A little over five years ago, humanity was yet to detect gravitational waves.

Now, observations are pouring in at an astonishing speed. In a six-month span last year, the LIGO-Virgo collaboration detected, on average, 1.5 gravitational wave events per week.

From 1 April to 1 October 2019, the upgraded LIGO and Virgo interferometers detected 39 new gravitational wave events: the shockwaves rippling out across spacetime from massive collisions between neutron stars or black holes. In total, the Gravitational-Wave Transient Catalog 2 (GWTC-2) now boasts 50 such events.

This has given us the most complete census of black holes in our toolkit, representing a range of black holes that not only had never been detected before, but can reveal previously unplumbed depths of the evolution and afterlives of binary stars.

"Gravitational-wave astronomy is revolutionary - revealing to us the hidden lives of black holes and neutron stars," said astronomer Christopher Berry of Northwestern University, a member of the LIGO Scientific Collaboration (LSC).

"In just five years we have gone from not knowing that binary black holes exist to having a catalog of over 40. The third observing run has yielded more discoveries than ever before. Combining them with earlier discoveries paints a beautiful picture of the Universe's rich variety of binaries."

You've already heard about some of the new discoveries made from the observing run.

GW 190412 (gravitational wave events are named for their date of detection) was the first black hole collision in which the two black holes had wildly mismatched masses; all other black hole collisions detected prior had involved more or less equal-mass binaries.

GW 190425 is thought to be from a collision between two neutron stars, only the second ever detected (the first was in August 2017).

GW 190521 finally confirmed the existence of the elusive 'middleweight' class of black holes, between those of stellar mass, and the supermassive behemoths.

And GW 190814 was the first collision that involved an object in the 'mass gap' between neutron stars and black holes.

"So far, LIGO and Virgo's third observing run has yielded many surprises," said astronomer Maya Fishbach of Northwestern University and LSC.

"After the second observing run, I thought we'd seen the whole spectrum of binary black holes, but the landscape of black holes is much richer and more varied than I imagined. I'm excited to see what future observations will teach us."

That's not all the new data haul had to offer. Two other events, GW 190426_152155 and GW 190924_021846, stood out as extraordinary. And yes, those names are longer: As we detect more and more events, the date may not be enough to distinguish them, so the new naming convention is to include the time in UTC.

"One of our new discoveries, GW 190426_152155, could be a merger of a black hole of around six solar masses with a neutron star. Unfortunately the signal is rather faint, so we cannot be entirely sure," said astronomer Serguei Ossokine of the Albert Einstein Institute Potsdam in Germany.

"GW 190924_021846 certainly is from the merger of the two lightest black holes we've seen so far. One had the mass of six Suns, the other that of nine Suns. There are signals from mergers with less massive objects like GW 190814 but we don't know for sure whether these are black holes."

The new population of black hole and neutron star mergers has been described in four preprint papers.

The first paper catalogues the 39 new events. The second paper reconstructs the mass and spin distributions of 47 merger events found in the entire GWTC-2 catalogue, and estimates the rate of black hole and neutron star collisions. The third paper painstakingly searches for gamma-ray bursts associated with merger events (it found none). And the fourth paper evaluates the data against predictions of general relativity; spoiler, general relativity holds up completely.

Overall, the new collection of merger events isn't just a way to study collisions. It gives us a way to directly study black holes, which - as they emit no detectable radiation - are notoriously difficult to probe.

Thanks to gravitational waves, we know much more about these objects than we did even a year ago. And it's going to snowball from here.

"Merging black hole and neutron star binaries are a unique laboratory," Berry said.

"We can use them to study both gravity - so far Einstein's general relativity has passed every test - and the astrophysics of how massive stars live their lives. LIGO and Virgo have transformed our ability to observe these binaries, and, as our detectors improve, the rate of discovery is only going to accelerate."

LIGO has uploaded the preprints to its website while they await peer-review. They can be found here, here, here and here.