Written by Michael Coughlin

The future of gravitational-wave astronomy after the first detection

Michael Coughlin is currently a post-doctoral fellow at Harvard University with Prof. Christopher Stubbs. In September 2016, he successfully defended his physics PhD at Harvard, titled “Gravitational-wave astronomy in the LSST era”. He began researching gravitational waves with LIGO over eight years ago as a college freshman at Carleton College in Northfield, MN and it was very exciting for him to be part of LIGO’s historical confirmation in February 2016. At Harvard, he added the Large Synoptic Survey Telescope (LSST), Pan-STARRS, and ATLAS to his research areas, including designing and building a prototype calibration system, which he nicknamed “CaBumP”.

Since LIGO announced the detection of gravitational waves from binary black hole mergers in its first observing run [1-2], the most common question I have received is “What was it like to be part of such a historic scientific discovery?” The second most common question has been: “So what happens now?” The answer is a lot of stuff! Here I’ll focus on three main goals:

1. Using LIGO to detect other sources of gravitational-waves
2. Improving the gravitational-wave detectors in order to probe farther into the cosmos
3. Electromagnetic follow-up of gravitational-wave events with telescopes to get a more complete picture

What else does nature have in store for us?

The detection of gravitational waves from binary black hole mergers has been incredibly exciting, and we look forward to the detection of more such systems. Of course, there are many other sources (pulsars, supernovae, binary neutron stars, etc.) that we hope to detect as well. As a member of the group in LIGO searching for a stochastic background of gravitational waves, I am particularly interested in the processes that could create such a signal. This includes backgrounds from compact binary coalescences, pulsars, magnetars, or core-collapse supernovae. A cosmological background (such as from inflation!) could be generated by various physical processes in the early universe. In particular, with the recent discovery of binary black-hole mergers, there is a really good chance of observing a stochastic gravitational-wave background from these systems [3].

There are other sources that are likely to produce long-lived transients, including emission from rotational instabilities in proto-neutron stars and black-hole accretion disk instabilities. There is ongoing significant effort to improve the algorithms used to search for such systems [4]. A detection of a stochastic background from any of these sources would enable us to explore the bulk properties of such systems at high redshift, most of which are not individually detectable, and therefore we could do population studies not otherwise possible.

Making the best even better – How to improve the sensitivity of LIGO?

I got my start in LIGO searching environmental sensors for coincident noise with the gravitational-wave detectors, and it is a problem near and dear to my heart (and is chronically underappreciated!). With the arrival of new data and the corresponding improvement in sensitivity, new challenges arise. The presence of noise in the detectors is one of the foremost challenges in detecting gravitational waves. Seismic noise from earthquakes and trucks driving near the site, magnetic noise from machines on site and lightning strikes, acoustic noise from airplanes, and many, many other sources contribute. Because there is way too much data for the LIGO scientists to analyze on their own, a citizen science program has been setup to ask the public to help identify transient noise sources in LIGO data [5].

I have been particularly interested in the effect of seismic and magnetic noise sources on the new detectors. Gravitational wave detectors have very complicated seismic isolation systems designed to decouple the interferometers from the ground, but there are types of seismic sources that couple directly to the mirrors, bypassing these systems. In [6], we explored the potential cancellation of Newtonian noise produced by seismic waves moving the ground below the gravitational-wave detectors changing the local gravitational field at the mirrors inside. The idea is to use arrays of seismometers to monitor and subtract the sources of gravity perturbations.

Because gravitational wave detectors are made out of magnetic materials, they are susceptible to the presence of magnetic fields at the detectors. In [7], we explored how global magnetic noise is a significant problem for gravitational-wave detectors. We used low-noise, extremely low-frequency magnetometers to explore the effects of global electromagnetic fields known as the “Schumann resonances” on searches for a stochastic background of gravitational waves.

Needle in a haystack – Electromagnetic follow-up of LIGO detections

There is a significant effort to perform follow-up measurements of gravitational-wave events with telescopes in order to study these astrophysical systems with both electromagnetic and gravitational-wave observatories (and most of my thesis research was dedicated to this and related problems!). Electromagnetic emission likely occurs on a variety of timescales and wavelengths ranging from seconds to months in X-ray to radio, respectively. The problem is that LIGO can only pinpoint the location of the source to about 100 square degrees on the sky, and with telescopes that usually have fields of view of 1 degree or less, it is like searching for a needle in a haystack!

Searches for an electromagnetic counterpart of the binary black hole systems were completed by twenty-five participating teams of observers, who responded to the gravitational-wave alerts to mobilize satellites and ground-based telescopes spanning 19 orders of magnitude in electromagnetic wavelength. Our lab at Harvard University collaborated with Pan-STARRS and ATLAS to follow-up these events [8-9], studying how to maximize the probability for joint detection of electromagnetic and gravitational-wave emission, both by choosing where to point the telescopes in an optimized way and taking images as soon as possible after the detection [10]. The LSST, with its large field of view and plan to image the entire visible southern sky every few nights, will be one of the best tools for follow-up of gravitational-wave events. It is currently under construction and expects to take first commissioning images in 2019.

Tying it all together

Coughlin dances on the Harvard ballroom dance team and enjoys the chaos of teaching 3rd and 4th graders in an after-school math and science program at a local elementary school.

I work on the interface between gravitational waves and optical astronomy. The field of gravitational-wave astronomy is just beginning and there are many, many things to be done. The above is a small (and biased) slice of some of the research being done in groups I am involved in, and there are many other areas that also deserve attention. In addition to the fundamental physics probed by binary black hole mergers, there are a significant variety of other sources of gravitational waves. Searches for these sources are greatly aided by efforts to limit the effects of terrestrial noise sources. In addition, imaging sources emitting both electromagnetic radiation and gravitational waves, and the observation of the gravitational waves in coincidence with electromagnetic observations, could give new insight about the source.

To learn more about my research on Newtonian Noise and its mitigation, read our paper in CQG.

[1] Abbott, B. P. et al. Observation of gravitational waves from a binary black hole merger. Phys. Rev. Lett.,116:061102, Feb 2016.
[2] Abbott, B. P. et al. GW151226: Observation of gravitational waves from a 22-solar-mass binary black hole coalescence. Phys. Rev. Lett., 116:241103, Jun 2016.
[3] Abbott, B. P. et al. GW150914: Implications for the stochastic gravitational-wave background from binary black holes. Phys. Rev. Lett. , 116:131102, Mar 2016.
[4] E Thrane and M Coughlin. Detecting gravitational-wave transients at 5 sigma: A hierarchical approach. Phys. Rev. Lett. , 115:181102, Oct 2015.
[5] https://www.gravityspy.org/
[6] M Coughlin, N Mukund, et al. Towards a first design of a Newtonian Noise cancellation system for Advanced LIGO. Class. and Quantum Grav., 33 244001, 2016.
[7] M Coughlin, N Christensen, at al. Subtraction of correlated noise in global networks of gravitational-wave interferometers. Class. and Quantum Grav. 33 224003, 2016.
[8] Smartt, S. J. et al. Pan-STARRS and PESSTO search for an optical counterpart to the LIGO gravitational-wave source GW150914. Monthly Notices of the Royal Astronomical Society , 462(4):4094-4116, 2016.
[9] S. J. Smartt et al. A search for an optical counterpart to the gravitational-wave event GW151226. The Astrophysical Journal Letters, 827(2):L40, 2016.
[10] M Coughlin and C Stubbs. Maximizing the probability of detecting an electromagnetic counterpart of gravitational-wave events. Experimental Astronomy, pages 1-14, 2016.

Read the full article in Classical and Quantum Gravity:
Towards a first design of a Newtonian-noise cancellation system for Advanced LIGO
M Coughlin et al 2016 Class. Quantum Grav. 33 244001

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