Cosmology

About: Cosmology is a research topic. Over the lifetime, 18004 publications have been published within this topic receiving 631028 citations. The topic is also known as: physical cosmology & cosmologies.

New agegraphic dark energy in Hořava-Lifshitz cosmology

Abstract: We investigate the new agegraphic dark energy scenario in a universe governed by Ho?ava-Lifshitz gravity. We consider both the detailed and non-detailed balanced version of the theory, we impose an arbitrary curvature, and we allow for an interaction between the matter and dark energy sectors. Extracting the differential equation for the evolution of the dark energy density parameter and performing an expansion of the dark energy equation-of-state parameter, we calculate its present and its low-redshift value as functions of the dark energy and curvature density parameters at present, of the Ho?ava-Lifshitz running parameter ?, of the new agegraphic dark energy parameter n, and of the interaction coupling b. We find that w0 = ?0.82+0.08?0.08 and w1 = 0.08+0.09?0.07. Although this analysis indicates that the scenario can be compatible with observations, it does not enlighten the discussion about the possible conceptual and theoretical problems of Ho?ava-Lifshitz gravity.

Constraints on the Coupling between Dark Energy and Dark Matter from CMB data

Abstract: We investigate a phenomenological non-gravitational coupling between dark energy and dark matter, where the interaction in the dark sector is parameterized as an energy transfer either from dark matter to dark energy or the opposite. The models are constrained by a whole host of updated cosmological data: cosmic microwave background temperature anisotropies and polarization, high-redshift supernovae, baryon acoustic oscillations, redshift space distortions and gravitational lensing. Both models are found to be compatible with all cosmological observables, but in the case where dark matter decays into dark energy, the tension with the independent determinations of H0 and σ8, already present for standard cosmology, increases: this model in fact predicts lower H0 and higher σ8, mostly as a consequence of the higher amount of dark matter at early times, leading to a stronger clustering during the evolution. Instead, when dark matter is fed by dark energy, the reconstructed values of H0 and σ8 nicely agree with their local determinations, with a full reconciliation between high- and low-redshift observations. A non-zero coupling between dark energy and dark matter, with an energy flow from the former to the latter, appears therefore to be in better agreement with cosmological data.

Bouncing cosmologies in Palatini f(R) gravity

Abstract: We consider the early time cosmology of fðRÞ theories in Palatini formalism and study the conditions that guarantee the existence of homogeneous and isotropic models that avoid the big bang singularity. We show that for such models the big bang singularity can be replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even for pressureless dust. We give a characterization of such models and discuss their dynamics in the region near the bounce. We also find that power-law Lagrangians with a finite number of terms may lead to nonsingular universes, which contrasts with the infinite-series Palatini fðRÞ Lagrangian that one needs to fully capture the effective dynamics of loop quantum cosmology. We argue that these models could also avoid the formation of singularities during stellar gravitational collapse. DOI: 10.1103/PhysRevD.80.024016

Type Ia supernovae, evolution, and the cosmological constant

Abstract: We explore the possible role of evolution in the analysis of data on Type Ia supernovae (SNe Ia) at cosmological distances. First, using a variety of simple sleuthing techniques, we find evidence that the properties of the high- and low-redshift SNe Ia observed so far differ from one another. Next, we examine the effects of allowing for an uncertain amount of evolution in the analysis, using two simple phenomenological models for evolution and prior probabilities that express a preference for no evolution but allow it to be present. One model shifts the magnitudes of the high-redshift SNe Ia relative to the low-redshift SNe Ia by a fixed amount. A second, more realistic, model introduces a continuous magnitude shift of the form δm(z) = β ln(1 + z) to the SNe Ia sample. The result is that cosmological models and evolution are highly degenerate with one another, so that the incorporation of even very simple models for evolution makes it virtually impossible to pin down the values of ΩM and ΩΛ, the density parameters for nonrelativistic matter and for the cosmological constant, respectively. The Hubble constant, H0, is unaffected by evolution. We evaluate the Bayes factor for models with evolution versus models without evolution, which, if one has no prior predilection for or against evolution, is the odds ratio for these two classes of models. The resulting values are always of order 1, in spite of the fact that the models that include evolution have additional parameters; thus, the data alone cannot discriminate between the two possibilities. Simulations show that simply acquiring more data of the same type as are available now will not alleviate the difficulty of separating evolution from cosmology in the analysis. What is needed is a better physical understanding of the SN Ia process, and the connections among the maximum luminosity, rate of decline, spectra, and initial conditions, so that physical models for evolution may be constructed, and confronted with the data. Moreover, we show that if SNe Ia evolve with time, but evolution is neglected in analyzing data, then, given enough SNe Ia, the analysis hones in on values of ΩM and ΩΛ that are incorrect. Using Bayesian methods, we show that the probability that the cosmological constant is nonzero (rather than zero) is unchanged by the SNe Ia data when one accounts for the possibility of evolution, provided that we do not discriminate among open, closed, and flat cosmologies a priori. The case for nonzero cosmological constant is stronger if the universe is presumed to be flat but still depends sensitively on the degree to which the peak luminosities of SNe Ia evolve as a function of redshift.