The area of a future holographic screen increases monotonically. Associating this area with entropy results in a generalized second law for Cosmology (GSLC). Unlike black hole horizons, screens relevant to Cosmology have no thermodynamical interpretation. We suggest relating the entropy of the screens to spacetime degrees of freedom surface density derived from a (seemingly) phase space. This construction enables us to identify the entropy of any holographic screen as the entropy detected by accelerating observers due to their acceleration. Using Unruh's temperature and the equivalence principle, this gives the holographic screens' temperature and yields a thermodynamical interpretation of the GSLC.
In this paper, we investigate thermal equilibrium in string gas cosmology which is dominated by closed string.We consider two interesting regimes, Hagedorn and radiation regimes.We find that for short strings in small radius of Hagedorn regime very large amount of energy requested to have thermal equilibrium but for long strings in such system a few energy is sufficient to have thermal equilibrium. On the other hand in the large radius of Hagedorn regime, which pressure is not negligible, we obtain a relation between the energy and pressure in terms of cosmic time which is satisfied by thermal…
Motivated by the recent work of Wang, Lin, Pavon, and Abdalla [1], we generalize their work to the non-flat case. In particular, we provide a thermodynamical interpretation for the holographic dark energy model in a non-flat universe. For this case, the characteristic length is no more the radius of the event horizon (R_E) but the event horizon radius as measured from the sphere of the horizon (L). Furthermore, when interaction between the dark components of the holographic dark energy model in the non-flat universe is present its thermodynamical interpretation changes by a stable thermal fluc…
The aim here is to provide a set of equations for cosmology in terms of information and thermodynamical parameters. The method we implement in order to describe the universe is a development of Padmanabhan\rq{}s approach which is based on the fact that emergence of the cosmic space is provided by the evolution of the cosmic time. In this line we obtain the Friedmann equation or its equivalent the conservation law in terms of information by the implementation of Laundauer\rq{}s principle or in other words the information loss/production rate. Hence, a self consistent description of the universe…
We study string-gas cosmology in dilaton gravity, inspired by the fact that it naturally arises in a string theory context. Our main interest is the thermodynamical treatment of the string-gas and the resulting implications for the cosmology. Within an adiabatic approximation, thermodynamical equilibrium and a small, toroidal universe as initial conditions, we numerically solve the corresponding equations of motions in two different regimes describing the string-gas thermodynamics: (i) the Hagedorn regime, with a single scale factor, and (ii) an almost-radiation dominated regime, which include…
The emergent gravity paradigm interprets gravitational field equations as describing the thermodynamic limit of the underlying statistical mechanics of microscopic degrees of freedom of the spacetime. The connection is established by attributing a heat density Ts to the null surfaces where T is the appropriate Davies-Unruh temperature and s is the entropy density. The field equations can be obtained from a thermodynamic variational principle which extremizes the total heat density of all null surfaces. The explicit form of s determines the nature of the theory. We explore the consequences of t…
In the present paper, we provide a thermodynamical interpretation for the holographic dark energy model in a non-flat universe. For this case, the characteristic length is no more the radius of the event horizon ($R_E$) but the event horizon radius as measured from the sphere of the horizon ($L$). Furthermore, when interaction between the dark components of the holographic dark energy model in the non-flat universe is present its thermodynamical interpretation changes by a stable thermal fluctuation. A relation between the interaction term of the dark components and this thermal fluctuation is…
Based on the assumptions of the standard model of cosmology, a phase transition associated with chiral symmetry breaking after the electroweak transition has occurred at approximately $10μ$ seconds after the Big Bang to convert a plasma of free quarks and gluons into hadrons. We consider such a phase transition in the context of a deformed Horava-Lifshitz cosmology. The Friedmann equation for the deformed Horava-Lifshitz universe is obtained using the entropic interpretation of gravity, proposed by Verlinde. We investigate the effects of the parameter $ω$ appearing in the theory on the evoluti…
The question of the interpretation of Wheeler-DeWitt solutions in the context of cosmological models is addressed by implementing the Hamiltonian constraint as a spinor wave equation in minisuperspace. We offer a relative probability interpretation based on a non-closed vector current in this space and a prescription for a parametrisation of classical solutions in terms of classical time. Such a prescription can accommodate classically degenerate metrics describing manifolds with signature change. The relative probability density, defined in terms of a Killing vector of the Dewitt metric on mi…
A general class of solutions of string background equations is studied and its physical interpretations are presented. These solutions correspond to generalizations of the standard black p-brane solutions to surfaces with curvature k=-1,0. The relation with the recently introduced S-branes is provided. The mass, charge, entropy and Hawking temperature are computed, illustrating the interpretation in terms of negative tension branes. Their cosmological interpretation is discussed as well as their potential instability under small perturbations.
In the last years, we saw more and more attempts to explain dark matter as a general relativistic effect, at least for some fraction. Following this philosophy, we considered the gravitational distortions due to the inhomogeneous distribution of matter in the universe, which we know from general relativity to be retarded distortions. This provides a magnification effect, since the distortions we feel now depend not on the present matter density, but on the past one, which is greater. The expansion rate of the universe is perturbed as well, in a not negligible way, despite matter inhomogeneitie…
We consider the problem of extracting physical predictions from the wave function of the universe in quantum cosmological models. We state the features of quantum cosmology an interpretational scheme should confront. We discuss the Everett interpretation, and extensions of it, and their application to quantum cosmology. We review the steps that are normally taken in the process of extracting predictions from solutions to the Wheeler-DeWitt equation for quantum cosmological models. Some difficulties and their possible resolution are discussed. We conclude that the usual wave function-based appr…
We make a critical review of the semiclassical interpretation of quantum cosmology and emphasise that it is not necessary to consider that a concept of time emerges only when the gravitational field is (semi)classical. We show that the usual results of the semiclassical interpretation, and its generalisation known as the Born-Oppenheimer approach to quantum cosmology, can be obtained by gauge fixing, both at the classical and quantum levels. By `gauge fixing' we mean a particular choice of the time coordinate, which determines the arbitrary Lagrange multiplier that appears in Hamilton's equati…
In this paper, we study thermodynamical properties of the apparent horizon in a universe governed by quasi-topological gravity. Our aim is twofold. First, by using the variational method we derive the general form of Friedmann equation in quasi-topological gravity. Then, by applying the first law of thermodynamics on the apparent horizon, after using the entropy expression associated with the black hole horizon in quasi-topological gravity, and replacing the horizon radius, $r_{+}$, with the apparent horizon radius, $\tilde{r}_{A}$, we derive the corresponding Friedmann equation in quasi-topol…
The features of the fundamental thermodynamical relation (expressing entropy as function of state variables) that arise from the self-gravitating character of a system are analyzed. The models studied include not only a spherically symmetric hot matter shell with constant particle number but also a black hole characterized by a general thermal equation of state. These examples illustrate the formal structure of thermodynamics developed by Callen as applied to a gravitational configuration as well as the phenomenological manner in which Einstein equations largely determine the thermodynamical e…
We present a model of dark energy based on the string effective action, and on the assumption that the dilaton is strongly coupled to dark matter. We discuss the main differences between this class of models and more conventional models of quintessence,uncoupled to dark matter. This paper is based on talks presented at the "VII Congresso Nazionale di Cosmologia" (Osservatorio Astronomico di Roma, Monte Porzio Catone, November 2002), and at the Meeting "Dark Energy Day" (University of Milano-Bicocca, November 2002). To appear in the Proc. of the International Conference on ``Thinking, Observing…
Conformally related metrics and Lagrangians are considered in the context of scalar-tensor gravity cosmology. After the discussion of the problem, we pose a lemma in which we show that the field equations of two conformally related Lagrangians are also conformally related if and only if the corresponding Hamiltonian vanishes. Then we prove that to every non-minimally coupled scalar field, we may associate a unique minimally coupled scalar field in a conformally related space with an appropriate potential. The latter result implies that the field equations of a non-minimally coupled scalar fiel…
We study a quantization via fractional derivative of a nonminimal derivative coupling cosmological theory, namely, the Fab Four John theory. Its Hamiltonian version presents the issue of fractional powers in the momenta. That problem is solved here by the application of the so-called conformable fractional derivative. This leads to a Wheeler-DeWitt equation of second order, showing that a Bohm-de Broglie interpretation can be constructed. That combination of fractional quantization and Bohmian interpretation provides us a new quantization method, in which the quantum potential is the criterion…
We give an explicit construction of a positive-definite invariant inner-product for the Klein-Gordon fields, thus solving the old problem of the probability interpretation of Klein-Gordon fields without having to restrict to the subspaces of the positive-frequency solutions. Our method has a much wider domain of application and may be used to obtain the most general invariant inner-product on the solution space of a broad class of Klein-Gordon type evolution equations. We explore its consequences for the solutions of the Wheeler-DeWitt equation associated with the FRW-massive-real-scalar-field…
We outline the key-steps towards the construction of a physical, fully relativistic cosmology, in which we aim to trace Dark Energy and Dark Matter back to physical properties of space. The influence of inhomogeneities on the effective evolution history of the Universe is encoded in backreaction terms and expressed through spatially averaged geometrical invariants. These are absent and interpreted as missing dark fundamental sources in the standard model. In the inhomogeneous case they can be interpreted as energies of an emerging scalar field (the morphon). These averaged invariants vanish fo…
The fact that one can associate thermodynamic properties with horizons brings together principles of quantum theory, gravitation and thermodynamics and possibly offers a window to the nature of quantum geometry. This review discusses certain aspects of this topic concentrating on new insights gained from some recent work. After a brief introduction of the overall perspective, Sections 2 and 3 provide the pedagogical background on the geometrical features of bifurcation horizons, path integral derivation of horizon temperature, black hole evaporation, structure of Lanczos-Lovelock models, the c…
The cosmological constant $Λ$ can be achieved as the result of entangled and statistically correlated minisuperspace cosmological states, built up by using a minimal choice of observable quantities, i.e. $Ω_{m}$ and $Ω_{k}$, which assign the cosmic dynamics. In particular, we consider a cosmological model where two regions, corresponding to two correlated eras, are involved; the present universe description would be, in this way, given by a density matrix $\hat ρ$, corresponding to an entangled final state. Starting from this assumption, it is possible to infer some considerations on the cosmi…
We start with a particular cosmological model derived from type IIB supergravity theory with fluxes, where usually the dilaton is interpreted as a Quintessence field. Instead of that, in this letter we interpret the dilaton as the dark matter of the universe. With this alternative interpretation we find that in this supergravity model gives a similar evolution and structure formation of the universe compared with the $Λ$CDM model in the linear regime of fluctuations of the structure formation. Some free parameters of the theory are fixed using the present cosmological observations. In the non-…
We discuss the physical interpretation of unparticles and review the constraints from cosmology. Unparticles may be understood in terms of confined states of a strongly-coupled scale-invariant theory, where scale-invariance implies that the confined states have continuous masses. This picture is consistent with the observation that unparticle operators can be represented in terms of continuous mass fields. Finite results in scattering processes are obtained by compensating the infinite number of unparticle final states with an infinitesimal coupling per unparticle. As a result, unparticles are…
In this paper, we take cosmological constant as a thermodynamical pressure and it's conjugate quantity as a thermodynamical volume. This expression help us to investigate the phase transition and holographic heat engine. So, in order to have Van der Waals fluid behaviour in Horava -Lifshitz (HL) black hole, we modified the solution of such black hole with some cosmology ansatz. Also from holographic heat engine, we obtain Carnot efficiency for the HL black hole. The phase transition of the system lead us to investigate the stability condition for the corresponding black hole. In that case, we …
We propose a new phenomenological model for quantum gravity. This is based on a new interpretation in which quantum gravity is not an interaction, rather it is just responsible for generation of space-time-matter. Then we show this model is capable of solving the important problems of cosmology and particle physics.
A so-called "entropy-corrected holographic dark energy" (ECHDE), was recently proposed to explain the dark energy-dominated universe with the help of quantum corrections to the entropy-area relation in the setup of loop quantum cosmology. Using this new definition, we investigate its thermodynamical features including entropy and energy conservation. We describe the thermodynamical interpretation of the interaction between ECHDE and dark matter in a non-flat universe. We obtain a relation between the interaction term of the dark components and thermal fluctuation. Our study further generalizes…
In this note, we investigate some simple generalizations of a bottom-up holographic approach to cosmology introduced in arXiv:1810.10601. Our models utilize the Karch/Randall/Takayanagi ansatz for the gravitational dual of a boundary conformal field theory, involving pure AdS gravity and an end-of-the-world brane. Following a suggestion made in arXiv:2102.05057, we consider models with an additional interface brane in the bulk. We find that solutions with a viable cosmological interpretation exist only if our model is further generalized, for example by including an Einstein-Hilbert term in th…
We introduce a duality relation between two distinct branes, a cosmological brane with macroscopic matter and a holographic brane with microscopic gauge fields. Using brane-world cosmology with a single brane in a 5-dimensional AdS5 background, we find an explicit time-dependent holographic correspondence between the bulk metric surrounding the cosmological brane and the N=4 gauge field theory living on the boundary of the Z2-symmetric mirror bulk, identified with the holographic brane. We then relate the cosmic acceleration on the cosmological brane to the conformal anomaly of the gauge theor…
