667 research outputs found
Still flat after all these years
The Universe could be spatially flat, positively curved or negatively curved.
Each option has been popular at various times, partly affected by an
understanding that models tend to evolve away from flatness. The curvature of
the Universe is amenable to measurement, through tests such as the
determination of the angles of sufficiently large triangles. The angle
subtended by the characteristic scale on the Cosmic Microwave sky provides a
direct test, which has now been realised through a combination of exquisite
results from a number of CMB experiments.
After a long and detailed investigation, with many false clues, it seems that
the mystery of the curvature of the Universe is now solved. It's an open and
shut case: the Universe is flat.Comment: 7 pages, 2 figures, submitted to the Gravity Research Foundation
Essay Competition for 200
Boomerang returns unexpectedly
Experimental study of the anisotropy in the cosmic microwave background (CMB)
is gathering momentum. The eagerly awaited Boomerang results have lived up to
expectations. They provide convincing evidence in favor of the standard
paradigm: the Universe is close to flat and with primordial fluctuations which
are redolent of inflation. Further scrutiny reveals something even more
exciting however -- two hints that there may be some unforeseen physical
effects. Firstly the primary acoustic peak appears at slightly larger scales
than expected. Although this may be explicable through a combination of mundane
effects, we suggest it is also prudent to consider the possibility that the
Universe might be marginally closed. The other hint is provided by a second
peak which appears less prominent than expected. This may indicate one of a
number of possibilities, including increased damping length or tilted initial
conditions, but also breaking of coherence or features in the initial power
spectrum. Further data should test whether the current concordance model needs
only to be tweaked, or to be enhanced in some fundamental way.Comment: 11 pages, 3 figures, final version accepted by Ap
Endocrine Disorders as a Contributory Factor to Neoplasia in SJL/J Mice
We studied the endocrine status of SJL/J mice. Light and electron microscopy revealed that the adenohypophyses of both sexes became progressively infiltrated with an abnormal number of gonadotropinproducing cells that probably secreted large amounts of luteotropic hormone. The ovaries had numerous large corpora lutea even in animals over 1 year of age with reticulum cell neoplasms. The adrenal cortexes of female mice showed no regression of the reticular zone. In accordance with the anomalous condition of the adenohypophysis and ovary, females had abnormal estrous cycles, with prolonged diestrus and consequent reduction in fertility. These data were discussed in the context of hormone environment versus onset of systemic neoplastic disease and the relationship between hormone dependence and leukemic virus expressio
Termination of the Phase of Quintessence by Gravitational Back-Reaction
We study the effects of gravitational back-reaction in models of
Quintessence. The effective energy-momentum tensor with which cosmological
fluctuations back-react on the background metric will in some cases lead to a
termination of the phase of acceleration. The fluctuations we make use of are
the perturbations in our present Universe. Their amplitude is normalized by
recent measurements of anisotropies in the cosmic microwave background, their
slope is taken to be either scale-invariant, or characterized by a slightly
blue tilt. In the latter case, we find that the back-reaction effect of
fluctuations whose present wavelength is smaller than the Hubble radius but
which are stretched beyond the Hubble radius by the accelerated expansion
during the era of Quintessence domination can become large. Since the
back-reaction effects of these modes oppose the acceleration, back-reaction
will lead to a truncation of the period of Quintessence domination. This result
impacts on the recent discussions of the potential incompatibility between
string theory and Quintessence.Comment: 7 pages a few clarifying comments adde
A preference for a non-zero neutrino mass from cosmological data
We present results from the analysis of cosmic microwave background (CMB),
large scale structure (galaxy redshift survey) and X-ray galaxy cluster (baryon
fraction and X-ray luminosity function) data, assuming a geometrically flat
cosmological model and allowing for tensor components and a non-negligible
neutrino mass. From a combined analysis of all data, assuming three degenerate
neutrinos species, we measure a contribution of neutrinos to the energy density
of the universe, Omega_nu h^2=0.0059^{+0.0033}_{-0.0027} (68 per cent
confidence limits), with zero falling on the 99 per cent confidence limit. This
corresponds to ~4 per cent of the total mass density of the Universe and
implies a species-summed neutrino mass \sum_i m_i =0.56^{+0.30}_{-0.26} eV, or
m_nu~0.2 eV per neutrino. We examine possible sources of systematic uncertainty
in the results. Combining the CMB, large scale structure and cluster baryon
fraction data, we measure an amplitude of mass fluctuations on 8h^{-1} Mpc
scales of sigma_8=0.74^{+0.12}_{-0.07}, which is consistent with measurements
based on the X-ray luminosity function and other studies of the number density
and evolution of galaxy clusters. This value is lower than that obtained when
fixing a negligible neutrino mass (sigma_8=0.86^{+0.08}_{-0.07}). The
combination of CMB, large scale structure and cluster baryon fraction data also
leads to remarkably tight constraints on the Hubble constant,
H_0=68.4^{+2.0}_{-1.4} km/s/Mpc, mean matter density, Omega_m =0.31\pm0.02 and
physical baryon density, Omega_b h^2=0.024\pm0.001, of the Universe.Comment: Final version. MNRAS, in press (9 pages, 6 figures, 1 table).
Includes small modification to the neutrino mass calculation and comment on
quintessence. Conclusions unchange
Constraining dark energy with Sunyaev-Zel'dovich cluster surveys
We discuss the prospects of constraining the properties of a dark energy
component, with particular reference to a time varying equation of state, using
future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute
the number of clusters expected for a given set of cosmological parameters and
propogate the errors expected from a variety of surveys. In the short term they
will constrain dark energy in conjunction with future observations of type Ia
supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR
The abundance of galaxy clusters in MOND: Cosmological simulations with massive neutrinos
We present a new Particle-Mesh cosmological N-body code for accurately
solving the modified Poisson equation of the Quasi Linear formulation of MOND.
We generate initial conditions for the Angus (2009) cosmological model, which
is identical to LCDM except that the cold dark matter is switched for a single
species of thermal sterile neutrinos. We set the initial conditions at z=250
for a (512 Mpc/h)^3 box with 256^3 particles and we evolve them down to z=0. We
clearly demonstrate the necessity of MOND for developing the large scale
structure in a hot dark matter cosmology and contradict the naive expectation
that MOND cannot form galaxy clusters. We find that the correct order of
magnitude of X-ray clusters (with T_X > 4.5 keV) can be formed, but that we
overpredict the number of very rich clusters and seriously underpredict the
number of lower mass clusters. The latter is a shortcoming of the resolution of
our simulations, whereas we suggest that the over production of very rich
clusters might be prevented by incorporating a MOND acceleration constant that
varies with redshift and an expansion history that cannot be described by the
usual Friedmann models. We present evidence that suggests the density profiles
of our simulated clusters are compatible with those of observed X-ray clusters
in MOND. It remains to be seen if the low mass end of the cluster mass function
can be reproduced and if the high densities of dark matter in the central 20
kpc of groups and clusters of galaxies, measured in the MOND framework, can be
achieved. As a last test, we computed the relative velocity between pairs of
halos within 10 Mpc and find that pairs with velocities larger than 3000 km/s
like the bullet cluster, can form without difficulty.Comment: 9 pages, 7 figur
Random noise in Diffusion Tensor Imaging, its Destructive Impact and Some Corrections
The empirical origin of random noise is described, its influence on DTI variables is illustrated by a review of numerical and in vivo studies supplemented by new simulations investigating high noise levels. A stochastic model of noise propagation is presented to structure noise impact in DTI. Finally, basics of voxelwise and spatial denoising procedures are presented. Recent denoising procedures are reviewed and consequences of the stochastic model for convenient denoising strategies are discussed
Measuring in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis
We extend our recent work on the effects of a time-varying fine-structure
constant in the cosmic microwave background, by providing a thorough
analysis of the degeneracies between and the other cosmological
parameters, and discussing ways to break these with both existing and/or
forthcoming data. In particular, we present the state-of-the-art CMB
constraints on , through a combined analysis of the BOOMERanG, MAXIMA
and DASI datasets. We also present a novel discussion of the constraints on
coming from large-scale structure observations, focusing in particular
on the power spectrum from the 2dF survey. Our results are consistent with no
variation in from the epoch of recombination to the present day, and
restrict any such (relative) variation to be less than about 4%. We show that
the forthcoming MAP and (particularly) Planck experiments will be able to break
most of the currently existing degeneracies between and other
parameters, and measure to better than percent accuracy.Comment: 11 pages in RevTex4 format. Low-quality figures to comply with arXiv
restrictions (better ones available from the authors). v2: Updated Oklo
discussion, plus other cosmetic changes. Version to appear in Phys Rev
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