1,632 research outputs found
The Evolution of Bias - Generalized
Fry (1996) showed that galaxy bias has the tendency to evolve towards unity,
i.e. in the long run, the galaxy distribution tends to trace that of matter.
Generalizing slightly Fry's reasoning, we show that his conclusion remains
valid in theories of modified gravity (or equivalently, complex clustered dark
energy). This is not surprising: as long as both galaxies and matter are
subject to the same force, dynamics would drive them towards tracing each
other. This holds, for instance, in theories where both galaxies and matter
move on geodesics. This relaxation of bias towards unity is tempered by cosmic
acceleration, however: the bias tends towards unity but does not quite make it,
unless the formation bias were close to unity. Our argument is extended in a
straightforward manner to the case of a stochastic or nonlinear bias. An
important corollary is that dynamical evolution could imprint a scale
dependence on the large scale galaxy bias. This is especially pronounced if
non-standard gravity introduces new scales to the problem: the bias at
different scales relaxes at different rates, the larger scales generally more
slowly and retaining a longer memory of the initial bias. A consistency test of
the current (general relativity + uniform dark energy) paradigm is therefore to
look for departure from a scale independent bias on large scales. A simple way
is to measure the relative bias of different populations of galaxies which are
at different stages of bias relaxation. Lastly, we comment on the possibility
of directly testing the Poisson equation on cosmological scales, as opposed to
indirectly through the growth factor.Comment: 8 pages, 2 figures. References added. Accepted for publication in
Physical Review
Cosmological Baryon Sound Waves Coupled with the Primeval Radiation
The fluid equations for the baryon-electron system in an expanding universe
are derived from the Boltzmann equation. The effect of the Compton interaction
is taken into account properly in order to evaluate the photon-electron
collisional term. As an application, the acoustic motions of the
baryon-electron system after recombination are investigated. The effective
adiabatic index is computed for sound waves of various wavelengths,
assuming the perturbation amplitude is small. The oscillations are found to be
dumped when changes from between 1 (for an isothermal process) to 5/3
(for an adiabatic process).Comment: 20 pages, Revtex, Accepted for publication in Phys. Rev.
Cosmological Higgs fields
We present a time-dependent solution to the coupled Einstein-Higgs equations
for general Higgs-type potentials in the context of flat FRW cosmological
models. Possible implications are discussed.Comment: 5 pages, no figures. Version to be published in Phys. Rev. Lett.
Changes: references and citations added; introduction partly modified;
expanded discussion of relations between parameters in the Higgs potentia
Inflationary and dark energy regimes in 2+1 dimensions
In this work we investigate the behavior of three-dimensional (3D)
cosmological models. The simulation of inflationary and dark-energy-dominated
eras are among the possible results in these 3D formulations; taking as
starting point the results obtained by Cornish and Frankel.
Motivated by those results, we investigate, first, the inflationary case
where we consider a two-constituent cosmological fluid: the scalar field
represents the hypothetical inflaton which is in gravitational interaction with
a matter/radiation contribution. For the description of an old universe, it is
possible to simulate its evolution starting with a matter dominated universe
that faces a decelerated/accelerated transition due to the presence of the
additional constituent (simulated by the scalar field or ruled by an exotic
equation of state) that plays the role of dark energy. We obtain, through
numerical analysis, the evolution in time of the scale factor, the
acceleration, the energy densities, and the hydrostatic pressure of the
constituents. The alternative scalar cosmology proposed by Cornish and Frankel
is also under investigation in this work. In this case an inflationary model
can be constructed when another non-polytropic equation of state (the van der
Waals equation) is used to simulate the behavior of an early 3D universe.Comment: Latex file, plus 9 figures. To appear in General Relativity and
Gravitatio
Halo Sampling, Local Bias and Loop Corrections
We develop a new test of local bias, by constructing a locally biased halo
density field from sampling the dark matter-halo distribution. Our test differs
from conventional tests in that it preserves the full scatter in the bias
relation and it does not rely on perturbation theory. We put forward that bias
parameters obtained using a smoothing scale R can only be applied to computing
the halo power spectrum at scales k ~ 1/R. Our calculations can automatically
include the running of bias parameters and give vanishingly small loop
corrections at low-k. Our proposal results in much better agreement of the
sampling and perturbation theory results with simulations. In particular,
unlike the standard interpretation of local bias in the literature, our
treatment of local bias does not generate a constant power in the low-k limit.
We search for extra noise in the Poisson corrected halo power spectrum at
wavenumbers below its turn-over and find no evidence of significant positive
noise (as predicted by the standard interpretation) while we find evidence of
negative noise coming from halo exclusion for very massive halos. Using
perturbation theory and our non-perturbative sampling technique we also
demonstrate that nonlocal bias effects discovered recently in simulations
impact the power spectrum only at the few percent level in the weakly nonlinear
regime.Comment: 25 pages, 14 figures; V2: significant revision including more details
about halo exclusion and low-k noise. Conclusions unchange
Ewald Sums for One Dimension
We derive analytic solutions for the potential and field in a one-dimensional
system of masses or charges with periodic boundary conditions, in other words
Ewald sums for one dimension. We also provide a set of tools for exploring the
system evolution and show that it's possible to construct an efficient
algorithm for carrying out simulations. In the cosmological setting we show
that two approaches for satisfying periodic boundary conditions, one overly
specified and the other completely general, provide a nearly identical
clustering evolution until the number of clusters becomes small, at which time
the influence of any size-dependent boundary cannot be ignored. Finally we
compare the results with other recent work with the hope of providing
clarification over differences these issues have induced. We explain that
modern formulations of physics require a well defined potential which is not
available if the forces are screened directly.Comment: 2 figures added references expanded discussion of algorithm corrected
figures added discussion of screened forc
Compact massive objects in Virgo galaxies: the black hole population
We investigate the distribution of massive black holes (MBHs) in the Virgo
cluster. Observations suggest that AGN activity is widespread in massive
galaxies (M>1e10 solar masses), while at lower galaxy masses star clusters are
more abundant, which might imply a limited presence of central black holes in
these galaxy-mass regimes. We explore if this possible threshold in MBH
hosting, is linked to nature, nurture, or a mixture of both. The nature
scenario arises naturally in hierarchical cosmologies, as MBH formation
mechanisms typically are efficient in biased systems, which would later evolve
into massive galaxies. Nurture, in the guise of MBH ejections following MBH
mergers, provides an additional mechanism that is more effective for low mass,
satellite galaxies. The combination of inefficient formation, and lower
retention of MBHs, leads to the natural explanation of the distribution of
compact massive ob jects in Virgo galaxies. If MBHs arrive to the correlation
with the host mass and velocity dispersion during merger-triggered accretion
episodes, sustained tidal stripping of the host galaxies creates a population
of MBHs which lie above the expected scaling between the holes and their host
mass, suggesting a possible environmental dependence.Comment: MNRAS letter
Cosmic axion thermalization
Axions differ from the other cold dark matter candidates in that they form a
degenerate Bose gas. It is shown that their huge quantum degeneracy and large
correlation length cause cold dark matter axions to thermalize through
gravitational self-interactions when the photon temperature reaches
approximately 500 eV. When they thermalize, the axions form a Bose-Einstein
condensate. Their thermalization occurs in a regime, herein called the
`condensed regime', where the Boltzmann equation is not valid because the
energy dispersion of the particles is smaller than their interaction rate. We
derive analytical expressions for the thermalization rate of particles in the
condensed regime, and check the validity of these expressions by numerical
simulation of a toy model. We revisit axion cosmology in light of axion
Bose-Einstein condensation. It is shown that axions are indistinguishable from
ordinary cold dark matter on all scales of observational interest, except when
they thermalize or rethermalize. The rethermalization of axions that are about
to fall in a galactic potential well causes them to acquire net overall
rotation as they go to the lowest energy state consistent with the total
angular momentum they acquired by tidal torquing. This phenomenon explains the
occurrence of caustic rings of dark matter in galactic halos. We find that
photons may reach thermal contact with axions and investigate the implications
of this possibility for the measurements of cosmological parameters.Comment: 38 pages, 1 figur
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