119188055
Semiclassical environment of collapsing shells
We explore in detail the semiclassical environment of collapsing shells of matter, and determine the semiclassical flux measured by a variety of observers. This study is a preliminary step in a broader investigation of thermodynamic properties of the geometry of collapsing objects. Specifically, in this paper we consider spherically symmetric null and timelike collapsing shells which form an event horizon, and calculate the flux measured by observers both inside and outside the shell, and both inside and outside the event horizon, and find nontrivial results in most of the cases. Additionally, we also investigate the environment of a shell which collapses but does not form a horizon, halting at some radius larger than the Schwarzschild radius, and find that such an object generically gives rise to a pulse of radiation which is sharply peaked as it travels inwards and is reflected at the origin, and eventually emerges from the shell in a thermalized form. Our results have potential consequences in addressing questions pertaining, e.g. to black hole entropy and backreaction.
118443233
Backreaction of Cosmological Perturbations in Covariant Macroscopic Gravity
The problem of corrections to Einstein's equations arising from averaging of inhomogeneities (backreaction) in the cosmological context has gained considerable attention recently. We present results of analyzing cosmological perturbation theory in the framework of Zalaletdinov's fully covariant macroscopic gravity. We show that this framework can be adapted to the setting of cosmological perturbations in a manner which is free from gauge related ambiguities. We derive expressions for the backreaction which can be readily applied in any situation (not necessarily restricted to the linear perturbations considered here) where the metric can be brought to the perturbed Friedmann-Lemaitre-Robertson-Walker form. In particular, these expressions can be employed in toy models studying nonlinear structure formation, and possibly also in N-body simulations. Additionally, we present results of example calculations which show that the backreaction remains negligible well into the matter dominated era.
19943745
Spatial averaging limit of covariant macroscopic gravity: Scalar corrections to the cosmological equations
It is known that any explicit averaging scheme of the type essential for describing the large scale behavior of the Universe must necessarily yield corrections to the Einstein equations applied in the cosmological setting. The question of whether or not the resulting corrections to the Einstein equations are significant is still a subject of debate, partly due to possible ambiguities in the averaging schemes available. In particular, it has been argued in the literature that the effects of averaging could be gauge artifacts. We apply the formalism of Zalaletdinov's macroscopic gravity (MG), which is a fully covariant and nonperturbative averaging scheme, in an attempt to construct gauge independent corrections to the standard Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) equations. We find that whereas one cannot escape the problem of dependence on one gauge choice\char22{}which is inherent in the assumption of large scale homogeneity and isotropy\char22{}it is however possible to construct space-time scalar corrections to the standard FLRW equations. This partially removes the criticism concerning the corrections being gauge artifacts. For a particular initial choice of gauge which simplifies the formalism, we explicitly construct these scalars in terms of the underlying inhomogeneous geometry, and incidentally demonstrate that the formal structure of the corrections with this gauge choice is identical to that of analogous corrections derived by Buchert in the context of spatial averaging of scalars.
119302434
Excursion sets and non-Gaussian void statistics
Primordial non-Gaussianity (NG) affects the large scale structure (LSS) of the Universe by leaving an imprint on the distribution of matter at late times. Much attention has been focused on using the distribution of collapsed objects (i.e. dark matter halos and the galaxies and galaxy clusters that reside in them) to probe primordial NG. An equally interesting and complementary probe however is the abundance of extended underdense regions or voids in the LSS. The calculation of the abundance of voids using the excursion set formalism in the presence of primordial NG is subject to the same technical issues as the one for halos, which were discussed e.g. in Ref. . However, unlike the excursion set problem for halos which involved random walks in the presence of one barrier δ c, the void excursion set problem involves two barriers δ v and δ c. This leads to a new complication introduced by what is called the "void-in-cloud" effect discussed in the literature, which is unique to the case of voids. We explore a path integral approach which allows us to carefully account for all these issues, leading to a rigorous derivation of the effects of primordial NG on void abundances. The void-in-cloud issue, in particular, makes the calculation conceptually rather different from the one for halos. However, we show that its final effect can be described by a simple yet accurate approximation. Our final void abundance function is valid on larger scales than the expressions of other authors, while being broadly in agreement with those expressions on smaller scales. © 2011 The American Physical Society.
17381559
Nonlinear Structure Formation, Backreaction and Weak Gravitational Fields
There is an ongoing debate in the literature concerning the effects of averaging out inhomogeneities (“backreaction”) in cosmology. In particular, some simple models of structure formation studied in the literature seem to indicate that the backreaction can play a significant role at late times, and it has also been suggested that the standard perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes nonlinear. In this work we use Zalaletdinov's covariant averaging scheme (macroscopic gravity or MG) to show that as long as the metric of the Universe can be described by the perturbed FLRW form, the corrections due to averaging remain negligibly small. Further, using a fully relativistic and reasonably generic model of pressureless spherical collapse, we show that as long as matter velocities remain small (which is true in our model), the perturbed FLRW form of the metric can be explicitly recovered. Together, these results imply that the backreaction remains small even during nonlinear structure formation, and we confirm this within the toy model with a numerical calculation.
18963472
Structure formation, backreaction and weak gravitational fields
There is an ongoing debate in the literature as to whether the effects of averaging out inhomogeneities ('backreaction') in cosmology can be large enough to account for the acceleration of the scale factor in the Friedmann–Lemaitre–Robertson–Walker (FLRW) models. In particular, some simple models of structure formation studied in the literature seem to indicate that this is indeed possible, and it has also been suggested that the perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes non-linear. In this work we attempt to clarify the situation to some extent, using a fully relativistic model of pressureless spherical collapse. We find that whereas averaging during structure formation can lead to acceleration via a selective choice of averaging domains, the acceleration is not present when more generic domains are used for averaging. Further, we show that for most of the duration of the collapse, matter velocities remain small, and the perturbed FLRW form of the metric can be explicitly recovered, in the structure formation phase. We also discuss the fact that the magnitude of the average effects of inhomogeneities depends on the scale of averaging, and while it may not be completely negligible on intermediate scales, it is expected to remain small when averaging on suitably large scales.
5059816
Cosmological sector for localized mass and spin in 2+1 dimensional topologically massive gravity
Abstract The cosmological sector to the full non-linear topologically massive gravity (TMG) is obtained for localized sources of mass m and spin σ besides the asymptotically spinning conical flat sector previously obtained [A. Edery, M.B. Paranjape, gr-qc/9606029, to be published]. In a small region near but outside the sources, the metric resembles the spinning conical flat metric but we find that the mass m creates a negative deficit angle of 3 m as opposed to m . Furthermore, it is not possible to recover the results of pure Einstein gravity in the limit μ →∞ unlike the flat sector.