2,942 research outputs found
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
A stacking method to study the gamma-ray emission of source samples based on the co-adding of Fermi LAT count maps
We present a stacking method that makes use of co-added maps of gamma-ray
counts produced from data taken with the Fermi Large Area Telescope. Sources
with low integrated gamma-ray fluxes that are not detected individually may
become detectable when their corresponding count maps are added. The combined
data set is analyzed with a maximum likelihood method taking into account the
contribution from point-like and diffuse background sources. For both simulated
and real data, detection significance and integrated gamma-ray flux are
investigated for different numbers of stacked sources using the public Fermi
Science Tools for analysis and data preparation. The co-adding is done such
that potential source signals add constructively, in contrast to the signals
from background sources, which allows the stacked data to be described with
simply structured models. We show, for different scenarios, that the stacking
method can be used to increase the cumulative significance of a sample of
sources and to characterize the corresponding gamma-ray emission. The method
can, for instance, help to search for gamma-ray emission from galaxy clusters.Comment: accepted for publication in Astronomy & Astrophysics, 10 pages, 12
figure
Wyman's solution, self-similarity and critical behaviour
We show that the Wyman's solution may be obtained from the four-dimensional
Einstein's equations for a spherically symmetric, minimally coupled, massless
scalar field by using the continuous self-similarity of those equations. The
Wyman's solution depends on two parameters, the mass and the scalar charge
. If one fixes to a positive value, say , and let
take values along the real line we show that this solution exhibits critical
behaviour. For the space-times have eternal naked singularities,
for one has a Schwarzschild black hole of mass and finally
for one has eternal bouncing solutions.Comment: Revtex version, 15pages, 6 figure
Charged Condensate and Helium Dwarf Stars
White dwarf stars composed of carbon, oxygen or heavier elements are expected
to crystallize as they cool down below certain temperatures. Yet, simple
arguments suggest that the helium white dwarf cores may not solidify, mostly
because of zero-point oscillations of the helium ions that would dissolve the
crystalline structure. We argue that the interior of the helium dwarfs may
instead form a macroscopic quantum state in which the charged helium-4 nuclei
are in a Bose-Einstein condensate, while the relativistic electrons form a
neutralizing degenerate Fermi liquid. We discuss the electric charge screening,
and the spectrum of this substance, showing that the bosonic long-wavelength
fluctuations exhibit a mass gap. Hence, there is a suppression at low
temperatures of the boson contribution to the specific heat -- the latter being
dominated by the specific heat of the electrons near the Fermi surface. This
state of matter may have observational signatures.Comment: 10 pages; v2: to appear in JCAP, brief comments and section titles
added, typos correcte
Gravitomagnetism, clocks and geometry
New techniques to evaluate the clock effect using light are described. These
are based on the flatness of the cylindrical surface containing the world lines
of the rays constrained to move on circular trajectories about a spinning mass.
The effect of the angular momentum of the source is manifested in the fact that
inertial observers must be replaced by local non rotating observers. Starting
from this an exact formula for circular trajectories is found. Numerical
estimates for the Earth environment show that light would be a better probe
than actual clocks to evidence the angular momentum influence. The advantages
of light in connection with some principle experiments are shortly reviewed.Comment: TCI Latex, 12 pages, 2 figures. To appear in European Journal of
Physic
The linear growth rate of structure in Parametrized Post Friedmannian Universes
A possible solution to the dark energy problem is that Einstein's theory of
general relativity is modified. A suite of models have been proposed that, in
general, are unable to predict the correct amount of large scale structure in
the distribution of galaxies or anisotropies in the Cosmic Microwave
Background. It has been argued, however, that it should be possible to
constrain a general class of theories of modified gravity by focusing on
properties such as the growing mode, gravitational slip and the effective, time
varying Newton's constant. We show that assuming certain physical requirements
such as stability, metricity and gauge invariance, it is possible to come up
with consistency conditions between these various parameters. In this paper we
focus on theories which have, at most, 2nd derivatives in the metric variables
and find restrictions that shed light on current and future experimental
constraints without having to resort to a (as yet unknown) complete theory of
modified gravity. We claim that future measurements of the growth of structure
on small scales (i.e. from 1-200 h^{-1} Mpc) may lead to tight constraints on
both dark energy and modified theories of gravity.Comment: 15 Pages, 11 Figure
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