365 research outputs found
Phantom dark energy from non-local infrared modifications of General Relativity
We discuss the cosmological consequences of a model based on a non-local
infrared modification of Einstein equations. We find that the model generates a
dynamical dark energy that can account for the presently observed value of
, without introducing a cosmological constant. Tuning a free
mass parameter to a value we reproduce the observed
value . This leaves us with no free parameter and
we then get a pure prediction for the EOS parameter of dark energy. Writing
, we find and ,
consistent with the Planck data, and on the phantom side. We also argue that
non-local equations of the type that we propose must be understood as purely
classical effective equations, such as those derived in semiclassical gravity
for the in-in matrix elements of the metric. As such, any apparent ghost
instability in such equations only affects the classical dynamics, but there is
no propagating degree of freedom associated to the ghost, and no issue of
ghost-induced quantum vacuum decay.Comment: 7 pages, 2 figures; v3: the version accepted in Phys Rev. D. Title
changed in journa
Spherically symmetric static solutions in a non-local infrared modification of General Relativity
We discuss static spherically symmetric solutions in a recently proposed
non-local infrared modification of Einstein equations induced by a term
, where is a mass scale. We find that, contrary
to what happens in usual theories of massive gravity, in this non-local theory
there is no vDVZ discontinuity and classical non-linearities do not become
large below a Vainshtein radius parametrically larger than the Schwarzschild
radius . Rather on the contrary, in the regime the
corrections to the metric generated by a static body in GR are of the form
and become smaller and smaller toward smaller values of
. The modification to the GR solutions only show up at . For
, as required for having interesting cosmological
consequences, the non-local theory therefore recovers all successes of GR at
the solar system and lab scales.Comment: 29 pages, 5 figures. v2: expanded discussion of conceptual aspects.
The version to appear in JHE
The Halo Mass Function from Excursion Set Theory with a Non-Gaussian Trispectrum
A sizeable level of non-Gaussianity in the primordial cosmological
perturbations may be induced by a large trispectrum, i.e. by a large connected
four-point correlation function. We compute the effect of a primordial
non-Gaussian trispectrum on the halo mass function, within excursion set
theory. We use the formalism that we have developed in a previous series of
papers and which allows us to take into account the fact that, in the presence
of non-Gaussianity, the stochastic evolution of the smoothed density field, as
a function of the smoothing scale, is non-markovian. In the large mass limit,
the leading-order term that we find agrees with the leading-order term of the
results found in the literature using a more heuristic Press-Schecther
(PS)-type approach. Our approach however also allows us to evaluate
consistently the subleading terms, which depend not only on the four-point
cumulant but also on derivatives of the four-point correlator, and which cannot
be obtained within non-Gaussian extensions of PS theory. We perform explicitly
the computation up to next-to-leading order.Comment: LaTeX file, 15 page
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