90 research outputs found
Friction in Gravitational Waves: a test for early-time modified gravity
Modified gravity theories predict in general a non standard equation for the
propagation of gravitational waves. Here we discuss the impact of modified
friction and speed of tensor modes on cosmic microwave polarization B modes. We
show that the non standard friction term, parametrized by , is
degenerate with the tensor-to-scalar ratio , so that small values of can
be compensated by negative constant values of . We quantify this
degeneracy and its dependence on the epoch at which is different
from the standard, zero, value and on the speed of gravitational waves .
In the particular case of scalar-tensor theories, is constant and
strongly constrained by background and scalar perturbations, and the degeneracy with is removed. In more general cases however
such tight bounds are weakened and the B modes can provide useful constraints
on early-time modified gravity.Comment: Minor changes after published version. One new figur
Constraints on coupled dark energy using CMB data from WMAP and SPT
We consider the case of a coupling in the dark cosmological sector, where a
dark energy scalar field modifies the gravitational attraction between dark
matter particles. We find that the strength of the coupling {\beta} is
constrained using current Cosmic Microwave Background (CMB) data, including
WMAP7 and SPT, to be less than 0.063 (0.11) at 68% (95%) confidence level.
Further, we consider the additional effect of the CMB-lensing amplitude,
curvature, effective number of relativistic species and massive neutrinos and
show that the bound from current data on {\beta} is already strong enough to be
rather stable with respect to any of these variables. The strongest effect is
obtained when we allow for massive neutrinos, in which case the bound becomes
slightly weaker, {\beta} < 0.084(0.14). A larger value of the effective number
of relativistic degrees of freedom favors larger couplings between dark matter
and dark energy as well as values of the spectral index closer to 1. Adding the
present constraints on the Hubble constant, as well as from baryon acoustic
oscillations and supernovae Ia, we find {\beta} < 0.050(0.074). In this case we
also find an interesting likelihood peak for {\beta} = 0.041 (still compatible
with 0 at 1{\sigma}). This peak comes mostly from a slight difference between
the Hubble parameter HST result and the WMAP7+SPT best fit. Finally, we show
that forecasts of Planck+SPT mock data can pin down the coupling to a precision
of better than 1% and detect whether the marginal peak we find at small non
zero coupling is a real effect.Comment: 22 pages, 17 figure
The darkness that shaped the void: dark energy and cosmic voids
Aims: We assess the sensitivity of void shapes to the nature of dark energy
that was pointed out in recent studies. We investigate whether or not void
shapes are useable as an observational probe in galaxy redshift surveys. We
focus on the evolution of the mean void ellipticity and its underlying physical
cause. Methods: We analyse the morphological properties of voids in five sets
of cosmological N-body simulations, each with a different nature of dark
energy. Comparing voids in the dark matter distribution to those in the halo
population, we address the question of whether galaxy redshift surveys yield
sufficiently accurate void morphologies. Voids are identified using the
parameter free Watershed Void Finder. The effect of redshift distortions is
investigated as well. Results: We confirm the statistically significant
sensitivity of voids in the dark matter distribution. We identify the level of
clustering as measured by \sigma_8(z) as the main cause of differences in mean
void shape . We find that in the halo and/or galaxy distribution it
is practically unfeasible to distinguish at a statistically significant level
between the various cosmologies due to the sparsity and spatial bias of the
sample.Comment: 22 pages, 23 figures, 3 tables; v2: added references and short
comparison of void size results; accepted for publication by MNRA
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