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Posts Tagged ‘LIGO’

On Wednesday of last week (15 June 2016), it was announced by the LIGO-Virgo collaboration that they had made their second detection of gravitational waves. This follows the announcement made by the same team in February of the first ever detection of the waves predicted by Einstein 100 years ago (see my blogposts here and here about that).

The fact that LIGO has now detected two gravitational wave events in the space of a few months suggests that there will be many more; and really does highlight how we are opening up a whole new window on the Universe, as I have said before. To me, this is akin to the development of radio astronomy in the 1950s or X-ray astronomy in the 1960s, when new sources were being detected several times a year. Or, one could say, to the development of the telescope in the early 1600s.

I think that we can not only expect to see more and more detections coming from the LIGO-Virgo team, but also an increase in sensitivity of the detections as time goes on. Even with ground-based detectors I expect the sensitivity to increase, but once we start doing this from space (as ESA plans to do with eLISA), the sensitivity will increase hugely.

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The LIGO collaboration has detected a second emission of gravitational waves. This detection, announced last week (15 June 2016), was made on 25 December 2015.

As with the first detection, this second detection seems to be of two black holes merging. However, unlike the first event, which lasted about a tenth of a second, this event was about 1-second long. Also, whereas it is calculated that the two black holes in the September 2015 event (announced in February) had masses of 29 and 36 times the mass of the Sun, the black holes in this event had masses of 11 and 8 times the mass of the Sun. It is the lower mass of the two black holes in this second event which leads to the merger taking longer, as their orbits about each other would have been slower.

We really are living at a very exciting time, to be witnessing this whole new window on the Universe opening up.

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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.

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The abstract of the paper by Abbott et al. announcing the first ever detection of gravitational waves (from Physical Review Letters)

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.

My book "The Cosmic Microwave Background - how it changed our understanding of the Universe" is published by Springer

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

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!

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A major science story broke on Thursday (11th of February 2016) – the first ever detection of gravitational waves. I am currently in South America giving astronomy talks on a cruise, and therefore my internet access is very slow and very very expensive.

So, I will do a more detailed blogpost about this later next week, hopefully by Tuesday (17th). In the meantime, you can find out more about this very exciting announcement and what it means for astronomy by going to your favourite news website. From the little that I have been able to see, the coverage and explanations on the BBC’s science pages is hard to beat. 

More next week!!

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The European Space Agency announced last week (28th of November) that a space-based gravitational wave observatory will form one of its next two large science missions. This is exciting news for Cardiff University, as it has a very active gravitational waves research group headed up by Professor B S Sathyaprakash (known to everyone as “Sathya”). The Cardiff group, along with others in the Disunited Kingdom, played an important role in persuading ESA to make this observatory one of its next large science missions. Just over a year ago I blogged about some theoretical modelling of gravitational waves the Cardiff group had done.

The ESA plan is to launch a space-based gravitational wave observatory in 2034, which will have much more sensitivity than any current or even future ground-based gravitational wave observatory. Although NASA also had plans to launch a space-based gravitational wave observatory, there is currently none in existence, so this is really ground-breaking technology that ESA is announcing. From what I can understand, the current announcement by ESA is their commitment to the proposed LISA (Laser Interferometer Space Antenna) gravitational wave observatory. It would seem NASA has withdrawn their commitment to what was originally going to be a joint NASA/ESA mission.



ESA has chosen a gravitational wave detector to be funded.

ESA has chosen a space-based gravitational wave detector to be funded as one of its two key future missions. It should go into operation in 2034.



What are gravitational waves?

Gravitational waves are ripples in the fabric of space. They were predicted by Einstein as part of his theory of general relativity, the best theory we currently have to describe gravity. In his theory, events which involve extreme gravitational forces (such as two neutron stars orbiting each other (or merging), or the creation of a black hole) will lead to the emission of these gravitational waves. As they spread out from the source at the speed of light, they literally deform space as they pass by, just as ripples deform the surface of a pond as they spread out from a dropped stone.



An artist's impression of gravitational waves being produced around two orbiting neutron stars.

An artist’s impression of gravitational waves being produced as two black holes orbit each other.



Current gravitational wave observatories

There are several current gravitational wave observatories, all ground-based. These include VIRGO (in Italy) and LIGO (Laser Interferometer Gravitational Wave Observatory) which is in the United States. LIGO is the most sensitive of the current generation of gravitational wave detectors. LIGO actually comprises three separate detectors; one in Livingston, Louisiana and two in Hanford, Washington State. Each of the three separate detectors consists of two long arms at right angles to each other, forming a letter “L”. The idea behind these detectors is that, if a gravitational wave were to pass the detector, each of the two arms would have its length changed differently by the deformation of space as the gravitational wave passes through. Thus the detectors work on the principle of an interferometer, looking for tiny changes in the relative length of the two arms. And, when I say tiny, I mean tiny. In a 4km arm they are looking for changes of the order of 10^{-18} \text{ m}, or about one thousandth the size of a proton!



The principle of a gravitational wave detector.

The principle of a gravitational wave detector. They are essentially “interferometers”, with two arms at right angles to each other. As the gravitational waves pass the detector, space will be deformed and alter the relative lengths of the two arms.



LIGO (Laser Interferometer Gravitational Wave Detector)

Currently the most sensitive gravitational wave detector is LIGO. LIGO consists of three separate detectors, one in Livingston, Louisiana and two in Hanford, Washington State. The detector in Louisiana is shown below.



The LIGO detector in Livingstone, Louisiana. Each arm is 4km in length, and can detect changes in the relative length of the two arms of less than the size of a proton.

The LIGO detector in Livingstone, Louisiana. Each arm is 4km in length, and can detect changes in the relative length of the two arms of less than the size of a proton.



The detector in Louisiana, and one of the two detectors in Washington State, consist of two 4km long arms at right angles to each other. An event like the collapse of a 10 solar-mass star into a black hole is expected to produce a change in length in a 4km arm of about 10^{-18} \text{ m}, which is about one thousandth the size of a proton. This is just at the limit of the detection capabilities of LIGO, which is why astrophysicists are wanting more sensitive detectors to be placed into space. The other detector in Washington State has arms which are 2km in length, but just as sensitive as the detector with 4km arms at frequencies above 200 Hz, due to a different design.

LISA – Laser Interferometer Space Antenna

The ESA plans just announced will be based on the NASA/ESA plans for LISA, which have been on the drawing board for most of the last 10 years. ESA’s plan is to build two space-based interferometers, which will be in the form of equilateral triangles as this artist’s description shows.



An artist's impression of LISA, the "Laser Interferometer Space Antenna". Each interferometer will consist of an equilateral triangle, with each side 5 million km in length.

An artist’s impression of LISA, the “Laser Interferometer Space Antenna”. Each interferometer will consist of an equilateral triangle, with each side 5 million km in length.

The plan for LISA is to have arms which are 5 million km long! Compare this to the 4km long arms of LIGO. The changes in the length of a 5 million km long arm would be roughly one million times more than for LIGO, so rather than 10^{-18} \text{ m it would be } 10^{-12} \text{ m}, which should be well within the capabilities of the detectors. This means that less energetic events than the collapse of a 10-solar mass star into a black hole would be detectable by LISA. All kinds of astrophysical events which involve large changes in gravitational fields should be detectable by LISA, including the afore-mentioned creation of black holes, but also the merging of neutron stars, and even the merging of less massive stars.

But, possibly most exciting is the opportunity that gravitational waves provide to probe the very earliest moments after the Big Bang. With normal electromagnetic radiation (light, x-rays, infrared light etc.), we can only see as far back as about 300,000 years after the Big Bang. This is when the Cosmic Microwave Background Radiation was produced. Prior to this time, the Universe was opaque to EM radiation of any wavelength, because it was full of unbound electrons, and the photons would just scatter off of them and not get anywhere (see my blog here about the CMB). But, it was not opaque to gravitational waves, so they provide a way for us to see back beyond the CMB, and a unique way to learn about the conditions of the Universe in its earliest moments.

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