A new window on the Universe
Last Thursday (11 February 2016) the very exciting news broke that scientists had directly detected gravitational waves for the very first time. I briefly blogged about it here, but now that I am back from giving talks in South America I have a little more time (and a much better internet connection) to write a more complete blogpost about it.
This direct detection, hopefully the first of many, was made by two international teams (including colleagues of mine at Cardiff University) using the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is in the United States. The link to the actual paper, which appears in Physical Review Letters, is here. As you can see from the abstract of the paper (see below), the signal was detected by both LIGO detectors simultaneously, which is a very important point as it strongly suggests that the detection is real.If you read the last sentence of the abstract, you will notice that it states that not only is this the first ever direct detection of gravitational waves, but is also the first ever observation of two black holes merging. This illustrates nicely that being able to detect gravitational waves opens up a whole new way of learning about the Universe; so this detection marks a significant step forward in our capabilities to “observe” our Universe. In my mind, it is as significant as William Herschel’s accidental discovery of infrared light in 1800, which was the first ever detection of radiation outside of the visible part of the spectrum.
What are gravitational waves?
Gravitational waves were predicted by Albert Einstein in 1916, as a natural consequence of his new theory of gravity – general relativity. Unlike Newton’s theory of gravity, which argued that gravity acts instantaneously, Einstein’s general theory of relativity predicts that gravity is due to a bending of spacetime. The bending (warping) is produced by masses (e.g. the Sun, black holes, galaxies, clusters of galaxies); and changes in any gravitational field travel through space at the speed of light from the place of change as ripples , distorting spacetime as they spread outwards.
Although predicted one hundred years ago by Einstein’s theory, up until now there has only been indirect evidence for gravitational waves. The best example of indirect evidence is from observations of pulsars, spinning neutron stars. Back in 1974 it was noticed by Russell Hulse and Joseph Taylor that the period of a particular pulsar, PSR B1913+16 (which happens to be one of a pair of orbiting neutron stars) was slowing down. It was argued that this was due to the neutron stars spiralling towards each other and losing energy, with this energy being taken away in the form of gravitational waves. The calculations matched, the loss of energy being measured agreed with the predicted energy which should be carried away as gravitational waves. Hulse and Taylor received the 1993 Nobel Prize in Physics for this work.
Since then, there have been other similar observations, all agreeing with the predictions of general relativity; but until now there had been no direct observations of the elusive gravitational waves.
‘Seeing’ back to the beginning of time
The cosmic microwave background, the subject of my recent book, comes about from when the Universe became transparent.But, when I say ‘transparent’, I mean transparent to electromagnetic (EM) radiation. Gravitational waves are not EM radiation, so the opacity of the Universe before about 400,000 years into its existence does not apply to gravitational waves. These should be detectable back to the very tiniest fraction of the first second, a time when gravity and the other three forces separated. The ‘surface of last scattering’ (from where the CMB comes), effectively acts as a wall to EM radiation, but it is transparent to gravitational waves.
We have a very long way to go before we are able to detect gravitational waves directly from anything but nearby objects. But, the potential is there at some point in the future to be able to use gravitational waves to probe back to the beginning of time. That is a truly exciting possibility!