Cosmological Energy Density Research Articles

Investigation on hyperbolic Ricci soliton in a perfect fluid spacetime

The aim of this paper is to inspect the geometrical aspects of a perfect fluid spacetime conveying hyperbolic Ricci soliton with torse-forming vector field [Formula: see text]. Next, we have investigated some physical perceptions of perfect fluid spacetime such as dark fluid spacetime, stiff matter fluid spacetime, dust fluid spacetime and radiation fluid spacetime with hyperbolic Ricci soliton in terms of isotropic pressure, the cosmological constant, energy density and gravitational constant. We have also provided two examples of hyperbolic Ricci soliton and hyperbolic Ricci–Bourguignon soliton. Later, we explore the relativistic magneto-fluid spacetime obeying hyperbolic Ricci soliton along with different vector fields.

Vacuum Energy, the Casimir Effect, and Newton’s Non-Constant

The idea of quantum mechanical vacuum energy contributing to the cosmological vacuum energy density is not new. However, despite the persisting cosmological constant problem, few investigations have focused on this subject. We explore the possibility that the quantum vacuum energy density contributes to the (local) gravitational energy density in the framework of a scale-dependent cosmological constant Λ and Newton’s constant G. This hypothesis has several important consequences, ranging from quantum scale-dependence to the hypothetical prospect of novel experimental insight concerning the quantum origin of cosmological energy density.

Novel theorems for a Bochner flat Lorentzian K[formula omitted]hler space-time manifold with [formula omitted]-Ricci-Yamabe solitons

In the current work, we examine η-Ricci-Yamabe solitons in a Bochner flat Lorentzian Ka¨hler space-time manifolds. We have addressed various circumstances for Ricci-Yamabe solitons to be steady, shrinking or expanding in terms of isotropic pressure, the cosmological constant, energy density and gravitational constant in different perfect fluids such as dark fluid, Stiff matter, dust fluid and radiation fluid.

Stability of Cauchy horizon in a charged de-Sitter spacetime with dark matter

The violation of strong cosmic censorship (SCC) in RNdS black holes by a minimally coupled neutral massless scalar field has recently been discovered. This paper investigates the stability of the Cauchy horizon of a spherically charged de-Sitter black hole surrounded by dark matter under perturbations from a massless scalar field. Our results show that SCC can also be destroyed in the nearly extremal region, regardless of the presence of dark matter. However, the existence of dark matter can mitigate the extent of SCC violation, particularly when the cosmological constant and dark matter energy density are both small. Notably, the violation region of SCC as a function of the dark matter state parameter does not exhibit a simple monotonic decrease, suggesting that the influence of dark matter on SCC is not straightforward and may be complex.

Solving the Problem of the Cosmological Constant and Vacuum Energy Density by Considering the Cosmic Quantum Number

Solving the Problem of the Cosmological Constant and Vacuum Energy Density by Considering the Cosmic Quantum Number

Non-unitary three-neutrino mixing in the early Universe

Deviations from unitarity in the three-neutrino mixing canonical picture are expected in many physics scenarios beyond the Standard Model. The mixing of new heavy neutral leptons with the three light neutrinos would in principle modify the strength and flavour structure of charged-current and neutral-current interactions with matter. Non-unitarity effects would therefore have an impact on the neutrino decoupling processes in the early Universe and on the value of the effective number of neutrinos, N eff. We calculate the cosmological energy density in the form of radiation with a non-unitary neutrino mixing matrix, addressing the possible interplay between parameters. Highly accurate measurements of N eff fromforthcoming cosmological observations can provide independent and complementary limits on thedepartures from unitarity. For completeness, we relate the scenario of small deviations fromunitarity to non-standard neutrino interactions and compare the forecasted constraints to otherexisting limits in the literature.

Observational Evidence for Cosmological Coupling of Black Holes and its Implications for an Astrophysical Source of Dark Energy

Observations have found black holes spanning 10 orders of magnitude in mass across most of cosmic history. The Kerr black hole solution is, however, provisional as its behavior at infinity is incompatible with an expanding universe. Black hole models with realistic behavior at infinity predict that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. We test this prediction by considering the growth of supermassive black holes in elliptical galaxies over 0 < z ≲ 2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z ≲ 7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that black hole production from the cosmic star formation history gives the value of ΩΛ measured by Planck while being consistent with constraints from massive compact halo objects. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z ∼ 0.7.

Cosmological Evolution With Negative Energy Densities

For general number of spatial dimensions we investigate the cosmological dynamics driven by a cosmological constant and by a source with barotropic equation of state. It is assumed that for both those sources the energy density can be either positive or negative. Exact solutions of the cosmological equations are provided for flat models. For models with curved space and with zero cosmological constant the general solutions are expressed in terms of the hypergeometric function. The qualitative evolution is described for all values of the equation of state parameter. We specify the values of that parameter and the combinations of the signs for the cosmological constant and matter energy density for which the cosmological dynamics is nonsingular. An example is considered with positive cosmological constant and negative matter energy density induced by the polarization of the hyperbolic vacuum.

Cosmological Evolution with Negative Energy Densities

For general number of spatial dimensions we investigate the cosmological dynamics driven by a cosmological constant and by a source with barotropic equation of state. It is assumed that for both those sources the energy density can be either positive or negative. Exact solutions of the cosmological equations are provided for flat models. For models with curved space and with zero cosmological constant the general solutions are expressed in terms of the hypergeometric function. The qualitative evolution is described for all values of the equation of state parameter. We specify the values of that parameter and the combinations of the signs for the cosmological constant and matter energy density for which the cosmological dynamics is nonsingular. An example is considered with positive cosmological constant and negative matter energy density induced by the polarization of the hyperbolic vacuum.

Darwinian standard model of physics obtains general relativity

A Darwinian perspective of the standard model of physics (SMP) quantum fields (QFs) is proposed, called the physics-cell (PC) approach. Because Darwinian evolution is not deterministic, the PC approach allows for the violation of the charge-parity-time symmetry. In the PC approach, the SMP laws are contained in the PCs which receive and emit QFs through the PCs’ outer surface which is necessarily constrained by Bekenstein’s surface-information limit. The establishment of gauge invariance-compatible communication protocol-agreements between the PCs obtains an average correlation of QFs that is equivalent to an asymmetric metric tensor with the symmetric component being equivalent to general relativity and the anti-symmetric component being very small but still large enough to allow for enough ex-nihilo mass-creation to explain dark matter. Based on experimental data, the PC minimum-size is 1.5cdot 10^{-31} m which is similar to the scale at which the grand unified theory force convergence occurs. Plus, the cosmological constant energy density is equal to the energy density of the discreteness-correction QF alterations that constitute the dark energy and are caused by the finiteness of the PC time-step which equals 5.0cdot 10^{-40} s, hence obtaining a PC maximum information processing rate of 6.6cdot 10^{47} qubit/s. Moreover, the PC approach obtains that the minimum mass for black holes is 2.1 cdot 10^{9} larger than the maximum mass for which the no-hiding theorem can apply and that the maximum capacity for quantum computers is about 29.0 cdot 10^{12} qubit.

Gravitational energy–momentum pseudo-tensor in Palatini and metric [formula omitted] gravity

We derive the gravitational energy–momentum pseudo-tensor τμνμ in both Palatini and metric approaches to f(R) gravity. We then obtain the related cosmological gravitational energy density. Considering a flat Friedmann–Lemaître–Robertson–Walker spacetime, the energy density complex of matter and gravitation vanishes in the metric approach, but results non-vanishing in the Palatini formalism. This feature could be relevant in order to physically discriminate between the two approaches.

PBH Evaporation, Baryon Asymmetry, and Dark Matter

Sufficiently light primordial black holes (PBH) could evaporate in the very early universe and dilute the preexisting baryon asymmetry and/or the frozen density of stable relics. The effect is especially strong in the case that PBHs decayed if and when they dominated the cosmological energy density. The size of the reduction is first calculated analytically under the simplifying assumption of the delta-function mass spectrum of PBH and in instant decay approximation. In the realistic case of exponential decay and for an extended mass spectrum of PBH the calculations are made numerically. Resulting reduction of the frozen number density of the supersymmetric relics opens for them a wider window to become viable dark matter candidate.

Spectral separation of the stochastic gravitational-wave background for LISA in the context of a modulated Galactic foreground

ABSTRACT Within its observational band the Laser Interferometer Space Antenna, LISA, will simultaneously observe orbital modulated waveforms from Galactic white dwarf binaries, a binary black hole produced gravitational-wave background, and potentially a cosmologically created stochastic gravitational-wave background (SGWB). The overwhelming majority of stars end their lives as white dwarfs, making them very numerous in the Milky Way. We simulate Galactic white dwarf binary gravitational-wave emission based on distributions from various mock catalogues and determine a complex waveform from the Galactic foreground with 3.5 × 107 binaries. We describe the effects from the Galactic binary distribution population across mass, position within the Galaxy, core type, and orbital frequency distribution. We generate the modulated Galactic white dwarf signal detected by LISA due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimation of the LISA noise and the parameters for the different signal classes. We estimate the detectable limits for the future LISA observation of the SGWB in the spectral domain with the three LISA channels A, E, and T. We simultaneously estimate the Galactic foreground, the astrophysical and cosmological backgrounds. Assuming the expected astrophysical background and a Galactic foreground, a cosmological background energy density of around ΩGW,cosmo ≈ 8 × 10−13 could be detected by LISA. LISA will either detect a cosmologically produced SGWB, or set a limit that will have important consequences.

Spectral separation of the stochastic gravitational-wave background for LISA: Observing both cosmological and astrophysical backgrounds

With the goal of attempting to observe a stochastic gravitational wave background (SGWB) with LISA, the spectral separability of the cosmological and astrophysical backgrounds is important to estimate. We attempt to determine the level with which a cosmological background can be observed given the predicted astrophysical background level. We predict detectable limits for the future LISA measurement of the SGWB. Adaptive Markov chain Monte-Carlo methods are used to produce estimates with the simulated data from the LISA Data challenge (LDC). We also calculate the Cramer-Rao lower bound on the variance of the SGWB parameter uncertainties based on the inverse Fisher Information using the Whittle Likelihood. The estimation of the parameters is done with the 3 LISA channels $A$, $E$, and $T$. We simultaneously estimate the noise using a LISA noise model. Assuming the expected astrophysical background, a cosmological background energy density of around $\Omega_{GW,Cosmo} \approx 1 \times 10^{-12}$ to $1 \times 10^{-13}$ can be detected by LISA.

Clustering and halo abundances in early dark energy cosmological models

ABSTRACT Cold Dark Matter with cosmological constant (ΛCDM) cosmological models with early dark energy (EDE) have been proposed to resolve tensions between the Hubble constant $H_0=100\, h$ km ṡ−1Ṁpc−1 measured locally, giving h ≈ 0.73, and H0 deduced from Planck cosmic microwave background (CMB) and other early-Universe measurements plus ΛCDM, giving h ≈ 0.67. EDE models do this by adding a scalar field that temporarily adds dark energy equal to about 10 per cent of the cosmological energy density at the end of the radiation-dominated era at redshift z ∼ 3500. Here, we compare linear and non-linear predictions of a Planck-normalized ΛCDM model including EDE giving h = 0.728 with those of standard Planck-normalized ΛCDM with h = 0.678. We find that non-linear evolution reduces the differences between power spectra of fluctuations at low redshifts. As a result, at z = 0 the halo mass functions on galactic scales are nearly the same, with differences only 1–2 per cent. However, the differences dramatically increase at high redshifts. The EDE model predicts 50 per cent more massive clusters at z = 1 and twice more galaxy-mass haloes at z = 4. Even greater increases in abundances of galaxy-mass haloes at higher redshifts may make it easier to reionize the universe with EDE. Predicted galaxy abundances and clustering will soon be tested by the James Webb Space Telescope (JWST) observations. Positions of baryonic acoustic oscillations (BAOs) and correlation functions differ by about 2 per cent between the models – an effect that is not washed out by non-linearities. Both standard ΛCDM and the EDE model studied here agree well with presently available acoustic-scale observations, but the Dark Energy Spectroscopic Instrument and Euclid measurements will provide stringent new tests.

The well-tempered cosmological constant: fugue in B♭

Zero point fluctuations of quantum fields should generate a large cosmological constant energy density in any spacetime. How then can we have anything other than de Sitter space without fine tuning? Well tempering—dynamical cancellation of the cosmological constant using degeneracy within the field equations— can replace a large cosmological constant with a much lower energy state. Here we give an explicit mechanism to obtain a Minkowski solution, replacing the cosmological constant with zero, and testing its attractor nature and persistence through a vacuum phase transition. We derive the general conditions that Horndeski scalar-tensor gravity must possess, and evolve in a fugue of functions, to deliver nothing and make the universe be flat.

Solitons of Kählerian space-time manifolds

We study solitons of almost pseudo symmetric Kählerian space-time manifold. It is considered that different curvature tensors like projective, conharmonic and conformal curvature tensors in almost pseudo symmetric Kählerian space-time manifolds are flat. It is shown that solitons are steady, expanding or shrinking under different relations of isotropic pressure, the cosmological constant, energy density and gravitational constant.

Constraining f(T, B) teleparallel gravity from energy conditions

f(T, B) teleparallel gravity is a recently proposed straightforward generalization of the popular f(T) teleparallel gravity by the incorporation of a boundary term B=2e∂i(eTi)=▽iTi where T denote the torsion scalar (Bahamonde et al., 2015). In this work, I investigate the viability of some well motivated f(T, B) teleparallel gravity models of the forms f=αBn+βTm,f=αBnTm and f=αlog(B)+βT where α, β, n and m are free parameters from the inequalities imposed the weak energy condition. I use the recent estimates of Hubble, deceleration, jerk and snap parameters in finding corners in parameter spaces for the cosmological models for which the energy density remain positive and the weak energy condition (i.e, ρ+p≥0, where p and ρ represent respectively the cosmological pressure and energy density) attains a minute positive value, as this implies the EoS parameter ω=p/ρ≃−1 and therefore consistent with an accelerating universe.

Coordinate-time lapse function of FLRW accelerated expanding universe in dRGT massive gravity theory

In this paper, we discuss the behavior of coordinate-time dependent lapse function of FLRW metric of an accelerated expanding universe in the de Rham–Gabadadze–Tolley massive gravity theory. Physically, this function is considered to connect a general observer coordinate-time to the related proper-time and also to another normal observer under specific condition. We find that in the standard dRGT formalism, the corresponding lapse function can exhibit unphysical behavior in the parameter space region with a negative cosmological-constant-like term. To solve this problem, we reconsider the cosmological constant energy density which induces negative pressure. It turns out that this setup could overcome the existence of singular and negative square lapse function in the related parameter space and restore all the parameter space to admit only the accelerated expanding universe model.

Active-sterile neutrino oscillations in the early Universe with the complete mixing matrix

In the framework of a 3+1 scheme with an additional inert state, we consider the thermalisation of sterile neutrinos in the early Universe taking into account the full 4 × 4 mixing matrix. The evolution of the neutrino energy distributions is found solving the momentum-dependent kinetic equations with full diagonal collision terms. The degree of thermalisation of the sterile state is shown in terms of the effective number of neutrinos, N eff, and its dependence on the three additional mixing angles (θ 14, θ 24, θ 34) and on the squared mass difference ∆m 2 41 is discussed. Our results are relevant for fixing the contribution of a fourth light neutrino species to the cosmological energy density, whose value is very well constrained by the final Planck analysis. For the preferred region of active-sterile mixing parameters from short-baseline neutrino experiments, we find that the fourth state is fully thermalised (N eff ≃ 4).