February 24, 2016
(Written by the ASTRO program organizing committee)
– A long time ago in a galaxy far, far away, two large black holes—each with a mass of about 30 suns—reached the end of an aeons-long orbital dance. In the final second of their separate existence, they spiraled toward each other, whirling with a frequency that quickly rose from tens to hundreds of cycles per second. At last they touched, then violently merged in the space of about twenty milliseconds, producing a single black hole that quickly settled down to a bloated, lone existence. Had a video camera been present in the vicinity, it would likely have seen little; black holes are black, after all, regions where gravity is so strong that not even light can escape. Yet during that final merger, the power emitted by this event was larger than all of the power being emitted in light by all of the stars in all of the galaxies in the observable universe. The merger shone, not in electromagnetic waves, but in gravitational waves. The black hole binary’s dance continually sloshed the fabric of space and time in its vicinity, sending out waves carrying news of the invisible event as fluctuations in the spatial separations of objects, and in the flow of time. The waves began as gentle ripples during the long inspiral, steadily climbing in frequency and amplitude; they roiled and crashed during the merger; and finally, they decayed away like the ring of a bell. They followed paths outward from the merger in all directions at the speed of light, diminishing in amplitude but maintaining their shape, an encoding of the story of the merger in the dynamics of spacetime. After a billion-year journey, the waves reached Earth.
This is not the start of a science fiction tale. An international team of over a thousand scientists has observed this merger, the culmination of over four decades of effort sponsored by the National Science Foundation (NSF) and international sources. And NSF’s Statistical and Applied Mathematical Sciences Institute (SAMSI) will soon help astronomers to take the next steps to make the most of this and future gravitational wave discoveries.
Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org)
Gravitational waves and LIGO
In 1916, Einstein realized that the theory of gravity he had proposed a year before—general relativity, a revolutionary reframing of gravitational interaction, not as the consequence of long-range forces, but rather as a consequence of curvature of spacetime—implied the existence of a new type of radiation, gravitational waves. But the theory revealed space to be incredibly stiff, so resistant to changes in curvature that even violent motions of large masses would produce what seemed to be immeasurably small waves. By the late 1970s, scientists in the U.S. and Europe had converged on a vision for how to make the immeasurable measurable. The Laser Interferometer Gravitational wave Observatory (LIGO) is the realization of this vision.
LIGO’s time series data from the binary black hole merger event GW150914; see the LIGO project page, Gravitational Waves, As Einstein Predicted, for details. Image Credit: Caltech/MIT/LIGO Lab
On September 14, 2015, the waves from that distant merger met LIGO and produced a signal. Months of analysis by many dozens of scientists confirmed its reality, and enabled detailed measurement of the properties of the merging black holes, and of the final hole. The LIGO project announced the discovery to the world on February 11, 2016, dubbing the event GW150914. The discovery marked the confirmation of Einstein’s century-old prediction. But more than that, it marked the opening of a new sense with which astronomers could examine the universe.
SAMSI ASTRO and LIGO
SAMSI is poised to help astronomers hone their new sense. In November 2014, SAMSI sought input from the astronomical community for a year-long program that would gather astronomers, statisticians, and applied mathematicians to address challenging interdisciplinary problems in astronomy. Led by statistician G. Jogesh Babu (Penn. State University), a team of scientists identified a set of timely and compelling research directions, under the overarching and overlapping themes of time-domain astronomy and survey-based astronomy. With renovations to LIGO nearing completion, gravitational wave data analysiswas quickly identified as a focus area, along with exoplanets (which are detected via time series measurements), synoptic surveys(an emerging mode of large-scale automated time-domain observing), and cosmology. In September 2015, scientists gathered at SAMSI to plan the 2016-17 Program on Statistical, Mathematical and Computational Methods for Astronomy (ASTRO). The planning team included LIGO scientists who had only just learned of the candidate detection, and had to keep it secret until confirmed.
Of five working groups planned for the ASTRO program, four will address LIGO data analysis challenges, in concert with related challenges in other areas of time-domain astronomy (a fifth working group will focus on statistical problems in cosmology). One working group will study the potential role of new stochastic process models for analysis of time series data from LIGO and exoplanet surveys, particularly models that abandon the simplifying assumptions of stationarity and Gaussianity underlying most currently-used methods. Another working group will focus on gravitational wave and exoplanet signal detection, and how best to use detected signals for demographic studies (for example, to infer the prevalence and diversity of binary black hole systems, and other sources of LIGO signals). A third working group will address the data-theory interface in the regime of computationally expensive theoretical calculations, where it is impossible to directly compute detailed predictions for every candidate model for the data. Numerical general relativity calculations of binary black hole mergers are a motivating example; similar challenges arise in cosmology.
Finally, a working group on synoptic time-domain surveys will address how to find electromagnetic counterparts to gravitational wave sources. Black hole binary mergers, by their very nature, are essentially invisible electromagnetically. But astronomers expect LIGO to detect other types of events that synoptic surveys could capture electromagnetically, providing opportunities for synergistic multimessenger astronomy. These include such exotic phenomena as merging binary neutron stars, and mergers between black holes and ordinary stars, neutron stars, or white dwarf stars. In addition, gigantic stellar explosions, such as those producing supernovae or gamma-ray bursts, may produce detectable gravitational waves. In a tantalizing twist of fate, astronomers have observed all of these types of objects, and presumed that the first LIGO events would come from such already-known systems. Instead, the first LIGO signal was from a type of system hitherto undetected. What other surprises might this new ear on the sky reveal to us?
SAMSI and Astronomy
The ASTRO program is just the latest of several productive programs SAMSI has hosted to build interdisciplinary partnerships between astronomers, statisticians, and mathematicians. The first such program was the 2006 Spring Program on Astrostatistics (also led by Babu). It, too, included working groups addressing problems in gravitational wave and exoplanet astronomy. Many participants built long-lived collaborations at SAMSI; several are helping to organize the forthcoming ASTRO program. SAMSI’s 2012-13 Program on Statistical and Computational Methodology for Massive Datasets included a week-long Workshop on Astrostatistics, organized by Babu, exploring the intersection of astronomy and “big data.” In the summer of 2013, exoplanet astronomer Eric Ford (Penn State University) led a three-week program, Modern Statistical and Computational Methods for Analysis of Kepler Data. It spawned an independent ExoStats2014 workshop, and one of that program’s working groups continues to meet two and a half years later. Finally, the ASTRO program’s working group on inference with computationally expensive models will build on expertise gained from the 2006-07 Program on Development, Assessment and Utilization of Complex Computer Models, and the 2011-12 Programs on Uncertainty Quantification; participants from both of those programs are on the ASTRO planning team.