Thursday, September 14, 2017

Second Anniversary of the First Detection and a New Job!

While posts here have been long awaited, I've been busy doing research, teaching, and changed my job!  I've just started a new job as an assistant professor at Villanova University and an academic job search is quite intensive (perhaps I will write about finding a faculty job soon).  Also, I've been writing some other things.  I've written a TED-Ed video/lesson on gravitational waves and it premiers today (you can view it here), I have a PhysicsWorld Discovery text on "Gravitational Waves" coming soon, and have written a few other things here and there.  So, now that I've moved from Baton Rouge to the suburbs of Philadelphia, I have some time to talk with all of you again.


Today is the second anniversary of the first detection of gravitational waves.  That got me thinking of about where the front of that wave is now...

The colored area on this map shows the most probable source of the detected gravitational wave where red is more likely than purple.  The location is shown against a map of the night sky centered on the Milky Way galaxy with constellations outlined.
[Credits: NASA Deep Star Maps (Visualization Credits, Ernie Wright (USRA): Lead Animator, Tom Bridgman (GST): Animator) by NASA/Goddard Space Flight Center Scientific Visualization Studio with constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg), and the source location based on Gravoscope screen grabs (LIGO & Nick Risinger,, all in galactic coordinates. Composition by University of Florida / S. Barke.]

The source of GW150914 was from the general vicinity of the constellations Volans and Carina.  That means that it is traveling towards the stars in the constellation Draco.  It hasn't encountered much.  Since it has traveled a distance of 2 light years from Earth, it is still in our Milky Way galaxy (the radius of the Milky Way 60,000 is ly and its disk is 2000 ly).  It has not encountered any other stars (the closest star in Draco is Struve 2398, a binary system of red dwarf stars 11.6 ly away) and therefore no other planets.  That means that no other life forms have detected GW150914 and won't reach Struve until early 2027 (give or take for the error in our understanding of its distance).


Since I wrote last, we announced the discovery of a third detection of gravitational waves from another binary black hole system dubbed GW170104.  A black hole 32 times the mass of our Sun merged with another black hole 19 times the mass of our Sun resulting in a single 49 solar mass black hole after radiating away 3 solar masses in gravitational waves.  Among other things, this detection helps to fill in the range of masses we've observed; gaps would imply that there is something preventing the formation of those kinds of systems and that would be unexpected.

Graphic representation of the known stellar mass black holes observed through X-ray observations (purple) and gravitational waves (blue).  [credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)]


This system originated from about 3 billion light-years away: farther out in the universe than any of our previous detections.  Using a gravitational wave that has traveled such a distance allows us to test a part of general relativity that we haven't been able to before: do gravitational waves disperse?  For example, when white light enters a prism, the different colors (frequencies) of light travel at slightly different speeds causing them to separate or disperse.  General relativity predicts that different frequencies of gravitational waves should not disperse.  There are alternate theories of gravity that make predictions of how dispersion will affect a gravitational wave.  We compared our observation to standard general relativity and the alternate theories' predictions and found our observations to be consistent with general relativity.  That is, we did not observe any significant dispersion of our gravitational wave!  We've also done all of the tests of general relativity that were done for the previous detections and this gravitational wave continues to affirm that general relativity is correct.


Artist's conception shows two merging black holes similar to those detected by LIGO. The black holes are spinning in a non-aligned fashion, which means they have different orientations relative to the overall orbital motion of the pair. LIGO found hints that at least one black hole in the system called GW170104 was non-aligned with its orbital motion before it merged with its partner. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

We also were able to investigate the spin of the black holes.  For as mind-blowing as a black hole is, it is completely described by three numbers: 1) its mass, 2) its spin, 3) its charge (this is postulated by the no-hair theorem).  Since we believe the electric charge of astrophysical black holes to be negligible, physicists get very excited about the mass and the spin.  The mass is not too hard to measure (we can get that pretty accurately from the waveform shape and frequency evolution), but the spin is all in the details of the signal which makes it more difficult to estimate.  But we always still mention it because any information we get about it will tell us more about the other half of the back hole's story.

For this detection, we were able to extract information about how the combined spin of the black hole system compares to the direction of its orbital angular momentum.  Basically, does the direction the effective spin of the black holes (on their internal axes) go in the same direction as their orbit?  For example, the Earth spins on its axis in the same direction as it orbits the Sun, so this is a positive alignment, but the Earth's axis is tilted so it isn't a perfect alignment.

If most of our black hole systems have small misalignments that would support their formation through something we call "common envelope evolution" (I wrote about this here), which is a complicated way of saying that the stars that formed the black holes were always paired together and once they both died you end up with a binary black hole system.  The interactions between those original stars will cause their spins to align giving the resulting binary black hole system only small misalignments.

Another formation mechanism for binary black holes is that they just happened to pass one another while drifting through space and became gravitationally bound together (this is called dynamical assembly).  We expect things like this to happen in dense stellar clusters or near the centers of galaxies.  Since these would have had no interaction with each other before they became a system, we expect random spin alignments from the black holes.

Ultimately we found that our system likely had a low total spin and was likely not aligned with the orbital angular momentum of the system.  It is also possible that our black holes had no spin to begin with.  So this isn't definitive, but we are starting to assemble the story of how black hole systems like this form.


With this discovery, we are adding to our understanding of the universe and testing general relativity in ways we've never been able to before.  The importance is that LIGO is truly operating as an observatory (that's what the 'O' in LIGO stands for after all) and building a database of observations.  In astronomy, you can never make a single observation and understand a system's history.  You need to take a large sample of observations from similar systems and find out what the patterns are.  That's how we understand how stars evolve since a human life, or even all of human existence, isn't long enough to have followed a single star's life cycle.  But because we have observed many stars in different stages of their lives we've discovered patterns.  That's how we know that the black holes we just observed are the collapsed corpses of the extremely massive stars.  Now we can collect observations of many of these black hole systems to learn more about black holes in general and how these pairs of black holes form.


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  2. Hi Amber,

    I stumbled upon your nice blog recently. I am wondering if you could elaborate on how LIGO tested whether the GWs it heard were dispersive? (I probably should read the LIGO papers more closely!) With more events, how much better would the constraints become? I assume when you say "non-dispersive", you are referring to the high frequency/JKWB/eikonal statement that the GW's (4-momentum)^2 = 0? I believe extremely low frequency GWs propagating in a curved background -- such as that of our expanding universe -- would actually interact with its geometric curvature and become dispersive, though I am guessing human GW experiments may never be directly sensitive to such effects...


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