117453366
Unified First Law and Some Comments
In this small article, unified first law has been analyzed and some results have been deduced from it. The results have been presented in the form of lemmas and some conclusions have been drawn from them. The joint venture of quantum mechanics and general relativity leads to a remarkable discovery that a ”black hole (BH) behaves like a black body”. The thermal radiation emitted by a BH has temperature proportional to its surface gravity at the horizon while entropy is proportional to the area of the horizon [1,2], i.e., T = � 2� , S = A 4G . Also this temperature entropy and the mass of the BH are related by the first law of thermodynamics [3]. Further, the equivalence of the thermodynamical quantities with the geometry of the horizon leads to speculate some inherent relationship between thermodynamical laws and Einstein equations. This speculation comes into true in 1995 when Jacobson [4] was able to formulate Einstein’s equations from the Clausius relation (�Q = TdS) using local Rindler Causal horizons with T, the Unruh temperature as seen by an accelerated observer just inside the horizon. The equivalence in other way round was shown by Padmanabhan [5] for a general spherical symmetric space time. However in other gravity theories, such type of equivalence is not possible. Eling et al [6] showed that the usual Clausius relation and the entropy assumption S = �f 0 (r)A(or S = �F (�)A) for f(R)gravity (or scalar tensor gravity) do not give the correct equations of motion-on entropy production term has to be added to the Clausius relation and as a result there will be non-equilibrium thermodynamics of space-time [6,7]. Similarly, in the context of BH thermodynamics most studies of BH thermodynamics are concentrated to stationary BHs. It is speculated that thermodynamics of dynamical (i.e., non stationary) BH is related to the non-equilibrium thermodynamics of the universe. Hayward [8] initiated the study of the thermodynamics of dynamical BH by proposing ”Unified first law”. He introduced the idea of the trapping horizon and was able to show the equivalence of Einstein equations and unified first law . He formulated the first law of thermodynamics for a dynamical BH by projecting unified first law along the trapping horizon. Also projecting along the tangent to the trapping horizon he was able to formulate the Clausius relation. Subsequently, Cai et al [7] studied in details the thermodynamics of FRW universe starting from the unified first law, in Einstein gravity, Lovelock gravity and in scalar-tensor theory of gravity and showed the necessity of introducing entropy production term in scalar tensor theory. In this small article we study the unified first law in details and reveal some of its properties in the form of lemmas. Mathematically, the unified first law can be written as
12672489
NAKED SINGULARITIES IN HIGHER DIMENSIONAL GRAVITATIONAL COLLAPSE
Spherically symmetric inhomogeneous dust collapse has been studied in higher dimensional space–time and the factors responsible for the appearance of a naked singularity are analyzed in the region close to the centre for the marginally bound case. It is clearly demonstrated that in the former case naked singularities do not appear in the space–time having more than five dimension, which appears to a strong result. The non-marginally bound collapse is also examined in five dimensions and the role of shear in developing naked singularities in this space–time is discussed in details. The five-dimensional space–time is chosen in the later case because we have exact solution in closed form only in five dimension and not in any other case.
120578159
Exact solutions of the field equations for Charap’s chiral invariant model of the pion dynamics
The field equations for the chiral invariant model of pion dynamics developed by Charap have been revisited. Two new types of solutions of these equations have been obtained. Each type allows infinite number of solutions. It has also been shown that the chiral invariant field equations admit invariance for a transformation of the dependent variables.
119261702
Emergent universe in brane world scenario with Schwarzschild–de Sitter bulk
A model of an emergent universe is obtained in brane world. Here the bulk energy is in the form of cosmological constant, while the brane consists of a fluid satisfying an equation of state of the form $${p_{b}=\frac{1}{3}\rho_{b}}$$ , which is effectively a radiation equation of state at high energies. It is shown that with the positive bulk cosmological constant, one of our models represents an emergent universe.
119296208
Interacting Holographic Dark Energy at the Ricci scale and Dynamical system
In this work, we consider homogeneous and isotropic FRW model of the universe, filled with interacting dark matter and dark energy. The dark matter is chosen as usual in the form of dust while dark energy is holographic in nature with IR cut off at the Ricci's length and it is in the form of a perfect fluid with variable equation of state. We have chosen the interaction term of the following two types: (i) a linear combination of the matter density of the two fluids, (ii) a product of the two matter densities. For both the choices the evolution equations are transformed to an autonomous system and the corresponding critical points are analyzed. Finally, for the first choice of the interaction term the evolution of the ratio of the energy densities has been studied from the point of view of the present coincidence problem.
120407295
Energy Conditions, Cosmological Solutions and Cosmic No-Hair Conjecture in Gauss-Bonnet Theory
A detailed investigation of Gauss-Bonnet theory has been done from a different perspective. At first, the standard energy conditions are discussed and modified forms have been presented. Then some cosmological solutions have been obtained in 5D for perfect fluid assuming that the extra dimensional metric coefficient decreases with time. For some particular choice of the parameters, exponential solutions are obtained and finally, Cosmic No-Hair Conjecture has been proved for Gauss-Bonnet dilatonic scalar coupled to Einstein gravity with coupling parameter growing linearly in time.
119399325
New possibilities in supernova accretion phase from dense matter effect
We carry out a detailed analysis of the supernova (SN) neutrino flavor evolution during the accretion phase (at post-bounce times tpb ≤ 500 ms), characterizing the SN ν signal by recent hydrodynamical simulations. We find that trajectory-dependent multi-angle effects, associated with the dense ordinary matter suppress collective oscillations, that would have been induced by ν-ν interactions in the deepest SN regions. The matter suppression implies that neutrino oscillations will start outside the neutrino decoupling region and therefore will have a negligible impact on the neutrino heating and the explosion dynamics. Furthermore, the possible detection of the next galactic SN neutrino signal from the accretion phase, based on the usual Mikheyev-Smirnov-Wolfenstein effect in the SN mantle and Earth matter effects, can reveal the neutrino mass hierarchy in the likely case that the mixing angle θ13 is not very small.
119098412
Analysis of matter suppression in collective neutrino oscillations during the supernova accretion phase
The usual description of self-induced neutrino flavor conversions in core collapse supernovae (SNe) is based on the dominance of the neutrino density n_nu over the net electron density n_e. However, this condition is not met during the post-bounce accretion phase, when the dense matter in a SN is piled up above the neutrinosphere. As recently pointed-out, a dominant matter term in the anisotropic SN environment would dephase the flavor evolution for neutrinos traveling on different trajectories, challenging the occurrence of the collective behavior in the dense neutrino gas. Using the results from recent long term simulations of core-collapse SN explosions, based on three flavor Boltzmann neutrino transport in spherical symmetry, we find that both the situations of complete matter suppression (when n_e >> n_nu) and matter-induced decoherence (when n_e \gtrsim n_nu) of flavor conversions are realized during the accretion phase. The matter suppression at high densities prevents any possible impact of the neutrino oscillations on the neutrino heating and hence on the dynamics of the explosion. Furthermore, it changes the interpretation of the Earth matter effect on the SN neutrino signal during the accretion phase, allowing the possibility of the neutrino mass hierarchy discrimination at not too small values of the leptonic mixing angle \theta_{13} (i.e. \sin^2{\theta}_{13} \gtrsim 10^{-3}).
55676024
Holographic quark-antiquark potential in hot, anisotropic Yang-Mills plasma
Abstract Using the gauge/gravity duality we calculate the heavy quark–antiquark potential in a hot, anisotropic and strongly coupled Yang–Mills plasma in ( 3 + 1 )-dimensions. As the anisotropic medium we take a deformed version of N = 4 super Yang–Mills theory at finite temperature following a recent work where the dual type IIB supergravity solution is also proposed. We turn on a small value of the anisotropy parameter, for which the gravity dual is known analytically (perturbatively), and compute the velocity-dependent quark–antiquark interaction potential when the pair is moving through the plasma with a velocity v . By setting v = 0 we recover the static quark–antiquark potential. We numerically study how the potential is modified in the presence of anisotropy. We further show numerically how the quark–antiquark separation (both in the static and the velocity-dependent case) and hence, the screening length gets modified by anisotropy. We discuss various cases depending upon the direction of the dipole and the direction of its propagation and make a comparative study of these cases. We are also able to obtain an analytical expression for the screening length of the dipole moving in a hot, anisotropic plasma in a special case.
118540091
Wilson loops in noncommutative Yang–Mills theory using gauge/gravity duality
Abstract By using the gauge/gravity duality and the Maldacena prescription we compute the expectation values of the Wilson loops in hot, noncommutative Yang–Mills (NCYM) theory in ( 3 + 1 ) dimensions. We consider both the time-like and the light-like Wilson loops. The gravity dual background is given by a particular decoupling limit of non-extremal (D1, D3) bound state of type IIB string theory. We obtain the velocity dependent quark–antiquark potential and numerically study how the dipole length and the potential change with velocity (for 0 v 1 , i.e., the Wilson loop is time-like) of the dipole as well as noncommutativity. We discuss and compare the results with the known commutative results. We also obtain an analytic expression for the screening length when the rapidity is large and the noncommutativity parameter is small with the product remaining small. When v → 1 , the time-like Wilson loop becomes light-like and in that case we obtain the form of the jet quenching parameter for the strongly coupled noncommutative Yang–Mills plasma which matches with our earlier results obtained using different approach.
53554327
7 keV sterile neutrino dark matter in U(1)R-lepton number model
A bstractWe study the phenomenology of a keV sterile neutrino in a supersymmetric model with U(1)R-lepton number in the light of a very recent observation of an X-ray line signal at around 3.5 keV, detected in the X-ray spectra of Andromeda galaxy and various galaxy clusters including the Perseus galaxy cluster. This model not only provides a small tree level mass to one of the active neutrinos but also renders a suitable warm dark matter candidate in the form of a sterile neutrino with negligible active-sterile mixing. Light neutrino masses and mixing can be explained once one-loop radiative corrections are taken into account. The scalar sector of this model can accommodate a Higgs boson with a mass of ∼125 GeV. In this model gravitino is the lightest supersymmetric particle (LSP) and we also study the cosmological implications of this light gravitino with mass ∼OGeV$$ \sim \mathcal{O}\left(\mathrm{GeV}\right) $$.