100 research outputs found
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
Non-local gravity with a Weyl-square term
Recent work has shown that modifications of General Relativity based on the
addition to the action of a non-local term , or on the addition
to the equations of motion of a term involving ,
produce dynamical models of dark energy which are cosmologically viable both at
the background level and at the level of cosmological perturbations. We explore
a more general class of models based on the addition to the action of terms
proportional to and , where is the Weyl
tensor. We find that the term does not give a
viable background evolution. The non-local Weyl-square term, in contrast, does
not contribute to the background evolution but we find that, at the level of
cosmological perturbations, it gives instabilities in the tensor sector. Thus,
only non-local terms which depend just on the Ricci scalar appear to be
cosmologically viable. We discuss how these results can provide a hint for the
mechanism that might generate these effective non-local terms from a
fundamental local theory.Comment: 25 pages, 6 figures. v2: the version to appear in PR
Non-local gravity and dark energy
We discuss a nonlocal modification of gravity obtained adding a term to the Einstein-Hilbert action. We find that the mass parameter
only affects the non-radiative sector of the theory, while the graviton
remains massless, there is no propagating ghost-like degree of freedom, no vDVZ
discontinuity, and no Vainshtein radius below which the theory becomes strongly
coupled. For the theory therefore recovers all successes of
GR at solar system and lab scales, and only deviates from it at cosmological
scales. We examine the cosmological consequences of the model and we find that
it automatically generates a dynamical dark energy and a self-accelerating
evolution. After fixing our only free parameter so to reproduce the
observed value of the dark energy density today, we get a pure prediction for
the dark energy equation of state, . This value is
consistent with the existing data, and could also resolve the possible tension
between the Planck data and local measurements of the Hubble parameter.Comment: 7 pages, 3 figures; v2: the version accepted in PR
Effective Theory of Dark Energy at Redshift Survey Scales
We explore the phenomenological consequences of general late-time
modifications of gravity in the quasi-static approximation, in the case where
cold dark matter is non-minimally coupled to the gravitational sector. Assuming
spectroscopic and photometric surveys with configuration parameters similar to
those of the Euclid mission, we derive constraints on our effective description
from three observables: the galaxy power spectrum in redshift space,
tomographic weak-lensing shear power spectrum and the correlation spectrum
between the integrated Sachs-Wolfe effect and the galaxy distribution. In
particular, with CDM as fiducial model and a specific choice for the
time dependence of our effective functions, we perform a Fisher matrix analysis
and find that the unmarginalized CL errors on the parameters describing
the modifications of gravity are of order --. We
also consider two other fiducial models. A nonminimal coupling of CDM enhances
the effects of modified gravity and reduces the above statistical errors
accordingly. In all cases, we find that the parameters are highly degenerate,
which prevents the inversion of the Fisher matrices. Some of these degeneracies
can be broken by combining all three observational probes.Comment: 41 pages, 5 figures, 2 tables, improved analysis of ISW-galaxy
correlation, matches published version on JCA
Forecasting the detection capabilities of third-generation gravitational-wave detectors using
We introduce , a novel Fisher-matrix code for
gravitational-wave studies, tuned toward third-generation gravitational-wave
detectors such as Einstein Telescope (ET) and Cosmic Explorer (CE). We use it
to perform a comprehensive study of the capabilities of ET alone, and of a
network made by ET and two CE detectors, as well as to provide forecasts for
the forthcoming O4 run of the LVK collaboration. We consider binary neutron
stars, binary black holes and neutron star-black hole binaries, and compute
basic metrics such as the distribution of signal-to-noise ratio (SNR), the
accuracy in the reconstruction of various parameters (including distance, sky
localization, masses, spins and, for neutron stars, tidal deformabilities), and
the redshift distribution of the detections for different thresholds in SNR and
different levels of accuracy in localization and distance measurement. We
examine the expected distribution and properties of `golden events', with
especially large values of the SNR. We also pay special attention to the
dependence of the results on astrophysical uncertainties and on various
technical details (such as choice of waveforms, or the threshold in SNR), and
we compare with other Fisher codes in the literature. In a companion paper we
discuss the technical aspects of the code. Together with this paper, we
publicly release the code at
https://github.com/CosmoStatGW/gwfast, and the library
implementing state-of-the-art gravitational-wave waveforms in pure
at https://github.com/CosmoStatGW/WF4Py.Comment: 43 + 9 pages, 24 + 3 Figures, available at
https://github.com/CosmoStatGW/gwfast, available at
https://github.com/CosmoStatGW/WF4P
Inferring, not just detecting: metrics for high-redshift sources observed with third-generation gravitational-wave detectors
The detection of black-hole binaries at high redshifts is a cornerstone of
the science case of third-generation gravitational-wave interferometers. The
star-formation rate peaks at z~2 and decreases by orders of magnitude by z~10.
Any confident detection of gravitational waves from such high redshifts would
imply either the presence of stars formed from pristine material originating
from cosmological nucleosynthesis (the so-called population III stars), or
black holes that are the direct relics of quantum fluctuations in the early
Universe (the so-called primordial black holes). Crucially, detecting sources
at cosmological distances does not imply inferring that sources are located
there, with the latter posing more stringent requirements. To this end, we
present two figures of merit, which we refer to as "z-z plot" and "inference
horizon", that quantify the largest redshift one can possibly claim a source to
be beyond. We argue that such inference requirements, in addition to detection
requirements, should be investigated when quantifying the scientific payoff of
future gravitational-wave facilities.Comment: 6 pages, 4 figure
Non-Local Gravity and Dark Energy
We discuss a non-local modification of gravity obtained adding a non-local term to the Einstein-Hilbert action, depending on a free mass parameter, m, and on the inverse d'Alembertian applied to the Ricci scalar. We find that the mass parameter m only affects the non-radiative sector of the theory, while the graviton remains massless, there is no propagating ghost-like degree of freedom, no vDVZ discontinuity, and no Vainshtein radius below which the theory becomes strongly coupled.
For m of the order of the present value of the Hubble constant, the theory therefore recovers all successes of GR at solar system and lab scales, and only deviates from it at cosmological scales. We examine the cosmological consequences of the model and we find that it automatically generates a dynamical dark energy and a self-accelerating evolution. After fixing our only free parameter m so to reproduce the observed value of the dark energy density today, we get a pure prediction for the dark energy equation of state,
w~-1.14. This value is in excellent agreement with the Planck result and would also resolve the existing tension between the Planck data and local measurements of the Hubble parameter
Adding Gamma-ray Polarimetry to the Multi-Messenger Era
The last decade has seen the emergence of two new fields within astrophysics:
gamma ray polarimetry and GW astronomy. The former, which aims to measure the
polarization of gamma rays in the energy range of 10s to 100s of keV, from
astrophysical sources, saw the launch of the first dedicated polarimeters such
as GAP and POLAR. On the other hand, GW astronomy started with the detection of
the first black hole mergers by LIGO in 2015, followed by the first multi
messenger detection in 2017. While the potential of the two individual fields
has been discussed in detail in the literature, the potential for joint
observations has thus far been ignored. In this article, we aim to define how
GW observations can best contribute to gamma ray polarimetry and study the
scientific potential of joint analyses. In addition we aim to provide
predictions on feasibility of such joint measurements in the near future. We
study which GW observables can be combined with measurements from gamma ray
polarimetry to improve the discriminating power regarding GRB emission models.
We then provide forecasts for the joint detection capabilities of current and
future GW detectors and polarimeters. Our results show that by adding GW data
to polarimetry, a single precise joint detection would allow to rule out the
majority of emission models. We show that in the coming years joint detections
between GW and gamma ray polarimeters might already be possible. Although these
would allow to constrain part of the model space, the probability of highly
constraining joint detections will remain small in the near future. However,
the scientific merit held by even a single such measurement makes it important
to pursue such an endeavour. Furthermore, we show that using the next
generation of GW detectors, such as the Einstein Telescope, joint detections
for which GW data can better complement the polarization data become possible.Comment: 19 pages, 10 figures, Accepted for publication in A&
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