Rewriting astronomy: have gravitational waves revealed an object to fill the mass gap?

An artist’s rendition of a mystery object in the ‘mass gap’ found by LIGO and Virgo. Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

When two massive objects collide in space, ripples in space-time are sent through the universe. These ripples – gravitational waves – can be detected, and give an insight into what they may have been. Last August, the LIGO-Virgo collaboration detected gravitational waves from what was reported as the collision of a black hole and a neutron star, called GW190814. However, analysis has shown that the supposed neutron star involved has a mass of 2.6 solar masses. This would make it either the heaviest neutron star yet discovered, or the lightest black hole ever observed. 

This mysterious object lies within the mass gap between the lightest black hole and the heaviest neutron star observed. Astronomers have been puzzling over the mass gap for a while now and it seems that this object could give some answers. To date, the heaviest neutron star observed had a mass of 2.3 solar masses and the smallest black hole has a mass of 5 solar masses. 

Questions remain as to whether this object is a black hole or a neutron star however. The Tolman-Oppenheimer-Volkoff (TOV) limit makes it harder to be sure about what the smaller object in the collision is. This limit determines the heaviest a neutron star can be before its mass is too great, and the outward pressure of neutrons can no longer repel each other against the gravitational pressure pushing them together. The object then collapses into a black hole. So far, calculations of the TOV limit have put it between 2.2 and 2.4 solar masses, though this was narrowed down in 2017 to 2.3 solar masses by the GW170817 event of two neutron stars colliding. 

Work on a LIGO mirror, part of the exquisite equipment involved in the detection of gravitational waves. Image credit: Matt Heintze/Caltech/MIT/LIGO Lab

If the object in the collision was the lightest black hole, then a new theory must be found in order to explain how such an object could be created. If it is a heavy neutron star, all theories about how neutron stars are created must be revised. It is thought that it is more likely to be a small black hole than a neutron star, but because of an uncertainty in the TOV limit, there is still a possibility of it being a neutron star.

If this object could be identified, it may give rise to more questions on neutron stars and binary system formation than it gives answers. Current models suggest a large star will collapse into a neutron star when it dies, and slowly dissipate until it is gone. If the star is large enough, the gravitational pressure will cause the star to collapse further, producing a black hole. By studying objects in the mass gap and the object involved in GW190814, astronomers hope to improve their theories regarding what goes on in a neutron star.

Another interesting aspect of the GW190814 event is the large mass ratio. As the mass of the black hole is calculated to be 23 solar masses, it is nine times larger than the unknown object. Every other event thus far observed has been between objects of similar masses, which causes scientists to revisit theories on how binary systems are created. Other binary systems have been of similar mass size, which this event shows may not always be the case.

As this event was observed not long after the first detection of gravitational waves, astronomers have come to the conclusion that they must be a common occurrence. The hope is that as the detectors are upgraded and become even more sensitive, more measurements can be made to pinpoint the populations of neutron stars and black holes in the universe. 

Written by Jessie Hammond and edited by Ailie McWhinnie.

Jessie’s thoughts… This article made me think a lot about how much we don’t know in physics. I had never heard of the mass gap between neutron stars or black holes or that it was so hard to tell the difference between the two within this mass gap. 
It also makes me excited for how many discoveries can now be made because of the detection of gravitational waves. This whole other field in physics has been opened up and it is just the start of many discoveries to come. 
The test of General relativity has come from the many observations made through gravitational waves, and it is exciting to see if it will continue to hold up. I wonder if more observations on binary systems will maybe need an alternative theory on gravity, and what this would look like.
I came away with more questions than answers writing this article. Is there a better theory for how gravity acts in our universe? Will another theory such as TeVes or f(R) theory win out? And how will observing more mass gap objects alter our theories on how neutron stars develop and operate?

Find me on… Twitter @jessietweet1998 and Instagram @jessieboo1998. Also check out my podcast Delving Into Academics on Twitter @Intodelving and Instagram @delvingintoacademics

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