Magazine article American Scientist

Gravitational Wave Astronomy

Magazine article American Scientist

Gravitational Wave Astronomy

Article excerpt

One hundred years ago, Albert Einstein presented his general theory of relativity, our foundational theory of gravity. A direct consequence of this theory is the prediction that violent cosmic events can be strong sources of gravitational waves-a new messenger with which to study our universe. (See "The Secret History of Gravitational Waves," March-April, for more on early predictions.) The first direct detection of gravitational waves by the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in 2015 and the first "multimessenger" simultaneous detection of electromagnetic and gravitational waves showed how this gravitational messenger can lead to transformative discoveries in astrophysics and cosmology.

The first handful of LIGO detections came from the merger of stellar mass black holes in distant galaxies, followed by the stunning 2017 observation of a binary neutron star collision and the associated gamma ray burst, along with a brief subsequent brightening known as a kilonova. Other predicted sources of gravitational waves detectable by LIGO include nearby supernovae, rapidly spinning neutron stars, an all-sky gravitational wave background, and a network of cosmic strings (hypothetical onedimensional structures of cosmological length that may have been created in the early universe). Reaching beyond the predicted physics, there is great potential for discovering unknown phenomena through their emission of gravitational waves.

The potential discovery space in gravitational wave astronomy relies on our ability to understand our current and future detectors and to maximize our sensitivity to signals from unexpected sources of gravitational waves. New technology will go hand in hand with a deeper understanding of where and how gravitational waves are produced.

Maximizing science capabilities for gravitational wave observations requires a new set of data science techniques and relies on active research and development. This is an exciting time, as we confront the limits of our observations and push into the unknown, paving the way for the next revolution in our understanding of the cosmos.

Gravitational wave observatories will take many forms. Just as there is a spectrum of light, or electromagnetic radiation, from long-wavelength radio waves to short-wavelength gamma rays, there is also a spectrum of gravitational waves. LIGO has observed events at the very highest gravitational wave frequencies, but new gravitational wave observatories will extend our access across the gravitational wave spectrum. We observe various astrophysical phenomena at different wavelengths of light; likewise, we will see radically different skies at different wavelengths of gravitational waves. There surely will be many surprises along the way. Astronomers have been using electromagnetic observations for hundreds of years to understand the cosmos, whereas observational gravitational wave astronomy, having debuted only in 2015, is in its infancy.

First Steps into Gravitational Waves

The era of gravitational wave astronomy began with the first detection of gravitational waves and the first observation of the collision and merger of a pair of black holes. On September 14, 2015, the two LIGO detectors registered the gravitational wave signal known as GW150914 (designating the year, month, and day of detection), created by the cataclysmic merger of two black holes in a galaxy more than one billion light-years from the Earth. The 2017 Nobel Prize in Physics was awarded for this groundbreaking discovery.

On August 17, 2017, astronomers were alerted to the gravitational wave event GW170817, observed by LIGO and by the Virgo detector in Italy. Less than two seconds after the GW170817 signal, NASA's Fermi satellite objoey served a gamma-ray burst catalogued as GRB170817A. Within minutes of these initial detections, telescopes around the world began an extensive observing campaign to identify the galaxy in which the event occurred and then to observe this location with instruments sensitive across the electromagnetic spectrum. …

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