S. Cantalupo, M. Colpi, M. Dotti, M. Fumagalli, D.Gerosa, B. Giacomazzo, M. Gervasi, F. Nati, A. Sesana, M. Zannoni

Reference: Alejandro Benítez-Llambay

Astronomical observations on large scales have enabled the establishment of the Lambda-Cold-Dark-Matter (ΛCDM) model as the standard paradigm of structure formation and the pillar of galaxy formation theory. ΛCDM makes specific predictions about the abundance, structure, and clustering of dark matter (DM) halos, the sites where galaxies form. In recent years, these predictions have been challenged on small scales by observations of low-mass DM-dominated galaxies. Well-known issues include (i) the missing satellites; (ii) too-big-to-fail; and (iii) cusp-core problems. Theoretical efforts have demonstrated that the discrepancies largely arise from naive comparisons between observations and collisionless N-body simulations that neglect the impact of galaxy formation on the host DM halo. Once baryonic effects are considered, these tensions alleviate. Detailed quantitative comparisons are, however, hindered by the uncertainties in the non-resolved (so-called subgrid) modeling of galaxy formation in simulations, or by observational uncertainties, making low-mass galaxies anything but robust cosmological probes. Testing ΛCDM on small scales thus requires pushing both astronomical observations and cosmological simulations to a regime that has not yet been explored, and in which galaxy formation and baryonic physics play a negligible role.

In this research area, we produce and explore the predictions of state-of-the-art numerical hydrodynamical simulations of galaxy formation, and learn novel aspects that might help to astrophysically constrain the nature of the elusive dark matter particle. We complement this endeavour with observations carried out with Space based facilities and the largest radio interferometers on earth.

S. Cantalupo

In our current paradigm of cosmological structure formation, gravitational collapse during the Universe’s first billion years transformed a nearly homogeneous matter distribution into a network of gaseous filaments astronomers call the “Cosmic Web". Galaxy formation occurs within the densest parts of these filamentary structures and galaxy evolution is expected to be sustained and regulated by gas infall from the Cosmic Web or the Intergalactic Medium (IGM).

We study the "Cosmic Web" and associated galaxies both through theoretical/numerical modelling and observations with the most advanced astronomical instruments on 8-​10m class telescopes. In particular, we focus on the direct detection and study in absorption and emission of the baryonic component of the "Cosmic Web" - the Intergalactic and Circumgalactic media - in order to unravel the physical properties and the three- dimensional morphology of Cosmic Structures and to address several fundamental questions:

• How do galaxies form within the Cosmic Web? What are the physical conditions for the formation of stars within the early, potentially dark, proto-galaxies?
• How do galaxies get their gas? What is the morphology and kinematics of the accreting gas and how does this affect the galaxy formation and evolution process?
• What are the physical and morphological properties of the Cosmic Web? How does this compare to our understanding of cosmological structure formation in the universe and what does it tell us about the nature of Dark Matter?

For more information about our research activities, methodologies and results see our Publication List.

M.Dotti, R.Buscicchio

Astrophysical black holes (BHs) are described by two parameters which, according toGeneral Relativity, uniquely determine the geometric structure of spacetime: mass and spin. These two quantities govern the luminosity and efficiency of accretion processes onto BHs, the rate at which they can grow in mass, and during the coalescence of two BHs the properties of the gravitational-wave signal and the recoil velocity due to anisotropic gravitational wave emission. In extreme cases, this recoil can eject BHs from their host galaxies. In particular, spin is believed to play a crucial role in the formation of relativistic jets, which can extend over millions of light-years. Our group focuses on modeling the cosmic evolution of massive black hole (MBH) spins, linking it to the dynamical processes that funnel intergalactic gas toward the galactic nucleus. We also test these models through measurements of both the spin magnitude and its orientation.

M. Dotti, R. Buscicchio

Galaxy mergers are a natural channel for the formation of supermassive black hole binaries (MBHBs). In the final stages of their evolution, MBHBs emit strong gravitational waves, making them detectable at low frequencies by current pulsar timing array campaigns and by the upcoming space-based interferometer LISA sensitive to gravitational waves a milli-Hertz frequencies .  To date, however, only one MBHB has been unequivocally identified in the radio band. Our group works on modeling the distinctive (electromagnetic) signatures of MBHBs, searching for them in observational catalogs, and developing a pipeline for the rapid identification of MBHBs in the data stream expected once LISA becomes operational.

A. Sesana

Pulsars are highly magnetized neutron stars that spin rapidly and act as cosmic clocks. By monitoring tiny variations in the ticking of a set of pulsars distributed throughout our galaxy, we can reconstruct minute distortions of spacetime caused by the propagation of gravitational waves (GWs) at nanohertz frequencies. Such waves are generated by supermassive black hole binaries (MBHBs), and their ‘signature’ in PTA data was first detected in 2023. In collaboration with the European and International PTAs (EPTA, IPTA), our group focuses on modeling, analyzing, and interpreting PTA data and their implications for the astrophysics of MBHBs and the mergers of their host galaxies.

M. Bonetti, M. Colpi, M. Dotti, A. Sesana

Supermassive black holes (MBHs) reside at the centers of galaxies and play a fundamental role in their formation and evolution. But how do these MBHs form? How do they reach masses of several billion times that of the Sun when the universe is still young? What happens when two or more galaxies merge and the MBHs within them encounter each other?

Our group develops theoretical models for the formation and evolution of MBHs to interpret the increasingly detailed observations coming from the young universe (for example, thanks to JWST). We also study the dynamical processes leading to the formation of MBH binaries (MBHBs) and the importance of these sources for gravitational wave observations with PTAs and LISA.

M. Bonetti, M. Colpi, M. Dotti, A. Sesana

Massive black holes (MBHs) reside in the nuclei of galaxies—very dense regions populated by millions of stars and compact objects within a very limited volume. Due to continuous mutual gravitational interactions, stars and stellar-mass black holes can pass very (sometimes too) close to the MBH, giving rise to unique and spectacular phenomena such as tidal disruption events (TDEs), the capture of black holes followed by inspiral through gravitational wave emission (phenomena known as extreme mass ratio inspirals—EMRIs), or the quasi-periodic radiation emission caused by the impact of these objects on an accretion disk (quasi-periodic eruptions QPEs). These phenomena have important implications for both LISA and future time-domain surveys, such as the one to be conducted by the Vera Rubin Observatory. Our group is recognized worldwide as one of the leading teams in the theoretical study of these phenomena.

M. Bonetti, R. Buscicchio, M. Colpi, M. Dotti, A. Sesana

The Laser Interferometer Space Antenna (LISA) is currently the most important project of the European Space Agency (ESA). Now under construction, LISA is scheduled to fly around 2035 and will unveil the universe of gravitational waves at millihertz frequencies, which are the richest in sources: from supermassive black hole binaries to extreme mass ratio inspirals, from stellar-mass black holes to white dwarf binaries in our galaxy. Identifying, separating, and interpreting this multitude of sources in LISA data is a highly complex task. A European team of over one hundred scientists is dedicated to developing the pipelines necessary to extract the maximum scientific output from the data. Our group is part of this team and develops innovative techniques for LISA data analysis, combining artificial intelligence and machine learning with more traditional methods.

Davide Gerosa (www.davidegerosa.com)

The first detections of black hole mergers by the LIGO and Virgo interferometers have marked the beginning of a golden age of gravitational-wave astronomy. With upgraded instruments and expanding international collaborations, thousands of new gravitational-wave events are expected in the near future. These observations promise transformative discoveries in both fundamental physics—such as testing general relativity in the strong-field regime—and astrophysics, including unveiling the origin and evolution of compact objects across cosmic time.

This rapidly growing data stream calls for equally sophisticated theoretical and computational tools. Accurate modeling of gravitational-wave signals, coupled with robust statistical and machine-learning frameworks, is essential to fully extract the science encoded in these events. My group works at the intersection of gravitational-wave phenomenology, inference, and data science, with a strong emphasis on developing innovative algorithms and applying them to real and simulated gravitational-wave data.

Looking ahead, next-generation detectors like the space-based LISA mission and ground-based observatories such as the Einstein Telescope and Cosmic Explorer will vastly expand our observational window—from stellar mass black holes to supermassive mergers and beyond. These instruments will probe entirely new source populations and cosmological epochs, opening avenues for multi-band gravitational-wave astronomy and unprecedented tests of gravity.

Students joining our group will contribute to these efforts by building novel models, running large-scale simulations, and developing inference pipelines to interpret current and future gravitational-wave data. The work is highly interdisciplinary, offering opportunities to engage with statistical methods, machine learning, computational physics, and astrophysical modeling—positioning students at the forefront of this rapidly evolving field. For possible thesis projects at the BSc, MSc, or PhD level, please do get in touch: davide.gerosa@unimib.it.