973 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
On boosted space-times with cosmological constant and their ultrarelativistic limit
The problem of deriving a shock-wave geometry with cosmological constant by
boosting a Schwarzschild-de Sitter (or anti-de Sitter) black hole is
re-examined. Unlike previous work in the literature, we deal with the exact
Schwarzschild-de Sitter (or anti-de Sitter) metric. By virtue of peculiar
cancellations in this exact calculation, where the metric does not depend
linearly on the mass parameter, we find a singularity of distributional nature
on a null hypersurface, which corresponds to a shock-wave geometry derived in a
fully non-perturbative way. The result agrees with previous calculations, where
the metric had been linearized in the mass parameter.Comment: 13 pages, Latex file. In the final version, the presentation has been
improved, and two references have been adde
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
Prescriptions for Off-Shell Bosonic String Amplitudes
We give, in the framework of the bosonic string theory, simple prescriptions
for computing, at tree and one-loop levels, off-shell string amplitudes for
open and closed string massless states. In particular we obtain a tree
amplitude for three open strings that in the field theory limit coincides with
the three-gluon vertex in the usual covariant gauge and two-string one-loop
amplitudes satisfying the property of transversality.Comment: 11 pages, LaTeX, to appear in the proceedings of the workshop
"Quantum Aspects of Gauge Theories, Supersymmetry and Unification", Corfu
(Greece), 20-26 September 1998. Eq. (12) and numerical factors in eqs. (13)
and (16) corrected; some minor changes and references adde
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