45 research outputs found
Large-k Limit of Multi-Point Propagators in the RG Formalism
Renormalized versions of cosmological perturbation theory have been very
successful in recent years in describing the evolution of structure formation
in the weakly non-linear regime. The concept of multi-point propagators has
been introduced as a tool to quantify the relation between the initial matter
distribution and the final one and to push the validity of the approaches to
smaller scales. We generalize the n-point propagators that have been considered
until now to include a new class of multi-point propagators that are relevant
in the framework of the renormalization group formalism. The large-k results
obtained for this general class of multi-point propagators match the results
obtained earlier both in the case of Gaussian and non-Gaussian initial
conditions. We discuss how the large-k results can be used to improve on the
accuracy of the calculations of the power spectrum and bispectrum in the
presence of initial non-Gaussianities.Comment: 30 page
Natural Entropy Production in an Inflationary Model for a Polarized Vacuum
Though entropy production is forbidden in standard FRW Cosmology, Berman and
Som presented a simple inflationary model where entropy production by bulk
viscosity, during standard inflation without ad hoc pressure terms can be
accommodated with Robertson-Walker's metric, so the requirement that the early
Universe be anisotropic is not essential in order to have entropy growth during
inflationary phase, as we show. Entropy also grows due to shear viscosity, for
the anisotropic case. The intrinsically inflationary metric that we propose can
be thought of as defining a polarized vacuum, and leads directly to the desired
effects without the need of introducing extra pressure terms.Comment: 7 pages including front one. Accepted to publication, Astrophysics
and Space Science, subjected to a minor correction, already submitte
Self-consistent stability analysis of spherical shocks.
In this paper, we study self-similar solutions, and their linear stability as well, describing the flow within a spherical shell with finite thickness, expanding according to a power law of time, t q , where q>0. The shell propagates in a medium with initially uniform density and it is bounded by a strong shock wave at its outer border while the inner face is submitted to a time-dependent uniform pressure. For q=2/5, the well-known Sedov–Taylor solution is recovered. In addition, although both accelerated and decelerated shells can be unstable against dynamic perturbations, they exhibit highly different behaviors. Finally, the dispersion relation derived earlier by Vishniac (Vishniac, E.T. in Astrophys. J. 274:152, 1983) for an infinitely thin shell is obtained in the limit of an isothermal shock wave
Next-to-leading resummation of cosmological perturbations via the Lagrangian picture: 2-loop correction in real and redshift spaces
We present an improved prediction of the nonlinear perturbation theory (PT)
via the Lagrangian picture, which was originally proposed by Matsubara (2008).
Based on the relations between the power spectrum in standard PT and that in
Lagrangian PT, we derive analytic expressions for the power spectrum in
Lagrangian PT up to 2-loop order in both real and redshift spaces. Comparing
the improved prediction of Lagrangian PT with -body simulations in real
space, we find that the 2-loop corrections can extend the valid range of wave
numbers where we can predict the power spectrum within 1% accuracy by a factor
of 1.0 (), 1.3 (1), 1.6 (2) and 1.8 (3) vied with 1-loop Lagrangian PT
results. On the other hand, in all redshift ranges, the higher-order
corrections are shown to be less significant on the two-point correlation
functions around the baryon acoustic peak, because the 1-loop Lagrangian PT is
already accurate enough to explain the nonlinearity on those scales in -body
simulations.Comment: 18pages, 4 figure
Evolution Equation for Non-linear Cosmological Perturbations
We present a novel approach, based entirely on the gravitational potential,
for studying the evolution of non-linear cosmological matter perturbations.
Starting from the perturbed Einstein equations, we integrate out the
non-relativistic degrees of freedom of the cosmic fluid and obtain a single
closed equation for the gravitational potential. We then verify the validity of
the new equation by comparing its approximate solutions to known results in the
theory of non-linear cosmological perturbations. First, we show explicitly that
the perturbative solution of our equation matches the standard perturbative
solutions. Next, using the mean field approximation to the equation, we show
that its solution reproduces in a simple way the exponential suppression of the
non-linear propagator on small scales due to the velocity dispersion. Our
approach can therefore reproduce the main features of the renormalized
perturbation theory and (time)-renormalization group approaches to the study of
non-linear cosmological perturbations. We conclude by a preliminary discussion
of the nature of the full solutions of the equation and their significance
Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts
Ionization fronts, the sharp radiation fronts behind which H/He ionizing
photons from massive stars and galaxies propagate through space, were
ubiquitous in the universe from its earliest times. The cosmic dark ages ended
with the formation of the first primeval stars and galaxies a few hundred Myr
after the Big Bang. Numerical simulations suggest that stars in this era were
very massive, 25 - 500 solar masses, with H II regions of up to 30,000
light-years in diameter. We present three-dimensional radiation hydrodynamical
calculations that reveal that the I-fronts of the first stars and galaxies were
prone to violent instabilities, enhancing the escape of UV photons into the
early intergalactic medium (IGM) and forming clumpy media in which supernovae
later exploded. The enrichment of such clumps with metals by the first
supernovae may have led to the prompt formation of a second generation of
low-mass stars, profoundly transforming the nature of the first protogalaxies.
Cosmological radiation hydrodynamics is unique because ionizing photons coupled
strongly to both gas flows and primordial chemistry at early epochs,
introducing a hierarchy of disparate characteristic timescales whose relative
magnitudes can vary greatly throughout a given calculation. We describe the
adaptive multistep integration scheme we have developed for the self-consistent
transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech,
March 15 - 18, 201
Lagrangian evolution of global strings
We establish a method to trace the Lagrangian evolution of extended objects
consisting of a multicomponent scalar field in terms of a numerical calculation
of field equations in three dimensional Eulerian meshes. We apply our method to
the cosmological evolution of global strings and evaluate the energy density,
peculiar velocity, Lorentz factor, formation rate of loops, and emission rate
of Nambu-Goldstone (NG) bosons. We confirm the scaling behavior with a number
of long strings per horizon volume smaller than the case of local strings by a
factor of 10. The strategy and the method established here are
applicable to a variety of fields in physics.Comment: 5 pages, 2 figure
Topological Defects as Seeds for Eternal Inflation
We investigate the global structure of inflationary universe both by
analytical methods and by computer simulations of stochastic processes in the
early Universe. We show that the global structure of the universe depends
crucially on the mechanism of inflation. In the simplest models of chaotic
inflation the Universe looks like a sea of thermalized phase surrounding
permanently self-reproducing inflationary domains. In the theories where
inflation occurs near a local extremum of the effective potential corresponding
to a metastable state, the Universe looks like de Sitter space surrounding
islands of thermalized phase. A similar picture appears even if the state is unstable but the effective potential has a discrete symmetry . In this case the Universe becomes divided into domains containing
different phases. These domains will be separated from each other by domain
walls. However, unlike ordinary domain walls, these domain walls will inflate,
and their thickness will exponentially grow. In the theories with continuous
symmetries inflation generates exponentially expanding strings and monopoles
surrounded by thermalized phase. Inflating topological defects will be stable,
and they will unceasingly produce new inflating topological defects. This means
that topological defects may play a role of indestructible seeds for eternal
inflation.Comment: 21 pages, 17 figures (not included), Stanford University preprint
SU--ITP--94--
DT/T beyond linear theory
The major contribution to the anisotropy of the temperature of the Cosmic
Microwave Background (CMB) radiation is believed to come from the interaction
of linear density perturbations with the radiation previous to the decoupling
time. Assuming a standard thermal history for the gas after recombination, only
the gravitational field produced by the linear density perturbations present on
a universe can generate anisotropies at low z (these
anisotropies would manifest on large angular scales). However, secondary
anisotropies are inevitably produced during the nonlinear evolution of matter
at late times even in a universe with a standard thermal history. Two effects
associated to this nonlinear phase can give rise to new anisotropies: the
time-varying gravitational potential of nonlinear structures (Rees-Sciama RS
effect) and the inverse Compton scattering of the microwave photons with hot
electrons in clusters of galaxies (Sunyaev-Zeldovich SZ effect). These two
effects can produce distinct imprints on the CMB temperature anisotropy. We
discuss the amplitude of the anisotropies expected and the relevant angular
scales in different cosmological scenarios. Future sensitive experiments will
be able to probe the CMB anisotropies beyong the first order primary
contribution.Comment: plain tex, 16 pages, 3 figures. Proceedings of the Laredo Advance
School on Astrophysics "The universe at high-z, large-scale structure and the
cosmic microwave background". To be publised by Springer-Verla
Gravitational Wave Emission From a Binary Black Hole System in Presence of an Accretion Disk
We study time evolution and gravitational wave emission properties of a black
hole orbiting {\it inside} an accretion disk surrounding a massive black hole.
We simultaneously solve the structure equations of the accretion disk in
presence of heating, cooling and viscosity as well as the equations governing
the companion orbit. The deviation from Keplerian distribution of angular
momentum of the disk due to pressure and advection effects causes a significant
exchange of angular momentum between the disk and the companion. This
significantly affects the gravitational wave emission properties from the
binary system. We show that when the companion is light, the effect is
extremely important and must be taken into account while interpreting
gravitational wave signals from such systems.Comment: 8 pages of Latex plus postscript fil