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Register a new userObservational constraints on Barrow holographic dark energy
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We use observational data from Supernovae (SNIa) Pantheon sample, as well as from direct measurements of the Hubble parameter from the cosmic chronometers (CC) sample, in order to extract constraints on the scenario of Barrow holographic dark energy. The latter is a holographic dark energy model based on the recently proposed Barrow entropy, which arises from the modification of the black-hole surface due to quantum-gravitational effects. We first consider the case where the new deformation exponent $Delta$ is the sole model parameter, and we show that although the standard value $Delta=0$, which corresponds to zero deformation, lies within the 1$sigma$ region, a deviation is favored. In the case where we let both $Delta$ and the second model parameter to be free we find that a deviation from standard holographic dark energy is preferred. Additionally, applying the Akaike, Bayesian and Deviance Information Criteria, we conclude that the one-parameter model is statistically compatible with $Lambda$CDM paradigm, and preferred comparing to the two-parameter one. Finally, concerning the present value of the Hubble parameter we find that it is close to the Planck value.
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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 sub-case, and we show that the scenario can describe the universe thermal history, with the sequence of matter and dark energy eras. Additionally, the new Barrow exponent $Delta$ 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.
We use Big Bang Nucleosynthesis (BBN) data in order to impose constraints on the exponent of Barrow entropy. The latter is an extended entropy relation arising from the incorporation of quantum-gravitational effects on the black-hole structure, parameterized effectively by the new parameter $Delta$. When considered in a cosmological framework and under the light of the gravity-thermodynamics conjecture, Barrow entropy leads to modified cosmological scenarios whose Friedmann equations contain extra terms. We perform a detailed analysis of the BBN era and we calculate the deviation of the freeze-out temperature comparing to the result of standard cosmology. We use the observationally determined bound on $ |frac{delta {T}_f}{{T}_f}|$ in order to extract the upper bound on $Delta$. As we find, the Barrow exponent should be inside the bound $Deltalesssim 1.4times 10^{-4}$ in order not to spoil the BBN epoch, which shows that the deformation from standard Bekenstein-Hawking expression should be small as expected.
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 quintessence-like, phantom-like, or be exactly equal to the cosmological-constant value. Finally, we extract the constraints on the model parameters that arise from Big Bang Nucleosynthesis.
In order to apply holography and entropy relations to the whole universe, which is a gravitational and thus nonextensive system, for consistency one should use the generalized definition for the universe horizon entropy, namely Tsallis nonextensive entropy. We formulate Tsallis holographic dark energy, which is a generalization of standard holographic dark energy quantified by a new dimensionless parameter $delta$, possessing the latter as a particular sub-case. We provide a simple differential equation for the dark energy density parameter, as well as an analytical expression for its equation-of-state parameter. In this scenario the universe exhibits the usual thermal history, namely the successive sequence of matter and dark-energy epochs, before resulting in a complete dark energy domination in the far future. Additionally, the dark energy equation-of-state parameter presents a rich behavior and, according to the value of $delta$, it can be quintessence-like, phantom-like, or experience the phantom-divide crossing before or after the present time. Finally, we confront the scenario with Supernovae type Ia and Hubble parameter observational data, and we show that the agreement is very good, with $delta$ preferring a value slightly larger than its standard value 1.
We use data from Supernovae (SNIa) Pantheon sample, from Baryonic Acoustic Oscillations (BAO), and from cosmic chronometers measurements of the Hubble parameter (CC), alongside arguments from Big Bang Nucleosynthesis (BBN), in order to extract constraints on Myrzakulov $F(R,T)$ gravity. This is a connection-based theory belonging to the Riemann-Cartan subclass, that uses a specific but non-special connection, which then leads to extra degrees of freedom. Our analysis shows that both considered models lead to $sim 1 sigma$ compatibility in all cases. For the involved dimensionless parameter we find that it is constrained to an interval around zero, however the corresponding contours are slightly shifted towards positive values. Furthermore, we use the obtained parameter chains so to reconstruct the corresponding Hubble function, as well as the dark-energy equation-of-state parameter, as a function of redshift. As we show, Model 1 is very close to $Lambda$CDM scenario, while Model 2 resembles it at low redshifts, however at earlier times deviations are allowed. Finally, applying the AIC, BIC and the combined DIC criteria, we deduce that both models present a very efficient fitting behavior, and are statistically equivalent with $Lambda$CDM cosmology, despite the fact that Model 2 does not contain the latter as a limit.
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