Researchers have announced that for the first time that they saw gravitational waves. They were generated by the collision of a pair of neutron stars. Although gravitational waves — or ripples in space-time — have been in the news since this year’s Nobel Prize in Physics, the latest announcement constitutes an exciting new discovery. Sparrho Hero, Miguel Zumalacarregui explains why.
We have just detected the first gravitational wave accompanied by a luminous signal, produced in the violent collision of two dead stars hundred million years ago. This newly discovered phenomenon will teach a great deal, from the formation of gold to the mechanism accelerating the universe’s expansion.
How is this different from other gravitational waves?
So far we have detected 4 gravitational waves coming from black holes. Not even light escapes a black hole, and hence no light signal accompanied those detections. This event is different, as it results from the collapse of neutron stars, two objects roughly as heavy as the sun but small as a city, produced after a massive star has exhausted its fuel. Neutron stars are incredibly dense but unlike black holes they’re still formed by matter. Light can escape, and a neutron star collision causes a spectacular explosion — also known as a gamma ray burst — that can be seen from distant regions of the universe.
A storm of light and gravity
We have observed an extremely violent process. Just as in a storm we see lightning and thunder, the embrace of neutron stars can be observed through light and gravity. For decades short bursts of gamma rays (very energetic light) from distant galaxies had been detected, but their origin remained elusive.
“This event is different, as it results from the collapse of neutron stars, two objects roughly as heavy as the sun but small as a city, produced after a massive star has exhausted its fuel […] Light can escape, and a neutron star collision causes a spectacular explosion that can be seen from distant regions of the universe.”
Thanks to the detection of gravitational waves we know now that these violent, short bursts are due to the collision of neutron stars: the gravitational wave brings detailed information about the last minute of these objects, in which they orbit over a hundred times before collapsing into each other.
Where did the gold come from?
A neutron star is much like a humongous atomic nucleus, heavier than our Sun and held together by gravity. Layers or the neutron star ejected during the collision rapidly decay into stable elements after escaping the gravity of the object. This process creates heavy elements including precious metals like gold, silver and platinum, while stars can create iron at most. An event like the one observed is estimated to produce about ten times the mass of the Earth in gold and platinum alone.
“This observation settles the issue of where these heavy elements come from: now we know that most of our jewellery was originally forged at a neutron star collision!”
How does the universe expand?
Normally we can’t say if we’re listening to a loud noise very far away or a quieter one closer to us. But having two signals helps a lot! The gravitational waves contain information about the neutron stars sizes and orbit, from which we can determine precisely how far they are. On the other hand, the light signal tells us how much the universe has expanded since the light was emitted. Putting all together we can measure the expansion of the universe with a completely new technique.
“…the light signal tells us how much the universe has expanded since the light was emitted.”
How fast is gravity?
Just as we can know if a storm is close of far away by timing the delay between the lightning and the thunder, we can learn about how fast gravity propagates by comparing the arrival time of light and gravitational waves. Both events have been observed simultaneously, confirming that both signals travel at the same speed, a basic prediction of Einstein’s general relativity. Because the signals took 100 million years to reach our detectors, the difference in speed, if any, has to be minuscule.
Gravity and the fate of the universe
Measuring the speed of gravity can teach us about dark energy: the mechanism causing the universe’s expansion to accelerate. This acceleration is weird because gravity is attractive: it tends to slow things down galaxies the same way we fall after we jump. Some dark energy models explain the acceleration by tweaking the properties of gravity.
“We can rule out that light and gravity [travel at different speeds] with a great degree of confidence”
Among those, some of the most interesting examples predict that light and gravity travel at different speeds (just like lighting and thunder), unlike what we have just observed. Thanks to this detection we can rule out these exotic models with a great degree of confidence.