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A special issue of Entropy (ISSN 1099-4300). This special issue belongs to the section "Astrophysics, Cosmology, and Black Holes".

Deadline for manuscript submissions: closed (30 June 2017) | Viewed by 19677

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Prof. Dr. Nikolaos K. Spyrou

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Guest Editor

Department of Astronomy, Aristoteleion University of Thessaloniki, 541.24 Thessaloniki, Greece
Interests: gravitational astrophysics, general theory of relativity, relativistic astrophysics, physical cosmology, environmental physics, history of astronomy, science education

Dr. Kostas Kleidis

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Department of Mechanical Engineering, Technological Education Institute of Central Macedonia, 621.24 Serres, Greece
Interests: general theory of relativity; mathematical cosmology; quantum field theory, quantum gravity

Special Issue Information

Dear Colleagues,

You are cordially invited to submit papers to the Special Issue Dark Energy, that focuses on understanding the nature of the constituent that causes the accelerated expansion of the Universe.

During the last 20 years, a continuously growing list of observational data has verified the existence of a distributed energy component in the Universe, i.e., one that does not cluster at any scale. Reflecting our ignorance on its exact nature, this new component—which constitutes about two-thirds of the Universe’s mass-energy content—was termed dark energy (DE).

The need for DE was first suggested by high-precision distance measurements, performed with the aid of the supernovae Ia standard candles. Today, there is also evidence from galaxy clusters, the integrated Sachs–Wolfe effect, baryon acoustic oscillations, weak gravitational lensing, gamma ray bursts, the Lyman-α forest, etc. A combination of these data with those from the cosmic microwave background survey has provided evidence for DE at the 5σ confidence level.

Although the notion of DE can be attributed to a non-vanishing cosmological constant, Λ, such a choice fails to explain the magnitude of Λ, with the corresponding theoretical predictions being 10¹²³ times larger than what is observed. As a consequence, many other physically-motivated models have appeared in the literature, including quintessence, phantom cosmology, braneworld scenarios, modified gravity theories, Chaplygin gas, Cardassian cosmology, unified dark matter models, and so on.

As none of these models meets observation at a 100% level, today, the determination of DE’s exact nature is one of the most intriguing challenges of theoretical physics and cosmology.

Dr. N. K. Spyrou
Dr. K. Kleidis
Guest Editors

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Keywords

dark energy
accelerating universe
cosmological constant
quintessence
cardassian cosmology
unified dark matter

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Published Papers (4 papers)

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Research

14 pages, 373 KiB

Open AccessArticle

Cosmographic Thermodynamics of Dark Energy

by Orlando Luongo

Entropy 2017, 19(10), 551; https://doi.org/10.3390/e19100551 - 19 Oct 2017

Cited by 3 | Viewed by 4278

Abstract

Dark energy’s thermodynamics is here revised giving particular attention to the role played by specific heats and entropy in a flat Friedmann-Robertson-Walker universe. Under the hypothesis of adiabatic heat exchanges, we rewrite the specific heats through cosmographic, model-independent quantities and we trace their evolutions in terms of z. We demonstrate that dark energy may be modeled as perfect gas, only as the Mayer relation is preserved. In particular, we find that the Mayer relation holds if

j - q > \frac{1}{2}

. The former result turns out to be general so that, even at the transition time, the jerk parameter j cannot violate the condition:

j_{t r} > \frac{1}{2}

. This outcome rules out those models which predict opposite cases, whereas it turns out to be compatible with the concordance paradigm. We thus compare our bounds with the

Λ

CDM model, highlighting that a constant dark energy term seems to be compatible with the so-obtained specific heat thermodynamics, after a precise redshift domain. In our treatment, we show the degeneracy between unified dark energy models with zero sound speed and the concordance paradigm. Under this scheme, we suggest that the cosmological constant may be viewed as an effective approach to dark energy either at small or high redshift domains. Last but not least, we discuss how to reconstruct dark energy’s entropy from specific heats and we finally compute both entropy and specific heats into the luminosity distance

d_{L}

, in order to fix constraints over them through cosmic data. Full article

(This article belongs to the Special Issue Dark Energy)

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19 pages, 492 KiB

Open AccessArticle

Coupled DM Heating in SCDEW Cosmologies

by Silvio Bonometto and Roberto Mainini

Entropy 2017, 19(8), 398; https://doi.org/10.3390/e19080398 - 2 Aug 2017

Cited by 5 | Viewed by 3520

Abstract

Strongly-Coupled Dark Energy plus Warm dark matter (SCDEW) cosmologies admit the stationary presence of ∼1% of coupled-DM and DE, since inflationary reheating. Coupled-DM fluctuations therefore grow up to non-linearity even in the early radiative expansion. Such early non-linear stages are modelized here through the evolution of a top-hat density enhancement, reaching an early virial balance when the coupled-DM density contrast is just 25–26, and the DM density enhancement is ∼10

%

of the total density. During the time needed to settle in virial equilibrium, the virial balance conditions, however, continue to modify, so that “virialized” lumps undergo a complete evaporation. Here, we outline that DM particles processed by overdensities preserve a fraction of their virial momentum. Although fully non-relativistic, the resulting velocities (moderately) affect the fluctuation dynamics over greater scales, entering the horizon later on. Full article

(This article belongs to the Special Issue Dark Energy)

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22 pages, 869 KiB

Open AccessFeature PaperArticle

A Thermodynamic Point of View on Dark Energy Models

by Vincenzo F. Cardone, Ninfa Radicella and Antonio Troisi

Entropy 2017, 19(8), 392; https://doi.org/10.3390/e19080392 - 29 Jul 2017

Cited by 6 | Viewed by 5666

Abstract

We present a conjugate analysis of two different dark energy models, namely the Barboza–Alcaniz parameterization and the phenomenologically-motivated Hobbit model, investigating both their agreement with observational data and their thermodynamical properties. We successfully fit a wide dataset including the Hubble diagram of Type Ia Supernovae, the Hubble rate expansion parameter as measured from cosmic chronometers, the baryon acoustic oscillations (BAO) standard ruler data and the Planck distance priors. This analysis allows us to constrain the model parameters, thus pointing at the region of the wide parameters space, which is worth focusing on. As a novel step, we exploit the strong connection between gravity and thermodynamics to further check models’ viability by investigating their thermodynamical quantities. In particular, we study whether the cosmological scenario fulfills the generalized second law of thermodynamics, and moreover, we contrast the two models, asking whether the evolution of the total entropy is in agreement with the expectation for a closed system. As a general result, we discuss whether thermodynamic constraints can be a valid complementary way to both constrain dark energy models and differentiate among rival scenarios. Full article

(This article belongs to the Special Issue Dark Energy)

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10 pages, 1090 KiB

Open AccessArticle

Testing the Interacting Dark Energy Model with Cosmic Microwave Background Anisotropy and Observational Hubble Data

by Weiqiang Yang, Lixin Xu, Hang Li, Yabo Wu and Jianbo Lu

Entropy 2017, 19(7), 327; https://doi.org/10.3390/e19070327 - 17 Jul 2017

Cited by 12 | Viewed by 5609

Abstract

The coupling between dark energy and dark matter provides a possible approach to mitigate the coincidence problem of the cosmological standard model. In this paper, we assumed the interacting term was related to the Hubble parameter, energy density of dark energy, and equation [...] Read more.

Q = 3 H ξ (1 + w_{x}) ρ_{x}

. Based on the Markov chain Monte Carlo method, we made a global fitting on the interacting dark energy model from Planck 2015 cosmic microwave background anisotropy and observational Hubble data. We found that the observational data sets slightly favored a small interaction rate between dark energy and dark matter; however, there was not obvious evidence of interaction at the

1 σ

level. Full article

(This article belongs to the Special Issue Dark Energy)

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