Not long ago, we published a study exploring a new way to measure how fast the Universe is expanding, not only today but also in the past. This method relied on sources of gravitational waves (GWs) emitted by the coalescences of black holes.

Some of these sources emit no light at all. We call them spectral sirens. Even without light, they can still tell us about the expansion of the Universe. In our previous work, we showed that the key information actually comes from how the mass distribution of BHs changes with distance. Certain characteristic mass scales act like landmarks, allowing us to trace how the Universe expands.

In our new preprint, we exploit this idea by introducing a new model designed to track an additional mass scale in the BH mass spectrum, around 60 times the mass of the Sun. We applied this model to the latest GW catalog released by the LIGO–Virgo–KAGRA collaboration.

The result is pretty nice: using spectral sirens alone, we obtained the tightest measurement of the Hubble constant to date, improving the error budget of previous analyses by 36%!
We also show that this gain in precision comes directly from our ability to measure this new high-mass feature, which complements the already known mass scales around 11 and 27 solar masses. This clearly demonstrates an important point: the most massive BHs play a crucial role in spectral siren cosmology.

In short, by better understanding how BH masses are distributed across the Universe, we can learn how fast the Universe itself is expanding, even when no light is involved.

Figure: Probability distribution of the Hubble constant inferred from GWs. Our new result (pink) is compared with the standard LIGO–Virgo–KAGRA analysis (blue). The vertical lines show the latest measurements from Planck and SH0ES defined the so-called Hubble tension.

Sometime ago, Gregoire and I argued for the possible evidence of a spin magnitude/mass correlation for binary black holes observed via Gravitational Waves. In a recent preprint, with colleagues from the University of Rome Sapienza, CERN and Johns Hopkins University, we tried to better understand this correlation by modelling the astrophysical formation channels that can create it.

We looked at possible spin/mass correlations introduced by black holes formed from isolated binary stars, dynamical formation in dense star clusters or in Active Galactic Nuclei (AGN), and primordial black holes. The main result of this study is that we still find this spin magnitude/mass correlation, but now we know a bit

We have found that Primordial black holes alone can definitely not be the only formation channel of black holes; instead, we need at least two formation channels to describe the data. Unfortunately, we can not say who these channels are at the moment.

Figure: Top row – spin magnitude distribution as a function of the source mass (colorbar). Bottom row – distribution of the cosine of the tilt angle of the black holes from the orbital angular momentum.

On August 17, 2017, we detected the first gravitational-wave signal from a binary neutron star merger, GW170817. For the first time in history, this event was also accompanied by an electromagnetic counterpart, marking the beginning of the multi-messenger astronomy era. Since then, only one additional binary neutron star merger has been observed—without an electromagnetic counterpart—suggesting that multi-messenger astronomy is currently experiencing a rather quiet phase. However, in about ten years, next-generation gravitational-wave detectors such as the Einstein Telescope and Cosmic Explorer will revolutionize the field, allowing us to detect tens of thousands of sources each year.

In this work accepted for publication in Astronomy and Astrophysics, Alberto has simulated populations of BNS and BHNS mergers to provide forecasts of what the multi-messenger universe will look like in the ET era. While the present may seem less promising, the future of this field is extraordinarily bright, with the potential to observe dozens of gamma-ray bursts and hundreds of kilonovae.

Figure caption: Geocentric universe in the ET era. Points represent gravitationally detected events after five years of ET observations. Red markers indicate potentially detectable kilonovae, while blue markers indicate gamma-ray bursts. Left: BNS mergers. Right: BHNS mergers (in the most pessimistic scenario).

PhD student Gabriele Perna, now a Postdoc,

Our research explores how future gravitational wave (GW) observations—especially those without electromagnetic counterparts—can still be used to infer key cosmological parameters like the Hubble constant (H₀). We investigate how incorrect assumptions about the GW host galaxy distribution, particularly regarding luminosity and redshift, can bias H₀ measurements. Using realistic galaxy and compact binary coalescence simulations, we show that such biases depend on both the observational footprint and distance of GW events. Our findings emphasise the need for accurate modelling of host probabilities when combining GW data with galaxy catalogues.

Please, feel free to read our paper and reach out to us for any question or suggestion.

Figure: Hubble constant posterior calculated using 200 well-localised dark sirens when assuming different models for the galaxies’ hosting probability of binary black holes. The true underlying astrophysical population is generated with the model indicated on the top left panel.

PhD student Sarah Ferraiuolo presented a new study on how the stochastic gravitational-wave background can improve the measures of the Hubble parameter.

The individual gravitationa-wave (GW) sources that we see in our detectors are just the tip of the iceberg. Below them, thousands of silent GW sources compose the stochastic GW background (SGWB). In this paper, we demonstrate that the SGWB can be exploited, along with resolved sources,  to measure cosmological expansion parameters.  Its inclusion can help us to constrain cosmological models where the Universe is very old.

Please, feel free to read our paper and reach out to us for any question or suggestion.

Figure: Hubble parameter reconstructed from only resolved GW sources (pink) and with the stochastic GW background (blue).