Latest Observational Data Research Articles

Search for sterile neutrinos in a universe of vacuum energy interacting with cold dark matter

We investigate the cosmological constraints on sterile neutrinos in a universe in which vacuum energy interacts with cold dark matter by using latest observational data. We focus on two specific interaction models, $Q=\beta H\rho_{\rm v}$ and $Q=\beta H\rho_{\rm c}$. To overcome the problem of large-scale instability in the interacting dark energy scenario, we employ the parametrized post-Friedmann (PPF) approach for interacting dark energy to do the calculation of perturbation evolution. The observational data sets used in this work include the Planck 2015 temperature and polarization data, the baryon acoustic oscillation measurements, the type-Ia supernova data, the Hubble constant direct measurement, the galaxy weak lensing data, the redshift space distortion data, and the Planck lensing data. Using the all-data combination, we obtain $N_{\rm eff}<3.522$ and $m_{\nu,{\rm sterile}}^{\rm eff}<0.576$ eV for the $Q=\beta H\rho_{\rm v}$ model, and $N_{\rm eff}=3.204^{+0.049}_{-0.135}$ and $m_{\nu,{\rm sterile}}^{\rm eff}=0.410^{+0.150}_{-0.330}$ eV for the $Q=\beta H\rho_{\rm c}$ model. The latter indicates $\Delta N_{\rm eff}>0$ at the 1.17$\sigma$ level and a nonzero mass of sterile neutrino at the 1.24$\sigma$ level. In addition, for the $Q=\beta H\rho_{\rm v}$ model, we find that $\beta=0$ is consistent with the current data, and for the $Q=\beta H\rho_{\rm c}$ model, we find that $\beta>0$ is obtained at more than 1$\sigma$ level.

Minimal gauge inflation and the refined Swampland conjecture

The refined de Sitter (dS) conjecture provides two consistency conditions for an effective theory potential of a quantum gravity theory. Any inflationary model can be checked by these conditions and minimal gauge inflation is not an exception. We develop a generic method to analyze a monotonically growing potential with an inflection point on the way to the plateau near the top such as the potential in minimal gauge inflation model and the Higgs inflation. Taking the latest observational data into account, we find the fully consistent parameter space where the model resides in the Landscape rather than in the Swampland.

Effects of Low Anisotropy on Generalized Ghost Dark Energy in Galileon Gravity

The definition of the Galileon gravity form is extended to the Brans-Dicke theory. Given, the framework of the Galileon theory, the generalized ghost dark energy model in an anisotropic universe is investigated. We study the cosmological implications of this model. In particular, we obtain the equation of state and the deceleration parameters and a differential equation governing the evolution of this dark energy in Bianchi type I model. We also probe observational constraints by using the latest observational data on the generalized ghost dark energy models as the unification of dark matter and dark energy. In order to do so, we focus on observational determinations of the Hubble expansion rate (namely, the expansion history) H(z). As a result, we show the influence of the anisotropy (although low) on the evolution of the universe in the statefinder diagrams for Galileon gravity.

Theoretical and observational constraints on Tachyon Inflation

We constrain several models in Tachyonic Inflation derived from the large-N formalism by considering theoretical aspects as well as the latest observational data. On the theoretical side, we assess the field range of our models by means of the excursion of the equivalent canonical field. On the observational side, we employ BK14+PLANCK+BAO data to perform a parameter estimation analysis as well as a Bayesian model selection to distinguish the most favoured models among all four classes here presented. We observe that the original potential V ∝ sech(T) is strongly disfavoured by observations with respect to a reference model with flat priors on inflationary observables. This realisation of Tachyon inflation also presents a large field range which may demand further quantum corrections. We also provide examples of potentials derived from the polynomial and the perturbative classes which are both statistically favoured and theoretically acceptable.

Anisotropic Cosmology with a Dilaton Field Coupled to Ghost Dark Energy

We study a dilaton scalar field coupled to ghost dark energy in an anisotropic universe. The evolution of dark energy, which dominates the universe, can be completely described by a single dilaton scalar field. This connection allows reconstructing the kinetic energy and also the dynamics of the dilaton scalar field according to the evolution of the energy density. Using the latest observational data, we obtain bounds on the ghost dark energy models and also on generalized dark matter and dark energy. For this, we investigate how the expansion history H(z) is determined by observational quantities. We calculate the evolution of density perturbations in the linear regime for both ghost and generalized ghost dark energy and compare the results with ΛCDM models. We discuss the justification of the generalized second law of thermodynamics in a Bianchi type-I universe. The obtained model is stable for large time intervals but is unstable at small times.

The effect of anisotropy on the thermodynamics of the interacting holographic dark energy model

By considering a holographic model for the dark energy in an anisotropic universe, the thermodynamics of a scheme of dark matter and dark energy interaction has been investigated. The results suggest that when holographic dark energy and dark matter evolve separately, each of them remains in thermodynamic equilibrium, therefore the interaction between them may be viewed as a stable thermal fluctuation that brings a logarithmic correction to the equilibrium entropy. Also the relation between the interaction term of the dark components and this thermal fluctuation has been obtained. Additionally, for a cosmological interaction as a free function, the anisotropy effects on the generalized second law of thermodynamics have been studied. By using the latest observational data on the holographic dark energy models as the unification of dark matter and dark energy, the observational constraints have been probed. To do this, we focus on observational determinations of the Hubble expansion rate H(z). Finally, we evaluate the anisotropy effects (although low) on various topics, such as the evolution of the statefinder diagnostic, the distance modulus and the spherical collapse from the holographic dark energy model and compare them with the results of the holographic dark energy of the Friedmann-Robertson-Walker and $\Lambda$ CDM models.

Warm inflation and WMAP9

A warm inflation model, drawn upon the vector cosmology, was applied in the present work to calculate cosmological fluctuations. Moreover, parameters of the model were constrained and compared using the latest observational data such as Planck and WMAP9.

Reconstruction of slow-roll [formula omitted] gravity inflation from the observational indices

In this work, we introduce a bottom-up F(R) gravity reconstruction technique, in which we fix the observational indices and we seek for the F(R) gravity which may realize them. Particularly, as an exemplification of our method, we shall assume that the scalar-to-tensor ratio has a specific form, and from it we shall reconstruct the F(R) gravity that may realize it, focusing on special values of the parameters in order to obtain analytical results. The observational indices we study are compatible with the latest observational data, and we discuss how the functional form of the observational indices may affect the viability of the model.

An analytical approach to the CMB polarization in a spatially closed background

Abstract The scalar mode polarization of the cosmic microwave background is derived in a spatially closed universe from the Boltzmann equation using the line of sight integral method. The EE and TE multipole coefficients have been extracted analytically by considering some tolerable approximations such as considering the evolution of perturbation hydrodynamically and sudden transition from opacity to transparency at the time of last scattering. As the major advantage of analytic expressions, C E E , l S and C T E , l S explicitly show the dependencies on baryon density Ω B , matter density Ω M , curvature Ω K , primordial spectral index n s , primordial power spectrum amplitude A s , Optical depth τ r e i o n , recombination width σ t and recombination time t L . Using a realistic set of cosmological parameters taken from a fit to data from Planck, the closed universe EE and TE power spectrums in the scalar mode are compared with numerical results from the CAMB code and also latest observational data. The analytic results agree with the numerical ones on the big and moderate scales. The peak positions are in good agreement with the numerical result on these scales while the peak heights agree with that to within 20 % due to the approximations have been considered for these derivations. Also, several interesting properties of CMB polarization are revealed by the analytic spectra.

Unification of constant-roll inflation and dark energy with logarithmic R2-corrected and exponential F(R) gravity

In this paper we investigate how to describe in a unified way a constant-roll inflationary era with a dark energy era, by using the theoretical framework of F(R) gravity. To this end, we introduce some classes of appropriately chosen F(R) gravity models, and we examine in detail how the unification of early and late-time acceleration eras can be achieved. We study in detail the inflationary era, and as we demonstrate it is possible to achieve a viable inflationary era, for which the spectral index of primordial curvature perturbations and the scalar-to-tensor ratio can be compatible with the latest observational data. In addition, the graceful exit issue is briefly discussed for a class of models. Finally, we discuss the dark energy oscillations issue, and we investigate which model from one of the classes we introduced, can produce oscillations with the smallest amplitude.

Search for sterile neutrinos in holographic dark energy cosmology: Reconciling Planck observation with the local measurement of the Hubble constant

We search for sterile neutrinos in the holographic dark energy cosmology by using the latest observational data. To perform the analysis, we employ the current cosmological observations, including the cosmic microwave background temperature power spectrum data from the Planck mission, the baryon acoustic oscillation measurements, the type Ia supernova data, the redshift space distortion measurements, the shear data of weak lensing observation, the Planck lensing measurement, and the latest direct measurement of $H_0$ as well. We show that, compared to the $\Lambda$CDM cosmology, the holographic dark energy cosmology with sterile neutrinos can relieve the tension between the Planck observation and the direct measurement of $H_0$ much better. Once we include the $H_0$ measurement in the global fit, we find that the hint of the existence of sterile neutrinos in the holographic dark energy cosmology can be given. Under the constraint of the all-data combination, we obtain $N_{\rm eff}= 3.76\pm0.26$ and $m_{\nu,\rm sterile}^{\rm eff}< 0.215\,\rm eV$, indicating that the detection of $\Delta N_{\rm eff}>0$ in the holographic dark energy cosmology is at the $2.75\sigma$ level and the massless or very light sterile neutrino is favored by the current observations.

An analytical approach to the CMB anisotropies in a spatially closed background

The scalar mode temperature fluctuations of the cosmic microwave background has been derived in a spatially closed universe from two different methods. First, by following the photon trajectory after the last scattering and then from the Boltzmann equation in a closed background and the line of sight integral method. An analytic expression for the temperature multipole coefficient has been extracted at the hydrodynamical limit, where we have considered some tolerable approximations. By considering a realistic set of cosmological parameters taken from a fit to data from Planck, the TT power spectrum in the scalar mode for the closed universe has been compared with numerical one by using the CAMB code and also latest observational data. The analytic result agrees with the numerical one on almost all scales. The peak positions are in very good agreement with numerical result while the peak heights agree with that to within 10% due to the approximations have been considered for this derivation.

Highlights in the Milky Way

AbstractWe discuss some important topics concerning the chemical evolution of the Milky Way. In particular, we compare the predictions of theoretical chemical models for our Galaxy with the latest observational data in order to derive constraint on the formation and evolution of the various Galactic components.

Tachyon logamediate inflation on the brane

According to a Barrow solution for the scale factor of the universe, the main properties of the tachyon inflation model in the framework of the RSII braneworld are studied. Within this framework the basic slow-roll parameters are calculated analytically. We compare this inflationary scenario to the latest observational data. The predicted spectral index and the tensor-to-scalar fluctuation ratio are in excellent agreement with those of Planck 2015. The current predictions are consistent with those of viable inflationary models.

Bulk viscosity, interaction and the viability of phantom solutions

We study the dynamics of a bulk viscosity model in the Eckart approach for a spatially flat Friedmann–Robertson–Walker (FRW) Universe. We have included radiation and dark energy, assumed as perfect fluids, and dark matter treated as an imperfect fluid having bulk viscosity. We also introduce an interaction term between the dark matter and dark energy components. Considering that the bulk viscosity is proportional to the dark matter energy density and imposing a complete cosmological dynamics, we find bounds on the bulk viscosity in order to reproduce a matter-dominated era (MDE). This constraint is independent of the interaction term. Some late time phantom solutions are mathematically possible. However, the constraint imposed by a MDE restricts the interaction parameter, in the phantom solutions, to a region consistent with a null value, eliminating the possibility of late time stable solutions with w<-1. From the different cases that we study, the only possible scenario, with bulk viscosity and interaction term, belongs to the quintessence region. In the latter case, we find bounds on the interaction parameter compatible with latest observational data.

Comparison between the Logotropic and ΛCDM models at the cosmological scale

We perform a detailed comparison between the Logotropic model [P.H. Chavanis, Eur. Phys. J. Plus, 130 (2015)] and the ΛCDM model. These two models behave similarly at large (cosmological) scales up to the present. Differences will appear only in the far future, in about 25 Gyrs, when the Logotropic Universe becomes phantom while the ΛCDM Universe enters in the de Sitter era. However, the Logotropic model differs from the ΛCDM model at small (galactic) scales, where the latter encounters serious problems. Having a nonvanishing pressure, the Logotropic model can solve the cusp problem and the missing satellite problem of the ΛCDM model. In addition, it leads to dark matter halos with a constant surface density Σ0=ρ0 rh, and can explain its observed value Σ0=141 M⊙/pc2 without adjustable parameter. This makes the logotropic model rather unique among all the models attempting to unify dark matter and dark energy. In this paper, we compare the Logotropic and ΛCDM models at the cosmological scale where they are very close to each other in order to determine quantitatively how much they differ. This comparison is facilitated by the fact that these models depend on only two parameters, the Hubble constant H0 and the present fraction of dark matter Ωm0. Using the latest observational data from Planck 2015+Lensing+BAO+JLA+HST, we find that the best fit values of H0 and Ωm0 are H0=68.30 km s−1 Mpc−1 and Ωm0=0.3014 for the Logotropic model, and H0=68.02 km s−1 Mpc−1 and Ωm0=0.3049 for the ΛCDM model. The difference between the two models is at the percent level. As a result, the Logotropic model competes with the ΛCDM model at large scales and solves its problems at small scales. It may therefore represent a viable alternative to the ΛCDM model. Our study provides an explicit example of a theoretically motivated model that is almost indistinguishable from the ΛCDM model at the present time while having a completely different (phantom) evolution in the future. We analytically derive the statefinders of the Logotropic model for all values of the logotropic constant B. We show that the parameter s0 is directly related to this constant since s0=−B/(B+1) independently of any other parameter like H0 or Ωm0. For the predicted value of B=3.53× 10−3, we obtain (q0,r0,s0)=(−0.5516,1.011,−0.003518) instead of (q0,r0,s0)=(−0.5427,1,0) for the ΛCDM model corresponding to 0B=.

Effect of anisotropy on the generalized Chaplygin gas scalar field and its interaction with other dark energy models

In this work, we establish a correspondence between the interacting holographic, new agegraphic dark energy and generalized Chaplygin gas model in Bianchi type I universe. Then, we reconstruct the potential of the scalar field which describes the generalized Chaplygin cosmology. Cosmological solutions are obtained when the kinetic energy of the phantom field is of the order of the anisotropy and dominates over the potential energy of the field. We investigate observational constraints on the generalized Chaplygin gas, holographic and new agegraphic dark energy models as the unification of dark matter and dark energy, by using the latest observational data. To do this we focus on observational determinations of the expansion history H(z) . It is shown that the HDE model is better than the NADE and generalized Chaplygin gas models in an anisotropic universe. Then, we calculate the evolution of density perturbations in the linear regime for three models of dark energy and compare with the results of the $ \Lambda$ CDM model. Finally, the analysis shows that the increase in anisotropy leads to more correspondence between the dark energy scalar field model and observational data.

On the expanding phase of a singular bounce and intermediate inflation: The modified gravity description

We demonstrate that the intermediate inflation scenario is a singular inflation cosmology, with the singularity at the origin t = 0 being a pressure and energy density singularity and particularly a Type III singularity. Also, we show that the expanding phase of a singular bounce can be identical to the intermediate inflation scenario, if the singular bounce has a Type III singularity at the origin. For the intermediate inflation scenario we examine the cosmological implications on the power spectrum in the context of various forms of modified gravity. Particularly, we calculate the power spectrum in the context of F(R), F(G) Gauss–Bonnet gravity and also for F(T) gravity and we discuss the viability of each scenario by comparing the resulting spectral index with the latest observational data.

Cosmology with Varying Constants from a Thermodynamic Viewpoint

We study the variation of fundamental constants in cosmology while dealing with thermodynamic aspects of gravity. We focus on the variation of the speed of light, c, and Newton’s gravitational constant, G, with respect to cosmic time. We find the constraints on the possible variation of these constants by comparing varying constants of cosmological models with the latest observational data.

Unification of late- and early-time acceleration, with that of the intermediate eras, by scalar fields

In this work we shall investigate how to realize a cosmological evolution which describes all the evolution eras of our Universe, by using scalar-tensor theories. Particularly, we shall use single scalar and two-scalar theories, and by employing several well known reconstruction schemes, we shall investigate which scalar-tensor theories can realize the unification cosmology. For both the single and two-scalar theories, we find the kinetic terms and the potential of the theory and we also address the issue of the stability of the solutions towards linear perturbations. As we show, in the two-scalar description no instabilities occur, but in the single scalar description, certain solutions are unstable. Also we demonstrate that in the single scalar case, the power spectrum corresponding to early times is nearly scale invariant and compatible with the latest observational data. Finally, we perform an analysis of the effective equation of state, and we show that the EoS describes in a unified way early and late-time acceleration, and in addition it also describes the radiation and matter domination era between the acceleration eras.