Value Of Cosmological Constant Research Articles

Observational Constraints and Cosmographic Analysis of f(T,TG) Gravity and Cosmology

We perform observational confrontation and cosmographic analysis of f(T,TG) gravity and cosmology. This higher-order torsional gravity is based on both the torsion scalar, as well as on the teleparallel equivalent of the Gauss–Bonnet combination, and gives rise to an effective dark-energy sector which depends on the extra torsion contributions. We employ observational data from the Hubble function and supernova Type Ia Pantheon datasets, applying a Markov chain Monte Carlo sampling technique, and we provide the iso-likelihood contours, as well as the best-fit values for the parameters of the power-law model, an ansatz which is expected to be a good approximation of most realistic deviations from general relativity. Additionally, we reconstruct the effective dark-energy equation-of-state parameter, which exhibits a quintessence-like behavior, while in the future the Universe enters into the phantom regime, before it tends asymptotically to the cosmological constant value. Furthermore, we perform a detailed cosmographic analysis, examining the deceleration, jerk, snap, and lerk parameters, showing that the transition to acceleration occurs in the redshift range 0.52≤ztr≤0.89, as well as the preference of the scenario for quintessence-like behavior. Finally, we apply the Om diagnostic analysis to cross-verify the behavior of the obtained model.

The state of the dark energy equation of state circa 2023

We critically examine the state of current constraints on the dark energy (DE) equation of state (EoS) w. Our study is motivated by the observation that, while broadly consistent with the cosmological constant value w = -1, several independent probes appear to point towards a slightly phantom EoS (w ∼ -1.03) which, if confirmed, could have important implications for the Hubble tension. We pay attention to the apparent preference for phantom DE from Planck Cosmic Microwave Background (CMB) data alone, whose origin we study in detail and attribute to a wide range of (physical and geometrical) effects. We deem the combination of Planck CMB, Baryon Acoustic Oscillations, Type Ia Supernovae, and Cosmic Chronometers data to be particularly trustworthy, inferring from this final consensus dataset w = -1.013+0.038 -0.043, in excellent agreement with the cosmological constant value. Overall, despite a few scattered hints, we find no compelling evidence forcing us away from the cosmological constant (yet).

Confinement from distance in metric space and its relation to cosmological constant

We argue that, in a theory of quantum gravity, the gauge coupling and the confinement scale of a gauge theory are related to distance in the space of metric configurations, and in turn to the cosmological constant. To support the argument, we compute the gauge kinetic functions in variuos supersymmetric Heterotic and type II string compactifications and show that they depend on distance. According to the swampland program, the distance between two (anti) de Sitter vacua in the space of metric configurations is proportional to the logarithm of the ratio of cosmological constants and thus the confinement scale depends on the value of the cosmological constant. In this framework, for de Sitter space, we revisit the swampland Festina Lente bound and gauge theories in the dark dimension scenario. We show that if the Festina Lente bound is realized in a de Sitter vacuum and dependence on distance is strong enough, it will be realized in vacua with higher cosmological constants. In dark dimension scenario, as the value of cosmological constant is related to the decompactifying dimension, we find that the confinement scale is indeed related to radius of dark dimension. We show that in this scenario the Festina Lente bound holds for the standard model QCD, as well as all confining gauge groups with Nc ≲ 103.

Stability of the de-Sitter spacetime. The anisotropic case

We elaborate on the cosmological implications of the recently established non-perturbative O(D,D)-symmetric approach. Following the α′-complete Friedmann equations, we provide a qualitative description of the dynamics of anisotropy perturbations. We find that the spacetime stability depends on the sign of the generalized dilaton time derivative. We consider isotropic limits and find possible mechanism for pre-big bang cosmology, governed by the value of cosmological constant.

Constrained [formula omitted]CDM dark energy models in higher derivative [formula omitted]-gravity theory

In the present paper, we have investigated dark energy scenarios in higher derivative f(R,Lm) gravity theory in a flat spacetime universe by constraining cosmographic series of Hubble function with Hubble constant H(z) datasets. We have considered an arbitrary function f(R,Lm)=γR2+λLmn, where R is the Ricci-scalar, Lm is matter Lagrangian, γ, λ and n are non-vanishing parameters. We have obtained contour plots of model parameters for the best fit values of (H0,q0,j0,s0,γ,λ,n,Ωm0) with 1−σ and 2−σ errors with H(z) datasets, and using these best fit model parameters, we have tried to explore the effective dark energy properties of the universe by defining the terms ρeff, peff and ωeff. We have found the values of effective equation of state parameter ωeff≈−0.96 at present and tends to cosmological constant value at late-time. We have found that the dark energy density term ρΛ∼Λ and it behaves just like time-dependent cosmological constant Λ-term. We have found a constrained ΛCDM transit phase expanding and currently accelerating dark energy model. Also, we have estimated the present age of the universe as t0=14.28 Gyrs.

Constraining Dark Energy from the Local Group Dynamics

This Letter develops a method to constrain the cosmological constant Λ from binary galaxies, focusing on the Milky Way and Andromeda galaxies. We provide an analytical solution to the two-body problem with Λ and show that the ratio between the Keplerian period and controls the importance of effects from the cosmological constant. The Andromeda–Milky Way orbit has a period of ∼17 Gyr, and so dark energy has to be taken into account. Using the current best mass estimates of the Milky Way and Andromeda galaxies, we find the cosmological constant value has an upper bound that is 5.44 times the value obtained by Planck. With future astrometric measurements, the bound on the cosmological constant can be reduced to . Our results offer the prospects of constraints on Λ over very different scales than previously. The Local Group provides also a completely novel platform to test alternative theories of gravity. We illustrate this by deriving bounds on scalar-tensor theories of gravity over megaparsec scales.

Massless minimal quantum scalar field with an asymmetric self interaction in de Sitter spacetime

Massless minimally coupled quantum scalar field with an asymmetric self interaction, V(ϕ) = λϕ 4/4! + βϕ 3/3! (with λ > 0) is considered in the (3+1)-dimensional inflationary de Sitter spacetime. This potential is bounded from below irrespective of the sign of β. Earlier computations mostly considered the quartic part. Our chief motivation behind this study is to assess the vacuum expectation values of V(ϕ) and ϕ, both of which can be negative, and hence may lead to some screening of the inflationary cosmological constant value. First using the in-in formalism, the renormalised quantum correction to the cubic potential appearing in the energy-momentum tensor is computed at two loop, which is the leading order in this case. The quantum correction to the kinetic term at two loop are subleading compared to the above result at late cosmological times. Next, using some of these results we compute the renormalised vacuum expectation value of ϕ, by computing the tadpoles at \U0001d4aa(β) and \U0001d4aa(λβ). Due to the appearance of the de Sitter isometry breaking logarithms, the tadpoles cannot be completely renormalised away in this case, unlike the flat spacetime. All these results, as expected, show secularly growing logarithms at late cosmological times. We next use a recently proposed renormalisation group inspired formalism to resum perturbative secular effects, to compute a non-perturbative 〈ϕ〉 at late cosmological times. 〈ϕ〉 turns out to be approximately one order of magnitude less compared to the position of the classical minima ϕ = -3β/λ of V(ϕ). Estimation on the possible screening of the inflationary cosmological constant due to this 〈ϕ〉 is also presented.

Timelike orbits around accelerating black holes

We study the geodesics of massive particles around an accelerating Schwarzschild black hole. We show that the radius of the innermost stable circular orbit and the angular momentum of a particle at this orbit decrease by increasing the acceleration. Apart from quantitative influence, the acceleration qualitatively changes the physics. We show that in accelerating black hole spacetime there exists an outermost stable circular orbit in flat, de Sitter, and anti--de Sitter backgrounds. Investigations of radial geodesics show that the acceleration acts like a repulsive force in the sense that test particles around accelerating black holes can move radially outward, unless there exists a large negative value of cosmological constant in the background to compensate the repulsive force. We also investigate the precession of perihelion of orbits around accelerating black holes. The precession would be larger compared to the nonaccelerating case. It is also shown that the precession in anti--de Sitter background could be opposite to the particle motion.

Radial oscillations and gravitational wave echoes of strange stars with nonvanishing lambda

We study the effect of the cosmological constant on radial oscillations and gravitational wave echoes (GWEs) of non-rotating strange stars. To depict strange star configurations we used two forms of equations of state (EoSs), viz., the MIT Bag model EoS and the linear EoS. By taking a range of positive and negative values of cosmological constant, the corresponding mass–radius relationships for these stars have been calculated. For this purpose, first we solved the Tolman–Oppenheimer–Volkoff (TOV) equations with a non-zero cosmological constant and then we solved the pressure and radial perturbation equations arising due to radial oscillations. The eigenfrequencies of the fundamental f-mode and first 22 pressure p-modes are calculated for each of these EoSs. Again considering the remnant of the GW170817 event as a strange star, the echo frequencies emitted by such stars in presence of the cosmological constant are computed. From these numerical calculations, we have inferred relations between cosmological constant and mode frequency, structural parameters, GWE frequencies of strange stars. Our results show that for strange stars, the effective range of cosmological constant is 10−15cm−2≤Λ≤3×10−13cm−2.

Energy and entropy compensation, phase transition and kinetics of four dimensional charged Gauss-Bonnet Anti-de Sitter black holes on the underlying free energy landscape

By treating black hole as a state in canonical ensemble or in grand canonical ensemble, we study the phase transition and the kinetics of the four dimensional charged Anti-de Sitter (AdS) black hole in Gauss-Bonnet (GB) gravity based on the free energy landscape. Below the critical temperature, the free energy landscape topography has the shape of double basins with each representing one stable/unstable black hole phase (the small or the large black hole). The thermodynamic small/large black hole phase transition is determined by the equal depths of the basins. We also demonstrate the underlying kinetics of the phase transition by studying the time evolution of the probability distribution of the state in the ensemble as well as the mean first passage time (MFPT) and the kinetic fluctuations of the state switching process caused by the thermal fluctuations. The results show that the final distribution is determined by the Boltzmann law and the MFPT and its fluctuation are closely related to the free energy landscape topography through the barrier heights between the small and the large black hole states and the ensemble temperature. Furthermore, we provide a complete description of the kinetics of phase transition by investigating the temperature dependence of the MFPT and the kinetic fluctuations with different physical parameters. It is shown that the free energy is the result of the delicate balance and competition between the two relatively large numbers, the energy and entropy multiplied by temperature compared to kT. Low energy (mass) and low entropy can give rise to a stable thermodynamic state in terms of free energy minimum (energy/mass preferred) while the high energy (mass) and high entropy (entropy preferred) can also give rise to a stable state in terms of free energy minimum. The comparable free energy barrier with respect to kT makes it possible for the switching from small size black hole state to the large size black hole state under thermal fluctuations and vice versa. When the GB coupling coupling constant increases, or the electric charge (potential) increases, or the pressure (absolute value of cosmological constant) decreases, it is easier for the small black hole state to escape to the large black hole state. Meanwhile, the inverse process becomes harder, i.e. the small (large) black hole state becomes less (more) stable.

The Universe acceleration from the Unimodular gravity view point: Background and linear perturbations

Abstract With the goal of studying the cosmological constant (CC) problem, we present an exhaustive analysis of unimodular gravity as a possible candidate to resolve the CC origin and with this, the current Universe acceleration. In this theory, a correction constant (CC-like) in the field equations sources the late cosmic acceleration. This constant is related to a new parameter, z i n i , which is interpreted as the redshift of CC-like emergence. By comparing with the CC value obtained from Planck and Supernovaes measurements, it is possible to estimate z i n i = 11 . 1 5 − 0 . 02 + 0 . 01 and z i n i = 11 . 4 3 − 0 . 06 + 0 . 03 respectively, which is close to the reionization epoch. Moreover, we use the observational Hubble data (OHD), Type Ia Supernovae (SnIa), Baryon Acoustic Oscillations (BAO) and the Cosmic Microwave Background Radiation (CMB) distance data to constrain the UG cosmological parameters. A Joint analysis (OHD+SnIa+BAO+CMB), results in z i n i = 11 . 4 7 − 0 . 073 + 0 . 074 within 1 σ confidence level consistent with our estimation from Planck and Supernovae measurements. We also include linear perturbations, starting with scalar and tensor perturbations and complementing with the perturbed Boltzmann equation for photons. We show that the 00 term in the UG field equations and the Boltzmann equation for photons contains corrections, meanwhile the other equations are similar as those obtained in standard cosmology.

Transit cosmological models coupled with zero-mass scalar field with high redshift in higher derivative theory

The present study deals with a flat FRW cosmological model filled with perfect fluid coupled with the zero-mass scalar field in the higher derivative theory of gravity. We have obtained two types of universe models, the first one is the accelerating universe (power-law cosmology), and the second one is the transit phase model (hyperbolic expansion-law). We have obtained various physical and kinematic parameters and discussed them with observationally constrained values of H0. The transit redshift value is obtained zt=0.414, where the transit model shows signature-flipping and is consistent with recent observations. In our models, the present values of EoS parameter ω0 crosses the cosmological constant value ω0=−1. Also, the present age of the universe is calculated.

On Einstein equations with cosmological constant in braneworld models

In this paper, we investigate the Einstein equations with cosmological constant for Randall-Sundrum (RS) and Dvali-Gabadadze-Porrati (DGP) models to determine the warp functions in the context of warp product spacetimes. In RS model, it is shown that Einstein's equation in the bulk is reduced into the brane as a vacuum equation, having vacuum solution, which is not affected by the cosmological constant in the bulk. In DGP model, it is shown that the Einstein's equation in the bulk is reduced into the brane and along the extra dimension, where both equations are affected by the cosmological constant in the bulk. We have solved these equations in DGP model, subject to vanishing cosmological constants on the brane and along extra dimension, and obtained exact solutions for the warp functions. The solutions depend on the typical values of cosmological constant in the bulk as well as the dimension of the brane. So, corresponding to the typical values, some solutions have exponential behaviours which may be set to represent warp inflation on the brane, and some other solutions have oscillating behaviours which may be set to represent warp waves or branes waves along the extra dimension.

Varying alpha generalized Dirac-Born-Infeld models

We study the cosmological consequences of a class of Dirac-Born-Infeld models, and assess their viability as a candidate for the recent acceleration of the Universe. The model includes both the rolling tachyon field and the generalized Chaplygin gas models as particular limits, and phenomenologically each of these provides a possible mechanism for a deviation of the value of the dark energy equation of state from its canonical (cosmological constant) value. The field-dependent potential that is characteristic of the rolling tachyon also leads to variations of the fine-structure constant $\alpha$, implying that the model can be constrained both by standard cosmological probes and by astrophysical measurements of $\alpha$. Our analysis, using the latest available low-redshfit data and local constraints from atomic clock and weak equivalence principle experiments, shows that the two possible deviations of the dark energy equation of state are constrained to be $\log_{10}{(1+w_0)_V}<-7.85$ and $\log_{10}{(1+w_0)_C}<-0.85$, respectively for the rolling tachyon and Chaplygin components, both being at the $95.4\%$ confidence level (although the latter depends on the choice of priors, in a way that we quantify). Alternatively, the $95.4\%$ confidence level bound on the dimensionless slope of the potential is $\log_{10}{\lambda}<-5.36$. This confirms previous analyses indicating that in these models the potential needs to be extremely flat.

Dark Energy Is a Phenomenal Effect of the Expanding Universe-Possibility for Experimental Verification

Vacuum energy density has been defined and mass formation from “space-time” has been viewed from a different perspective. This explanation for vacuum energy is based on “space-time” and conversion of space into time keeping “space-time density” as constant. Equations for “space-time” and mutual conversion of space and time have been derived. As a result, new concept of mass creation has been explained. By postulating that space time density of universe is constant, low and high values of cosmological constant has been shown as the exchange of energy between space, time and matter. The concept has been used to explain dark energy concept of the universe. It concluded a result that velocity of light is changing with the apparent expansion of the universe. The derived equation is possible for experimental verification. Obviously it is a contradiction to Big bang model. So the derived equation with the help of quantum concepts explained the 2.7 K cosmic micro wave background radiation. Theoretical value of Hubble’s constant has been proposed which is useful for calculation of experimental results appropriately.

Traversable wormholes supported by non-exotic matter in general relativity

Natural horizonless object and its astrophysical signatures have been proposed in various aspects. In this paper, the traversable wormhole solutions are investigated for Einstein’s field equations with cosmological constant. Using redshift function Φ(r)and shape function b(r)as Φ(r)=−1r2and b(r)=r0log(r+1)log(r0+1)in the static and spherically symmetric metric of wormholes, the spherical regions are determined, where the energy conditions are satisfied for positive value of cosmological constant. Further, horizonless compact objects with light rings (or photon spheres) are becoming more popular in recent years for numerous motives. We observed that a horizonless object such as a traversable wormhole can have a photon sphere outside the throat. So, the photon spheres are detected due to the effect of strong gravitational lensing. Furthermore, the deflection angle is found to diverge for the impact parameter corresponding to the photon sphere.

(In)stability of black holes in the 4D Einstein–Gauss–Bonnet and Einstein–Lovelock gravities

A (3+1)-dimensional Einstein-Gauss-Bonnet effective description of gravity has been recently formulated as the $D \to 4$ limit of the higher dimensional field equations after the rescaling of the coupling constant. This approach has been recently extended to the four-dimensional Einstein-Lovelock gravity. Although validity of the regularization procedure has not been shown for the general case, but only for a wide class of metrics, the black-hole solution obtained as a result of such a regularization is also an exact solution in the well defined 4D Einstein-Gauss-Bonnet theory suggested by Aoki, Gorji and Mukohyama [arXiv:2005.03859] and in the scalar-tensor effective classical theories. Here we study the eikonal gravitational instability of asymptotically flat, de Sitter and anti-de Sitter black holes in the four dimensional Einstein-Gauss-Bonnet and Einstein-Lovelock theories. We find parametric regions of the eikonal instability for various orders of the Lovelock gravity, values of coupling and cosmological constants, and share the code which allows one to construct the instability region for an arbitrary set of parameters. For the four-dimensional Gauss-Bonnet black holes we obtain the region of stability in analytic form. Unlike the higher dimensional Einstein-Lovelock case, the eikonal instability serves as an effective cut-off of higher curvature Lovelock terms for the 4D black holes.

Chiral gauge theory and gravity from unconventional supersymmetry

From a gauge SU (2, 2|2) model with broken supersymmetry, we construct an action for SU(2)×U (1) Yang-Mills theory coupled to gravity and matter in four dimensions. The connection components for AdS boosts and special conformal translations are auxiliary fields and their fixing reduces the theory to two distinct sectors: a vector-like gauge theory with general relativity and a chiral gauge theory where gravity drops out. We discuss some of the main classical features of the model such as the predicted tree level gauge couplings, cosmological constant value, mass-like terms and the Einstein equations.

Transit cosmological model with specific Hubble parameter in F(R, T) gravity

In this paper, we have investigated a flat FLRW (Friedmann Lemaitre Robertson Walker metric) cosmological model in f(R, T) gravity with specific Hubble parameter. We have considered f(R,T)=f(R)+2f(T) where f(R)=R+αR2,f(T)=λT and R is Ricci scalar, T the trace of energy momentum tensor. To obtain exact solutions of modified Einstein’s field equation, we have considered a specific Hubble parameter H(a)=l(a−n+k) which gives the scale factor a(t)=[ekln(t+m)−1k]1n where n > 0, k, l, m are arbitrary constants called model parameters. With this scale factor, we have obtained a transit phase cosmological model with variable deceleration parameter (DP). We have compared our results with observational constraints obtained from various observations by using the curve-fitting techniques and found that the derived model is a transit dark energy model quintessence (ω>−1) at present (ω0≈−0.7406) and approaches to cosmological constant value (ω=−1) at late time universe. The transition point is found as ztc≈0.676 which is in good agreement with the recent observations. Also, the derived model explains formation of different structures in the universe from past to present scenarios.

Irrational monodromies of vacuum energy

We present a theory with axion flux monodromies coupled to gravity, that reduces to the local vacuum energy sequester below the axion mass scales. If the axion potentials include a term generated by nonperturbative couplings to gauge sectors, with a decay constant incommensurate with monodromy periods, the low energy potential germinates a landscape of irrational axion vacua, with arbitrarily small cosmological constants. The sensitivity of the values of cosmological constants to unknown UV physics can be greatly reduced. The variation of the cosmological constant in each vacuum, from one order in perturbation theory to the next, can be much smaller than the naïve cut­off. The nonperturbative transitions in the early universe between the vacua populate this landscape, similar to the case of irrational axion. In such a landscape of vacua a small cosmological constant can naturally emerge.