1,385 research outputs found
Dark matter powered stars: Constraints from the extragalactic background light
The existence of predominantly cold non-baryonic dark matter is unambiguously
demonstrated by several observations (e.g., structure formation, big bang
nucleosynthesis, gravitational lensing, and rotational curves of spiral
galaxies). A candidate well motivated by particle physics is a weakly
interacting massive particle (WIMP). Self-annihilating WIMPs would affect the
stellar evolution especially in the early universe. Stars powered by
self-annihilating WIMP dark matter should possess different properties compared
with standard stars. While a direct detection of such dark matter powered stars
seems very challenging, their cumulative emission might leave an imprint in the
diffuse metagalactic radiation fields, in particular in the mid-infrared part
of the electromagnetic spectrum. In this work the possible contributions of
dark matter powered stars (dark stars; DSs) to the extragalactic background
light (EBL) are calculated. It is shown that existing data and limits of the
EBL intensity can already be used to rule out some DS parameter sets.Comment: Accepted for publication in ApJ; 7 pages, 5 figure
Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy
The previous proposal (by two of us) of chain inflation with the QCD axion is
shown to fail. The proposal involved a series of fast tunneling events, yet
here it is shown that tunneling is too slow. We calculate the bubble nucleation
rates for phase transitions in the thick wall limit, approximating the barrier
by a triangle. A similar problem arises in realization of chain inflation in
the string landscape that uses series of minima along the monodromy staircase
around the conifold point. The basic problem is that the minima of the
potential are too far apart to allow rapid enough tunneling in these two
models. We entertain the possibility of overcoming this problem by modifying
the gravity sector to a Brans-Dicke theory. However, one would need extremely
small values for the Brans-Dicke parameter. Many successful alternatives exist,
including other "axions" (with mass scales not set by QCD) or potentials with
comparable heights and widths that do not suffer from the problem of slow
tunneling and provide successful candidates for chain inflation.Comment: 6 pages, 1 figur
A black hole solution to the cosmological monopole problem
We propose a solution to the cosmological monopole problem: Primordial black
holes, produced in the early universe, can accrete magnetic monopoles before
the relics dominate the energy density of the universe. These small black holes
quickly evaporate and thereby convert most of the monopole energy density into
radiation. We estimate the range of parameters for which this solution is
possible: under very conservative assumptions we find that the black hole mass
must be less than 10^9 gm.Comment: accepted for publication in Phys. Lett.
Observational constraints on supermassive dark stars
Some of the first stars could be cooler and more massive than standard
stellar models would suggest, due to the effects of dark matter annihilation in
their cores. It has recently been argued that such objects may attain masses in
the 10^4--10^7 solar mass range, and that such supermassive dark stars should
be within reach of the upcoming James Webb Space Telescope. Notwithstanding
theoretical difficulties with this proposal, we argue here that some of these
objects should also be readily detectable with both the Hubble Space Telescope
and ground-based 8--10 m class telescopes. Existing survey data already place
strong constraints on 10^7 solar mass dark stars at z~10. We show that such
objects must be exceedingly rare or short-lived to have avoided detection.Comment: 6 pages, 4 figures. v3: erratum incorporate
Devaluation: a dynamical mechanism for a naturally small cosmological constant
We propose a natural solution to the cosmological constant problem consistent
with the standard cosmology and successful over a broad range of energies. This
solution is based on the existence of a new field, the devaluton, with its
potential modeled on a tilted cosine. After inflation, the universe reheats and
populates the devaluton's many minima. As the universe cools, domain walls form
between different regions. The domain wall network then evolves and sweeps away
regions of higher vacuum energy in favor of lower energy ones. Gravitation
itself provides a cutoff at a minimum vacuum energy, thus leaving the universe
with a small cosmological constant comparable in magnitude to the present day
dark energy density.Comment: 6 pages and prepared in ReV-TeX added notes on eltro-weak breaking
and ds vacu
Inflating with the QCD Axion
We show that the QCD axion can drive inflation via a series of tunneling
events. For axion models with a softly broken symmetry, the axion
potential has a series of local minima and may be modeled by a tilted
cosine. Chain inflation results along this tilted cosine: the field tunnels
from an initial minimum near the top of the potential through a series of ever
lower minima to the bottom. This results in sufficient inflation and reheating.
QCD axions, potentially detectable in current searches, may thus simultaneously
solve problems in particle physics and provide inflation.Comment: 5 pages, 1 figure, revised for submission to PR
Protogalactic Extension of the Parker Bound
We extend the Parker bound on the galactic flux of magnetic
monopoles. By requiring that a small initial seed field must survive the
collapse of the protogalaxy, before any regenerative dynamo effects become
significant, we develop a stronger bound. The survival and continued growth of
an initial galactic seed field G demand that . For a given
monopole mass, this bound is four and a half orders of magnitude more stringent
than the previous `extended Parker bound', but is more speculative as it
depends on assumptions about the behavior of magnetic fields during
protogalactic collapse. For monopoles which do not overclose the Universe
(), the maximum flux allowed is now cm^{-2}
s^{-1} sr^{-1}, a factor of 150 lower than the maximum flux allowed by the
extended Parker bound.Comment: 9 pages, 1 eps figur
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