Energy Density Materials Research Articles

Gravitational waves for some dark energy models in FRW Universe

We study the gravitational waves phenomena for generalized Chaplygin gas, variable modified Chaplygin gas and some parametrizations of dark energy models. Motivated by their works, here we consider the Friedmann–Robertson–Walker (FRW) model of the non-flat Universe in the presence of dark matter with dark energy and we investigate the gravitational waves phenomena for some other candidates of dark energy like tachyonic field, generalized cosmic Chaplygin gas, new variable modified Chaplygin gas and Van der Waals fluid. By considering the individual conservation laws of dark matter and the dark energy components, we obtain the solutions of energy densities of the dark matter and dark energy components separately in terms of redshift. Next, we get the evolution equation for gravitational waves in FRW Universe. Then we analyze the natures of wave curves and their power spectrums for tachyonic field, generalized cosmic Chaplygin gas, new variable modified Chaplygin gas, and Van der Waals fluid.

Baryogenesis through asymmetric Hawking radiation from primordial black holes as dark matter

We examine the extent to which primordial black holes (PBHs) can constitute the observed dark matter while also giving rise to the measured matter-antimatter asymmetry and account for the observed baryon abundance through asymmetric Hawking radiation generated by a derivative coupling of curvature to the baryon-lepton current. We consider both broad and monochromatic mass spectra for this purpose. For the monochromatic spectrum we find that the correct dark matter and baryon energy densities are recovered for peak masses of the spectrum of $M_{\rm pk} \geq 10^{12}$ kg whereas for the broad case the observed energy densities can be reproduced regardless of peak mass. Adopting some simplifications for the early-time expansion history as a first approximation, we also find that the measured baryon asymmetry can be recovered within an order of magnitude. We argue furthermore that the correct value of the baryon-lepton yield can in principle be retrieved for scenarios where a significant amount of the radiation is produced by PBH decay during or after reheating, as is expected when the decaying PBHs also cause reheating, or when an early matter-dominated phase is considered. We conclude from this first analysis that the model merits further investigation.

Perturbations in a scalar field model with virtues of ΛCDM

In the era of precision cosmology, the cosmological constant Λ gives quite an accurate description of the evolution of the Universe, but it is still plagued with the fine-tuning problem and the cosmic coincidence problem. In this work, we investigate the perturbations in a scalar field model that drives the recent acceleration in a similar fashion that the cosmological constant does and has the dark energy (DE) density comparable to the dark matter (DM) energy density at the recent epoch starting from arbitrary initial conditions. The perturbations show that this model, though it keeps the virtues of a ΛCDM model, has a distinctive qualitative feature, particularly it reduces the amplitude of the matter power spectrum on a scale of 8 h-1 Mpc, σ8 at the present epoch.

Vector dark matter production from inflation with symmetry breaking

We present a scenario of vector dark matter production from symmetry breaking at the end of inflation. In this model, the accumulated energy density associated with the quantum fluctuations of the dark photon accounts for the present energy density of dark matter. The inflaton is a real scalar field while a heavy complex scalar field, such as the waterfall of hybrid inflation, is charged under the dark gauge field. After the heavy field becomes tachyonic at the end of inflation, rolling rapidly towards its global minimum, the dark photon acquires mass via the Higgs mechanism. To prevent the decay of the vector field energy density during inflation, we introduce couplings between the inflaton and the gauge field such that the energy is pumped to the dark sector. The setup can generate the observed dark matter abundance for a wide range of the dark photon's mass and with the reheat temperature around $10^{12}$ GeV. The model predicts the formation of cosmic strings at the end of inflation with the tensions which are consistent with the CMB upper bounds.

Stealth dark energy in scordatura DHOST theory

A stealth de Sitter solution in scalar-tensor theories has an exact de Sitter background metric and a nontrivial scalar field profile. Recently, in the context of Degenerate Higher-Order Scalar-Tensor (DHOST) theories it was shown that stealth de Sitter solutions suffer from either infinite strong coupling or gradient instability for scalar field perturbations. The sound speed squared is either vanishing or negative. In the first case, the strong coupling scale is zero and thus lower than the energy scale of any physical phenomena. From the viewpoint of effective field theory, this issue is naturally resolved by introducing a controlled detuning of the degeneracy condition dubbed scordatura, recovering a version of ghost condensation. In this paper we construct a viable dark energy model in the scordatura DHOST theory based on a stealth cosmological solution, in which the metric is the same as in the standard ΛCDM model and the scalar field profile is linearly time-dependent. We show that the scordatura mechanism resolves the strong coupling and gradient instability. Further, we find that the scordatura is also necessary to make the quasi-static limit well-defined, which implies that the subhorizon observables are inevitably affected by the scordatura. We derive the effective gravitational coupling and the correction to the friction term for the subhorizon evolution of the linear dark matter energy density contrast as well as the Weyl potential and the gravitational slip parameter. In the absence of the scordatura, the quasi-static approximation would break down at all scales around stealth cosmological solutions even if the issue of the infinite strong coupling is unjustly disregarded. Therefore previous estimations of the subhorizon evolution of matter density contrast in modified gravity in the literature need to be revisited by taking into account the scordatura effect.

Kaluza–Klein minimally interacting dark energy model in the presence of massive scalar field

In this paper, our purpose is to discuss the dynamical aspects of Kaluza–Klein five-dimensional cosmological model filled with minimally interacting baryonic matter and dark energy (DE) in the presence of an attractive massive scalar field. We obtain a determinate solution of the Einstein field equations using (i) a relation between the metric potentials and (ii) a power law relation between the average scale factor of the universe and the massive scalar field. We have determined scalar field, matter energy density, DE density, equation of state (EoS) [Formula: see text], deceleration [Formula: see text] and statefinder [Formula: see text] parameters of our model. We also develop [Formula: see text]–[Formula: see text] phase, squared sound speed, statefinders and [Formula: see text]–[Formula: see text] planes in the evolving universe. It is observed that the EoS parameter exhibits quintom-like behavior from quintessence to phantom epoch by crossing the vacuum era of the universe. The squared speed of sound represents the instability of the model, whereas the [Formula: see text]–[Formula: see text] plane shows both thawing and freezing regions. The [Formula: see text]CDM limit is attained in both [Formula: see text]–[Formula: see text] and statefinder planes. We have also discussed the cosmological importance of the above parameters with reference to modern cosmology. It is found that the dynamics of these cosmological parameters indicate the accelerated expansion of the universe which is consistent with the current cosmological observations.

Dynamical system analysis of FLRW models with Modified Chaplygin gas

We investigate Friedmann–Lamaitre–Robertson–Walker (FLRW) models with modified Chaplygin gas and cosmological constant, using dynamical system methods. We assume p=(gamma -1)mu -dfrac{A}{mu ^alpha } as equation of state where mu is the matter-energy density, p is the pressure, alpha is a parameter which can take on values 0<alpha le 1 as well as A and gamma are positive constants. We draw the state spaces and analyze the nature of the singularity at the beginning, as well as the fate of the universe in the far future. In particular, we address the question whether there is a solution which is stable for all the cases.

Gravitational wave friction in light of GW170817 and GW190521

We use the gravitational wave (GW) events GW170817 and GW190521, together with their proposed electromagnetic counterparts, to constrain cosmological parameters and theories of gravity beyond General Relativity (GR). In particular we consider models with a time-varying Planck mass, large extra-dimensions, and a phenomenological parametrization covering several beyond-GR theories. In all three cases, this introduces a friction term into the GW propagation equation, effectively modifying the GW luminosity distance. We set constraints on \\LCDM parameters and GR deviation parameters using two sets of priors on the Hubble constant and matter energy density. With priors set to the measured Planck's mission values, we find that with the inclusion of GW190521, the two GR deviation parameters constraints improve by a factor ∼ 10. We report a number of space-time dimensions compatible with 4 with an precision of 2.5% (at 95% CL) and an upper limit to the variation of Newton's constant at the epoch of GW170817 of <20%. With wide priors on the Hubble constant and matter energy density, we show that it is still possible to constrain the \\LCDM parameters and GR deviation parameters conjointly from GW170817 and GW190521, obtaining constraints on GR deviation parameters which are a factor 2–6 worse than the results using restricted priors on \\LCDM parameters.

Transit cosmological models in modified f(Q,T) gravity

[Formula: see text] gravity is a recently proposed modified theory of gravity due to Xu et al. ([Formula: see text]). It is an extension of the symmetric teleparallel gravity, in which the gravitational action is given by an arbitrary function [Formula: see text] of the non-metricity tensor [Formula: see text] and the trace of energy-momentum tensor [Formula: see text]. In this paper also, we have investigated the cosmological model with Friedmann–Lemaitre–Robertson–walker (FLRW) Universe in [Formula: see text] theory with [Formula: see text]. Applying the energy conservation condition [Formula: see text], we have obtained the various cosmological parameters viz. Hubble parameter [Formula: see text], deceleration parameter [Formula: see text], etc. in terms of redshift [Formula: see text]. Using the available observational Hubble datasets [Formula: see text], the best fit values of the model parameters are determined by the [Formula: see text]-test formula. For these obtained values of model parameters, our model represents a transit cosmological model with past decelerating to present accelerating expansion phase of the Universe with the present value [Formula: see text]. The point of signature-flipping (transition) is calculated as [Formula: see text]. We have analyzed the variations of physical parameters viz. matter energy density [Formula: see text], isotropic pressure [Formula: see text], equation of state [Formula: see text] with respect to redshift [Formula: see text] and coupling parameter [Formula: see text]. For checking the viability of our derived model, we have tested energy conditions and also performed statefinder diagnosis. The age of the Universe is also calculated.

Analysis of Bianchi Type V Holographic Dark Energy Models in General Relativity and Lyra’s Geometry

In this paper, we analyze anisotropic and homogeneous Bianchi type V spacetime in the presence of dark matter and holographic dark energy model components in the framework of general relativity and Lyra’s geometry. The solutions of differential equation fields have been obtained by considering two specific cases, namely, the expansion scalar θ in the model is proportional to the shear scalar σ and the average scale factor taken as hybrid expansion form. The solutions for field equations are obtained in general relativity and Lyra’s geometry. The energy density of dark matter in both natures was obtained and compared so that the energy density of dark matter in general relativity is slightly different from the energy density of dark matter in Lyra’s geometry. A similar behavior occurred in case of pressure and EoS parameter of holographic dark energy model in respective frameworks. Also, it is concluded that the physical parameters such as the average Hubble parameter, spatial volume, anisotropy parameter, expansion scalar, and shear scalar are the same in both frameworks. Moreover, it is observed that the gauge function β t is a decreasing function of cosmic time in Lyra’s geometry, and for late times, the gauge function β t converges to zero and Lyra’s geometry reduces to general relativity in all respects. Finally, we conclude that our models are a close resemblance to the Λ CDM cosmological model in late times and consistent with the recent observations of cosmological data.

Unrivaled quantum vacuum in the primordial universe

In a primordial universe pre- (post-)inflationary era, there could be phases of early universe made of cold gas baryons, radiation and early post-inflationary cosmological constant. I showed that in the baryonic epoch, the quantum vacuum is unique. By using the standard quantization scheme for a massive minimally coupled scalar field with maximal conformal symmetry in the classical space–time, I demonstrated that the scalar modes had an effective mass [Formula: see text] (or [Formula: see text]). This argument was validated when the conformal time [Formula: see text] kept so close to the inflation ending time [Formula: see text]. The energy density of the baryonic matter diverged at the inflation border and vanishes at the late time future. Furthermore, I argued that at very early accelerating epoch when the radiation was the dominant part in the close competition with the early time cosmological constant, fine-tuned mass of the scalar field [Formula: see text] also provided a unique quantum vacuum. The reason is that the effective mass eventually is vanished. A remarkable observation was that all the other possible vacuum states “squeezed” eternally.

A stability analysis of the static Einstein‐Klein‐Gordon Boson Stars

AbstractThe stability of the recently proposed static solutions for boson stars is analyzed. These solutions of Einstein‐Klein‐Gordon (EKG) equations are obtained by considering the interaction of a real scalar field with matter. We assume that the inclusion of the scalar field in addition to matter allows one to justify that stability implies that the total mass of the solution should grow when the initial condition for the density of matter at the origin is also increased. Employing numerical values for the static boson star based on a linear relation between the source and the energy density and between this and the pressure, we found the relation that linked the scalar field at the origin with the matter energy density at the same point. We also determine the behavior of the total mass with the matter energy density in the origin by obtaining through this and the weak energy condition two possible ranges for stable solutions of static boson stars.

Varying $$\boldsymbol{\Lambda}$$ Theory Revisited

We revisit the variable $$\Lambda$$ theory recently developed by Alexander et al. [1, 2]. We implement it to explain the current expansion of the Universe. The varying $$\Lambda$$ theory reduces one degree of freedom. The varying $$\Lambda$$ term (cosmological term) follows the energy density of the Universe. We consider such a theory which does not respect self-dual symmetry. The Bianchi identity enforces the $$\Lambda$$ term to be a constant for a theory without torsion. However, torsion can incorporate a varying $$\Lambda$$ term in the action. We consider a form of the action which has such a varying $$\Lambda$$ term, and the presence of only nonrelativistic matter energy density. We fit the luminosity distance according to the model and compare it with that in the $$\Lambda\textrm{{CDM}}$$ model and find their close behavior. However, we find a current value of matter energy density $$\rho_{m0}$$ larger than the critical energy density. Using the supernova data from Union 2.1 compilation, we further consider a different set of parameters, for which $${\kappa\rho_{m0}}/{3H_{0}^{2}}$$ becomes close to unity but, however, greater. For this set of parameters, we find a negative value in the effective equation of state, however, the model has a very different pattern of the Hubble parameter from the standard scenario.

Reconstructing a non-linear interaction in the dark sector with cosmological observations

Abstract In this work we model two non-linear directly interacting scenarios in dark sector of the universe with the dimensionless parameter α and β , which dominate the energy exchange between dark energy and dark matter. The central goal of this investigation is to research the interacting model and discuss the cosmological implications based on the current observational datasets. The class of the interaction is generally characterized by a coupling function Q ∝ H ( z ) ρ x , x denotes the energy density of dark matter or dark energy. The constrained results we obtained indicate that the direct interaction in cosmic dark sector is favored by various observational data and the key effects on CMB power spectrum and linear matter power spectrum appear compared to Λ CDM standard paradigm. Finally, we discuss in depth the effect of different neutrino mass hierarchy on matter power spectrum and the variation of the ratio of CMB temperature power spectrum C l T T and matter power spectrum P ( k ) when the Δ N e f f = N − 3 . 046 from 0.5 to 2, respectively.

Big Rip and Big Crunch Cosmological Models in a Gravitational Field with Torsion

The gravitational field with torsion is being constructed by using the parametrized absolute parallelism geometry. A generalized law of variation of Hubble’s parameter in evolutionary cosmological models is used. The cosmological models under the influence of the gravitational field with torsion are obtained and discussed. A new model of the Universe is presented using a special class of Riemann–Cartan geometry. This model is oscillating with expansion and contraction at different stages. It behaves normally as the conventional Big Bang model in the first half-age until it reaches the moment of a Big Rip, then reverses its behavior as a result of a changes in the pressure and torsion until it reaches a Big Crunch at the end of the second half-age. We suppose that the Big Rip singularity is replaced by a regular maximum of the scale factor at the Big Rip due to a possible physical mechanism of quantum nature. The positivity condition for the energy density of matter leads to exclusion of open and closed universes.

Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN6, X = Ca, Sr and Ba): a first-principles study.

Multi-nitrogen or polynitrogen compounds can be used as potential high energy-density materials, so they have attracted great attention. Nitrogen can exist in alkaline earth metal nitrogen-rich (N-rich) compounds in the form of single or double bonds. In recent years, to explore N-rich compounds which are stable and easy to synthesize has become a new research direction. The N-rich compounds XN6 (X = Ca, Sr and Ba) have been reported under normal pressure. In order to find other stable crystal structures, we have performed XN6 (X = Ca, Sr and Ba) exploration under high pressure. We found that SrN6 has a new P1̄ phase at a pressure of 22 GPa and an infinite nitrogen chain structure, and BaN6 has a new C2/m phase at 110 GPa, with an N6 ring network structure. Further, we observed that the infinite nitrogen chain and the N6 ring network structure contain typical covalent bonds formed by the hybridization of the sp2 and sp3 orbitals of N, respectively. It is found that both SrN6 and BaN6 are semiconductor materials and the N-2p orbital plays an important role in the stability of the crystal structure for P1̄-SrN6 and C2/m-BaN6. Because of the polymerization of nitrogen in the two compounds and their stabilities under high pressure, they can be used as potential high energy-density materials. The research in this paper further promotes the understanding of alkaline earth metal N-rich compounds and provides new information and methods for the synthesis of alkaline earth metal N-rich compounds (XN6, X = Ca, Sr and Ba).

Lyra’s cosmology of homogeneous and isotropic universe in Brans–Dicke theory

In this paper, we investigate a scalar field Brans–Dicke cosmological model in Lyra’s geometry which is based on the modifications in a geometrical term as well as energy term of Einstein’s field equations. We have examined the validity of the proposed cosmological model on the observational scale by performing statistical analysis from the latest [Formula: see text] and SN Ia observational data. We find that the estimated values of Hubble’s constant and matter energy density parameter is in agreement with their corresponding values, obtained from recent observations of Wilkinson Microwave Anisotropy Probe (WMAP) and Plank collaboration. We also derived the deceleration parameter, age of the universe and jerk parameter in terms of red-shift and computed its present values. The dynamics of the deceleration parameter in the derived model of the universe show a signature flipping from positive to a negative value and also indicate that the present universe is in the accelerating phase.

Resolving Hubble tension with quintom dark energy model * *S.P. (first author) is supported by Rachadapisek Sompote Fund for Postdoctoral Fellowship, Chulalongkorn University. P.B. is supported in part by the Thailand Research Fund (TRF), Office of Higher Education Commission (OHEC) and Chulalongkorn University under grant (RSA6180002). L.T. is supported by Postdoctoral Fellowship of King Mongkut's University of Technology Thonburi

Recent low-redshift observations have yielded the present-time Hubble parameter value . This value is approximately 10% higher than the predicted value of , based on Planck's observations of the Cosmic Microwave Background radiation (CMB) and the CDM model. Phenomenologically, we show that, by adding an extra component, X, with negative density to the Friedmann equation, it can address the Hubble tension without changing the Planck's constraint on the matter and dark energy densities. To achieve a sufficiently small extra negative density, its equation-of-state parameter must satisfy . We propose a quintom model of two scalar fields that realizes this condition and potentially alleviate the Hubble tension. One scalar field acts as a quintessence, while another “phantom” scalar conformally couples to matter such that a viable cosmological scenario is achieved. The model only depends on two parameters, and , which represent the rolling tendency of the self-interacting potential of the quintessence and the strength of the conformal phantom-matter coupling, respectively. The toy quintom model with (Quintom I) yields a good Supernovae-Ia luminosity fit and acceptable fit but slightly small acoustic multipole . A full parameter scan revealed that the quintom model was superior to the CDM model in certain regions of the parameter space, , while significantly alleviating the Hubble tension, although it is not completely resolved. A benchmark quintom model, Quintom II, is presented as an example.

Loop quantum cosmology from an alternative Hamiltonian. II. Including the Lorentzian term

The scheme of using the Chern-Simons action to regularize the gravitational Hamiltonian constraint is extended to including the Lorentzian term in the $k=0$ cosmological model. The Euclidean term and the Lorenzian term are thus regularized separately mimicking the treatment of full loop quantum gravity. The new quantum dynamics for the spatially flat Friedmann-Robertson-Walker model with a massless scalar field as an emergent time is studied. By semiclassical analysis, the effective Hamiltonian constraint is obtained, which indicates that the new quantum dynamics has the correct classical limit. The classical big-bang singularity is again replaced by a quantum bounce. Similar to the case of quantizing only the Euclidean term, the backward evolution of the cosmological model determined by the new effective Hamiltonian will be bounced to an asymptotic de Sitter universe coupled to a massless scalar field, while the problem of negative energy density of matter in the former case is resolved.

Interacting generalized anisotropic universe model

In this paper, we consider the generalized anisotropic model in Einstein's theory of gravity to study evolution of anisotropic universe. For this purpose, we consider interacting form of dark energy model and evaluate some exact solutions corresponding to Bianchi type-III, V and VI models. We study the behavior of these solutions through some standard cosmological parameters such as Hubble, squared speed of sound, equation of state and state-finder parameters. We also discuss the evolution of fractional energy densities of ordinary matter and dark energy. The graphical behavior of these parameters against red-shift parameter provides consistency with current observations.