Gravitation and Cosmology最新文献

From a classical analysis, we show that gravitational waves in a cosmological medium with the equation of state parameter (w=-1/3) can follow a London-like equation, implying that some gravitational wave solutions present a decay for certain wavelengths. This scenario, corresponding to a cosmic string cosmology, induces an attenuation temporal scale on the gravitational wave propagation. We discuss on how these solutions impose a limit on the wavelength of the waves that can propagate, which depends on the type of spatial curvature and the energy density content of this type of cosmology.

We introduce a new, well-mannered and stable model. To create the model, we used well-known Finch–Skea [1] ansatz for the metric potential (g_{rr}). For our model, we look into a number of physical variables, including the energy density, pressures, mass, anisotropy factor, gravitational redshifts, energy conditions, and stability analysis by utilizing graphs. This made sure that our suggested solutions behave as expected and represent physically plausible models for anisotropic matter distributions. We consider compact stars to compare our model with observational data: SAX J1808.4-3658, Her X-1, 4U 1608-52, PSR B1913 (+) 16, Cen X-3, LMC X-4, PSR J1614-2230, 4U1538-52, PSR J1903 (+) 327, SMC X-1, 4U 1820-30, Vela X-1.

The present investigation is focused on a curvatureless and torsionless modified nonmetricity theory of gravity in the context of string fluid. For that, we use an anisotropic, locally rotationally symmetric (LRS), Bianchi Type I space-time universe with the arbitrary function (f(Q)=Q+Lambda), where (Q) is the nonmetricity scalar and (Lambda) is the cosmological constant. We solve the field equations using a time-dependent deceleration parameter and obtain various cosmological parameters. To obtain the best-fit values of the model parameters, we use the observational constraints on Hubble function (H(z)) and the apparent magnitude (m(z)) using the observational datasets (H(z)) and SNe Ia. Using these values of model parameters, we investigate cosmological scenarios of the model. We observe the behavior of the variable dark energy equation of state (EoS) parameter (omega) in the context of a string fluid universe and find the present value of the effective EoS parameter (omega^{textrm{eff}}<-1) that corresponds to phantom dark energy. Also, we analyze the statefinder parameters and estimate the present age of the universe.

We consider the inverse function of the scale factor (a(t)) of the Friedmann equations, discussing also the asymptotics of cosmological parameters under various assumptions on the (Lambda)CDM model. Using algebraic dependencies between the cosmological parameters and the representation of the inverse function of the scale factor (a(t)) as an elliptic integral with a parameter, we compute in a uniform way some special events in the universe’s evolution.

We investigate the possibility of a future singularity due to accelerating expansion of the Universe in a gravitational theory that comprises the Ricci scalar (R) and the Gauss–Bonnet invariant (mathcal{G}), known as (F(R,mathcal{G})) gravity, which can be viewed in the quadratic form. Three models are presented using Hubble parameters to represent a finite and infinite future. The model parameters are analyzed on the basis of their physical and geometrical properties. This study also explores the properties of the modified gravitational theory, and neither a future singularity nor a little or pseudo-rip are posed as threats to the fate of the Universe. We present scalar perturbation approaches to perturbed evolution equations and demonstrate their stability.

We have constructed a dark energy cosmological model in the framework of the generalized Brans Dicke (GBD) theory with a self-interacting potential. The source of dark energy is considered through a unified dark fluid (UDF) characterized by a linear equation of state (EoS). A time-varying deceleration parameter simulating the cosmic transit behavior has been introduced to get the dynamical behavior of the model. The (H(z)) data have been explored to constrain the model parameters and to study the dynamical aspects of the Brans-Dicke parameter and the scalar field.

We revisit the cyclic Universe scenario in scalar field FRW cosmology and check its applicability for a nonminimally coupled scalar field. We show that for the most popular case of a quartic potential and the standard nonminimal coupling this scenario does work. On the other hand, we identify certain cases where the cyclic model fails to work and present the corresponding reasons for that.

Using the Radon–Nikodym theorem concerning the relation between any two measures, as well as the methods employed in loop quantum gravity, it is shown that in gravitation one can quantize any measure which is not even associated with metric. We have considered the simplest case where the proportionality coefficient between two operators of measure (the Radon–Nikodym derivative) is some function. The result is that in the right-hand side of the commutation relations for the measure the fundamental length becomes a variable quantity, and this can lead to smoothing the singularity. The case where the Radon–Nikodym derivative is an operator is also under discussion. Classical and quantum theories in the background of space endowed with quantum measure are under consideration.

A model of a thick fluid disk around a binary black hole is considered. A binary black hole is described by the Majumdar-Papapetrou solution. The hydrodynamic equations in this metric are written out. Exact analytical solutions are presented. A generalization to the case of a toroidal magnetic field is carried out.

We consider a Bianchi Type I universe filled with cold dark matter and non-interacting Tsallis holographic dark energy in the framework of (f(R)) theory of gravity. We take the GO scale as an IR cutoff and find exact solutions of the field equations by considering a linearly varying deceleration parameter. The evolution of different cosmologically relevant parameters are studied graphically, and Statefinder diagnostics is performed in the light of recent cosmological observations. We find that our model corresponds to current accelerated expansion scenarios, and the function (f(R)approx R) implies that our model resembles General Relativity.