CQG

… and we’re back to selection effects. That means modeling what you cannot see. The black holes that gravitational-wave detectors observe are not representative of those that are out there in the Universe. Some are easier to see, some are harder. Quantifying how much easier and harder is crucial to properly understand the underlying astrophysics. In this paper (which came out of Malvina’s BSc student project!), we go back to the basics and work out gravitational-wave selection effects one step after the other, using and refining the most common approximation. Two things to remember: including noise fluctuations is easy, and a signal-to-noise ratio threshold of 11 is probably ok.

D. Gerosa, M. Bellotti.
Classical and Quantum Gravity 41 (2024) 125002. arXiv:2404.16930 [astro-ph.HE].

Spinning black holes are weird (well, all black holes are weird but those that spin are the worse!). They have a funny thing called ergoregion where orbiting particles can have negative energy. Penrose was the first to realize that this can be exploited to extract energy from the black hole itself. The thing is, even if you figure out how to do it, you’re inevitably going to spin the black hole down. At the end of the day, you’re left with a fossil black hole that does not have any spin. The mass of that leftover black hole (“ What’s for lunch dear? Fancy some sushi or prefer a black hole?”) is called irreducible mass. Hawking (another giant!) figured out this has to do with thermodynamics.

Long story short, in this paper we compute the irreducible mass of the black holes detected in gravitational waves by LIGO. It was funny to re-discover that gravitational wave detection was indeed the motivation behind Hawking original proof of the area theorem (he had Weber‘s claimed detection in mind at the time). The story behind our paper starts as a toy calculation with my undergraduate student Cecilia and ended up in a neat, hopefully informative exploitation of LIGO data. We reparametrized LIGO’s black-hole properties using the rotational and rotational contributions to their total energy, we ranked current gravitational-wave events according to their “irreversibility”, and we compute a sort of population version of the area law. Enjoy!

D. Gerosa, C. M. Fabbri, U. Sperhake.
Classical and Quantum Gravity 39 (2022) 175008. arXiv:2202.08848 [gr-qc].

A black-hole binary starts its life as two single black holes, and finish it as a single black hole. In between there’s all the complicated dynamics predicted by General Relativity: many orbits, dissipation of energy via gravitational waves, spins that complicate the whole business, and finally the merger which leaves behind a remnant. In this paper we put together different techniques to map this entire story beginning to end, connecting the two asymptotic conditions of a black-hole binary. This work started as a summer project of my student Luca: well done!

L. Reali, M. Mould, D. Gerosa, V. Varma.
Classical and Quantum Gravity 37 (2020) 225005. arXiv:2005.01747 [gr-qc].

As you can imagine, I’m kind of obsessed with black hole binaries. So easy (let’s face it, a black hole is easy! Just mass and spin), but at the same time so terribly complicated… Happy to present our attempt to see the binary dynamics in real time. Technical blah blah: we attach a visualization tool to a numerical relativity surrogate model. Are you ready to be a binary black hole explorer? Here!

ps. Folks are having fun with this! From mikesmathpage.

V. Varma, L. C. Stein, D. Gerosa.
Classical and Quantum Gravity 36 (2019) 095007. arXiv:1811.06552 [astro-ph.HE].

Latest in the series of our spin-precession papers, here we found a thing that was worthy of a new name: wide nutation(we had wide precession before, but this is better). These are black-hole binary configurations where the angle between any of the two spins and the orbital angular momentum changes a lot. Can’t change more actually: spins goes from full alignment to full anti-alignment. And they do it many times.

We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech!

D. Gerosa, A. Lima, E. Berti, U. Sperhake, M. Kesden, R. O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 105003. arXiv:1811.05979 [gr-qc].

This is a massive review born out of the European COST Action CA16104 Gravitational waves, black holes and fundamental physics (GWverse). We summarize the status of the field of gravitational-wave astronomy and lie down a roadmap for the immediate future.

L. Barack, et al. (199 authors incl. D. Gerosa).
Classical and Quantum Gravity 36 (2019) 143001. arXiv:1806.05195 [gr-qc].

Editor’s coverage in physicsworld.com.

Equal-mass binaries correspond to a discontinuous limit in the spin precession equations. A new constant of motion pops up, which can be exploited to study the dynamics. This is a really neat calculation done with Jakub, a Cambridge undergraduate student. Also, my first paper at Caltech!

D. Gerosa, U. Sperhake, J. Vosmera.
Classical and Quantum Gravity 34 (2017) 064004. arXiv:1612.05263 [gr-qc].

Here we present 1+1 numerical-relativity simulation of stellar collapse in scalar-tensor theories, where gravity is mediated by the usual metric coupled to an additional scalar field. Bottom line: you can test General Relativity with supernovae explosions!

D. Gerosa, U. Sperhake, C. D. Ott.
Classical and Quantum Gravity 33 (2016) 135002. arXiv:1602.06952 [gr-qc].

What happens if you throw a scalar field into General Relativity? And if you throw more than one? Here is a paper on the phenomenology of neutron stars in theories with more than one scalar field coupled to gravity.

M. Horbatsch, H. O. Silva, D. Gerosa, P. Pani, E. Berti, L. Gualtieri, U. Sperhake.
Classical and Quantum Gravity 32 (2015) 204001. arXiv:1505.07462 [gr-qc].
IoP Editor’s choice (CQG++, IOPselect).

A massive review on testing gravity, which came out of a nice meeting we had at University of Mississippi in January 2014. See the numbers.

E. Berti, et al. (53 authors incl. D. Gerosa).
Classical and Quantum Gravity 32 (2015) 243001. Topical Review. arXiv:1501.07274 [gr-qc].