Navograv Astrophysics Team Makes History With Detection Of Black Hole Collision Using Gravitational Waves

 

Recall the original report to do with detecting gravitational waves from two colliding black holes in late 2015, appeared in an abstract in  Physical Review Letters, in February, 2016.   

This was based on measurements from the Laser Interferometer Gravitational Wave Observatory (LIGO) which basic system is depicted below:

Based on the data cited in the Phys. Rev. link, the two black holes were each roughly 30 times the mass of the Sun. They evidently merged some 1.3 billion light years from Earth. The  gravitational waves themselves were generated in the final moments before the black holes merged. The signal was brief but definitive and we on Earth have since received it.

More recently, in May, 2022, the Event Horizon Telescope project obtained an image of the supermassive black hole at the center of the Milky Way for the first time.  The results were presented during a press conference livestreamed on the US National Science Foundation website, Recall  back in April 2019, the EHT collaboration unveiled the very first image of a different black hole, the supermassive entity at the center of the Messier 87 galaxy, dubbed M87*.  Then the Event Horizon Telescope was able to generate an image by using eight radio telescopes from across the planet. Synchronizing these instruments helped produce an image, i.e.

which the mainstream media called a "black hole", but which I noted in a post at the time was not the black hole per se but its event horizon. No one, nothing - no instrument - can 'see' an actual black hole - which is why it's called a black hole because no detectable radiation can escape.

Now, for the latest iteration, more than 100 astrophysicists have gathered data suggesting that the population of massive black hole pairs are merging by the hundreds of thousands—perhaps even millions. The 'nano' scale  (nanohertz frequency)  gravitational waves detected from these mergers are all contributing to an underlying background hum of the universe that researchers can detect from Earth. This has been reported now in Astrophysical Journal Letters, See e.g.

The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background - IOPscience

The findings have arrived from a collaboration of more than 100 scientists, and help to confirm what will one day happen to the super-massive black hole at our galaxy’s center known as Sagittarius A*, as it crashes into the black hole at the heart of the Andromeda galaxy.  According to Joseph Simon, a University of Colorado, Boulder astrophysicist and a member of the North American Nanohertz Observatory for Gravitational Waves, or Nanograv:

“The Milky Way galaxy is on a collision course with the Andromeda galaxy, and in about 4.5 billion years, the two galaxies are set to merge. That merger will eventually result in the black hole at the center of Andromeda and Sagittarius A* sinking into the center of the newly combined galaxy and forming what is known as a binary system. "

Another Nanograv member, Chiara Mingarelli, a Yale University astrophysicist - quoted in a WSJ piece- noted:

“Before now, we didn’t even know if supermassive black holes merged, and now we have evidence that hundreds of thousands of them are merging,”

This latest research with Nanograv could answer two key questions:  

(i)How do these black holes grow, and 

(ii)  How often their host galaxies merge, 

According to Masha Baryakhtar, a physicist at the University of Washington in Seattle, who wasn’t involved in the research. 

"If scientists understand more about the history of merging supermassive black holes, it could help reveal how they form in the first place."

Well, true!  Because origins and history are intermeshed in the same physical process.  The physical dynamics of origin, then, could likely shed light on the ultimate merger into a supermassive black hole-including understanding what mass limits - if any- apply.

Essential to these findings is the detection of elusive gravitational waves, and understanding how they are produced.  In 1915  Albert Einstein, in a remarkable achievement of theoretical physics, used an abstruse form of math known as tensor calculus to predict the existence of gravitational waves. This was by way of showing how gravitational distortions arise when mass or energy warp space-time.  The ground breaking field equation that relates these parameters was summarized:

G _mn   =  - ½ g _mn  G=  - 8 p T _mn   

Where the  T _mn    denotes the associated  “stress-energy” tensor which incorporates internal stresses, the density of matter and its component velocities (u, v, w or in some texts: u1, u2 and u3).   From this one can see that if no matter is present, one would have:  G _mn   =  0

If matter is present there must then be internal stresses and velocities so that:  G _mn   =  K _mn  where  (as seen from the field equations):  K _mn   = - 8 p T _mn   

This implies that any massive object moving in space-time can cause these waves.  The distortion in space-time from mass motion is bound up in the stress-energy tensor: T _mn

 To fix ideas, LIGO in 2015 detected how short, high-frequency  gravitational waves from one merger between less massive black holes "jiggled" the Earth by less than the width (10 ^-15  m )of a single subatomic particle. The effort won the scientists involved a Nobel Prize.

 

There are still other things in the universe producing gravitational waves that haven’t yet been detected.  This according to Julie Comerford, an astrophysicist at the University of Colorado, Boulder, and Nanograv member. One of those other sources, she said, could be ripples in space time from the big bang itself. Stay tuned.

 See Also:

Astro2020_Paper1.pdf (nanograv.org)