15 research outputs found
Does Planck mass run on the cosmological horizon scale?
Einstein's theory of general relativity, which contains a universal value of
the Planck mass, has been so far successfully invoked to explain gravitational
dynamics from sub-millimeter scales to the scale of the cosmological horizon.
However, one may envisage that in alternative theories of gravity, the
effective value of the Planck mass (or Newton's constant), which quantifies the
coupling of matter to metric perturbations, can run on the cosmological horizon
scale. In this letter, we study the consequences of a glitch in the Planck mass
from sub-horizon to super-horizon scales. We first give three examples of
models that naturally exhibit this feature, and then show that current
cosmological observations severely constrain this glitch to less than 1.2%.
This is the strongest constraint to date, on natural (i.e. non-fine-tuned)
deviations from Einstein gravity on the cosmological horizon scale.Comment: 5 pages, 1 figur
Properties of Quintessence
We study cosmological models with quintessence, a form of energy that is dis- tributed almost homogeneously in the Universe, and today makes up about three fourths of its energy density. We use observational data in order to infer upper bounds for the energy density of quintessence also during early times, and analyze the impact of this early quintessence on the interpretation of observa- tional data. Furthermore, we investigate deviations from the law of gravity as described by the theory of General Relativity on very large scales. Finally, we consider models in which the mass of the dark matter or of neutrinos depends on the quintessence field, and present extensions of the software Cmbeasy, that allow to compute the cosmological predictions of these models
Phenomenology of Gravitational Aether as a solution to the Old Cosmological Constant Problem
One of the deepest and most long-standing mysteries in physics has been the
huge discrepancy between the observed vacuum density and our expectations from
theories of high energy physics, which has been dubbed the Old Cosmological
Constant problem. One proposal to address this puzzle at the semi-classical
level is to decouple quantum vacuum from space-time geometry via a modification
of gravity that includes an incompressible fluid, known as Gravitational
Aether. In this paper, we discuss classical predictions of this theory along
with its compatibility with cosmological and experimental tests of gravity. We
argue that deviations from General Relativity (GR) in this theory are sourced
by pressure or vorticity. In particular, the theory predicts that the
gravitational constant for radiation is 33% larger than that of
non-relativistic matter, which is preferred by (most) cosmic microwave
background (CMB), Lyman-Alpha forest, and Lithium-7 primordial abundance
observations, while being consistent with other cosmological tests at ~2-sigma
level. It is further shown that all Parametrized Post-Newtonian (PPN)
parameters have the standard GR values aside from the anomalous coupling to
pressure, which has not been directly measured. A more subtle prediction of
this model (assuming irrotational aether) is that the (intrinsic)
gravitomagnetic effect is 33% larger than GR prediction. This is consistent
with current limits from LAGEOS and Gravity Probe B at ~2-sigma level
Reconstructing signals from noisy data with unknown signal and noise covariance
We derive a method to reconstruct Gaussian signals from linear measurements
with Gaussian noise. This new algorithm is intended for applications in
astrophysics and other sciences. The starting point of our considerations is
the principle of minimum Gibbs free energy which was previously used to derive
a signal reconstruction algorithm handling uncertainties in the signal
covariance. We extend this algorithm to simultaneously uncertain noise and
signal covariances using the same principles in the derivation. The resulting
equations are general enough to be applied in many different contexts. We
demonstrate the performance of the algorithm by applying it to specific example
situations and compare it to algorithms not allowing for uncertainties in the
noise covariance. The results show that the method we suggest performs very
well under a variety of circumstances and is indeed qualitatively superior to
the other methods in cases where uncertainty in the noise covariance is
present.Comment: 12 pages, 6 figures; 1D example added; accepted for publication in
Phys. Rev.
Shifting the Universe: Early Dark Energy and Standard Rulers
The presence of dark energy at high redshift influences both the cosmic sound
horizon and the distance to last scattering of the cosmic microwave background.
We demonstrate that through the degeneracy in their ratio, early dark energy
can lie hidden in the CMB temperature and polarization spectra, leading to an
unrecognized shift in the sound horizon. If the sound horizon is then used as a
standard ruler, as in baryon acoustic oscillations, then the derived
cosmological parameters can be nontrivially biased. Fitting for the absolute
ruler scale (just as supernovae must be fit for the absolute candle magnitude)
removes the bias but decreases the leverage of the BAO technique by a factor 2.Comment: 6 pages, 3 figure
Early Dark Energy Cosmologies
We propose a novel parameterization of the dark energy density. It is
particularly well suited to describe a non-negligible contribution of dark
energy at early times and contains only three parameters, which are all
physically meaningful: the fractional dark energy density today, the equation
of state today and the fractional dark energy density at early times. As we
parameterize Omega_d(a) directly instead of the equation of state, we can give
analytic expressions for the Hubble parameter, the conformal horizon today and
at last scattering, the sound horizon at last scattering, the acoustic scale as
well as the luminosity distance. For an equation of state today w_0 < -1, our
model crosses the cosmological constant boundary. We perform numerical studies
to constrain the parameters of our model by using Cosmic Microwave Background,
Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that
the fractional dark energy density at early times Omega_early < 0.06. This
bound tightens considerably to Omega_early < 0.04 when the latest Boomerang
data is included. We find that both the gold sample of Riess et. al. and the
SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer
w_0 < -1, but are well compatible with a cosmological constant.Comment: 6 pages, 3 figures; references added, matches published versio
Averaging Robertson-Walker Cosmologies
The cosmological backreaction arises when one directly averages the Einstein
equations to recover an effective Robertson-Walker cosmology, rather than
assuming a background a priori. While usually discussed in the context of dark
energy, strictly speaking any cosmological model should be recovered from such
a procedure. We apply the Buchert averaging formalism to linear
Robertson-Walker universes containing matter, radiation and dark energy and
evaluate numerically the discrepancies between the assumed and the averaged
behaviour, finding the largest deviations for an Einstein-de Sitter universe,
increasing rapidly with Hubble rate to a 0.01% effect for h=0.701. For the LCDM
concordance model, the backreaction is of the order of Omega_eff~4x10^-6, with
those for dark energy models being within a factor of two or three. The impacts
at recombination are of the order of 10^-8 and those in deep radiation
domination asymptote to a constant value. While the effective equations of
state of the backreactions in Einstein-de Sitter, concordance and quintessence
models are generally dust-like, a backreaction with an equation of state
w_eff<-1/3 can be found for strongly phantom models.Comment: 18 pages, 11 figures, ReVTeX. Updated to version accepted by JCA
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
Hydrodynamical N-body simulations of coupled dark energy cosmologies
If the accelerated expansion of the Universe at the present epoch is driven
by a dark energy scalar field, there may well be a non-trivial coupling between
the dark energy and the cold dark matter (CDM) fluid. Such interactions give
rise to new features in cosmological structure growth, like an additional
long-range attractive force between CDM particles, or variations of the dark
matter particle mass with time. We have implemented these effects in the N-body
code GADGET-2 and present results of a series of high-resolution N-body
simulations where the dark energy component is directly interacting with the
cold dark matter. As a consequence of the new physics, CDM and baryon
distributions evolve differently both in the linear and in the nonlinear regime
of structure formation. Already on large scales a linear bias develops between
these two components, which is further enhanced by the nonlinear evolution. We
also find, in contrast with previous work, that the density profiles of CDM
halos are less concentrated in coupled dark energy cosmologies compared with
LCDM, and that this feature does not depend on the initial conditions setup,
but is a specific consequence of the extra physics induced by the coupling.
Also, the baryon fraction in halos in the coupled models is significantly
reduced below the universal baryon fraction. These features alleviate tensions
between observations and the LCDM model on small scales. Our methodology is
ideally suited to explore the predictions of coupled dark energy models in the
fully non-linear regime, which can provide powerful constraints for the viable
parameter space of such scenarios.Comment: 21 pages, 18 figures, 4 tables, title changed, several references
added. Revised version accepted for publication in MNRAS. Main conclusions
unchange