Photo Courtesy of NASA and ESA

A view of the fading Kilonova, as captured by the Hubble Space Telescope.

October 30, 2017

Cornell Physicists Contribute to Discovery of Colliding Neutron Stars

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On Oct. 16, astronomers announced that they had viewed a cosmic event, the collision of two neutron stars, through both light and gravitational waves. Over a thousand scientists working with LIGO, the U.S.-based Laser Interferometer Gravitational Wave Observatory, and Virgo, an Italy based observatory, contributed to the ground-breaking discovery. Three of these researchers were Prof. Saul Teukolsky, physics and astrophysics, research associate Prayush Kumar and senior research associate Larry Kidder.

Gravitational waves were first detected in late 2015. The discovery won the chief scientists of LIGO and Virgo the Nobel Prize in Physics this year and confirmed yet another consequence of Einstein’s Theory of General Relativity.

Teukolsky was first notified of the newest gravitational wave observations by a text alert that was sent out to all scientists working on LIGO and Virgo. At the same time, gamma ray bursts were detected by space-based telescopes. According to LIGO scientists, these events only randomly coincide once in 80,000 years and thus, the scientists were fairly confident that the source was a cosmic collision.

Kumar, who says he’s “interested in exotic things in the universe,” said that his job was to help interpret the gravitational wave signal and pinpoint its source.

“We started with Einstein’s theory. The models we make can predict any kind of collision and our job is to find out which prediction fits the data the best,” Kumar said.

Gravitational waves were first detected in late 2015 as a consequence of two merging black holes. As a result, Kumar and other scientists began by examining the possibility that this observation was identical to those that they had already witnessed. The only problem was the fireball of light that accompanied the waves, something that merging black holes do not emit.

Kumar and other scientists came to the conclusion that the gravitational wave pattern indicated a neutron star collision, a kilonova. The smallest, densest stars known, neutron stars form during supernova explosions. Kumar says that approximately 130 million years ago, the two stars spiralled closer together and distorted the spacetime around them, emitting gravitational waves. Two seconds later, at the precise moment of collision, the two stars fused, releasing energy in the form of gamma rays.

Thousands of astronomers working on 70 ground and space-based telescopes then attempted to locate the source of the explosion. Because of several different factors, they only had an hour to find the light from among thousands or even millions of galaxies. That the signal seemed to come from one of Virgo’s blindspots helped scientists narrow down the location to 15 different galaxies.

So far most astronomical discoveries have been based on observing some wavelength of light. But with this discovery, astronomers now know that gravitational waves are another tool they have in their arsenal.

“Since Galileo, astronomy has used one form of energy at a time,” Kumar said. “This is the beginning of a whole new era of multi-messenger astronomy, where we can look at cosmic phenomena through different forms of energy at the same time.”

Future discoveries could help explain what happens to neutron stars after a collision and provide insight on the interactions between gravity and light, Kumar said. He is also looking forward to seeing a supernova explosion, which has not been directly observed yet.

Kumar’s contributions have also helped confirm a belief that scientists have long held, that heavy metals, like silver, gold and platinum are produced during neutron star collisions. Among others, NASA’s Hubble Space Telescope observed signatures of newly formed platinum and gold during the collision.

“Neutron stars are the highest density objects in the universe. A spoonful of star matter weighs as much as a mountain. To form metals heavier than iron, we need extreme density, pressure, and temperature and neutron star collisions provide these conditions,” Kumar said. “Earth is formed from stardust and this is where it’s manufactured.”