It’s been a busy few weeks for LIGO and Virgo. Not only did the two interferometer observatories announce a fourth detection of gravitational waves, three of their scientists won a Nobel Prize in Physics for their efforts.
Now, via a press conference in Washington, the LIGO (Laser Interferometer Gravitational Wave Observatory) and Virgo teams, alongside researchers from 70 organisations, have announced their latest “unprecedented discovery” in the form of a new source of gravitational waves and an astronomical object called a “kilonova” – the cosmic explosion of two colliding neutron stars.
The kilonova was spotted using a burst of gravitational waves detected on 17 August 2017. Following the detection by the LIGO and Virgo observatories in California and Italy, astronomers began pointing telescopes at the possible source and found the object in a nearby galaxy called NGC4993, 130 million light years from Earth.
This is the first time gravitational waves have been detected following the collision of neutron stars and gives unparalleled insight into our understanding of the Big Bang theory, the universe and even gravity.
“It doesn’t get more exciting that this for an astronomer,” Bob Nichol, director of the Institute of Cosmology and Gravitation (ICG) at the University of Portsmouth and member of the Dark Energy Survey (DES) said. “At sunset, the DES team was ready to scan the position of the gravitational waves for a new source”.
The Dark Energy Survey (DES) is an international effort to map hundreds of millions of galaxies, detect supernovae, and find the mysterious dark energy said to be driving the expansion of our universe.
(Source: ‘Neutron Star Merge and the Gravity Waves it Produces’, NASA Goddard Space Flight Center)
According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet astronomers in 1998 found that the expansion of the universe is, instead, accelerating. There are two possible explanations – either 70% of the universe exists in an “exotic form”, referred to as dark energy that can be seen by how its gravitational force impacts visible objects, or General Relativity is wrong and needs to be replaced by a new theory of gravity.
Neutron stars and the kilonova
Rumours of gravitational waves being detected by a new source, namely neutron stars, started circling ahead of last month’s announcement.
Neutron stars are produced when the cores of giant stars (10-29 solar masses) collapse, and the mass’s protons and electrons merge to form neutrons. Unlike gravitational waves borne of black hole collisions, the merging of neutron stars produce visible light, in turn giving scientists unprecedented observational capacity.
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A kilonova has been theorised for many years as the merger of neutron stars, but this is the first time such an event has been observed.
Immediately after the detection, astronomers studied the kilonova event from across the electromagnetic spectrum from gamma-rays to radio waves. The gravitational wave source was named GW170817, and the optical source was named Swope Supernova Survey 2017a (SSS17a). Within a week, the source had faded and could no longer be seen in visible light but while it was visible, astronomers could gather a treasure trove of data on this phenomenon. ICG researcher Chris Frohmaier was part of a team led by Caltech and Berkeley who wanted to know the rate and light-curves of this kilonova, in particular how many we expect in the local universe, and how the explosion changed over time.
Their work, published today in the journal Science, led to an unexpected solution to a long-standing problem; kilonovae could be the source of half the heavy elements in the universe. There are a total of seven papers published on these findings.
Hunting the universe’s missing elements
The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram
It is well established that chemical elements up to iron in the periodic table are produced either in the Big Bang, in the cores of stars, or in supernovae explosions. However, the origin of half the elements heavier than iron, including gold, platinum and uranium, has remained a mystery.
“Someone in the team noted that if you multiply the rate of kilonova events expected, with the yield of heavy elements like uranium, gold and platinum per explosion, then you obtained a rather large number, basically enough to explain half the abundance of such elements in the universe,” explained Frohmaier.
A team from UC Santa Cruz found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope
“It’s one of those eureka moments” added Frohmaier. “It’s fantastic when such different areas of astronomy just come together. I started out studying the rates of kilonova and we found half the gold in the universe!”
The discovery of the kilonova and the fact that gravitational waves can lead astronomers to such events opens a new window on the Universe. The combined observing strength of LIGO+Virgo and other telescopes will allow astronomers to discover further strange phenomena and solve remaining mysteries of the Universe.
“The press conference began with an overview of latest findings from LIGO, Virgo and partners that span the globe, followed by details from telescopes that work with the LIGO and Virgo collaborations to study extreme events in the cosmos. Among those speaking on the panel were LIGO’s executive director, David Reitze, Fermi Project scientist at NASA’s Goddard Space Flight Center, Julie McEnery, and spokesperson for the Virgo Collaboration, Jo van den Brand.
What are gravitational waves?
Gravitational waves occur on an infinitesimal scale whenever there are gravitational interactions, but the easiest to detect are when explosive clashes happen in the universe. Until now, this has been predominantly seen when black holes collide. The waves are essentially large-scale ripples in space-time, and they provide us with an entirely different way of perceiving the universe.
They were first predicted by Albert Einstein in 1905 and form part of his theory of special relativity and general relativity published a year later. This theory has been used to explain how objects behave in space and time and has been used to predict the existence of black holes, to gravitational lensing and even Mercury’s orbit.
When it comes to gravitational waves, Einstein’s theories claim that “the most powerful processes” in the universe can create ripples in the curvature of space-time that travel outward from the source that created them, and these ripples move across the universe at the speed of light.
When were gravitational waves first detected?
Gravitational waves were first spotted in data analysed by LIGO in September 2015; these space ripples were produced when, 1.3 billion years ago, two black holes collided. It then took six months for the researchers working with LIGO to confirm they had spotted these gravitational waves, and the groundbreaking announcement was made in February 2016. A second set of waves was confirmed in June the same year.
The initial waves were the result of two black holes, 36 and 29 times the mass of our sun, crashing into each other to create a spinning black hole 21 times the mass of our sun. The second set of waves were created when two black holes, this time eight and 14 times the mass of our sun, collided.
Since these initial findings, scientists have detected a further two gravitational wave events, both of which were similarly caused by the collisions of black holes.
What is LIGO?
The 2015 detection and subsequent 2016 papers involved more than 90 institutions across 15 countries, including MIT and Caltech, working with LIGO data and its interferometers are spread across two sites in the US, in Washington and Louisiana.
LIGO is essentially a large-scale physics experiment, where astrophysicists convene to monitor and observe gravitational waves. It is funded by the National Science Fund (NSF), and holds the distinction of being the largest and most ambitious project to be funded by the body.
The Advanced LIGO project, which began shortly after the first detections were made in October 2015, features an interferometer with two 2.5-mile-long “arms”. Laser beams are passed across these arms and hit mirrors at each end. Changes in the way these beams travel along these arms generate patterns that hint at the presence of gravitational waves, while changes in these patterns can suggest different types of gravitational wave source.
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For the recent, fourth detection (announced last month), the signal was detected by the two advanced LIGO sensors in the US, and, for the first time, by the Virgo detector near Pisa, Italy.
Virgo isn’t as sensitive as LIGO but the combination of all three magnified the precision by a factor of 10. Virgo was completed in 2003. It features a €300 million interferometer with 2-mile-long arms and was funded by the French National Center for Scientific Research (CNRS) and the Italian National Institute of Nuclear Physics (INFN).
Researchers from the LIGO and Virgo projects joined a data-sharing agreement in 2007 and then in August, Virgo joined LIGO in the search for gravitational waves.
Why should I care?
Following the first detection last year, we put together a list of six reasons why you should be excited about gravitational waves. And you really should. But astrophysicist Katie Mack sums up the significance of this latest discovery much more succintly than we can:
Images: Murguia-Berthier et al., Science/Hubble/UC Santa Cruz & Carnegie Observatories/Ryan Foley
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