71 research outputs found
Chirality of the gravitational-wave background and pulsar-timing arrays
We describe the signatures of a circularly polarized gravitational-wave
background on the timing residuals obtained with pulsar-timing arrays. Most
generally, the circular polarization will depend on the gravitational-wave
direction, and we describe this angular dependence in terms of spherical
harmonics. While the amplitude of the monopole (the overall chirality of the
gravitational-wave background) cannot be detected, measures of the anisotropy
are theoretically conceivable. We provide expressions for the minimum-variance
estimators for the circular-polarization anisotropy. We evaluate the smallest
detectable signal as a function of the signal-to-noise ratio with which the
isotropic GW signal is detected and the number of pulsars (assumed to be
roughly uniformly spread throughout the sky) in the survey. We find that the
overall dipole of the circular polarization and a few higher overall
multipoles, are detectable in a survey with pulsars if their
amplitude is close to maximal and once the isotropic signal is established with
a signal-to-noise ratio . Even if the anisotropy can be
established, though, there will be limited information on its direction.
Similar arguments apply to astrometric searches for gravitational waves.Comment: 7 pages, 2 figure
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
Testing nonlocal gravity with Lunar Laser Ranging
We study the impact of the limit on from Lunar Laser Ranging on
"nonlocal gravity", i.e. on models of the quantum effective action of gravity
that include nonlocal terms relevant in the infrared, such as the "RR" and "RT"
models proposed by our group, and the Deser-Woodard (DW) model. We elaborate on
the analysis of Barreira et al. [1] and we confirm their findings that (under
plausible assumptions such as the absence of strong backreaction from
non-linear structures), the RR model is ruled out. We also show that the
mechanism of "perfect screening for free" suggested for the DW model actually
does not work and the DW model is also ruled out. In contrast, the RT model
passes all phenomenological consistency tests and is still a viable candidate.Comment: 46 pages, 4 figure
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
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
Teoria efficace dell'inflazione con simmetria galileiana
La teoria dell'inflazione cosmologica nasce per fornire una spiegazione dell'estrema uniformità della temperatura della CMB e della geometria sostanzialmente piatta dell'Universo attuale che, all'interno della cosmologia relativistica standard, richiederebbero un “fine tuning” delle condizioni iniziali dell'Universo. Tali problemi vengono risolti in modo naturale se si ammette l'esistenza di un rapido ma intenso periodo di espansione accelerata nelle primissime fasi di vita dell'Universo, alla fine del quale l'evoluzione sarebbe proseguita come descritto dalla cosmologia standard. Un ottimo candidato a guidare l'inflazione è un campo scalare che in opportune condizioni simula gli effetti di una costante cosmologica e produce dunque un'espansione accelerata. Il più grande successo dell'inflazione sta comunque nella sua capacità di fornire una spiegazione della formazione delle perturbazioni di densità primordiali, i germi delle grandi strutture formate in seguito dal collasso gravitazionale. Le informazioni su tali perturbazioni primordiali provengono dallo studio delle piccole anisotropie nella radiazione di fondo, che rivela una loro distribuzione approssimativamente gaussiana con piccole deviazioni. Lo studio di queste ultime non-gaussianità ci permette di discriminare tra vari modelli di inflazione. Il modello più semplice di inflazione è quello di slow-roll, dove la dinamica di un campo scalare è guidata da un potenziale molto piatto che domina sul termine cinetico; tuttavia un modello di questo tipo predice non-gaussianità troppo piccole per essere osservabili nel prossimo futuro, giustificando così l'introduzione di modelli più complessi.
In questo lavoro di tesi si prende in considerazione un modello di inflazione descritto da una teoria efficace con interazioni “higher derivative”, quindi con più di una derivata per campo, che restituisca però equazioni del moto del secondo ordine in modo da non contenere gradi di libertà ulteriori e instabili. Tale condizione è raggiungibile considerando una teoria per un campo scalare invariante sotto la simmetria interna detta galileiana. Si considera dapprima la situazione in assenza di gravità, dove la simmetria vincola molto la struttura della teoria, tanto che sono possibili solo un numero finito di termini lagrangiani. Le correzioni radiative sono in grado di generare solo operatori con almeno due derivate per campo (che rispettano quindi la simmetria) soppressi ad energie minori del cutoff della teoria, mentre i potenziali operatori dello stesso ordine di quelli originari, ma che non rispettano la simmetria galileiana, non vengono generati. Quando si estende la teoria al caso di spazio curvo attraverso il metodo di accoppiamento minimale, la simmetria ne risulta rotta, ma tale rottura è caratterizzata da una scala di energia molto maggiore del cutoff ultravioletto della teoria stessa. In questo senso si dirà che la simmetria galileiana è rotta solo debolmente e si parlerà di “weakly broken galileon (WBG) invariance”.
Considereremo una classe di teorie WBG accoppiate alla gravità e studieremo le equazioni di Friedmann corrispondenti che governano la dinamica del background, mostrando l'esistenza di soluzioni cosmologiche inflazionarie. Per affrontare lo studio delle fluttuazioni attorno alla soluzione uniforme per il campo scalare galileonico (e di tipo FRW per la metrica), è conveniente utilizzare la gauge unitaria in cui il grado di libertà scalare proveniente dalla fluttuazione del campo è inglobato dalle fluttuazioni della metrica. Questa gauge rompe l'invarianza della teoria sotto diffeomorfismi richiesta dalla relatività generale, in quanto implica la scelta di una coordinata temporale ben precisa. Tuttavia l'invarianza può essere ripristinata mediante la reintroduzione di un grado di libertà scalare corrispondente al bosone di Goldstone che realizza i diffeomorfismi temporali. Con tale procedura lo studio delle conseguenze cosmologiche della teoria risulta semplificato grazie al disaccoppiamento della dinamica del Goldstone da quella della metrica al di sopra di una certa energia minore della scala H
data dal parametro di Hubble durante l'inflazione. Possiamo dunque calcolare le non-gaussianità previste dal modello, le più rilevanti delle quali sono fornite dalla funzione di correlazione a tre punti. Le non-gaussianità sono calcolate in termini dei parametri della teoria e, sotto ipotesi naturali che vengono discusse, è possibile ottenere valori compatibili con i limiti attuali, dati dal satellite Planck, e abbastanza grandi da essere esplorati nel prossimo futuro
Quantum origin of dark energy and the Hubble tension
Local measurements of the Hubble parameter obtained from the distance ladder at low redshift are in tension with global values inferred from cosmological standard rulers. A key role in the tension is played by the assumptions on the cosmological history, in particular on the origin of dark energy. Here we consider a scenario where dark energy originates from the amplification of quantum fluctuations of a light field in inflation. We show that spatial correlations inherited from inflationary quantum fluctuations can reduce the Hubble tension down to one standard deviation, thus relieving the problem with respect to the standard cosmological model. Upcoming missions, like Euclid, will be able to test the predictions of models in this class
Spatial correlations of dark energy from quantum fluctuations during inflation
This paper contains a detailed study of the properties of a simple model attempting to explain dark energy as originated from quantum fluctuations of a light spectator scalar field in inflation. In Belgacem and Prokopec [Phys. Lett. B 831, 137174 (2022)] we recently outlined how Starobinsky's stochastic formalism can be used to study the spatial correlations imprinted on dark energy by its quantum origin in this model and we studied their possible role in relieving the Hubble tension. Here we provide a more comprehensive derivation of the results in Belgacem and Prokopec and we refine some of our estimates, comparing to the approximate results obtained previously. Among the main results, we analyze the noncoincident correlators predicted by a full field theoretical treatment and their relation with those computed within the stochastic formalism. We find that in the region where stochastic theory predicts significant sub-Hubble correlators it is in disagreement with field theoretical predictions. However, agreement can be restored by introducing a reduced speed of sound for the scalar field. We also discuss an alternative approach to the problem of studying correlators within the stochastic formalism based directly on the evolution of probability distributions. We find that the two approaches give the same answer for 2-point functions of the field, but not for 4-point functions relevant to density correlators and we discuss the behavior of the two methods with respect to Wick's theorem
New horizons for fundamental physics with LISA
The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas
Gravity in the infrared and effective nonlocal models
We provide a systematic and updated discussion of a research line carried out
by our group over the last few years, in which gravity is modified at
cosmological distances by the introduction of nonlocal terms, assumed to emerge
at an effective level from the infrared behavior of the quantum theory. The
requirement of producing a viable cosmology turns out to be very stringent and
basically selects a unique model, in which the nonlocal term describes an
effective mass for the conformal mode. We discuss how such a specific structure
could emerge from a fundamental local theory of gravity, and we perform a
detailed comparison of this model with the most recent cosmological datasets,
confirming that it fits current data at the same level as CDM.
Most notably, the model has striking predictions in the sector of tensor
perturbations, leading to a very large effect in the propagation of
gravitational wave (GWs) over cosmological distances. At the redshifts relevant
for the next generation of GW detectors such as Einstein Telescope, Cosmic
Explorer and LISA, this leads to deviations from GR that could be as large as
, and could be verified with the detection of just a single coalescing
binary with electromagnetic counterpart. This would also have potentially
important consequences for the search of the counterpart since, for a given
luminosity distance to the source, as inferred through the GW signal, the
actual source redshift could be significantly different from that predicted by
CDM. At the redshifts relevant for advanced LIGO/Virgo/Kagra the
effect is smaller, but still potentially observable over a few years of runs at
target sensitivity.Comment: 84 pages, 22 figure
- …