Found 615 results, showing the newest relevant preprints. Sort by relevancy only.Update me on new preprints

f(R)-gravity **models** constrained with **cosmological** data

In this work, we look at the cosmological constraints of four different f(R)-gravity models, which include 2 toy models and 2 more realistic models, such as the Starobinsky and Hu-Sawicki models. Expand abstract.

In this work, we look at the

**cosmological**constraints of four different f(R)-gravity models, which include 2 toy**models**and 2 more realistic models, such as the Starobinsky and Hu-Sawicki**models**. We use 359 low- and intermediate-redshift Supernovae Type 1A data obtained from the SDSS-II/SNLS3 Joint Light-curve Analysis (JLA). We then develop a Markov Chain Monte Carlo (MCMC) simulation to determine the best-fit for each f(R)-gravity model, as well as for the Lambda Cold Dark Matter ($\Lambda$CDM) model, to obtain the**cosmological**parameters ($\Omega_{m}$ and $\bar{h}$). We assume a flat universe with negligible radiation. Therefore, the only difference between these**model**are the dark energy term and the arbitrary free parameters. When do a statistical analysis on these models, (where we used the $\Lambda$CDM**model**as the "true model"), we found that the Starobinsky**model**obtained a larger likelihood function that the $\Lambda$CDM model, while still obtaining the**cosmological**parameters to be $\Omega_{m} = 0.268^{+0.027}_{-0.024}$ and $\bar{h} = 0.690^{+0.005}_{-0.005}$. We also found a reduced Starobinsky model, that explained the data, as well as being statistically significant. We also found a further 3**models**that can explain the data, even though they are not statistically significant, while also finding 3**models**that did not explain the data and was statistically rejected.8 days ago

4/10 relevant

arXiv

4/10 relevant

arXiv

Extended FLRW **Models**: dynamical cancellation of **cosmological**
anisotropies

We investigate a corner of the Bianchi models that has not received much attention: "extended FLRW models" (eFLRW) defined as a cosmological model with underlying anisotropic Bianchi geometry that nevertheless expands isotropically and can be mapped onto a reference FLRW model with the same expansion history. Expand abstract.

We investigate a corner of the Bianchi

**models**that has not received much attention: "extended FLRW models" (eFLRW) defined as a**cosmological****model**with underlying anisotropic Bianchi geometry that nevertheless expands isotropically and can be mapped onto a reference FLRW**model**with the same expansion history. In order to investigate the stability and naturalness of such**models**in a dynamical systems context, we consider spatially homogeneous**models**that contain a massless scalar field $\varphi$ and a non-tilted perfect fluid obeying an equation of state $p=w\rho$. Remarkably, we find that matter anisotropies and geometrical anisotropies tend to cancel out dynamically. Hence, the expansion is asymptotically isotropic under rather general conditions. Although extended FLRW**models**require a special matter sector with anisotropies that are 'fine-tuned" relative to geometrical anisotropies, our analysis shows that such solutions are dynamically preferred attractors in general relativity. Specifically, we prove that all locally rotationally symmetric Bianchi type III universes with space-like $\nabla_\mu\varphi$ are asymptotically shear-free, for all $w\in[-1,1]$. Moreover, all shear-free equilibrium sets with anisotropic spatial curvature are proved to be stable with respect to all homogeneous perturbations for $w\geq -1/3$.9 days ago

4/10 relevant

arXiv

4/10 relevant

arXiv

A Map Between Primordial Power Spectra and the Effective Field Theory of Inflation

We have developed a precise dictionary between the spectrum of primordial density fluctuations and the parameters of the effective field theory (EFT) of inflation that determine the primordial power spectrum (PPS). Expand abstract.

We have developed a precise dictionary between the spectrum of primordial density fluctuations and the parameters of the effective field theory (EFT) of inflation that determine the primordial power spectrum (PPS). At lowest order the EFT contains two parameters: the slow-roll parameter $\epsilon$, which acts as an order parameter, and the speed of sound $c_s$. Applying second-order perturbation theory, we provide maps from the PPS to the EFT parameters that are precise up to the cube of the fractional change in the PPS $(\Delta \mathcal{P}/\mathcal{P})^3$, or less than $1\%$ for spectral features that modulate the PPS by $20\%$. While such features are not required when the underlying

**cosmological****model**is assumed to be $\Lambda$CDM they are necessary for alternative**models**that have no**cosmological**constant/dark energy. We verify the dictionary numerically and find those excursions in the slow-roll parameter that reproduce the PPS needed to fit Planck data for both $\Lambda$ and no-$\Lambda$**cosmological****models**.9 days ago

5/10 relevant

arXiv

5/10 relevant

arXiv

Standard Sirens as a novel probe of dark energy

Cosmological models with a dynamical dark energy field typically lead to a modified propagation of gravitational waves via an effectively time-varying gravitational coupling $G(t)$. Expand abstract.

**Cosmological**

**models**with a dynamical dark energy field typically lead to a modified propagation of gravitational waves via an effectively time-varying gravitational coupling $G(t)$. The local variation of this coupling between the time of emission and detection can be probed with standard sirens. Here we discuss the role that Lunar Laser Ranging (LLR) and binary pulsar constraints play in the prospects of constraining $G(t)$ with standard sirens. In particular, we argue that LLR constrains the matter-matter gravitational coupling $G_N(t)$, whereas binary pulsars and standard sirens constrain the quadratic kinetic gravity self-interaction $G_{gw}(t)$. Generically, these two couplings could be different in alternative

**cosmological**models, in which case LLR constraints are irrelevant for standard sirens. We use the Hulse-Taylor pulsar data and show that observations are highly insensitive to time variations of $G_{gw}(t)$, and we thus conclude that future gravitational waves data will become the best probe to test $G_{gw}(t)$, and will hence provide novel constraints on dynamical dark energy

**models**.

30 days ago

5/10 relevant

arXiv

5/10 relevant

arXiv

Connecting early and late epochs by f(z)CDM cosmography

The cosmographic approach is gaining considerable interest as a model-independent technique able to describe the late expansion of the universe. Expand abstract.

The cosmographic approach is gaining considerable interest as a

**model**-independent technique able to describe the late expansion of the universe. Indeed, given only the observational assumption of the**cosmological**principle, it allows to study the today observed accelerated evolution of the Hubble flow without assuming specific**cosmological****models**. In general, cosmography is used to reconstruct the Hubble parameter as a function of the redshift, assuming an arbitrary fiducial value for the current matter density, $\Omega_m$, and analysing low redshift**cosmological**data. Here we propose a different strategy, linking together the parametric cosmographic behavior of the late universe expansion with the small scale universe. In this way, we do not need to assume any "a priori" values for the**cosmological**parameters, since these are constrained at early epochs using both the Cosmic Microwave Background Radiation (CMBR) and Baryonic Acoustic Oscillation (BAO) data. In order to test this strategy, we describe the late expansion of the universe using the Pad\'e polynomials. This approach is discussed in the light of the recent $H(z)$ values indicators, combined with Supernovae Pantheon sample, galaxy clustering and early universe data, as CMBR and BAO. We found an interesting dependence of the current matter density value with cosmographic parameters, proving the inaccuracy of setting the value of $\Omega_m$ in cosmographic analyses, and a non-negligible effect of the cosmographic parameters on the CMBR temperature anisotropy power spectrum. Finally, we found that the cosmographic series, truncated at third order, shows a better $\chi^2$ best fit value then the vanilla $\Lambda$CDM**model**. This can be interpreted as the requirement that higher order corrections have to be considered to correctly describe low redshift data and remove the degeneration of the**models**.31 days ago

4/10 relevant

arXiv

4/10 relevant

arXiv

Interaction of Bianchi Type-I Anisotropic Cloud String **Cosmological**
**Model** Universe with Electromagnetic Field

Here, we have investigated the interaction of Bianchi type I anisotropic cloud string

**cosmological****model**universe with electromagnetic field in the context of general relativity. Expand abstract. Here, we have investigated the interaction of Bianchi type I anisotropic cloud string

**cosmological****model**universe with electromagnetic field in the context of general relativity. In this paper, the energy momentum tensor is assumed to be the sum of the rest energy density and string tension density with an electromagnetic field. To obtain exact solutions of Einstein field equations, we take average scale factor as an integrating function of time. Also, the dynamics and significance of various physical parameters of**model**are discussed.38 days ago

7/10 relevant

arXiv

7/10 relevant

arXiv

Constraining the anisotropy of the Universe via Pantheon supernovae sample

We test the possible dipole anisotropy of a Finslerian cosmological model and other three dipole-modulated cosmological models, i.e., the dipole-modulated $\rm{\Lambda}$CDM, $w$CDM and Chevallier--Polarski--Linder (CPL) model by using the recently released Pantheon sample of SNe Ia. Expand abstract.

We test the possible dipole anisotropy of a Finslerian

**cosmological****model**and other three dipole-modulated**cosmological**models, i.e., the dipole-modulated $\rm{\Lambda}$CDM, $w$CDM and Chevallier--Polarski--Linder (CPL)**model**by using the recently released Pantheon sample of SNe Ia. The Markov chain Monte Carlo (MCMC) method is used to explore the whole parameter space. We find that the dipole anisotropy is very weak in all**cosmological****models**used. Although the dipole amplitudes of four**cosmological****models**are consistent with zero within $1\sigma$ uncertainty, the dipole directions are close to the axial direction to the plane of SDSS subsample among Pantheon. It may imply that the weak dipole anisotropy in the Pantheon sample originates from the inhomogeneous distribution of the SDSS subsample. More homogeneous distribution of SNe Ia is necessary to constrain the cosmic anisotropy.38 days ago

9/10 relevant

arXiv

9/10 relevant

arXiv

Non-Standard **Cosmological** **Models** and the trans-Planckian Censorship
Conjecture

The trans-Planckian censorship conjecture (TCC) puts an upper bound on the Hubble parameter during inflation $H_{\rm inf}$ of order $0.1$ GeV. Expand abstract.

The trans-Planckian censorship conjecture (TCC) puts an upper bound on the Hubble parameter during inflation $H_{\rm inf}$ of order $0.1$ GeV. This conclusion is based on the assumption that the Universe undergoes the standard thermal history such that it is radiation dominated after the end inflation until the time of radiation-matter equality. In this note, we depart from this assumption and study implications of the TCC when the Universe follows a non-standard history {\it e.g.} early matter domination or secondary stages of inflation. String theory compactifications and axiverse scenarios generically predict epochs when the modulus/axion fields rule the dynamics of the Universe as matter or dark energy. We observe that in these non-standard pictures, the TCC bound on $H_{\rm inf}$ can be raised up to 3 orders of magnitude in moduli cosmology. In multiple inflationary scenario the upper bound on the first (observable) inflation can be raised up to the {\it Planck 2018} upper bound.

38 days ago

7/10 relevant

arXiv

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arXiv

Constraining a causal dissipative **cosmological** **model**

In this paper a cosmological solution of polynomial type $H \approx ( t + const.)^{-1}$ for the causal thermodynamical approach of Isarel-Stewart, found in \cite{MCruz:2017, Cruz2017}, is constrained using the joint of the latest measurements of the Hubble parameter (OHD) and Type Ia Supernovae (SNIa). Expand abstract.

In this paper a

**cosmological**solution of polynomial type $H \approx ( t + const.)^{-1}$ for the causal thermodynamical approach of Isarel-Stewart, found in \cite{MCruz:2017, Cruz2017}, is constrained using the joint of the latest measurements of the Hubble parameter (OHD) and Type Ia Supernovae (SNIa). Since the expansion described by this solution does not present a transition from a decelerated phase to an accelerated one, both phases can be well modeled connecting both phases by requiring the continuity of the Hubble parameter at $z=z_{t}$, the accelerated-decelerated transition redshift. Our best fit constrains the main free parameters of the**model**to be $A_1= 1.58^{+0.08}_{-0.07}$ ($A_2=0.84^{+0.02}_{-0.02}$) for the accelerated (decelerated) phase. For both phases we obtain $q=-0.37^{+0.03}_{-0.03}$ ($0.19^{+0.03}_{-0.03}$) and $\omega_{eff} = -0.58^{+0.02}_{-0.02}$ ($-0.21^{+0.02}_{-0.02}$) for the deceleration parameter and the effective equation of state, respectively. Comparing our**model**and LCDM statistically through the Akaike information criterion and the Bayesian information criterion we obtain that the LCDM**model**is preferred by the OHD+SNIa data. Finally, it is shown that the constrained parameters values satisfy the criterion for a consistent fluid description of a dissipative dark matter component, but with a high value of the speed of sound within the fluid, which is a drawback for a consistent description of the structure formation. We briefly discuss the possibilities to overcome this problem with a non-linear generalization of the causal linear thermodynamics of bulk viscosity and also with the inclusion of some form of dark energy.57 days ago

5/10 relevant

arXiv

5/10 relevant

arXiv

**Cosmological** Parameters from the BOSS Galaxy Power Spectrum

We present cosmological parameter measurements from the publicly available Baryon Oscillation Spectroscopic Survey (BOSS) data on anisotropic galaxy clustering in Fourier space. Expand abstract.

We present

**cosmological**parameter measurements from the publicly available Baryon Oscillation Spectroscopic Survey (BOSS) data on anisotropic galaxy clustering in Fourier space. Compared to previous studies, our analysis has two main novel features. First, we use a complete perturbation theory**model**that properly takes into account the non-linear effects of dark matter clustering, short-scale physics, galaxy bias, redshift-space distortions, and large-scale bulk flows. Second, we employ a Markov-Chain Monte-Carlo technique and consistently reevaluate the full power spectrum likelihood as we scan over different cosmologies. Assuming a minimal $\Lambda$CDM cosmology with massive neutrinos, fixing the primordial power spectrum tilt, and imposing the big bang nucleosynthesis (BBN) prior on the physical baryon density $\omega_b$, we find the following late-Universe parameters: Hubble constant $H_0=(67.89\pm 1.06)$ km$\,$s$^{-1}$Mpc$^{-1}$, matter density fraction $\Omega_m=0.295\pm 0.010$, and the mass fluctuation amplitude $\sigma_8=0.721\pm 0.043$. These parameters were measured directly from the BOSS data and independently of the Planck cosmic microwave background observations. Varying the power spectrum tilt or changing the $\omega_b$ prior do not significantly alter our main results. Finally, we discuss the information content of the BOSS power spectrum and show that it is dominated by the location of the baryon acoustic oscillations and the power spectrum shape. We argue that the contribution of the Alcock-Paczynski effect is marginal in $\Lambda$CDM, but becomes important for non-minimal**cosmological****models**.72 days ago

4/10 relevant

arXiv

4/10 relevant

arXiv