179 research outputs found
General Relativistic Cosmological N-body Simulations I: time integration
This is the first in a series of papers devoted to fully general-relativistic
-body simulations applied to late-time cosmology. The purpose of this paper
is to present the combination of a numerical relativity scheme, discretization
method and time-integration algorithm that provides satisfyingly stable
evolution. More precisely, we show that it is able to pass a robustness test
and to follow scalar linear modes around an expanding homogeneous and isotropic
space-time. Most importantly, it is able to evolve typical cosmological initial
conditions on comoving scales down to tenths of megaparsecs with controlled
constraint and energy-momentum conservation violations all the way down to the
regime of strong inhomogeneity.Comment: 28 pages, 16 figure
A numerical relativity scheme for cosmological simulations
Cosmological simulations involving the fully covariant gravitational dynamics
may prove relevant in understanding relativistic/non-linear features and,
therefore, in taking better advantage of the upcoming large scale structure
survey data. We propose a new 3+1 integration scheme for General Relativity in
the case where the matter sector contains a minimally-coupled perfect fluid
field. The original feature is that we completely eliminate the fluid
components through the constraint equations, thus remaining with a set of
unconstrained evolution equations for the rest of the fields. This procedure
does not constrain the lapse function and shift vector, so it holds in
arbitrary gauge and also works for arbitrary equation of state. An important
advantage of this scheme is that it allows one to define and pass an adaptation
of the robustness test to the cosmological context, at least in the case of
pressureless perfect fluid matter, which is the relevant one for late-time
cosmology.Comment: 14 pages, 3 figures, matches published versio
Modified gravitational-wave propagation and standard sirens
Studies of dark energy at advanced gravitational-wave (GW) interferometers
normally focus on the dark energy equation of state . However,
modified gravity theories that predict a non-trivial dark energy equation of
state generically also predict deviations from general relativity in the
propagation of GWs across cosmological distances, even in theories where the
speed of gravity is equal to . We find that, in generic modified gravity
models, the effect of modified GW propagation dominates over that of , making modified GW propagation a crucial observable for dark energy
studies with standard sirens. We present a convenient parametrization of the
effect in terms of two parameters , analogue to the
parametrization of the dark energy equation of state, and we give a limit from
the LIGO/Virgo measurement of with the neutron star binary GW170817. We
then perform a Markov Chain Monte Carlo analysis to estimate the sensitivity of
the Einstein Telescope (ET) to the cosmological parameters, including
, both using only standard sirens, and combining them with other
cosmological datasets. In particular, the Hubble parameter can be measured with
an accuracy better than already using only standard sirens while, when
combining ET with current CMB+BAO+SNe data, can be measured to
. We discuss the predictions for modified GW propagation of a specific nonlocal
modification of gravity, recently developed by our group, and we show that they
are within the reach of ET. Modified GW propagation also affects the GW
transfer function, and therefore the tensor contribution to the ISW effect.Comment: 25 pages, 23 figures: v3: several significant improvement
Nonlocal gravity. Conceptual aspects and cosmological predictions
Even if the fundamental action of gravity is local, the corresponding quantum
effective action, that includes the effect of quantum fluctuations, is a
nonlocal object. These nonlocalities are well understood in the ultraviolet
regime but much less in the infrared, where they could in principle give rise
to important cosmological effects. Here we systematize and extend previous work
of our group, in which it is assumed that a mass scale is dynamically
generated in the infrared, giving rise to nonlocal terms in the quantum
effective action of gravity. We give a detailed discussion of conceptual
aspects related to nonlocal gravity and of the cosmological consequences of
these models. The requirement of providing a viable cosmological evolution
severely restricts the form of the nonlocal terms, and selects a model (the
so-called RR model) that corresponds to a dynamical mass generation for the
conformal mode. For such a model: (1) there is a FRW background evolution,
where the nonlocal term acts as an effective dark energy with a phantom
equation of state, providing accelerated expansion without a cosmological
constant. (2) Cosmological perturbations are well behaved. (3) Implementing the
model in a Boltzmann code and comparing with observations we find that the RR
model fits the CMB, BAO, SNe, structure formation data and local
measurements at a level statistically equivalent to CDM. (4) Bayesian
parameter estimation shows that the value of obtained in the RR model is
higher than in CDM, reducing to the tension with the value
from local measurements. (5) The RR model provides a prediction for the sum of
neutrino masses that falls within the limits set by oscillation and terrestrial
experiments. (6) Gravitational waves propagate at the speed of light, complying
with the limit from GW170817/GRB 170817A.Comment: 60 pages, 12 figures; v2: references adde
The gravitational-wave luminosity distance in modified gravity theories
In modified gravity the propagation of gravitational waves (GWs) is in
general different from that in general relativity. As a result, the luminosity
distance for GWs can differ from that for electromagnetic signals, and is
affected both by the dark energy equation of state and by a
function describing modified propagation. We show that the effect
of modified propagation in general dominates over the effect of the dark energy
equation of state, making it easier to distinguish a modified gravity model
from CDM. We illustrate this using a nonlocal modification of gravity,
that has been shown to fit remarkably well CMB, SNe, BAO and structure
formation data, and we discuss the prospects for distinguishing nonlocal
gravity from CDM with the Einstein Telescope. We find that, depending
on the exact sensitivity, a few tens of standard sirens with measured redshift
at , or a few hundreds at , could suffice.Comment: 6 pages, 3 figures; v4: minor modifications; the version to appear in
PR
- …