Barrow holographic dark energy Saridakis, Emmanuel N.
Physical review. D,
12/2020, Volume:
102, Issue:
12
Journal Article
Peer reviewed
Open access
We formulate Barrow holographic dark energy, by applying the usual holographic principle at a cosmological framework, but using the Barrow entropy instead of the standard Bekenstein-Hawking one. The ...former is an extended black-hole entropy that arises due to quantum-gravitational effects which deform the black-hole surface by giving it an intricate, fractal form. We extract a simple differential equation for the evolution of the dark-energy density parameter, which possesses standard holographic dark energy as a limiting subcase, and we show that the scenario can describe the thermal history of the universe, with the sequence of matter and dark-energy eras. Additionally, the new Barrow exponent Δ significantly affects the dark-energy equation of state, and according to its value it can lead it to lie in the quintessence regime, in the phantom regime, or experience the phantom-divide crossing during the evolution.
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We investigate the validity of the generalized second law of thermodynamics, applying Barrow entropy for the horizon entropy. The former arises from the fact that the black-hole surface may be ...deformed due to quantum-gravitational effects, quantified by a new exponent
Δ
. We calculate the entropy time-variation in a universe filled with the matter and dark energy fluids, as well as the corresponding quantity for the apparent horizon. We show that although in the case
Δ
=
0
, which corresponds to usual entropy, the sum of the entropy enclosed by the apparent horizon plus the entropy of the horizon itself is always a non-decreasing function of time and thus the generalized second law of thermodynamics is valid, in the case of Barrow entropy this is not true anymore, and the generalized second law of thermodynamics may be violated, depending on the universe evolution. Hence, in order not to have violation, the deformation from standard Bekenstein–Hawking expression should be small as expected.
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We show that the well-known problem of frame dependence and violation of local Lorentz invariance in the usual formulation of f(T) gravity is a consequence of neglecting the role of spin connection. ...We re-formulate f(T) gravity starting from, instead of the 'pure tetrad' teleparallel gravity, the covariant teleparallel gravity, using both the tetrad and the spin connection as dynamical variables, resulting in a fully covariant, consistent, and frame-independent version of f(T) gravity, which does not suffer from the notorious problems of the usual, pure tetrad, f(T) theory. We present the method to extract solutions for the most physically important cases, such as the Minkowski, the Friedmann-Robertson-Walker (FRW) and the spherically symmetric ones. We show that in covariant f(T) gravity we are allowed to use an arbitrary tetrad in an arbitrary coordinate system along with the corresponding spin connection, resulting always in the same physically relevant field equations.
We present modified cosmological scenarios that arise from the application of the “gravity-thermodynamics” conjecture, using the Barrow entropy instead of the usual Bekenstein-Hawking one. The former ...is a modification of the black hole entropy due to quantum-gravitational effects that deform the black-hole horizon by giving it an intricate, fractal structure. We extract modified cosmological equations which contain new extra terms that constitute an effective dark-energy sector, and which coincide with the usual Friedmann equations in the case where the new Barrow exponent acquires its Bekenstein-Hawking value. We present analytical expressions for the evolution of the effective dark energy density parameter, and we show that the universe undergoes through the usual matter and dark-energy epochs. Additionally, the dark-energy equation-of-state parameter is affected by the value of the Barrow deformation exponent and it can lie in the quintessence or phantom regime, or experience the phantom-divide crossing. Finally, at asymptotically large times the universe always results in the de-Sitter solution.
We present a model of holographic dark energy in which the infrared cutoff is determined by both the Ricci and the Gauss-Bonnet invariants. Such a construction has the significant advantage that the ...infrared cutoff, and consequently the holographic dark energy density, does not depend on the future or the past evolution of the universe, but only on its current features, and moreover it is determined by invariants, whose role is fundamental in gravitational theories. We extract analytical solutions for the behavior of the dark energy density and equation-of-state parameters as functions of the redshift. These reveal the usual thermal history of the universe, with the sequence of radiation, matter and dark energy epochs, resulting in the future to a complete dark energy domination. The corresponding dark energy equation-of-state parameter can lie in the quintessence or phantom regime, or experience the phantom-divide crossing during the cosmological evolution, and its asymptotic value can be quintessencelike, phantomlike, or be exactly equal to the cosmological-constant value. Finally, we extract the constraints on the model parameters that arise from big bang nucleosynthesis.
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We apply Gaussian processes and Hubble function data in f(T) cosmology to reconstruct for the first time the f(T) form in a model-independent way. In particular, using H(z) data sets coming from ...cosmic chronometers as well as from the method of radial baryon acoustic oscillations, alongside the latest released local value of H0 = 73.52 1.62 km s−1 Mpc−1, we reconstruct H(z) and its derivatives, resulting eventually in a reconstructed region for f(T), without any assumption. Although the cosmological constant lies in the central part of the reconstructed region, the obtained mean curve follows a quadratic function. Inspired by this we propose a new f(T) parameterization, i.e., f(T) = −2Λ + T2, with the sole free parameter that quantifies the deviation from ΛCDM cosmology. Additionally, we confront three viable one-parameter f(T) models from the literature, which are the power-law, the square-root exponential, and the exponential models, with the reconstructed f(T) region, and then we extract significantly improved constraints for their model parameters, comparing to the constraints that arise from the usual observational analysis. Finally, we argue that since we are using the direct Hubble measurements and the local value for H0 in our analysis, the H0 tension can be efficiently alleviated with the above reconstruction of f(T).
We present a modified cosmological scenario that arises from the application of non-extensive thermodynamics with varying exponent. We extract the modified Friedmann equations, which contain new ...terms quantified by the non-extensive exponent, possessing standard
Λ
CDM cosmology as a subcase. Concerning the universe evolution at late times we obtain an effective dark energy sector, and we show that we can acquire the usual thermal history, with the successive sequence of matter and dark-energy epochs, with the effective dark-energy equation-of-state parameter being in the quintessence or in the phantom regime. The interesting feature of the scenario is that the above behaviors can be obtained even if the explicit cosmological constant is set to zero, namely they arise purely from the extra terms. Additionally, we confront the model with Supernovae type Ia and Hubble parameter observational data, and we show that the agreement is very good. Concerning the early-time universe we obtain inflationary de Sitter solutions, which are driven by an effective cosmological constant that includes the new terms of non-extensive thermodynamics. This effective screening can provide a description of both inflation and late-time acceleration with the same parameter choices, which is a significant advantage.
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We construct modified cosmological scenarios through the application of the first law of thermodynamics on the universe horizon, but using the generalized, nonextensive Tsallis entropy instead of the ...usual Bekenstein–Hawking one. We result to modified cosmological equations that possess the usual ones as a particular limit, but which in the general case contain extra terms that appear for the first time, that constitute an effective dark energy sector quantified by the nonextensive parameter
δ
. When the matter sector is dust, we extract analytical expressions for the dark energy density and equation-of-state parameters, and we extend these solutions to the case where radiation is present too. We show that the universe exhibits the usual thermal history, with the sequence of matter and dark-energy eras, and according to the value of
δ
the dark-energy equation-of-state parameter can be quintessence-like, phantom-like, or experience the phantom-divide crossing during the evolution. Even in the case where the explicit cosmological constant is absent, the scenario at hand can very efficiently mimic
Λ
CDM
cosmology, and is in excellent agreement with Supernovae type Ia observational data.
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We investigate the generation of baryon asymmetry from the corrections brought about in the Friedman equations due to Barrow entropy. In particular, by applying the gravity-thermodynamics conjecture ...one obtains extra terms in the Friedmann equations that change the Hubble function evolution during the radiation-dominated epoch. Hence, even in the case of standard coupling between the Ricci scalar and baryon current they can lead to a non-zero baryon asymmetry. In order to match observations we find that the Barrow exponent should lie in the interval
0.005
≲
Δ
≲
0.008
, which corresponds to a slight deviation from the standard Bekenstein–Hawking entropy. The upper bound is tighter than the one of other observational constraints, however the interesting feature is that in the present analysis we obtain a non-zero lower bound. Nevertheless this lower bound would disappear if the baryon asymmetry in Barrow-modified cosmology is generated by other mechanisms, not related to the Barrow modification.
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10.
Cosmology in cubic and f ( P ) gravity Erices, Cristian; Papantonopoulos, Eleftherios; Saridakis, Emmanuel N.
Physical review. D,
06/2019, Volume:
99, Issue:
12
Journal Article
Peer reviewed
We construct cubic gravity and its f(P) extension and we investigate their early- and late-time cosmological applications. Cubic gravity is based on a particular invariant P, constructed from cubic ...contractions of the Riemann tensor, under three requirements: (i) the resulting theory possesses a spectrum identical to that of general relativity, (ii) it is neither topological nor trivial in four dimensions, and (iii) it is defined such that it is independent of the dimensions. Relaxing the last condition and restricting the parameters of cubic gravity we can obtain second-order field equations in a cosmological background. We show that at early times one can obtain inflationary, de Sitter solutions, which are driven by an effective cosmological constant constructed purely from the cubic terms of the simple cubic or f(P) gravity. Concerning late-time evolution, the new terms constitute an effective dark-energy sector and we show that the Universe experiences the usual thermal history and the onset of late-time acceleration. In the case of f(P) gravity, depending on the choice of parameters, we find that the dark-energy equation-of-state parameter can be quintessencelike or phantomlike or it can experience the phantom-divide crossing during the evolution, even if an explicit cosmological constant is absent.
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