Friedmann-Robertson-Walker Research Articles

Observational Constraining Study of New Deceleration Parameters in FRW Universe

AbstractIn the current paper, a dark energy (DE) model reconstructed from the well‐motivated deceleration parameter (DP) is analyzed. A flat FRW Universe filled with radiation, dark matter (DM), and dark energy fluids is considered. The free parameters are constrained using measurements from Supernovae, Hubble, Gamma Ray Bursts, Quasars, and Baryon acoustic Oscillations. The model under study is found to be very supported by observation with respect to ΛCDM since . Besides, a cosmographic analysis is performed showing that the reconstructed model behaves similarly as ΛCDM does. Finally, a diagnostic analysis is performed reporting that the studied model behaves quintessence type at a late time.

Dark energy nature in logarithmic f(R,T) cosmology

This research paper is an investigation of dark energy nature of logarithmic [Formula: see text]-gravity cosmology in a flat FLRW space–time universe. We have derived modified Einstein’s field equations for the function [Formula: see text] where [Formula: see text] is the Ricci scalar curvature, [Formula: see text] is the trace of the stress energy momentum tensor, and [Formula: see text] is a model parameter. We have solved field equations in the form of two fluid scenarios as perfect fluid and dark fluid, where dark fluid term is derived in the form of perfect fluid source. We have made an observational constraint on the cosmological parameters [Formula: see text] and [Formula: see text] using [Formula: see text] test with observational datasets like Pantheon sample of SNe Ia and [Formula: see text]. With these constraints, we have discussed our model with deceleration parameter [Formula: see text], energy parameters [Formula: see text], EoS parameter [Formula: see text], etc. Also, we have done Om diagnostic analysis. The derived [Formula: see text] model shows a quintessence dark energy model [Formula: see text] and late-time universe approaches to [Formula: see text] CDM model.

Little Rip, Pseudo Rip and bounce cosmology from generalized equation of state in the universe with spatial curvature

We consider the Little Rip (LR), Pseudo Rip (PR) and bounce cosmological models in the Friedmann–Robertson–Walker (FRW) metric with nonzero spatial curvature. We describe the evolution of the universe using a generalized equation of state (EoS) in the presence of a viscous fluid. The conditions of the occurrence of the LR, PR and bounce were obtained from the point of view of the parameters of the generalized EoS for the cosmic dark fluid, taking into account the spatial curvature. The analytical expressions for the spatial curvature were obtained. Asymptotic cases of the early and late universe are considered. A method of Darboux transformation (DT) was proposed in the case of models of an accelerating universe with viscosity.

New emergent observational constraints in f(Q,T) gravity model

In the current study, we inspect the late-time acceleration of the universe in f(Q,T) gravity. In this theory, Q represents the non-metricity term and with Tμν the energy-momentum tensor, with T its trace. To explore the solutions in the theory for the Friedmann-Lemaitre-Robertson-Walker (FLRW) model, we propose a new scale factor called the emergent scale factor, which yields the Hubble parameter, H(z), having the form H(z)=nb(−B+1(A(z+1))1b) with a new relation a(t)=11+z). By using the SNIa from Pantheon, BAO from (6dFGS, BOSS-LOWZ, SDSS DR7 MGS, BOSS-DR12, BOSS-CMASS, WiggleZ and Lya), CMB from Planck 2018, and the 36 data points from Hubble datasets using the Monte-Carlo Markov Chain (MCMC) approach, we estimate the parameters of the model. The variation in time of the deceleration parameter in the model shows that a phase of deceleration transits to a phase of acceleration. Further, we examine the behaviour of the jerk parameter and carry out a statefinder analysis. In order to complete the current study, we consider two forms for f(Q,T), specifically, f(Q,T)=a1Qm+a2T and f(Q,T)=γQ2+ηQ+λT where a1, a2, m, γ, η, and λ are the model's parameters that need to satisfy observations. Finally, we present an appraisal of our current analysis.

Cosmic implications of generalized HDE model in FRW universe

In this study, we investigate a generalized holographic dark energy model within the framework of dynamical Chern–Simons modified gravity theory. By incorporating the Hubble parameter H and its higher order derivatives, we explore the equation of state and its implications for the expansion of the known universe using the Friedmann–Robertson–Walker metric. Our analysis reveals three distinct cases arising from the solutions to the Chern–Simons field equations, which depend on the chosen parameters. In the first two cases, we observe confirmation of the accelerated expansion of the universe, consistent with the predicted range of −1<ω<0. However, in the third case, the depicted behavior exhibits a phantom-like nature. It is important to note that irrespective of the case, the generalized holographic dark energy model supports the transition from deceleration to acceleration, aligning well with observational data. Furthermore, our findings establish that the generalized holographic dark energy model endorses the universe’s accelerated expansion within the framework of dynamical Chern–Simons modified gravity theory.

Electromagnetic Waves in Cosmological Spacetime

We consider the propagation of electromagnetic waves in the Friedmann–Lemaître–Robertson–Walker metric. The exact solutions for plane and spherical wave are written down. The corresponding redshift, amplitude change, and dispersion are discussed. We also speculate about the connection of the electromagnetic wave equation to the Proca equation and its significance for the early Universe.

Dynamical stability in presence of non-minimal derivative dependent coupling of k-essence field with a relativistic fluid

In this paper we investigate a non-minimal, space-time derivative dependent, coupling between the k-essence field and a relativistic fluid using a variational approach. The derivative coupling term couples the space-time derivative of the k-essence field with the fluid 4-velocity via an inner product. The inner product has a coefficient whose form specifies the various models of interaction. By introducing a coupling term at the Lagrangian level and using the variational technique we obtain the k-essence field equation and the Friedmann equations in the background of a spatially flat Friedmann–Lemaitre–Robertson–Walker (FLRW) metric. Explicitly using the dynamical analysis approach we analyze the dynamics of this coupled scenario in the context of two kinds of interaction models. The models are distinguished by the form of the coefficient multiplying the derivative coupling term. In the simplest approach we work with an inverse square law potential of the k-essence field. Both of the models are not only capable of producing a stable accelerating solution, they can also explain different phases of the evolutionary universe.

A constrained cosmological model in [formula omitted] gravity

In this article, we study the expanding nature of the universe in the context of f(R,Lm) gravity theory, here R represents the Ricci scalar and Lm is the matter Lagrangian density. With a specific form of f(R,Lm), we obtain the field equations for flat FLRW metric. We parametrize the deceleration parameter in terms of the Hubble parameter and from here we find four free parameters, which are constraints and estimated by using H(z), Pantheon, and their joint data sets. Further, we investigate the evolution of the deceleration parameter which depicts a transition from the deceleration to acceleration phases of the universe. The evolutionary behaviour of energy density, pressure, and EoS parameter shows that the present model is an accelerated quintessence dark energy model. To compare our model with the ΛCDM model we use some of the diagnostic techniques. Thus, we find that our model in f(R,Lm) gravity supports the recent standard observational studies and delineates the late-time cosmic acceleration.

Scale-invariant inflation

We examine a scalar-tensor model of gravity that is globally scale-invariant. When adapted to a spatially flat Robertson-Walker metric, the equations of motion describe a dynamical system that flows from an unstable de Sitter space to a stable one. We show that during this transition inflation can occur. Moreover, at the final fixed point, a mass scale naturally emerges that can be identified with the Planck mass. We compute the inflationary spectral indices and the tensor perturbation and we compare them with observations. We also study the possibility that primordial magnetic fields are generated during inflation.

Fermion mixing in curved spacetime

We develop the quantum field theory of fermion mixing in curved spacetime and discuss the role of unitarily inequivalent representations in the particle interpretation of the theory. We derive general oscillation formulae and apply them to specific spcetimes of interest, such as spatially flat FRW metrics and the Schwarzschild spacetime. We exhibit the main deviations from the usual quantum mechanical approach.

An update on adiabatic modes in cosmology and δN formalism

In this paper, we generalize the Weinberg's procedure to determine the comoving curvature perturbation ℛ to non-attractor inflationary regimes. We show that both modes of ℛ are related to a symmetry of the perturbative equations in the Newtonian gauge. As a byproduct, we clarify that adiabaticity does not generally imply constancy of ℛ, not even in the k ⟶ 0 limit. We then show that there exist non-equivalent definitions of δN that would reproduce ℛ or the uniform density curvature perturbation ζ at linear order. We have then shown that the perturbative δN definition in terms of difference between the number of e-foldings of different gauges, can be extended non-perturbatively at leading order in gradient expansion. Nevertheless, the computer friendly definition in terms of the difference of e-foldings obtained from the evolution of a local FRW Universe, respectively with perturbed and un-perturbed initial conditions, might only give information about the linear order curvature perturbations, contrary to what is stated in the literature.

The Evolution of a Higher-Dimensional FRW Universe with Variable G and Λ and Particle Creation

Using an open thermodynamic systems theory, the effect of particle creation on the evolution and dynamics of the standard cosmological FLRW model in a higher-dimensional spacetime with functionally dependent cosmological and gravitational constants Λ and G is investigated. The gravitational field equations have been transformed into a dimensionless system of non-linear, first-order, coupled differential equations (DEs) as functions of the universe’s density parameters Ωi and rate of particle creation Ψ in redshift space, which can be numerically casted. Two cosmological models are obtained, depending on the choice of particle creation rate—Ψ∼H2 and Ψ∼n2 for dust-, radiation- and dark-energy-dominated universes, respectively. The dynamic behaviour of each model is discussed.

A new approach to the study of spacelike submanifolds in a spherical Friedmann–Lemaître–Robertson–Walker spacetime: characterization of the stationary spacelike submanifolds as an application

A natural codimension one isometric embedding of each -dimensional spherical Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime in the -dimensional Lorentz–Minkowski spacetime permits to contemplate as a rotation Lorentzian hypersurface in . After a detailed study of such Lorentzian hypersurfaces, any k-dimensional spacelike submanifold of such an FLRW spacetime can be contemplated as a spacelike submanifold of . Then, we use that situation to study k-dimensional stationary (i.e. of zero mean curvature vector field) spacelike submanifolds of the FLRW spacetime. In particular, we prove a wide extension of the Lorentzian version of the classical Takahashi theorem, giving a characterization of stationary spacelike submanifolds of when contemplating them as spacelike submanifolds of .

Legendre scalarization in gravity and cosmology

We propose a new formulation of f(R) gravity, dubbed scalarized f(R) gravity, in which the Legendre transform is included as a dynamical term. This leads to a theory with second-order field equations that describes general relativity with a self-interacting scalar field, without requiring the introduction of conformal frames. We demonstrate that the quadratic version of scalarized f(R) gravity reduces to general relativity with a massive scalar field, and we explore its implications for Friedmann cosmology. Our findings suggest that scalarized f(R) gravity may lead to simplified descriptions of cosmological applications, while the proposed formulation could offer a new perspective on the relationship between f(R) gravity and scalar–tensor theories.

Spinor Field in FLRW Cosmology

Within the scope of a Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model we study the role of a nonlinear spinor field in the evolution of the universe. In doing so, we exploit the FLRW models given in both Cartesian and spherical coordinates. It is found that if the FLRW model is given in the spherical coordinates the energy-momentum tensor (EMT) of the spinor field possesses nontrivial non-diagonal components, which is not the case for Cartesian coordinates. These non-diagonal components do not depend on either the spinor field nonlinearity or the parameter k that defines the type of curvature of the FLRW model. The presence of such components imposes some restrictions on the spinor field. The problem is studied for open, flat and close geometries and the spinor field is used to simulate different types of sources including dark energies. Some qualitative numerical solutions are given.

Cosmological gravitational particle production of massive spin-2 particles

The phenomenon of cosmological gravitational particle production (CGPP) is expected to occur during the period of inflation and the transition into a hot big bang cosmology. Particles may be produced even if they only couple directly to gravity, and so CGPP provides a natural explanation for the origin of dark matter. In this work we study the gravitational production of massive spin-2 particles assuming two different couplings to matter. We evaluate the full system of mode equations, including the helicity-0 modes, and by solving them numerically we calculate the spectrum and abundance of massive spin-2 particles that results from inflation on a hilltop potential. We conclude that CGPP might provide a viable mechanism for the generation of massive spin-2 particle dark matter during inflation, and we identify the favorable region of parameter space in terms of the spin-2 particle’s mass and the reheating temperature. As a secondary product of our work, we identify the conditions under which such theories admit ghost or gradient instabilities, and we thereby derive a generalization of the Higuchi bound to Friedmann-Robertson-Walker (FRW) spacetimes.

Lagrangian Reconstruction of Barrow Holographic Dark Energy in Interacting Tachyon Model

We consider a correspondence between the tachyon dark energy model and Barrow holographic dark energy (BHDE). The latter is a modified scenario based on the application of the holographic principle with Barrow entropy instead of the usual Bekenstein–Hawking one. We reconstruct the dynamics of the tachyon scalar field T in a curved Friedmann–Robertson–Walker universe both in the presence and absence of interactions between dark energy and matter. As a result, we show that the tachyon field exhibits non-trivial dynamics. In a flat universe, T˙2 must always be vanishing, independently of the existence of interaction. This implies ωD=−1 for the equation-of-state parameter, which in turn can be used for modeling the cosmological constant behavior. On the other hand, for a non-flat universe and various values of the Barrow parameter, we find that T˙2 decreases monotonically for increasing cos(Rh/a) and cosh(Rh/a), where Rh and a are the future event horizon and the scale factor, respectively. Specifically, T˙2≥0 for a closed universe, while T˙2&lt;0 for an open one, which is physically not allowed. We finally comment on the inflation mechanism and trans-Planckian censorship conjecture in BHDE and discuss observational consistency of our model.

Reconstruction of an observationally constrained f(R,T) gravity model

In this paper, an attempt is made to construct a Friedmann–Lemaitre–Robertson–Walker model in [Formula: see text] gravity with a perfect fluid that yields acceleration at late times. We take [Formula: see text] as [Formula: see text]. As in the [Formula: see text]CDM model, we take the matter to consist of two components, viz., [Formula: see text] and [Formula: see text] such that [Formula: see text]. The parameter [Formula: see text] is the matter density (baryons [Formula: see text] dark matter), and [Formula: see text] is the density associated with the Ricci scalar [Formula: see text] and the trace [Formula: see text] of the energy–momentum tensor, which we shall call dominant matter. We find that at present [Formula: see text] is dominant over [Formula: see text], and that the two are in the ratio 3:1–3:2 according to the three data sets: (i) 77 Hubble OHD data set, (ii) 580 SNIa supernova distance modulus data set and (iii) 66 pantheon SNIa data which include high red shift data in the range [Formula: see text]. We have also calculated the pressures and densities associated with the two matter densities, viz., [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], respectively. It is also found that at present, [Formula: see text] is greater than [Formula: see text]. The negative dominant matter pressure [Formula: see text] creates acceleration in the universe. Our deceleration and snap parameters show a change from negative to positive, whereas the jerk parameter is always positive. This means that the universe is at present accelerating and in the past it was decelerating. State finder diagnostics indicate that our model is at present a dark energy quintessence model. The various other physical and geometric properties of the model are also discussed.

Revisiting cosmography via Gaussian process

In this paper, we revisit the kinematical state of our Universe via the cosmographic approach by using Gaussian process, where the minimum assumption is the cosmological principle, i.e. the Friedmann–Lemaître–Robertson–Walker metric. A obviously distinguished feature is that these cosmography parameters are free of any gravity theories and cosmological models. And Gaussian process is independent of any specific parameterized forms of function. Thus by transformations these generic cosmography parameters can be used to constrain a cosmological model and dark energy model directly at the kinematics level of our Universe. As a result, a series of cosmography parameters up to the fifth oder, i.e. the Hubble parameter H(z), the deceleration parameter q(z), the jerk parameter j(z), the snap parameter s(z) and the lerk parameter l(z), evolve with respect to the redshift z are reconstructed from the cosmic observations which include the recently released Pantheon+ SN Ia samples and the observational Hubble data H(z), also dubbed as cosmic chronometers. The result shows the transition redshift from a decelerated expansion to an accelerated expansion z_t=0.652^{+0.054}_{-0.043} which is consistent with the previous results.

Constraining barrow entropy-based cosmology with power-law inflation

We study the inflationary era of the Universe in a modified cosmological scenario based on the gravity-thermodynamics conjecture with Barrow entropy instead of the usual Bekenstein–Hawking one. The former arises from the effort to account for quantum gravitational effects on the horizon surface of black holes and, in a broader sense, of the Universe. First, we extract modified Friedmann equations from the first law of thermodynamics applied to the apparent horizon of a Friedmann–Robertson–Walker Universe. Assuming a power-law behavior for the scalar inflaton field, we then investigate how the inflationary dynamics is affected in Barrow cosmological setup. We find that the inflationary era may phenomenologically consist of the slow-roll phase, while Barrow entropy is incompatible with kinetic inflation. By demanding observational consistency of the scalar spectral index and tensor-to-scalar ratio with recent Planck data, we finally constrain Barrow exponent to Delta lesssim mathcal {O}(10^{-4}), which is the most stringent bound in so-far literature.