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.
In this article, we elaborate further on the
Λ
CDM “tension”, suggested recently by the authors Lusso et al. (Astron Astrophys 628:L4, 2019) and Risaliti and Lusso (Nat Astron 3(3):272, 2019). We ...combine Supernovae type Ia (SNIa) with quasars (QSO) and Gamma Ray Bursts (GRB) data in order to reconstruct in a model independent way the Hubble relation to as high redshifts as possible. Specifically, in the case of either SNIa or SNIa/QSO data we find that the
current values of the
cosmokinetic parameters extracted from the Gaussian process are consistent with those of
Λ
CDM. Including GRBs in the analysis we find a tension, which lies between
2
σ
and
3
σ
levels respectively. Finally, we find that at high redshifts (
z
>
1
) the corresponding cosmokinetic parameters significantly deviate from those of
Λ
CDM, hence the possibility of new Physics is not precluded by the present analysis.
We propose a novel model in the framework of f(Q) gravity, which is a gravitational modification class arising from the incorporation of non-metricity. The model has General Relativity as a ...particular limit, it has the same number of free parameters to those of ΛCDM, however at a cosmological framework it gives rise to a scenario that does not have ΛCDM as a limit. Nevertheless, confrontation with observations at both background and perturbation levels, namely with Supernovae type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), cosmic chronometers (CC), and Redshift Space Distortion (RSD) data, reveals that the scenario, according to AIC, BIC and DIC information criteria, is in some datasets slightly preferred comparing to ΛCDM cosmology, although in all cases the two models are statistically indiscriminate. Finally, the model does not exhibit early dark energy features, and thus it immediately passes BBN constraints, while the variation of the effective Newton's constant lies well inside the observational bounds.
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
Δ
is the sole model parameter, and we show that although the standard value
Δ
=
0
, which corresponds to zero deformation, lies within the
1
σ
region, a deviation is favored. In the case where we let both
Δ
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
Λ
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.
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 Δ. 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 |δTfTf| in order to extract the upper bound on Δ. As we find, the Barrow exponent should be inside the bound Δ≲1.4×10−4 in order not to spoil the BBN epoch, which shows that the deformation from standard Bekenstein-Hawking expression should be small as expected.
In this paper we present conjoined constraints on several cosmological models from the expansion history H(z) and cosmic growth fσ8. The models we study include the CPL w0wa parametrization, the ...holographic dark energy (HDE) model, the time-varying vacuum (ΛtCDM) model, the Dvali, Gabadadze and Porrati (DGP) and Finsler-Randers (FRDE) models, a power-law f(T) model, and finally the Hu-Sawicki f(R) model. In all cases we perform a simultaneous fit to the SnIa, CMB, BAO, H(z) and growth data, while also following the conjoined visualization of H(z) and fσ8 as in Linder (2017). Furthermore, we introduce the figure of merit (FoM) in the H(z)−fσ8 parameter space as a way to constrain models that jointly fit both probes well. We use both the latest H(z) and fσ8 data, but also LSST-like mocks with 1% measurements, and we find that the conjoined method of constraining the expansion history and cosmic growth simultaneously is able not only to place stringent constraints on these parameters, but also to provide an easy visual way to discriminate cosmological models. Finally, we confirm the existence of a tension between the growth-rate and Planck CMB data, and we find that the FoM in the conjoined parameter space of H(z)−fσ8(z) can be used to discriminate between the ΛCDM model and certain classes of modified gravity models, namely the DGP and f(T).
Modified cosmology through Kaniadakis horizon entropy Lymperis, Andreas; Basilakos, Spyros; Saridakis, Emmanuel N.
European physical journal. C, Particles and fields,
11/2021, Volume:
81, Issue:
11
Journal Article
Peer reviewed
Open access
We apply the gravity-thermodynamics conjecture, namely the first law of thermodynamics on the Universe horizon, but using the generalized Kaniadakis entropy instead of the standard Bekenstein–Hawking ...one. The former is a one-parameter generalization of the classical Boltzmann–Gibbs–Shannon entropy, arising from a coherent and self-consistent relativistic statistical theory. We obtain new modified cosmological scenarios, namely modified Friedmann equations, which contain new extra terms that constitute an effective dark energy sector depending on the single model Kaniadakis parameter
K
. We investigate the cosmological evolution, by extracting analytical expressions for the dark energy density and equation-of-state parameters and we show that the Universe exhibits the usual thermal history, with a transition redshift from deceleration to acceleration at around 0.6. Furthermore, depending on the value of
K
, the dark energy equation-of-state parameter deviates from
Λ
CDM cosmology at small redshifts, while lying always in the phantom regime, and at asymptotically large times the Universe always results in a dark-energy dominated, de Sitter phase. Finally, even in the case where we do not consider an explicit cosmological constant the resulting cosmology is very interesting and in agreement with the observed behavior.
We study how the cosmological constraints from growth data are improved by including the measurements of bias from Dark Energy Survey (DES). In particular, we utilize the biasing properties of the ...DES Luminous Red Galaxies (LRGs) and the growth data provided by the various galaxy surveys in order to constrain the growth index (
γ
) of the linear matter perturbations. Considering a constant growth index we can put tight constraints, up to
∼
10
%
accuracy, on
γ
. Specifically, using the priors of the Dark Energy Survey and implementing a joint likelihood procedure between theoretical expectations and data we find that the best fit value is in between
γ
=
0.64
±
0.075
and
0.65
±
0.063
. On the other hand utilizing the Planck priors we obtain
γ
=
0.680
±
0.089
and
0.690
±
0.071
. This shows a small but non-zero deviation from General Relativity (
γ
GR
≈
6
/
11
), nevertheless the confidence level is in the range
∼
1.3
-
2
σ
. Moreover, we find that the estimated mass of the dark-matter halo in which LRGs survive lies in the interval
∼
6.2
×
10
12
h
-
1
M
⊙
and
1.2
×
10
13
h
-
1
M
⊙
, for the different bias models. Finally, allowing
γ
to evolve with redshift Taylor expansion:
γ
(
z
)
=
γ
0
+
γ
1
z
/
(
1
+
z
)
we find that the
(
γ
0
,
γ
1
)
parameter solution space accommodates the GR prediction at
∼
1.7
-
2.9
σ
levels.
We study the dynamical properties of a large body of varying vacuum cosmologies for which dark matter interacts with vacuum. In particular, performing the critical point analysis we investigate the ...existence and the stability of cosmological solutions which describe de-Sitter, radiation and matter dominated eras. We find several cases of varying vacuum models that admit stable critical points, hence they can be used in describing the cosmic history.
We present a model for the Universe in which quantum anomalies are argued to play an important dual role: they are responsible for generating matter-antimatter asymmetry in the cosmos, but also ...provide time-dependent contributions to the vacuum energy density of "running-vacuum" type, which drive the Universe's evolution. According to this scenario, during the inflationary phase of a string-inspired Universe, and its subsequent exit, the existence of primordial gravitational waves induces gravitational anomalies, which couple to the Kalb-Ramond (KR) axion field emerging from the antisymmetric tensor field of the massless gravitational multiplet of the string. Such anomalous CP-violating interactions have two important effects. First, they lead to contributions to the vacuum energy density of the form appearing in the "running vacuum model" (RVM) framework, which are proportional to both, the square and the fourth power of the effective Hubble parameter, H2 and H4 respectively. The H4 terms may lead to inflation, in a dynamical scenario whereby the role of the inflaton is played by the effective scalar-field ("vacuumon") representation of the RVM. Second, there is an undiluted KR axion at the end of inflation, which plays an important role in generating matter-antimatter asymmetry in the cosmos, through baryogenesis via leptogenesis in models involving heavy right-handed neutrinos. As the Universe exits inflation and enters a radiation-dominated era, the generation of chiral fermionic matter is responsible for the cancellation of gravitational anomalies, thus restoring diffeomorphism invariance for the matter/radiation (quantum) theory, as required for consistency. Chiral U(1) anomalies may remain uncompensated, though, during matter/radiation dominance, providing RVM-like H2 and H4 contributions to the Universe energy density. Finally, in the current era, when the Universe enters a de Sitter phase again, and matter is no longer dominant, gravitational anomalies resurface, leading to RVM-like H2 contributions to the vacuum energy density, which are however much more suppressed, as compared to their counterparts during inflation, due to the smallness of the present era's Hubble parameter H0. In turn, this feature endows the observed dark energy with a dynamical character that follows the RVM pattern, a fact which has been shown to improve the global fits to the current cosmological observations as compared to the concordance ΛCDM model with its rigid cosmological constant, Λ>0. Our model favors axionic dark matter, the source of which can be the KR axion. The uncompensated chiral anomalies in late epochs of the Universe are argued to play an important role in this, in the context of cosmological models characterized by the presence of large-scale cosmic magnetic fields at late eras.
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