arXiv subjects
Publications and source records attributed to Umesh Kumar Sharma.
Is thermodynamics consistent with the quantum gravity reconciliation hypothesis [A. G. Cohen et al. Phys. Rev. Lett. 82, 4971 (1999)], which establishes holographic dark energy models? Here, we have attempted to address this issue in the affirmative by concentrating on the first law of thermodynamics.
Casimir energy is always suggested as a possible source to create a traversable wormhole. It is also used to demonstrate the existence of negative energy, which can be created in a lab. To generalize, this idea, Yukawa modification of Casimir source has been considered in Remo Garattini (Eur. Phys. J. C 81 no.9, 824, 2021). In this work, we explore the Yukawa Casimir wormholes in symmetric teleparallel gravity. We have taken four different forms of $f(Q)$ to obtain wormhole solutions powered by the original Casimir energy source and Yukawa modification of the Casimir energy source. In power law form $f(Q)= αQ^2 + β$ and quadratic form $f(Q)= αQ^2 + βQ + γ$, where $α, β, γ$ are constants and $Q$ is non-metricity scalar, we analyze that wormhole throat is filled with non-exotic matter. We find self-sustained traversable wormholes in the Casimir source where null energy conditions are violated in all specific forms of $f(Q)$, while after Yukawa modification it is observed that violation of null energy conditions is restricted to some regions in the vicinity of the throat.
By extending the standard holographic principle to a cosmological framework and combining the non-flat condition with the Kaniadakis entropy, we construct the non-flat Kaniadakis holographic dark energy model. The model employs Kaniadakis parameter $K$ and a parameter $c$. Derivation of the differential equation for KHDE density parameter to describe the evolutionary behavior of the universe is obtained. Such a differential equation could explain both the open as well as closed universe models. The classification based on matter and dark energy (DE) dominated regimes show that the KHDE scenario may be used to specify the Universe's thermal history and that a quintom regime can be encountered. For open and closed both the cases, we find the expressions for the deceleration parameter and the equation of state (EoS) parameter. Also, by varying the associated parameters, classical stability of the method is established. On considering the curvature to be positive, the universe favors the quintom behavior for substantially smaller values as opposed to the flat condition, when only quintessence is attained for such $K$ values. Additionally, we see a similar behavior while considering the curvature to be negative for such $K$ values. Therefore, adding a little bit of spatial geometry that isn't flat to the KHDE enhances the phenomenology while maintaining $K$ values at lower levels. To validate the model parameters, the most recent $30\;H(z)$ dataset measurements, in the redshift range $0.07 \leq z \leq 1.965$ are utilized. In addition, the distance modulus measurement from the current Union 2.1 data set of type Ia supernovae are employed.
The proposed model is a study of the nature of dark energy through non-extensive Tsallis entropy. The method is based on the Karolyhazy relation which is a combined idea from quantum physics and general relativity. Dark energy is the energy density of quantum fluctuations in space-time. This is the key idea behind proposing agegraphic dark energy (ADE) models here. The parameter $δ$ is used to measure the quantitative distinction from the standard scenario. To look at the cosmological implications of the hypothesized dark energy model, as well as the expansion of the Universe filled with zero pressure matter and the resulting dark energy alternatives, the role of IR cutoff is played by age of the universe. The dynamic behavior of dark energy density parameter is carried out. The expressions for the equation of state parameter and deceleration parameter are obtained. The analysis is performed by taking into account a no flow as well as a flow of energy among the dark matter and dark energy sectors of the universe.
In this manuscript, we present a novel dark energy model to study the nature of dark energy. Non-extensive Kaniadakis entropy and, timescale as infrared cutoff are the major tools of the current study. Age of the Universe will serve the purpose of infrared cutoff. The cosmological characteristics of the proposed dark energy model, as well as the evolution of the cosmos filled with pressure-free matter and the ensuing dark energy candidates, are investigated. The interaction as well as non-interaction among the two sectors will also be considered. The differential equation for the dark energy density parameter, including the expression of the equation of state and deceleration parameters, are derived. The analysis of deceleration parameter clearly shows the universe to transit from decelerated to accelerated phase around $z\approx 0.6$. The squared sound speed is also plotted against redshift $z$ to check the stability behavior of the model for both the cases.
Tsallis entropy is a generalization of the Boltzmann-Gibbs entropy in statistical theory which uses a parameter $δ$ to measure the deviation from the standard scenario quantitatively. Using concepts of Tsallis entropy and future event horizon, we construct a new Tsallis holographic dark energy model. The parameters $c$ and $δ$ will be used to characterize various aspects of the model. Analytical expressions for various cosmological parameters such as the differential equation describing the evolution of the effective dark energy density parameter, the equation of state parameter and the deceleration parameter are obtained. The equation of state parameter for the current model exhibits the pure quintessence behaviour for $c>1$, quintom behaviour for $c<1$ whereas the $Λ$CDM model is recovered for $c=1$. To analyze the thermal history of the universe, we obtained the expression for the deceleration parameter and found that for $z \approx 0.6$, the phase transits from deceleration to acceleration.
New Tsallis holographic dark energy with apparent horizon as IR-cutoff studied by considering non-flat Friedmann-Lemaitre-Robertson-Walker metric. The accelerating expansion phase of the Universe is described by using deceleration parameter, equation of state parameter and density parameter by using different values of NTHDE parameter "$δ$". The NTHDE Universe's transition from a decelerated to an accelerated expanding phase is described by the smooth graph of deceleration parameter. Depending on distinct values of Tsallis parameter "$δ$", we have explored the quintessence behavior of the equation of state parameter. We used Hubble data sets obtained using Cosmic Chronometric methods and distance modulus measurement of Type Ia Supernova to fit the NTHDE parameters. Stability of our model by analyzing the squared speed of sound investigated as well.
The presence of exotic matter for the existence of the wormhole geometry has been an unavoidable problem in GR. In recent studies researchers have tried to deal with this issue using modified gravity theories where the WH geometry is explained by the extra curvature terms and NEC's are not violated signifying the standard matter in the WH geometry. In the present article we are trying to find the solutions of traversable wormholes with normal matter in the throat within the framework of symmetric teleparallel gravity $f(Q)$ where $Q$ is the non metricity scalar which defines the gravitational interaction. We will examine the wormhole geometries for three forms of function $f(Q)$. First is the linear form $f(Q)=αQ$, second a non -linear form $f(Q)=αQ^2 + β$ and third one a more general quadratic form $f(Q)=αQ^2 + βQ + γ$ with $α$, $β$ and $γ$ being the constants. For all the three cases the shape function is taken as $b(r) = {\frac{r_{0}\ln(r+1)}{\ln({r_0}+1)}}$ where $r_0$ is the throat radius. A special variable redshift function is considered for the discussion. All the energy conditions are then examined for the existence and the stability of the wormhole geometry.
The current research investigates the behavior of the Tsallis holographic dark energy (THDE) model with quintessence in a homogeneous and anisotropic Bianchi type-III (B-III) space-time. We construct the model by using two conditions (i) expansion scalar ($θ$) is proportionate to shear scalar ($σ$) in the model and (ii) hybrid expansion law $a = t^βe^{γt}$, where $β>0$, $γ>0$. Our study is based on Type Ia supernovae (SNIa) data in combination with CMB and BAO observations (Giostri et al, JCAP 3, 27 (2012), arXiv:1203.3213v2[astro-ph.CO]), the present values of Hubble constant and deceleration parameter are $H_{0} = 73.8$ and $q_{0} = -0.54$ respectively. Compiling our theoretical models with this data, we obtain $β= 2.1445~ \& ~ 2.1154$ for $γ= 0.5 ~ \& ~ 1$ respectively. We have completed a new type of cosmic model for which the expansion occurs to the current accelerated phase for the restraints. We have discussed the conformity among the scalar field model of quintessence and THDE model. To understand the Universe, we have also established the relations for Distance modulus, Luminosity Distance, and Angular-diameter distance. Some geometric and physical aspects of the THDE model are also highlighted.
In the framework of the generalized Rastall theory (GRT), we study the ability of a non-minimal coupling between geometry and matter fields in order to provide a setting which allows for a variable G during the cosmic evolution. In this regard, the compatibility of this theory with Dirac hypothesis on the variations of G is investigated, and additionally, the possibility of obtaining the current accelerated universe is also addressed. In summary, our study indicates that, in GRT, having in hand the G profile, one may find the corresponding non-minimal coupling between the energy source and geometry and vise versa, in a compatible way with the current accelerated universe.
In present research, we construct Kaniadakis holographic dark energy (KHDE) model within a non-flat Universe by considering the Friedmann-Robertson-Walker (FRW) metric with open and closed spatial geometries. We therefore investigate cosmic evolution by employing the density parameter of the dark energy (DE), the equation of state (EoS) parameter and the deceleration parameter (DP). The transition from decelerated to accelerated expanding phase for the KHDE Universe is explained through dynamical behavior of DP. With the classification of matter and DE dominated epochs, we find that the Universe thermal history can be defined through the KHDE scenario, and moreover, a phantom regime is experienceable. The model parameters are constrained by applying the newest $30$ data cases of $H(z)$ measurements, over the redshift span $0.07 \leq z \leq 2.36$, and the distance modulus measurement of the recent Union $2.1$ data set of type Ia supernovae. The classical stability of KHDE model has also been addressed.
In the present paper, we investigate wormholes in 4D-Einstein-Gauss-Bonnet gravity without the requirement of exotic matters. We have taken the radial dependent red-shift function $ϕ=\ln \left( {\frac {r_{0}}{r}}+1 \right)$ and shape function $b(r)={\frac{r_{0}\ln(r+1)}{\ln({r_0}+1)}}$ as well as anisotropic matter sources through equation of state (EoS) $p_r(r) = ωρ(r)$. We examine the energy conditions, flaring out condition, throat condition and anisotropic parameter. The volume integral quantifier is also analyzed to validate the existence and stability of the WH solution. We find, for -ve branch wormhole solutions satisfy the null energy conditions (NEC) throughout the entire space time and +ve branch reduces exactly to Morris-Thorne of GR.
In this work, we have considered the recently proposed new Tsallis Agegraphic Dark Energy model (NTADE) (Mod. Phys. Lett. A 34, 1950086, 2019) within the framework of a flat Friedmann-Robertson-Walker(FRW) Universe by taking various values of the parameter $δ$. The NTADE model shows the current phase transition of the Universe from decelerated to accelerated phase. The NTADE EoS parameter shows a rich behaviour as it can be quintessence-like or phantom-like depending on the value of $δ$. For discriminating the NTADE model from $Λ$CDM, we have plotted the statefinder parameters $r(z)$, $s(z)$ and $(r, s)$, $(r, q)$ pair. The NTADE model shows distinct evolutionary trajectories of their evolution in ($ r, s$) and ($ r, q$) plane. An analysis using the snap parameter and the $ω_{D}-ω_{D}^{'}$ pair dynamical analysis have also been performed.
In this work, we explore the recently proposed new Tsallis agegraphic dark energy model in a flat FLRW Universe by taking the conformal time as IR cutoff with interaction. The deceleration parameter of the interacting new Tsallis agegraphic dark energy model provides the phase transition of the Universe from decelerated to accelerated phase. The EoS parameter of the model shows a rich behaviour as it can be quintessence-like or phantom-like depending on the interaction ($b^2$) and parameter $B$. The evolutionary trajectories of the statefinder parameters and $(ω_D, ω_D^{'})$ planes are plotted by considering the initial condition $Ω_{D}^{0} =0.73$, $H_{0}= 67$ according to $Λ$CDM observational Planck 2018 data for different $b^2$ and $B$. The model shows both quintessence and Chaplygin gas behaviour in the statefinder $(r, s)$ and $(r, q)$ pair planes for different $b^2$ and $B$.
In this work, we propose a new dark energy model by applying the Barrow entropy and the holographic principle, with a time scale as IR cut off. Analysing the conformal time as well as universe's age as infrared cut-offs, we explore the cosmological importance of the suggested dark energy models and examine the universe evolution filled with the proposed DE applicants and a pressure-less matter. We observe that the equation of state, deceleration, the density parameters can present adequate nature, and these models may also explain the late-time acceleration though, the proposed models are unstable except some values of Barrow exponent $Δ$. Furthermore, we mention the consequences of the presence of interaction among the universe sectors.
In the present work, we study the R$\acute{e}$nyi holographic dark energy model (RHDE) in a flat FRW Universe where the infrared cut-off is taken care by the Hubble horizon and also by taking three different parametrizations of the interaction term between the dark matter and the dark energy. Analyzing graphically, the behavior of some cosmological parameters in particular deceleration parameter, equation of state (EoS) parameter, energy density parameter and squared speed of sound, in the process of the cosmic evolution, is found to be leading towards the late-time accelerated expansion of the RHDE model.
In this work, we propose a non-interacting model of Barrow holographic dark energy (BHDE) using Barrow entropy in a spatially flat FLRW Universe considering the IR cutoff as the Hubble horizon. We study the evolutionary history of important cosmological parameters, in particular, EoS $(ω_{B})$, deceleration parameter and, the BHDE and matter density parameter and also observe satisfactory behaviours in the BHDE the model. In addition, to describe the accelerated expansion of the Universe the correspondence of the BHDE model with the quintessence scalar field has been reconstructed.
In this work, we use the statefinder parameter diagnostic to the Holographic principal inspired dark energy models, taking into consideration, the Tsallis Holographic dark energy (THDE) model, the standard Holographic dark energy (HDE) model, and the R$\acute{e}$nyi holographic dark energy (RHDE) model. The evolutionary behaviour of first statefinder $r(z)$, second statefinder parameter $s(z)$, the statefinder parameter pairs $(r, s)$ and $(r, q)$ as well as the deceleration parameter $q(z)$ are plotted for comparison for the various parameter values of the respective dark energy models. In the low redshift region, it is observed from these plots that the Tsallis holographic dark energy (THDE) model and R$\acute{e}$nyi holographic dark energy (RHDE) model approach to the $Λ$CDM model. While for the HDE model, the evolutionary behaviour can be differentiated from the $Λ$CDM model in the low-redshift region. For all three dark energy models, a direct comparison in $q(z)$, $r(z)$, $s(z)$, $(r, s)$ and $(r, q)$ plane have also been done, in which the discrimination between these three models wif the $Λ$CDM model) may be easily seen.