Hubble Horizon Research Articles

Large-misalignment mechanism for the formation of compact axion structures: Signatures from the QCD axion to fuzzy dark matter

Axions are some of the best motivated particles beyond the Standard Model. We show how the attractive self-interactions of dark matter (DM) axions over a broad range of masses, from $10^{-22}$ eV to $10^7$ GeV, can lead to nongravitational growth of density fluctuations and the formation of bound objects. This structure formation enhancement is driven by parametric resonance when the initial field misalignment is large, and it affects axion density perturbations on length scales of order the Hubble horizon when the axion field starts oscillating, deep inside the radiation-dominated era. This effect can turn an otherwise nearly scale-invariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalar-field configurations such as solitons and oscillons. We call this class of cosmological scenarios for axion DM production "the large-misalignment mechanism." We explore observational consequences of this mechanism for axions with masses up to $10$ eV. For axions heavier than $10^{-5}$ eV, the compact axion halos are numerous enough to significantly impact Earth-bound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade and crossing times less than a day. These episodic increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including for the QCD axion. Dense structures made up of axions from $10^{-22}$ eV to $10^{-5}$ eV are detectable through gravitational lensing searches, and their gravitational interactions can also perturb baryonic structures and alter star formation. At very high misalignment amplitudes, the axion field can undergo self-interaction-induced implosions long before matter-radiation equality, producing potentially-detectable low-frequency stochastic gravitational waves.

Trans-Planckian censorship conjecture from the swampland distance conjecture

Starting from the swampland distance conjecture, and using the species bound for a large number of weakly-coupled particles, we give a derivation of the recently proposed trans-Planckian censorship conjecture. Our argument demonstrates how a quantum gravity principle requires that trans-Planckian quantum fluctuations should never cross the Hubble horizon. We also comment on how logarithmic corrections to the de-Sitter conjecture arise naturally from such an approach when one relaxes the requirement of traversing parametrically large distances on field space.

Cosmological evolution of Tsallis holographic dark energy model in LTB inhomogeneous gravity

The aim of this work is to analyze the modified form of holographic dark energy (DE) density suggested by Tsallis and Cirto within inhomogeneous models of the universe. This DE model is applied to IR cutoff equals the Hubble horizon. Some basic cosmological functions, such as the Hubble function, the deceleration function and the equation of state parameter, are reconstructed by choosing specific values of certain numerical parameters in this model. The stability of the model is studied by considering the square of sound speed and the strong, the weak and the dominant energy conditions (DEC). In addition, the variation of the diagnostic Omega, [Formula: see text] and [Formula: see text] parameters with the redshift and space are studied. The obtained results show a close behavior to that of the cosmological constant DE universe so it can be compared well with the recent observations.

Tsallis holographic dark energy models in axially symmetric space time

In this work, we have examined the behavior of Bianchi-I (axially symmetric) matter-dominated and the anisotropic Universe with the proposed dark energy, Tsallis holographic dark energy (THDE), with the Hubble horizon as infrared cut-off [Tavayef et al., Tsallis holographic dark energy, Phys. Lett. B 781 (2018) 195–200]. The Universe evolution from matter-dominated epoch to dark energy dominated epoch is described by our proposed THDE model. The EoS parameter in our THDE model explains the evolution of the Universe according to the value of nonextensive or Tsallis parameter [Formula: see text], phantom era ([Formula: see text]) or quintom (phantom line crossing) and the quintessence era ([Formula: see text]), before reaching to completely dark energy-dominated era in the future. Additionally, we also plan to reconcile the dark energy by the method of reconstructing the evolution of the scalar field potential. For the analysis, we take into account the quintessence field and phantom scalar field for this reconstruction, which at present shows the accelerated expansion.

Present value of the Universe's acceleration

The present value of the Universe's acceleration is deduced using the uncertainty principle of Heisenberg and the vacuum fluctuations that cause the appearance of a particle-antiparticle pair close to cosmic or Hubble horizon. Moreover using the generalized uncertainty principle the first correction to the present value of the Universe's acceleration is provided.

Trans-Planckian Censorship and single-field inflaton potential

It was recently proposed that a field theory cannot be consistent with quantum gravity if it allows a mode shorter than the Planck length to exit the Hubble horizon. This is called the Trans-Planckian Censorship Conjecture (TCC). We discuss the implications of the TCC on the possible shape of the inflaton potential in single-field slow-roll inflation. We point out that (1) there is generically an initial condition in which the total e-folding number Ntotal is doubled or more compared to the e-folds necessary for the cosmic microwave background fluctuations, and (2) a sizable negative running of spectral index is generically expected to make Ntotal small. In concrete setups, we find a stringent constraint on the inflationary energy scale, Vinf1/4 < \U0001d4aa(10) TeV with r < \U0001d4aa(10−50), and the running parameter is bounded above as αs ≲ − 4 × 10−3.

Cosmological and thermodynamics analysis in Weyl gravity

In the framework of modified Weyl gravity, we observe the equilibrium picture of the thermodynamical laws for flat Friedmann–Robertson–Walker metric with chameleon scalar field and analyze the validity of the generalized second law of thermodynamics and thermal equilibrium condition for Hubble horizon along with Bekenstien–Hawking entropy. Also, we examine the effective equation of state parameter as well as the square speed of sound. By assuming four different choices of deceleration parameter, we investigate the behavior of equation of state parameter as well as the square speed of sound. The validity of generalized second law of thermodynamics and thermal equilibrium condition is also checked by taking the observational values of the model parameters from CC+H_o dataset.

Observational constraint on interacting Tsallis holographic dark energy in logarithmic Brans\u2013Dicke theory

In this paper, we investigate the dark energy phenomenon by studying the Tsallis holographic dark energy within the framework of Brans–Dicke (BD) scalar–tensor theory of gravity (Brans and Dicke in Phys. Rev. 124:925, 1961). In this context, we choose the BD scalar field phi as a logarithmic function of the average scale factor a(t) and Hubble horizon as the IR cutoff (L=H^{-1}). We reconstruct two cases of non-interacting and interacting fluid (dark sectors of cosmos) scenario. The physical behavior of the models are discussed with the help of graphical representation to explore the accelerated expansion of the universe. Moreover, the stability of the models are checked through squared sound speed v_s^2. The well-known cosmological plane i.e., omega _{de}-omega ^{prime }_{de} is constructed for our models. We also include comparison of our findings of these dynamical parameters with observational constraints. It is also quite interesting to mention here that the results of deceleration, equation of state parameters and omega _{de}-omega ^{prime }_{de} plane coincide with the modern observational data.

Linear perturbations of an anisotropic Bianchi\xa0I model with a uniform magnetic field

In this work, we study the effect of a magnetic field on the growth of cosmological perturbations. We develop a mathematical consistent treatment in which a perfect fluid and a uniform magnetic field evolve together in a Bianchi I universe. We then study the energy density perturbations on this background with particular emphasis on the effect of the background magnetic field. We develop a full relativistic solution which refines previous analysis in the relativistic limit, recovers the known ones in the Newtonian treatment with adiabatic sound speed, and it adds anisotropic effects to the relativistic ones for perturbations with wavelength within the Hubble horizon. This represents a refined approach on the perturbation theory of an isotropic universe in GR, since most of the present studies deal with fully isotropic systems.

Viability of specific reconstructed f(T,𝒯 ) models

The reconstruction scenario of well-established dark energy models such as pilgrim dark energy model and generalized ghost dark energy with Hubble horizon and [Formula: see text] models is being considered. We have established [Formula: see text] models and analyzed their viability through equation of state parameter and [Formula: see text] (where prime denotes derivative with respect to [Formula: see text]) plane. The equation of state parameter evolutes the universe in three different phases such as quintessence, vacuum and phantom. However, the [Formula: see text] plane also describes the thawing as well as freezing region of the universe. The recent observational data also favor our results.

Observational constraints on interacting Tsallis holographic dark energy model

In this paper, we investigate a recent proposed model – so called the Tsallis holographic dark energy (THDE) model with consideration of the Hubble and the event future horizon as IR cutoffs. In this case, we consider the non-gravitational and phenomenological interaction between dark sectors. We fit the free parameters of the model using Pantheon Supernovae Type Ia data, Baryon Acoustic Oscillations, Cosmic Microwave Background, Gamma-Ray burst and the the local value of the Hubble constant. We examine the THDE model to check its compatibility with observational data using objective Information Criterion (IC). We find that the THDE models cannot be supported by observational data once the Lambda CDM is considered as the referring model. Therefore we re-examine the analysis with the standard holographic dark energy model (HDE) as another reference. Changing the Lambda CDM to main standard dark energy model (HDE), we observe the compatibility of the THDE models. Using the Alcock–Paczynski (AP) test we check the deviation of the model compared to Lambda CDM and HDE. Surveying the evolution of squared of sound speed v^2_s as an another test we check the stability of the interacting and non-interacting THDE models and we find that while the THDE model with the Hubble horizon as IR cutoff is unstable against the background perturbation, the future event horizon as IR cutoff show stability at the late time. In addition, using the modified version of the CAMB package, we observe the suppressing the CMB spectrum at small K-modes and large scale.

Tsallis holographic dark energy in Bianchi-I Universe using hybrid expansion law with k-essence

In this paper, the proposed dark energy, Tsallis holographic dark energy (THDE), infrared cut-off with the Hubble horizon has been investigated in the Bianchi-I (axially symmetric) anisotropic model with a hybrid expansion law. It has been observed that the THDE is in tune with the accelerating Universe with equation of state (EoS) parameter ( $$\omega _{\mathrm {T}}<-1/3$$ ) in the k-essence region. We have used the statefinder diagnostic in our model. In addition, we try to accommodate the perspective of dark energy by the avenue of reconstructing the evolution of scalar field potential. We have considered the k-essence for the analysis of this reconstruction, showing the accelerated expansion at present.

Enhancement of gravitational waves induced by scalar perturbations due to a sudden transition from an early matter era to the radiation era

We study gravitational waves induced from the primordial scalar perturbations at second order around the reheating of the Universe. We consider reheating scenarios in which a transition from an early matter dominated era to the radiation dominated era completes within a timescale much shorter than the Hubble time at that time. We find that an enhanced production of induced gravitational waves occurs just after the reheating transition because of fast oscillations of scalar modes well inside the Hubble horizon. This enhancement mechanism just after an early matter-dominated era is much more efficient than a previously known enhancement mechanism during an early matter era, and we show that the induced gravitational waves could be detectable by future observations if the reheating temperature $T_{\text{R}}$ is in the range $T_\text{R} \lesssim 7\times 10^{-2}$GeV or $20 \, \text{GeV} \lesssim T_\text{R} \lesssim 2 \times 10^7 \, \text{GeV}$. This is the case even if the scalar perturbations on small scales are not enhanced relative to those on large scales, probed by the observations of the cosmic microwave background.

Cosmological implications of Tsallis dark energy in modified Brans–Dicke theory

Tsallis entropy has been widely applied to analyze the gravitational and cosmological setups. We discuss the dark energy (DE) model by its cosmological consequences using Tsallis holographic entropy in the framework of modified Brans–Dicke (BD) gravity. We consider the Hubble horizon as infrared cutoff to study the nature of DE that is responsible for current cosmic acceleration. We focus on flat FRW universe in interacting and noninteracting scenarios between DE and dark matter (DM). In this framework, we discuss the cosmological parameters like equation of state parameter, deceleration parameter, Om-diagnostic, squared speed of sound and planes like evolving equation of state parameter and statefinders. We discuss graphical presentation of these parameters and planes. We compare the results with observation data to check the consistency of results.

Cosmological consequences and thermodynamics of modified gravity with extended nonminimal derivative couplings

We consider the newly proposed gravitational modifications that go beyond Horndeski’s theory, named as theories with extended nonminimal derivative couplings. By these modifications, the coefficient functions depend on the scalar field and its kinetic energy. These theories become ghost-free in cosmological background. We consider the flat FRW universe and explore the equation-of-state parameter, [Formula: see text]–[Formula: see text] plane and the squared speed of sound. The equation-of-state parameter exhibits phantom behavior of the universe, [Formula: see text]–[Formula: see text] plane represents the freezing region of the universe while the squared speed of sound denotes the stability of the model for the specific choice of constant parameters. Also, we investigate the validity of generalized second law of thermodynamics on the Hubble horizon taking into account the Bekenstein, power-law, Renyi and logarithmic corrections to the horizon entropy.

Tsallis, Renyi and Sharma–Mittal holographic dark energy models in DGP brane-world

Abstract Newly proposed holographic dark energy models such as Tsallis, Renyi and Sharma–Mittal are being studied in order to evaluate the accelerated expansion of the universe. For this purpose, we consider the flat FRW universe filled with interacting dark energy models and cold dark matter within the framework of DGP braneworld. By assuming the Hubble horizon as an infrared cutoff, we explore the different cosmological quantities like the equation of state parameter, deceleration parameter, squared speed of sound, statefinder parameters, ω − ω ′ plane and Om diagnostic. The equation of state parameter and deceleration parameter support observational data in all cases. The squared speed of sound exhibits the stability of all the models. The statefinder parameters, ω − ω ′ plane and Om diagnostic exhibit the accelerated expansion of the universe in all models and also correspond to Λ CDM model.

Tsallis holographic dark energy in FRW universe with time varying deceleration parameter

By considering the time-varying deceleration parameter (DP), we have investigated Tsallis holographic dark energy (THDE), infrared cut-off with the Hubble horizon proposed by Tavayef, et al. (2018) in the framework of Friedmann Robertson Walker universe. The proposed THDE model describes that the universe is in an accelerating expansion mode. Interestingly enough, the EoS parameter in our model is representing the outstanding evolution of the universe as, the quintessence era (ωT≥−1), phantom era (ωT<−1), according to the value of dimensionless parameter δ. To analyze the different structure of the model, we try to accommodate the two perspectives of (DE) by reconstructing the scalar field potential, which depicts the accelerated expansion at present. We examine the validity of the model by drawing the statefinder parameters and comparing them with the recent observational outcomes of cosmography.

Quantum Hubble horizon

The article addresses the possibility of obtaining cosmologically relevant effects from the quantum nature of the Hubble horizon. Following the observation made by Bianchi and Rovelli (Phys Rev D 84:027502, 2011) we explore the relationship between the Planck scale discreteness of the Hubble horizon and deformations of the symmetry of rotations. We show that the so-called q-deformations in a natural way lead to a mechanism of condensation in the very early Universe. We argue that this provides a possible resolution of the problem of the initial homogeneity at the onset of inflation. Furthermore, we perform an analysis in terms of entropy of the quantum Hubble horizon and show that the Lambda CDM model may arise from a linearly corrected Bekenstein–Hawking entropy (that is, linearly in area of the horizon). Based on this, we show that the current accelerating expansion can be associated with the entropy decrease in the Hubble volume. The results presented open new ways to explore the relation between the Planck scale effects and observationally relevant features of our Universe.

Thermodynamic analysis of modified teleparallel gravity involving higher-order torsion derivative terms

The present study is elaborated to investigate the validity of thermodynamical laws in a modified teleparallel gravity based on higher-order derivative terms of torsion scalar. For this purpose, we consider spatially flat FRW model filled with perfect fluid matter contents. Firstly, we explore the possibility of existence of equilibrium as well as non-equilibrium picture of thermodynamics in this extended version of teleparallel gravity. Here, we present the first law and the generalized second law of thermodynamics (GSLT) using Hubble horizon. It is found that non-equilibrium description of thermodynamics exists in this theory with the presence of an extra term called as entropy production term. We also establish GSLT using the logarithmic corrected entropy. Further, by taking the equilibrium picture, we discuss validity of GSLT at Hubble horizon for two different F models. Using Gibbs law and the assumption that temperature of matter within Hubble horizon is similar to itself, We use different choices of scale factor to discuss the GSLT validity graphically in all scenarios. It is found that the GSLT is satisfied for a specified range of free parameters in all cases.

Holographic dark energy: constraints on the interaction from diverse observational data sets

The present work deals with holographic dark energy models with Hubble horizon as the infra-red cut-off. The interaction rate between dark energy and dark matter has been reconstructed with three different choices of the interaction term. It is shown that the coupling parameter of the interaction term should evolve with redshift to allow the successful transition from decelerated to accelerated phase of expansion. Constraints on the model parameters are obtained from Markov Chain Monte Carlo (MCMC) analysis using the supernova distance modulus data and observational measurements of the Hubble parameter. Results show that the model with the coupling parameter increasing with redshift (z) or equivalently decreasing with the evolution, are ruled out. On the other hand, coupling parameters, increasing or slowly varying with the evolution, are consistent with the observed evolution scenario. A Bayesian evidence calculation has been carried out for statistical selection of the reconstructed models. Though the kinematical parameters are well behaved for these models, the physical variables which determine the nature of the components in the matter sector, are not at all realistic. We have concluded that the existence of spatial curvature is essential for this particular type of dark energy models.