Cosmic archeology with gravitational waves

It has been a while that I have not discussed gravitational waves. Triggered by a scientific article that I have read last week, here we go again! This article can be found here, and one may immediately notice the reason why it triggered my curiosity: it has an interesting title. We had by the way some fun about it on the steemSTEM chat as well.

Cosmic archeology is just a fancy way to say that one targets the study of the early universe.

In the same way archeology addresses what was going on in the early days of the human race (and later too, but this is a kind of detail for this post), cosmic archeology focuses on what was going on in the early moments of the universe.

Nothing more, nothing less.

As archeology is a science associated with a dedicated set of tools allowing to dig into the old days of human history, cosmic archeology comes with a variety of tools allowing physicists to study what happened billions years go.

Gravitational waves consist of one of these tools.

_{I hope I didn’t say wrong things about archeology… I am sure that @zest will correct me heavily otherwise :p}

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GRAVITATIONAL WAVES IN A NUTSHELL

Let us start with a definition. Gravitational waves are small ripples distorting spacetime that are generated by ‘catastrophic’ events in the universe.

In other words, once produced, gravitational waves slightly distort spacetime during their propagation, in the same way ripples can be observed when one throws a stone in water (as illustrated on the picture).

Gravitational waves consist of a prediction of the theory of general relativity proposed by Einstein in the first part of the 20^th century.

They have been observed for the first time exactly 100 years later by the LIGO and VIRGO collaborations, and the observations were a sign of the merging of either two black holes or of two neutron stars.

And this detection yielded a Nobel prize some weeks ago :)

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THE CHALLENGE: DETECTING GRAVITATIONAL WAVES

Let us now come back to one point.

I have mentioned in the previous section that one needed catastrophic events in the universe to get some gravitational waves that are observables.

This stems from the fact that significant gravitational interactions are needed to get anything detectable on Earth.

In other words, we need events involving the gravitational interactions of super-massive objects like black holes or neutron stars to get something noticeable.

And even in this case, the ripples in spacetime that are generated are extremely small. When two huge black holes or neutron stars merge, one gets tiny variations of spacetime of about 10^-19m.

This is is equal to. 0.0000000000000000001 meters, and this is the variation of spacetime that one needs to measure! The good news is that we managed to do it! Humans are sometimes suprizing, in particular when interferometry is used.

After a catastrophic event takes place in the universe, one needs to wait a little bit before being able to observe it here. The reason is that we need to give to the gravitational waves enough time to propagate.

Like electromagnetic radiation, gravitational waves travel at the speed of light. It will thus take some time between the moment the waves have been emitted and the moment they are recorded. Traveling in space indeed takes time…

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DETECTING GRAVITATIONAL WAVES - THE BASICS

Gravitational wave detection relies to interferometry. One of the LIGO detectors (we have two of those, located in places separated by a couple of thousands of kilometers from each other) is shown in the picture below. In addition, we also have the European VIRGO detector that works in the same way.

Three is the magic number ^^

A specific gravitational wave detector is made of two arms (as shown in the picture), in which a laser beam circulates in a system completely disconnected from the external world.

This allows one to avoid, or at least reduce the noise that would plague the measurements.

This external noise is also the reason why the detectors are located in very remote place, to get rid of any vibration that would originate from a too intense human activity.

As a result, the apparatus is shielded enough to be able to detect modifications of 0.0000000000000000001 meters, which matches the signal that would be expected from a gravitational wave.

In order to be sure that a record is not a local event, we need at least two detectors so that we could rely on coordination.

As gravitational waves travel at the speed of light, the signal associated with a real gravitational wave should be simultaneously recorded by all detectors (in contrast to the vibration issued from a truck passing close to one of the detectors).

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DETECTING GRAVITATIONAL WAVES IN PRACTICE

In practice, the principle behind the detection of a gravitational wave is pretty easy. We can start with the picture below, where the red lines describe the path of the laser beam emitted by the laser source LD.

The laser LD starts by hitting a beam splitter (the diagonal thick line in the middle of the picture) so that we get two beams issued from a single one. They circulate each in one arm of the detector (that has an ‘L’ shape).

One beam goes straight to the target mirror, and the other one is deviated to the reference mirror. The distance traveled by the beam is enforced to be identical.

When hitting the respective mirrors, the two beams are reflected and travel back to the splitter. The splitter however acts this time as a merger, and the merged beam is finally deviated to a detector PD.

If the two beams travel the same distance, the PD detector records a single dot (as when one would point a laser to a screen).

However, if the spacetime is distorted by a gravitational wave on the path of the beams, one of them would travel a larger distance than the other. We would consequently observe an interference pattern on the detector, i.e., a succession of bright and shaded areas.

From this, we can derive the initial distortion at an unprecedented precision, and actually measure variation of space time of 0.0000000000000000001 meters.

In the real life, the things are slightly more complicated, but the key principle is as sketched above.

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REFERENCES AND DISCUSSION: COSMIC ARCHEOLOGY AGAIN

It is now time to go back to the topic. Gravitational waves issued from the merging of two black holes or two neutron stars have been recorded so far. But those are not the only events that are expected to generate waves.

Gravitational waves can actually be used as tools for testing the physics of the early universe.

In standard cosmology, many things that have happened during the very first moments of the life of the universe are expected to produce gravitational waves.

This includes, for instance, inflation and cosmic strings (to go back to the title of the paper which I have initially mentioned in this post).

In the cosmic string scenario, the universe is somehow populated by a network of strings, made of both long open strings and closed loops of strings. Those strings are extremely thin but they are very dense.

Therefore, they can be seen as a potential source of gravitational waves.

Thanks to the future gravitational wave experiments, physicists expect to get data to constrain these cosmic string scenarios through their capacity of producing gravitational waves. As a consequence. we could hence get more insight on the early times of the universe.

More information can be found here, as well as here and there for the gravitational wave detection articles. Plenty of information on LIGO is available here, and on VIRGO there.

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