Ebook: Gravitational Waves and Cosmology

The past twenty years have witnessed a number of breakthroughs in Astrophysics and Cosmology, that were awarded Nobel prizes: in 2019 “for theoretical discoveries in physical cosmology” and “for the discovery of an exoplanet orbiting a solar-type star”; in 2017 “for decisive contributions to the LIGO detector and the observation of gravitational waves”; in 2011 “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae”. How could we not mention the other two related breakthroughs in physics, also awarded with Nobel prizes: in 2015 “for the discovery of neutrino oscillations, which shows that neutrinos have mass”; in 2013 “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”.

These physics triumphs serve to highlight our awareness that a Varenna School in “Gravitation and Cosmology” should provide the students with a solid and broad knowledge of the fundamentals of astrophysics and cosmology, without losing sight of the basics of the fundamental interactions in Physics. Thus, the aim of the School, held in the beautiful location of Villa Monastero from the 3rd to 12th of July 2017, was to expose students to state-of-the-art research in the field of Gravitational Waves and Cosmology, from both theoretical and experimental points of view. The choice of the subjects seemed particularly timely, given the discovery of gravitational waves by the LIGO-Virgo collaboration and the release of the Planck data in 2015, that so strongly contributed to the era of precision cosmology and to the adoption of a standard model of cosmology.

Twenty-four speakers participated in the School. We are very grateful to all of our colleagues, both to those who contributed written papers to this volume, as well as to those who participated in the School by just providing lectures. The level of the speakers was very high, with their presentations offering a broad overview of the subject matter.

The lectures were organized in a way designed to foster the interactions between two different communities. This has been, in our opinion, one of the most notable added values of the School. The School admitted more than 50 PhD students, selected from all over the world, and also some Master students of the Erasmus Joint Master Program “Astromundus”, funded by the EU and jointly provided by five European universities (Innsbruck, Gottingen, Padua, Tor Vergata and Belgrade). The environment of Villa Monastero naturally facilitates informal interactions between students and teachers. This added value of the “Enrico Fermi” Schools has been particularly important in enabling the case for integrating groups of students working in different fields and communities.

As will be obvious by going through the volume, there was a wide range of topics addressed by our lecturers. For the Gravitational Waves section, the lectures covered the experimental issues connected with gravitational wave detection and the new field of multi-messenger astronomy, as well as more astrophysical aspects. Lectures in the first category were provided by Fulvio Ricci, Viviana Fafone and Francesco Fidecaro, while the contributions of Marica Branchesi on multi-messenger astronomy and of Michela Mapelli about the open questions of black hole binaries covered the complementary aspects. In the realm of Cosmology, there are contributions on the early universe, on the Cosmic Microwave Background (CMB) and on redshift surveys. For the Early Universe, Jerome Martin provided a review of the inflationary scenarios, Sabino Matarrese discussed the non-Gaussian features of primordial density fluctuations, while Jens Chluba concentrated on the physical mechanisms responsible for the spectral distortions of the blackbody spectrum of the CMB. Regarding the CMB proper, the results of the Planck Legacy were presented by Carlo Burigana, and their theoretical implications were discussed by Douglas Scott, while the polarization modes of the CMB and the physical mechanisms underlying them were introduced by Wayne Hu. On the redshift survey side, Will Percival presented the wealth of information encoded in the large-scale structure of the universe, while David Mota reviewed the effects of modified gravity theories, with a particular focus on the effects on the non-linear regime on the structure formation process.

In conclusion, we are indebted to all our colleagues for their availability to participate in the School and for delivering outstanding lectures. We are also indebted to all the students that participated to the School with great enthusiasm and for expressing their positive opinion about the choice of the topics to which they were exposed. We are particularly thankful to the Italian Physical Society for hosting our School in the context of the program of the “Enrico Fermi” Schools, and for the professional support we received during our stay in Varenna.

Eugenio Coccia, Joe Silk and Nicola Vittorio

The principles of gravitational wave detection are presented. The measurement of the propagating minute deformation of space-time remains an extraordinary experimental challenge. After a brief introduction to signals and noise, fundamental disturbances in the measurement are discussed, outlining how these have been sufficiently reduced to achieve detection.

Despite the simple detection principle, a gravitational wave interferometer is an extremely complex instrument. In these lectures we will try to summarise some of the main features of the detector. We will start from the very basic concepts and gradually advance to a more general treatment. We will focus our discussion on the solutions adopted in the second generation of instruments, by means of which now the era of the gravitational astronomy has started. In the final sections, in view of the development of the future gravitational wave detectors, we will discuss how to reduce two main noise sources: the thermal noise via the use of cryogenic techniques and the readout noise via the strategies conceived to circumvent the quantum limit of the detector.

In this lecture we will focus our attention on a specific issue connected with the optical properties of the test masses in ground-based interferometers, namely the presence of optical aberrations that can have a relevant impact on the possibility to operate the detector. The second part will present an overview on the medium- and long-term evolution of terrestrial interferometers.

On September 14, 2015, the LIGO interferometers captured a gravitational wave (GW) signal from two merging black holes (BHs), opening the era of GW astrophysics. Five BH mergers have been reported so far, three of them involving massive BHs (>30M_⊙). According to stellar evolution models, such massive BHs can originate from massive relatively metal-poor stars. Alternatively, gravitational instabilities in the early Universe were claimed to produce BHs in this mass range. The formation channels of merging BH binaries are still an open question: a plethora of uncertainties affect the evolution of massive stellar binaries (e.g. the process of common envelope) and their dynamics. This review is intended to discuss the open questions about BH binaries, and to present the state-of-the-art knowledge about the astrophysics of black holes for non-specialists, in light of the first LIGO detections

On 2017 August 17 the Advanced LIGO and Advanced Virgo detectors detected for the first time the signal, GW170817, from the coalescence of a binary system of neutron stars (Abbott B.P. et al., Phys. Rev. Lett., 119 (2017) 161101). Exactly 1.7 s after the merger time (12:41:04 UTC) the Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst, GRB170817A (Abbott B.P. et al., Astrophys. J. Lett., 848 (2017) L13; L14). An extensive observing campaign involving more than 70 world-wide ground and space observatories was performed leading to the discovery of the counterpart signals across all the electromagnetic spectrum (Abbott B.P. et al., Astrophys. J. Lett., 848 (2017) L12). This observational campaign marks the birth of multi-messenger astronomy, which uses gravitational waves and electromagnetic emission. The collected multi-messenger data confirmed ten-year-old theoretical models. And at the same time, the richness of details of the taken data will require to develop new theory and to make other observations in the coming years to be interpreted

The status of the standard cosmological model, also known as “ΛCDM” is described. With some simple assumptions, this model fits a wide range of data, with just six (or seven) free parameters. One should be skeptical about this claim, since it implies that we now have an astonishingly good picture of the statistical properties of the large-scale Universe. However, the successes of the model cannot be denied, including more than 1000 σ worth of detection of CMB anisotropy power. The model is older than most modern astrophysicists seem to appreciate, and has not fundamentally changed for more than a quarter of a century. Tensions and anomalies are often discussed, and while we should of course be open to the possibility of new physics, we should also be skeptical about the importance of 2–3 σ differences between data sets until they become more significant. Still, today’s SMC is surely not the full story and we should be looking for extensions or new ingredients to the model, guided throughout by a skeptical outlook.

This article contains a concise review of the theory of inflation. We discuss its main theoretical aspects as well as its observational predictions. We also explain how the most recent astrophysical observations constrain the inflationary scenario.

Here we review the present status of modelling of and searching for primordial non-Gaussianity of cosmological perturbations. After introducing the models for non-Gaussianity generation during inflation, we discuss the search for non-Gaussian signatures in the Cosmic Microwave Background and in the Large-Scale Structure of the Universe.

I review the sources of CMB polarization in the acoustic regime, reionization and gravitational waves from inflation and their distortion by gravitational waves using the ΛCDM model as an illustration.

While the final release of products and papers from the Planck mission is forthcoming, new much more ambitious projects dedicated to the Cosmic Microwave Background (CMB) are in preparation for the next decade and beyond. We review the Planck products, in form of source catalogs and maps, briefly discussing the methods to produce them and extracting the various types of astrophysical and cosmological signals, presenting their main implications. The limits on primordial B-modes associated to stochastic field of gravitational waves expected in inflationary scenarios and the perspectives open by the investigations of expected tiny spectral distortions are discussed. Finally, the studies carried out in the last decade towards future CMB space missions are described.

Since the measurements of COBE/FIRAS in the mid-90’s we know that the energy spectrum of the cosmic microwave background (CMB) is extremely close to that of a perfect blackbody at an average temperature T₀≃2.726 K. However, a number of early-universe processes are expected to create CMB spectral distortions —departures of the average CMB energy spectrum from a blackbody— at a level that is within reach of present-day technology. This provides strong motivation to study the physics of CMB spectral distortions and ask what these small signals might be able to tell us about the Universe we live in. In this lecture, I will give a broad-brush overview of recent theoretical and experimental developments, explaining why future spectroscopic measurements of the CMB will open an unexplored new window to early-universe and particle physics. I will give an introduction about the different types of distortions, how they evolve and thermalize and highlight some of the physical processes that can cause them. I hope to be able to convince you that CMB spectral distortions could open an exciting new path forward in CMB cosmology, which is complementary to planned and ongoing searches for primordial B-mode polarization signals. Spectral distortions should thus be considered very seriously as part of the activities in the next decades.

These are advanced lecture notes covering recent developments in the methodology used to analyse galaxy surveys. The focus is particularly on direct measurements of the galaxy power spectrum although we also discuss its Fourier transform, the correlation function for comparison. These 2-point statistics, under the assumption that the overdensity field has Gaussian statistics on large scales, contain the majority of the cosmological signal available from the galaxy distribution. Recent developments in multipole measurements, dealing with systematics, convolving theoretical models with the survey window function, the approximation of covariance matrices, and weighting schemes for measuring evolution with redshift are considered. The focus is on analytic explanation of the issues involved rather than on recent analyses or simulation results. These notes only loosely follow the lectures I gave in Varenna, which were more wide ranging and contained more introductory material. However, I have in the not-too-distant past created lecture notes for an introductory course on galaxy survey analysis, and I did not want to duplicate those notes, but instead write something new for these proceedings.

We review the effects of modified gravity theories, in particular the symmetron and f(R) gravity, on the nonlinear regime of structure formation. In particular, we investigate the velocity dispersion of galaxy clusters as a function of the halo masses, how the matter power spectra depends on the coupling, range, and screening scale of the fifth force, and on possible ways of detecting violations of the equivalence principle using the mass inferred via lensing methods versus the mass inferred via dynamical methods. Furthermore, we show how one could use different voids statistics as one of the most promising probes of modified gravity.