Gravitation

About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.

Stability of the Einstein static universe in f(R) gravity

Abstract: We analyze the stability of the Einstein static universe by considering homogeneous scalar perturbations in the context of f(R) modified theories of gravity. By considering specific forms of f(R), the stability regions of the solutions are parametrized by a linear equation of state parameter w=p/{rho}. Contrary to classical general relativity, it is found that in f(R) gravity a stable Einstein cosmos with a positive cosmological constant does indeed exist. Thus, we are lead to conclude that, in principle, modifications in f(R) gravity stabilize solutions which are unstable in general relativity.

Odd-parity stability of a Reissner-Nordström black hole

Abstract: The odd-parity perturbations of a Reissner-Nordstr\"om black hole are studied through the use of a variational principle for the perturbation equations. The Hamiltonian for the perturbations is expressed in terms of a natural set of canonical variables and shown to be (weakly) positive-definite. This result is used to prove the nonexistence of unstable normalmode solutions of the perturbation equations. The two wave equations governing gravitational and electromagnetic perturbations are decoupled by a simple transformation.

Parametrized post-Newtonian limit of fourth order gravity inspired by scalar-tensor gravity

Abstract: Based on the dynamical equivalence between higher order gravity and scalar-tensor gravity the parametrized post-Newtonian (PPN) limit of fourth order gravity is discussed. We exploit this analogy developing a fourth order gravity version of the Eddington PPN parameters. As a result, Solar System experiments can be reconciled with higher order gravity, if physical constraints descending from experiments are fulfilled.

Galactic angular momentum in the illustris simulation: feedback and the hubble sequence

Abstract: We study the stellar angular momentum of thousands of galaxies in the Illustris cosmological simulation, which captures gravitational and gas dynamics within galaxies, as well as feedback from stars and black holes. We find that the angular momentum of the simulated galaxies matches observations well, and in particular two distinct relations are found for late-type versus early-type galaxies. The relation for late-type galaxies corresponds to the value expected from full conservation of the specific angular momentum generated by cosmological tidal torques. The relation for early-type galaxies corresponds to retention of only ?30% of that, but we find that those early-type galaxies with low angular momentum at z = 0 nevertheless reside at high redshift on the late-type relation. Some of them abruptly lose angular momentum during major mergers. To gain further insight, we explore the scaling relations in simulations where the galaxy formation physics is modified with respect to the fiducial model. We find that galactic winds with high mass-loading factors are essential for obtaining the high angular momentum relation typical for late-type galaxies, while active galactic nucleus feedback largely operates in the opposite direction. Hence, feedback controls the stellar angular momentum of galaxies, and appears to be instrumental for establishing the Hubble sequence.