GraSP24 - Gravity Shape Pisa 2024

23 Oct 2024, 08:15 → 25 Oct 2024, 19:00 Europe/Rome

Aula Gerace (University of Pisa, Polo Fibonacci Ed. C, Dipartimento di Informatica, Aula Gerace)

Lucia Papalini (University of Pisa and INFN Pisa), Matteo Della Rocca, Denis Bitnaya (Università di Pisa), Gabriele Demasi, Giuseppe Di Somma, Ilaria Caporali, Rhona McTeague (University of Glasgow), Maria Antonietta Palaia (Università di Pisa and INFN-Pisa), Joachim Pomper, Michele Vacatello (Istituto Nazionale di Fisica Nucleare)

Gravity Shape Pisa 2024:  

Exploring New Sources of Gravitational Waves

Gravity Shape Pisa (GraSP) 2024 is the 2^nd edition of the International Conference GraSP, entirely organised by PhD students, that supports the active participation of young researchers.

The event is organized to balance “senior” and “young” talk sessions in the fields of observative and theoretical gravitational wave physics. This will give the opportunity to early-stage researchers to spread their work, but also to experienced scientists to be aware about original research inputs coming from younger physicists. The conference will focus on still undetected sources of gravitational waves and their study in current and future detectors. Presentations will cover aspects of astrophysics, cosmology and fundamental physics.

GraSP 2024 will take place in the Physics Department “E. Fermi” of the University of Pisa.

No FEE required.

 

Organizing committee contact information:

 

Lucia Papalini

lucia.papalini@phd.unipi.it

Matteo Della Rocca

matteo.dellarocca@phd.unipi.it

Denis Bitnaya

denis.bitnaya@phd.unipi.it

Ilaria Caporali

ilaria.caporali@phd.unipi.it

Gabriele Demasi

gabriele.demasi@unifi.it

Giuseppe Di Somma

giuseppe.disomma@phd.unipi.it

Rhona McTeague

r.mcteague.1@research.gla.ac.uk

Maria Antonietta Palaia

mariaantonietta.palaia@phd.unipi.it

Joachim Pomper

joachim.pomper@phd.unipi.it

Michele Vacatello 

michele.vacatello@phd.unipi.it

Wednesday 23 October

Registration

Welcoming

grasp_intro.pdf

Next Generation Detector Science

1 Forecasting detection and parameter estimation capabilities for the Einstein Telescope

The Einstein Telescope (ET) is a European project for a next-generation gravitational-wave detector. ET is expected to explore a broad range of science case studies. The reference design is based on a triangular shape of three nested detectors with 10 km arms, where each arm consists of a 'xylophone' configuration made of one interferometer tuned towards high frequencies and the other, cryogenic, towards low frequencies. Evaluating ET's detection and parameter estimation capabilities is a mandatory step in its planning process.
The talk will overview the science perspectives of ET reference design, considering different shapes (triangle versus 2L), different choices of arm length, and the use of only the high-frequency instrument. A broad class of scientific output is examined, ranging from compact binary coalescences to multi-messenger astronomy and cosmology.

Speaker: Ulyana Dupletsa (Istituto Nazionale di Fisica Nucleare)

Dupletsa_GRASP24.pdf

Fundamental Physics

fund_physics_GRASP.pdf

GWECS @GraSP 23_07_2024.pdf

2 The black hole ringdown and its quasinormal modes

In the ringing of a black hole, the nonlinearity of Einstein’s theory can appear in weak and subtle ways. The importance of these nonlinearities has only been appreciated in the past few years, thanks to accurate numerical relativity simulations, and in anticipation of the next generation of gravitational-wave detectors. I will review our current understanding of quasinormal modes, the characteristic modes of black hole ringing, and the current status of black hole perturbation theory for the ringdown, up to third order. Motivated by this and other problems in black hole physics, I will introduce new methods to tackle black-hole perturbation theory using quasinormal modes.

Speaker: Dr Laura Sberna (University of Nottingham)

3 Gravitational wave lensing beyond General Relativity: geometric optic expansion and dispersive phenomena

The nature of gravity can be tested by how gravitational waves (GWs) are emitted, detected, and propagate through the universe. Propagation tests are powerful, as small deviations compound over astronomical distances. However, tests of theories beyond Einstein's general relativity (GR) are limited by the high degree of symmetry of the cosmological spacetime. Deviations from homogeneity, such as those caused by gravitational lenses, allow for new interactions to emerge, offering a path toward novel tests of gravity through GW propagation.

In this talk, I will present the theory of GW propagation beyond GR (i.e., GW lensing beyond GR) in the short-wave expansion, including, for the first time, corrections to the leading-order amplitude and phase. I will introduce the formalism valid for a general scalar-tensor theory and subsequently present the computation of the dispersive (i.e., frequency-dependent) corrections to all metric and scalar field perturbations in Brans-Dicke, the simplest modified theory exhibiting GW dispersion. I will discuss in detail the structure of these effects, their potential impact on the phase of the GW signal, and how such lensing scenarios can give rise to non-tensorial polarizations. Although these effects are highly suppressed in Brans-Dicke, our formalism opens the possibility of novel tests of gravity, including dark-energy models and theories with screening mechanisms.

Speaker: Nicola Menadeo (Scuola Superiore Meridionale (SSM))

Grasp 2024_Menadeo.pdf

4 Theory-agnostic searches for non-gravitational modes in black hole ringdown

In any extension of General Relativity (GR), extra fundamental degrees of freedom couple to gravity. Besides deforming GR forecasts in a theory-dependent way, this coupling generically introduces extra modes in the gravitational-wave signal. We propose a novel theory-agnostic test of gravity to search for these nongravitational modes in black hole merger ringdown signals. To leading order in the GR deviations, their frequencies and damping times match those of a test scalar, vector, or tensor field in a Kerr background, with only amplitudes and phases as free parameters. This test will be highly valuable for future detectors, which will achieve signal-to-noise ratios higher than 100 (and as high as 1000 for space-based detectors such as LISA). By applying this test to GW150914, GW190521, and GW200129, we find that the current evidence for an extra mode is comparable to that for the first gravitational overtone, but its inclusion modifies the inferred remnant spin.

Speaker: Paolo Pani (Sapienza University of Rome & INFN Roma1)

ExtraModes_Pani.pdf

11:00 Coffee Break

Cosmology

5 Measuring Stochastic GW Background (SGWB) with LISA

My talk summarizes the results of several works to assess the prospects of measuring Stochastic GW Background (SGWB) with LISA. After a brief introduction to the topic, I will present some of the expected astrophysical and cosmological SGWBs at LISA scales and discuss the possible implications of their measurement. The last part of my seminar will be devoted to conclusions and future perspectives.

Speaker: Mauro Pieroni (CERN)

mauro_pieroni_slides.pdf

6 Primordial Black Holes in globular clusters

Even if Primordial Black Holes (PBHs) represent only a subdominant component of the dark matter sector, their mere presence can inform us about physical processes that happened at energy scales not reachable by future Particle Physics experiments. Therefore, it is extremely interesting to investigate whether a small fraction of these objects can be detectable by future gravitational wave experiments. In this talk I will discuss the role of PBHs of few to hundreds solar masses in globular cluster. In particular I will show how such dense systems can boost PBH binary formation rate to the point of impacting the gravitational wave background detected by next-generation ground-based interferometers.

Speaker: Nicola Bellomo (Istituto Nazionale di Fisica Nucleare)

Bellomo_PBH_in_GC.pdf

7 Can we identify primordial black holes? Analysis and physical implications of candidate subsolar gravitational-wave events

The detection of a subsolar object in a compact binary merger is regarded as one of the smoking gun signatures of a population of primordial black holes (PBHs). We critically assess whether these systems could be distinguished from stellar binaries, for example composed of white dwarfs or neutron stars, which could also populate the subsolar mass range. At variance with PBHs, the gravitationalwave signal from stellar binaries is affected by tidal effects, which dramatically grow for moderately compact stars as those expected in the subsolar range. We forecast the capability of constraining tidal effects of putative subsolar neutron star binaries with current and future LIGO-Virgo-KAGRA (LVK) sensitivities as well as next-generation experiments. We show that, should LVK O4 run observe subsolar neutron-star mergers, it could measure the (large) tidal effects with high significance. Also, we show possible consequences of the detection of such important candidates, both from the cosmological and the nuclear-physics points of view.

Speaker: Francesco Crescimbeni (Istituto Nazionale di Fisica Nucleare)

GRASP presentation.pdf

13:00 Lunch Break

not provided by the organisation

Astrophysics

8 Massive black hole binaries in the era of multimessenger astronomy

Following detection by LIGO and Virgo, gravitational wave (GW) stocks are on the rise. Despite their outstanding capabilities, ground based detectors are only sensitive to GW sources in the audio band. Conversely, the low frequency GW Universe, from nHz to mHz, is the exploration playground of future spaceborne interferometers such as the Laser Interferometer Space Antenna (LISA) and ongoing and future pulsar timing arrays (PTAs). In particular, massive black hole binaries (MBHBs) forming in the aftermath of galaxy mergers are expected to be the loudest low frequency sources in the GW universe, and possibly bright in the electromagnetic (EM) spectrum. In fact, the recent evidence of a GW signal reported by PTAs around the world is likely due to a cosmic population of these objects. I will discuss the recent PTA results, the expected joint GW and EM emission of MBHBs and future opportunities for multimessenger observations with LISA and PTA and future EM facilities such as LSST, SKA, Athena.

Speaker: Alberto Sesana (Universita` di Milano Bicocca)

9 Gravitational lensing applied to gravitational waves

In my talk, I will provide an overview about gravitational lensing applied to gravitational wave emission. I will start with the definition of lensing, discussing how it affects the gravitational signal from a source, with a focus on the differences between geometrical and wave optics regime. Next, I will describe what we expect to see in the cases of strongly and weakly lensed signals and why it is important to recognise these effects among the data. Finally, I will present the main findings in the literature about two types of sources, that we might observed lensed in the upcoming years: black hole mergers and extreme mass ratio inspirals, highlighting their importance for gravitational wave astronomy.

Speaker: Martina Toscani

lensing_toscani.pdf

10 Machine Learning Identification of Strongly Lensed Gravitational Waves Events

I will review the physics of gravitational lensing and its impact on the propagation of gravitational wave (GW) signals. I will discuss the current methods for identifying strongly lensed GW events, including posterior overlap integral estimation.

I will then introduce a new machine learning-based approach designed to distinguish strongly lensed GW events from independent signals. Given the high noise levels in GW detectors, data compression and parameter space analysis are essential to test the strong lensing hypothesis. To address the significant non-Gaussian nature of parameter posterior distributions, I employ normalizing flows to model the difference distribution of parameters between events. This technique allows us to statistically quantify the significance of potential lensed event pairs.

I will demonstrate the application of this method and present the results of the analysis on both simulated catalogs and real events from the Ligo-Virgo-Kagra O3 catalog.

Speaker: Giulia Campailla (Istituto Nazionale di Fisica Nucleare)

Talk_GraSP24_Pisa.pdf

Poster Advertisement Session I

advertisement_session_October23rd.pdf

Fundamental Physics: Panel Session

Conveners: Prof. Andrea Maselli (Gran Sasso Science Institute), Dr Elisa Maggio (Max Planck Institute)

fund_physics_GRASP.pdf

GWECS @GraSP 23_07_2024.pdf

11 Panel Session

Speakers: Andrea Maselli (Gran Sasso Science Institute), Elisa Maggio (Max Planck Institute)

fund_physics_GRASP.pdf

GWECS @GraSP 23_07_2024.pdf

Poster session: Welcoming Drink & Poster Session

Thursday 24 October

Next Generation Detector Science

12 Overview of the LISA detector and its future observational capabilities

In this talk, we will provide an overview of the Laser Interferometer Space Antenna (LISA) and its future observational capabilities. LISA, recently adopted by the European Space Agency (ESA) and scheduled to launch in 2035, will consist of a constellation of three spacecraft arranged in an almost equilateral triangle, with millions of kilometers separating each pair. This very long baseline will allow LISA to detect gravitational waves (GW) in the mHz range. This frequency band is expected to be extremely rich in GW sources but completely in-accessible to current ground-based detectors. Therefore, LISA is expected to provide advancements across a wide range of subjects, from cosmology and astrophysics to fundamental physics.

Since LISA is the first mission of its kind, achieving the required sensitivity for this significant scientific return involves overcoming a number of technical challenges. We will review key aspects of LISA's instrument design and operational principles, including the overall mission architecture, the laser interferometry technique at its core, the main limiting noise sources, and the complex data processing pipeline needed to extract meaningful signals from the raw measurements.

Speaker: Dr Olaf Hartwig

2024-10-23-grasp24-pdf.pdf

Astrophysics

13 The precision science of extreme mass-ratio inspirals

Extreme mass-ratio inspirals (EMRIs) of a stellar-mass compact object into a massive black hole are a unique science target of LISA. As a consequence of their mass ratios, these systems complete tens of thousands of orbits, and have intricate gravitational-wave signals. This means that we can make precision measurements of EMRI sources. Using these precise measurements, we can probe the strong-field spacetime around massive black holes, and study the astrophysics of the evolution of the source population. In this talk, we will review some of the scientific potential of EMRIs, and highlight some of the challenges in analysing these gravitational-wave signals.

Speaker: Christopher Berry (University of Glasgow)

GRASP24.pdf

14 Performing precision gravitational-wave science with LISA using extreme-mass-ratio inspiral observations

The formation and evolution of massive black holes (MBHs) is an
unresolved mystery in astrophysics.
A primary goal of the Laser Interferometer Space Antenna (LISA) mission
is therefore to study massive black holes (MBHs) and their environments
from their gravitational wave emission in binary systems.
One class of these systems are extreme-mass-ratio inspirals (EMRIs), in
which a stellar-mass compact object (CO) completes tens of thousands of
orbital cycles around a MBH over a timescale of years before merging.
The precession of the CO trajectory produces a waveform rich in harmonic
structure, enabling LISA to measure source parameters to one part in a
milion.
However, this waveform complexity and extreme measurement precision
makes EMRI data analysis a computationally expensive procedure.
In this talk, we will show how these exquisite waveforms can be
generated in milliseconds with the FastEMRIWaveforms (FEW) package.
We will then explore what future improvements in waveform generation
with FEW are required to fully exploit the science potential of EMRI
signals.
Lastly, we will demonstrate how the measurement precision of EMRIs makes
them ideal candidates for investigating environmental effects and
performing cosmology/lensing studies.

Speaker: Christian Chapman-Bird (University of Birmingham)

GraSP24 - Christian Chapman-Bird.pdf

15 Cyclostationary signals in LISA: a practical application to Milky Way satellites

One of the primary sources of gravitational waves (GWs) anticipated to be detected by the Laser Interferometer Space Antenna (LISA) are galactic double white dwarf binaries (DWDs). However, most of these binaries will be unresolved, and their GWs will overlap incoherently, creating a stochastic noise known as the galactic foreground. Similarly, the population of unresolved systems in the Milky Way's (MW) satellites is expected to contribute to a stochastic gravitational wave background (SGWB). Due to their anisotropy and the annual motion of LISA constellation, both the galactic foreground and the satellite SGWB fall into the category of cyclostationary processes. Leveraging this property, we develop a purely frequency-based method to study LISA's capability to detect the SGWB from the principal MW satellites. To achieve this, we first analyze mock data generated by an astrophysical motivated SGWB spectrum, and then examine realistic data from a DWD population generated via binary population synthesis.
Our findings highlight the significance of the interaction between the astrophysical spectrum and LISA's sensitivity for detecting the satellite SGWB. Furthermore, we explore the potential to observe a hypothetical satellite located behind the galactic disk. Our results suggest that an LMC-like satellite could indeed be observable by LISA.

Speaker: Federico Pozzoli (Istituto Nazionale di Fisica Nucleare)

GraSP24_pozzoli.pdf

11:00 Coffee Break

Cosmology

16 Scalar-tensor perturbation mixing in Cosmology

I will review the several new effects emerging from the combined non-linear action of scalar and tensor modes. Among these: scalar-induced gravitational waves, scalar fluctuation modulation of gravitational waves, anisotropies of the gravitational-wave background induced by large-scale scalar and tensor cosmological inhomogeneities, tensor-induced scalar perturbations. I'll also discuss the wave-optics limit of gravitational waves.

Speaker: Prof. Sabino Matarrese (Istituto Nazionale di Fisica Nucleare)

Matarrese_GraSP24 (1).pdf

Matarrese_GraSP24.pdf

17 The effect of quantum decoherence on inflationary gravitational waves

The theory of inflation provides a mechanism to explain the structures we observe today in the Universe, starting from quantum-mechanically generated fluctuations. However, this leaves the question of: how did the quantum-to-classical transition, occur? During inflation, tensor perturbations interact (at least gravitationally) with other fields, meaning that we need to view these perturbations as an open system that interacts with an environment, which leads to quantum decoherence. In this talk I will show how this interaction can lead to a change in the gravitational wave power spectrum. By using current upper bounds on the gravitational wave power spectrum from inflation, obtained from CMB and the LIGO-Virgo-KAGRA constraints, we find an upper bound on the interaction strength between the system and the environment. We also show how using sensitivity forecasts for future gravitational-wave detectors, such as LISA and ET, could further constrain the decoherence parameter space. Furthermore, we indicate the minimal interaction strength needed for a specific scale to have successfully decohered by the end of inflation. This allows us to indicate a lower bound on the interaction strength, assuming CMB modes have completely decoherence. Additionally, this allows us to look at which scales might not have fully decohered and could still show some relic quantum signatures.

Speaker: Jessie Arnoldus De Kruijf (Istituto Nazionale di Fisica Nucleare)

Decoherence_presentation_Pisa.pdf

18 Gravitational waves from early universe first order phase transitions

While cooling down, the early universe is believed to have witnessed symmetry breaking phase transitions. With the increasing need to look at beyond the Standard Model theories, one particularly attractive possibility is extending the Standard Model such that phase transitions are of the first order. Not only could they explain e.g. the baryon asymmetry, they are also expected to produce gravitational waves. Were the future detectors to give us evidence of a first order phase transition, it would be a sign of new physics. In this talk I will discuss the process of early universe phase transitions concentrating on the details of the field evolution and the effects they have on the gravitational wave spectrum.

Speaker: Satumaaria Sukuvaara (University of Helsinki)

grasp_sukuvaara.pdf

13:00 Lunch break

Not provided by the organisation

Fundamental Physics

fund_physics_GRASP.pdf

GWECS @GraSP 23_07_2024.pdf

19 Extreme mass-ratio inspirals into black holes surrounded by boson clouds

Boson clouds can form through superradiant instabilities of ultralight bosons around spinning black holes. The formation of boson clouds leads to a number of potentially detectable signatures, among them the possibility that they can affect the dynamics of binary black hole systems. In this talk I will review recent work aiming at studying extreme-mass-ratio systems in which a small compact object evolves around a massive black hole surrounded by a boson cloud.

Speaker: Richard Brito

brito_GraSPA.pdf

20 Measuring scalar charge with extreme mass ratio inspirals

I will first discuss under which circumstances can black holes carry scalar charge and how such charge would scale with the mass of the black hole. I will use this insight to argue that EMRIs are an ideal system for searches of new fundamental scalars and lay out the framework for modelling EMRIs. I will then present forecasts on LISA's ability to measure scalar charge.

Speaker: Thomas Sotiriou (University of Nottingham)

21 EMRIs and new fundamental fields

Extreme Mass Ratio Inspirals (EMRIs) are asymmetric binary systems composed by a stellar mass compact object inspiralling around a central massive black hole while emitting gravitational waves (GWs), which are among the main targets of the future space detector LISA. By tracking the emitted GWs with LISA, it will be eventually possible to recover with extreme accuracy the intrinsic source parameters. This potential renders EMRIs highly promising candidates for detecting deviations from general relativity or to derive bounds on theories predicting such deviations.
In this talk I will present a theoretical model of EMRIs in gravity theories with new fundamental scalar fields, showing how, for a vast class of such theories, great simplifications occur in the EMRI description. Indeed, at leading order in the binary mass ratio, the primary scalar charge is suppressed, so that the background spacetime is simply described by the Kerr metric. Moreover, the imprint of the scalar field on the waveform is fully captured by two parameters: the scalar charge carried by the secondary and the scalar field mass. Using these simplifications, I will show how these parameters affect the EMRI orbital evolution, and how such changes get imprinted on the emitted waveforms. By analysing such signals, I will finally present the encouraging results on the LISA ability to detect the scalar charge and scalar field mass.

Speaker: Susanna Barsanti

GraSP24_Barsanti.pdf

Poster Advertisement Session II

advertisement_session_October24th.pdf

Astrophysics: Panel Session

22 Panel Session

Speakers: Dr Riccardo Buscicchio (Universitá di Milano-Bicocca), Stefano Rinaldi (Universität Heidelberg)

Astrophysics panel slides

17:30 Wine and cheese & Poster Session

20:00 Social Dinner

NOT PROVIDED BY THE ORGANISATION

Friday 25 October

Cosmology

23 Cosmology with standard sirens - what's the state of play?

Gravitational wave signals from compact binary mergers are of huge interest to the cosmology community due to their ability to act as standard sirens, providing measurements of luminosity distance which are independent of the cosmic distance ladder. Using standard sirens to measure the Hubble constant could shed light on the current tension, which lies above 4σ between early-universe and local measurements, and the cause of which is as-yet unknown. In this talk I will give an overview of the current ways in which standard sirens are being used for cosmological measurements, including the bright siren method (making use of electromagnetic counterparts), the dark siren method (using galaxy surveys to provide information about potential host galaxies) and the spectral siren method (utilising features in the compact binary mass distribution to break the mass-redshift degeneracy of detected signals). I will look at some of the latest measurements in the field of gravitational-wave cosmology and at how the field may change as we look to the future.

Speaker: Rachel Gray (University of Glasgow)

GraSP24.pdf

24 'Spectral Siren' Cosmology with a Combined-Population Mass Distribution

The ‘spectral siren’ method is a powerful probe of cosmology using gravitational waves observed from compact binary coalescences (CBCs). By exploiting the features of the mass spectrum of CBCs, the degeneracy between mass and redshift can be broken - allowing for the use of such events to constrain cosmological parameters. Previously, these mass spectra have been separated by binary type, with different priors for analysis of BBH, NSBH and BNS binaries. We present a new, unified approach which combines the mass spectra of different binary types to constrain cosmological parameters, the shape of the compact object mass spectrum, and the astrophysical fractions of each binary type. Using the gwcosmo cosmological inference pipeline, we apply this new spectral siren method to mock data representing the observations of a three-detector (LHV) network operating at O5 sensitivity. This demonstrates the constraining power of this combined CBC population method with future gravitational wave observations.

Speaker: Alexander Papadopoulos (University of Glasgow)

GraSP Presentation-1.pdf

25 Cosmic Variance of the Hellings and Downs Correlation and Source Anisotropies

Gravitational waves (GWs) induce correlated perturbations to the arrival times of pulses from an array of galactic millisecond pulsars. The expected correlations, obtained by averaging over many pairs of pulsars having the same angular separation (pulsar averaging) and over an ensemble of model universes (ensemble averaging), are described by the Hellings and Downs curve. As shown by Allen [Phys. Rev. D 107, 043018 (2023)], the pulsar-averaged correlation will not agree exactly with the expected Hellings and Downs prediction if the gravitational-wave sources interfere with one another, differing instead by a "cosmic variance" contribution. The precise shape and size of the cosmic variance depends on the statistical properties of the ensemble of universes used to model the background. Here, we extend the calculations of the cosmic variance for the standard Gaussian ensemble to an ensemble of model universes which collectively has rotationally invariant correlations in the GW power on different angular scales (described by an angular power spectrum, $C_\ell$ for $\ell=0,1,\dots$). We obtain an analytic form for the cosmic variance in terms of the $C_\ell$'s and show that for realistic values $C_\ell/C_0≲10−3$, there is virtually no difference in the cosmic variance compared to that for the standard Gaussian ensemble (which has a zero angular power spectrum).

Speaker: Dr Deepali Agarwal (CP3- UCLouvain, Belgium)

Deepali_GraSP.pdf

Next Generation Detector Science

26 Challenges of pulsar timing array (PTA) data analysis

I will introduce the challenges of pulsar timing array (PTA) data analysis, leading up to the methods and techniques we use for the GW searches in PTAs. I will then provide a status update on PTAs, and where current work stands with the new and upcoming data releases from the international and European PTAs.

Speaker: Golam Shaifullah ('G. Occhialini' Dipartimento di Fisica, Università degli Studi di Milano-Bicocca)

PTAChallenges_GolamShaifullah

11:00 Coffee Break

Astrophysics

27 Individual neutron stars as GW sources: continuous and long-transient signals

One of the longest-standing science targets of gravitational-wave detetors are spinning deformed neutron stars. While exceedingly weak and hence still eluding detection, "continuous waves" from such individual objects will bring a new regime of gravitational astrophysics where we can keep observing the same source over and over and perform rich multi-messenger studies. In addition, neutron stars can also emit long transient signals triggered by a variety of energetic events. Together, both new types of gravitational-wave signals promise an unprecedented probe into the structure, interior, and dynamics of the densest stellar objects in the Universe.

Speaker: Dr David Keitel (Universitat de les Illes Balears)

20241025_GraSP24_dkeitel_G2401742-v2.pdf

28 Astrometric signatures of gravitational waves from compact binaries

Gravitational waves (GWs) induce characteristic oscillations in the observed positions of distant stars over time. This effect can potentially be captured by astrometric observations, such as those conducted by the Gaia mission, offering a promising method for detecting GWs by measuring tiny changes in the angular separations between pairs of point-like sources—effectively acting as large-scale detector arms. I explore the potential of this approach for detecting GWs, both from individually resolvable supermassive black hole binaries and from a stochastic background of GWs. I also examine how this method complements existing GW detection techniques, such as pulsar timing arrays, providing a novel means to probe gravitational waves across a broad range of scales and sources.

Speaker: Massimo Vaglio (SISSA)

DetectingGWswithAstrometry-Vaglio.pdf

29 Cliffhanger EMRIs: a new GW source for LISA

In nuclear clusters, massive black holes (MBHs) are surrounded by numerous stars and compact objects. In these high-density environments, two-body interactions between orbiting objects occur frequently, potentially leading to the formation of extreme mass ratio inspirals (EMRIs). In this work, we present a novel post-Newtonian Monte Carlo approach to locally account for the effects of two-body relaxation on objects orbiting the MBH, without leveraging on the common approximation of orbit-averaging. We apply our method to study the formation ratio of EMRIs to direct plunges (DPs) as a function of the initial semi-major axis of the orbit around the MBH. While it is generally believed that this ratio approaches zero for large initial semi-major axes, where only DPs are expected, a recent study challenges this notion for low-mass MBHs. Our simulations confirm the existence of cliffhanger EMRIs, forming from initially wide orbits around MBHs with masses less than 10^6 solar masses. These EMRIs arise from failed DPs, as their orbits significantly shrink and circularise following a single pericentre passage of few gravitational radii. We test how the EMRI-to-DP ratio is influenced by different assumptions on the dynamics used to evolve the system and treatment of two-body relaxation. We find that post-Newtonian corrections greatly enhance the number of EMRIs formed, while the departure from orbit-averaging does not notably affect the EMRI-to-DP ratio. Our findings call for a reassessment of future LISA detection rates to account for this new gravitational wave source.

Speaker: Davide Mancieri (University of Trento / Milano-Bicocca)

GraSP24-Mancieri.pdf

13:00 Lunch Break

Not provided by the organisation

Fundamental Physics

fund_physics_GRASP.pdf

GWECS @GraSP 23_07_2024.pdf

30 Breaking black-hole uniqueness at supermassive scales

In general relativity, all vacuum black holes are described by the Kerr metric. However, beyond general relativity, there is a prevailing expectation that deviations from the Kerr solution are more likely to manifest with increasing horizon curvature, making solar-mass black holes more promising grounds to test general relativity. In this talk I will challenge this expectation and discuss a model where black holes differ from Kerr only in a finite mass range, bounded from above and below. In particular, black-hole uniqueness can be broken at supermassive black-hole scales, while solar-mass black holes remain well-described by the Kerr solution.

Speaker: Pedro Fernandes (University of Heidelberg)

Talk_Fernandes.pdf

31 Probing the regularization of spacetime singularities

Spacetime singularities are usually considered the proof of the intrinsic incompleteness of General Relativity (GR). The common belief is that their formation will be prevented in a full, potentially quantum, completion of GR. In this view, it is reasonable to assume that regular metrics can provide an effective description of the outcome of gravitational collapse. We have then two possible regular alternatives to describe the ultracompact objects that we saw in our universe: regular black holes and horizonless compact objects. I will talk about the possible structures of these black hole mimickers and the gravitational waves ringdown signal that we expect from their coalescence. In particular I will focus on the deviations in the spectrum of QNMs with respect to singular BHs, their possible detectability and the structure of echoes when backreaction effects and propagation in the interior of the object are taken into account.

Speaker: Vania Vellucci (SISSA)

Grasp24.pdf

32 Tests of General Relativity with GW230529: a neutron star merging with a lower mass-gap compact object

On 29 May 2023, the LIGO Livingston observatory detected the gravitational-wave signal GW230529_181500 from the merger of a neutron star with a lower mass-gap compact object. Its long inspiral signal provides a unique opportunity to test General Relativity (GR) in a parameter space previously unexplored by strong-field tests. In this work, we performed parameterized inspiral tests of GR with GW230529_181500. Specifically, we search for deviations in the frequency-domain GW phase by allowing for agnostic corrections to the post-Newtonian coefficients. We performed tests with the Flexible Theory Independent (FTI) and Test Infrastructure for General Relativity (TIGER) frameworks using several quasi-circular waveform models that capture different physical effects (higher modes, spins, tides). We find that the signal is consistent with GR for all deviation parameters. Assuming the primary object is a black hole, we obtain particularly tight constraints on the dipole radiation at $-1$PN order of $|\delta\hat{\varphi}_{-2}| \lesssim 8 \times 10^{-5}$, which is a factor $\sim17$ times more stringent than previous bounds from the neutron star--black hole merger GW200115_042309, as well as on the 0.5PN and 1PN deviation parameters. We discuss some challenges that arise when analyzing this signal, namely biases due to correlations with tidal effects and the degeneracy between the 0PN deviation parameter and the chirp mass. To illustrate the importance of GW230529_181500 for tests of GR, we mapped the agnostic $-1$PN results to a class of Einstein-scalar-Gauss-Bonnet (ESGB) theories of gravity. We also conducted an analysis probing the specific phase deviation expected in ESGB theory and obtain an upper bound on the Gauss-Bonnet coupling of $\ell_{\rm GB} \lesssim 0.51~\rm{M}_\odot$ ($\sqrt{\alpha_{\rm GB}} \lesssim 0.28$ km), which is better than any previously reported constraint.

Speaker: Elise Sänger (Max Planck Institute for Gravitational Physics (AEI Potsdam))

presentation_GraSP24_GW230529_TGR.pdf

Cosmology: Panel Session

33 Panel Session

Speakers: Nicola Bartolo (Istituto Nazionale di Fisica Nucleare), Simone Mastrogiovanni (Istituto Nazionale di Fisica Nucleare)

Pres

Final remarks and cheers

cheers113.pdf

17:30 Tea Break