Adel Awad

Nonsingular rainbow universes

In this work, we study FRW cosmologies in the context of gravity rainbow. We discuss the general conditions for having a nonsingular FRW cosmology in gravity rainbow. We propose that gravity rainbow functions can be fixed using two known modified dispersion relation (MDR), which have been proposed in literature. The first MDR was introduced by Amelino-Camelia, et el. in [9] and the second was introduced by Magueijo and Smolin in [24]. Studying these FRW-like cosmologies, after fixing the gravity rainbow functions, leads to nonsingular solutions which can be expressed in exact forms.

Holographic Stress Tensors for Kerr-AdS Black Holes

We use the counterterm subtraction method to calculate the action and stress-energy-momentum tensor for the (Kerr) rotating black holes in AdS_{n+1}, for n=2, 3 and 4. We demonstrate that the expressions for the total energy for the Kerr-AdS_3 and Kerr-AdS_5 spacetimes, in the limit of vanishing black hole mass, are equal to the Casimir energies of the holographically dual n-dimensional conformal field theories. In particular, for Kerr-AdS_5, dual to the case of four dimensional N=4 supersymmetric Yang-Mills theory on the rotating Einstein universe, we explicitly verify the equality of the zero mass stress tensor from the two sides of the correspondence, and present the result for general mass as a prediction from gravity. Amusingly, it is observed in four dimensions that while the trace of the stress tensor defined using the standard counterterms does not vanish, its integral does, thereby keeping the action free of ultraviolet divergences. Using a different regularisation scheme another stress tensor can be defined, which is traceless.

Gauge Theories with Time Dependent Couplings and their Cosmological Duals

We consider the N = 4 super Yang-Mills theory in flat 3 + 1-dimensional space-time with a time dependent coupling constant which vanishes at t = 0, like g 2 YM = t p . In an analogous quantum mechanics toy model we find that the response is singular. The energy diverges at t = 0, for a generic state. In addition, if p > 1 the phase of the wave function has a wildly oscillating behavior, which does not allow it to be continued past t = 0. A similar effect would make the gauge theory singular as well, though nontrivial effects of renormalization could tame this singularity and allow a smooth continuation beyond t = 0. The gravity dual in some cases is known to be a time dependent cosmology which exhibits a spacelike singularity at t = 0. Our results, if applicable in the gauge theory for the case of the vanishing coupling, imply that the singularity is a genuine sickness and does not admit a meaningful continuation. When the coupling remains nonzero and becomes small at t = 0, the curvature in the bulk becomes of order string scale. The gauge theory now admits a time evolution beyond this point. In this case, a finite amount of energy is produced which possibly thermalizes and leads to a black hole in the bulk.

Gauge theory duals of cosmological backgrounds and their energy momentum tensors

We revisit type IIB supergravity backgrounds with null and spacelike singularities with natural gauge theory duals proposed in [S. R. Das, J. Michelson, K. Narayan, and S. P. Trivedi, Phys. Rev. D 74, 026002 (2006)] and [S. R. Das, J. Michelson, K. Narayan, and S. P. Trivedi, Phys. Rev. D 75, 026002 (2007)]. We show that for these backgrounds there are always choices of the boundaries of these deformed AdS{sub 5}xS{sup 5} space-times, such that the dual gauge theories live on flat metrics and have space-time dependent couplings. We present a new time dependent solution of this kind where the effective string coupling is always bounded and vanishes at a spacelike singularity in the bulk, and the space-time becomes AdS{sub 5}xS{sup 5} at early and late times. The holographic energy momentum tensor calculated with a choice of flat boundary is shown to vanish for null backgrounds and to be generically nonzero for time dependent backgrounds.

Muon anomalous magnetic moment and μ → eγ in B − L model with inverse seesaw

We study the anomalous magnetic moment of the muon, aμ, and lepton flavor violating decay μ→eγ in TeV scale B−L extension of the Standard Model (SM) with inverse seesaw mechanism. We show that the B−L contributions to aμ are severely constrained; therefore, the SM contribution remains intact. We also emphasize that the current experimental limit of BR(μ→eγ) can be satisfied for a wide range of parameter space and it can be within the reach of MEG experiment.

Minimal Length, Friedmann Equations and Maximum Density

A bstractInspired by Jacobson’s thermodynamic approach [4], Cai et al. [5, 6] have shown the emergence of Friedmann equations from the first law of thermodynamics. We extend Akbar-Cai derivation [6] of Friedmann equations to accommodate a general entrop-yarea law. Studying the resulted Friedmann equations using a specific entropy-area law, which is motivated by the generalized uncertainty principle (GUP), reveals the existence of a maximum energy density closed to Planck density. Allowing for a general continuous pressure p(ρ, a) leads to bounded curvature invariants and a general nonsingular evolution. In this case, the maximum energy density is reached in a finite time and there is no cosmological evolution beyond this point which leaves the big bang singularity inaccessible from a spacetime prospective. The existence of maximum energy density and a general nonsingular evolution is independent of the equation of state and the spacial curvature k. As an example we study the evolution of the equation of state p = ωρ through its phase-space diagram to show the existence of a maximum energy which is reachable in a finite time.

Generalized teleparallel cosmology and initial singularity crossing

We present a class of cosmological solutions for a generalized teleparallel gravity with,

Planck-scale corrections to Friedmann equation

f(T)=T+\tilde{\alpha}(-T)^n

Phase portraits of general f(T) cosmology

, where

D-dimensional charged Anti-de-Sitter black holes in f (T) gravity

\tilde{\alpha}