Work Packages (WPs):
WP1: Matter under extreme conditions. GWs from binary neutron star mergers provide a way to investigate the behaviour of strongly interacting matter at ultra-high density, temperature, and isospin. Outstanding open issues that may be addressed encompass the equation of state of neutron-star matter and its phase structure, e.g. the determination of possible existence of hyperons and quark matter in the neutron star core. Temperatures and densities in different phases of the coalescence may reach those attained in relativistic heavy-ion collision experiments, providing complementarity and strong synergy between these fields.
WP2: Nuclear and atomic physics and their role in multi-messenger astronomy. The discovery of electromagnetic radiation in the optical and near-infrared band following the neutron-star merger event GW170817 is the strongest evidence to date of heavy-element nucleosynthesis through the rapid neutron-capture (r-) process. The extreme environments associated with GWs, like x-ray binaries, are sources of short-wavelength radiation emitted by highly ionized matter, e.g. in accretion disks and jets. Electromagnetic spectra also contain information about the composition, temperatures, and densities of the observed objects that complement GW detection. The electromagnetic-transients are sensitive to the nuclear physics and the neutron dripline. However, the nuclear data needed to understand the r-process nucleosynthesis is to a large extent missing. Moreover, nuclear theory is not yet able to provide input with the required precision. Huge efforts on the nuclear-physics side are now emerging to address these theoretical and experimental challenges, with the aim to greatly improve our understanding of the r-process in binary-neutron-star- and black-hole-neutron-star merger events. As for nuclear data, not all necessary atomic data is currently available with the needed accuracy. Understanding spectra and the mechanisms behind them requires efforts from atomic structure theory, in combination with laboratory benchmarking experiments, and with existing and upcoming x-ray satellite missions and future GW data.
WP3: Fundamental problems in high-energy and gravitational physics. GWs provide novel portals to explore foundational physics in various flavours. From the particle physics viewpoint, they can provide novel information on the nature and phenomenology of dark matter (e.g., whether it is made of heavy particles, light fields, compact objects, or a mixture thereof), and on the existence of new fundamental fields (e.g. GW searches for axion-like particles and dark-photons which extend beyond the range of ongoing lab searches). From the gravitational viewpoint, violent GW events like mergers will elucidate the nature of black-holes and the fate of spacetime singularities, as well as providing novel probes of possible extensions (classical and quantum) of Einstein's General Relativity. At the same time, the discovery potential in GW science has reinvigorated the ongoing efforts to construct high-accuracy waveforms, in particular using sophisticated computational methods from particle physics that have been instrumental to search for “new physics” at colliders through precision data.
WP4: GWs & Cosmology. GWs will also help in accessing standard sirens at large redshifts leading to exquisite measurements of the expansion rate of the universe, thus shedding light on fundamental cosmological questions, such as what is the nature of the dark energy or whether gravity is modified at cosmological distances. At the same time, the detection of a stochastic background of GWs will open a window into the primordial universe and give us the opportunity to study fundamental phenomena like primordial inflation and phase transitions.
WP5: Synergies between particle accelerators and GWs. New experimental and theoretical challenges emerging in GW science call for a cross-cutting and synergic approach aimed at exporting, extending, and further developing the decade-long expertise built by the particle-physics community. Our goal is to explore and foster this synergy both at the experimental (e.g. instrumentation and hardware development) and theoretical levels. In particular, we will explore synergies with nuclear and (astro-)particle physics for what concerns vacuum and cryogenic technologies, and underground infrastructures.
Attacking these grand problems requires a multidisciplinary, cross-cutting effort at the interface of different communities. Recently, specific initiatives to connect affine subareas (e.g. nuclear and GW astrophysics or black-hole and particle physics) were established, but lack the required depth and stability. We propose to support and train the next generation of leaders in GW physics, who will be able to communicate across a spectrum of sub-fields. This is instrumental to maximize the benefit from theoretical developments and from the wealth of data, now available from current (LIGO/Virgo/KAGRA) and soon to be taken by future (LISA, Einstein Telescope, Cosmic Explorer) GW interferometers, radio and X-ray observatories (e.g., Event Horizon Telescope, NICER, GRAVITY, Athena, eXTP), cosmological observations by the JWST, and from particle accelerators facilities (e.g., CERN, GSI/FAIR).
With this Expression of Interest, we aim to:
Foster synergies among different communities, in particular astroparticle, atomic, nuclear, high-energy, and gravitational physics, cosmology, and GW and multi-messenger astronomy
Strengthen the connection between the theoretical and experimental/observational communities in these areas, in synergy with the European Center for Astroparticle Theory (EuCAPT)
Share expertise, tools, cutting edge (detector and accelerator) technologies and knowledge to attack multidisciplinary problems in an innovative way
Organize interdisciplinary training programs for students, to train a new generation of researchers with diverse expertise and background
Contribute to establish and exploit a dedicated cross-disciplinary open science platform where relevant data can be easily gathered, analysed and modelled holistically, in the framework of the Test Science Project “Extreme Universe” of the European Science Cluster of Astronomy & Particle physics ESFRI research infrastructures (ESCAPE) project
Create a common platform for cooperation with industries and non-academic sectors, and for the dissemination of the results to different stakeholders, including the general public and funding agencies
- Organize regular workshops and conferences, including sustainable and telematic ones
- Create opportunities for early-career researchers (e.g. visiting programs)
- Organize interdisciplinary training programs and schools
- Create and maintain a repository to share codes & tools and other data that can be useful in interdisciplinary studies, e.g., state-of-the-art equations of state, observational/experimental data, numerical codes, analytic methods and waveforms models, data-analysis tools, cosmological models
- Stimulate the use and development of machine-learning techniques and big-data analysis, which are now ubiquitous in these areas, also linking to existing initiatives such as ESCAPE
- Create a portal collecting the above activities and maintain a mailing list among members of the initiative to share news, positions, conference informations, etc
- Reach out to the general public by organizing multidisciplinary public lectures (e.g. speakers from different areas presenting their perspective on a common topic), and round tables, and by communicating to kids through interactive master classes to raise their awareness for our exciting researc