1,470 research outputs found
Reproducing the observed Cosmic microwave background anisotropies with causal scaling seeds
During the last years it has become clear that global O(N) defects and U(1)
cosmic strings do not lead to the pronounced first acoustic peak in the power
spectrum of anisotropies of the cosmic microwave background which has recently
been observed to high accuracy. Inflationary models cannot easily accommodate
the low second peak indicated by the data. Here we construct causal scaling
seed models which reproduce the first and second peak. Future, more precise CMB
anisotropy and polarization experiments will however be able to distinguish
them from the ordinary adiabatic models.Comment: 6 pages 2 figures, revtex; minor corrections and references adde
Cosmic Microwave Background Temperature at Galaxy Clusters
We have deduced the cosmic microwave background (CMB) temperature in the Coma
cluster (A1656, ), and in A2163 () from spectral
measurements of the Sunyaev-Zel'dovich (SZ) effect over four passbands at radio
and microwave frequencies. The resulting temperatures at these redshifts are
K and K, respectively. These values confirm the expected
relation , where K is the value
measured by the COBE/FIRAS experiment. Alternative scaling relations that are
conjectured in non-standard cosmologies can be constrained by the data; for
example, if or , then
and (at 95% confidence). We
briefly discuss future prospects for more precise SZ measurements of at
higher redshifts.Comment: 13 pages, 1 figure, ApJL accepted for publicatio
Towards a future singularity?
We discuss whether the future extrapolation of the present cosmological state
may lead to a singularity even in case of "conventional" (negative) pressure of
the dark energy field, namely . The discussion is based on an
often neglected aspect of scalar-tensor models of gravity: the fact that
different test particles may follow the geodesics of different metric frames,
and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on
Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and
selected for "Honorable Mention
Constraints on a New Post-General Relativity Cosmological Parameter
A new cosmological variable is introduced which characterizes the degree of
departure from Einstein's General Relativity (GR) with a cosmological constant.
The new parameter, \varpi, is the cosmological analog of \gamma, the
parametrized post-Newtonian variable which measures the amount of spacetime
curvature per unit mass. In the cosmological context, \varpi measures the
difference between the Newtonian and longitudinal potentials in response to the
same matter sources, as occurs in certain scalar-tensor theories of gravity.
Equivalently, \varpi measures the scalar shear fluctuation in a dark energy
component. In the context of a "vanilla" LCDM background cosmology, a non-zero
\varpi signals a departure from GR or a fluctuating cosmological constant.
Using a phenomenological model for the time evolution \varpi=\varpi_0
\rho_{DE}/\rho_{M} which depends on the ratio of energy density in the
cosmological constant to the matter density at each epoch, it is shown that the
observed cosmic microwave background (CMB) temperature anisotropies limit the
overall normalization constant to be -0.4 < \varpi_0 < 0.1 at the 95%
confidence level. Existing measurements of the cross-correlations of the CMB
with large-scale structure further limit \varpi_0 > -0.2 at the 95% CL. In the
future, integrated Sachs-Wolfe and weak lensing measurements can more tightly
constrain \varpi_0, providing a valuable clue to the nature of dark energy and
the validity of GR.Comment: 9 pages, 7 figures; added reference
Limitations to the Accuracy of Cosmic Background Radiation Anisotropy Measurements: Atmospheric Fluctuations
We discuss the ultimate limits posed by atmospheric fluctuations to observations of cosmic background anisotropies (CBAs) in ground-based and balloon-borne experiments both in the radio and millimetric regions. We present correlation techniques useful in separating CBAs from atmospheric fluctuations. An experimental procedure is discussed for testing a site in view of possible CBA observations. Four sites with altitudes ranging from 0 up to 3.5 km have been tested
Triple Experiment Spectrum of the Sunyaev-Zeldovich Effect in the Coma Cluster: H_0
The Sunyaev-Zeldovich (SZ) effect was previously measured in the Coma cluster
by the Owens Valley Radio Observatory and Millimeter and IR Testa Grigia
Observatory experiments and recently also with the Wilkinson Microwave
Anisotropy Probe satellite. We assess the consistency of these results and
their implications on the feasibility of high-frequency SZ work with
ground-based telescopes. The unique data set from the combined measurements at
six frequency bands is jointly analyzed, resulting in a best-fit value for the
Thomson optical depth at the cluster center, tau_{0}=(5.35 \pm 0.67) 10^{-3}.
The combined X-ray and SZ determined properties of the gas are used to
determine the Hubble constant. For isothermal gas with a \beta density profile
we derive H_0 = 84 \pm 26 km/(s\cdot Mpc); the (1\sigma) error includes only
observational SZ and X-ray uncertainties.Comment: 11 pages, 1 figur
Planck-scale modifications to Electrodynamics characterized by a space-like symmetry-breaking vector
In the study of Planck-scale ("quantum-gravity induced") violations of
Lorentz symmetry, an important role was played by the deformed-electrodynamics
model introduced by Myers and Pospelov. Its reliance on conventional effective
quantum field theory, and its description of symmetry-violation effects simply
in terms of a four-vector with nonzero component only in the time-direction,
rendered it an ideal target for experimentalists and a natural concept-testing
ground for many theorists. At this point however the experimental limits on the
single Myers-Pospelov parameter, after improving steadily over these past few
years, are "super-Planckian", {\it i.e.} they take the model out of actual
interest from a conventional quantum-gravity perspective. In light of this we
here argue that it may be appropriate to move on to the next level of
complexity, still with vectorial symmetry violation but adopting a generic
four-vector. We also offer a preliminary characterization of the phenomenology
of this more general framework, sufficient to expose a rather significant
increase in complexity with respect to the original Myers-Pospelov setup. Most
of these novel features are linked to the presence of spatial anisotropy, which
is particularly pronounced when the symmetry-breaking vector is space-like, and
they are such that they reduce the bound-setting power of certain types of
observations in astrophysics
An improved cosmological bound on the thermal axion mass
Relic thermal axions could play the role of an extra hot dark matter
component in cosmological structure formation theories. By combining the most
recent observational data we improve previous cosmological bounds on the axion
mass m_a in the so-called hadronic axion window. We obtain a limit on the axion
mass m_a < 0.42eV at the 95% c.l. (m_a < 0.72eV at the 99% c.l.). A novel
aspect of the analysis presented here is the inclusion of massive neutrinos and
how they may affect the bound on the axion mass. If neutrino masses belong to
an inverted hierarchy scheme, for example, the above constraint is improved to
m_a < 0.38eV at the 95% c.l. (m_a < 0.67eV at the 99% c.l.). Future data from
experiments as CAST will provide a direct test of the cosmological bound.Comment: 5 Pages, 3 Figure
The power spectrum of systematics in cosmic shear tomography and the bias on cosmological parameters
Cosmic shear tomography has emerged as one of the most promising tools to
both investigate the nature of dark energy and discriminate between General
Relativity and modified gravity theories. In order to successfully achieve
these goals, systematics in shear measurements have to be taken into account;
their impact on the weak lensing power spectrum has to be carefully
investigated in order to estimate the bias induced on the inferred cosmological
parameters. To this end, we develop here an efficient tool to compute the power
spectrum of systematics by propagating, in a realistic way, shear measurement,
source properties and survey setup uncertainties. Starting from analytical
results for unweighted moments and general assumptions on the relation between
measured and actual shear, we derive analytical expressions for the
multiplicative and additive bias, showing how these terms depend not only on
the shape measurement errors, but also on the properties of the source galaxies
(namely, size, magnitude and spectral energy distribution). We are then able to
compute the amplitude of the systematics power spectrum and its scaling with
redshift, while we propose a multigaussian expansion to model in a
non-parametric way its angular scale dependence. Our method allows to
self-consistently propagate the systematics uncertainties to the finally
observed shear power spectrum, thus allowing us to quantify the departures from
the actual spectrum. We show that even a modest level of systematics can induce
non-negligible deviations, thus leading to a significant bias on the recovered
cosmological parameters.Comment: 19 pages, 5 tables, 4 figure
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