2,429 research outputs found
On nonlocally interacting metrics, and a simple proposal for cosmic acceleration
We propose a simple, nonlocal modification to general relativity (GR) on
large scales, which provides a model of late-time cosmic acceleration in the
absence of the cosmological constant and with the same number of free
parameters as in standard cosmology. The model is motivated by adding to the
gravity sector an extra spin-2 field interacting nonlocally with the physical
metric coupled to matter. The form of the nonlocal interaction is inspired by
the simplest form of the Deser-Woodard (DW) model, ,
with one of the Ricci scalars being replaced by a constant , and gravity
is therefore modified in the infrared by adding a simple term of the form
to the Einstein-Hilbert term. We study cosmic expansion
histories, and demonstrate that the new model can provide background expansions
consistent with observations if is of the order of the Hubble expansion
rate today, in contrast to the simple DW model with no viable cosmology. The
model is best fit by and . We also compare the
cosmology of the model to that of Maggiore and Mancarella (MM),
, and demonstrate that the viable cosmic histories
follow the standard-model evolution more closely compared to the MM model. We
further demonstrate that the proposed model possesses the same number of
physical degrees of freedom as in GR. Finally, we discuss the appearance of
ghosts in the local formulation of the model, and argue that they are
unphysical and harmless to the theory, keeping the physical degrees of freedom
healthy.Comment: 47 pages in JCAP style, 7 figures. Some discussions extended in
response to referee's comments. Version accepted for publication in JCA
Unattainability of the Trans-Planckian regime in Nonlocal Quantum Gravity
Based on the ultraviolet asymptotic freedom of nonlocal quantum gravity, we
show that the trans-Planckian energy regime is unattainable in laboratory
experiments. As physical implications, it turns out that the violation of
causality, typical of nonlocal field theories, can never be detected in
particle accelerators, while the asymptotic freedom of the theory provides an
elegant solution to the so called trans-Planckian cosmological problem.Comment: 13 pages, version published in JHE
Anisotropic Cosmology in the Local Limit of Nonlocal Gravity
Within the framework of the local limit of nonlocal gravity (NLG), we
investigate a class of Bianchi type I spatially homogeneous but anisotropic
cosmological models. The modified field equations are presented in this case
and some special solutions are discussed in detail. This modified gravity
theory contains a susceptibility function S(x) such that general relativity
(GR) is recovered for S = 0. In the modified anisotropic cosmological models,
we explore the contribution of S(t) and its temporal derivative to the local
anisotropic cosmic acceleration. The implications of our results for
observational cosmology are briefly discussed.Comment: 25 page
Conformal symmetry and nonlinear extensions of nonlocal gravity
We study two nonlinear extensions of the nonlocal gravity
theory. We extend this theory in two different ways suggested by conformal
symmetry, either replacing with , which is the
operator that enters the action for a conformally-coupled scalar field, or
replacing with the inverse of the Paneitz operator, which is a
four-derivative operator that enters in the effective action induced by the
conformal anomaly. We show that the former modification gives an interesting
and viable cosmological model, with a dark energy equation of state today
, which very closely mimics CDM and evolves
asymptotically into a de Sitter solution. The model based on the Paneitz
operator seems instead excluded by the comparison with observations. We also
review some issues about the causality of nonlocal theories, and we point out
that these nonlocal models can be modified so to nicely interpolate between
Starobinski inflation in the primordial universe and accelerated expansion in
the recent epoch.Comment: 27 pages, 4 figure
Instabilities in tensorial nonlocal gravity
We discuss the cosmological implications of nonlocal modifications of general
relativity containing tensorial structures. Assuming the presence of standard
radiation- and matter-dominated eras, we show that, except in very particular
cases, the nonlocal terms contribute a rapidly growing energy density. These
models therefore generically do not have a stable cosmological evolution.Comment: 10 pages, 2 figures. v2: version published in PR
On Nonlocal Modified Gravity and its Cosmological Solutions
During hundred years of General Relativity (GR), many significant
gravitational phenomena have been predicted and discovered. General Relativity
is still the best theory of gravity. Nevertheless, some (quantum) theoretical
and (astrophysical and cosmological) phenomenological difficulties of modern
gravity have been motivation to search more general theory of gravity than GR.
As a result, many modifications of GR have been considered. One of promising
recent investigations is Nonlocal Modified Gravity. In this article we present
a brief review of some nonlocal gravity models with their cosmological
solutions, in which nonlocality is expressed by an analytic function of the
d'Alembert-Beltrami operator . Some new results are also presented.Comment: 16 page
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
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