11 research outputs found
Gauges and Cosmological Backreaction
We present a formalism for spatial averaging in cosmology applicable to
general spacetimes and coordinates, and allowing the easy incorporation of a
wide variety of matter sources. We apply this formalism to a
Friedmann-LeMaitre-Robertson-Walker universe perturbed to second-order and
present the corrections to the background in an unfixed gauge. We then present
the corrections that arise in uniform curvature and conformal Newtonian gauges.Comment: 13 pages. Updated: reference added, typos corrected, exposition
clarified. Version 3: Replaced with version published by JCA
Stability of Closed Timelike Curves in Goedel Universe
We study, in some detail, the linear stability of closed timelike curves in
the Goedel metric. We show that these curves are stable. We present a simple
extension (deformation) of the Goedel metric that contains a class of closed
timelike curves similar to the ones associated to the original Goedel metric.
This extension correspond to the addition of matter whose energy-momentum
tensor is analyzed. We find the conditions to have matter that satisfies the
usual energy conditions. We study the stability of closed timelike curves in
the presence of usual matter as well as in the presence of exotic matter
(matter that does satisfy the above mentioned conditions). We find that the
closed timelike curves in Goedel universe with or whithout the inclusion of
regular or exotic matter are also stable under linear perturbations. We also
find a sort of structural stability.Comment: 12 pages, 11 figures, RevTex, several typos corrected. GRG, in pres
Accelerating the Universe with Gravitational Waves
Inflation generically produces primordial gravitational waves with a red
spectral tilt. In this paper we calculate the backreaction produced by these
gravitational waves on the expansion of the universe. We find that in radiation
domination the backreaction acts as a relativistic fluid, while in matter
domination a small dark energy emerges with an equation of state w=-8/9.Comment: 18 pages, 4 figures. Replaced with version published by JCAP - some
discussion and references added concerning second-order gravitational waves,
typeset in JHEP styl
Averaging Robertson-Walker Cosmologies
The cosmological backreaction arises when one directly averages the Einstein
equations to recover an effective Robertson-Walker cosmology, rather than
assuming a background a priori. While usually discussed in the context of dark
energy, strictly speaking any cosmological model should be recovered from such
a procedure. We apply the Buchert averaging formalism to linear
Robertson-Walker universes containing matter, radiation and dark energy and
evaluate numerically the discrepancies between the assumed and the averaged
behaviour, finding the largest deviations for an Einstein-de Sitter universe,
increasing rapidly with Hubble rate to a 0.01% effect for h=0.701. For the LCDM
concordance model, the backreaction is of the order of Omega_eff~4x10^-6, with
those for dark energy models being within a factor of two or three. The impacts
at recombination are of the order of 10^-8 and those in deep radiation
domination asymptote to a constant value. While the effective equations of
state of the backreactions in Einstein-de Sitter, concordance and quintessence
models are generally dust-like, a backreaction with an equation of state
w_eff<-1/3 can be found for strongly phantom models.Comment: 18 pages, 11 figures, ReVTeX. Updated to version accepted by JCA
Stability of circular orbits of spinning particles in Schwarzschild-like space-times
Circular orbits of spinning test particles and their stability in
Schwarzschild-like backgrounds are investigated. For these space-times the
equations of motion admit solutions representing circular orbits with particles
spins being constant and normal to the plane of orbits. For the de Sitter
background the orbits are always stable with particle velocity and momentum
being co-linear along them. The world-line deviation equations for particles of
the same spin-to-mass ratios are solved and the resulting deviation vectors are
used to study the stability of orbits. It is shown that the orbits are stable
against radial perturbations. The general criterion for stability against
normal perturbations is obtained. Explicit calculations are performed in the
case of the Schwarzschild space-time leading to the conclusion that the orbits
are stable.Comment: eps figures, submitted to General Relativity and Gravitatio
Spherically Symmetric Solutions in Macroscopic Gravity
Schwarzschild's solution to the Einstein Field Equations was one of the first
and most important solutions that lead to the understanding and important
experimental tests of Einstein's theory of General Relativity. However,
Schwarzschild's solution is essentially based on an ideal theory of
gravitation, where all inhomogeneities are ignored. Therefore, any
generalization of the Schwarzschild solution should take into account the
effects of small perturbations that may be present in the gravitational field.
The theory of Macroscopic Gravity characterizes the effects of the
inhomogeneities through a non-perturbative and covariant averaging procedure.
With similar assumptions on the geometry and matter content, a solution to the
averaged field equations as dictated by Macroscopic Gravity are derived. The
resulting solution provides a possible explanation for the flattening of
galactic rotation curves, illustrating that Dark Matter is not real but may
only be the result of averaging inhomogeneities in a spherically symmetric
background.Comment: 14 pages, added and updated references, some paragraphs rewritten for
clarity, typographical errors fixed, results have not change
Dark energy as a mirage
Motivated by the observed cosmic matter distribution, we present the
following conjecture: due to the formation of voids and opaque structures, the
average matter density on the path of the light from the well-observed objects
changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in
the clumpy late universe, so that the average expansion rate increases along
our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free
expansion Ht ~ 1 at low redshifts. To calculate the modified observable
distance-redshift relations, we introduce a generalized Dyer-Roeder method that
allows for two crucial physical properties of the universe: inhomogeneities in
the expansion rate and the growth of the nonlinear structures. By treating the
transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position
of the baryon oscillation peak, the magnitude-redshift relations of type Ia
supernovae, the local Hubble flow and the nucleosynthesis, resulting in a
concordant model of the universe with 90% dark matter, 10% baryons, no dark
energy, 15 Gyr as the age of the universe and a natural value for the
transition redshift z_0=0.35. Unlike a large local void, the model respects the
cosmological principle, further offering an explanation for the late onset of
the perceived acceleration as a consequence of the forming nonlinear
structures. Additional tests, such as quantitative predictions for angular
deviations due to an anisotropic void distribution and a theoretical derivation
of the model, can vindicate or falsify the interpretation that light
propagation in voids is responsible for the perceived acceleration.Comment: 33 pages, 2 figs; v2: minor clarifications, results unchanged; v3:
matches the version published in General Relativity and Gravitatio
Dark Energy from structure: a status report
The effective evolution of an inhomogeneous universe model in any theory of
gravitation may be described in terms of spatially averaged variables. In
Einstein's theory, restricting attention to scalar variables, this evolution
can be modeled by solutions of a set of Friedmann equations for an effective
volume scale factor, with matter and backreaction source terms. The latter can
be represented by an effective scalar field (`morphon field') modeling Dark
Energy.
The present work provides an overview over the Dark Energy debate in
connection with the impact of inhomogeneities, and formulates strategies for a
comprehensive quantitative evaluation of backreaction effects both in
theoretical and observational cosmology. We recall the basic steps of a
description of backreaction effects in relativistic cosmology that lead to
refurnishing the standard cosmological equations, but also lay down a number of
challenges and unresolved issues in connection with their observational
interpretation.
The present status of this subject is intermediate: we have a good
qualitative understanding of backreaction effects pointing to a global
instability of the standard model of cosmology; exact solutions and
perturbative results modeling this instability lie in the right sector to
explain Dark Energy from inhomogeneities. It is fair to say that, even if
backreaction effects turn out to be less important than anticipated by some
researchers, the concordance high-precision cosmology, the architecture of
current N-body simulations, as well as standard perturbative approaches may all
fall short in correctly describing the Late Universe.Comment: Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 59
pages, 2 figures; matches published versio
Light propagation in the averaged universe
Bagheri S, Schwarz D. Light propagation in the averaged universe. Journal of Cosmology and Astroparticle Physics. 2014;2014(10):073.Cosmic structures determine how light propagates through the Universe andconsequently must be taken into account in the interpretation of observations.In the standard cosmological model at the largest scales, such structures areeither ignored or treated as small perturbations to an isotropic andhomogeneous Universe. This isotropic and homogeneous model is commonly assumedto emerge from some averaging process at the largest scales. We study theeffects of averaging the geometry of space-time and derive an averaged versionof the null geodesic equation of motion. For the averaged geometry we thenassume a flat Friedmann-Lema\^{i}tre (FL) model and find that light propagationin this averaged FL model is not given by null geodesics of that model, butrather by a modified light propagation equation that contains an effectiveHubble expansion rate, which differs from the Hubble rate of the averagedspace-time
True Hardness Evaluation of Bulk Metallic Materials in the Presence of Pile Up:Analytical and Enhanced Lobes Method Approaches
In the last few decades, a great deal of attention has been paid to understanding the factors
controlling the detailed shape of loading-unloading curves obtained by the Oliver and Pharr’s
nanoindentation analysis, to enable us estimate material parameters such as true contact area,
Young’s modulus, and hardness. In fact, it is well known that the Oliver and Pharr’s analysis
can overestimate hardness, especially in such cases where the material plastically deforms by
piling up around the indentation. In recent years, different direct and analytic methods have
been proposed. Direct visual methods are based on measurements of the produced indentation
by scanning probe microscopy (SPM) or by atomic force microscopy (AFM). Some other
analytic methods are based on the work of indentation analysis, that is to say, the analysis of the
area of the load-displacement curve. In the case of the current study, DHP-copper hardness
measurements using nanoindentation, in both H58 and annealed metallurgical conditions, were
investigated by both direct and analytic methods. Three different SPM-based methods and work
of indentation analysis methods were used to determine the true hardness. Among the three
SPM-based methods, one showed quite a good agreement with the literature data and micro-
hardness tests. This method was therefore developed and improved to make it dependent on
curve parameters and no longer dependent on SPM or AFM, the measurements of the real
contact area. The correlation between the pile-up phenomenon and the m exponent of the P = B(h - h f )^m relationship is also discussed