Wednesday, February 19, 2020

Secret Science Club Post Lecture Recap: Taking the Pulse of Gravitational Waves

Last night, I headed down to the beautiful Bell House, for this month's Secret Science Club lecture featuring Dr Chiara Mingarelli of the University of Connecticut and the Flatiron Institute’s Center for Computational Astrophysics. Dr Mingarelli is also a member of NANOGrav, the North American Nanohertz Observatory for Gravitational Waves. Taking the stage, Dr Mingarelli started off with a joke: "Thank you for clapping for my new tenure track position."

Dr Mingarelli started her lecture by acknowledging Black History Month and calling attention to the accomplishments of three African-American women... Katherine Johnson, is a former NASA computer who calculated orbital mechanics necessary for NASA's early forays into space, and was awarded the Presidential Medal of Freedom in 2015. Dr Jedidah Isler, is Yale University's first African-American woman PhD in astrophysics, and is an expert on blazars, the supermassive black hole powered galactic nuclei. Shirley Ann Jackson, MIT's first African-American woman PhD recipient, who was Chair of the NRC, president of RPI, and invented a portable fax machine.

After this topical introduction, Dr Mingarelli took us back to the deep past, the days soon after the Big Bang, when dark matter filaments spread throughout a very early universe. One billion years after the Big Bang, galaxies began to form as matter in halo clusters. Then galaxies began to form in spiral and elliptical shapes, the elliptical galaxies being the most massive galaxies, which have lost their spiral structure. Galaxies have supermassive black holes in their centers. Twenty four thousand light years from Earth, the supermassive black hole Sagittarius A*, thought to have 4.6 million solar masses, lies at the center of the Milky Way. Stars near Sagittarius A* have randomly distributed orbits, but further stars have regular orbits. Dr Mingarelli showed us an animation by Dr Andrea Ghez depicting the orbit of stars around the Milky Way's center, similar to this video:

Galaxies merge, and Dr Mingarelli noted that the supermassive black holes at the center of galaxies merge as well. This merging creates gravitational waves, ripples in spacetime which actually cause distance to stretch and squash. To illustrate this, she performed a funny 'gravitational wave dance'. She then turned the topic of the lecture to pulsars, which she likened to cosmic lighthouses. Pulsars are excellent clocks, as accurate as all but the most modern atomic clocks, flashing radio waves at regular intervals (though gravitational waves can change the distance between a pulsar and Earth). Pulsars were discovered by Jocelyn Bell Burnell, whose supervisors at Cambridge University tasked her with looking for quasars. She discovered a periodic radio signal and was told to drop study of it, but she suspected that she would be kicked out of the program soon, so she studied this radio source with diligence. She initially called this radio source a 'periodic star' but a reporter gave the object the catchier name 'pulsar'. Her male supervisor won the Nobel prize for this discovery.

Pulsars serve as clocks, accurate to withing 100 nanoseconds over a decade. Redshift and blueshift in pulsars is caused by gravitation waves stretching and squashing distance. This doesn't happen very frequently, in fifteen years, no gravitational wave activity has been noted, suggesting longer wave periods or weaker gravitational waves. It's possible that, with better instruments, the entire galaxy may be purposed as a gravitational wave detector.

There are other means for detecting gravitational waves, such as LIGO, a ground-based laser interferometer. Dr Mingarelli had another dance to demonstrate how LIGO works, with one arm extending while another arm at a 90 degree angle contracts. She noted that the signal from two black holes colliding was about 100 hertz in frequency, which is audible. The colliding black holes which triggered LIGO were stellar mass black holes. When two supermassive black holes merge, they emit radiation from all over the electromagnetic spectrum: x-rays, UV light, radio waves. The LISA (Laser Interferometer Space Antenna) mission, slated for 2034, is designed to create a new avenue to explore the universe, meant to detect galaxy formation and mergers, subject important to cosmology and basic physics.

Dr Mingarelli then returned to the subject of timed pulsar arrays- the data sets are sufficiently long and of sufficient quality that the lack of results in signal detection is significant. Are there some sort of hangups in black hole mergers that reduce merger rates? Could black holes not be as massive as thought? One of the big problems of astrophysics is the Final Parsec Problem (PDF). Black holes sink to the bottom of gravitational wells until they get within a parsec of each other. Using only gravitational waves, it would take longer than the age of the universe to get the black holes to merge. The centers of galaxies are also filled with gas and stars, which might interact with black holes which 'slingshot' out matter, shrink, and lose energy so the black holes can finally merge after billions of years. If there are not enough stars, no merger will take place. Dr Mingarelli showed us a NASA video simulation of two black holes colliding:

As the black holes merge, their outer gas rings merge to form a circumbinary accretion disk. The merging black holes send out 'messengers', radiation and gravitational waves- by measuring the amplitude of these waves, we might be able to find out how the black holes merge, overcoming the final parsec problem. Dr Mingarelli posed a mind-bender of a question: do local conditions apply everywhere? It sounds ridiculous, it may be ridiculous, maybe there is no solution to the final parsec problem. In galactic mergers, a third galaxy results, with matter and energy ejected in the merger.

If no results are found through pulsar timing arrays by 2030, scientists will have to ask some hard questions- do all galaxies have supermassive black holes at their centers? Dr Mingarelli asserted that the science is doable, and scientists are doing it. She hinted at something exciting occurring, with detection possibly occurring within three years. She posed the question, do the models reflect reality? Do black holes have less mass than thought? Are their 'weirder' factors at play? The binary supermassive black hole problem should be resolvable, and one possible binary has been found using 'maps' of the cosmos. Not a lot of weird stuff has been detected by using pulsar timing arrays, but they may help astrophysicists figure out which galaxies harbor supermassive black holes, though such supermassive black holes might be decoupled from their gas accretion disks when we find them. There is probably a 'sweet spot' for supermassive black holes, bigger might not be better. At ten to the tenth power solar masses, mergers might take place too quickly, at lower masses, the energy of mergers might be hard to detect. The 'goldilocks zone' might be around one billion solar masses.

Dr Mingarelli noted that the Sombrero Galaxy might be one of the best candidates for the search while M87 might be too big, to difficult to detect a supermassive black hole in. The object of the search is to learn how galaxies develop and grow.

Dr Mingarelli followed the lecture up with a Q&A session. The first question regarded LIGO, and its limitation to low-frequency waves- Dr Mingarelli joked that if the wavelengths are too long, LIGO is no-go. Another question regarded dark matter- if dark matter follows the MACHO model, it could interact with black holes, but such interaction is hard to detect.

Asked about the ten year time frame, Dr Mingarelli noted that the most pessimistic model has the gravitational waves discovered withing ten years, if not, then something is broken with the model.

A question about Jocelyn Bell Burnell's biography elicited a sad tale of sexism... she had a child while getting her PhD, so she worked an array of temp jobs, as did her husband. Her salary was capped because, combined with her husband's salary, the family income would have exceeded that of her department head. She was also an Irish woman working in the UK during the Troubles. When she was finally awarded the Breakthrough Prize, she donated the winnings for a scholarship for refugees and migrants. Dr Mingarelli half-jokingly referred to her as St Jocelyn of the Pulsar.

Asked whether she herself faced discrimination, Dr Mingarelli said yes, but not as much as women of color typically face. She noted a subtle discrimination, having comments ignored, then attributed to male colleagues. She stated that things could get better, but that topic is a different talk.

I make no bones about having a bio bias, so it's important for me to pay heed to the physics and astrophysics lectures. Dr Mingarelli spoke about some heady subjects and leavened her talk with humor, so the brain-bending was accompanied by funny bone tickling. Her lecture also combined both the joy of discovery and the righteous indignation of an advocate for better representation of underserved populations in the STEM fields. This combination of hard science and sound policy is exactly what I expect from the Secret Science Club. Kudos to Dr Mingarelli, Dorian and Margaret, and the staff of the beautiful Bell House.

Here's a video of Dr Mingarelli lecturing about pulsar timing array and black hole mergers. This video is bound to stretch and squash your brain like gravitational waves stretch and squash signals from pulsars:

Pour yourself a nice beverage and soak in that SCIENCE!!!

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