1,144 research outputs found
Supermassive Black Holes from Ultra-Strongly Self-Interacting Dark Matter
We consider the cosmological consequences if a small fraction () of the dark matter is ultra-strongly self-interacting, with an elastic
self-interaction cross-section per unit mass .
This possibility evades all current constraints that assume that the
self-interacting component makes up the majority of the dark matter.
Nevertheless, even a small fraction of ultra-strongly self-interacting dark
matter (uSIDM) can have observable consequences on astrophysical scales. In
particular, the uSIDM subcomponent can undergo gravothermal collapse and form
seed black holes in the center of a halo. These seed black holes, which form
within several hundred halo interaction times, contain a few percent of the
total uSIDM mass in the halo. For reasonable values of , these black
holes can form at high enough redshifts to grow to quasars
by , alleviating tension within the standard CDM
cosmology. The ubiquitous formation of central black holes in halos could also
create cores in dwarf galaxies by ejecting matter during binary black hole
mergers, potentially resolving the "too big to fail" problem.Comment: submitted to Ap
The mystery of the cosmic vacuum energy density and the accelerated expansion of the Universe
After a short history of the -term it is explained why the
(effective) cosmological constant is expected to obtain contributions from
short-distance-physics, corresponding to an energy scale of at least 100 GeV.
The actual tiny value of the cosmological constant in any natural scale of
units represents, therefore, one of the deepest mysteries of present day
fundamental physics. We also briefly discuss recent astronomical evidence for a
cosmologically significant vacuum energy density causing an accelerating
expansion of the universe. This arises mainly from the Hubble diagram of type
Ia supernovae and from the observed temperature fluctuations of the cosmic
microwave background radiation. If this should become an established fact, we
are also confronted with a disturbing {\it cosmic coincidence} problem.Comment: 12 pages, 2 figures, iopart macros include
Preventing eternality in phantom inflation
We have investigated the necessary conditions that prevent phantom inflation
from being eternal. Allowing additionally for a nonminimal coupling between the
phantom field and gravity, we present the slow-climb requirements, perform an
analysis of the fluctuations, and finally we extract the overall conditions
that are necessary in order to prevent eternality. Furthermore, we verify our
results by solving explicitly the cosmological equations in a simple example of
an exponential potential, formulating the classical motion plus the stochastic
effect of the fluctuations through Langevin equations. Our analysis shows that
phantom inflation can be finite without the need of additional exotic
mechanisms.Comment: 8 pages, V2 references added. V3 version published in Phys. Rev.
Halo Properties in Cosmological Simulations of Self-Interacting Cold Dark Matter
We present a comparison of halo properties in cosmological simulations of
collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM)
for a range of dark matter cross sections. We find, in agreement with various
authors, that CDM yields cuspy halos that are too centrally concentrated as
compared to observations. Conversely, SIDM simulations using a Monte Carlo
N-body technique produce halos with significantly reduced central densities and
flatter cores with increasing cross section. We introduce a concentration
parameter based on enclosed mass that we expect will be straightforward to
determine observationally, unlike that of Navarro, Frenk & White, and provide
predictions for SIDM and CDM. SIDM also produces more spherical halos than CDM,
providing possibly the strongest observational test of SIDM. We discuss our
findings in relation to various relevant observations as well as SIDM
simulations of other groups. Taking proper account of simulation limitations,
we find that a dark matter cross section per unit mass of sigma_DM ~=
10^{-23}-10^{-24} cm^2/GeV is consistent with all current observational
constraints.Comment: 14 pages, submitted to Ap
A smooth bouncing cosmology with scale invariant spectrum
We present a bouncing cosmology which evolves from the contracting to the
expanding phase in a smooth way, without developing instabilities or
pathologies and remaining in the regime of validity of 4d effective field
theory. A nearly scale invariant spectrum of perturbations is generated during
the contracting phase by an isocurvature scalar with a negative exponential
potential and then converted to adiabatic. The model predicts a slightly blue
spectrum, n_S >~ 1, no observable gravitational waves and a high (but model
dependent) level of non-Gaussianities with local shape. The model represents an
explicit and predictive alternative to inflation, although, at present, it is
clearly less compelling.Comment: 20 pages, 1 fig. v2: references added, JCAP published versio
The Cosmic Triangle: Revealing the State of the Universe
The "cosmic triangle" is introduced as a way of representing the past,
present, and future status of the universe. Our current location within the
cosmic triangle is determined by the answers to three questions:
How much matter is in the universe?
Is the expansion rate slowing down or speeding up?
And, is the universe flat? A review of recent observations suggests a
universe that is lightweight (matter density about one-third the critical
value), is accelerating, and is flat. The acceleration implies the existence of
cosmic dark energy that overcomes the gravitational self-attraction of matter
and causes the expansion to speed up.Comment: 36 pages, 8 eps figures, use epsfig.sty, final draft of paper to
appear in Science 284, 1481-1488, (1999) Change to CMB plot, Fig. 6 (third
MSAM data point corrected) Addition of second CMB plot with more recent data
added Minor typo in figs. 1 and 2 correcte
Optical Absorption Characteristics of Silicon Nanowires for Photovoltaic Applications
Solar cells have generated a lot of interest as a potential source of clean
renewable energy for the future. However a big bottleneck in wide scale
deployment of these energy sources remain the low efficiency of these
conversion devices. Recently the use of nanostructures and the strategy of
quantum confinement have been as a general approach towards better charge
carrier generation and capture. In this article we have presented calculations
on the optical characteristics of nanowires made out of Silicon. Our
calculations show these nanowires form excellent optoelectronic materials and
may yield efficient photovoltaic devices
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