2,519 research outputs found
Measuring the Topology of the Universe
Observations of microwave background fluctuations can yield information not
only about the geometry of the universe, but potentially about the topology of
the universe. If the universe is negatively curved, then the characteristic
scale for the topology of the universe is the curvature radius. Thus, if we are
seeing the effects of the geometry of the universe, we can hope to soon see
signatures of the topology of the universe. The cleanest signature of the
topology of the universe is written on the microwave sky: there should be
thousands of pairs of matched circles. These circles can be used to determine
the precise topology and volume of the universe. Since we see hundreds of
slices through the fundamental domain of the universe, we can use the microwave
observations to reconstruct the initial conditions of the entire universe on
the scale of a few Megaparsecs.Comment: 11 pages, LaTex, Talk at NAS Cosmology Conference, Irvine, CA, March
199
Top-Down Fragmentation of a Warm Dark Matter Filament
We present the first high-resolution n-body simulations of the fragmentation
of dark matter filaments. Such fragmentation occurs in top-down scenarios of
structure formation, when the dark matter is warm instead of cold. In a
previous paper (Knebe et al. 2002, hereafter Paper I), we showed that WDM
differs from the standard Cold Dark Matter (CDM) mainly in the formation
history and large-scale distribution of low-mass haloes, which form later and
tend to be more clustered in WDM than in CDM universes, tracing more closely
the filamentary structures of the cosmic web. Therefore, we focus our
computational effort in this paper on one particular filament extracted from a
WDM cosmological simulation and compare in detail its evolution to that of the
same CDM filament. We find that the mass distribution of the halos forming via
fragmentation within the filament is broadly peaked around a Jeans mass of a
few 10^9 Msun, corresponding to a gravitational instability of smooth regions
with an overdensity contrast around 10 at these redshifts. Our results confirm
that WDM filaments fragment and form gravitationally bound haloes in a top-down
fashion, whereas CDM filaments are built bottom-up, thus demonstrating the
impact of the nature of the dark matter on dwarf galaxy properties.Comment: 7 pages, 7 figures, replaced with MNRAS accepted version (minor
revisions
Neutrinos and Primordial Nucleosynthesis
The importance of the Big Bang Nucleosynthesis (BBN) as a unique tool for
studying neutrino properties is discussed, and the recent steps towards a
self-consistent and robust handling of the weak reaction decoupling from the
thermal bath as well as of the neutrino reheating following the e+e-
annihilation are summarized. We also emphasize the important role of the Cosmic
Microwave Background (CMB) anisotropy in providing an accurate and independent
determination of the baryon density parameter omegab. The BBN is presently a
powerful parameter-free theory that can test the standard scenario of the
neutrino decoupling in the early Universe. Moreover it can constrain new
physics in the neutrino sector. The perspectives for improvements in the next
years are outlined.Comment: Talk given by G. Mangano at NOW2004, Conca Specchiulla, Otranto
Italy, september 2004. To appear in the Proceedings of the Worksho
Natural extension of the Generalised Uncertainty Principle
We discuss a gedanken experiment for the simultaneous measurement of the
position and momentum of a particle in de Sitter spacetime. We propose an
extension of the so-called generalized uncertainty principle (GUP) which
implies the existence of a minimum observable momentum. The new GUP is directly
connected to the nonzero cosmological constant, which becomes a necessary
ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as
published in CQ
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
Weak Gravity Conjecture and Holographic Dark Energy Model with Interaction and Spatial Curvature
In the paper, we apply the weak gravity conjecture to the holographic
quintessence model of dark energy. Three different holographic dark energy
models are considered: without the interaction in the non-flat universe; with
interaction in the flat universe; with interaction in the non-flat universe. We
find that only in the models with the spatial curvature and interaction term
proportional to the energy density of matter, it is possible for the weak
gravity conjecture to be satisfied.Comment: 14 pages, 7 figures, typographical errors are corrected; conclusin is
unchange
Exact Polynomial Eigenmodes for Homogeneous Spherical 3-Manifolds
Observational data hints at a finite universe, with spherical manifolds such
as the Poincare dodecahedral space tentatively providing the best fit.
Simulating the physics of a model universe requires knowing the eigenmodes of
the Laplace operator on the space. The present article provides explicit
polynomial eigenmodes for all globally homogeneous 3-manifolds: the Poincare
dodecahedral space S3/I*, the binary octahedral space S3/O*, the binary
tetrahedral space S3/T*, the prism manifolds S3/D_m* and the lens spaces
L(p,1).Comment: v3. Final published version. 27 pages, 1 figur
Anisotropy of the Cosmic Neutrino Background
The cosmic neutrino background (CNB) consists of low-energy relic neutrinos
which decoupled from the cosmological fluid at a redshift z ~ 10^{10}. Despite
being the second-most abundant particles in the universe, direct observation
remains a distant challenge. Based on the measured neutrino mass differences,
one species of neutrinos may still be relativistic with a thermal distribution
characterized by the temperature T ~ 1.9K. We show that the temperature
distribution on the sky is anisotropic, much like the photon background,
experiencing Sachs-Wolfe and integrated Sachs-Wolfe effects.Comment: 5 pages, 2 figures / updated references, discussion of earlier wor
Corrections to Gravity due to a Sol Manifold Extra Dimensional Space
The corrections to the gravitational potential due to a Sol extra dimensional
compact manifold, denoted as , are studied. The total spacetime is of
the form . The range of the Sol corrections is investigated
and compared to the range of the corrections.Comment: 13 pages, 10 figures, published versio
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