Filling in the Holes

Following up on some previous posts about gravitational wave detections, this week I have some questions from Papou:

How many current black hole collisions are we aware?
Last year, the LIGO/Virgo Collaboration released the first Gravitational-Wave Transient Catalog, covering all the detections from the first and second observing runs. That includes 10 binary black hole (BBH) events, and 1 binary neutron star (BNS) merger. The third observing run is split in two pieces. Results from the first half, O3a, are available on GraceDb, and include 37 BBHs, 6 BNSs, 5 neutron star black hole (NSBH) mergers, and 4 events that fall in the "mass gap".

The mass gap represents a range of masses where we have never seen a black hole or neutron star. This image shows a summary of the compact objects observed by LIGO and electromagnetic astronomers as of the end of O2:

via Northwestern

The empty space between 2 and 5 solar masses is the mass gap. It is unknown what kind of bodies were involved in those mass gap collisions.

What happens to "Matter and Antimatter" when two black holes collide?
Black holes are made of matter. When matter and antimatter combine they make energy, but as with most physical processes, this is reversible: energy can create matter/antimatter pairs. Since the universe contains residual heat energy from the Big Bang, these pairs are constantly forming and annihilating back into energy in space. If one of these pairs forms near a black hole though, the antiparticle can fall into it, while the matter particle escapes. This process is called Hawking radiation, and can lead to a black hole losing mass.

Are photons escaping due to the collision energy?
In the case of black holes, no. Light can't escape a black hole's event horizon, and when two collide, their horizons merge. Neutron star collisions, however, can release photons in the form of a gamma-ray burst (GRB). In an earlier post, I mentioned LIGO's detection of GW170817, which showed a correlation between a binary neutron star merger, and a GRB.

Are the combining gravities simply arithmetic additions or does the total gravity grow in multiples?
Neither! Mass and energy are connected through E = mc^2, so when energy is released in the form of gravitational waves, part of that comes from the masses of the black holes. Despite the extreme sensitivity needed to detect gravitational waves, they carry an enormous amount of energy. The first detection, GW150914, lost about 3 suns worth of mass-energy. I tried to find a way to put that number in perspective, like "X billion nuclear bombs", but it's so huge that it dwarfs even a measure like that. Spacetime is exceptionally stiff stuff, and wrinkling it even a little needs an amount of energy that we don't normally encounter.

Thanks for more great questions, Papou!