonian result in the squeezed limit. In that limit relativistic one-loop contributions, which we compute for the first time, can be as large as tree level results and have the same 1/𝑘2 dependence as a primordial local non-Gaussianity signal where 𝑘 is the momentum approaching zero. Moreover, we find the time dependence of the relavistic corrections to the bispectrum to be the same as that of a primordial non-Gaussianity signal.
An excess of cosmic positrons above 10 GeV with respect to the spallation reaction of cosmic rays with the interstellar medium has been measured by AMS-02 with unprecedented precision. Recently, a gamma-ray halo in the direction of Geminga and Monogem pulsars has been detected by HAWC. These observations can be interpreted with positrons and electrons accelerated by pulsar wind nebulae (PWNe), released in a Galactic environment with a low diffusion and inverse Compton scattering (ICS) with the interstellar radiation fields.
We confirm the detection of a gamma-ray halo around Geminga analyzing almost 10 years of Fermi-LAT data above 8 GeV.
We inspect how the morphology of the ICS gamma-ray halos depends on the energy, the pulsar age and distance and the strength and extension of the low-diffusion bubble. In particular we demonstrate that gamma-ray experiments with a peak of sensitivity at about TeV energies are the most promising ones since, at these energies, the ICS halos are expected to be relatively small and the pulsar proper motion does not affect significantly the spatial morphology.
Then, we select a sample of PWNe reported in the HESS Galactic plane survey. Using the information available in this catalog for the gamma-ray spatial morphology, we find that the diffusion coefficient is two orders of magnitude smaller than the value considered to be the average in the Galaxy. Finally, we report the consequences for the contribution of PWNe to the positrons excess and for the propagation of these particles in the Galactic environment.
Light antinuclei may be generated in dark matter annihilations or decays, offering a potential method of identifying the nature of dark matter. However, current estimations of the antinucleus fluxes has large uncertainties due to, amongst other, the antinucleon formation models. Today it is common to use the coalescence model on an event-by-event basis in a Monte Carlo framework when estimating the antinucleus production in both various dark matter models and the astrophysical background. However, this model is classical and lacks a microphysical picture. We therefore develop a new coalescence model for deuteron, helium-3, tritium and their antinuclei based on the Wigner function representations of the produced nuclei states. This approach includes both the size of the formation region, which is process dependent, and momentum correlations in a semi-classical picture. We compare the predictions of this model with experimental data from $e^+e^-$ collisions at LEP and $pp$ collisions at LHC and find in general good agreement with the data. Finally, we comment on the detection prospects of cosmic ray antideuteron and antihelium.
In recent years, several investigations have pointed towards an excess in the cosmic-ray antiproton data reported by the AMS-02 Collaboration. The interpretation of this result, which could potentially represent a dark matter signal, requires a thorough understanding of the systematic uncertainties associated with it. In this talk I will focus on one of these uncertainties, the one arising from our limited knowledge of the cross section describing the production of secondary antiprotons. In particular, I will illustrate how the modelling of this cross section at very low center-of-mass energies can play an important role in the fit to experimental data, with important consequences on the search for dark matter hints in the antiproton spectrum.
Global fits of primary and secondary cosmic-ray (CR) fluxes measured by the AMS-02 experiment provide a powerful tool to probe the existence of exotic sources of antimatter in our Galaxy, such as annihilation or decay of dark matter (DM). Previous analyses derived strong constraints on the annihilation cross section of a potential DM particle with masses above a couple of hundred GeV, while between 10 and 20 GeV a small excess over the expected astrophysical background was observed in the antiprotons spectrum, which is compatible with a 70 GeV DM particle and a thermal annihilation cross section. To establish or disprove the existence of this potential excess requires a better understanding and consideration of systematic uncertainties. We will review the most important effects, in particular focusing on uncertainties arriving from the production cross section of secondary antiprotons. By performing a joint fit, simultaneously to CR and cross section data, we are able to explore correlations and marginalize over the cross section uncertainties. Furthermore, we will discuss the effects of a potential correlation in the CR data. The most direct but complementary way to test the DM interpretation would be the observation of low-energy antinuclei in CRs. We will shortly review the prospects to observe antideuteron or antihelium with AMS-02 and the future experiment GAPS.
Dark matter in cosmic structures is expected to produce signals
originated from its particle physics nature, among which the electromagnetic
emission represents a relevant opportunity, whose intensity is directly linked
to the amount of dark matter in galaxies and clusters. On the other hand, this
emission is very faint, thus contributing only at the unresolved level. These
unresolved radiation backgrounds are isotropic at first order, but must
exhibit a degree of anisotropy since they originate from clustered dark matter
haloes. This fact implies also that the anisotropies in the radiation fields
should be correlated to the same matter distribution in the Universe.
In this talk we propose to exploit this correlation by using the intensity
mapping of the 21cm emission line of neutral hydrogen as the tracer of matter
distribution, and gamma-rays as the tracer of particle dark matter
annihilation. Intensity mapping has the advantage of not being flux limited in
the measurement of the matter distribution (as instead galaxy catalogs are)
since it does not need to identify individual galaxies, and offers excellent
redshift information being a line emission. We show the expected level for
this cross-correlation signal and we derive forecasts for the study of this
novel signature through the combination of Fermi-LAT gamma-rays data and SKA
intensity mapping capabilities.
The dissection of the gamma-ray sky into point sources and diffuse components is a valuable tool to search for new physics, such as dark matter signals. In the recent past, it has been shown that statistical analysis methods can excel the sensitivity of classic source detection approaches. In this contribution we discuss the application of photon count statistics to dark matter searches in Fermi-LAT data at different Galactic latitudes. We analyze eight years of Fermi-LAT data by considering the 1-point photon counts statistics. We aim at resolving the population of point sources and decomposing the diffuse component into Galactic foreground emission and isotropic diffuse background emission. The analysis is employed to incorporate a potential contribution from annihilating dark matter (DM), investigating the sensitivity reach of 1-point photon counts statistics for the DM thermally-averaged self-annihilation cross section. We find that the sensitivity of 1-point statistics at high Galactic latitudes is highly competitive to upper limits recently obtained with other indirect detection methods. Moreover, we illustrate the results of this method applied to low Galactic latitudes.
Hidden fields present during single-field inflation can affect CMB observables through quantum vacuum fluctuations. Besides the renormalization of background quantities, loop corrections of these fields induce Planck-suppressed logarithmic runnings in correlation functions of curvature and tensor perturbations. In this talk we consider the impact of a large number of such field degrees of freedom on inflationary observables, and show that one can extract bounds on the hidden field content of the universe from bounds on violations of the consistency relation. Our approach will be based on the 1-loop effective action, and we shall show how this can shortcut the computation of diagrams in the effective theory of inflation involving loops of particles of arbitrary spin.
Recent swampland conjectures highlight again the importance of finding viable scenarios for inflation that are not strictly single-field. In particular, one may wonder whether there are multi-field inflationary scenarios that have a similar phenomenology to single field inflation. We present a family of exact models of inflation - dubbed Orbital Inflation - in which the multi-field effects are significant, but the phenomenology remains similar to single field inflation. This simple predictions have a dynamic origin, and are non-trivial, as the isocurvature perturbations are exactly massless. The effective action of perturbations inherits a symmetry from an equivalence between background solutions. We comment on how our results could be connected to symmetries of the UV theory.
Observations of Planck's CMB favours a canonical slow-roll single-field scenario for inflation. However, simple multi-field extensions can also explain the current data, and on top some of the existing anomalies not accounted for by the canonical scenario. We present our search for multi-field-motivated extra degrees of freedom in the context of an effective single field theory with a varying speed of sound $c_s$ of the adiabatic mode. Transient reductions in $c_s$ produce deviations (or "features") in the primordial power spectrum of scalar perturbations. Features of sufficient intensity may be observed in the CMB angular power spectrum (T and E), and in the power spectrum of galaxy clustering and weak lensing. Moreover, our theory predicts also correlated features in higher-order correlators (e.g. the bispectrum), greatly enhancing the constraining power of the data. We present a standard methodology based on Gaussian Processes for general Bayesian reconstruction of primordial dynamics that imprint primordial features, accounting for theoretical priors in a natural way, and show some preliminary results when we reconstruct the reduction of the speed of sound using cosmological data.
In this talk I will introduce tools for the construction of effective field theories (EFTs) that involve the breaking of spacetime symmetries. Nowadays, symmetry breakdown is part and parcel of the study of EFTs, with group theory allowing us to derive the most generic action for the Goldstone bosons—but only if the group is internal. When spacetime symmetries are broken, the game becomes much harder: the counting of Goldstones becomes nontrivial, they can be massive, and the EFT construction might fail altogether. Yet it's precisely the breakdown of spacetime symmetries that is so important for cosmology, because the universe expands over time. I will outline the challenges when breaking spacetime symmetries and discuss new techniques to tackle them and to classify EFTs. The goal is to develop an improved framework for the constructions of EFTs in the context of cosmology, particularly inflation, and other gravitational systems.
The viability of a given model for inflation is determined not only by the form of the inflaton potential, but also by the initial inflaton field configuration. In many models, otherwise well-motivated inflaton field configurations fail to induce inflation, or fail to produce sufficient inflation to solve the horizon problem. In this talk, we propose a mechanism which enables inflation to occur even with such initial conditions. This mechanism involves multiple scalar fields which experience a time-dependent mixing. This in turn leads to a "re-overdamping" phase as well as a parametric resonance which "slingshots" the inflaton field from regions of parameter space that fail to induce inflation to regions that do. Our mechanism is flexible, dynamical, and can easily yield an inflationary epoch of sufficiently long duration. This slingshot mechanism can therefore be utilized in a variety of settings and thereby enlarge the space of potentially viable inflation models.
I will discuss a recent construction wherein an effective de Sitter spacetime appears as a non-supersymmetric AdS vacuum decays into the supersymmetric vacuum. Four-dimensional observers are localized on a spherical brane which nucleates to facilitate the decay. The expansion of the brane bubble then leads to an effective four-dimensional de Sitter space. This construction side-steps the difficulties of constructing scale-separated de Sitter vacua or inflating spacetimes in traditional flux compactifications. I will discuss new developments concerning four-dimensional phenomenology and the embedding of the scenario in string theory.
We develop a cosmological parameter estimation code for (tomographic) angular power spectra analyses of galaxy number counts, for which we include, for the first time, redshift-space distortions (RSD) in the Limber approximation. This allows for a speed-up in computation time, and we emphasise that only angular scales where the Limber approximation is valid are included in our analysis. Our main result shows that a correct modelling of RSD is crucial not to bias cosmological parameter estimation. This happens not only for spectroscopy-detected galaxies, but even in the case of galaxy surveys with photometric redshift estimates. Moreover, a correct implementation of RSD is especially valuable in alleviating the degeneracy between the amplitude of the underlying matter power spectrum and the galaxy bias. We argue that our findings are particularly relevant for present and planned observational campaigns, such as the Euclid satellite or the Square Kilometre Array, which aim at studying the cosmic large-scale structure and trace its growth over a wide range of redshifts and scales.
In this talk I will present new cosmic shear results based on the combined KiDS optical and VIKING infrared data over an area of 450 square degrees. I will show how the crucial redshift calibration benefits from the extended wavelength coverage and how this leads to more robust cosmological conclusions. The results will be put into context and compared to findings from the two other big cosmic shear experiments (HSC and DES) and other cosmological probes, most importantly the Planck Legacy CMB results that show some tension to recent cosmic shear measurements. I will present brand-new results from a Self-Organised-Map-based calibration of the KiDS+VIKING photometric redshifts that implements the baseline plan for the Euclid space mission, tests on realistic simulations, and how all of this influences the cosmological conclusions. Through a careful re-assessment of the HSC and DES results I will show that the discrepancies in large-scale-structure parameters we are seeing today are approaching a level of significance that is similar to the tension in the Hubble constant. Taken together this might hint at a serious problem of the standard $\Lambda$CDM paradigm in simultaneously explaining early- and late-time cosmic structure formation.
We present an iterative method to reconstruct the linear-theory initial conditions from the late-time cosmological matter density field, with the intent of improving the recovery of the cosmic distance scale from the baryon acoustic oscillations (BAOs) and making the distance measurements more reliable in upcoming redshift surveys, e.g., PFS, DESI and Euclid. We apply the iterative method to the dark matter density field and galaxy mocks, in both real and redshift space, generated from N-body simulations and then compute the two-point correlation function and the power spectrum for the reconstructed density field. Comparing with the standard reconstruction method, which has been widely used in previous BAO distance measurements, we find that our iterative method can reconstruct the initial linear density field more precisely, especially on small scales (< 40 Mpc/h). Furthermore, we measure the distance scale by fitting for the position of the acoustic signature in the correlation function and evaluate the performance of the iterative reconstruction method. We also discuss the effects of number density, smoothing filtering, and galaxy bias on the reconstruction process.
I will review the status of cosmological searches for dark matter-baryon interactions, summarizing the best current limits on scattering of sub-GeV particle candidates with protons derived from the CMB anisotropy measurements. I will then present stringent new bounds on the same physics, inferred recently from the observed population of the Milky Way satellite galaxies. I will highlight complementarities between different observations and laboratory searches for dark matter, and discuss the prospects for the coming decade.
We present MadDM v.3.0, a numerical tool to compute particle dark matter observables. The new version features a comprehensive and automated framework for dark matter searches at the interface of collider physics, astrophysics and cosmology and is deployed as a plugin of the MadGraph5_aMC@NLO platform, inheriting most of its features. With respect to the previous version, MadDM v.3.0 now provides predictions for indirect dark matter signatures in astrophysical environments, such as the annihilation cross section at present time and the energy spectra of prompt photons, cosmic rays and neutrinos resulting from dark matter annihilation. MadDM indirect detection features support both 2 → 2 and 2 → n dark matter annihilation processes. In addition, the ability to compare theoretical predictions with experimental constraints is extended by including the Fermi-LAT likelihood for gamma-ray constraints from dwarf spheroidal galaxies.
Paleo-detectors are a proposed experimental technique where one would search for the traces of nuclear recoils in ancient minerals. Modern read out technologies should allow to reach ~ 1 keV nuclear recoil energy thresholds for exposures as large as 100 g Gyr = 100 kt yr. Recently, we investigated the sensitivity of paleo detectors for dark matter. In this talk, I will demonstrate that paleo-detectors could also be used for the detection of neutrinos from a range of sources. For example, paleo-detectors could be used to measure neutrinos from core collapse supernovae occurring in our galaxy. This would allow for the first direct measurement of the galactic core collapse supernova rate. Further, I will discuss how paleo-detectors could be used to gain some information about the time-dependence of the galactic supernova rate. This would provide a unique opportunity to measure the star formation history of the Milky Way over the past ~1 Gyr.
A large experimental program is underway to extend the sensitivity of direct detection experiments, searching for interaction of Dark Matter with nuclei, down to the neutrino floor. However, such experiments are becoming increasingly difficult and costly due to the large target masses and exquisite background rejection needed for the necessary improvements in sensitivity. We investigate an alternative approach to the detection of Dark Matter-nucleon interactions: Searching for the persistent traces left by Dark Matter scattering in ancient minerals obtained from much deeper than current underground laboratories. We estimate the sensitivity of paleo-detectors, which extends far beyond current upper limits for a wide range of Dark Matter masses.
Primordial SU(2) gauge fields coupled to axions can contribute to the physics of inflation. Their rich phenomenology and unique observational features, e.g., chiral primordial gravitational waves, turned this class of models to a hot topic of study since their discovery in 2011. In this talk, I will briefly review the models in this class, which so far have been studied in the literature. Then, I will talk about the three different types of particles produced by the SU(2) gauge field in this setup, i.e., scalar, fermion, and spin-2 particles. I will explain how the size of the backreaction constrains the parameter space of the models. Next, I will talk about the chiral gravitational waves and how it is produced by the extra spin-2 particle, which is the generic feature of this class of models. Finally, I will finish my talk by the natural inflationary leptogenesis setting provided by this class as their yet another generous opportunity!
The coupling of an axion-like particle driving inflation to the Standard Model particle content through a Chern-Simons term generically sources a dual production of massless helical gauge fields and chiral fermions. We demonstrate that the interplay of these two components results in a highly predictive baryogenesis model, which requires no further ingredients beyond the Standard Model. If the helicity stored in the hyper magnetic field and the effective chemical potential induced by the chiral fermion production are large enough to avoid magnetic diffusion from the thermal plasma but small enough to sufficiently delay the chiral plasma instability, then the non-vanishing helicity survives until the electroweak phase transition and sources a net baryon asymmetry which is in excellent agreement with the observed value. If any of these two conditions is violated, the final baryon asymmetry vanishes.
The observed baryon asymmetry can be reproduced if the energy scale of inflation is around $H_\text{inf} \sim 10^{10}$-$10^{12}$ GeV with moderate dependence on inflation model parameters.
In this talk I will consider an inflationary universe with non-Abelian gauge fields and axion fields that are in part identified with the standard model of particle physics.
In particular I consider possibilities of an enlarged color group with extra heavy quarks that solves the strong CP problem. When the heavy quarks are integrated out below the Peccei-Quinn symmetry breaking scale, they generate an axion coupling which makes the non-Abelian gauge field develop slowly during inflation and solves the strong CP problem of QCD after inflation.
In this class of models, the axion mass receives a new non-perturbative contribution from the new confinement scale, which is larger than the inflationary scale.
I then discuss the running of the gauge coupling constants that realizes the observable signal as chiral gravitational waves from inflation.
Finally, I constrain the number of extra heavy quarks in this scenario by future CMB observations.
We revisit the clustering of relic neutrinos in the gravitational potential of the Milky Way. Previous work was based on forward-tracking of particles from high redshift. As the orbits of the neutrinos depend quite sensitively on their initial conditions, determining their density at a particular position, e.g. the sun, is however computationally inefficient. Consequently, the equations of motion could only be solved in a 1D, spherically symmetric approximation whereas both baryons in the Galaxy as well as the presence of the Virgo cluster break the spherical symmetry. Here, we present the results from a 3D modelling of the gravitational potential of the dark matter and baryons in the Galaxy as well as dark matter in the Virgo cluster. For the first time, we employ back-tracking of neutrinos and compute their phase-space density today through Liouville's theorem. We find that the baryonic contribution to neutrino clustering has been underestimated in 1D approaches and that the presence of the Virgo cluster further enhances the local density of relic neutrinos.
My talk has two parts. First, I talk about second-order lensing of 21cm intensity mapping (IM). Like the CMB, 21cm IM temperature fluctuations have second and higher order lensing and no first-order lensing. We find a new (third order) lensing term that is neglected in the CMB lensing but is important for 21cm IM. We study the detectability of 21cm IM lensing with a Fisher matrix approach for the redshift range of z=2 to z=6 and find that with optimistic assumptions, we obtain a signal-to-noise of ~10 for futuristic surveys like SKA2.
In the second part, I talk about our current project on estimating the first order lensing from cross-correlation of 21cm IM and galaxy clustering surveys. We introduce a new lensing estimator which has higher signal-to-noise compared to the estimator that is currently used for magnification bias detection.
In the next decade we will almost certainly measure the neutrino mass sum to high significance. Neutrino cosmology faces a revolution due to upcoming large-scale structure surveys such as Euclid and DESI. Although accurate modelling of non-linear scales will be crucial for reaching high levels of significance, we will have a wealth of data from other sources as well: future CMB experiments will not only complement large-scale structure data through observations of the primary CMB anisotropies and lensing, but will allow us to probe the large-scale distribution of matter through the Sunyaev-Zeldovich effect.
Primordial gravitational waves arising during inflation are expected to imprint a B-mode polarization pattern on the cosmic microwave background (CMB). The BICEP/Keck experiments target this primordial signature by observing the polarized microwave sky at degree-scale resolution from the South Pole. Attempting to observe the very faint primordial B-mode signal requires an instrument with exquisite sensitivity and tight control of systematics. Bright Galactic emission at the same observing frequencies, along with polarization distortion due to gravitational lensing of CMB photons by large-scale structure, make this measurement extremely challenging. Distinguishing the primordial signal from these foregrounds requires a wide frequency coverage. I will present the latest constraints on the tensor-to-scalar ratio r from the BICEP/Keck experiments, using data taken from 2010 up to 2015 (BK15) in combination with data from the Planck and WMAP satellites. Future observations with the “Stage-3” BICEP Array experiment will expand in frequency range, steadily improving our sensitivity to r by an order of magnitude over the next few years and thus constraining natural inflation and most single-field models. Finally, I will outline how these efforts inform CMB "Stage 4” experiments, which will also probe the thermal history of our Universe, investigate Dark energy and general relativity, and study neutrino properties.
One of the promising cosmological probes in the next decades is the CMB polarization. While CMB temperature anisotropies have been already measured very precisely, CMB polarization, in particular a twisting pattern in the polarization map (B mode) is still dominated by the statistical noise at most of the scales. The precise measurements of B mode will enable us to explore not only the primordial gravitation waves but also to measure gravitational lensing, and cosmic birefringence. I will talk about a recent work on data analysis using high precision B-mode data, including CMB-galaxy lensing cross-correlation by Subaru HSC and POLARBEAR.
Exploiting the weak gravitational lensing signal of the CMB has become one of the primary targets of current and upcoming CMB observatories, since it allows to tighten constraints on the physics of structure formation in a more direct way than by CMB power spectrum measurements. In this context, future CMB experiments target a sub-percent measurement of the CMB lensing power spectrum, aiming for example at a detection of the absolute mass scale of neutrinos. Furthermore, the lensing deflection appears as a crucial foreground in the quest to detect primordial gravitational waves with future high-sensitivity CMB polarization measurements. I will discuss robustness of common estimators for auto- and cross-correlation CMB lensing spectra to the modeling of the non-linear matter distribution and post-Born CMB lensing in our Universe. I will demonstrate that a good understanding of these higher-order effects are crucial to reach the targets of the next-generation CMB experiments, e.g. an unbiased estimate of the total mass of neutrinos on the level of 100 meV. I will also present results on the impact of galactic foregrounds on the CMB lensing estimation, as well as the removal of the lensing deflection effect from measured CMB B-mode maps, the so-called delensing. I will show that taking both, foreground removal with multi-frequency observations and delensing, into account at the same time with realistic simulations, it is possible to reach the proposed target of a detection of a tensor-to-scalar ratio larger than $r \sim 10^{-3}$ of CMB-S4.
Gravitational lensing of the cosmic microwave background (CMB) encodes information from the low-redshift universe. Therefore, its measurement is useful for constraining cosmological parameters that describe structure formation, e.g. $\Omega_M$, $\sigma_8$ and the sum of neutrino masses. In this talk, I will present a measurement of the CMB lensing potential and its power spectrum using data from the SPTpol 500 deg$^2$ survey. From the minimum-variance combination of the lensing estimators from all combinations of SPTpol temperature and polarization data, we measure the lensing amplitude $A_{\rm MV} = 0.944 \pm 0.058 ({\rm Stat.}) \pm 0.025 ({\rm Sys.})$, which constitutes the tightest lensing amplitude measurement using ground-based CMB data alone. Restricting to only polarization data, we measure the lensing amplitude $A_{\rm Pol} = 0.906 \pm 0.090 ({\rm Stat.}) \pm 0.040 ({\rm Sys.})$, which is more constraining then our measurement using only temperature data. As SPT-3G, the successor to SPTpol, and other CMB experiments continue to lower the CMB map noise levels, polarization data will dominate the signal-to-noise of lensing measurements for angular multipoles below at least several hundred. Looking to the future, high signal-to-noise measurements of lensing enabled by deep polarization maps is crucial for constraining the sum of neutrino masses and the amplitude of inflationary gravitational waves through delensing.
Abstract:
The inflationary paradigm, already in its simplest disguises, has been spectacularly successful when it comes to agreement with observations. However, there’s a lot we do not yet know about inflation:
- what is its energy scale?
- how about its particle content?
- how did inflation begin?
…
New cosmological probes (at all scales, from CMB to interferometers) will soon put some of our best ideas to the test.
The answers to these questions are bound to be transformative of our understanding of cosmology and, possibly, also particle physics. A high-scale inflation, for example, would automatically be a portal to otherwise unaccessible energy scales.
In this talk I will review some recent work on the inflationary particle content and then focus on a model that includes a pseudo scalar field coupled with SU(2) gauge fields. This setup can generate a chiral gravitational waves signal. I will then detail on how the parameter space of the theory supports a blue tensor spectrum and large tensor as well as mixed non-Gaussianities.
The search for primordial gravitational waves through their imprint on the polarization of the CMB is one of the most promising avenues for new discoveries in cosmology. The next generation of measurements will be carried out by ground-based facilities, which must face a number of observational challenges. In this talk I will review the current state of the art and discuss the main strategies that will be used by future experiments, with a particular emphasis on the Atacama-based Simons Observatory.
GW170817 with its coincident optical counterpart led to a first "standard siren" measurement of the Hubble constant independent of the cosmological distance ladder. The Schutz "statistical" method with galaxy catalogues, which is expected to work in the absence of uniquely identified hosts, has also started bringing in its first estimates. In this talk we report the latest results of the gravitational-wave measurement of the Hubble constant and discuss the prospects with observations during the upcoming runs of the Advanced LIGO-Virgo detector network.
The groundbreaking progresses in the detection of gravitational waves, and the possibility to gain insight into the black holes that populate our Universe, have recently attracted attention on the proposal of Primordial Black Holes, which could constitute the dark matter. If such objects were generated during the early stages of the cosmological history, they would be accompanied by a stochastic background of gravitational waves, potentially detectable with many recently proposed experiments.
We will illustrate this connection, with a particular application to the scenario in which the perturbations responsible for the generation of these signatures are generated by the Standard Model Higgs.
The low-energy dynamics of a generic self-gravitating media can be studied by using effective field theory in terms four derivatively coupled scalar fields and naturally gives rise to an interesting model of dark energy. Depending on internal symmetries, the theory describes fluids, superfluids, solid and supersolids. Dynamical and thermodynamical properties are also dictated by internal symmetries. In the unitary gauge, where the scalar fields' fluctuations are gauged away, the most general medium can be equivalently described as rotational invariant massive gravity with six propagating degrees of freedom. In the scalar sector, besides the gravitational potential, a non-adiabatic mode $\delta \sigma$
corresponding to entropy per particle perturbations is present. Perfect fluids and solids are adiabatic with constant in time $\delta \sigma$, while for superfluids and supersolids $\delta \sigma$ has non-trivial dynamics. Tensor perturbations are massive for solids and supersolids. A special subclass of media with an exact equation of state $w=-1$ but with nontrivial perturbations can be consistently constructed.
The physical reason for the observed acceleration of the Universe is one of the most important mysteries in cosmology. This is also one motivation for the next generation of large galaxy surveys like Euclid, LSST or SKA that will observe billions of galaxies to provide galaxy number counts and weak lensing measurements.
In the first part of my talk, I'm going to show a systematic extension of the Effective Field Theory of Dark Energy framework to non-linear clustering. As a first step, we have studied the k-essence model and have developed a relativistic N-body code, k-evolution. I'm going to talk about the k-evolution results, including the effect of k-essence perturbations on the matter and gravitational potential power spectra and the k-essence structures formed around the dark matter halos.
In the second part of my talk, I'm going to show for some choice of parameters the k-essence non-linearities suffer from a new instability and blow up in finite time.
Perturbations on a static and spherically symmetric solution in the Horndeski theory were studied by previous researches. Their radial stability conditions were calculated but angular stability condition of the even-parity mode, which has more complicated form of perturbative equations of motions than the odd-parity mode, was not obtained. We have calculated it with high multipole limit and gotten its expression explicitly. We will use it to discuss angular stability of the black holes which are emerged in modified gravity.
In my talk, I will present the impact of general, model independent conditions of theoretical stability and cosmological viability on the analysis of scalar-tensor theories with cosmological data. These conditions account for the avoidance of ghost and gradient instabilities as well as exponential growth of the scalar perturbations in the Dark Energy sector.
As an example, I will show the role of such conditions in the computation of emblematic Large Scale Structure observables and what we can learn from them in order to efficiently constrain Dark Energy and Modified Gravity against data.
With the next generation of CMB surveys promising to map linear perturbation modes (in both temperature and E-mode polarization) down to the cosmic variance limit for $\ell$ below $\sim 3000$, cosmologists are turning to different avenues to further constrain the $\Lambda CDM$ model. Primordial gravitational waves, in the form of linear B-modes polarization, could be detected in the near future by upcoming ground-based experiments. Secondary anisotropies are another more recently studied potential source of cosmological information. CMB lensing for example, has been successfully measured and used to obtain stringent constraints on neutrino masses. The Sunayev – Zel’dovich (SZ) Effects, both kinetic and thermal, although they heavily depend on astrophysical processes, carry their own cosmological information.
Among these secondaries, Rayleigh scattering of the CMB is a less studied yet potentially powerful probe of the recombination history. Scattering of CMB photons off neutral species right after recombination presents a distinctive $\nu^4$ scaling with frequency as well as a strong correlation with the primary CMB. These unique features should guarantee its detection by the next generation of ground based CMB experiments. We will present detectability forecasts combining the Simons Observatory and CCAT-prime telescopes as well as more futuristic space missions. Finally, we will present potential cosmological implications of the detection of this signal by studying improvement of parameter constraints.
Primordial Magnetic Fields (PMFs), being present before the epoch of cosmic recombination, induce small-scale baryonic density fluctuations. These inhomogeneities lead to an inhomogeneous recombination process which alters the peaks and heights of the large-scale anisotropies of the Cosmic Microwave Backround (CMB) radiation. Utilizing numerical compressible MHD calculations, and a Monte Carlo Markov Chain analysis, which compares calculated CMB anisotropies with those observed by the WMAP and Planck satellites, we derive limits on the magnitude of putative PMFs. We find that the total remaining present day field, integrated over all scales, cannot exceed 47 pG for scale-invariant PMFs and 8.9 pG for PMFs with a violet Batchelor spectrum at 95% confidence level. These limits are more than one order of magnitude more stringent than any prior stated limits on PMFs from the CMB which have not accounted for this effect.
In this talk, I will detail two ways to search for low-mass axion dark matter using cosmic microwave background (CMB) polarization measurements. These appear, in particular, to be some of the most promising ways to directly detect fuzzy dark matter. Axion dark matter causes rotation of the polarization of light passing through it. This gives rise to two novel phenomena in the CMB. First, the late-time oscillations of the axion field today cause the CMB polarization to oscillate in phase across the entire sky. Second, the early-time oscillations of the axion field wash out the polarization produced at last-scattering, reducing the polarized fraction (TE and EE power spectra) compared to the standard prediction. Since the axion field is oscillating, the common (static) ‘cosmic birefringence’ search is not appropriate for axion dark matter. These two phenomena can be used to search for axion dark matter at the lighter end of the mass range, with a reach several orders of magnitude beyond current constraints. I will present a limit from the washout effect using existing Planck results, and discuss the significant future discovery potential for CMB detectors searching in particular for the oscillating effect.
Most of the modern Boltzmann solvers are based on seminal work by Ma and Bertschinger (Astrophys.J. 455 (1995) 7-25). We found that in the work as well as in the code, the baryon equations of motions breaks general covariance. There are terms missing at the order of $c_s^2$. Considering a covariant action for baryon perfect fluid with tiny temperature which has non vanishing $c_s^2$, we show that these problems can be solved. The correction in the equations of motion are similar order as second order tight coupling approximation. We also study tight coupling approximation up to second order, with out choosing any gauge. We see that on making baryon equation of motion explicitly covariant does not make the code stiff or slow. There are some parameters whose best fit values are deviating by, at most, one percent. This deviation is a contribution coming from the covariant equations of motion. We believe this deviation has to be taken in to account in the context of precision cosmology.
We present the computation of the spin of primordial black holes produced by the collapse of large inhomogeneities in the early universe. Since such primordial black holes originate from overdensity peaks, we resort to peak theory to obtain the probability distribution of the spin at formation. We show that the spin is a first-order effect in perturbation theory: it results from the action of first-order tidal gravitational fields generating first-order torques upon horizon-crossing, and from the asphericity of the collapsing object. The typical value of the dimensionless Kerr parameter takes values which are at the percent level. This is a clear prediction of the primordial formation scenario which can be compared with the astrophysical one in explaining the observation of the effective spin of binary mergers observed with gravitational waves at LIGO.
Current observational constraints leave only a few mass ranges for the primordial black holes (PBHs) to be the totality of dark matter in the universe.
One of them is around $10^{-12}$ solar masses. If PBHs with this mass are formed due to an enhanced scalar-perturbation amplitude, their formation is inevitably accompanied by the generation of gravitational waves (GWs) with frequency peaked in the mHz range, precisely around the maximum sensitivity of the LISA mission. We discuss whether LISA will be able to observe the associated GWs. Although they are intrinsically non-Gaussian, LISA can measure only the power spectrum, since the detectable signal is a sum of GWs from a large number of independent sources suppressing the non-Gaussianity at detection to an unobservable level. We will also discuss the effect of the GW propagation in the perturbed universe.
Although Cosmic Microwave Background and Large Scale Structure probe the largest scales of our universe with ever increasing precision, our knowledge about the smaller scales is still very limited other than the bounds on Primordial Black Holes(PBHs). We show that the statistical properties of the small scale quantum fluctuations can be probed via the stochastic gravitational wave background, which is induced as the scalar modes re-enter the horizon. We found that even if scalar curvature fluctuations have a subdominant non-Gaussian component, these non-Gaussian perturbations can source a dominant portion of the induced GWs. Moreover, the GWs sourced by non-Gaussian scalar fluctuations peaks at a higher frequency and this can result in distinctive observational signatures. If the induced GW background is detected, but not the signatures arising from the non-Gaussian component, this translates into stringent bounds on non-Gaussianity depending on the amplitude of the GW signal. The induced GWs can also give valuable information about the accretion and merging histories of the black holes as they are associated with the formation properties of the PBH.
We explore gravitational wave signals arising from first-order phase transitions occurring in a secluded hidden sector, allowing for the possibility that the hidden sector may have a different temperature than the Standard Model sector. We present the sensitivity to such scenarios for both current and future gravitational wave detectors in a model-independent fashion. Since secluded hidden sectors are of particular interest for dark matter models at the MeV scale or below, we pay special attention to the reach of pulsar timing arrays. Cosmological constraints on light degrees of freedom restrict the number of sub-MeV particles in a hidden sector, as well as the hidden sector temperature. Nevertheless, we find that observable first-order phase transitions can occur. To illustrate our results, we consider two minimal benchmark models: a model with two gauge singlet scalars and a model with a spontaneously broken U(1) gauge symmetry in the hidden sector.
There is a well known degeneracy between the enhancement of the growth of large-scale structure produced by modified gravity models and the suppression due to the free-streaming of massive neutrinos at late times. This makes the matter power-spectrum alone a poor probe to distinguish between modified gravity and the concordance ΛCDM model when neutrino masses are not strongly constrained.
In this talk, I will examine the potential of using redshift-space distortions (RSD) to break this degeneracy when the modification to gravity is scale-dependent in the form of Hu-Sawicki f(R). I will discuss our findings that if the linear growth rate can be recovered from the RSD signal, the degeneracy can be broken at the level of the dark matter field. However, this requires accurate modelling of the non-linearities in the RSD signal, and I will also introduce an extension of the standard perturbation theory-based model for non-linear RSD that includes both Hu-Sawicki f(R) modified gravity and massive neutrinos. Finally, I shall examine how we intend to develop our method to deal with biased tracers of the underlying dark matter in order to bring us closer to applying this analysis to galaxy clustering data.
We present a complete analysis of the observational constraints and cosmological implications of our Bound Dark Energy (BDE) model aimed to explain the late-time cosmic acceleration of the universe. BDE is derived from particle physics and corresponds to the lightest meson field $\phi$ dynamically formed at low energies due to the strong gauge coupling constant. The evolution of BDE is determined by the scalar potential $V(\phi)=\Lambda_c^{4+2/3}\phi^{-2/3}$ arising from non-perturbative effects at a condensation scale $\Lambda_c$ and scale factor $a_c$, related each other by $a_c\Lambda_c/\mathrm{eV}=1.0934\times 10^{-4}$. We present the full background and perturbation evolution at a linear level. Using current observational data, we obtain the constraints $a_c=(2.48 \pm 0.02)\times10^{-6}$ and $\Lambda_c=(44.09 \pm 0.28) \textrm{ eV}$, which is in complete agreement with our theoretical prediction $\Lambda_c^{th}=34^{+16}_{-11}\textrm{ eV}$. The bounds of the EoS $w$, the DE density and the expansion rate are $w_\mathrm{BDE 0}=-0.929\pm 0.007$, $\Omega_\mathrm{BDE0}=0.696\pm0.007$ and $H_0=67.82\pm 0.05$ km s$^{-1}$Mpc. Even though the constraints on the Planck base parameters are consistent at 1$\sigma$ level between BDE and the concordance $\Lambda$CDM model, BDE improves the likelihood ratio by 2.1 of BAO measurements with respect to $\Lambda$CDM and has an equivalent fit SNIa and CMB data. We present the constraints on the different cosmological parameters, and particularly we show the tension between BDE and $\Lambda$CDM in the BAO distance ratio $r_\mathrm{BAO}$ vs $H_\mathrm{0}$ and the growth index $\gamma$ at different redshifts, as well as the DM density at present time $\Omega_ch^2$ vs $H_0$. These results allow us to discriminate between these two models.
We discuss the embedding of the Horndeski model into supergravity. In the case of linearly realized supersymmetry, higher derivative interaction often leads to ghost and propagating auxiliary field problems. Therefore, supergravity realization of the Horndeski model has not been known so far.
These issues can be circumvented in the recently proposed framework, called pure de Sitter supergravity, where supersymmetry is nonlinearly realized. The pure de Sitter supergravity is also known to be an effective description of anti-D3 brane in superstring theory. We will show how the Horndeski Lagrangian can be realized within the nonlinearly realized supergravity. We will also discuss the implication of the Horndeski model in supergravity.
We discuss the ability of a dark fluid becoming relevant around the time of matter radiation equality to significantly relieve the tension between local measurements of the Hubble constant and CMB inference, within the $\Lambda$CDM model.
We show the gravitational impact of acoustic oscillations in the dark fluid balance the effects on the CMB and result in an improved fit to CMB measurements themselves while simultaneously raising the Hubble constant.
The required balance favors a model where the fluid is a scalar field that converts its potential to kinetic energy around matter radiation equality which then quickly redshifts away.
We derive the requirements on the potential for this conversion mechanism and find that a simple canonical scalar with two free parameters for its local slope and amplitude robustly improves the fit to the combined data by $\Delta\chi^2 \approx 12.7$ over $\Lambda$CDM.
We uncover the CMB polarization signatures that can definitively test this scenario with future data.
Local measurements of the Hubble parameter are increasingly in tension with the value inferred from a LCDM fit to the cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data. A general class of solutions to this tension involves temporarily increasing the energy density of the Universe close to the epoch of matter-radiation equality to reduce the size of the baryon-photon sound horizon at recombination. In the literature, various models for this energy injection have been proposed, ranging from rolling and oscillating scalar fields, new relativistic species with nonstandard properties, or extra matter components that subsequently decay. We describe the appealing and problematic features of these proposed solutions, showing that it is in general challenging to resolve the tension between CMB, BAO, and distance ladder measurements without either introducing new tensions with other cosmological datasets or requiring particle physics models that are significantly fine-tuned. We argue that none of the currently proposed solutions are entirely satisfactory, but identify important properties that a complete solution should have.
According to cosmological low and intermediate-redshift data, what is the statistical evidence in favor of the current speed-up of the Universe? Although this question seems to be kind of outdated, a review to the many papers that address this pivotal question in the literature tells us that the answer is not obvious at all. Determining the value of the deceleration parameter, i.e. q0=q(z=0), in the context of particular cosmological models, concrete parametrizations of the cosmographical functions, or even using truncated cosmographical expansions in which the truncation order is set in an ad hoc way can lead to biased estimations of both, q0 and its uncertainty. In this talk (based on arXiv:1810.02278, JCAP 05 (2019) 026) I present a new determination of q0 obtained with data from the Pantheon+MCT compilation of SnIa, cosmic chronometers and BAOs. I apply the so-called Weighted Function Regression method to reconstruct q(z) and the jerk in a more model-independent way than many other analyses in the literature, improving thereby the usual cosmograhical approach. We will see e.g., that using only the first two data sets and Jeffreys' scale and jargon the evidence for the current positive acceleration of the universe is only moderate, contrary to the more than 17\sigma-evidence found in the framework of the flat LCDM model. The level of evidence grows up to the very strong one when BAOs are also considered, giving rise to q0= -0.60 +- 0.10, with a deceleration-acceleration transition redshift at z_t = 0.80 +- 0.10.
Measurements of the CMB temperature anisotropies on large angular scales have uncovered a number of anomalous features of marginal statistical significance: a hemispherical power asymmetry, lack of power on large angular scales, and features in the power spectrum. Because the primary CMB temperature has been measured at the cosmic variance limit, determining if these anomalies are hints of new physics as opposed to foregrounds, systematics, or simply statistical flukes, requires new observables. We highlight the potential contribution that future measurements of the kinetic Sunyaev-Zel’dovich effect (kSZ) and the polarized Sunyaev Zel’dovich effect (pSZ) could make in determining the physical nature of several CMB anomalies. The kSZ and pSZ effects, temperature and polarization anisotropies induced by scattering from free electrons in the reionized Universe, are the dominant blackbody contribution to the CMB on small angular scales. Using the technique of SZ tomography, measurements of these effects can be combined with galaxy surveys to reconstruct the remote CMB dipole and quadrupole fields, providing a 3-dimensional probe of large scale modes inside our Hubble volume. We forecast the additional constraining power that these observables might offer for a representative set of anomaly models and find that the information from CMB temperature, polarization, and the remote dipole and quadrupole fields is complementary, and the full set of observables can improve constraints on anomaly models by a factor of ~ 2 − 4 using next-generation CMB experiments and galaxy surveys. This could be sufficient to definitively establish the physical origin of several CMB anomalies.
New physics in the neutrino sector might be necessary to address anomalies between different neutrino oscillation experiments. Intriguingly, it also offers a possible solution to the discrepant cosmological measurements of $H_0$. We show that delaying the onset of neutrino free-streaming until close to the epoch of matter-radiation equality can naturally accommodate a larger value for the Hubble constant, while not degrading the fit to the cosmic microwave background (CMB) damping tail. We achieve this by introducing neutrino self-interactions in the presence of a non-vanishing sum of neutrino masses. This "strongly interacting" neutrino cosmology prefers a 3+1 neutrino scenario, which has interesting implications for particle model-building and neutrino oscillation anomalies. We show that the absence of the neutrino free-streaming phase shift on the CMB can be compensated by shifting the value of several cosmological parameters, hence providing an important caveat to the detections made in the literature. Due to their impact on the evolution of the gravitational potential at early times, self-interacting neutrinos and their subsequent decoupling leave a tell-tale structure on the matter power spectrum. Our analysis shows that it is possible to find radically different cosmological models that nonetheless provide excellent fits to the data, hence providing an impetus to thoroughly explore alternate cosmological scenarios.
The effects of gravitational waves on the arrival times of pulses from pulsars produce a characteristic angular correlation in the pulsar timing residuals. It is still not understood, however, whether the local GW signal due to super massive black hole (SMBH) mergers will be the type of stochastic background that arises as the sum of a large number of cosmological sources, or whether it will be dominated by just a handful — or even just one — source. Hence a first obvious step after the initial detection of a gravitational wave signal will therefore be to seek the statistical anisotropy in the background that may arise from a finite number of sources. Following our work in arXiv:1904.05348, we will discuss the problem in a conceptually straightforward manner and provide results on the smallest detectable amplitude of statistical anisotropy.
Microlensing of gravitational waves by can result in interference patterns in the observed strain. The spècific form of these interference patterns depend on the mass of the microlens and the abundance of microlenses. We demonstrate how microleneses with masses of a few tens of solar masses (similar to the masses of the black holes observed by LIGO-Virgo) can produce observable effects in the frequency range of LIGO-Virgo. A detailed analysis of these distortions can reveal the abundance of black holes in this mass regime.
I will discuss recent collaborative studies on the potential of advanced GW detectors for improving the current knowledge of cosmological parameters and testing the dark-energy sector, using standard sirens. We consider second generation network made by the two advanced LIGO detectors+advanced Virgo+LIGO India+Kagra, and third-generation detectors such as the Einstein Telescope and Cosmic Explorer. We construct mock catalogs of standard sirens, considering different scenarios for the local merger rate and for the detection of an electromagnetic counterpart. We first study how standard sirens can improve the determination of H0 and OmegaM in LCDM, with respect to the current results from Planck CMB data, BAO and SNe. We then study how standard sirens at second- or third-generation GW detectors, alone or in combination with other cosmological datasets, can give information not only on H0 and Omega_M, but also on the dark energy sector, considering both a non-trivial dark energy equation of state and modified GW propagation.
Independently of the order of the phase transition, topology of the defects, and nature, global or gauge, of the symmetry broken, defect networks emit gravitational waves (GWs). In this talk I will review how any scaling defect network emits an irreducible GW background, which has scale-invariant amplitude for $f\gg f_{\rm eq}$. I will show results of numerical experiments where we compute, using different techniques, the GW signal generated by the scalar dynamics of a global theory. I will also briefly discuss the ability of direct detection GW observatories to detect this background.
The stochastic gravitational wave background (SGWB) expected from cosmic string loops contains relevant information on the properties of the string network itself. In this talk, we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background under different hypothesis, determining the relevant parameter information that can be extracted from it.
In this talk I present a novel model of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework. We will start by reviewing the dark matter superfluid model and show how it describes the dynamics of dark matter in large and small scales. Then we will show that if the superfluid consists of a mixture of two distinguishable states with a small energy gap, such as the ground state and an excited state of dark matter, interacting through a contact interaction a new dynamics of late-time accelerated expansion emerges in this system, without the need of dark energy, coming from a universe containing only this two-state DM superfluid. I will show the expansion history and growth of linear perturbations, and show that the difference in the predicted growth rate in comparison to ΛCDM is significant at late times. The present theory nicely complements the recent proposal of dark matter superfluidity to explain the empirical success of MOdified Newtonian Dynamics (MOND) on galactic scales, thus offering a unified framework for dark matter, dark energy, and MOND phenomenology.
I will present current observational bounds on general Horndeski scalar-tensor theories of gravity, using data from the Planck, SDSS/BOSS and 6dF surveys. Using such theories as an example, I will also show how combining these observational bounds with insights from theoretical particle physics (e.g. stability criteria and positivity bounds) can drastically improve constraints and therefore allows us to test gravity with unprecedented precision.
In this talk I will discuss a scalar field model of dark energy that exhibits essentially no sensitivity to initial conditions and possesses a naturally suppressed effective mass and interactions in the late Universe. The magnitude of dark energy today is generated via an intricate conspiracy of numbers related to inflation, gravity and electroweak physics. arXiv:1905.00045
We consider a subclass of degenerate higher-order scalar-tensor (DHOST) theories in which gravitational waves propagate at the speed of light and do not decay into scalar fluctuations. The screening mechanism in DHOST theories evading these two gravitational wave constraints operates very differently from that in generic DHOST theories. We derive a spherically symmetric solution in the presence of nonrelativistic matter. General relativity is recovered in the vacuum exterior region provided that functions in the Lagrangian satisfy a certain condition, implying that fine-tuning is required. Gravity in the matter interior exhibits novel features: although the gravitational potentials still obey the standard inverse power law, the effective gravitational constant is different from its exterior value, and the two metric potentials do not coincide. We discuss possible observational constraints on this subclass of DHOST theories, and argue that the tightest bound comes from the Hulse-Taylor pulsar.
The by far strongest bound on the sum of the neutrino masses today comes from cosmological observations. Future surveys promise to even tighten this bound significantly and will be realized within the next decade. It is therefore crucial to be aware of parameter degeneracies and the main assumptions hiding behind the cosmological mass bound. We study the impact of non-standard momentum distributions of cosmic neutrinos on the anisotropy spectrum of the cosmic microwave background and the matter power spectrum of the large scale structure. We show that the neutrino distribution has almost no unique observable imprint, as it is almost entirely degenerate with the the neutrino mass and the effective number of neutrino flavours. Performing a Markov chain Monte Carlo analysis with current cosmological data, we demonstrate that the neutrino mass bound therefore heavily depends on the assumed momentum distribution of relic neutrinos.
Going beyond the primary CMB, there is overwhelming evidence that measuring the late time effects on the CMB photons (secondaries) will provide new and valuable information for cosmological inference, in particular upon cross-correlating with large-scale structure surveys. It has been shown recently (arXiv:1812.03167) that the near-future CMB surveys and galaxy surveys will have the statistical power to make a first detection of the moving lens effect, a CMB modulation due to changing gravitational potentials as a result of cosmological structure moving transverse to the line of sight. We will describe the velocity reconstruction method we developed and discuss applications for the reconstructed transverse velocities. The large-scale velocity modes reconstructed with the moving lens effect can be used to cancel cosmic variance for the purpose of constraining local non-Gaussianity, and in principle, they can be used to measure quantities such as the absolute growth rate, which is useful for studying dark energy, modified gravity, and the effects of neutrino mass. Our study opens up a new and promising channel of investigation for near-future surveys.
Next generation cosmic microwave background (CMB) experiments will allow us to put the most stringent constraints on primordial non-Gaussianity (PNG). However as these experiments push to increasingly small scales we become increasingly sensitive to biases and noise from other sky signals, such as extragalactic sources and gravitational lensing. In this work we explore how lensing acts as a source of noise for PNG searches. This effect, originally reported in Babich and Zaldarriaga (2004), dominates the bispectrum noise for squeezed shapes at small scales. We explore how important this effect will be for the Simons Observatory and stage 4 experiments, and how delensing can be used to reduce this effect.
Understanding the thermal properties of the cosmic gas is vital in order to mitigate important astrophysical systematics for cosmology. The relation between mass and gas pressure is a particularly relevant one, which affects both Sunyaev-Zel'dovich (SZ) cluster studies and weak lensing analyses. In this work, we use the SZ effect, quantified through the Planck Compton-y maps, and galaxy number counts from a set of low-redshift photometric surveys to make a tomographic measurement of the cosmic gas pressure at low redshifts. We compare our results with the constraints found by Planck based on cluster counts, as well as other existing analyses using alternative datasets.
Although in the Standard Model the electroweak phase transition is a crossover, many well-motivated extensions can generate a first-order phase transition at the electroweak scale. For a sufficiently strong phase transition, LISA would be able to observe gravitational waves sourced by plasma motion generated by expanding and colliding bubbles. While numerical simulations have examined ‘weak’ and ‘intermediate’ strength phase transitions, we conduct the first 3-dimensional simulations of strong first-order thermal phase transitions in the early Universe. We examine two types of transition, deflagrations and detonations. Detonations in strong transitions behave similarly to their weak and intermediate counterparts. In deflagrations, substantial vorticity is generated, hot droplets form, and the gravitational wave signal is reduced compared to previous models.
We revisit the effects of an early matter dominated era on gravitational waves induced by scalar perturbations. We carefully take into account the evolution of the gravitational potential, the source of these induced gravitational waves, during a gradual transition from an early matter dominated era to the radiation dominated era, where the transition timescale is comparable to the Hubble time at that time. Realizations of such a gradual transition include the standard perturbative reheating with a constant decay rate. Contrary to previous works, we find that the presence of an early matter dominated era does not necessarily enhance the induced gravitational waves due to the decay of the gravitational potential around the transition from an early matter dominated era to the radiation dominated era. This talk will be based on our paper, arXiv:1904.12878.
We study gravitational waves induced from the primordial scalar perturbations at second order around the reheating of the Universe. We consider reheating scenarios in which a transition from an early matter dominated era to the radiation dominated era completes within a timescale much shorter than the Hubble time at that time. We find that an enhanced production of induced gravitational waves occurs just after the reheating transition because of fast oscillations of scalar modes well inside the Hubble horizon. This enhancement mechanism just after an early matter-dominated era is much more efficient than a previously known enhancement mechanism during an early matter era, and we show that the induced gravitational waves could be detectable by future observations if the reheating temperature $T_{\text{R}}$ is in the range $T_{\text{R}} $\lesssim $7\times 10^{-2}\,\text{GeV}$ or $20 \, \text{GeV}$ \lesssim $T_\text{R}$ \lesssim $2 \times 10^7 \, \text{GeV}$. This is the case even if the scalar perturbations on small scales are not enhanced relative to those on large scales, probed by the observations of the cosmic microwave background. This talk will be based on our paper, arXiv:1904.12879.
Although Weakly Interacting Massive Particles (WIMPs) are promising candidates of dark matter, null results from various experiments cast doubt on WIMPs, implying the need to search for other candidates. Ultralight scalar fi?eld is one of the other dark matter candidates that is motivated by string theory. Interestingly, if it couples with Standard Model particles, it oscillates mirrors in gravitational-wave detectors and generates detectable signals. To extract information on ultralight scalar ?field dark matter from real data as much as possible, we studied its signal's characteristics in detail and developed
a suitable data-analysis method [1].
As a result, we found that the morphology of the signal's spectra is characterized by the frequency dispersion of the scalar ?field in the Galaxy and the period of the detector's motion. Then, we proposed two data analysis methods for that signal: (1)Incoherent sum of the spectra and (2)Narrow band stochastic gravitational-wave background search. Finally, we estimated its detectability with our analysis methods. We found that our methods can improve the existing constraints given by ?fifth-force experiments on one of the scalar fi?eld's coupling constants by a factor of O(10) to O(100) depending on its mass. Our study also demonstrated that experiments with gravitational-wave detectors play a complementary role to the Equivalence Principle tests.
[1]: Soichiro Morisaki and Teruaki Suyama. On the detectability of ultralight scalar fi?eld dark matter with gravitational-wave detectors. arXiv:1811.05003 [hep-ph].
The astrophysical stochastic gravitational-wave background is created by incoherent superposition of sources, such as merging binary black holes and binary neutron stars. The estimated merger rate of binary compact objects suggests that this background may be detected with ongoing and future gravitational-wave experiments. In this talk I will describe the theoretical predictions for the stochastic background from binary black holes and the astrophysical parameters we will be able to constrain with future observations. In particular, I will show that this background is expected to have an anisotropic component, which depends on the properties of binary black holes and their host galaxies. I will also discuss the possibility of cross-correlating the anisotropies of the stochastic gravitational-wave background with electromagnetic tracers of the large scale structure, such as galaxy number counts, and the complementarity of this signal with other gravitational-wave observations.
Astrophysical tests of the stability of fundamental couplings such as the fine-structure constant $\alpha$ and the proton-to-electron mass ratio $\mu$ are a key probe of fundamental physics and cosmology. A new generation of high-resolution spectrographs and improved statistical analysis techniques are enabling tests with improved sensitivities and larger redshift ranges. I will present new astrophysical measurements of $\alpha$, from the ESPRESSO collaboration and other facilities, and discuss their impact on models of dark energy. Time permitting I may also briefly highlight how the field will evolve in the coming years.
In this talk, I will discuss the decay of gravitational waves (GWs) into dark energy fluctuations $\gamma \rightarrow \pi\pi$ taking into account the large occupation numbers. We study the decay due to the $m_3^3$- and $\tilde{m}_4^2$-operators in the context of the effective field theory (EFT) of dark energy. It turns out that, in the regime of small GW amplitude corresponding to narrow resonance, the produced waves $\pi$ feature an instability that grows exponentially. However, once the $\pi$ non-linearities become important the previous analysis fails to describe the productions. This non-linear effect is relevant for the $m_3^3$-operator, while it can be neglected for the $\tilde{m}_4^2$-operator.
In this talk I will discuss the classical decay of gravitational waves into dark energy fluctuations $\pi$ in the context of the EFT of Dark Energy. For cubic Horndeski and beyond Horndeski theories, the gravitational wave acts as a classical background for $\pi$ and thus modifies its dynamics. In particular, for a sufficiently large amplitude of the wave, the kinetic term of $\pi$ becomes pathological, featuring gradient and ghost instabilities. For smaller gravitational wave amplitude, $\pi$ fluctuations are described by a Mathieu equation and feature instability bands that grow exponentially. The gravitational wave signal is affected by the $\pi$ back-reaction and this provides very stringent bounds on cubic and quartic GLPV theories.
Although quadratic curvature terms are well-motivated by leading quantum corrections to gravity and can be responsible for inflation, they generally lead to a massive spin-2 ghost. In this talk, instead of Riemannian geometry, we study quadratic curvature theories in four-dimensional Riemann-Cartan geometry where the torsion tensor does not vanish and can carry new degrees of freedom. Including all possible terms up to mass dimension four, we find a ghost-free quadratic curvature theory under some conditions where higher derivatives of the graviton perturbation are degenerate via curvature-torsion derivative couplings at least around the Minkowski background. The ghost-free theory has a massive spin-2 particle and a massive spin-0 particle in addition to the massless graviton. In the limit of the infinite mass of the torsion, these particles coincide with the well-known massive spin-2 ghost and massive spin-0 particle of the quadratic curvature theory in the metric formalism, while these can be non-ghost particles in a finite mass of the torsion.