International Journal of Modern Physics D

Recent observations, especially by the Fermi satellite, point out the importance of the thermal component in GRB spectra. This fact revives strong interest in photospheric emission from relativistic outflows. Early studies already suggested that the observed spectrum of photospheric emission from relativistically moving objects differs in shape from the Planck spectrum. However, this component appears to be subdominant in many GRBs and the origin of the dominant component is still unclear. One of the popular ideas is that energy dissipation near the photosphere may produce a nonthermal spectrum and account for such emission. Before considering such models, though, one has to determine precise spectral and timing characteristics of the photospheric emission in the simplest possible case. Hence this paper focuses on various physical effects which make the photospheric emission spectrum different from the black body spectrum and quantifies them.

In the quest of understanding gravity, binary pulsars provide indispensable laboratories for precision tests of gravity. Effects that can be studied in great detail include the emission of gravitational waves, Shapiro delay, orbital precession and more. But also fundamental differences between general relativity and alternative theories of gravity can be probed, such as possible violations of the strong equivalence principle, preferred frame effects or the existence of gravitational dipole radiation or scalar fields. Also the effects of spin precession in strongly self-gravitating bodies can be studied by observing effects of geodetic precession.

This paper is a broadband review of the current status of nonbaryonic dark matter (DM) research, starting from a historical overview of the evidences of existence of DM, then discussing how DM is distributed from small scale to large scale, continuing with a discussion on DM nature, DM candidates and their detection. I finally discuss some of the limits of the ΛCDM model, with particular emphasis on the small scale problems of the paradigm.

The dynamical behaviors of Brans–Dicke gravity with the interacting new agegraphic dark energy (NADE) model are studied in this paper. First of all, by considering the dark energy density ρ_de = 3n²Φη^-2, the evolutions of the equation of state and the deceleration parameter can be described in the framework of Friedmann–Robertson–Walker universe. Moreover, the field equations of the Brans–Dicke gravity can be cast to the form of the first law of thermodynamics with the so-called entropy production term . Furthermore, the generalized second law of thermodynamics can be given in nonflat Brans–Dicke gravity with the interacting NADE enclosed by the dynamical apparent horizon with Hawking temperature. Finally, when the interaction between NADE and dark matter is considered as a fluctuation around the thermal equilibrium, it can be written to a form of logarithmic correction, and the expression of the interaction term can be obtained.

We show that the dark energy (DE) effects can be modeled by using an Ising perfect fluid with network interactions, whose low redshift equation of state (EoS), i.e. ω₀, becomes ω₀ = -1 as in the ΛCDM model. In our picture, DE is characterized by a barotropic fluid on a lattice in the equilibrium configuration. Thus, mimicking the spin interaction by replacing the spin variable with an occupational number, the pressure naturally becomes negative. We find that the corresponding EoS mimics the effects of a variable DE term, whose limiting case reduces to the cosmological constant Λ. This permits us to avoid the introduction of a vacuum energy as DE source by hand, alleviating the coincidence and fine tuning problems. We find fairly good cosmological constraints, by performing three tests with supernovae Ia (SNeIa), baryonic acoustic oscillation (BAO) and cosmic microwave background (CMB) measurements. Finally, we perform the Akaike information criterion (AIC) and Bayesian information criterion (BIC) selection criteria, showing that our model is statistically favored with respect to the Chevallier–Polarsky–Linder (CPL) parametrization.

In this paper, we have considered the modified holographic Ricci dark energy interacting with dark matter through a nongravitational coupling. We took three phenomenological forms for the interaction term Q in the model, where, in general, it is proportional to the Hubble parameter and densities of the dark sectors, that Q = ρ_de+ρ_m, ρ_m and ρ_de, respectively. We have obtained analytical solutions for the three interacting models, and studied the evolutions of equations of state parameter and deceleration parameter. The results are compared with the observationally constrained values for the best parameters of the model. In general, we have shown that the equation of state of models are showing a de Sitter type behavior, in the far future evolution of the universe. We have also done the statefinder analysis of the model to discriminate it from other standard models of dark energy and have shown that the present r–s values of the models were in the range of the r–s value of the Chaplygin gas model of dark energy.

While it is widely believed that the gravitational collapse of a sufficiently large mass will lead to a density singularity and an event horizon, we propose that this never happens when quantum effects are taken into account. In particular, we propose that when the conditions become ripe for the formation of a trapped surface, a quantum critical firewall sweeps over the collapsing body, transforming the nucleons in the collapsing matter into a lepton/photon gas together with droplets of a positive vacuum energy. This will happen regardless of the matter density at the time a trapped surface starts to form, and as a result, we predict that at least in all cases of gravitational collapse involving ordinary matter, a large fraction of the rest mass of the collapsing matter will be converted into a burst of neutrinos and γ-rays. We predict that the peak luminosity of these bursts is only weakly dependent on the mass of the collapsing object, and on the order of (ϵ_q/m_Pc²)^1/4c⁵/G where ϵ_q is the mean energy of a nucleon parton and m_P is the Planck mass. The duration of the bursts will depend on the mass of the collapsing object; in the case of stellar core collapse, we predict that the duration of both the neutrino and γ-ray bursts will be on the order of 10s.

Unlike flat space quantum field theories that focus on scattering amplitudes, the main observables in quantum cosmology are correlation functions. The systematic way of calculating correlators is called in–in formalism, which requires only a single asymptotic region, i.e. past infinity. The rules in perturbation theory and the path integral measure are very different for in–in and in–out formalisms, and thus the results which are standard in one approach may not necessarily hold in the other one. We show that stationary phase approximation works completely different for a scalar in–in path integral. Hence, in a cosmological background there are solutions, pseudo-instantons, that allow tunneling between locally stable vacua even in infinite volume, which is counterintuitive from an in–out perspective. We argue that various familiar notions of in–out formalism must be re-examined in the in–in formalism, which might have important consequences for quantum cosmology.

Hořava–Lifshitz theory as a renormalizable model of gravity might be an ultraviolet (UV) completion of general relativity (GR) and it reduces to Einstein gravity with a nonvanishing cosmological constant in infrared (IR) approximation. Kehagias and Sfetsos have added a relevant operator proportional to the three-dimensional (3D) geometry Ricci scalar to the original Hořava–Lifshitz theory action and obtained a spherically symmetric asymptotically flat black hole solution called Kehagias–Sfetsos (KS) black hole. Nonequilibrium thermodynamic fluctuations based on the metric of a KS black hole in IR modified Hořava–Lifshitz gravity have been calculated. It is concluded that the second-order momentum of mass flux is nondivergent and phase transition does not occur at the extremal case, while phase transition occurs at some other case, which is also different from the common case when the heat capacity is divergent.