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 was a limited number of review talks by leading experts in each field and selected contributed talks, fostering thorough debates. Some time was supposed to be 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)
Géraldine Servant (DESY Hamburg)
Nicola Tamanini (CEA IPhT Saclay)
Quintessential Inflation attempts to account for the tunings of the Hot Big Bang and for Dark Energy, using a single degree of freedom and a single theoretical framework, with the aim to avoid the extreme fine tuning needed for the cosmological constant in LambdaCDM. However, in the past the task proved to be very difficult. We will present how modern developments, e.g. in the context of Gauss-Bonnet inflation or with alpha-attractors in supergravity theories, can decisively overcome the problems of Quintessential Inflation and render it natural and compelling.
We consider an eight-dimensional Einstein-Yang-Mills theory to study whether Yang-Mills instantons formed in extra-dimensions can trigger cosmic inflation in our four-dimensional spacetime. We observe that the Yang-Mills instantons in extra dimensions and homogeneous in four dimensional spacetime act as a cosmological constant for the four-dimensional Einstein gravity. As a result,we study whether the cosmic inflation in our four-dimensional spacetime can be triggered by the Yang-Mills instantons.
We consider the most minimal scale invariant extension of the standard model that allows for successful radiative electroweak symmetry breaking and inflation. The framework involves an extra scalar singlet, that plays the role of the inflaton, and is compatibile with current experimental bounds owing to the non-minimal coupling of the latter to gravity.
This inflationary scenario predicts a very low tensor-to-scalar ratio $r \approx 10^{-3}$, typical of Higgs-inflation models, but in contrast yields a scalar spectral index $n_s \simeq 0.97$ which departs from the Starobinsky limit. We briefly discuss the collider phenomenology of the framework.
Current measurements of the Higgs boson and top quark mass favor metastability of the electroweak vacuum in the Standard Model. This raises some questions when we consider the evolution of our universe: how did it end up in such an energetically disfavored state? Why it remained there during inflation? These problems can be addressed by assuming for the Higgs a direct coupling with the inflaton and/or a non-minimal coupling to gravity. In this talk I will review the effects of these interactions on the Higgs dynamics during the inflationary period and the subsequent period of particle production, namely reheating.
I will present a minimal extension of the Standard Model that addresses dark matter, the strong CP problem, the smallness of neutrino masses, baryogenesis and primordial inflation. The model contains a new U(1) symmetry and a single new physics scale of the order of 10^11 GeV. Dark matter is made of axions, whose mass in predicted to be in a narrow range, which will be probed in the near future. Remarkably, inflation does not suffer from unitarity issues, unlike in other minimal proposals, and reheating can be computed in detail since all the particle content up to the Planck scale is assumed to be known. This allows to draw sharp predictions for the CMB properties.
I will discuss the appeal of pseudo-Goldstone bosons (pGBs) for the generation of scales in Early Universe cosmology. In particular, I will show how Goldstone Inflation addresses the inflationary hierarchy problem (the tension between the Lyth bound and the scale of inflation as preferred by CMB anisotropies), while avoiding the problems with trans-Planckian scales that are typically associated with related models.
I will explore compact models based on the coset SO(n+1)/SO(n) and non-compact models based on a SO(n,1)/SO(n). I will show how both setups can give rise to inflation compatible with the current data, and discuss different scenarios for reheating in both setups.
In the conventional Big Bang picture, our current (perturbative) theories of gravity are not powerful enough to reliably capture the earliest moments of our Universe – they break down due to the large curvatures and high energies. An alternative early Universe scenario is a 'non-singular bounce’, in which an initially contracting phase bounces into an expanding Universe like the one we live in today. The bounce can take place at sub-Planckian energy scales, allowing us to apply all of our perturbative field theory techniques. However, attempts to realise such bouncing behaviour in General Relativity have encountered problems: in order for the matter sector to drive a bounce, it must violate the Null Energy Condition - which generically leads to unstable modes which quickly grow out of control. This talk reviews the recent progress in taming these instabilities in P(X) scalar field theories, and shows how higher derivative modifications to the theory in the UV can preserve stability and unitarity throughout the bounce. Significantly, the resulting theory is capable of describing an early Universe bounce entirely within a perturbative regime, is free from classical instabilities, and obeys tree-level unitarity constraints.
If the electroweak sector of the standard model is described by classically conformal dynamics, we show that the electroweak phase transition can be triggered by the chiral condensation of six massless quarks in the standard model in the supercooled universe.
The phase transition is first-order and occurs below the QCD scale temperature.
One of the phenomenological consequences of this scenario is sizable gravitational waves from the bubble collisions.
We derive the necessary conditions for the scenario to occur, using the specific example of the classically conformal B-L model.
We also briefly mention other cosmological implications, such as possibilities for electroweak baryogenesis and altered dark matter productions.
Rotational superradiance was theoretically shown to occur in black hole spacetimes; in the presence of massive bosonic degrees of freedom, superradiance triggers an instability that leads to peculiar gravitational-wave signatures and black hole distribution in the spin-mass plane, which in turn can impose stringent constraints on ultralight fields. In this talk, I will demonstrate that a similar effect occurs with rotating conducting spheres, and I will discuss rotational superradiance effects around conducting stars. Our results can also be applied to understand the interaction of stars with massive hidden photons. In this case, the rotating stars are unstable on timescales that depend on the mass of the hidden photon, and on the rotation rate, compactness, and conductivity of the star.
The discovery of the accelerated expansion of the universe triggered an intense activity in infrarred modifications of gravity with an additional scalar degree of freedom. This scalar is then used to replace the cosmological constant as the responsible for the cosmic acceleration. A common problem in these models is that this scalar must be very light to have cosmological effects today. However, it typically mediates a long-range force that has not been observed in local gravity tests and this severely constrains such models. A resolution to this problem came about with the implementation of screening mechanisms that allow to avoid local gravity tests while still having relevant cosmological effects. I will review some models featuring the different screening mechanism existing in the literature and how they work to evade local gravity tests. However, evading Solar System bounds leads in many cases to tight constraints for the cosmological evolution of the scalar field. I will pay special attention to the so-called chameleon and Vainshtein mechanisms. For the chameleon, the local gravity constraints prevents the scalar to drive self-accelerated solutions and, furthermore, to have an impact in structure formation at linear scales. For a class of theories featuring a Vainshtein mechanism, I will argue how the cosmological evolution of the field can induce non-screenable effects in local gravity observables, mainly a time-variation in Newton's constant and an anomalous propagation speed of gravitational waves. These effects are then constrained using solar system and binary pulsar constraints.
Most existing theories of dark energy and/or modified gravity, involving a scalar degree of freedom, can be conveniently described within the framework of the Effective Theory of Dark Energy. After reviewing this approach, I will extend it to consider Higher-Order Scalar Tensor Theories and discuss their degeneracy and phenomenological viability at the linear level.
The "Effective theory of dark energy" is a simple, general and effective way to bridge theory and observations in dark energy and modified gravity scenarios based on a single scalar field. I will illustrate its application to models that admit a kinetic mixing between matter and the scalar field, which I’ll call "Kinetic Matter Mixing". I will argue that this is a truly physical effect independent of the metric used to describe the action and show that it has the peculiar consequence of weakening gravity on short scales. Finally, I will discuss the impact on the matter power spectrum and the angular power spectrum of the CMB computed with a Boltzmann code, without resorting to the quasi-static approximation, and comment on the validity of the latter.
Many viable Scalar-Tensor theories for modified gravity introduce scalar fields that are coupled to matter. The equations that describe the evolution of the scalar fields are field equations similar to the Klein-Gordon equation, with additional source terms depending on the specific model. The usual way to solve this equation has been to apply the quasi-static approximation, neglecting the time derivatives and solve it like a Poisson equation. We have developed a method to integrate the full field equation numerically, allowing for new phenomena not seen when using the quasi-static approximation.
We present results from our latest research, studying waves arising when solving the full field equations in spherical symmetry. We present results from the Symmetron model and the Disformally coupled model, where the propagation of waves has surprising effects which can lead to further constraints on the models, and to new observables.
Constraining the dark energy equation of state, w, as well as modified growth of large structures predicted by alternatives to GR, are among the primary science goals of ongoing and future cosmological surveys. We derive the theoretical prior covariance for w predicted by a general class of theories of a scalar field dark energy (Horndeski theories). This is achieved by generating a large ensemble of possible scalar-tensor theories using a Monte Carlo methodology, including the application of physical viability conditions. We also use the same technique to study correlations between the frequently used modified growth parameters mu and Sigma (also known as G_matter/G and G_light/G) and confirm the previously made conjecture that the combination (mu-1)(Sigma-1) is non-negative for viable models.
In an attempt to explain dark energy numerous models and modified gravity theories have been constructed in order to better describe current cosmological observations. An obvious way to modify gravity is to introduce a new field other than the metric and make dark energy a dynamical component. In this talk I will present Generalized Einstein-Aether, a vector-tensor theory of gravity where the vector field is constrained to be of time-like unit norm, and discuss its dynamics at the level of background cosmology and linear perturbations. Using this as an example I will also discuss the Equation of State approach to parameterizing perturbations and how it applies to other general models.
We present an application of the equation of state approach to dark sector perturbations as a comprehensive phenomenological framework to understand the evolution of perturbations in dark energy and modified gravity models. The
approach is based on the observation that any modified gravity theory can be recast into an effective dark energy fluid. By eliminating the internal degree of freedom of the given theory, the gauge invariant entropy perturbation and
anisotropic stress can be expressed in terms of the fluid variables. They represent the equations of state at the perturbed level and describe how matter and dark energy perturbations evolve. In this work we specialise to
$f(\mathcal{R})$ gravity theories. By incorporating the equations of motion in a suitably modified version of the Boltzmann code CLASS, we follow the evolution of background and perturbed quantity. By parametrizing this class of models via the parameter $B_0$, we present the impact of $f(\mathcal{R})$ models on several observables, such as the CMB and matter power spectrum and the lensing potential. With the help of the analytic expressions for the perturbed equations of state, we explain the different evolution and features appearing in the fluid variables and in the observables considered. We show that our approach is numerically stable and fast to be used in parameter constraints studies and we provide approximated expressions for the entropy perturbation and anisotropic stress which capture the
physics involved.
In the next few years, we are going to probe the low-redshift universe with unprecedented accuracy. Among the various fruits that this will bear, it will greatly improve our knowledge of the dynamics of dark energy, though for this there is a strong theoretical preference for a cosmological constant. We assume that dark energy is described by the so-called Effective Field Theory of Dark Energy, which assumes that dark energy is the Goldstone boson of time translations. Such a formalism makes it easy to ensure that our signatures are consistent with well-established principles of physics. Since most of the information resides at high wavenumbers, it is important to be able to make predictions at the highest wavenumber that is possible. The Effective Field Theory of Large-Scale Structure (EFTofLSS) is a theoretical framework that has allowed us to make accurate predictions in the mildly non-linear regime. In this paper, we derive the non-linear equations that extend the EFTofLSS to include the effect of dark energy both on the matter fields and on the biased tracers. For the specific case of clustering quintessence, we then perturbatively solve to cubic order the resulting non-linear equations and construct the one-loop power spectrum of the total density contrast.
Nombre de places limité, réservation conseillée.
Pour information voir le lien suivant:
https://indico.cern.ch/event/527550/page/9261-conference-grand-public
Cosmological inflation is one of the leading paradigms for explaining the physical conditions that prevailed in the early Universe. It consists in a phase of very high energy accelerated expansion that solves the hot big bang model problems. When combined with quantum mechanics, it also provides a causal mechanism for generating cosmological fluctuations on large scales.
In this talk I will explain how future CMB missions will allow one to extract a few favoured models among hundreds of currently proposed scenarios. I will insist on the role played by reheating, which connects inflation to the subsequent radiation era and during which ordinary matter as we see it today is produced; and show how this epoch will also be constrained. I will finally discuss some extensions to the minimal single-field setups, where extra scalar fields are added and play a role both during inflation and reheating.
The Wiener filter has emerged as a standard tool for the inference of high dimensional signals, such as the large scale structures and cosmic microwave background (CMB) problems. Some particularly key applications of the Wiener filter in CMB data analysis include power spectrum estimation, map-making and the reconstruction of lensing potential. We present a new fast and robust iterative solver, via a formulation that is dual to the recently developed messenger technique, to efficiently calculate the Wiener filter solution of large and complex data sets. Like its predecessor, this new dual messenger algorithm does not require an ingenious choice of preconditioner and can account for inhomogeneous noise distributions and arbitrary mask geometries, while being unconditionally stable. We demonstrate the capabilities of this hierarchical scheme in signal reconstruction by applying it on a simulated CMB temperature data set to investigate the effectiveness of reconstruction and convergence properties. The dual messenger algorithm outperforms the standard messenger and the popular preconditioned conjugate gradient (PCG) schemes in terms of execution time, being roughly a factor of 2 and 4 times faster than the respective methods, for the specific problem considered. We also showcase the application of the dual messenger algorithm on polarised CMB data sets, where traditional techniques such as the PCG run into numerical difficulties due to the significant increase in the condition number of the matrices involved. This new high-performance algorithm is particularly adapted to cope with the complex numerical challenges posed by state-of-the-art data sets and is therefore relevant for current and future high-resolution CMB missions such as Planck, South Pole Telescope, Advanced ACTPol, Simons Observatory and CMB S-4.
Measurements by WMAP and Planck have indicated nearly a $3\sigma$ departure from statistical isotropy in the temperature field of cosmic microwave background (CMB) at large scales, which is popularly known as Hemispherical Asymmetry. Such an anomalous signal is beyond the standard LCDM cosmological model and can lead to important consequences on cosmological parameters. Cosmological origin of hemispherical asymmetry must leave its imprints on the matter distribution of the Universe at large scales. We demonstrate that weak lensing of the CMB due to statistical isotropy violated density field produces an imprint on the CMB B-mode at small angular scales. Measurability of this phenomenon can confirm its cosmological origin from scalar perturbations and can impose an independent bound on the scale dependence of the hemispherical asymmetry. Next generation CMB missions can validate this effect from small-scale B-mode polarization and can shed light to this decade long enigma.
References: Phys. Rev. Lett. 116, 221301 (2016), JCAP 09 (2016) 029.
The last decades witnessed huge progress in understanding the large-scale structure of the Universe. While homogeneous and isotropic on the largest scales, the matter and galaxy distributions display complex patterns on smaller scales where we observe elongated filaments, compact clusters and volume-filling underdense regions. These features are not captured by studies of two-point statistics like the power spectrum that does not retain information on the phases of the Fourier modes of the density field. Therefore, higher-order statistics like the bispectrum should provide additional information. The Euclid galaxy redshift survey will cover a large enough volume to provide robust measurements of the galaxy bispectrum as a function of redshift. The potential of these measurements as a mean to extract additional cosmological information has never been investigated properly.
In this talk we present detailed forecasts for the Euclid mission. Our study shows that there is a clear advantage in combining the power spectrum and the bispectrum to infer the galaxy bias parameters and constrain the dark-energy equation of state.
The current and future galaxy surveys give us several challenge both in terms of data management and interpretation. The interpretation is made particularly complex by observational limitations, such as selection and foreground issues, and the extreme dynamical non-linearity of the galaxies. Such problems are generally taken into account a posteriori in the analysis, and are covered by putting some weights on the galaxies calibrated using large number mock galaxy catalogs, designed to resemble as close as possible to the observations. Then the considered summary statistics, such as the power-spectrum, have to be closely examined and corrected for non-linearities.
We will showcase here our two frameworks, ARES (Algorithm for REconstruction and Sampling) and BORG (Bayesian Origin Reconstruction from Galaxies), to analyze deep and wide galaxy surveys, to reconstruct the initial conditions of our Local Universe and to deliver cosmological measurements. These statistical and computational frameworks are designed at their base for performance and statistical accuracy. The a posteriori measurements include already all the adequate uncertainties and corrections. I will discuss here the results of the tests that we conducted on present data-sets like 2M++ (Lavaux & Hudson 2011) and the Sloan Digital Sky Survey (SDSS3/BOSS, Dawson et al. 2013). I will move then to the current limitations of these frameworks and the future directions we intend to take.
Consistency relations of large-scale structures provide exact nonperturbative results for cross-correlations of cosmic fields in the squeezed limit. They only depend on the equivalence principle and the assumption of Gaussian initial conditions, and remain nonzero at equal times for cross-correlations of density fields with velocity or momentum fields, or with the time derivative of density fields. In this talk I will introduce consistency relations for large scale structure and apply them to observational probes that involve the integrated Sachs-Wolfe effect or the kinematic Sunyaev-Zeldovich effect. In particular, I will show how cross-correlations with the integrated Sachs-Wolfe effect show a specific angular dependence which could be used to test the equivalence principle and the primordial Gaussianity, or to check the modeling of large-scale structures.
The Cosmological Principle is one of the main pillars of the Friedman-Lemaître-Robertson-Walker models that constitute the the theoretical basis of ΛCDM concordance model. The Cosmological Principle states that the universe becomes homogeneous and isotropic at very large scale. While the isotropy of our universe is tested from several different probes and surveys, with the most well known, the Cosmic Microwave Background Radiation (CMB), the property of homogeneity is mode complicated to establish observationally with comparable accuracy and is usually implied through the Copernican Principle.
In this talk, I will present a study of the transition to homogeneity with the latest SDSS3-BOSS galaxy sample spanning a redshift range [0.43,0.7] using a technique based on the evolution of the fractal dimension as a function of scale. I will show that besides finding the expected transition to homogeneity, the scale at which occurs is in excellent agreement with the expectations from ΛCDM. Furthermore, the transition to homogeneity can also be seen as a new standard scale in Cosmology and therefore be used to constrain further the ΛCDM model. I will present forecasts of the constraints that can be obtained with this new probe with the data from incoming large scale structure surveys.
To extract cosmological information from large-scale galaxy clustering, we need accurate modeling of the relationship between dark matter and galaxies (galaxy bias). Recently, field-theory techniques have been used to provide a new description of galaxy biasing in terms of renormalised operators and counter terms, i.e. to build quantities that are not UV sensitive. We test these definitions of the leading-order non-linear bias coefficients (quadratic and tidal bias) against a set of numerical simulations. As a byproduct of our analysis we also discuss the accuracy of the kernels of standard perturbation theory.
In this talk I will describe progresses in considering GR effects in the dynamics of structure formation. First I will present results of a nonlinear post-Friedman approximation, a kind of post-Newtonian formalism. Then I will focus on recent fully nonlinear numerical relativity simulations. Numerical relativity is a fundamental tool in the modelling of gravitational waves sources, but its application to cosmology is in its infancy. As more interdisciplinary work between the gravitational waves and the cosmology communities will develop, in the next few years numerical relativity may become a fundamental tool for understanding the extent to which we can trust standard newtonian N-body simulations on the largest scales. First results of simulations representing the full GR nonlinear evolution of initial perturbations in a Einstein de Sitter background are: 1) back-reaction effects on the overall expansion of the model are very small; 2) voids expansion rate is significantly higher than that of the background; 3) over-densities can reach turn-around much earlier than predicted by the standard top-hat model. To establish the significance of these results is the goal of future work.
It is known that cold dark matter candidates lead to the structuring of matter on scales much smaller than typical galaxies. This clustering translates into a very large population of subhalos in galaxies, which must impact predictions for indirect searches of annihilating dark matter. I present a model (arXiv:1610:02233) consistently describing the subhalo population in a dynamically constrained Galaxy. I will show application of this model to indirect searches of dark matter via antiprotons cosmic-rays using the latest data from AMS-02.
In this presentation, I will demonstrate the unprecedented capabilities of the Event Horizon Telescope (EHT) to image the innermost dark matter profile in the vicinity of the supermassive black hole at the center of the M87 radio galaxy. I will present the first model of the synchrotron emission induced by dark matter annihilations from a spiky profile in the close vicinity of a supermassive black hole, accounting for strong gravitational lensing effects. I will show that the photon ring surrounding the silhouette of the black hole is enhanced in the presence of a dark matter spike, which introduces observable small-scale structure into the signal. This will lead me to the observational constraints on dark matter we obtain from EHT data. More specifically, I will discuss how current EHT data constrain very weakly annihilating dark matter, and how future EHT observations will further constrain the DM scenario.
Although the existence of Dark Matter (DM) is by now well-established thanks to a variety of observations on many different scales, its nature is still unknown and so are many of its most basics properties, such as its lifetime. Moreover, even if obvious arguments require that most of the DM is stable on timescales of (at least) the lifetime of the universe, a fraction of it could be in the form of unstable exotic particles that are free to decay at much shorter times. In the literature, numerous relics from the early universe have been proposed in many extensions of the standard model of particle physics, in some cases unstable to processes injecting electromagnetic (e.m.) forms of energy (e.g. ‘superWIMP’, R-parity breaking SUSY models, Sterile neutrinos, Primordial black holes …).
In this talk, I would like to review how the study of CMB temperature and polarization anisotropies can be used to put stringeant constraints on the abundance of such electromagnetically decaying exotic particles, as a function of their lifetime. I will then emphasize the synergy with CMB spectral distortions and Big Bang Nucleosynthesis studies and illustrate how the 21cm signal, one of the main target of future experiments, could be used in order to improve (but not always !) over these bounds.
According to the Standard Model of Cosmology, about 25% of the content of the universe is composed of Dark Matter (DM). From a theoretical point of view, there are many possible alternatives to explain its origin and composition, ranging from ultralight axions to supermassive black holes. However, despite many experimental efforts, the nature of DM is still obscure. One interesting possibility is that DM is composed of Primordial Black Holes (PBHs), arising from high peaks in the matter power spectrum of some inflationary models. In this talk, I will show that models of axion-inflation in which the inflaton is coupled to abelian massless gauge fields can generate peaks in the matter power spectrum, giving rise to the formation of PBHs. I will discuss the possibility that such PBHs compose a fraction of the DM observed in the universe, and I will present some ideas about a possible UV completion of such inflationary model.
I will talk about a DM scenario in which the self-scattering rate today is regulated by kinematics and/or the abundance ratio, through the mass-splitting of two nearly degenerate states, emphasising the implications of the considered models and their prospect of solving astrophysical small-scale structure problems.
Peculiar velocities affects the redshifts of distant galaxies and introduces distortions in all statistical measures of the reconstructed large-scale structure. These distortions are in general complex to model. In this talk, we focus on the streaming equation which is often used to model these distortions in configuration space. Current phenomenological models based on the streaming equation do not work well at small scales. Using N-body simulations, we investigate and study the impact of how the flipping of pairs by peculiar velocities affect the streaming equation at those scales. We also take a deeper look into the pairwise velocity distributions and show how field and halo particles contribute to these distribution. With a firm understanding of these effects, we hope to pave way for better phenomenological models which could be utilisied to obtain precise cosmological constraints from future telescopes like EUCLID.
The exponentially expanding de Sitter solution is frequently encountered in cosmology due to its significance for the late and early Universe. However, its stability in a quantised theory has for a long time been the source of debate. In this talk we discuss this issue in the framework of semi-classical gravity. Based on a first principle calculation, we argue that when one considers only those degrees of freedom that are accessible to a local observer, the quantum back reaction destabilizes de Sitter space leading to a gradual continuous increase of the Horizon
We consider a model of a holographic braneworld universe in which a cosmological fluid occupies a 3+1 dimensional brane located at the boundary of the asymptotic AdS bulk. We combine the AdS/CFT correspondence and the second Randall-Sundrum
(RSII) model to establish a relationship between the RSII braneworld cosmology and
the boundary metric induced by the time dependent bulk geometry. In the framework
of the Friedmann Robertson Walker cosmology we discuss some physically interesting
scenarios involving the RSII and holographic braneworlds.
The trajectory of light in a flat Robertson-Walker universe is presented taking due account of its spin. The off-set between the trajectories of positive and negative helicity states (birefringence) is of the order of a wave length and depends on the acceleration parameter. In 2008, using techniques of weak quantum measurement, an analogous birefringence in reflection, the Federov-Imbert effect, was observed for the first time. Observation of gravitational birefringence could offer an independent measurement of the acceleration of our universe.
Birefringence might also be induced by gravitational waves and allow for new detection techniques of these waves.