May 21, 2019,  the Laser Interferometer Gravitational-Wave Observatory (or LIGO) detected a signal that came from the merging of two black holes.

Wow! Where to begin. One of the world’s first gravitational wave detectors, LIGO, is a pair of laser interferometers located 3,000 km apart or about 1,800 miles. Locations are the Hanford Observatory in Washington and the Livingston Observatory in Louisiana.  This detector is incredibly sensitive, detecting some of the weakest signals known to man. A short video summarizes the first observation made by LIGO told directly by the leading scientists in the endeavor. These scientists, Rainer Weiss, Kip Thorne, and Barry Barish, won the 2017 Nobel Prize in Physics for the discovery. LIGO is supported by the National Science Foundation (NSF), Caltech, and MIT.
We have come a long way with our understanding of the Universe and general relativity. Gravitational waves (GWs) are detected as disturbances in the interferometer. We can measure the duration and the frequency evolution of the signal and from that, we can apply general relativity theory to understand what took place and where for the GW event to occur. In 2015, LIGO reported the first ever detection of such an event, and it was determined to be from the merging of two black holes. These black holes merged to form an ever larger black hole.

Since then, LIGO and the twin observatory, VIRGO in Italy, have made several more observations of GW events! Isn’t that amazing? Most of the observations are consistent with general relativity (GR) modeling for binary black hole mergers with a single binary neutron star merge event, GW170814. All of these events are unique for different reasons but I will focus on the most recent detection yet, GW190521.

GW190521 is another binary black hole merge event, with the observed measurements being completely consistent with GR numerical simulations where they have assumed a quasi-circular compact binary coalescence – this is basically fancy talk describing two black holes slowly spiraling together in their respective circular orbits until collapsing and merging together. So, they were able to determine the most likely scenario to generate a GW signal with the same properties as GW190521 was a binary black hole system that ended with the two black holes merging together to form an altogether larger black hole.

First of all, this is amazing that this kind of research is possible. I just – wow. I mean, right? This is just incredible. The unimaginable has been imagined and then brought to life! Thank you, Albert Einstein. Second of all, this binary black hole (BBH from now on) merge event is special for two main reasons.
1) The two black holes that merged together have been measured to be 85 and 66 times the mass of the Sun, which we denote using M ☉. The primary black hole (BH), the 85 M ☉ one, is especially intriguing. The current understanding of stellar evolutionary theory for very massive stars predict that no black holes should be formed from stellar collapse between about 55 to 120 M ☉. Of course, there are uncertainties surrounding where exactly this mass “gap” really lies but, the physics and modeling all appear to predict the same thing — no black hole remnants should be formed from stars between 55-120 M ☉, give or take. But how much give or take? In the second report characterizing the astrophysical properties of GW190521, they report that the probability one of the black holes that merged together has a mass within the mass gap is 99%.
2) The mass of the final BH from the merge event is estimated to be ~150 M ☉. This is the first strong evidence for the existence of an intermediate black hole. An intermediate black hole is a black hole that ranges in size from ~100s to ~100,000s the times the mass of the Sun. LIGO has detected several BH merge events, but only from stellar-mass black hole systems so far. GW190521 is the most massive BBH merge event observed to date! 

Illustration showing the masses of the two black holes that formed the intermediate black hole. How did the two black holes form before merging? [Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC).]
The LIGO community announced the detection only just Sept. 2, 2020. Below is the GW detection signal from both LIGO detectors and VIRGO, and the signal represented in the time-frequency domain in the images displayed in the bottom panels. The signal was very short, with a duration of only 0.1 seconds and ranged in frequency from 30-80 Hz.
The GW event GW190521 observed by the LIGO Hanford (left), LIGO Livingston (middle), and Virgo (right) detectors. Times are shown relative to May 21, 2019 at 03:02:29 UTC. s. The bottom row displays the time-frequency representation of the whitened data using the Q transform. From Abbott + 2020
I believe it was first reported in Physical Review Letters here, of which the above image is adapted from. A really cool short video is shared below that nicely reiterates what I’ve just mentioned (and then some!).
Other cool materials are available here: including a video of a numerical simulation of a binary coalescence that reproduces GW190521 (which I also display directly below because its COOL).
Above: Numerical simulation of two black holes that inspiral and merge, emitting gravitational waves. The black holes have large and nearly equal masses, with one only 3% more massive than the other. The simulated gravitational wave signal is consistent with the observation made by the LIGO and Virgo gravitational wave detectors on May 21st, 2019 (GW190521).

A New Frontier: Gravitational Wave Astronomy

Einstein predicted in general relativity theory that changes in the gravitational field will travel through the universe at the speed of light. These are gravitational waves and were only first confirmed in 2015. In 2020, we discover the most massive black hole merge event observed to date, and it happens to be pushing the limits of what we understand about the evolution of massive stars and, the nature and formation of intermediate sized black holes. Today, there are currently no confirmed intermediate black holes (IMBHs). There are several candidates, most of which reside in dwarf galaxies and are associated to low-luminous active Galactic centers. However, having no firmly identified IMBHs leaves many questions unanswered. How do IMBHs form? What conditions are necessary? The final BH responsible for GW190521 may give us clues and perhaps, future detections by LIGO and VIRGO may further reveal the Universe’s secrets.
I’ve been reading into this some and will discuss a short letter that investigates the origin of the primary 85  M ☉ BH of GW190521 in the coming blog post!

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