Inflationary quantum dynamics and backreaction using a classical-quantum correspondence

We study inflationary dynamics using a recently introduced classical-quantum correspondence for investigating the backreaction of a quantum mechanical degree of freedom to a classical background. Using specifically a coupled Einstein–Klein–Gordon system, an approximation that holds well during the very early inflationary era when modes are very deep inside the Hubble horizon, we show that the backreaction of a mode of the quantum field will renormalize the Hubble parameter only if the mode’s wavelength is longer than some threshold Planckian length scale. Otherwise, the mode will destabilize the inflationary era. We also present an approximate analytical solution that supports the existence of such short-wavelength threshold and compare the results of the classical-quantum correspondence with the traditional perturbative-iterative method in semiclassical gravity.

Growth rate and configurational entropy in Tsallis holographic dark energy

In this work, we analyzed the effect of different prescriptions of the IR cutoffs, namely the Hubble horizon cutoff, particle horizon cutoff, Granda and Oliveros horizon cut off, and the Ricci horizon cutoff on the growth rate of clustering for the Tsallis holographic dark energy (THDE) model in an FRW universe devoid of any interactions between the dark Universe. Furthermore, we used the concept of configurational entropy to derive constraints (qualitatively) on the model parameters for the THDE model in each IR cutoff prescription from the fact that the rate of change of configurational entropy hits a minimum at a particular scale factor $$a_{DE}$$ a DE which indicate precisely the epoch of dark energy domination predicted by the relevant cosmological model as a function of the model parameter(s). By using the current observational constraints on the redshift of transition from a decelerated to an accelerated Universe, we derived constraints on the model parameters appearing in each IR cutoff definition and on the non-additivity parameter $$\delta $$ δ characterizing the THDE model and report the existence of simple linear dependency between $$\delta $$ δ and $$a_{DE}$$ a DE in each IR cutoff setup.

Modeling the dynamic evolution of the Universe under the action of the entropic force associated with the modified Sharma-Mittal entropy

Using the Verlind formalism, the paper considers several scenarios of the evolution of the Friedman-Robertson-Walker Universe, which arise in the framework of entropic cosmology based on the formulated new modification of the Sharma-Mittal entropy. The research, carried out in the framework of non-Gaussian statistical theory, uses several entropies associated with the surface of the horizon of the Universe due to the holographic information stored there. A set of new generalized Friedmann equations is obtained, in which, instead of the cosmological constant, control forces appear based on the Bekenstein-Hawking, Tsallis-Chirto and Barrow entropies, as well as modified Sharma-Mittal and Renyi entropies containing additional nonextensity parameters. The proposed approach, associated with the use of probabilistic nonextensive aspects of the Hubble horizon of the surface of the Universe, meets all the basic requirements for thermodynamic modeling of the dynamic behavior of outer space without involving the concept of dark energy.

Reconstruction of quintessence field for the THDE with swampland correspondence in f(R,T) gravity

In the present work, we construct the Tsallis holographic quintessence model of dark energy in [Formula: see text] gravity with Hubble horizon as infrared (IR) cut-off. In a flat Friedmann–Robertson–Walker (FRW) background, the correspondence among the energy density of the quintessence model with the Tsallis holographic density permits the reconstruction of the dynamics and the potentials for the quintessence field. The suggested Hubble horizon IR cut-off for the Tsallis holographic dark energy (THDE) density acts for two specific cases: (i) THDE 1 and (ii) THDE 2. We have reconstructed the Tsallis holographic quintessence model in the region [Formula: see text] for the equation of state (EoS) parameter for both the cases. we investigate the behavior of several well-known statefinder quantities, like the deceleration parameter, the jerk and the parameter [Formula: see text]. In addition, the quintessence phase of the THDE models is analyzed with swampland conjecture to describe the accelerated expansion of the Universe.

Barrow holographic dark energy with hubble horizon as IR cutoff

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, deceleration parameter, equation of state (EoS) parameter, the BHDE and matter density parameter, and also observe satisfactory behaviors in the BHDE model. The stability of the BHDE model has been examined by squared sound speed [Formula: see text]. In addition, to describe the accelerated expansion of the Universe, the correspondence of the BHDE model with the quintessence scalar field has been reconstructed.

Rényi holographic dark energy in the Brans–Dicke cosmology

In this paper, we construct a holographic dark energy (HDE) model considering the IR cut-off as Hubble horizon, holographic hypothesis, and using the generalized Rényi entropy, and investigate its cosmological outcomes in Brans–Dicke gravity without interaction. We observe the suitable behavior for the cosmological parameters, involving the deceleration parameter, the equation of state (EoS) parameter, and the density parameter in both flat and non-flat Universes. It is also concluded by the stability analysis that the Rényi holographic dark energy (RHDE) model is classically stable at present and future for the Rényi parameter [Formula: see text] in both flat and non-flat Universe.

Screening away the H0 tension

This Essay explores consequences of a dark nonlinear electromagnetic sector in a universe with a net dark charge for matter. The cosmological dynamics can be described by a Lemaître model and can be understood, thanks to a screening mechanism driven by the electromagnetic nonlinearities that suppress the dark force on small scales. Only at low redshift, when the screening scale enters the Hubble horizon, do cosmological structures commence to feel the dark repulsion. This repulsive force enhances the local value of the Hubble constant, thus providing a promising scenario for solving the Hubble tension. Remarkably, the dark electromagnetic interaction can have a crucial impact on peculiar velocities, i.e. introducing a bias in their reconstruction methods, and having the potential to explain the presence of a dark flow.

Trans-Planckian Censorship and the Swampland

In this paper, we propose a new Swampland condition, the Trans-Planckian Censorship Conjecture (TCC), based on the idea that in a consistent quantum theory of gravity sub-Planckian quantum fluctuations should remain quantum and never become larger than the Hubble horizon and freeze in an expanding universe. Applied to the case of scalar fields, it leads to conditions that are similar to the refined dS Swampland conjecture. For large field ranges, TCC is stronger than the dS Swampland conjecture but it is weaker for small field ranges. In particular for asymptotic regions of field space, TCC leads to a bound $$ \left|V^{\prime}\right|\ge \frac{2}{\sqrt{\left(d-1\right)\left(d-2\right)}}V, $$ V ′ ≥ 2 d − 1 d − 2 V , which is consistent with all known cases in string theory. Like the dS Swampland conjecture, the TCC forbids long-lived meta-stable dS spaces, but it does allow sufficiently short-lived ones.

Role of trans-Planckian modes in cosmology

Motivated by the old trans-Planckian (TP) problem of inflationary cosmology, it has been conjectured that any consistent effective field theory should keep TP modes ‘hidden’ behind the Hubble horizon, so as to prevent them from turning classical and thereby affecting macroscopic observations. In this paper we present two arguments against the Hubble horizon being a scale of singular significance as has been put forward in the TP Censorship Conjecture (TCC). First, refinements of TCC are presented that allow for the TP modes to grow beyond the horizon while still keeping the de-Sitter conjecture valid. Second, we show that TP modes can turn classical even well within the Hubble horizon, which, as such, negates this rationale behind keeping them from crossing it. The role of TP modes is known to be less of a problem in warm inflation, because fluctuations start out usually as classical. This allows warm inflation to be more resilient to the TP problem compared to cold inflation. To understand how robust this is, we identity limits where quantum modes can affect the primordial power spectrum in one specific case.

Swampland criteria and cosmological behavior of Tsallis holographic dark energy in Bianchi -III Universe

In this paper, a new form of dark energy, known as Tsallis holographic dark energy (THDE), with IR cutoff as Hubble horizon proposed by Tavayef et al. Tsallis holographic dark energy, Phys. Lett. B 781 (2018) 195 has been explored in Bianchi-III model with the matter. By taking the time subordinate deceleration parameter, the solution of Einstein’s field equation is found. The Universe evolution from earlier decelerated to the current accelerated phase is exhibited by the deceleration parameter acquired in the THDE model. It can be seen that the derived THDE model is related to an accelerating Universe with quintessence ([Formula: see text]). The squared sound speed [Formula: see text] also suggests that the THDE model is classically stable at present. In addition, the quintessence phase of the THDE model is analyzed with swampland conjecture to reformulate the accelerating expansion of the Universe.