We should be grateful that we’re alive to see the birth of a new type of astronomy.
On 16 October 2017, astronomers from the LIGO-VIRGO collaboration announced that they had detected — for the first time — gravitational waves from the collision of two neutron stars. The detection was made two months earlier, on 17 August so the event was given the name GW170817. But the announcement was of something much, much more exciting than another observation of gravitational waves (tremendously exciting though that is in itself — it is, after all, only the fifth observation of gravitational waves!)
Information from all three LIGO/VIRGO detectors enabled astronomers to localise GW170817 to a patch of sky in the constellation Hydra. However, about 1.74 seconds after detectors were shaken by gravitational waves, the Fermi Space Telescope registered the detection of gamma rays from a gamma-ray burst — the event GRB 170817A, which was in the same patch of sky as the merger. The chances of observing GW170817 and GRB 170817A at the same time and in the same place were tiny — unless, of course, they were the same event!
Astronomers around the world were notified, and about 70 telescopes were trained on Hydra. And so, in the weeks following the merger, astronomers were able to observe the after effects at all the different electromagnetic wavelengths, from gamma rays and X-rays all the way down through visible and radio. This is the most studied event in the history of astrophysics!
It’s difficult to keep up with what this event tells us. To pick three items at random, from the combined observations we now know that:
- colliding neutron stars power short gamma-ray bursts
- elements heavier than iron, such as gold and platinum, form in these collisions
- gravitational waves travel at the speed of light
But it’s the promise of seeing more of these events that is so exciting, because they will enable us to learn so much more about the universe. A merger of two neutron stars is a “standard siren“, which gives us a new method of directly measuring cosmic distances — and thus of measuring the Hubble constant; we can use neutron star mergers to investigate relativity; we can learn much more about the behaviour of matter in these intense conditions; the possibilities are huge. We have now entered the era of multi-messenger astronomy — and it’s going to be wonderful!
On 27 September 2017, astronomers announced the fourth detection of gravitational waves. This was the first time the Italian-based VIRGO observatory has detected gravitational waves; the same waves, of course, were picked up by the two LIGO observatories.
The VIRGO observatory in Italy consists of two 3km-long arms, which together form an interferometer. (Credit: VIRGO Collaboration)
We are now entering the era of gravitational wave astronomy. When the number of observatories detecting a gravitational wave signal increases from two to three, scientists can glean much more information about the source. For example, they can gain information about the polarization of the wave. (It looks as if the general relativistic prediction about the number of polarizations is correct; Einstein wins again.) And, as the image below demonstrates, they can localise the source much more accurately. (Astronomers pointed 25 optical telescopes in the direction suggested by the LIGO/VIRGO discovery, but saw nothing – as was to be expected if the source of the gravitational waves was the collision of a pair of inspiralling black holes.)
When VIRGO works in tandem with the two LIGO observatories, astronomers can localise a source with much more accuracy. (Credit: VIRGO Collaboration)
So what was the source of this gravitational wave event? It seems to have been the merger of a pair of black holes, with mass 31 and 25 times that of the Sun, about 1.8 billion light years away in the constellation of Eridanus. The merger created a single black hole with a mass 53 times that of the Sun; thus three solar masses was radiated away in gravitational waves. That’s a huge amount of energy. LIGO and VIRGO caught a tiny fraction of it.
We now have data from four events. That’s too few to start drawing conclusions, but these four events are interesting. They all come from the merger of two quite large-mass black holes, as the table below shows:
||Mass of BH1 (solar masses)
||Mass of BH2 (solar masses)
As I made clear in my book New Eyes on the Universe, I was sure that gravitational waves from merging black holes would soon be found. LIGO found them slightly more quickly than I anticipated, but I wasn’t surprised by the announcement. But I am surprised at the masses of the black holes that are involved: these seem to me to be on the high side. As more events are discovered in the coming years, it will be interesting to see whether these four events are typical.
I have just finished checking page proofs of an article that will appear in the 2015 Yearbook of Astronomy. The article is entitled “Ripples from the start of time?” and it discusses what had the potential to be one of the most important and exciting cosmological discoveries in decades: B-mode polarization of the cosmic microwave background. Earlier this year, the BICEP2 experiment claimed to have found just such polarization and argued that their measurements could only be explained in terms of primordial gravitational waves – ripples of space made large by inflation, an event that took place when the universe was only a trillionth of a trillionth of a trillionth of a second old.
The BICEP2 team made a bold claim, and bold claims require a lot of solid evidence before they can be accepted by the scientific community. Soon after the team made their announcement of B-mode polarization, scientists raised doubts about the interpretation of the measurements (though not of the measurements themselves: everyone acknowledges that the scientists involved here are extremely capable astronomers). One problem was that the BICEP2 experiment observed the sky at only one frequency: when you have only one data point, any curve can be made to go through it. When you observe the cosmic microwave background you need measurements at several different frequencies before you can be sure that your signal really does come from the distant cosmos and not somewhere nearby. A second, more pernicious, problem was that it was not at all clear that the BICEP2 team had properly accounted for dust.
The issue is that dust grains in the galaxy can polarize light – and in particular it can give rise to a B-mode pattern of polarization. If the BICEP2 team had underestimated the amount of dust emission then their interpretation of their observed signal had to be under suspicion. It’s why I added a question mark to the title of my article: BICEP2 might have been seeing a signal from the dawn of time, but it might not.
A recent paper by the Planck collaboration suggests that the BICEP2 result might well have been the result of dust. It turns out that there is much more dust in the area of sky observed by BICEP2 than was originally thought. That in turn means that the BICEP2 results are entirely consistent with observations of dust. This doesn’t mean that B-mode polarization of the cosmic microwave background does not exist, nor even that BICEP2 didn’t spot such polarization; but it does mean that we don’t need to invoke inflation in order to explain the BICEP2 results.
A joint paper by the Planck and BICEP2 teams, due for publication later this year, should clarify the situation further. But at the moment it seems that our dreams of being able to look back to the very start of the universe must be put on hold. Shame. Those dreams were beautiful while they lasted