October 19, 2017

Colliding Neutron Stars – New Era of Astronomy

August 17, 2017, was the night that the next chapter of gravitational wave astronomy was written. Earth’s gravitational wave detectors spotted a blip in the sky that came from two merging neutron stars. These stars were circling each other at close tot he speed of light and in their death throes released a huge amount of energy. Enough to make gold, platinum, lead, even shake up the fabric of spacetime itself.

NGC4993 from Hubble Space Telescope and Swope Telescope | D.A. Coulter

Whilst not the first time a gravitational wave has been sighted, what makes this event a sit-up-and-listen moment is that there was a gravitational wave… and then an electromagnetic wave. Specifically, a short gamma-ray burst. This is the first time that this event has ever been witnessed.
A few hours later, 130 million light years from Earth, a new kilonova appeared in the sky. This is like a nova but packs much more of a punch and is responsible for the production of a lot of the heavier elements on the periodic table. This event had never been observed before but has now been studied in detail.
The wide range of telescopes spread around Earth’s surface meant that this event could be studied, from the first gravitational wave to the last red-end-spectrum electromagnetic waves. Until now, the only gravitational waves that we have observed have come from black holes. Since black holes do not emit any light, there are no electromagnetic signals that go with the gravitational wave. This is why this neutron star event is so important as scientists can use the gravitational and electromagnetic wave information to build a more-complete picture of what is happening in the collision.

Why do Gravitational Waves arrive before Electromagnetic Waves?

Gravitational waves were seen first, followed by the electromagnetic (EM) waves. This does not mean that gravitational waves move faster than the speed of light, rather, that they can escape the explosion more quickly.
Think of a panda and a rat making their way through a dense bamboo forest – the panda has to push its way through dense trees, but the rat can scurry between with relative ease. They can both move at the same speed, but there are fewer barriers impeding the rat so it can escape earlier. (This isn’t the best example since it implies that gravitational waves are smaller than EM waves, and that EM waves have a penchant for bamboo).
The implication is that gravitational waves will expand the horizon of our universe. The cosmic microwave background (CMB) is considered the furthest sphere of information that we can see – the earliest light in the universe. Now, we can stretch this dome a little further out by looking at gravitational waves. We may one day be treated to a new map, the cosmic gravitational wave background (CGB).

r microwave sky – NASA / WMAP Science Team WMAP # 121238 Image Caption 9 year WMAP image of background cosmic radiation (2012)


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