The aim of this workshop is to bring together theorists and observers working on the broad topic of multi-messenger astronomy and astrophysics. The focus will be on how to best access and test fundamental physics by observing compact objects, taking advantage of current and future data. The workshop will connect experts from disjoint but related fields and the format will provide ample time for informal interaction and discussion.
The list of topics includes compact objects in General Relativity and beyond, gravitational waves obsevations and theory, pulsar timing, accretion physics, and multimessenger tests of fundamental physics and cosmology, from both data and theory perspectives.
|Eugeny Babichev||LPT, Orsay||
Modified gravity and compact objects I will review black hole and star solutions in modified gravity theories, with the focus on Horndeski, beyond Horndeski, DHOST theories as well as massive and bi-gravity. Modified gravity allows for a wide range of black hole solutions, in particular stealth, self-tuning and non-GR configurations. The properties of these solutions may be quite different from the GR counterparts. The Vainshtein mechanism, crucial for the recovery of General Relativity in Solar system, applies for some theories/solutions, but breaks down for others.
|Bruny Baret||APC, Paris (CNRS)||
Fundamental physics and astrophysics with high-energy neutrino telescopes Neutrino astronomy has begun with the detection by the Gton scale IceCube telescope in 2013 of the first high energy neutrino cosmic candidates followed until today by around 100 others which have confirmed the existence of a high energy cosmic diffuse radiation in the so called cascade event sample with low angular resolution but good energy estimation. So far this signal has not shown any sign of clusterisation and its origin remains unknown but it has been confirmed by the so called muon track sample benefiting from good pointing but loose energy reconstruction, albeit with some tensions in the spectral shape of the signal. Very recently In 2017 the detection by the MAGIC and FERMI telescopes of an AGN flare thanks to a high energy neutrino alert sent by IceCube led to the identification of TXS-0506+056 as the first serious candidate as neutrino transient identified source. Meanwhile, other detectors of tens of Mton scale, like ANTARES in the Mediterranean Sea and Baikal in the Russian homonymous lake, have been taking data aiming at confirming these detection and constraining their origin, opening the way for their Gton scale successors, KM3NeT and Baïkal-GVD both currently under construction and installation. We will review the most recent results, paying attention to the instrumental characteristics affecting the different available datasets, and their implications on our understanding (or misunderstanding) of the underlying astrophysical processes as well as the perspectives on the non thermal Universe or fundamental physics opened by future data in a multi-messenger context.
|Luc Blanchet||IAP, Paris||
High-order post-Newtonian calculations for gravitational waves We discuss theoretical aspects of gravitational waves needed to build accurate and reliable templates for the data analysis of gravitational waves generated by the inspiral and merger of compact binary systems. In particular we focus on the post-Newtonian approximation of general relativity and review the latest developments in the field.
|Diego Blas||King's College, London||
Probing light dark matter with pulsar observations I’ll describe new developments to probe light dark matter with pulsar timing observations. My focus will be on modifications of the trajectories of pulsars and of the propagating of the radio signal in different dark matter models.
|Mauricio Bustamante||Bohr Institute, Copenhagen||
Fundamental physics with high-energy cosmic neutrinos There is vast potential in high-energy cosmic neutrinos to test particle physics. The cosmic neutrinos recently discovered by IceCube have the highest detected neutrino energies --- up to a few PeV --- and travel the longest distances --- up to a few Gpc, the size of the observable Universe. These features make them attractive probes of particle-physics properties, possibly tiny in size, at energy scales unreachable by other means. The decades before the IceCube discovery saw many proposals of particle-physics studies in this direction. Today, these proposals have become a reality, as we are finally turning them into data-driven tests, in spite of astrophysical unknowns. I will showcase examples of testing neutrino physics at these scales, including stringent tests of physics beyond the Standard Model. For the coming decade, prospects are encouraging, thanks to upcoming experiments to detect neutrinos with energies a thousand times higher.
|Vítor Cardoso||Instituto Superior Técnico, Lisbon||
Black holes as particle detectors Black holes are the simplest yet most extreme macroscopic objects one can conceive of. They trap light and can even ``drag'' spacetime, giving rise to new spacetime features, including photon spheres and ergoregions. These properties can turn astrophysical black holes into perfect particle accelerators and detectors. I will discuss how one can use supermassive spinning objects to probe new fundamental fields or to confirm some of the amazing predictions of General Relativity.
|Jordy Davelaar||IMAPP, Radboud University, Nijmegen||
Modeling accreting supermassive black holes on event horizon scales On April 10th 2019, The Event Horizon Telescope Collaboration (EHTC) revealed the first image of the shadow of the black hole in the center of Messier 87. This provides direct evidence for the existence of supermassive black holes. The resulting image shows a black hole shadow of 42 micro-arcseconds, implying a black hole mass of approximately 6.5 billion solar masses. Besides the mm-observations the EHT campaign was a multi-wavelength campaign, including telescopes from radio to gamma-rays wavebands. In this talk, I will highlight both the observational and theoretical efforts made by the EHTC to obtain and interpret this spectacular image, and summarize my own work on modeling multi-wavelength spectra of accreting black holes.
|Paulo Freire||MPIR, Bonn||
Constraining the nature of gravitational waves and alternative theories of gravity using binary pulsars In this talk, I will first briefly review some of the high precision tests of general relativity that have been done with binary pulsars, with a special emphasis on the double pulsar system, PSR J0737-3039A.B. Then, I will focus on tests of the properties of gravitational waves, in particular a search for a dipolar component to gravitational radiation, that have been done with a set of pulsar- white dwarf systems. The results of our measurements always agree with the predictions of GR, which implies that GWs really have a quadrupolar structure, with at most a tiny dipolar contribution. These fundamental limits on the nature of gravitational waves impose stringent constraints on any alternative theories of gravity where matter couples to extra scalar fields, we exemplify this using the Scalar-Tensor theories of Damour and Esposito-Farese.
|Vladimir Karas||Astronomical Institute, Czech Academy of Sciences, Prague||
Electromagnetic signatures of strong-field gravity: light curves, spectra and polarization signal from black-hole accretion discs Observations help us to test general relativity in the strong-field regime. Whereas the No-hair Theorem states that "classical", stationary, isolated black holes can be fully characterized by a small number of parameters, cosmic black holes are not isolated and they grow in time due to their interaction with gaseous and stellar environment. Hence, the astrophysically realistic models require many more parameters.
Luckily, new techniques allow us to obtain further independent constraints from observations: non-electromagnetic messengers have emerged (gravitational waves and neutrinos) and novel electromagnetic observations have been developed (polarimetry). We will review a fruitful approach to study variety of electromagnetic radiation signatures from accretion disks in strong gravity. To generate synthetic spectra and light curves the method of transfer functions has been developed. This approach can be also employed to predict the polarimetric properties, which we expect to detect with the upcoming satellite missions IXPE and eXTP.
|Mikhail Kuznetsov||ULB, Brussels, and INR, Moscow||
Multimessengers and fundamental physics with ultra-high energy cosmic rays I will present a review of some multimessenger and fundamental physics scenarios related to ultra-high energy cosmic rays (UHECR). Namely, I will discuss the possibility to probe UHECR sources and composition with secondary cosmogenic signals. Then I will review possible UHECR tests of two fundamental scenarios: first the hypothesis of photon-ALP conversion of gamma-rays from distant blazars which explains the HiRes experiment result of UHECR correlations with BLLacs, and second the heavy decaying dark matter models, including those explaining IceCube PeV neutrinos.
|Ilídio Lopes||CENTRA, Lisbon||
Stars and Dark Matter I will briefly review the current status of the experimental and theoretical research of dark matter, followed by a discussion about the impact of dark matter in stars. Moreover, I will also talk about the constraints on the dark matter properties imposed by solar neutrinos, helioseismology and asteroseismology. Finally, I will discuss how stars can contribute to the resolution of the dark matter problem.
|Alessandro Nagar||INFN, Torino, and IHES, Bures sur Yvette||
Impact of waveform modeling on analysis of gravitational waves emitted by coalescing neutron star binaries The analysis of gravitational wave events like GW170817 relies on accurate waveform models. First, I will summarize the state of the art, discussing effective-one-body based tidal waveform models for coalescing neutron stars, and then I will point out the kind of bias in the recovered parameters that one may expect, at high signal-to-noise ratio, when less complete waveform models are used.
|Paolo Pani||University of Rome La Sapienza||
Gravitational-wave echoes Gravitational wave (GW) astronomy allows us for unprecedented tests of the nature of dark compact objects and to probe into outstanding foundational issues, such as the fate of spacetime singularities and the loss of unitarity in Hawking evaporation. In this context, I will discuss a striking signature of new physics at the horizon scale: GW “echoes” in the postmerger ringdown phase of a binary coalescence. The ringdown waveform of exotic ultracompact objects is initially identical to that of a black hole, and putative corrections at the horizon scale appear only at later times as a modulated and distorted train of echoes of the modes of vibration associated with the photon sphere. These corrections display a universal logarithmic dependence on the location of the surface in the black-hole limit, allowing to probe even Planckian corrections. I will discuss challenges in modelling this signal and the ability of present and future GW detectors to measure this effect.
|Georgios Pappas||Aristotle University, Thessaloniki||
Neutron stars as gravity laboratories Neutron stars are some of the most compact objects observed, second only to black holes, where strong gravity effects are important. Studying their structure could in principle be used to probe the strong gravity regime and test General Relativity. Unfortunately so far, the exact equation of state that describes matter at supranuclear densities remains unknown. This uncertainty has been an obstacle in using neutron stars as gravity laboratories (with the bright exception of binary pulsars). The past few years though a new approach has emerged in describing neutron stars, i.e., using equation of state insensitive, or "universal", relation between appropriately normalised observables. In my talk I will give a brief and incomplete review of some of these relations and outline how they could be used to test gravity and even, counterintuitively, help narrow down the equation of state.
|Elena Rossi||Leiden Observatory||
The LISA mission: a multimessenger perspective In this review talk, I will present the LISA mission and Science, focusing on the multimessenger possibilities that LISA data will open up to the astronomical and fundamental physics communities.
|Subir Sarkar||University of Oxford||
Probing cosmic acceleration with standard candles (and standard sirens) Recent observations reveal a bulk (non-Hubble) flow in our local Universe which is faster and extends to larger scales than is expected around a typical observer in the standard ΛCDM cosmology. From a maximum-likelihood analysis of the Joint Lightcurve Analysis catalogue of Type Ia supernovae we find that the deceleration parameter, in addition to a small monopole (consistent with no acceleration at 1.4σ) indeed has a much bigger dipole component (rejecting isotropy at 3.9σ) which points in approximately the same direction. Most of the supernovae are in fact contained within the bulk flow so the cosmic acceleration deduced from supernovae may just be an artefact of our being non-Copernican observers. Detecting gravitational waves from black hole mergers will in future enable their luminosity distance to be measured with per cent level accuracy out to much larger distances thus providing an independent test.
|Sergey Sibiryakov||EPFL, Lausanne, and CERN, Geneva, and INR, Moscow||
Probing dark matter with diffractive gravitational lensing of gamma and radio bursts Gravitational lensing of distant sources may lead to a characteristic interference pattern in the frequency spectrum of the observed signal. This happens when the angular separation between the two lensed images is too small to be resolved by the observer. I will discuss how this phenomenon can be used to probe the nature of dark matter at the cosmological scales by looking for the lensing signal in the spectra of gamma-ray bursts (GRBs) and fast radio bursts (FRBs). I will show that the existing GRB data are not sufficient to place any significant constraints on dark matter. A future improvement of the sensitivity will crucially hinge on a better understanding of the angular sizes of the GRB sources. For FRBs the main complication is posed by scintillation of the radio waves due rescattering in the interstellar and intergalactic medium. I’ll argue that within reasonable assumptions about the latter, the lensing signal can be disentangled from scintillation. A preliminary analysis suggests that using the FRB data expected in the near future one may be able to put constraints on dark matter that are competitive with the best current bounds.
|Nicola Tamanini||AEI, Potsdam||
Cosmology with gravitational wave standard sirens: current results and future prospects I will introduce the concept of standard siren, reviewing the methodologies that one can apply to probe the cosmic expansion using gravitational wave observations. I will then outline the gravitational wave sources that can be used as standard sirens for both Earth-based (LIGO/Virgo) and space-based (LISA) detectors, pointing out for which of them an electromagnetic counterpart is expected to be observed. I will then discuss the constraints on the Hubble constant obtained with the recent LIGO/Virgo observations and what they will be able to tell us in the future. Finally I will present cosmological forecasts for LISA, which will be able to probe the expansion of the universe at high redshift.