628 research outputs found
Dynamical Dark Matter: II. An Explicit Model
In a recent paper (arXiv:1106.4546), we introduced "dynamical dark matter," a
new framework for dark-matter physics, and outlined its underlying theoretical
principles and phenomenological possibilities. Unlike most traditional
approaches to the dark-matter problem which hypothesize the existence of one or
more stable dark-matter particles, our dynamical dark-matter framework is
characterized by the fact that the requirement of stability is replaced by a
delicate balancing between cosmological abundances and lifetimes across a vast
ensemble of individual dark-matter components. This setup therefore
collectively produces a time-varying cosmological dark-matter abundance, and
the different dark-matter components can interact and decay throughout the
current epoch. While the goal of our previous paper was to introduce the broad
theoretical aspects of this framework, the purpose of the current paper is to
provide an explicit model of dynamical dark matter and demonstrate that this
model satisfies all collider, astrophysical, and cosmological constraints. The
results of this paper therefore constitute an "existence proof" of the
phenomenological viability of our overall dynamical dark-matter framework, and
demonstrate that dynamical dark matter is indeed a viable alternative to the
traditional paradigm of dark-matter physics. Dynamical dark matter must
therefore be considered alongside other approaches to the dark-matter problem,
particularly in scenarios involving large extra dimensions or string theory in
which there exist large numbers of particles which are neutral under
Standard-Model symmetries.Comment: 45 pages, LaTeX, 10 figures. Replaced to match published versio
Dark Radiation Emerging After Big Bang Nucleosynthesis?
We show how recent data from observations of the cosmic microwave background
may suggest the presence of additional radiation density which appeared after
big bang nucleosynthesis. We propose a general scheme by which this radiation
could be produced from the decay of non-relativistic matter, we place
constraints on the properties of such matter, and we give specific examples of
scenarios in which this general scheme may be realized.Comment: v3: 5 pages, 1 figure. References added, typos corrected, notation
changed throughout. v2: 5 pages, 1 figure. Reformatted, references added,
acknowledgments updated, effect of radiation on CMB clarified. v1: 11 pages,
1 figur
Bulk Viscosity, Decaying Dark Matter, and the Cosmic Acceleration
We discuss a cosmology in which cold dark-matter particles decay into
relativistic particles. We argue that such decays could lead naturally to a
bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the
present hubble age, this bulk viscosity enters the cosmic energy equation as an
effective negative pressure. We investigate whether this negative pressure is
of sufficient magnitude to account fo the observed cosmic acceleration. We show
that a single decaying species in a flat, dark-matter dominated cosmology
without a cosmological constant cannot reproduce the observed
magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk
viscosity, possibly due to a cascade of decaying particles may be able to
account for a significant fraction of the apparent cosmic acceleration.
Possible candidate nonrelativistic particles for this scenario include sterile
neutrinos or gauge-mediated decaying supersymmetric particles.Comment: 7 pages, 4 figure
Statistical characteristics of formation and evolution of structure in the universe
An approximate statistical description of the formation and evolution of
structure of the universe based on the Zel'dovich theory of gravitational
instability is proposed. It is found that the evolution of DM structure shows
features of self-similarity and the main structure characteristics can be
expressed through the parameters of initial power spectrum and cosmological
model. For the CDM-like power spectrum and suitable parameters of the
cosmological model the effective matter compression reaches the observed scales
20 -- 25Mpc with the typical mean separation of
wall-like elements 50 -- 70Mpc. This description can be
directly applied to the deep pencil beam galactic surveys and absorption
spectra of quasars. For larger 3D catalogs and simulations it can be applied to
results obtained with the core-sampling analysis.
It is shown that the interaction of large and small scale perturbations
modulates the creation rate of early Zel'dovich pancakes and generates bias on
the SLSS scale. For suitable parameters of the cosmological model and reheating
process this bias can essentially improve the characteristics of simulated
structure of the universe.
The models with give the best description of the
observed structure parameters. The influence of low mass "warm" dark matter
particles, such as a massive neutrino, will extend the acceptable range of
and .Comment: 20pages, 7 figures, MNRAS in pres
Gravitational Instability in Collisionless Cosmological Pancakes
The gravitational instability of cosmological pancakes composed of
collisionless dark matter in an Einstein-de Sitter universe is investigated
numerically to demonstrate that pancakes are unstable with respect to
fragmentation and the formation of filaments. A ``pancake'' is defined here as
the nonlinear outcome of the growth of a 1D, sinusoidal, plane-wave, adiabatic
density perturbation. We have used high resolution, 2D, N-body simulations by
the Particle-Mesh (PM) method to study the response of pancakes to perturbation
by either symmetric (density) or antisymmetric (bending or rippling) modes,
with corresponding wavevectors k_s and k_a transverse to the wavevector k_p of
the unperturbed pancake plane-wave. We consider dark matter which is initially
``cold'' (i.e. with no random thermal velocity in the initial conditions). We
also investigate the effect of a finite, random, isotropic, initial velocity
dispersion (i.e. initial thermal velocity) on the fate of pancake collapse and
instability. Pancakes are shown to be gravitationally unstable with respect to
all perturbations of wavelength l<l_p (where l_p= 2pi/k_p). These results are
in contradiction with the expectations of an approximate, thin-sheet energy
argument.Comment: To appear in the Astrophysical Journal (1997), accepted for
publication 10/10/96, single postscript file, 61 pages, 19 figure
Limits on MeV Dark Matter from the Effective Number of Neutrinos
Thermal dark matter that couples more strongly to electrons and photons than
to neutrinos will heat the electron-photon plasma relative to the neutrino
background if it becomes nonrelativistic after the neutrinos decouple from the
thermal background. This results in a reduction in N_eff below the
standard-model value, a result strongly disfavored by current CMB observations.
Taking conservative lower bounds on N_eff and on the decoupling temperature of
the neutrinos, we derive a bound on the dark matter particle mass of m_\chi >
3-9 MeV, depending on the spin and statistics of the particle. For p-wave
annihilation, our limit on the dark matter particle mass is stronger than the
limit derived from distortions to the CMB fluctuation spectrum produced by
annihilations near the epoch of recombination.Comment: 5 pages, 1 figure, discussion added, references added and updated,
labels added to figure, to appear in Phys. Rev.
Interior of Distorted Black Holes
We study the interior of distorted static axisymmetric black holes. We obtain
a general interior solution and study its asymptotics both near the horizon and
singularity. As a special example, we apply the obtained results to the case of
the so-called `caged' black holes.Comment: 12 pages, 16 figure
Responses of the Brans-Dicke field due to gravitational collapses
We study responses of the Brans-Dicke field due to gravitational collapses of
scalar field pulses using numerical simulations. Double-null formalism is
employed to implement the numerical simulations. If we supply a scalar field
pulse, it will asymptotically form a black hole via dynamical interactions of
the Brans-Dicke field. Hence, we can observe the responses of the Brans-Dicke
field by two different regions. First, we observe the late time behaviors after
the gravitational collapse, which include formations of a singularity and an
apparent horizon. Second, we observe the fully dynamical behaviors during the
gravitational collapse and view the energy-momentum tensor components. For the
late time behaviors, if the Brans-Dicke coupling is greater (or smaller) than
-1.5, the Brans-Dicke field decreases (or increases) during the gravitational
collapse. Since the Brans-Dicke field should be relaxed to the asymptotic value
with the elapse of time, the final apparent horizon becomes time-like (or
space-like). For the dynamical behaviors, we observed the energy-momentum
tensors around ~ -1.5. If the Brans-Dicke coupling is greater than
-1.5, the component can be negative at the outside of the black hole.
This can allow an instantaneous inflating region during the gravitational
collapse. If the Brans-Dicke coupling is less than -1.5, the oscillation of the
component allows the apparent horizon to shrink. This allows a
combination that violates weak cosmic censorship. Finally, we discuss the
implications of the violation of the null energy condition and weak cosmic
censorship.Comment: 28 pages, 14 figure
On the Possibility of Anisotropic Curvature in Cosmology
In addition to shear and vorticity a homogeneous background may also exhibit
anisotropic curvature. Here a class of spacetimes is shown to exist where the
anisotropy is solely of the latter type, and the shear-free condition is
supported by a canonical, massless 2-form field. Such spacetimes possess a
preferred direction in the sky and at the same time a CMB which is isotropic at
the background level. A distortion of the luminosity distances is derived and
used to test the model against the CMB and supernovae (using the Union
catalog), and it is concluded that the latter exhibit a higher-than-expected
dependence on angular position. It is shown that future surveys could detect a
possible preferred direction by observing ~ 20 / (\Omega_{k0}^2) supernovae
over the whole sky.Comment: Extended SNe analysis and corrected some CMB results. Text also
extended and references added. 8 pages, 5 figure
The Statistics of Density Peaks and the Column Density Distribution of the Lyman-Alpha Forest
We develop a method to calculate the column density distribution of the
Lyman-alpha forest for column densities in the range . The Zel'dovich approximation, with appropriate smoothing, is used to
compute the density and peculiar velocity fields. The effect of the latter on
absorption profiles is discussed and it is shown to have little effect on the
column density distribution. An approximation is introduced in which the column
density distribution is related to a statistic of density peaks (involving its
height and first and second derivatives along the line of sight) in real space.
We show that the slope of the column density distribution is determined by the
temperature-density relation as well as the power spectrum on scales . An expression relating the three is given. We
find very good agreement between the column density distribution obtained by
applying the Voigt-profile-fitting technique to the output of a full
hydrodynamic simulation and that obtained using our approximate method for a
test model. This formalism then is applied to study a group of CDM as well as
CHDM models. We show that the amplitude of the column density distribution
depends on the combination of parameters , which is not well-constrained by independent observations. The
slope of the distribution, on the other hand, can be used to distinguish
between different models: those with a smaller amplitude and a steeper slope of
the power spectrum on small scales give rise to steeper distributions, for the
range of column densities we study. Comparison with high resolution Keck data
is made.Comment: match accepted version; discussion added: the effect of the shape of
the power spectrum on the slope of the column density distributio
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