Recent research directions

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research related to
Einstein Telescope research related
to LISA cosmology with
standard sirens modified GW
propagation nonlocal infrared
modifications of GR quantum aspects of
black holes

Research related to the Einstein Telescope

A large part of my current research, and of the research of my group, is devoted to studying the science that can be made with the Einstein Telescope. A selection of significant works:

Research related to other GW experiments

Cosmology with standard sirens

The observation of the GW signal emitted by the coalescence of a binary system allows one to reconstruct directly the (luminosity) distance dL of the source. In this context, these systems are called ''standard sirens'' (by analogy with the ''standard candles", such as type Ia supernovae, that provide absolute cosmological distance scales in electromagnetic observations). To extract cosmological information one also needs the redshift z of the source, since the cosmology affects the dL-z relation. However, the redshift is not directly obtained from GW observations; then, there are in general two options for using coalescing binaries as probes of cosmology: either one determines z thanks to the observation of an electromagnetic counterpart (''bright sirens"), or one performs a statistical analysis of a large number of standard, correlating them spatially with the position of galaxies, or using informations from the mass distribution or merger rate of the sources (''dark sirens''). With our group we have intensely worked on both aspects of standard sirens cosmology. A selection of significant papers:

Modified GW propagation

In theories where gravity is modified at cosmological distances, the propagation equation of tensor perturbations is modified. As a result, the quantity extracted from the GW signal of coalescing binaries is no longer the luminosity distance of the source, but rather a different quantity, that we called the ''GW luminosity distance''. Its difference with respect to the standard (''electromagnetic'') luminosity distance can be a smoking-gun signature of modifications of General Relativity (GR) at cosmological scales. We first stumbled on this effect in the context of our nonlocal modification of GR , but in fact the effect is generic to theories where GR is modified at cosmological scales. We have devoted much work to understanding this effect and devising methods to extract it from current and future GW data. In particular, in:

Nonlocal infrared modifications of GR

A natural possibility, to explain the origin of dark energy and of the observed accelerated expansion of the Universe, is that General Relativity is modified at cosmological distances. Typical modified gravity models are obtained adding extra degrees of freedom, such as extra scalar fields. In a research line that we pushed forward mostly in the period 2013-2020, we have developed a different idea, namely that, even when the fundamental theory is standard gravity governed, at the classical level, by the Einstein-Hilbert action, still quantum effects associated to infrared divergences can change the long distance behavior of the theory. Such terms appear as nonlocal terms (relevant in the infrared) in the quantum effective action. On spacetimes of cosmological interest, such as de Sitter, the strongest infrared divergences are associated to the conformal mode of the metric. One can then start by postulating that a dynamical mass is generated for the conformal mode, at the level of the quantum effective action. The covariantization of such a mass term gives rise to nonlocal terms, and there are basically two natural covariantizations that emerge. The first gives rise to the model that I originally proposed in

called the 'RT' model, while the second gives rise to the model that we proposed in

that we call the 'RR' model (sometimes quoted in the literature as the Maggiore-Mancarella model). Over several years, we performed a systematic investigation of models of this class, and of other nonlocal models. In general, in modified gravity, it is hard to get an acceptable cosmological behavior. A viable model must display a FRW solution with accelerated expansion in the recent epoch; it must have stable cosmological perturbations in the scalar and tensor sectors; and the background evolution and the scalar perturbations must be sufficiently close to \(\Lambda\)CDM in order to fit current cosmological data (CMB, SNe, BAO, structure formation,...); finally, the theory must reduce to GR at short scales.
After extensive studies we found that, among all possible nonlocal models, only the RT and RR models found in the two papers above pass all these constraints. It is quite interesting that the only phenomenologically viable models are precisely those that have a physical origin in a dynamical mass generation for the conformal mode. Eventually, we found that the RR model does not pass a constraint from Lunar Laser Ranging, while the RT does, so, in the end, the RT model remains our only candidate model. Turning our attention to the tensor perturbations, we further found that the model displays the phenomenon of modified GW propagation. This was our first encounter with this phenomenon, and led to the more general research line on modified GW propagation, that we discussed above . A selection of papers where we developed conceptual and phenomenological aspects of these nonlocal models can be found here. The review paper:

summarizes our current best understanding of the conceptual aspects of the model, and provides the most updated discussion of the phenomenological predictions.

A particularly interesting feature of the model is that (without any fine tuning) in the background evolution and in the scalar perturbation sector it differs from \(\Lambda\)CDM at the level of a few percent (which allows it to pass current constraints; a model that would differ much more from \(\Lambda\)CDM in the background evolution and in the scalar perturbations would be ruled out). Nevertheless, in the tensor perturbation sector it can differ much more, and the parameter \(\Xi_0\) that describes modified GW propagation can be as large as 1.8 (depending on the initial conditions of the model), corresponding to a 80% deviations from GR. Such an effect would be clearly seen in future GW observations.

An overall assessment of the status of this research line is that, first of all, the model is fully viable phenomenologically. Its differences from \(\Lambda\)CDM in the background evolution and in scalar perturbations are at the level of a few percent, and could be tested in the next generation of galaxy surveys, such as the Vera Rubin-LSST (see e.g. M. Ishak et al. ). The difference in the tensor sector can be much larger and can be tested using modified GW propagation , especially with the next generation of GW experiments. No other known model can predict such a large deviation of \(\Xi_0\) from the value \(\Xi_0=1\) of GR, so this would be a clear signature, not only of modified gravity, but specifically of our non-local model.
At the conceptual level, the model is based on the assumption that a dynamical mass for the conformal mode is generated by infrared effects. The most important open problem is to put this assumption on a more solid ground. This is, however, a very difficult problem, since it involves infrared divergences and non-perturbative effects in gravity, for which current theoretical tools have limited power (see, however, sect. 2.4.3 of our review arXiv:2001.07619 for a discussion of promising lines of attack).

Quantum aspects of black holes

Black holes are a natural playground for testing ideas in quantum gravity, from the existence of a Generalized Uncertainty Principle, to possible quantum interpretations of the structure of their quasinormal modes. These are topics on which I worked already many years ago, and to which I still occasionally come back. You can find here a selection of old and more recent papers that I wrote on the subject.