Science Spotlight: UCD Scientists Involved in Detecting Gravitational Waves from Fusion of Neutron Stars

Image: Artist's illustration of the fusion of neutron stars. Aoife Hardesty meets one of the UCD scientists behind the latest discovery in gravitational waves, and explains the importance of this discovery. On a clear night, if you were to travel outside of Dublin into the countryside, to an area relatively free of light pollution, and look up, you would be greeted with the inky dark night sky, dotted with billions upon billions of stars. Amongst the night sky you look at would be planets, galaxies, and invisible to your naked eyes, the answers to mysteries of the universe.Without stars, life as we know it would not exist. All the elements in the universe are made inside the cores of stars.Within stars’ cores, light elements fuse together to create heavier elements. For most of a star’s life, these fusion reactions consist of hydrogen fusing to create helium. As a star ages, the amount of hydrogen decreases as it has been used up and fusion occurs with helium atoms making heavier elements. Larger stars are capable of producing heavier elements through nuclear fusion. When stars explode, the elements are released and become part of the universe.We, and all we know, are made of stardust.As nuclear reactions in stars’ cores create heavier and heavier elements, the stars inevitably burst, and calculations have shown that stars reach exploding point before they would have been able to create heavy elements such as gold, platinum, and uranium. Scientists have theorised that such elements could be created by kilonova.A kilonova is the explosion caused by the fusion of two neutron stars, star remnants densely packed with neutrons.On Monday, October 16th scientists around the world announced they had witnessed a kilonova for the first time ever. Also witnessed was evidence of heavy elements forming as neutrons were released from the explosion and collided with nearby atoms.For the first time in the history of astronomy, scientists had detected gravitational waves from the fusion of two neutron stars. Previously gravitational waves had only been detected from the merger of black holes.Gravitational waves are ripples through space-time caused by cataclysmic events such as the fusion of massive bodies in space like neutron stars or black holes. They were first predicted by Albert Einstein in his theory of general relativity.Several UCD scientists were involved in the discovery including Professor Lorraine Hanlon, Associate Professor Sheila McBreen, and Dr. Antonio Martin-Carrillo. Dr. Martin-Carrillo told the University Observer why the discovery is so important.“In this case it is the first detection ever of two neutron stars merging.”
“It is the first detection ever of two neutron stars merging.”
Martin-Carrillo describes the importance of gravitational waves “imagine you put your finger on a pond of water. You start moving it around. As you move [your finger] you create waves and these waves start expanding out, and those are the gravitational waves. So while electromagnetic light is at the finger and tells what the finger is doing, the gravitational waves tell what the finger is doing to the space-time continuum.”“Gravitational waves have kicked off a brand new field of astronomy. We have detected five [gravitational waves] before ours… The first five detections have been from black hole mergers.”
“Gravitational waves have kicked off a brand new field of astronomy.”
This gravitational wave was detected on August 17th. Compared to other detections of gravitational waves, this one lasted much longer at 56 seconds compared to previous detections that had lasted less than 2 seconds. “This tells you that there is something different,” Martin-Carrillo explains, “and that [it is caused] by two objects of less mass, a black hole has much more mass [causing shorter detection periods].”Instruments set up across the globe; LIGO, Virgo, INTEGRAL, and Fermi all detected the gravitational waves. Hanlon and Martin-Carrillo are part of the INTEGRAL collaboration and McBreen is part of the Fermi collaboration.Due to the amount of instruments that detected the waves, it was possible to pinpoint the area of the sky from where the source emitting the waves by looking at the overlaps of areas of detection by the instruments.Martin-Carrillo explains how the wave source was found. “From that area we knew of about 50 galaxies that could host this event. So all we had to do was go to each of those galaxies and see if there was something new that had popped up. And in one of them, called NGC4993, we saw a [light] source that wasn’t there before.”“If you have two neutron stars [merging] you would expect to see light, and we saw it.” This is where Martin-Carrillo played his part in the discovery. The Watcher telescope used by UCD Physics in South Africa was down at the time “because of people breaking in and stealing stuff.” Martin-Carrillo had to convince colleagues at Boyden Observatory to observe a certain area of the sky and send him the data. “By Sunday I saw the light source for the first time, which later disappeared. I was one of the first few hundreds in the world to see [a kilonova] for the first time ever.According to Martin-Carrillo, this discovery has paved the way for “a new era of astronomy, using multi-messenger systems. Before we received messages from light, but now we can receive messages from gravity also.”As you gaze up at the night sky, remember this, and all the astronomical discoveries that have taken us one step closer to unravelling the mysteries about the universe, and our place in it.