The first detection of gravitational waves (GWs) in 2015 ushered in the golden era of GW astronomy. We can now observe the Universe not only with photons but through tiny ripples of space-time itself. In the same way that observing beyond the optical band (e.g., in X-rays, gamma-rays, radio waves) has offered a deeper and more complete picture of the Universe, opening new windows on the GW spectrum will revolutionise our understanding of the dynamical cosmos. In fact, as of this summer, Pulsar Timing Arrays (PTAs) have found evidence for a nanohertz GW background, offering a first glimpse into yet another band of the GW spectrum.

A PTA is a galactic-scale GW detector that relies on precise measurements of an array of millisecond pulsars. These magnetised neutron stars have collimated emission along the magnetic axis, and are very stable rotators. As the radiation beam periodically sweeps our line of sight, we observe a series of precisely repeating radio pulses. But if GWs with a very low frequency (nHz) traverse the cosmos and reach our Galaxy, they perturb the Earth-pulsar distances and induce coherent deviations in the arrival time of these radio signals. Since GWs affect all pulsars in the galaxy, the deviations should manifest themselves in all pulsars in the array and they are predicted to be spatially correlated with a characteristic pattern, known as the Hellings-Downs curve.

Very recently, all major PTA collaborations have reported evidence for a stochastic GW background with a significance for the characteristic Hellings & Downs inter-pulsar correlations between 3 and 4 sigmas. The origin of the signal is still uncertain, and interpretations range from GWs produced beyond the cosmic microwave background (including signals from cosmic inflation, cosmic strings, and tensor-mode scalar perturbations) to alternative dark matter candidates. However, the most likely source for the GW background (or at least a fraction of it) is a population of unresolved supermassive black hole binaries (SMBHBs) throughout the Universe. These systems can form as a result of galaxy-galaxy mergers and their evolution prior to coalescence has remained elusive despite decades-long observational and theoretical efforts. Confirming the binary origin of this GW background and pinning down its properties will provide new constraints for cosmological models of galaxy evolution, supermassive black hole growth, binary pairing and dynamical evolution across 10 orders of magnitude in separation. As such, this signal is also of great interest to a wide community of theoretical and numerical astrophysicists developing semi-analytic models for galaxy evolution, cosmological hydrodynamical simulations and numerical investigation of the dynamics in galactic nuclei.

In addition, if the background originates from SMBHBs, signals from individually resolved sources will be eventually detected on top of this background in just a few years. Such systems may produce bright and long-lived electromagnetic counterparts and will open up the possibility for multi-messenger observations combining electromagnetic and GW data. Therefore, the detection of individually resolved SMBHBs and the subsequent multi-messenger observations will allow us to probe in detail models of binary emission, the interaction of the binary with its galactic environment and the association of PTA observations with AGNs, which are currently active topics of research that appeal a wide community of observational astronomers and numerical astrophysicists.

The goal of this workshop is to bring together a variety of top researchers in the field of PTA (theorists, electromagnetic and GW observers, and cosmologists) and the larger theoretical and observational astrophysics community. The symposium is extremely timely, given the recent developments in the field. Since the recent PTAs? breakthrough results were published only a few days before EAS 2023, this will be the first opportunity to properly introduce the EAS community to them and to its astrophysical implications. We note that only a couple of PTA talks were presented in EAS 2023, and were very popular. Moreover, we plan to discuss the astrophysical interpretation and its implication for galaxy formation and evolution, as well as potential scenarios for cosmology and new physics. Next, we will explore synergies between PTAs and electromagnetic observations, especially in light of upcoming massive surveys, such as SDSS-V, the Rubin Observatory/LSST, Roman, etc. Finally, we highlight exciting prospects for the future of PTA in the era of the Square Kilometre Array, present new concepts like a gamma-ray PTA and explore synergies with missions that can be used as GW probes thanks to their precise astrometric measurements like Gaia and the Roman telescope. We will also examine connections with upcoming GW observatories like LISA.

Programme

• Block 1: results on the evidence for a GW background from the major PTA collaborations and upcoming constraints from the International Pulsar Timing Array collaboration
• Block 2: astrophysical implications of the GW background on the astrophysics of supermassive black hole binaries and galaxy formation models
• Block 3: individual source and multimessenger aspects, part I. Targeted searches and synergies with electromagnetic facilities
• Block 4: individual source and multimessenger aspects, part II. Connection with dual AGNs and electromagnetic counterparts modelling
• Block 5: new physics with PTAs: alternative interpretations to the binary origin of the GW background (dark matter, pre-CMB and inflation signatures) and their implications
• Block 6: the future of PTA detections: from novel detection techniques (with SKA, gamma ray facilities, Gaia, Roman) to multi-band GW observations with LISA

Invited speakers

• TBD

Scientific organisers

• Alberto Sesana (Chair; University of Milano-Bicocca, Italy)
• Maria Charisi (co-Chair; Washington State University, USA; University of Crete, Greece)
• Huanchen Hu (co-Chair; MPI for Radio Astronomy, Germany)
• Elisa Bortolas (University of Milano-Bicocca, Italy)
• Aurelien Chalumeau (University of Milano-Bicocca, Italy)
• Andrea Possenti (INAF, Osservatorio Astronomico di Cagliari, Italy)
• Golam Shaifullah (University of Milano-Bicocca, Italy)
• Gilles Theureau (Paris Observatory and CNRS, France)

Contact

Alberto Sesana (alberto.sesana @ unimib.it)