Flat Universe Research Articles

Mass superinflation in the Reissner-Nordström black hole

Ever since Penrose and Simpson's prediction contradicted Novikov's idea that an infalling person would emerge into an asymptotically flat universe, there have been a continued interest in predicting the nature of singularity at the inner horizon of a Reissner-Nordström black hole. This prediction was first confirmed by Poisson and Israel using cross-stream of massless particles, leading to the phenomenon coined as mass inflation. On the other hand, Ori obtained a weaker singularity using a null shell of radiation. Thus it is important to capture the nature of singularity at the inner horizon. We therefore consider in this work a massive scalar field coupled to the Reissner-Nordström spacetime. The ensuing field equations lead to a coupled set of nonlinear dynamical equations. For definiteness, we analytically solve these equations employing the Adomian decomposition method. This facilitates in obtaining a closed form solution that exhibits an unbounded double-exponential growth in the mass function, giving rise to a novel phenomenon coined herein as mass superinflation. The scalar field is also found to undergo a very strong blueshift at the inner horizon.

Quantum and classical properties of the universe according to unimodular quantum cosmology

Abstract We investigate the dynamical behaviour of wavepacket solutions due to unimodular quantum cosmology. We find early and late time quantum eras of the universe.
An Ehrenfest theorem ensures quasi-classical behaviour for the late time universe if the quantum uncertainties are small. The surprising prediction of late time quantum effects for a flat, homogeneous and isotropic universe with a scalar field with an exponential potential is obtained by the analysis of the uncertainty dynamics using the results of the corresponding classical dynamical system. Furthermore quantum correction terms to the Hubble parameter and the matter density are derived. A positive matter expectation value for the de-Sitter universe raises the question if correction terms due to quantum gravity could contribute to solve the dark matter problem.

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.

Modified Gravity in the Presence of Matter Creation: Scenario for the Late Universe.

We consider a dynamic scenario for characterizing the late Universe evolution, aiming to mitigate the Hubble tension. Specifically, we consider a metric f(R) gravity in the Jordan frame which is implemented to the dynamics of a flat isotropic Universe. This cosmological model incorporates a matter creation process, due to the time variation of the cosmological gravitational field. We model particle creation by representing the isotropic Universe (specifically, a given fiducial volume) as an open thermodynamic system. The resulting dynamical model involves four unknowns: the Hubble parameter, the non-minimally coupled scalar field, its potential, and the energy density of the matter component. We impose suitable conditions to derive a closed system for these functions of the redshift. In this model, the vacuum energy density of the present Universe is determined by the scalar field potential, in line with the modified gravity scenario. Hence, we construct a viable model, determining the form of the f(R) theory a posteriori and appropriately constraining the phenomenological parameters of the matter creation process to eliminate tachyon modes. Finally, by analyzing the allowed parameter space, we demonstrate that the Planck evolution of the Hubble parameter can be reconciled with the late Universe dynamics, thus alleviating the Hubble tension.

Can Stiff Matter Solve the Hubble Tension?

A new form of the mathematical expression for the co-moving volume element of a flat universe with cosmological constant, cold matter, and stiff matter is presented. It is used to determine the constraints from the Planck measurements of the Hubble parameter on the amount of stiff matter in the universe. These constraints are used to investigate whether the presence of stiff matter can solve the Hubble tension. It is found that the Planck measurements lead to an upper bound on the present value of the density parameter of stiff matter ΩS0<5⋅10−23, and that this is too small to solve the Hubble tension. Report. The main objective of this article is to introduce a novel mathematical expression for the co-moving volume element in a flat universe that includes a cosmological constant, cold matter, and stiff matter. This expression is utilized to derive constraints on the amount of stiff matter in the universe based on the Planck measurements of the Hubble parameter. These constraints are then examined to assess whether stiff matter could potentially resolve the Hubble tension. The findings indicate that the Planck measurements impose an upper limit on the current value of the density parameter of stiff matter, ΩS0<5⋅10−23, which is insufficient to resolve the Hubble tension.

Dynamical stability and phase space analysis of an emergent Universe with non-interacting and interacting fluids

Abstract We investigate the evolution of a flat Emergent Universe obtained with a non-linear equation of state (nEoS) in Einstein’s general theory of Relativity. The nEoS is equivalent to three different types of barotropic cosmic fluids, which are found from the nEoS parameter. The EU began expanding initially with no interactions among the cosmic fluids. However, one can assume an interaction that sets in at a time t ⩾ t i in the cosmic fluids, we study the evolution of the EU that accommodates the present observed Universe. We adopt dynamical system analysis to study the critical points of the autonomous system and the evolutionary behavior of an EU with or without interaction among the cosmic fluid components. We also study the stability of critical points and plot the phase portraits. The density parameters and the corresponding cosmological parameters are determined for both the non-interacting and interacting phases of the evolutionary dynamics of the Universe.

Observational constraints on the emergent universe with non-linear equation of state and interacting fluids

We investigate a flat emergent universe (EU) with a nonlinear equation of state equivalent to three different composition of fluids. In the EU initially, the evolution of the universe began with no interaction but as time evolves an interaction sets in among the three fluids leading to the observed universe. The characteristic of an EU is that it is a singularity free universe that evolves with all the basic features of the early evolution. For a given nonlinear equation of state parameter, it permits a universe with three different fluids, we get a universe with dark energy, cosmic string, and radiation domination to begin with which at a later epoch transits into a universe with three different fluids with matter domination (baryonic as well as dark matter) and dark energy for a given interaction strength among the cosmic fluids. The evolution of the universe is probed with exponential interactions to obtain a universe with late acceleration. The model parameters are constrained using the observed Hubble data and Type Ia Supernova (SnIa) data from the Pantheon data set. An interacting EU transforms to a matter dominated phase with DE accommodating the present universe satisfactorily. The stability of the cosmological model is also discussed.

Research on the Flat Universe and the Core of Galaxy Cluster

Research on the Flat Universe and the Core of Galaxy Cluster

The Cosmic Black Hole

Concurrent with advancements in cosmology, various theories have been proposed about the origin of the universe or its ultimate fate. These theories include the Steady State Theory, Oscillating Universe Theory or Cyclic model, Multiverse Theory, String Theory/M-Theory, Quantum Gravity or Loop Quantum Cosmology, The Big Bang Theory, Inflationary Universe Theory, and the No-Boundary Proposal. Among these, the Big Bang theory has been widely accepted by most scientists, with the Inflation theory serving as its complementary addition. On the other hand, theories such as the big rip, flat universe, and big crunch have also addressed the ultimate fate of the cosmos. However, T-Consciousness Cosmology presents a new hypothesis to explain how the universe was born from a black hole named the ‘Cosmic Black Hole,’ or the initial seed of the universe. This hypothesis not only addresses how this particular type of black hole forms, contingent on the reversion of the cosmos according to the Spherical Cosmos model but also details its fundamental differences from known black holes, referred to as “intra-cosmic black holes.” The reversion of the cosmos in the ‘Spherical Cosmos Model,’ is described through a mechanism known as space Rebound, which is distinct from the Big Crunch. The Terminal Edge of the cosmos is defined as the maximum radius at which space mesh is capable of rebound during the universe's volume increase. In the process of the rebound of space to its ultimate extent, objects within the cosmos face complete disintegration and transform into waves called absolute waves. Due to the inherent rotation of the cosmos and its reversion from the Terminal Edge, these waves collide and create gravitational centers in the central regions of the spherical cosmos, forming new types of matter known as light-dark matter, dark-dark matter, and thermal matter. Each of these mentioned materials represents a new type of matter introduced by T-Consciousness Cosmology. Additionally, according to this model, the cosmos contracts into a very tiny point with a final quench, where all types of matter and fundamental forces unite, forming a new type of absolute matter called ‘Taheri Absolute Matter’ (TAM). The spherical cosmos model also divides time into various types: ‘Longitudinal’ and ‘Transverse,’ and unlike the theory of relativity, where time is considered a dimension, it classifies it as one of the types of transverse time introduced as an entropic force. In essence, this type of force (time) acts against the force of gravity and, by disintegrating all types of objects from fundamental to large scale, acts as an agent of stress or tension release from the space mesh.

Effects of particle creation rate in two-fluid interacting cosmologies

ABSTRACT In this work, a two-fluid interacting model in a flat FLRW universe has been studied considering particle creation mechanism with a particular form of particle creation rate $\Gamma =\Gamma _0 H+\frac{\Gamma _1}{H}$ from different aspects. Statistical analysis with a combined data set of SNe Ia (Supernovae Type Ia) and Hubble data is performed to achieve the best-fitting values of the model parameters, and the model is compatible with current observational data. We also perform a dynamical analysis of this model to get an overall qualitative description of the cosmological evolution by converting the governing equations into a system of ordinary differential equations considering a proper transformation of variables. We find some non-isolated sets of critical points, among which some usually are normally hyperbolic sets of points that describe the present acceleration of the universe dominated by dark energy mimicking cosmological constant or phantom fluid. Scaling solutions are also obtained from this analysis, and they can alleviate the coincidence problem successfully. Statefinder diagnosis is also carried out for this model to compare it with the ΛCDM, and any other dark energy models byfinding various statefinder parameters. Finally, the thermodynamic analysis shows that the generalized second law of thermodynamics is valid in an irreversible thermodynamic context.

Euclid preparation

This work considers which higher order modeling effects on the cosmic shear angular power spectra must be taken into account for Euclid. We identified the relevant terms and quantified their individual and cumulative impact on the cosmological parameter inferences from Euclid. We computed the values of these higher order effects using analytic expressions and calculated the impact on cosmological parameter estimations using the Fisher matrix formalism. We reviewed 24 effects and determined the ones that potentially need to be accounted for, namely: the reduced shear approximation, magnification bias, source-lens clustering, source obscuration, local Universe effects, and the flat Universe assumption. After computing these effects explicitly and calculating their cosmological parameter biases, using a maximum multipole of ℓ = 5000, we find that the magnification bias, source-lens clustering, source obscuration, and local Universe terms individually produce significant (> 0.25σ) cosmological biases in one or more parameters; accordingly, these effects must be accounted for and warrant further investigation. In total, we find biases in Ωm, Ωb, h, and σ8 of 0.73σ, 0.28σ, 0.25σ, and −0.79σ, respectively, for the flat ΛCDM. For the w0waCDM case, we found biases in Ωm, Ωb, h, ns, σ8, and wa of 1.49σ, 0.35σ, −1.36σ, 1.31σ, −0.84σ, and −0.35σ, respectively. These are increased relative to the ΛCDM due to additional degeneracies as a function of redshift and scale.

Symmetry-based study and dynamics of casual bulk viscous matter-dominated universe

The dynamics of cosmological expansion is studied for a spatially flat matter-dominated universe filled with barotropic casual bulk viscous fluid. The evolution equation for the Hubble parameter is derived using Friedmann equations and conservation law for the viscous fluid, in which the viscous pressure is modeled by truncated version of the full Israel-Stewart theory of bulk viscosity. We use a Lie symmetry-based approach to study the evolution of the expansion rate of the Universe. The model equation possesses eight-parameter Lie symmetry generators and gives two different analytic group invariant solutions for the Hubble parameter. Expressions for various important cosmological parameters are obtained and values are estimated analytically for the explanation of the evolution dynamics of the Universe. The interesting features of the obtained symmetry-based solutions are that, one satisfies the present acceleration of the Universe, while the other could not support present accelerating phase, rather represents a universe dominated by non-viscous stiff fluid. We make useful checks on the observational constraints on the model parameters from the latest released observational data. The accelerating phase of the current universe is investigated using observational bounds and compared the model under investigation with the Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document}CDM model. The evolutionary cosmic phases obtained from the analysis of the Lie symmetry-based solutions correspond to the association with the critical points in the equivalent phase space. These interesting features are investigated and analyzed using dynamical systems approach.

A flat FLRW dark energy model in f(Q,C)-gravity theory with observational constraints

In the recently suggested modified non-metricity gravity theory with boundary term in a flat FLRW spacetime universe, dark energy scenarios of cosmological models are examined in this study. An arbitrary function, [Formula: see text], has been taken into consideration, where [Formula: see text] is the non-metricity scalar, [Formula: see text] is the boundary term denoted by [Formula: see text], and [Formula: see text] is the model parameter, for the action that is quadratic in [Formula: see text]. The Hubble function [Formula: see text], where [Formula: see text] is the current value of the Hubble constant and [Formula: see text] and [Formula: see text] are arbitrary parameters with [Formula: see text], has been used to examine the dark energy characteristics of the model. We discovered a transit phase expanding universe model that is both decelerated in the past and accelerated in the present, and we discovered that the dark energy equation of state (EoS) [Formula: see text] behaves as [Formula: see text]. The Om diagnostic analysis reveals the quintessence behavior in the present and the cosmological constant scenario in the late-time universe. Finally, we calculated the universe’s current age, which was found to be quite similar to recent data.

Emergence of cosmic space and the maximization of horizon entropy

The spatial expansion of the universe can be described as the emergence of space with the progress of cosmic time, through a simple equation, ΔV=ΔtNsurf−Nbulk. This law of emergence suggested by Padmanabhan in the context of general relativity for a flat universe has been generalized by Sheykhi to Gauss Bonnet and Lovelock gravity for a universe with any spatial curvature. We investigate whether this generalized holographic equipartition effectively implies the maximization of horizon entropy. First, we obtain the constraints imposed by the maximization of horizon entropy in Einstein, Gauss Bonnet and Lovelock gravities for a universe with any spatial curvature. We then analyze the consistency of the law of emergence in Ahmad Sheykhi (2013), with these constraints obtained. Interestingly, both the law of emergence and the horizon entropy maximization demands an asymptotically de Sitter universe with ω≥−1. More specifically, when the degrees of freedom in the bulk (Nbulk) becomes equal to the degrees of freedom on the boundary surface (Nsurf), the universe attains a state of maximum horizon entropy. Thus, the law of emergence can be viewed as a tendency for maximizing the horizon entropy, even in a non flat universe. Our results points at the deep connection between the law of emergence and horizon thermodynamics, beyond Einstein gravity irrespective of the spatial curvature.

Flat-sky angular power spectra revisited

We revisit the flat-sky approximation for evaluating the angular power spectra of projected random fields by retaining information about the correlations along the line of sight. For the case of projections with broad, overlapping radial window functions, these line-of-sight correlations are suppressed and are ignored in the commonly adopted Limber approximation. However, retaining the correlations is important for narrow window functions or unequal-time spectra but introduces significant computational difficulties due to the highly oscillatory nature of the integrands involved. We deal with the integral over line-of-sight wave-modes in the flat-sky approximation analytically, using the FFTlog expansion of the 3D power spectrum. This results in an efficient computational method, which is a substantial improvement compared to any full-sky approaches. We apply our results to galaxy clustering (with and without redshift-space distortions), CMB lensing and galaxy lensing observables in a flat ΛCDM universe. In the case of galaxy clustering, we find excellent agreement with the full-sky results on large (percent-level agreement) and intermediate or small (subpercent agreement) scales, dramatically out-performing the Limber approximation for both wide and narrow window functions, and in equal- and unequal-time cases. In the cases of lensing, we show on the full-sky that the angular power spectrum of the lensing convergence can be very well approximated by projecting the 3D Laplacian (rather than the correct angular Laplacian) of the gravitational potential, even on large scales. Combining this approximation with our flat-sky techniques provides an efficient and accurate evaluation of the CMB lensing angular power spectrum on all scales. We further analyse the clustering and lensing angular power spectra by isolating the projection effects due to the observable- and survey-specific window functions, separating them from the effects due to integration along the line of sight and unequal-time mixing in the 3D power spectrum. All of the angular power spectrum results presented in this paper are obtained using a Python code implementation, which we make publicly available.

Cosmological parameters derived from the final Planck data release (PR4)

We present cosmological parameter constraints using maps from the last Planck data release (PR4). In particular, we detail an upgraded version of the cosmic microwave background likelihood, HiLLiPoP, that is based on angular power spectra and relies on a physical modeling of the foreground residuals in the spectral domain. This new version of the likelihood retains a larger sky fraction (up to 75%) and uses an extended multipole range. Using this likelihood, along with low-ℓ measurements from LoLLiPoP, we derived constraints on ΛCDM parameters that are in good agreement with previous Planck 2018 results, but with smaller uncertainties by 10% to 20%. We demonstrate that the foregrounds can be accurately described in the spectral domain, with a negligible impact on ΛCDM parameters. We also derived constraints on single-parameter extensions to ΛCDM, including AL, ΩK, Neff, and ∑mν. Noteworthy results from this updated analysis include a lensing amplitude value of AL = 1.039 ± 0.052, which is more closely aligned with theoretical expectations within the ΛCDM framework. Additionally, our curvature measurement, ΩK = −0.012 ± 0.010, is now fully consistent with a flat universe and our measurement of S8 is closer to the measurements derived from large-scale structure surveys (at the 1.5σ level). We also added constraints from PR4 lensing, making this combination the most tightly constrained data set currently available from Planck. Additionally, we explored the addition of baryon acoustic oscillation data, which tightens the limits on some particular extensions to the standard cosmology.

Generalized linearly varying deceleration parameter solutions of higher dimensional universe in f(R,T) theory

In this paper, generalized Friedmann–Robertson–Walker universe for Strange Quark Matter (SQM) and Normal Matter (NM) coupled with Domain Wall (DW) in form perfect fluid have been investigated in [Formula: see text] modified theory. [Formula: see text] function has been assumed as [Formula: see text] and Modified Field Equations (MEFEs) have been attained. The MEFEs of the constructed model do not allow solutions of open and closed FRW universe. Obtained modified field equations of higher dimensional flat FRW universe have been solved by using Linearly Varying Deceleration Parameter (LVDP) suggested by Akarsu and Dereli [Int. J. Theor. Phys. 51, 612 (2012)]. Pressure and energy density of the matter distributions for the Linearly Expansion Model (LEM) and Constant Expansion Model (CEM) have been attained as decreasing values by the cosmic time t and constant [Formula: see text] in [Formula: see text] gravity. But for the Exponential Expansion Model (XEM), these quantities have been obtained as time-independent. It is found out that the domain wall behaves like invisible matter when [Formula: see text] due to negative domain wall tension ([Formula: see text]) for SQM with DW of LEM. Strange quark matters for LDW and CDW models behave like cold dark matter when [Formula: see text] and [Formula: see text], respectively. One can say that SQM with DW for LEM and CEM may cause the expansion of the universe in these special situations. Also, solutions of XEM have shown that domain wall matter behaves like stiff matter when [Formula: see text]. All solutions of the constructed model in [Formula: see text] theory have been reduced to General Relativity (GR) theory solutions by assuming [Formula: see text]. Finally, all solutions obtained have been discussed in detail.

Simultaneous measurements on cosmic curvature and opacity using latest HII regions and H(z) observations

The different spatial curvatures of the universe affect the measurement of cosmological distances, which may also contribute to explaining the observed dimming of type Ia supernovae. This phenomenon may be caused by the opacity of the universe. Similarly, the opacity of the universe can lead to a bias in our measurements of curvature. Thus, it is necessary to measure cosmic curvature and opacity simultaneously. In this paper, we propose a new model-independent method to simultaneously measure the cosmic curvature and opacity by using the latest observations of HII galaxies acting as standard candles and the latest Hubble parameter observations. The machine learning method-Artificial Neural Network is adopted to reconstruct observed Hubble parameter H(z) observations. Our results support a slightly opaque and flat universe at 1sigma confidence level by using previous 156 HII regions sample. However, the negative curvature is obtained by using the latest 181 HII regions sample in the redshift range zsim 2.5. More importantly, we obtain the simultaneous measurements with precision on the cosmic opacity mathrm Delta tau sim 10^{-2} and curvature mathrm Delta Omega _Ksim 10^{-1}. A strong degeneracy between the cosmic opacity and curvature parameters is also revealed in this analysis.

Phase‐space analysis of the anisotropies in Weyl integrable spacetime: Kantowski–Sachs cosmology

We consider anisotropic Kantowski–Sachs cosmology in Weyl integrable spacetime with an ideal gas. We perform a detailed analysis of the dynamics by investigating the stationary points and reconstruct the cosmological history. This analysis gives important information for the initial value problem in Weyl integrable spacetime. We study three cases, in which the scalar field mass depends only on the density of the ideal gas, with or without cosmological constant term, and when there exists an exponential scalar field potential. We conclude that the existence of the nonzero potential function is essential in order that a spatially flat FLRW universe which can describe inflation can be a future attractor.

Cosmography for various parametrizations of dark energy equation of state

The concept of dark energy (DE) emerged as a result of confirming the accelerated expansion of the universe. Since then, numerous models have been developed to explore the origin and nature of DE. In this study, we investigate several recent cosmological models (Models 1–9) based on the parametrization of the DE equation of state. Our analysis focuses on a homogeneous, isotropic flat universe comprising DE, dark matter (DM), and radiation. We assume the separate conservation of the dark components (DE and DM) and radiation. By employing various parametrizations of [Formula: see text], we derive the corresponding Hubble function [Formula: see text]. To understand the cosmic expansion history of the universe in a model-independent manner, we employ cosmography as an approach. We express important cosmographic parameters such as deceleration, jerk, snap, and lerk parameters in terms of the Hubble rate [Formula: see text] and its derivative up to the fourth order. Additionally, we examine the statefinder parameter and [Formula: see text] diagnostics to distinguish between different types of DE models. Finally, we compare the physical interpretations of these diagnostic parameters with the standard [Formula: see text]CDM model to assess the viability of each model.