1,614 research outputs found
Constraints on Gauss-Bonnet Gravity in Dark Energy Cosmologies
Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear
naturally from Kaluza-Klein compactifications of pure higher-dimensional
gravity. We study linear, cosmological perturbations in the limits of weak
coupling and slow-roll, and derive simple expressions for the main observable
sub-horizon quantities: the anisotropic stress factor, the time-dependent
gravitational constant, and the matter perturbation growth factor. Using
present observational data, and assuming slow-roll for the dark energy field,
we find that the fraction of energy density associated with the coupled
Gauss-Bonnet term cannot exceed 15%. The bound should be treated with caution,
as there are significant uncertainies in the data used to obtain it. Even so,
it indicates that the future prospects for constraining the coupled
Gauss-Bonnet term with cosmological observations are encouraging.Comment: 15 pages. v3: extended analysis, conclusions change
Adiabatic and non-adiabatic perturbations for loop quantum cosmology
We generalize the perturbations theory of loop quantum cosmology to a
hydrodynamical form and define an effective curvature perturbation on an
uniform density hypersurfaces . As in the classical cosmology,
should be gauge-invariant and conservation on the large scales. The
evolutions of both the adiabatic and the non-adiabatic perturbations for a
multi-fluids model are investigated in the framework of the effective
hydrodynamical theory of loop quantum cosmology with the inverse triad
correction. We find that, different from the classical cosmology, the evolution
of the large-scales non-adiabatic entropy perturbation can be driven by an
adiabatic curvature perturbation and this adiabatic source for the
non-adiabatic perturbation is a quantum effect. As an application of the
related formalism, we study a decay model and give out the numerical results.Comment: 10 pages, 3 figure
Imaging Polarimeter Arrays for Near-Millimeter Waves
An integrated-circuit antenna array has been developed that images both polarization and intensity. The array consists of a row of antennas that lean alternately left and right, creating two interlaced sub-arrays that respond to different polarizations. The arrays and the bismuth bolometer detectors are made by a photoresist shadowing technique that requires only one photolithographic mask. The array has measured polarization at a wavelength of 800 µm with an absolute accuracy of 0.8° and a relative precision of 7 arc min. and has demonstrated nearly diffraction-Iimited resolutiort of a 20° step in polarization
A non-geodesic motion in the R^-1 theory of gravity tuned with observations
In the general picture of high order theories of gravity, recently, the R^-1
theory has been analyzed in two different frameworks. In this letter a third
context is added, considering an explicit coupling between the R^-1 function of
the Ricci scalar and the matter Lagrangian. The result is a non-geodesic motion
of test particles which, in principle, could be connected with Dark Matter and
Pioneer anomaly problems.Comment: Accepted for Modern Physics Letters
Interacting Dark Matter and Dark Energy
We discuss models for the cosmological dark sector in which the energy
density of a scalar field approximates Einstein's cosmological constant and the
scalar field value determines the dark matter particle mass by a Yukawa
coupling. A model with one dark matter family can be adjusted so the
observational constraints on the cosmological parameters are close to but
different from what is predicted by the Lambda CDM model. This may be a useful
aid to judging how tightly the cosmological parameters are constrained by the
new generation of cosmological tests that depend on the theory of structure
formation. In a model with two families of dark matter particles the scalar
field may be locked to near zero mass for one family. This can suppress the
long-range scalar force in the dark sector and eliminate evolution of the
effective cosmological constant and the mass of the nonrelativistic dark matter
particles, making the model close to Lambda CDM, until the particle number
density becomes low enough to allow the scalar field to evolve. This is a
useful example of the possibility for complexity in the dark sector.Comment: 15 pages, 6 figures; added a reference and a minor correctio
Cosmological Baryon Sound Waves Coupled with the Primeval Radiation
The fluid equations for the baryon-electron system in an expanding universe
are derived from the Boltzmann equation. The effect of the Compton interaction
is taken into account properly in order to evaluate the photon-electron
collisional term. As an application, the acoustic motions of the
baryon-electron system after recombination are investigated. The effective
adiabatic index is computed for sound waves of various wavelengths,
assuming the perturbation amplitude is small. The oscillations are found to be
dumped when changes from between 1 (for an isothermal process) to 5/3
(for an adiabatic process).Comment: 20 pages, Revtex, Accepted for publication in Phys. Rev.
Universality of power law correlations in gravitational clustering
We present an analysis of different sets of gravitational N-body simulations,
all describing the dynamics of discrete particles with a small initial velocity
dispersion. They encompass very different initial particle configurations,
different numerical algorithms for the computation of the force, with or
without the space expansion of cosmological models. Despite these differences
we find in all cases that the non-linear clustering which results is
essentially the same, with a well-defined simple power-law behaviour in the
two-point correlations in the range from a few times the lower cut-off in the
gravitational force to the scale at which fluctuations are of order one. We
argue, presenting quantitative evidence, that this apparently universal
behaviour can be understood by the domination of the small scale contribution
to the gravitational force, coming initially from nearest neighbor particles.Comment: 7 pages, latex, 3 postscript figures. Revised version to be published
in Europhysics Letters. Contains additional analysis showing more directly
the central role of nearest neighbour interactions in the development of
power-law correlation
Can Structure Formation Influence the Cosmological Evolution?
The backreaction of structure formation influences the cosmological evolution
equation for the homogenous and isotropic average metric. In a cold dark matter
universe this effect leads only to small corrections unless a substantial
fraction of matter is located in regions where strong gravitational fields
evolve in time. A``cosmic virial theorem'' states that the sum of gravitational
and matter pressure vanishes and therefore relates the average kinetic energy
to a suitable average of the Newtonian potential. In presence of a scalar
``cosmon'' field mediating quintessence, however, cosmology could be modified
if local cosmon fluctuations grow large. We speculate that this may trigger the
accelerated expansion of the universe after the formation of structure.Comment: new "cosmic virial theorem",new references,LaTex,11 page
Noether symmetry approach in phantom quintessence cosmology
In the framework of phantom quintessence cosmology, we use the Noether
Symmetry Approach to obtain general exact solutions for the cosmological
equations. This result is achieved by the quintessential (phantom) potential
determined by the existence of the symmetry itself. A comparison between the
theoretical model and observations is worked out. In particular, we use type Ia
supernovae and large scale structure parameters determined from the 2-degree
Field Galaxy Redshift Survey (2dFGRS)and from the Wide part of the VIMOS-VLT
Deep Survey (VVDS). It turns out that the model is compatible with the
presently available observational data. Moreover we extend the approach to
include radiation. We show that it is compatible with data derived from
recombination and it seems that quintessence do not affect nucleosynthesis
results.Comment: 26 pages, 13 figure
Can a galaxy redshift survey measure dark energy clustering?
(abridged) A wide-field galaxy redshift survey allows one to probe galaxy
clustering at largest spatial scales, which carries an invaluable information
on horizon-scale physics complementarily to the cosmic microwave background
(CMB). Assuming the planned survey consisting of z~1 and z~3 surveys with areas
of 2000 and 300 square degrees, respectively, we study the prospects for
probing dark energy clustering from the measured galaxy power spectrum,
assuming the dynamical properties of dark energy are specified in terms of the
equation of state and the effective sound speed c_e in the context of an
adiabatic cold dark matter dominated model. The dark energy clustering adds a
power to the galaxy power spectrum amplitude at spatial scales greater than the
sound horizon, and the enhancement is sensitive to redshift evolution of the
net dark energy density, i.e. the equation of state. We find that the galaxy
survey, when combined with Planck, can distinguish dark energy clustering from
a smooth dark energy model such as the quintessence model (c_e=1), when
c_e<0.04 (0.02) in the case of the constant equation of state w_0=-0.9 (-0.95).
An ultimate full-sky survey of z~1 galaxies allows the detection when c_e<0.08
(0.04) for w_0=0.9 (-0.95). We also investigate a degeneracy between the dark
energy clustering and the non-relativistic neutrinos implied from the neutrino
oscillation experiments, because the two effects both induce a scale-dependent
modification in the galaxy power spectrum shape at largest spatial scales
accessible from the galaxy survey. It is shown that a wider redshift coverage
can efficiently separate the two effects by utilizing the different redshift
dependences, where dark energy clustering is apparent only at low redshifts
z<1.Comment: 14 pages, 7 figures; minor changes to match the published versio
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