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[7.12.2020]: The conference started on this day! A Zoom link was sent by email every evening to all registered participants. You were invited to please contact the Organizers if you had not received it.
NEWS [25.11.2020]: The conference was announced to be held fully online via Zoom. All registrants were informed they would receive the Zoom link just before the sessions. An email message was sent to all registered participants on this day.
NEWS [03.11.2020]: The conference was confirmed for 7-11 December 2020. The format was predicted to be most probably a fully online one.
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NEWS [28.9.2020]: The conference was confirmed for 7-11 December 2020 in person in Avignon (unless further changes). A web broadcasting was however also announced.
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NEWS [17.3.2020]: Because of the COVID-19 outbreak and in compliance with public health policies, the conference was moved to 7-11 December 2020.
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The conference addressed the cardinal issues of the dark universe today, gathering a selected number of scientists working in cosmology and particle physics in the inspiring and monumental setting of Avignon. There were a limited number of review talks by leading experts in each field and selected contributed talks, fostering thorough debates. Some time was allocated to discussion sessions.
Organizers:
Philippe Brax (CEA IPhT Saclay)
Chiara Caprini (CNRS APC Paris)
Marco Cirelli (CNRS LPTHE Jussieu Paris)
Christian Marinoni (CPT Marseille)
The standard LCDM cosmological model of structure formation provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few statistically significant anomalies and tensions between observations at early and late cosmological time that can indicate a failure of the LCDM model. I will show briefly these tensions, with a particular focus on the curvature of the universe, and the indication for a closed universe at a few percent level coming from the Planck 2018 data. This picture calls for a more conservative approach when discussing cosmological bounds on the parameters, and the necessity of further data and investigations to fully confirm a flat universe.
We consider a cosmological lepton asymmetry in the form of neutrinos and impose new expected sensitivities on such asymmetry through the degeneracy parameter ($\xi_\nu$) by using some future CMB experiment configurations, such as CORE and CMB-S4. Taking the default scenario with three neutrino states, we find $\xi_\nu =0.05±0.10(±0.04)$, from CORE (CMB-S4) at 95 per cent CL, respectively. Also, within this scenario, we evaluate the neutrino mass scale, obtaining that the normal hierarchy mass scheme is privileged. Our results are an update concerning on the cosmological lepton asymmetry and the neutrino mass scale within this context, from which can bring a perspective on the null hypothesis for $\xi_\nu$ (and its effects on $\Delta N_{eff}$), where perhaps, $\xi_\nu$ may take a non-null value up to 95 per cent CL from future experiments such as CMB-S4. Sensitivity results for CMB-S4 obtained here not including all expected systematic errors.
The LOFAR Two-metre Sky Survey (LoTSS) will be the worlds largest catalogue of radio sources at low radio frequencies for the next decades. It will also have photo-z information from the cross-matching with optical and infrared surveys for most radio sources. This allows us to address cosmological questions via a new frequency window to the Universe, among them the cosmic radio dipole and the measurement of angular correlation function for shells at different redshifts. Siewert et al. have demonstrated that already the products of the LoTSS-DR1 catalogue are consistent with the Planck 2018 results. I will discuss the observational and theoretical challenges for cosmological studies of the large scale structure at low radio frequencies and will show first results from the analysis of LoTSS-DR2.
The Hubble tension is relieved if the sound horizon at decoupling, r, is reduced. This can happen as a result of the enhanced recombination induced by primordial magnetic fields or caused by more exotic new physics. However, when trying to bring the H0 value measured from CMB into a complete agreement with SH0ES by reducing r alone, one runs into a disagreement with either the BAO or the weak lensing data.
We present a detailed investigation of a sub-dominant oscillating scalar field ('early dark energy', EDE) in the context of resolving the Hubble tension. Consistent with earlier work, but without relying on fluid approximations, we find that a scalar field frozen due to Hubble friction until log10($z_c$)∼3.5, reaching ρ$_{\rm EDE}$($z_c$)/ρ$_{\rm tot}$∼10%, and diluting faster than matter afterwards can bring cosmic microwave background (CMB), baryonic acoustic oscillations, supernovae luminosity distances, and the late-time estimate of the Hubble constant from the SH0ES collaboration into agreement. A scalar field potential which scales as $V(ϕ)∝ϕ^{2n}$ with 2≲n≲3.4 around the minimum is preferred at the 68% confidence level, and the Planck polarization places additional constraints on the dynamics of perturbations in the scalar field. In particular, the data prefers a potential which flattens at large field displacements. An MCMC analysis of mock data shows that the next-generation CMB observations (i.e., CMB-S4) can unambiguously detect the presence of the EDE at very high significance. This projected sensitivity to the EDE dynamics is mainly driven by improved measurements of the E-mode polarization. We also explore new observational signatures of EDE scalar field dynamics: (i) We find that depending on the strength of the tensor-to-scalar ratio, the presence of the EDE might imply the existence of isocurvature perturbations in the CMB. (ii) We show that a strikingly rapid, scale-dependent growth of EDE field perturbations can result from parametric resonance driven by the anharmonic oscillating field for n≈2. This instability and ensuing potentially nonlinear, spatially inhomogenoues, dynamics may provide unique signatures of this scenario.
Dark matter problem is a key problem in astrophysics. Recent gamma-ray and anti-proton detections suggest that dark matter annihilating via b quark channel can explain the Galactic center gamma-ray excess and the anti-proton excess simultaneously. Besides, recent studies show that radio data can also give stringent constraints for dark matter. Based on our recent studies using the radio continuum spectral data of the Ophiuchus cluster and the Abell 4038 cluster, we have figured out some potential signals of dark matter annihilation. The constrained mass range (~40-50 GeV) and annihilation channel (b quark) are consistent with the previous studies. This provides some hints for detecting dark matter signals by radio observational data.
Despite their incredible precision, both concluded and upcoming CMB missions (such as Planck, CMB-S4, or LiteBIRD) still face several intrinsic limitations that can only be overcome with the help of complementary probes. One particularly interesting avenue to extract more information from the CMB is given by its spectral distortions (SDs). Since these distortions are created whenever the energy or number density of the CMB photons is modified, they are an ideal candidate to constrain both exotic and non-exotic energy injection scenarios. In this talk, following the novel CLASS implementation of SDs, I will provide a brief pedagogical introduction to the topic of SDs, and discuss their application to a selection of examples including decaying dark matter and primordial black holes. The presented results will show the far-reaching possibilities of combining CMB anisotropies and SDs.
The origin of the baryon asymmetry of the Universe (BAU) and the nature of dark matter are two of the most challenging problems in cosmology. I will present a scenario in which the gravitational collapse of large inhomogeneities at the QCD epoch generates both the baryon asymmetry and the dark matter in the form of primordial black holes (PBHs). It would naturally explains the observed BAU and why the baryons and dark matter have comparable densities. No parameter fine-tuning is required if the PBH originate from the fluctuations of a light stochastic spectator field during inflation. The predicted wide mass distribution of PBH ranges from sub-solar to several hundred solar masses. It can evade the current limits on the PBH abundance and explain the mass, rate and low effective spins of the black hole mergers detected by LIGO-Virgo, as well as a series of cosmic conundra.
Profound evidence for the existence of dark matter has been collected throughout the past 100 years. However, its exact nature remains elusive. A large effort is being put into the search for direct detection of weakly interacting massive particles (WIMPs), which arise as dark matter particle candidates in various theories. The search is led by dual-phase liquid xenon time projection chambers for masses above 5 GeV/c2. The most sensitive experiment, XENON1T, probes spin-independent (SI) WIMP-nucleon interactions down to 4.1 × 10−47 cm2 for 30 GeV/c2 WIMP mass. This limit refers to the SI isoscalar channel, which, for vanishing momentum transfer q, scales quadratically with the number of nucleons A. The SI interaction thus yields the dominant nuclear response, making it the standard search channel in the field.
We will browse through the different technical approaches used in the present dark matter direct detection scenario as well as its future prospects.
In this work we estimated the cosmological constant in a pioneering approach by using galactic superclusters in the layout of $f(R,T)$ gravity. We set $f(R,T) = R + 2\lambda T$ where $\lambda$ is the model parameter. We report that appropriate values of $\lambda$ generate cosmological constant ($\Lambda$) values in harmony with observational value of $1.1056\times10^{-52}m^{-2}$. We also delineate that for $\lambda = 0$ which corresponds to GR, yields physically unacceptable results.
In a variety of theories, DM interacts with gauge bosons or scalars that induce long-range interactions because of repeated soft exchanges. Remarkably, the inclusion of bound-state effects for DM annihilation has been recently shown to have a large impact on the relic density and, therefore, on the parameters of a given model to be compatible with observations. At the same time, it is manifestly subtle and complicated to include bound-state dynamics in a thermal medium due to the intricate interplay between non-relativistic and thermal energy scales. Starting from a thermal field theoretic formulation of the problem, we use an effective field theory approach to describe bound-state formation and dissociation, Sommerfeld effect, DM thermal masses and interaction rates. We show the phenomenological impact of such framework for wimp-like models with mediators to the visible sector. Moreover, we discuss some shortcomings in the current rate equations that limit the validity of existing results in the literature for even simpler dark matter models.
We consider scalar field models of dark matter, with a mass in the range 10^{-21} << m << 10^{-3} eV. In the nonrelativistic regime, derivative or potential self-interactions can give rise to an effective pressure that builds equilibrium configurations (solitons) in galactic halos. We extend the analysis to the relativistic regime, down to the horizon of the supermassive galactic Black Hole (BH). We discuss when the large-scale soliton is eaten by the central BH or survives on timescales much greater than the age of the Universe.
Theoretical models and observations suggest that large fractions of Pop III stars end as black holes (BHs) in High-Mass-X-ray Binaries (BH-HMXBs), which are sources of hard X-rays and synchrotron relativistic jets called Microquasars (MQs ref.1). The hard X-rays from these accreting BHs of Pop III have long free paths, pre-heat the Intergalactic Medium (IGM), and lead to a smooth end of the re-ionization epoch (ref. 2).
We will show that the relativistic jets from the remnants of Pop III stars, namely, the BH-HMXB-MQs of Pop III, produce a Synchrotron Cosmic Radio Background (CRB) that can account for the excess amplitude and bottom-flat absorption of atomic hydrogen at z~17 (78 MHz), tentatively reported by EDGES (ref. 3). In fact, the existence of a Synchrotron CRB had been inferred from the NASA ARCADE 2 Experiment (ref. 4). Recently, it was proposed that --along with the hard X-ray cosmic background-- that CRB is the smoking gun of Pop III stars (ref. 5).
References:
1) Mirabel, I.F. & Rodríguez, L.F. 1998, Nature, 392, 673-677
2) Mirabel I. F., Dijkstra M., Laurent Ph., Loeb A., Pritchard J. R., 2011, A&A, 528, A149
3) Bowman J. D., Rogers A. E. E., Monsalve R. A., Mozdzen T. J., Mahesh N., 2018, Nature, 555, 67 EP
4) Fixsen D.J., Kogut A., Levin, S. et. al. 2011, ApJ, 734, 5
5) Mirabel (2019): Review at IAU Symp. 346 (arXiv#1902.00511)
Primordial black holes can form in the early Universe from the collapse of cosmological perturbations after the cosmological horizon crossing. They are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed today in the centre of galaxies. In calculations of spherically symmetric collapse, a Lagrangian relativistic hydrodynamical code is used to follow the non linear evolution. If the perturbation is larger than a threshold depending on the equation of state and on the specific shape of the perturbation, a black hole is formed. In this talk I will discuss the dependence of PBH formation from the initial shape of the curvature profile, showing the relation with the shape of the inflationary power spectrum. This allows to compute consistently the abundance of PBHs. Depending on the model, a proper calculation shows that the abundance of PBHs might be significantly increased by several order of magnitudes compared to previous estimations.
The chirp mass spectrum, revealed by LIGO, demonstrates impressive agreement with the prediction that the black holes forming the original binaries have log-normal mass spectrum. The central mass of the log-normal spectrum nicely fits the theoretically favored value around 10 solar masses.
On the other hand, the assumption of astrophysical origin of the primary black holes forming binaries quite poorly describes the data for several traditional models of their formation.
With the found values of the spectrum parameters (almost) all black holes in the universe in all mass ranges can be primordial. This conjecture allows to resolve a big lot of cosmological and astrophysical conundra.
The light mediator scenario of self-interacting dark matter is strongly constrained in many ways. After summarizing the various constraints, we discuss minimal options and models which allow to nevertheless satisfy all these constraints.
The calculation of tunneling actions, that control the exponential
suppression of the decay of metastable phases (like the unstable
electroweak vacuum), can be reformulated as an elementary variational
problem in field space. This alternative approach circumvents the use of
bounces in Euclidean space by introducing an auxiliary function, a
tunneling potential Vt that connects smoothly the metastable and stable
phases of the field potential V. The tunneling action is obtained as the
integral in field space of an action density that is a simple function
of Vt and V and can be considered as a generalization of the thin-wall
action to arbitrary potentials. This formalism provides new handles for
the theoretical understanding of different features of vacuum decay, can
be easily extended to include gravitational effects in an elegant way
and has a number of useful applications that I will discuss.
Generalized coupling theories are characterized by a nontrivial coupling between the gravitational metric and matter, which is mediated by an auxiliary rank-2 tensor. The actions generating the field equations are constructed so that these theories are equivalent to general relativity in a vacuum, and only differ from Einstein's theory within a matter distribution. This talk will focus on one of the simplest realizations of these theories, termed the MEMe model. The MEMe model admits an exact solution for the coupling for a single perfect fluid. An analysis of the evolution of homogeneous and isotropic spacetimes in the MEMe model reveals the existence of cosmic histories with both an inflationary phase and a dark era characterized by a different expansion rate. I also discuss the propagation speed of GWs through matter and some recent work on the PPN analysis for the MEMe model.
The dipole anisotropy of the CMB is believed to be due to our motion with respect to the `CMB rest frame' at 369 km/s. This should also cause a dipolar modulation in the number counts of distant sources, through aberration & Doppler boosting. We test this with various all-sky catalogues: NVSS & SUMSS radio galaxies, WISE galaxies & quasars, as well as GAIA-unWISE AGNs, consistently finding a significantly larger dipole than expected (implying velocities >1000 km/s) with statistical significance upto 3.3 sigma. These observations indicate a bulk flow between the matter & radiation rest frames in the local Universe, extending out to scales larger than is expected in LCDM. An observational effect of such a bulk flow would be a scale-dependent dipolar modulation in the deceleration parameter. We look for this in the SDSS-II/SNLS-III Joint lightcurve Analysis catalogue of SN Ia and find such a dipole with 3.9 sigma significance, while the evidence for isotropic acceleration simultaneously drops to 1.4 sigma. This talk will conclude by reviewing the history of supernova data fitting, focussing on statistical methods & data quality issues. Both dark energy & the Hubble tension seem to be artefacts of fitting data to an idealized model of the universe.
If the length scale of possible extra dimensions is large enough, the effective Planck scale is lowered such that microscopic black holes could be produced in collisions of high-energy particles, which opens up a plethora of novel phenomena in terrestrial detectors and in the early Universe. Microscopic black holes from high-energy cosmic neutrino-nucleon collisions are characterized by unique topologies, distinct energy distributions and unusual ratios of hadronic-to-electronic energy deposition, visible through Cherenkov light echos due to delayed neutron recombination in IceCube-like detectors. In addition, these black holes evaporate through the emission of all particles that are kinematically and thermally allowed, including dark matter. This enables us to study the properties of dark sector from the missing momentum signatures at the next generation of colliders, regardless of the strength of the coupling between dark matter and the Standard Model. The dark matter produced from microscopic black hole decay in the early Universe may account for part or all the dark matter relic density today if the reheating temperature is close to the Planck scale in the bulk, which serves as a new dark matter production mechanism even in the absence of non-gravitational dark matter-Standard Model coupling.
This talk will be mainly based on the paper; R. Erdem and K. Gultekin. JCAP 10 (2019) 061. By considering dynamics of a scalar Bose-Einstein condensation at microscopic level, we study the initial phase of formation of condensation in cosmology. To this end, first we introduce an effective Minkowski space formulation that enables considering only the effect of particle physics processes, excluding the effect of gravitational particle production and enabling to see cosmological evolution in an easier way. Then, by using this formulation we study a model with trilinear coupling $ϕ^2 χ$ that induces the processes χχ⟶ϕϕ. After considering the phase evolution of the produced ϕ particles, we find that they evolve towards formation of a Bose-Einstein condensate provided some conditions are satisfied. In principle, the effective Minkowski space formulation introduced in this study can be applied to particle physics processes in any spacetime that is sufficiently smooth.
We show that Dark Matter consisting of ultralight bosons in a Bose-Einstein condensate induces, via its quantum potential, a small positive cosmological constant which is close to the observed value. This explains why the densities of Dark Matter and Dark Energy are approximately equal.
[NOTE ADDED MC 2020.03.03 if abstract is selected, talk scheduled for 27, 28 or 29 April (i.e. Monday, Tuesday or Wednesday, and not on Thursday the 30th April)]
If we consider sterile neutrino with a O(keV) mass as Warm Dark Matter candidates, produced in the early universe through admixtures with the active neutrinos, via the Dodelson-Widrow or the Shi-Fuller mechanisms, strong constraints on the active-sterile mixing angle are imposed by the observations in the X-ray band and the measurements of the total DM abundance. These constraints, that in a standard scenario would seriously put at risk the possibility of getting a signal of the existence of such sterile neutrinos in laboratory experiments in the near future, can be largely relaxed in low reheating temperature scenarios in which $\Omega_{DM}$ is constituted by a cocktail of different candidates, among which there are also sterile neutrinos, or in the case in which the decay rate of sterile neutrinos is reduced by the action of new physics.
The properties of universes are explored that are entirely in the interior of black holes in another universe, a `mother universe'. It is argued that these models offer a paradigm that may shed a new light on old cosmological problems. The geometry of such a universe is discussed including how it would appear to the observer. The Hubble parameter is direction dependent, but it is argued that the interpretation of any such dependence will be hard to separate from local inhomogeneities. The models do not originate from a big bang, but rather from an initial collapse and subsequent infall, that started probably a very long time ago, presumably much earlier than the accepted age of the universe. The relation to the concordance model is discussed and it is shown that a lot of the existing theory can be taken over into the proposed models. The universe has an edge, which is an ordinary sphere in 3 dimensions. That sphere acts as a gravitational mirror as seen from inside the universe, but it does not mirror redshift. The same object can thus be seen in direct sight and in reflection, although with different redshifts, different ages and different aspect angles. The models do not need dark energy, but they need dark matter, of course. Since the models are closed and neutrino's are nowadays believed to have mass, neutrino's can be reconsidered as candidates for the dark matter. As a bonus result from this paradigm, mass ejection from black holes is shown to be possible, which links that process to the controversial anomalous galaxy redshifts. Finally, we show that gravitational mass and inertial mass are proportional, and that the inertial acceleration scales as $c^2/M$, with $M$ a characteristic scale of the universe.
We consider a model for dark matter with a dark U(1) charge and where the gauge mediator is described by a non-linear electromagnetic theory, being the Born-Infeld theory a paradigmatic example. These theories are equipped with a K-mouflage screening mechanism so that only at late times and on small scales will the dark electromagnetic force have a non-negligible effect. This scenario naturally leads to a cosmological evolution where the late-time universe is described by a Lemaître inhomogeneous model. The potential role of this model regarding the H0 tension will be discussed.
I will introduce the concept of standard siren, reviewing the methodologies that one can apply to probe the cosmic expansion using gravitational wave observations. I will then outline the gravitational wave sources that can be used as standard sirens for both Earth-based (LIGO/Virgo/3G) and space-based (LISA) detectors, pointing out for which of them an electromagnetic counterpart is expected to be observed. I will then discuss the constraints on the Hubble constant obtained with the recent LIGO/Virgo observations and what they will be able to tell us in the future. Finally I will present cosmological forecasts for LISA, which will be able to map the expansion of the universe at high redshift and probe cosmological models beyond LCDM in yet untested regimes.
We investigate the correlation between the distribution of galaxies and the predicted gravitational wave background of astrophysical origin. We show that the average contribution to the background as a function of redshift can be easily constrained by cross-correlating with galaxy catalogs at different redshifts. Furthermore, the interpretation of this signal allows us to address the discrepant predictions for the autocorrelation signal available in the literature. Because we show that the impact of shot noise is negligible, our results suggest that the gravitational-wave background, when combined with near-future galaxy surveys, is a powerful probe for both gravitational-wave merger physics and cosmology.
We will talk about Primordial Gravitational Waves (PGW): particularly we discuss the PGW spectrum in non-standard cosmology and in modified gravity theories, in early Universe cosmology, specifically in scalar-tensor and extra dimensional gravity scenarios, investigating the detection prospects in current and future GW observatories which can be potentially observed by laser interferometers operating in the high-frequency range and at low frequencies with pulsar timing arrays respectively. We will see that data from the planned network of several GW detectors operating across various frequency ranges will be able to distinguish between various modified gravity and non-standard cosmological history scenarios.
Solutions of string theory with a four-dimensional de Sitter space-time could serve as an interesting starting point to build cosmological models of the early universe, or the present one. Obtaining such solutions in a well-controlled manner is however notoriously difficult. This has recently led to various conjectures, in the context of the swampland program, that strongly constrain the existence of de Sitter solutions in string effective models, with drastic consequences regarding single field inflation or quintessence. Focusing on classical string backgrounds, these conjectures can be precisely tested, with sharp no-go theorems, as well as promising counter-examples, that could eventually serve in stringy cosmological models. We will present an overview and new results on this topic.
We revisit the scalar singlet dark matter (DM) scenario with a pair of dark lepton partners which form a vector-like Dirac fermionic doublet. The extra doublet couples with the SM leptonic doublet and the scalar singet via a non-SM-like Yukawa structure. As a result, (1) since the extra fermionic states interact with other dark sector particles as well as the SM via gauge and Yukawa interactions, it gives rise to new DM annihilation processes including pair annihilation as well as coannihilation channels, allowed by the existing experimental constraints and (2) such a Yukawa structure opens up new production channels for leptonic final states giving much enhancement in cross sections to search for dark matter in the LHC. The DM freeze-out scenario becomes more interesting if a dark singlet fermion is added in the particle spectrum. In the cases of mixing between the charged dark leptons, the mixing angle gives rise to interesting features in DM coannihilation. It further dictates the dominant coannihilating partner from an ensemble of nearly degenerate dark leptons. We further show how the mixing leads to interesting signatures in the distributions of weak gauge boson mediated processes in an collider environment.
Despite its remarkable success, the standard LCDM paradigm has been challenged lately by significant tensions between different datasets. This has boosted interest in non-minimal dark sectors, which are theoretically well-motivated and inspire new search strategies for DM. With this in mind, we have developed a new and efficient version of the Boltzmann code CLASS that allows for one DM species to have multiple interactions with photons, baryons, and dark radiation simultaneously. In this talk I will present our new results obtained using this framework, where we have reassessed existing cosmological bounds on the various interaction coefficients in multi-interacting DM scenarios, as well as investigating the possibility of these models to alleviate the cosmological tensions. The upcoming public release of our code will pave the way for the study of various rich dark sectors.
Early Dark Energy (EDE) contributing a fraction f_EDE(z_c) ∼ 10% of the energy density of the universe around z_c = 3500 and diluting as or faster than radiation afterwards, can provide a simple resolution to the Hubble tension, the ∼ 5σ discrepancy – in the ΛCDM context – between the H0 value derived from early- and late-universe observations. However, the inclusion of Large-Scale Structure (LSS) data, which are in ∼ 3σ tension with both ΛCDM and EDE cosmologies, might break some parameter degeneracy and alter these conclusions.
I will discuss the viability of the EDE scenario in view of a host of high- and low-redshift measurements, including LSS observations from recent weak lensing surveys, CMB, Baryon Acoustic Oscillation (BAO), growth function and Supernova Ia (SNIa) data, as well as the full-shape galaxy power spectrum from BOSS/SDSS, analyzed using the effective field theory (EFT) of LSS. I will show that the EDE cosmology still provides a potential resolution to the Hubble tension when confronted against current LSS data, though upcoming spectroscopic galaxy surveys, such as Euclid and DESI, will put it under stringent new tests. Finally, I will reassess the EDE scenario in light of the CMB lensing anomalies in Planck data, and I will outline further theoretical extensions that could allow to fully restore cosmological concordance.
Recent weak lensing surveys have revealed that the direct measurement of the parameter combination S8 = σ8 (Ωm/0.3)^0.5-- measuring the amplitude of matter fluctuations on 8 Mpc/h scales -- is ∼3σ discrepant with the value reconstructed from cosmic microwave background (CMB) data assuming the ΛCDM model. In this talk, I discuss that it is possible to resolve the tension if dark matter (DM) decays with a lifetime of Gamma^{-1} ∼ 55 Gyrs into one massless and one massive product, and transfers a fraction ε ∼ 0.7 % of its rest mass energy to the massless component. The velocity-kick received by the massive daughter leads to a suppression of gravitational clustering below its free-streaming length, thereby reducing the σ8 value as compared to that inferred from the standard ΛCDM model, in a similar fashion to massive neutrino and standard warm DM. Contrarily to the latter scenarios, the time-dependence of the power suppression and the free-streaming scale allows the 2-body decaying DM scenario to accommodate CMB, baryon acoustic oscillation, growth factor and uncalibrated supernova Ia data.
Dark matter (DM) in cosmic structures is expected to produce signals originating from its particle physics nature, among which the electromagnetic emission represents a relevant opportunity. One of the major candidates for DM are weak-scale particles, however no convincing signal of them has been observed so far. For this reason, alternative candidates are getting increasing attention, notably sub-GeV particles, which are the subject of our work. The challenge in indirect detection of sub-GeV DM is that there is scarcity of competitive experiments in the energy range between 1 MeV and hundreds of MeV, hence we need to find alternative ways to study DM candidates with mass in this energy window. In our work we proposed to look at energies much lower than the mass of the sub-GeV DM particles by including the contribution from Inverse-Compton scattering (ICS) in the total flux. In particular, the electrons and positrons produced by DM particles give rise to X-rays by upscattering the low-energy photons of the radiation fields in the Galaxy (CMB, infrared from dust, optical starlight). These X-rays fall in the energy range covered by the INTEGRAL data, which we used to determine conservative bounds on the DM annihilation cross-section. We considered three annihilation channels: electron, muon and pion. As a result, we derived competitive constraints for DM particles with a mass between 150 MeV and 1.5 GeV.
Thermal freeze-in is a simple scenario of dark matter production that can however be very difficult to probe. We discuss a decaying dark matter setup in which the feeble coupling required to reproduce the observed relic abundance gives rise to transitions between dark matter components, resulting in gamma ray spectral features.
In the present work, we have presented and analyzed the cosmological models of the universe with an anisotropic variable parameter. We have set up the ?field equations with the space time in the form of Bianchi I metric with an f(R;T) gravity. The functional form for the f(R;T) gravity assumed to be f(R;T) = R + 2f(T), where R and T respectively the Ricci scalar and trace of energy momentum tensor. Two different models are constructed with respect to the scale factors, such as Power law scale factor and Hybrid scale factor. Moreover, the anisotropic parameter taken here in the form of hyperbolic function that further gives clarity on the behaviour of Equation of State (EOS) parameter. The models can be reduced to isotropic universe when the coefficient constant vanishes. For both the cases,the deceleration parameter, state fi?nder diagnostic pairs and energy conditions have been obtained and analyzed which provide physical plausibility of the models.
Keywords: Modified Gravity; Cosmology; Anisotropy; Equation of State; Deceleration Parameter.
Particle production by oscillating curvature in $R+R^2$ cosmology is considered. It is shown that the cosmological density of massive stable relics may be close to the observed density of dark matter. The proper range of mass values depends on the channel of the scalaron decay. In particular it opens the window for heavy supersymmetric particles to be viable dark matter constituents.
The large-scale structure growth index γ provides a consistency test of the standard cosmology and is a potential indicator of modified gravity. We investigate the constraints on γ from next-generation spectroscopic surveys (like SKA, Euclid and DESI), and possible improvements from combining these using a multi-tracer technique. Using the angular power spectrum, which is observed in redshift space, we avoid the need for an Alcock-Packzynski correction. It also naturally incorporates cosmic evolution and wide angle effects, without any approximation. We include the cross-correlations between redshift bins, using a hybrid approximation
when the total number of bins is computationally unfeasible.
We derive the modified Friedmann equations based on non-extensive Tsallis thermodynamics. This model admits an accelerated expansion for the universe filled by ordinary matter and without needing dark energy. The age problem can also be alleviated in this model.
The generation of primordial magnetic fields and its interaction with the primordial plasma during cosmological phase transitions is turbulent in nature. I will describe and discuss results of direct numerical simulations of magnetohydrodynamic (MHD) turbulence in the early universe and the resulting stochastic gravitational wave background (SGWB). In addition to the SGWB, the primordial magnetic field will evolve up to our present time and its relics can explain indirect observations of weak magnetic fields coherent on very large scales. I will apply the numerical results to magnetic fields produced at the electroweak and the QCD phase transitions and show that these signals may be detectable by the planned Laser Interferometer Space Antenna and by Pulsar Timing Array. The detection of these signals would lead to the understanding of cosmological phase transition physics, which can have consequences on the baryon asymmetry problem and on the origin seed of observed magnetic fields coherent over very large scales at the present time.
Extra dimensions (ED) have been used as attempts to explain several phenomena in particle physics, such as the hierarchy and flavor problems. The interaction between new mediators in the bulk (vector, scalar of fermion fields) and the Standard Model (SM) particles can be naturally suppressed if one employs a single, flat ED. In this setup, the SM fields are localized in a finite width ‘fat’ brane, similar to models of Universal Extra Dimensions. A dark matter (DM) candidate is confined to a thin brane at the opposite end of the ED interval. Including brane localized kinetic terms on the fat brane for the mediator fields, the resulting coupling between the SM and these mediators can be several orders of magnitude smaller than the corresponding ones between the mediators and DM. The implications of this scenario is investigated for both vector (dark photon, DP) and scalar mediator fields in the 5-D bulk. The SM particles couple to the DP via their $B − L$ charges while the DP couples to the DM via a dark charge. Both the vector DP couplings and the corresponding Higgs portal couplings with the SM are shown to be naturally small in magnitude with a size dependent on ratio of the 5-D compactification radius and the SM brane thickness. This mechanism is also studied in 6-D. Finally, if a Dirac fermion is present in the bulk, it results (in 4-D) in two towers of Kaluza-Klein Majorana sterile neutrinos, whose mass mixing with the SM neutrinos is also suppressed. The seesaw mechanism is therefore obtained, and sterile neutrino masses of order $\mathcal{O}(1-10)$ TeV naturally explain the small SM neutrino mass.
If inflation is followed by a period of kinetic domination, as is common in quintessential inflation scenarios, internal symmetries of non-minimally coupled spectator fields will be generically broken. This generates a complex dynamics for this field and leads to the formation of defects. One of the most relevant consequences is that this mechanism will tend to produce a, potentially detectable, gravitational waves background. Interestingly, the symmetry will be eventually restored implying the eventual decay of the defects.
Degenerate scalar-tensor theories of gravity extend general relativity by a single degree of freedom, despite their equations of motion being higher than second order, a virtue made possible by the existence of an additional constraint that removes the would-be ghost. This constraint can however be obstructed by matter fields, even when minimally coupled to the metric. In this talk I will present this issue in detail, explaining through some illustrative examples the precise ways in which the extra degree of freedom may reappear. I will next turn to the more physically relevant case of fermionic matter, and show that spin-1/2 fermions evade these issues and can thus be consistently coupled to degenerate theories of scalar-tensor gravity.
We propose a 1/N expansion of Starobinsky and Yokoyama’s effective stochastic approach for light quantum fields on superhorizon scales in de Sitter spacetime. We explicitly compute the spectrum and the eigenfunctions of the Fokker-Planck operator for a O(N)-symmetric theory with quartic selfinteraction at leading and next-to-leading orders in this expansion. We obtain simple analytical expressions valid in various nonperturbative regimes in terms of the interaction coupling constant.
The IIB matrix model has been suggested as a a particular formulation
of nonperturbative superstring theory (M-theory).
It has now been realized that an emerging classical spacetime must
reside in its large-N master field.
The master field of the Lorentzian IIB matrix model can, in principle,
give rise to the regularized-big-bang metric of general relativity.
The length parameter of the regularized-big-bang metric is then calculated
in terms of the IIB-matrix-model length scale.