To be or not to be, said a very famous character. Even for gravitational wave cosmology, this question is becoming more and more crucial. We have been observing hundreds of gravitational waves from compact binaries, and we think that those binaries are made of black holes and neutron stars. So far, it is was common in our field to say that a compact object with masses above 5 solar masses is a black hole and with masses between 1 and ~3 solar masses is a neutron star. However, we do observe objects with masses between 3 and 5 solar masses! What are they?

In this preprint, Alberto Colombo, Sonia Triscari-Benimavo, and I developed a framework for gravitational wave cosmology that can help us when dealing with this problem. We have shown that neglecting the nature of the compact objects might result in a biased reconstruction of the compact objects’ masses, and maybe even a little bit of bias in the inference of the Hubble constant.

With this study, we also release two major software packages that we developed: mocking-gw and bright-sirens-tools. The former is used to quickly simulate the observed population of GW sources, and the latter to calculate what type of electromagnetic emission the binaries might have. Check below for our preprint and on the software page for our release.

Figure: Probability of emitting an electromagnetic counterpart (gamma-ray burst and Kilonova) as function of the masses of the two compact objects.

If you have been following this group for a while, you know that we are really trying hard to find new ways for localising binary black holes (dark sirens) in redshift space. This is crucial for gravitational-wave cosmology, as with a redshift measure we can measure the cosmic expansion.

In a recent collaboration led by Ulyana Dupletsa, we built a new methodology to use neutral hydrogen intensity maps that will be observed by radio telescopes such as the Square Kilometre Array Observatory to identify the possible redshifts of binary black holes detected from their gravitational emission.

We have shown that this methodology can be used to probe the cosmic expansion. We have also seen what the effect on gravitational wave cosmology is of assuming a homogeneous and isotropic universe when actually it isn’t. If you want to know more, check out our latest preprint.

Figure: Density contrast of neutral hydrogen at low redshifts. You can see that the hydrogen density (might) track(s) the dark matter filaments, and in these filaments, it is more likely to find gravitational wave sources.

With the release of the largest Gravitational Wave (GW) catalog so far — the LIGO-Virgo-KAGRA collaboration’s GWTC-4.0, containing more than 200 candidates — the race to measure the Hubble constant with dark sirens has accelerated.
But as our measurements of the universe’s expansion improve, new questions emerge: How much do the data themselves drive the results? How robust are our inferences? And to what extent does the choice of cosmological model shape what we conclude?

In our recent work, we take the dark siren approach one step further by moving beyond the usual ΛCDM (Dark Energy and Dark Matter dominated universe) assumption. Instead, we introduce a fully non-parametric method that lets the data speak for themselves. Using 137 binary BH events from GWTC-4.0, we directly reconstruct the cosmic expansion history, the Hubble parameter H(z).

This work provides the first non-parametric GW-based reconstruction of the Hubble parameter, but it also highlights something deeper: the cosmological model we assume shapes the inferred expansion rate. To make this clearer, we combine our reconstruction with independent measurements from baryon acoustic oscillations in DESI DR2. These external “anchors” help us identify where GW data alone are driving the inference, and where model choices matter most.

Together with this publication, we also release Gsirens a version of icarogw written in Jax and numpyro to allow for the possibility of sampling hundreds of population parameters.

Figure: Inferred H(z) from the 137 GW sources of the GWTC-4.0 catalog, using our non-parametric approach (left) and the standard ΛCDM model (right).

On August 26th, the LIGO-Virgo-KAGRA collaboration released its fourth Gravitational Wave Transient Catalog (GWTC-4), reporting more than two hundred GW signals. This is an impressive number of GW sources, if you consider that the first GW detection was only 10 years ago. And with such and impressive number of sources comes an amazing potential for science.

One of the flagship studies to which myself the GravitySirens group contributed is the measurement of the cosmic expansion with these sources. This herculean effort has been carried on the shoulders of Gregoire Pierra (GravitySirens postdoc) and Ulyana Dupletsa (postdoc in Vienna and a close collaborator of us). Kudos to them for all the patience and enthusiasm shown in these crazy two years.

Going to science now, what we can learn from this latest study is that GW sources without an electromagnetic counterpart are almost as precise in measuring the Hubble constant (cosmic expansion today), almost at the level of GW170817, the only GW source with an electromagnetic counterpart. We should start wandering about systematics for these sources. I am feeling like we are finally becoming real observational cosmologists!

The cherry on top of all of this is that we released a new version of icarogw, the code used to make the inference. The new release includes updated mass models for binary black holes and also modified gravity models of cosmology!

Check out the study below

Figure: Marginalized Hubble constant posteriors generated with GWTC-4 using several sources and models. Check the publication for more details.