Maya Fishbach

How are black holes made?

LIGO and Virgo have revealed a previously-unobserved population of black holes and neutron stars that collide into one another, emitting a burst of energy in the form of gravitational-waves. Meanwhile, the origin of these systems remains an open question. Several formation scenarios have been proposed, from primordial black holes that make up a fraction of the dark matter (“primordial”), to the ends states of binary stellar evolution in the galactic field (”isolated”), to stellar remnants that form binaries through dynamical interactions in dense environments (“dynamical”), with each category consisting of many proposed variations. In my research, I study the population properties of black holes and neutron stars observed in gravitational-waves, in order to learn about the formation history of these systems.

I have pointed out several powerful features in the distribution of spins, masses, mass ratios, and redshifts of LIGO/Virgo's observations. Using LIGO's first four observations, I found evidence for missing big black holes and pointed out the connection to the theoretically proposed pair-instability supernova mass gap for the first time. I also showed that we may have observed a black hole on the far side of the black hole mass gap in LIGO and Virgo's third observing run. I performed the first measurement of the redshift evolution of the merger rate and the mass distribution. I studied the pairing between the two black holes in a binary, analyzing whether black holes are picky about their partners. I performed the first full fit to the gravitational-wave compact binary population, including neutron stars and black holes, quantifying the evidence for a mass gap between neutron stars and black holes. These population properties probe the origin of black holes; for example, whether LIGO's black holes are made from smaller black holes. My research also has implications for the physics of massive stars, binary interactions, and nuclear reaction rates.

As we observe more and more black holes and neutron stars, we can precisely resolve features in the population and push our theoretical understanding. We analyzed the population properties of LIGO/Virgo's first 10 binary black holes in this LIGO/Virgo Collaboration paper. Recently I led the population analysis on the ~50 events in the LIGO/Virgo's second gravitational-wave transient catalog. Check out our detailed results about the Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog, or read about the highlights in the introduction I wrote for the Astrophysical Journal Letters' Focus on Gravitational-wave Astrophysics from the Second LIGO-Virgo Transient Catalog.

Where are black holes made?

In order to understand the history of black holes and neutron stars in merging binaries, we must understand where and when they formed. I carried out the first measurement of the evolution of the black hole merger rate as a function of redshift, or cosmic time. By including information from the unresolved stochastic background, we can even constrain the peak merger rate. Contextualizing black holes and neutron stars in cosmic time is a first step towards probing the coevolution of these binaries with the global population of stars, galaxies, and the chemical composition of the universe. For example, studying the evolution of the merger rate reveals the delay times between star formation and black hole merger, which can discriminate between proposed formation channels. Using the second LIGO/Virgo catalog, I measured the binary black hole time delay distribution the first time. We can learn even more about the history of gravitational-wave sources by identifying and characterizing their host galaxies.

How big is the universe?

The luminosity distance to a compact binary merger is directly encoded in its gravitational-wave signal, earning these systems the name “standard sirens.” This feature makes compact binary mergers extremely powerful probes of the size of the universe and the nature of gravity. I have developed analyses to measure the expansion rate of the universe –the Hubble parameter– by combining gravitational-wave measurements of distance with redshifts supplied either by electromagnetic counterparts, a catalog of galaxies, the redshifted pair-instability feature in the black hole mass spectrum, or prior knowledge of the peak merger redshift (cosmic noon for merging binaries). Standard sirens can also be used to probe the nature of gravity itself; for example, by measuring the number of spacetime dimensions or the running of the Planck mass. For a recent overview of standard siren cosmology, check out the standard sirens chapter I wrote for this Living Review on emerging cosmological probes.