Hubble Parameter Research Articles

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.

Flexible extra dimensions

This paper discusses the origin of the small parameters with the aim of explaining the Hierarchy problem. The flexible extra dimensions are an essential tool in the process by which physical parameters are formed. The evolution of a multidimensional metric starts at the Planck scale and is completed with the static extra-dimensional metric and the 4-dim de Sitter space at high energies, where the exponential production of causally disconnected universes begins. Quantum fluctuations independently distort the metric within these universes, causing inflationary processes within them. Some of these universes tend asymptotically towards states characterised by small Hubble parameters. The effective parameter reduction applied to the Higgs sector of the Standard Model is explained by the presence of small-amplitude distributions of a scalar field in a fraction of these universes.

Holographic realization of constant roll inflation and dark energy: An unified scenario

In the formalism of generalized holographic dark energy, the infrared cut-off LIR is generalized to the form, LIR=LIR(Lp,L˙p,L¨p,⋯,Lf,L˙f,L¨f,⋯,a,H,H˙,H¨,⋯), where Lp and Lf are the particle horizon and the future horizon, respectively (moreover, a is the scale factor and H is the Hubble parameter of the universe). Based on such formalism, we establish a holographic realization of constant roll inflation during the early universe, where the corresponding cut-off depends on the Hubble parameter and its derivatives (up to the second order). The viability of this holographic constant roll inflation with respect to the Planck data in turn puts a certain bound on the infrared cut-off at the time of horizon crossing. Such holographic correspondence of constant roll inflation is extended to the scenario where the infrared cut-off is corrected by the ultraviolet one, which may originate due to quantum effects. Besides the mere inflation, we further propose the holographic realization of an unified cosmic scenario from constant roll inflation (at the early time) to the dark energy era (at the late time) with an intermediate radiation dominated era followed by a Kamionkowski like reheating stage. In such a unified holographic scenario, the inflationary quantities (like the scalar spectral index and the tensor-to-scalar ratio) and the dark energy quantities (like the dark energy EoS parameter and the present Hubble rate) prove to be simultaneously compatible with observable constraints for suitable ranges of the infrared cut-off and the other model parameters. Moreover the curvature perturbations at super-Hubble scale prove to be a constant (with time) during the entire cosmic era, which in turn ensures the stability of the model under consideration.

A truncated scale factor to realize cosmological bounce under the purview of modified gravity

AbstractThe present paper reports a study on bounce realization with a truncated scale factor and the holographic fluid taken in the form of holographic Ricci Dark Energy, which is a particular case of holographic Nojiri–Odintsov model (Nojiri & Odintsov, 2006, Gen. Relativ. Gravit., 38(8), 1285). We have investigated the realization of bounce due to the application of a holographic fluid in the early scenario of the universe. We started with a de‐Sitter scale factor in our study and derived two modified forms using Taylor's series expansion. After studying the behavior of the resulting Hubble parameter, we identified as the suitable form of the scale factor to realize the bounce. It satisfied the conditions , , and before, at, and after the turn‐around point. We have also observed that the resulting equation of state (EoS) parameter violates the null energy condition required by the bounce realization. We have studied the bounce scenario in a modified gravity framework and derived the slow roll parameters. We have studied the behavior of the potential along with the slow‐roll parameters in this bouncing scenario.

Cosmological implications of an interacting model of dark matter & dark energy

In this paper, we have studied an interacting dark energy model. We have assumed the gravitational interaction between the matter fields i.e. between barotropic fluid and the dark energy. The dark energy evolution within the framework of spatially homogeneous and isotropic Friedmann-Robertson-Walker space-time. Therefore, we examine the cosmic evolution from the perspective of interacting scenario by selecting a suitable ansatz for the scale factor resulting from a parametrization of Hubble parameter. The evolution of the cosmological parameters are discussed in some details in the considered interacting scenario by calculating parameters and quantities such as deceleration parameter, energy density, pressure, equation of state (EoS) etc. Also, we have performed some cosmological tests and analysis in support of our obtained interacting model. Finally, we reconstruct the potential of the scalar field and refute the refined swampland conjecture using the equation of state of dark energy and the relationship between energy density and pressure with the scalar field and potential, and then thoroughly describe the findings.

Anisotropic LRS-BI Universe with f(Q) gravity theory

The possible anisotropic nature in the early phases of the Universe is one of the interesting aspects of study in cosmology. We investigate the evolution of the Universe in terms of few cosmological parameters considering an anisotropic locally rotationally symmetric (LRS) Bianchi type-I spacetime (LRS-BI) under the $f(Q)$ gravity of symmetric teleparallel theory equivalent to the GR (STEGR). In this study we consider two $f(Q)$ gravity models, viz., $f(Q) = -\,(Q+2\Lambda)$, a simple model with the cosmological constant $\Lambda$ and the power law model, $f(Q) = -\, \alpha Q^n$ with two constant parameters $\alpha$ and $n$. Considering a proportionality relation between the directional Hubble parameters with a proportionality constant parameter $\lambda$ we find a significant contribution of the anisotropic factor in the evolution of the early Universe for both models. The power law model shows the more dominating effect of anisotropy in comparison to the simple model depending on model parameters, especially on the parameter $n$. The power law model also shows some possible effects of anisotropy on the Universe's evolution in the near future. In addition, both models confirm that the anisotropy does not obtain any appreciable signature in the current stage of the Universe. From our analysis we also set rough constraints on our model parameters as $0.5 \le \lambda \le 1.25$, $0.75 \le \alpha \le 1.5$ and $0.95 \le n \le 1.05$.

EXPLORING THE INTERCONNECTEDNESS OF COSMOLOGICAL PARAMETERS AND OBSERVATIONS: INSIGHTS INTO THE PROPERTIES AND EVOLUTION OF THE UNIVERSE

This research aims to investigate the relationship between Confidence Interval, Hubble Parameter, Comoving Distance, and Distance-Volume Relationship, which are important equations in cosmology. The Confidence Interval equation is used to estimate the range of values for the difference between the mean redshift and Hubble parameter. The Hubble Parameter equation is used to measure the expansion rate of the universe, while the Comoving Distance equation is used to calculate the distance between two objects in the expanding universe, and the Distance-Volume Relationship equation is used to calculate the distance between an observer and a cosmic object based on the object's redshift. This study seeks to address several research questions, including the accuracy of estimating parameters using these equations and the potential for developing more precise equations. The study employs cosmological data analysis using the R program to analyze existing data and gain a better understanding of cosmological parameters. The results of this research contribute to our understanding of the nature and evolution of the universe, providing insights into the distribution of matter and the role of dark matter and dark energy in shaping the universe's evolution. By examining the relationship between cosmological parameters, this study enables us to make predictions about cosmic phenomena and improve the accuracy of future measurements. The findings of this research have implications for cosmological research and can aid in the development of more accurate models and theories in the field of cosmology. Overall, this study provides valuable insights into the fundamental equations in cosmology and their relationships, advancing our understanding of the universe's dynamics and evolution.

Exact Solutions and Cosmological Constraints in Fractional Cosmology

This paper investigates exact solutions of cosmological interest in fractional cosmology. Given μ, the order of Caputo’s fractional derivative, and w, the matter equation of state, we present specific exact power-law solutions. We discuss the exact general solution of the Riccati Equation, where the solution for the scale factor is a combination of power laws. Using cosmological data, we estimate the free parameters. An analysis of type Ia supernovae (SNe Ia) data and the observational Hubble parameter data (OHD), also known as cosmic chronometers, and a joint analysis with data from SNe Ia + OHD leads to best-fit values for the free parameters calculated at 1σ, 2σ and 3σ confidence levels (CLs). On the other hand, these best-fit values are used to calculate the age of the Universe, the current deceleration parameter (both at 3σ CL) and the current matter density parameter at 1σ CL. Finding a Universe roughly twice as old as the one of ΛCDM is a distinction of fractional cosmology. Focusing our analysis on these results, we can conclude that the region in which μ>2 is not ruled out by observations. This parameter region is relevant because fractional cosmology gives a power-law solution without matter, which is accelerated for μ>2. We present a fractional origin model that leads to an accelerated state without appealing to Λ or dark energy.

Planck Constants in the Symmetry Breaking Quantum Gravity

We consider the theory of quantum gravity in which gravity emerges as a result of the symmetry-breaking transition in the quantum vacuum. The gravitational tetrads, which play the role of the order parameter in this transition, are represented by the bilinear combinations of the fermionic fields. In this quantum gravity scenario the interval ds in the emergent general relativity is dimensionless. Several other approaches to quantum gravity, including the model of superplastic vacuum and BF theories of gravity support this suggestion. The important consequence of such metric dimension is that all the diffeomorphism invariant quantities are dimensionless for any dimension of spacetime. These include the action S, cosmological constant Λ, scalar curvature R, scalar field Φ, wave function ψ, etc. The composite fermion approach to quantum gravity suggests that the Planck constant ℏ can be the parameter of the Minkowski metric. Here, we extend this suggestion by introducing two Planck constants, bar ℏ and slash /h, which are the parameters of the correspondingly time component and space component of the Minkowski metric, gMinkμν=diag(−ℏ2,/h2,/h2,/h2). The parameters bar ℏ and slash /h are invariant only under SO(3) transformations, and, thus, they are not diffeomorphism invariant. As a result they have non-zero dimensions—the dimension of time for ℏ and dimension of length for /h. Then, according to the Weinberg criterion, these parameters are not fundamental and may vary. In particular, they may depend on the Hubble parameter in the expanding Universe. They also change sign at the topological domain walls resulting from the symmetry breaking.

Perturbative approach to f(R)-gravitation in FLRW cosmology

The f(R) theory of gravitation developed perturbatively around the general theory of relativity with cosmological constant (the varLambda CDM model) in a flat FLWR geometry is considered. As a result, a general explicit cosmological solution that can be used for any model with an arbitrary, but well-defined, f(R) function (just satisfying given perturbation conditions) is derived. This perturbative solution shows how the Hubble parameter H(t) depends on time (along with the cosmological constant and the matter density) to adapt to the evolution of the Universe. To illustrate, this approach is applied to some specific test models. One of these models appears to be more realistic as it could describe three phases of the Universe’s evolution. Despite the fact that the perturbation is applied for a flat FLWR geometry (according to the current cosmological observation) indicates that the obtained solution can mainly describe the evolution of the late Universe, it may also work for an early Universe. As a next step, the present method can be applied to the case with a more general FLRW geometry to increase the precision of the description of different stages in the evolution of the Universe. Finally, it is shown that in a desription of the Universe’s evolution the perturbative f(R)-theory can be considered as an effective GR with the cosmological constant varLambda replaced by an effective parameter varLambda _{eff}[rho (t)] . This trick leads to a simpler way of solving an f(R)-theory regardless its specific form.

Stability and singularity analysis of the cosmologies with different scenarios for deceleration parameter in the presence of torsion

In the current paper, path deviation equations in absolutely parametric parallel geometries are derived. It is considered as a geodesic deviation equation. Additionally, it is modified by a torsion term. It proposes the deviation path equation that describes the trajectory deviation of a particle under the influence of the gravitational field. To examine the singularity of the Cosmological models, the modified version of the Raychaudhuri equation is utilized. The generalized law of the variation of Hubble's parameter is utilized to achieve some Cosmological models.

Observationally constrained accelerating cosmological model with higher power of non-metricity and squared trace

In this paper, a cosmological model of the Universe is presented in f(Q,T) gravity and the parameters are constrained by cosmological data sets. Initially, a generalised form of f(Q,T) model is used as f(Q,T)=−λ1Qm−λ2T2, where λ1, λ2 and m are model parameters. With some algebraic manipulation, the Hubble parameter is obtained in terms of redshift. Then, using MCMC analysis, the model parameters are constrained using the most current Hubble and Pantheon+ data. The model parameters are also verified through the BAO data set. The model shows an early deceleration transitioning to an accelerating phase of the Universe. The Om(z) diagnostic indicates a positive slope, favouring the model to be in a phantom field dominated phase.

Neural network reconstructions for the Hubble parameter, growth rate and distance modulus

This paper introduces a new approach to reconstruct cosmological functions using artificial neural networks based on observational measurements with minimal theoretical and statistical assumptions. By using neural networks, we can generate computational models of observational datasets, and then we compare them with the original ones to verify the consistency of our method. This methodology is applicable to even small-size datasets. In particular, we test the proposed method with data coming from cosmic chronometers, fsigma _8 measurements, and the distance modulus of the Type Ia supernovae. Furthermore, we introduce a first approach to generate synthetic covariance matrices through a variational autoencoder, using the systematic covariance matrix of the Type Ia supernova compilation.

Acoustic metric and Planck constants

Based on Akama–Diakonov (AD) theory of emergent tetrads, it was suggested that one can introduce two Planck constants,and, which are the parameters of the corresponding components of Minkowski metric,. In the Akama–Diakonov theory, the intervalis dimensionless, as a result the metric elements and thus the Planck constants have nonzero dimensions. The Planck constanthas dimension of time, and the Planck constanthas dimension of length. It is natural to comparewith the Planck length. However, this connection remains an open question, because the microscopic (trans-Planckian) physics of the quantum vacuum is not known. Here we study this question using the effective gravity emerging for sound wave quanta (phonons) in superfluid Bose liquid, where the microscopic physics is known, and the elements of the effective acoustic metric are determined by the parameters of the Bose liquid. Since the acoustic interval is dimensionless, one may introduce the effective “acoustic Planck constants.” The acoustic Planck constanthas dimension of length and is on the order of the interatomic distance. This supports the scenario in which. We also use the acoustic metric for consideration of dependence ofon the Hubble parameter in expanding Universe.

Constraints on the transition redshift using Hubble phase space portrait

One of the most significant discoveries in modern cosmology is that the universe is currently in a phase of accelerated expansion after a switch from a decelerated expansion. The redshift corresponding to this epoch is commonly referred to as the transition redshift [Formula: see text]. In this work, we put constraints on the transition redshift with both model-independent and model-dependent approaches. We consider the recently compiled database of 32 Hubble parameter measurements and the Pantheon sample of Type Ia Supernovae (SNe). In order to include the possible systematic effects in this analysis, we use the full covariance matrix of systematic uncertainties for the Hubble parameter measurements. We plot a Hubble Phase Space Portrait (HPSP) between [Formula: see text] and [Formula: see text] in a model-independent way. From this HPSP diagram, we estimate the transition redshift as well as the current value of the equation of state parameter [Formula: see text] in a model-independent way. By considering H(z) measurements, we find the best fit value of [Formula: see text] and [Formula: see text]. We obtain the best fit value of [Formula: see text] and [Formula: see text] using the Pantheon database. Further, we also use a model-dependent approach to determine [Formula: see text]. Here, we consider a nonflat [Formula: see text]CDM model as a background cosmological model. We reconstruct the cosmic triangle plot among [Formula: see text], [Formula: see text] and [Formula: see text] where the constraints of each parameter are determined by the location in this triangle plot. By using [Formula: see text] and [Formula: see text] values, we find the best value of the transition redshift [Formula: see text], which is in good agreement with the Planck 2018 results at [Formula: see text] confidence level. We also simulate the observed Hubble parameter measurements in the redshift range [Formula: see text] and perform the same analysis to estimate the transition redshift.

Reconstruction methods and the amplification of the inflationary spectrum

We analyze the consequences of different evolutions of the Hubble parameter on the spectrum of scalar inflationary perturbations. The analysis is restricted to inflationary phases described by a transient evolution, when uncommon features arise in the inflationary spectra that may lead to an amplitude enhancement. We then discuss how the spectrum is, respectively, amplified or blue-tilted in the presence or absence of a growing solution of the Mukhanov-Sasaki equation. The cases of general relativity with a minimally coupled inflaton and that of induced gravity are considered explicitly. Finally, some remarks on constant roll inflation are discussed.

Scale covariant theory as a dark energy model

We consider it worthy if we could construct a realistic model universe that would enable us to identify a clue about the source of dark energy. So, we develop a Scale Covariant Theory model universe considering a 5D spherically symmetric space-time. It is predicted that the constructed model itself behaves as a phantom energy model/ source that tends to a de Sitter phase avoiding the finite-time future singularity (big rip). The model universe is isotropic and is free from an initial singularity. The gravitational constant [Formula: see text] decreases with a variation of [Formula: see text][Formula: see text] and the Hubble parameter is estimated to be [Formula: see text]. We also provide a thorough analysis of the cosmological findings with graphical representations.

Phantom Cosmological Model with Observational Constraints in f(Q)$f(Q)$ Gravity

AbstractIn this paper, the cosmological model of the Universe has been presented in gravity and the parameters are constrained from the cosmological data sets. At the beginning, a well motivated form of has been employed, where α, β, and n are model parameters. The Hubble parameter is obtained in redshift with some algebraic manipulation from the considered form of . Then it is parameterized with the recent data and data using Markov chain Monte Carlo analysis. The obtained model parameter values are validated with the baryon acoustic oscillation data set. A parametrization of the cosmographic parameters shows the early deceleration and late time acceleration with the transition at . The diagnostics gives positive slope which shows that the model is in the phantom phase. Also the current age of the Universe has been obtained as, . Based on the present analysis, it indicates that the gravity may provide an alternative to dark energy for addressing the current cosmic acceleration.

Cosmic voids and the kinetic analysis

We consider a principal problem, that of the possible dominating role of self-consistent gravitational interaction in the formation of cosmic structures: voids and their walls in the local Universe. It is in the context of the Hubble tension as a possible indication of the difference in the descriptions of the late (local) and early (global) Universe. The kinetic Vlasov treatment enables us to consider the evolution of gravitating structures where the fundamental role has the modified gravitational potential with a cosmological constant, leading to the prediction of a local flow with a Hubble parameter that is nonidentical to that of the global Hubble flow. The Poisson equation for a potential with an additional repulsive term, including an integral equation formulation, is analyzed, and we predict the appearance of multiply connected two-dimensional gravitating structures and voids in the local Universe. The obvious consequence of the developed mechanism is that the cosmological constant poses a natural scaling for the voids, along with the physical parameters of their local environment, which can be traced in observational surveys.

Cosmological-model-independent Determination of Hubble Constant from Fast Radio Bursts and Hubble Parameter Measurements

We establish a cosmological-model-independent method to determine the Hubble constant H 0 from the localized fast radio bursts (FRBs) and the Hubble parameter measurements from cosmic chronometers and obtain a first such determination H 0 = 71 ± 3 km s−1 Mpc−1, with an uncertainty of 4%, from the eighteen localized FRBs and nineteen Hubble parameter measurements in the redshift range 0 < z ≤ 0.66. This value, which is independent of the cosmological model, is consistent with the results from the nearby Type Ia supernovae (SNe Ia) data calibrated by Cepheids and the Planck cosmic microwave background radiation observations at the 1σ and 2σ confidence level, respectively. Simulations show that the uncertainty of H 0 can be decreased to the level of that from the nearby SNe Ia when mock data from 500 localized FRBs with 50 Hubble parameter measurements in the redshift range of 0 < z ≤ 1 are used. Since localized FRBs are expected to be detected in large quantities, our method will be able to give a reliable and more precise determination of H 0 in the very near future, which will help us to figure out the possible origin of the Hubble constant disagreement.