Friedmann-Robertson-Walker Research Articles

On the convergence of cosmographic expansions in Lemaitre-Tolman-Bondi models

Abstract We study cosmographic expansions of the luminosity distance for a variety of Lemaitre-Tolman-Bondi models which we specify inspired by local large-scale structures of the universe. We consider cosmographic expansions valid for general spacetimes and compare to the Friedmann-Lemaitre-Robertson-Walker (FLRW) limit of the expansions as well as to its naive isotropic extrapolation to an inhomogeneous universe. The FLRW expansions are often poor near the observer but become better at higher redshifts, where the light rays have reached the FLRW background. In line with this we find that the effective Hubble, deceleration and jerk parameters of the general cosmographic expansion are often very different from the global $\Lambda$CDM values, with deviations up to several orders of magnitude. By comparing with the naive isotropic extrapolation of the FLRW expansion, we assess that these large deviations are mainly due to gradients of the shear. Very close to the observer, the general cosmographic expansion is always best and becomes more precise when more expansion terms are included. However, we find that the convergence radius of the general cosmographic expansion is small for all studied models and observers and the general cosmographic expansion becomes poor for most of the studied observers already before a single LTB structure has been traversed. The small radius of convergence of the general cosmographic expansion has also been indicated by earlier work and may need careful attention before we can safely apply the general cosmographic expansion to real data.

Theoretical insights and implications of bouncing cosmology in f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},\ extrm{T}^2)$$\\end{document} theory

In this paper, we explore cosmological bouncing solutions to investigate the cosmic evolution in the framework of energy-momentum squared gravity. We consider flat Friedmann–Robertson–Walker spacetime with a perfect matter distribution. We assume two different functional forms of f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},T^2)$$\\end{document} gravity model to examine the impact of this modified framework in the evolution of the universe. Furthermore, we consider a specific scale factor to investigate different cosmological parameters, analyzing the evolutionary behavior of the universe in this gravity. We also perform stability analysis using the perturbation technique. Our findings indicate that the null energy condition violates at the bounce point and equation of state parameter exhibits characteristics of a quintessence era or phantom regimes. These aspects highlight the complex interplay between energy conditions and stability in bouncing cosmological model. We conclude that the f(R,T2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(\\mathcal {R},T^2)$$\\end{document} theory successfully provides viable alternatives to the standard cosmological scenarios, offering insights into the early universe and the nature of gravity.

Cooling-heating properties of the FRW universe in gravity with a generalized conformal scalar field

In this paper, within the framework of modified gravity involving a conformal scalar field, we investigate the Joule-Thomson expansion of the Friedmann-Robertson-Walker (FRW) universe to identify cooling and heating regions. We observe a divergence in the Joule-Thomson coefficient (μ), precisely at the apparent horizon (RA=−2α) of the FRW universe, under conditions of negative gravitational coupling (α<0), which aligns with a thermodynamic singularity. Furthermore, we determine the inversion temperature and pressure for the FRW universe, and elucidate the salient features of inversion and isenthalpic curves in the temperature-pressure (T-P) plane. We compare these findings with results obtained under Einstein gravity, discussing the influence of the modification term on the cooling and heating properties of the FRW universe. This work presents insights into the formation of cooling and heating regions within the FRW universe, contributing to a deeper understanding of the physical mechanisms behind cosmic expansion history.

Holographic bounce cosmological models induced by viscous dark fluid from a generalized non-singular entropy function

Bounce cosmological models containing a dark viscous fluid in a spatially flat Friedmann–Robertson–Walker (FRW) universe are considered. The universe’s evolution is described in terms of generalized equation of state (EoS) parameters, in the presence of the bulk viscosity. Entropic cosmology plays a key role in the discussion, and the matter bounce behavior is described based on a nonsingular, generalized entropy function, recently proposed by S. D. Odintsov and T. Paul. Three different forms of the scale factor are investigated: an exponential, a power-law and a double-exponential function, respectively. Appropriate bounce cosmological models are formulated, via the relevant parameters of the modified EoS, and analytical expressions for the corresponding infrared cut-off are obtained, via the particle horizon. Results are displayed in holographic form, making use of generalized holographic cut-offs first introduced by S. Nojiri and S. D. Odintsov. In addition, the viability of the corresponding bounce cosmological models is investigated, taking into account the actual thermodynamic properties of our universe, by means of a no-singular, generalized entropy function. In the asymptotic case, an expression for the generalized entropy is obtained, which remarkably has the additivity property.

Analyzing inflationary parameters and swampland conjectures of modified cosmology according to various entropies

This paper investigates the inflationary phase of the FRW universe within the context of generalized Chaplygin gas. We explore modified cosmological scenarios incorporating two entropy corrections, namely Tsallis and Barrow. These proposed entropies lead to modifications in the Friedmann equations. By incorporating the generalized Chaplygin gas into these equations, we determine various parameters including the slow roll parameters, number of e-folds, curvature perturbation, tensor-to-scalar ratio, and spectral index. We observe variations in the values of the tensor-to-scalar ratio r and the scalar spectral index ns with respect to the Tsallis and Barrow parameters. Additionally, we plot the r−ns plane. For the Tsallis entropy, we obtain r≤0.012, r≤0.027, and r≤0.008, with ns lying between (0.961,0.964), (0.957,0.966), and (0.956,0.964), respectively. In Barrow cosmology, we find r≤0.0037, r≤0.12, and r≤0.010, while ns falls within (0.92,0.97), (0.855,0.995), and (0.960,0.966) for the chaotic, SFI, and exponential potentials, respectively. These graphical analyses demonstrate the compatibility of our models with the latest Planck data. Moreover, by considering these entropy corrections, we determine the bounds of the Swampland de Sitter conjecture for generalized Chaplygin gas. In all cases, this bound remains less than unity, satisfying the required condition for the Swampland de Sitter conjecture.

Azimuthal geodesics in closed FLRW cosmological models

We study geodesics in Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models and give the full set of solutions. For azimuthal geodesics, in a closed universe, we give the angular distance traveled by a test particle moving along such a geodesic during one cycle of expansion and recollapse of the universe. We extend previous results regarding the path followed by light rays to the two-fluid case, also including a cosmological constant, as well as to massive test particles. Our work contains various new results and explicit formulas, often using special functions which naturally appear in this setting. Published by the American Physical Society 2024

Cosmological study in Myrzakulov [formula omitted] quasi-dilaton massive gravity

This study explores the cosmological implications of the Myrzakulov F(R,T) quasi-dilaton massive gravity theory, a modification of the de Rham-Gabadadze-Tolley (dRGT) massive gravity theory. Our analysis focuses on the self-accelerating solution of the background equations of motion, which are shown to exist in the theory. Notably, we find that the theory features an effective cosmological constant corresponding to the massive graviton, which has important implications for our understanding of the universe’s accelerated expansion. To assess the validity of the Myrzakulov F(R,T) quasi-dilaton massive gravity theory, we employ two datasets: the Union2 Type Ia Supernovae (SNIa) dataset, consisting of 557 observations, and the Pantheon SNIa data, which includes 1048 SNe I-a events gathered from diverse SN I-a samples. Our results demonstrate that the theory is capable of explaining the accelerated expansion of the universe without requiring the presence of dark energy. This finding supports the potential of the Myrzakulov F(R,T) quasi-dilaton massive gravity theory as an alternative explanation for the observed cosmic acceleration. Moreover, we investigate the properties of tensor perturbations within the framework of this theory and derive a novel expression for the dispersion relation of gravitational waves. Our analysis reveals interesting features of the modified dispersion relation in the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology, providing new insights into the nature of gravitational waves in the context of the Myrzakulov F(R,T) quasi-dilaton massive gravity theory.

Traces of Quantum Gravity Effects at Late-time Cosmological Dynamics via Distance Measures

Inspired by the entropy–area relation of black hole thermodynamics, we study the thermodynamics of the cosmological apparent horizon in a spatially flat Friedmann–Robertson–Walker universe in the framework of an extended uncertainty principle (EUP). The adopted EUP naturally admits a minimal measurable momentum (equivalently a maximal measurable length), as an infrared cutoff in the theory. We derive the modified Friedmann equations in this setup and explore some predictions of these equations for the late-time universe via distance measures. We show that in this framework it is possible to realize the late-time cosmic speedup and transition to the phantom phase of the equation-of-state parameter of the effective cosmic fluid without recourse to any dark energy component or modified gravity. Inspection of various distance measures in this framework shows that an EUP with a negative deformation parameter suffices for the interpretation of the late-time asymptotically de Sitter universe with standard nonrelativistic matter.

Renyi Type Holographic Dark Energy

Dark energy is one of the prominent mysteries of the universe that still awaits a solution. One of the plausible ways to collect data about any formation or understand its information capacity is to investigate the entropy of that formation. In this study, Renyi Holographic Dark Energy (RHDE) matter distribution is analyzed within the framework of General Relativity Theory, considering homogeneous and isotropic Friedmann-Robertson-Walker (FRW) space-time. Hubble parameter and RHDE density were used to obtain exact solutions of Einstein field equations. The analysis of the obtained solutions was performed by drawing evolution graphs for redshift $z$.

Phase space analysis of interacting and non-interacting models in f(Q, C) gravity

Abstract Using a dynamical system method, we have studied a Friedmann-Robertson-Walker (FRW) cosmological model within the context of $f(Q,C)$ gravity, where $Q$ is the non-metricity scalar and $C$ represents the boundary term, considering both interacting and non-interacting models. A set of autonomous equations is derived, and solutions are calculated accordingly. We assessed the critical points obtained from these equations, identified their characteristic values, and explored the physical interpretation of the phase space for this system. Two types of $f(Q,C)$ are assumed to be $(i)$ $f(Q,C)=Q+\alpha Q+\beta C logC$ and $(ii)$ $f(Q,C)=Q+\alpha Q+\frac{\beta}{C}$, where $\alpha$ and $\beta$ are the parameters. In Model I, we obtain two stable critical points, while in Model II, we identified three stable critical points for both interacting and non-interacting models. We look into how the phase space trajectories behave at every critical points. We calculate the values of the physical parameters for both systems at each critical point, indicating the Universe's accelerated expansion.

The analytical approach in testing the Kaniadakis cosmology

Abstract We know that a quantum-corrected cosmological scenario can emerge based on its corrected Friedmann equations corresponding to the corrected entropy of cosmic-horizon using the gravity-thermodynamics conjecture in deriving those equations. In this right, the Kaniadakis entropy associated with the apparent horizon of Friedmann-Robertson-Walker(FRW) Universe leads to a corrected Friedmann equation which contains a correction term as $\Omega_{Ka}=\frac{\alpha\left(H^2+\frac{k}{a^2}\right)^{-1}}{H^{2}}$ where $\alpha\equiv\frac{K^2 \pi^2}{2 G^2}$ ($K$ is the Kaniadakis parameter). Here, we derive the analytical relations between the energy density parameters $\Omega_{m},\Omega_{\Lambda}$ (and the ratio density $\Omega_{Ka}$ of correction term) and the geometrical cosmological parameters $\{q, j\}$. This leads to getting $\Omega_{Ka}=\frac{j-1}{4(q+1)^{2}-(j-1)}$ which enables us to put constrains on $\Omega_{Ka_{0}}$ using the measurable parameters $\{q_{0},j_{0}\}$ and $H_{0}$. It also reveals some interesting aspects of the Kaniadakis cosmology by explaining that the correction term plays different roles both in the presence and in the absence of the cosmological constant $\Lambda$.&amp;#xD;This term plays the role of dark energy in the absence of the cosmological constant $\Lambda$, while in the presence of this component, it plays the role of a small correction to dark energy. The value of $\Omega_{Ka}$ then determines the amount of the deviation of Kaniadakis model from the concordance $\Lambda CDM$ model. As a result, the value of $j_{0}$ is found to be close to unit for vanishing cosmological constant $\Lambda=0$ and the maximum value of $\Omega_{Ka0}$; thereby we get $(j_{0}-1)\simeq 0.137$ and $(j_{0}-1)\simeq&lt; 10^{-3}$ for the case of $\Lambda=0$ and $\Lambda\neq0$, respectively&amp;#xD;

Late time cosmic acceleration through parametrization of Hubble parameter in [formula omitted] gravity

This paper explores the rapid expansion of the Universe during its later stages within the context of f(R,Lm) gravity theory. We present a novel parametrization of the Hubble parameter in a manner independent of any specific model and employ it to analyze the Friedmann equations within the FLRW Universe framework. Using a Markov Chain Monte Carlo (MCMC) approach, we derive the model parameters by analyzing a composite dataset comprising 31 Cosmic Chronometers (CC) data points, 26 independent Baryonic Acoustic Oscillations (BAO) measurements, and 1701 data points from Pantheon+SH0ES dataset. The trajectory of the deceleration parameter highlights the shift from deceleration to acceleration in the evolution of the Universe. Additionally, we delve into the dynamics of fundamental cosmological variables for two nonlinear f(R,Lm) models, including energy density, pressure, equation of state (EoS) parameter, and energy conditions.

Euclid preparation

Verifying the fully kinematic nature of the long-known cosmic microwave background (CMB) dipole is of fundamental importance in cosmology. In the standard cosmological model with the Friedman–Lemaitre–Robertson–Walker (FLRW) metric from the inflationary expansion, the CMB dipole should be entirely kinematic. Any non-kinematic CMB dipole component would thus reflect the preinflationary structure of space-time probing the extent of the FLRW applicability. Cosmic backgrounds from galaxies after the matter-radiation decoupling should have a kinematic dipole component identical in velocity to the CMB kinematic dipole. Comparing the two can lead to isolating the CMB non-kinematic dipole. It was recently proposed that such a measurement can be done using the near-infrared cosmic infrared background (CIB) measured with the currently operating Euclid telescope, and later with Roman. The proposed method reconstructs the resolved CIB, the integrated galaxy light (IGL), from Euclid’s Wide Survey and probes its dipole with a kinematic component amplified over that of the CMB by the Compton–Getting effect. The amplification coupled with the extensive galaxy samples forming the IGL would determine the CIB dipole with an overwhelming signal-to-noise ratio, isolating its direction to sub-degree accuracy. We developed details of the method for Euclid’s Wide Survey in four bands spanning from 0.6 to 2 μm. We isolated the systematic and other uncertainties and present methodologies to minimize them, after confining the sample to the magnitude range with a negligible IGL–CIB dipole from galaxy clustering. These include the required star–galaxy separation, accounting for the extinction correction dipole using the new method developed here achieving total separation, and accounting for the Earth’s orbital motion and other systematic effects. Finally, we applied the developed methodology to the simulated Euclid galaxy catalogs, successfully testing the upcoming applications. With the techniques presented, one would indeed measure the IGL–CIB dipole from Euclid’s Wide Survey with high precision, probing the non-kinematic CMB dipole.

Constraining hybrid potential scalar field cosmological model in Lyra’s geometry with recent observational data

In this study, we investigate a scalar field cosmological model with Lyra’s geometry to explain the present cosmic expansion in a homogeneous and isotropic flat FRW universe. In Einstein’s field equations, we presupposed a variable displacement vector as an element of Lyra’s geometry. In the context of the conventional theory of gravity, we suggest a suitable parametrization of the scalar field’s dark energy density in the hybrid function of redshift [Formula: see text], confirming the essential transition behavior of the universe from a decelerating era to the present accelerated scenario. We present constraints on model parameters using the most recent observational datasets from OHD, BAO/CMB and Pantheon, taking Markov Chain Monte Carlo (MCMC) analysis into account. For the proposed model, the best estimated values of parameters for the combined dataset (OHD, BAO/CMB and Pantheon) are [Formula: see text][Formula: see text]km/s/Mpc, [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. The model exhibits a flipping nature, and the redshift transition occurs at [Formula: see text]. The current value of the decelerated parameter for the proposed model is calculated as [Formula: see text] for the combined dataset. Some dynamical properties of the model like energy density ([Formula: see text]), scalar field pressure ([Formula: see text]), EoS parameter of scalar field ([Formula: see text]), and effective EoS parameter ([Formula: see text]) are analyzed and presented. Further, we have also examined the statefinder diagnosis and jerk parameters of the derived model. The total density parameter for the derived model is found to be unity which is in nice agreement with recent standard findings.

Polytropic gas cosmology and late-time acceleration

The accelerated expansion of the Universe has sparked significant interest in the mysterious concept of dark energy within cosmology. Various theories have been proposed to explain dark energy, and many models have been developed to understand its origins and properties. This research explores cosmic expansion using the Polytropic Gas (PG) approach, which combines Dark Matter (DM) and Dark Energy (DE) into a single mysterious fluid. We used the principles of general relativity and built our model within the homogeneous and isotropic framework of Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. We revised the Original Polytropic Gas (OPG) model to expand its applicability beyond the OPG, to the ΛCDM model. Our model's parameters were carefully adjusted to reflect key cosmological features of the variable PG approach. To validate our model, we performed a Markov chain Monte Carlo analysis using recent Supernova data from the Pantheon+ survey, 36 observational data points, 162 Gamma-Ray Bursts, and 24 binned Quasars distance modulus data. The AIC and BIC criteria indicate that our model is slightly preferred over the ΛCDM model based on observational data. We also tested our model with data, Supernova, Gamma-Ray Bursts, and Quasars and found that it exhibits a transition from a quintessential to phantom regime. The Polytropic dark fluid model (PDFM) is a promising candidate that effectively addresses the interplay between cosmic acceleration and dark energy.

Cosmological solution through gravitational decoupling in [formula omitted] gravity

This paper aims to formulate anisotropic cosmological solution of a non-static spherical structure with the help of gravitational decoupling scheme through minimal geometric deformation in f(G,T) gravity. This technique transforms only the radial metric function while the temporal component remains unchanged. Consequently, the field equations are separated into two independent arrays: one is related to the seed source and the other characterizes the extra sector. In order to derive the solution corresponding to the isotropic sector, we use the Friedmann–Lemaitre–Robertson–Walker cosmic model and employ the barotropic equation of state as well as power-law model. Finally, we study the impact of decoupling parameter to describe different eras of the universe through graphical analysis. It is found that physically viable and stable trends of the resulting solution are achieved for both radiation-dominated as well as matter-dominated epochs in this modified theory.

Prediction of Super-Exponentially Accelerated Universe in a Friedmann–Lemaitre–Robertson–Walker Metric

Prediction of Super-Exponentially Accelerated Universe in a Friedmann–Lemaitre–Robertson–Walker Metric

Emergence of cosmic space and horizon thermodynamics from Kaniadakis entropy

Abstract Utilizing Kaniadakis entropy associated with the apparent horizon of the Friedmann-Robertson-Walker (FRW) Universe and applying the emergence of cosmic space paradigm, we deduce the modified Friedmann equation for a non-flat (n+1)-dimensional universe. Employing the first law of thermodynamics, we arrive at the same modified Friedmann equation, showing the connection between emergence of cosmic space and first law of thermodynamics. We also establish the condition to satisfy the Generalized second law of thermodynamics within the Kaniadakis framework. Our study illuminates the intricate connection between the law of emergence and horizon thermodynamics, offering a deeper insight through the lens of Kaniadakis entropy.

New characterization of Robertson–Walker geometries involving a single timelike curve

Abstract Our aim in this paper is two-fold. We establish a novel geometric characterization of the Robertson–Walker (RW) spacetime and, along the process, we find a canonical form of the RW metric associated to an arbitrary timelike curve and an arbitrary space frame. A known characterization establishes that a spacetime foliated by constant curvature leaves whose orthogonal flow (the cosmological flow) is geodesic, shear-free, and with constant expansion on each leaf, is RW. We generalize this characterization by relaxing the condition on the expansion. We show it suffices to demand that the spatial gradient and Laplacian of the cosmological expansion on a single arbitrary timelike curve vanish. In General Relativity these local conditions are equivalent to demanding that the energy flux measured by the cosmological flow, as well as its divergence, are zero on a single arbitrary timelike curve. The proof allows us to construct canonically adapted coordinates to the arbitrary curve, thus well-fitted to an observer with an arbitrary motion with respect to the cosmological flow.

Dynamical Black holes in the FLRW universe

Observations show that massive black holes, thought to be at the center of galaxies and galaxy clusters, are dynamical objects and interact with their environments. Black holes are characterized by their mass and angular momentum, and they are time-varying objects embedded in the cosmological Friedman-Lemaitre-Robertson-Walker (FLRW) universe. Finding analytical solutions for the dynamical black holes with all these properties is very important in theoretical physics. In the present work, we obtain Einstein’s field equations for our predicted dynamical black hole geometry and obtain analytical solutions using the energy-momentum tensor obeying imperfect fluid dynamics. Misner-Sharp-Hernandez mass, turnaround radius, and apparent horizon are analyzed and compared with the recent observations.