Narrow Results

We review some recent efforts in determining the absolute neutrino mass scale in cosmology. We illustrate in particular how distance measurements such as the baryon acoustic oscillations and the galaxy weak lensing can break the degeneracy between the neutrino mass and dark energy equation of state parameters.

We discuss the self-accelerating universe solutions in the framework of the potentially ghost-free, nonlinear massive gravity theory recently proposed by de Rham–Gabadadze–Tolley. The theory allows general Friedmann–Robertson–Walker solutions with negative curvature. The contribution of the mass terms at the background level is an effective cosmological constant term depending on the parameters of the theory. We also discuss the cosmological perturbations in these backgrounds, as well as similar solutions of the extended versions of the theory; the actions of the scalar and vector degrees do not undergo a modification with respect to general relativity, while the two polarizations of gravity waves acquire a time-dependent effective mass term. This may lead to a modification of the stochastic gravitational wave spectrum.

We construct (m, n)-type holographic dark energy models at a phenomenological level, which can be viewed as a generalization of agegraphic models with the conformal-like age as the holographic characteristic size. For some values of (m, n) the holographic dark energy can automatically evolve across ω = -1 into a phantom phase even without introducing an interaction between the dark energy and background matter. Our construction is also applicable to the holographic dark energy with generalized future event horizon as the characteristic size. Finally, we address the issue on the stability of our model and show that they are generally stable under the scalar perturbation.

In this paper, we propose a test to detect the linearity of the dark energy equation of state, and apply it to the SNLS3 Type Ia Supernova (SN Ia) data set. We find that: (a) current SN Ia data are well described by a dark energy equation of state linear in the cosmic scale factor a, at least up to a redshift z = 1, independent of the pivot points chosen for the linear relation; (b) there is no significant evidence of any deviation from linearity. This apparent linearity may reflect the limit of dark energy information extractable from current SN Ia data.

Brans–Dicke (BD) cosmology is reconsidered from an approach in which the model can be formulated in $\mathrm{\Lambda }$CDM form, but at the expense of replacing the rigid cosmological constant $\mathrm{\Lambda }$ with a dynamical quasivacuum component which depends on a small parameter $𝜖$. The product of $𝜖$ times the BD-parameter ${\omega }_{\mathrm{\text{BD}}}$ becomes exactly determined in terms of the ordinary cosmological parameters. The GR-limit is recovered for ${\omega }_{\mathrm{\text{BD}}}\to \infty$ when $𝜖\to 0$. We solve the background cosmology of the model as well as the perturbations equations. When fitted to the cosmological data, we find that the BD-cosmology, transcribed in such an effective GR-form, appears to be competitive with the $\mathrm{\Lambda }$CDM and emulates the running vacuum model.

We work out an effective theory of accelerated expansion for the background to describe general phenomena of inflation and acceleration (dark energy) in the universe. Our aim is to determine from theoretical grounds, in a physically motivated and model-independent way, which and how many (free) parameters are needed to broadly capture the physics of a theory describing the background of cosmic acceleration. Our goal is to make as much transparent as possible the physical interpretation of the parameters describing the expansion. We show that, at leading order, there are five independent parameters, of which one can be constrained via general relativity tests. The other four parameters need to be determined by observing and measuring the cosmic expansion rate only, H(z). Therefore, we suggest that future cosmology surveys focus on obtaining an accurate value as possible measurement of H(z) to constrain the nature of accelerated expansion.

Recent analyses of modern cosmological data show that the $\mathrm{\Lambda }=\mathrm{\text{const}}$ hypothesis, despite being the canonical framework, might well not be the most favored one from the phenomenological point of view. A number of persisting tensions with the observations (particularly with the data on structure formation and the local measurements of the Hubble parameter) indicate that the concordance or $\mathrm{\Lambda }$CDM model of cosmology might be in trouble. The combined fit to SNIa+BAO+H(z)+LSS+BBN+CMB observables shows that the performance of the $\mathrm{\Lambda }$CDM is not as good as could be expected, and in fact it may lag behind the fitting outcome from dynamical dark energy (DDE) models. A simple XCDM parameterization seems to perform better than the $\mathrm{\Lambda }$CDM. Furthermore, the running vacuum model (RVM) could also be superior in fitting the overall data and in smoothing out some of the mentioned tensions.

We discuss the Buchert equations, which describe the average expansion of an inhomogeneous dust universe. In the limit of small perturbations, they reduce to the Friedmann–Robertson–Walker equations. However, when the universe is very inhomogeneous, the behavior can be qualitatively different from the FRW case. In particular, the average expansion rate can accelerate even though the local expansion rate decelerates everywhere. We clarify the physical meaning of this paradoxical feature with a simple toy model, and demonstrate how acceleration is intimately connected with gravitational collapse. This provides a link to structure formation, which in turn has a preferred time around the era when acceleration has been observed to start.

A cosmological model with anisotropic dark energy is analyzed. The amount of deviation from isotropy of the equation of state of dark energy, the skewness δ, generates an anisotropization of the large-scale geometry of the Universe, quantifiable by means of the actual shear Σ₀. Requiring that the level of cosmic anisotropization at the time of decoupling be such that we can solve the "quadrupole problem" of cosmic microwave background radiation, we find that |δ| ~ 10^-4 and |Σ_0| ~10^-5, compatible with existing limits derived from the magnitude redshift data on Type Ia supernovae.

We study the phase-space dynamics of cosmological models in the theoretical formulations of nonminimal metric-torsion couplings with a scalar field, and investigate in particular the critical points (CPs) which yield stable solutions exhibiting cosmic acceleration driven by the dark energy (DE). The latter is so defined that it effectively has no direct interaction with the cosmological fluid, although in an equivalent scalar–tensor cosmological setup, the scalar field interacts with the fluid (which we consider to be the pressureless dust). Determining the conditions for the existence of the stable CPs, we check their physical viability in both Einstein and Jordan frames. We also verify that in either of these frames, the evolution of the universe at the corresponding stable points matches with that given by the respective exact solutions we have found in an earlier work [S. Sur and A. S. Bhatia, arXiv:1611.00654 [gr-qc]]. We not only examine the regions of physical relevance in the phase-space when the coupling parameter is varied, but also demonstrate the evolution profiles of the cosmological parameters of interest along fiducial trajectories in the effectively noninteracting scenarios, in both Einstein and Jordan frames.

We generalize the basic theory of mimetic gravity by extending its purview to the general metric-compatible geometries that admit torsion, in addition to curvature. This essentially implies reinstating the mimetic principle of isolating the conformal degree of freedom of gravity in presence of torsion, by parametrizing both the physical metric and torsion in terms of the scalar ‘mimetic’ field and the metric and torsion of a fiducial space. We assert the requisite torsion parametrization from an inspection of the fiducial space Cartan transformation which, together with the conformal transformation of the fiducial metric, preserves the physical metric and torsion. In formulating the scalar–tensor equivalent Lagrangian, we consider an explicit contact coupling of the mimetic field with torsion, so that the former can manifest itself geometrically as the source of a torsion mode, and most importantly, give rise to a viable ‘dark universe’ picture from a mimicry of an evolving dust-like cosmological fluid with a nonzero pressure. A further consideration of higher derivatives of the mimetic field in the Lagrangian leads to physical bounds on the mimetic–torsion coupling strength, which we determine explicitly.

We consider the impact of dynamical dark energy (DDE) in the possible solution of the existing tensions in the ΛCDM. We test both interacting and non-interacting DE models with dark matter (DM). Among the former, the running vacuum model (RVM) interacting with DM appears as a favored option. The non-interacting scalar field model based on the potential V ∼ ϕ^−α, and the generic XCDM parametrization, also provide consistent signs of DDE. The important novelty of our analysis with respect to the existing ones in the literature is that we use the matter bispectrum, together with the power spectrum. Using a complete and updated set of cosmological observations on SNIa+BAO+H(𝓏)+LSS+CMB, we find that the crucial triad BAO+LSS+CMB (i.e. baryonic acoustic oscillations, large scale structure formation data and the cosmic microwave background) provide the bulk of the signal. The bispectrum data is instrumental to get hold of the DDE signal, as our analysis shows. If the bispectrum is not included, the DDE signal could not be currently perceived at a significant confidence level.

In this contribution we apply model selection approach based on Akaike criterion as an estimator of Kullback-Leibler entropy. In particular, we present the proper way of ranking the competing models based on Akaike weights (in Bayesian language - posterior probabilities of the models). This important ingredient is missing in alternative studies dealing with cosmological applications of Akaike criterion.

Out of many particular models of dark energy we focus on four: quintessence, quintessence with time varying equation of state, brane-world and generalized Chaplygin gas model and test them on Riess' Gold sample.

As a result we obtain that the best model - in terms of Akaike Criterion - is the quintessence model with evolving equation of state. The odds suggest that although there exist differences in the support given to specific scenarios by supernova data all models considered receive similar support. One can also notice that models similar in structure i.e. ΛCDM, quintessence and quintessence with variable equation of state are closer to each other in terms of Kullback-Leibler entropy. Models having different structure i.e. Chaplygin gas or brane-world scenario are more distant (in Kullback-Leibler sense) from the best one.

We study static configurations of dark matter coupled to a scalar field responsible for the dark energy of the Universe. The dark matter is modelled as a Fermi gas within the Thomas-Fermi approximation. The mass of the dark matter particles is a function of the scalar field. We analyze the profile of the dark matter halos in galaxies. In this case our framework is equivalent to the model of the isothermal sphere. In the presence of a scalar field, the velocity of a massive object orbiting the galaxy is not of the order of the typical velocity of the dark matter particles, as in the conventional picture. Instead, it is reduced by a factor that quantifies the dependence of the dark matter mass on the scalar field. This has implications for dark matter searches.

This project proposes to discriminate in the wealth of models for dark energy using the formation of non-linear dark matter structures. In particular,it focuses on structures traced by the mass function of dark matter haloes.

In this paper, we present the effects of polynomial f(R) model on the stability of homogeneous energy density in self-gravitating spherical stellar object. For this purpose, we construct modified version of Ellis equations. We explore different factors responsible for energy density inhomogeneities with non-dissipative dust, isotropic and dissipative dust cloud.

We discuss the self-accelerating universe solutions in the framework of the potentially ghost-free, nonlinear massive gravity theory recently proposed by de Rham–Gabadadze– Tolley. The theory allows general Friedmann–Robertson–Walker solutions with negative curvature. The contribution of the mass terms at the background level is an effective cosmological constant term depending on the parameters of the theory. We also discuss the cosmological perturbations in these backgrounds, as well as similar solutions of the extended versions of the theory; the actions of the scalar and vector degrees do not undergo a modification with respect to general relativity, while the two polarizations of gravity waves acquire a time-dependent effective mass term. This may lead to a modification of the stochastic gravitational wave spectrum.

In this work we consider a gravitating scalar field χ with a non-conventional kinetic term, as in the string theory tachyon, and two measures – a geometric measure, , and a non-metric measure, Φ. This model gives a unified picture of dark energy and dark matter. It is also possible to realize dynamical dark energy and dark matter by a mechanism similar to that studied for branes and strings with dynamical tension; in particular an “inverse quintessence scenario” can be obtained by introducing an additional ghost fieldη into the dark energy/dark matter dynamics.