Einstein Telescope: a new-generation gravity wave detector

3r3-31. Longer, more powerful, more precisely - Europe is going to build a new generation of gravitational-wave detector called 3r3188. Einstein Telescope [/b] .
Einstein Telescope: a new-generation gravity wave detector  
Einstein Telescope concept art, credit: www.gwoptics.org
The AdvancedLIGO detector just started working a couple of years ago, and has not even reached its planned sensitivity yet. However, it is obvious to scientists that the sensitivity of LIGO will not be enough for real gravitational-wave astronomy.
I will talk about what limits LIGO, and how an underground cryogenic detector 2.5 times longer than LIGO can bypass these restrictions.
KAGRA will join the observations next year. [/i]
So, the new detector will be located underground. This will reduce seismic noise ? and, most importantly, Newtonian noise 2:
the main contribution to it is caused by surface waves, which are practically not underground.
Depending on where the detector will be built (now there are two main options - in the Netherlands or in Sardinia, and possibly in Hungary).
Seismic comparison in different possible locations with AdvancedVirgo detector in Italy.
Of course, the most obvious technical steps to suppress seismic will be taken: a new suspension system for passive isolation and heavier mirrors of 200 kg each to suppress all power noise.
One of the corner stations of the Einstein telescope with a variety of vacuum chambers. Credit: gwoptics.org
The problem of thermal noise of mirrors is more complicated. The obvious solution would be to cool the mirrors, thereby reducing the Brownian noise.
However, cooling will lead to a change in the optical properties of the mirrors, and increase the absorption. In addition, with cold mirrors it is impossible to use large light powers: the absorption in the mirrors will heat them up and reduce cooling to nothing. That is, you need to cool the detector and reduce the power of the light? It also does not work out this way - the shot noise will increase (4), and will spoil the sensitivity at low frequencies.
Scientists have come to another solution: use two interferometers in one place.

"Xylophone" detector configuration with two interferometers nested into each other. Credit:[1]
One will be optimized for low frequencies, work with mirrors cooled to 20K, and use low light power. Shot noise will increase, but the detector will not be used at frequencies where shot noise is important. The second detector will operate at room temperature at high power: this will suppress shot noise at high frequencies, but will spoil the sensitivity at low frequencies with increased radiation pressure noise. But this detector will not be used at low frequencies. As a result, the combined sensitivity will be optimal at all frequencies.
ET-D-LF low-frequency detector with chilled mirrors and low power (and low radiation pressure noise), and high power ET-D-HF high power (and low shot noise). Credit:[1]
Another problem with the new generation of detectors: at the time of construction, it will be the only one with such sensitivity. First, it will not be possible to distinguish a random burst from a signal if it is not possible to check the coincidence between the detectors. Secondly, it will not be possible to measure different polarizations of gravitational waves. Scientists propose to build not one detector, but three with different orientations (in the form of a triangle, as in the picture).

The concept of a triangular detector configuration (left); tunnels with different shoulders (right).
This will improve the detector's radiation pattern and record much more events: 3r3452.  
Comparison of the radiation pattern of one detector (left) and three detectors in a triangular configuration (right).
Let me remind you that each of them will consist of two: one for low frequencies and the other for high frequencies. As a result, six detectors will be located a triangle.
All these tricks will increase the sensitivity of the detectors by at least an order of magnitude.
Such sensitivity will allow to increase the observation range practically to the border of the visible Universe, to see the BH fusions of the first generation of stars and to observe the fusions of black holes and neutron stars constantly.
Increasing the sensitivity at low frequencies will allow one to observe the earlier stages of fusion of objects, and obtain more information about their parameters.
High frequencies will allow to observe the evolution of a black hole or a neutron star formed as a result of a merger. This mode is most interesting for checking GTR and possible alternatives. For example, 3r3327. gravitational wave echo
can be observed at high frequencies.
Comparison of the sensitivity of ET and LIGO-Virgo
But the most important thing is that it will be not just a detector, but a whole infrastructure that will allow the detector to increase its sensitivity for many decades.
4. Conclusion 3r33347.
3r3402. What I did not mention
I have not yet discussed such an important part of ET as a system for suppressing quantum noise using frequency-dependent squeezed light. You can read more about compressed light in 3r33354. excellent article on Habré
. I plan to tell you more about the quantum noise in the detector in the next article.
In addition, the ET will use the so-called optical rigidity - signal amplification due to the nonlinear interaction between the mechanical oscillator and the light inside the resonators. Read more about quantum optomechanics - the science of the interaction between mechanical systems and light - soon on a habre;)
Of course, I touched on only the most basic features of ET, there are a lot of details - welcome to the comments.
In addition, I did not mention that in the USA it is planned to build an even longer 40km ground-based telescope, 3r-3368. Cosmic Explorer
, but its design is still less developed than the ET, so I will not tell any interesting details.
3r3402. Einstein Telescope status
Currently, ET has not yet received the approval of the European Commission. Individual countries invest in preliminary research. Collaboration is gradually being formed. You can read official website 3r3451. and even join the collaboration by signing 3r33385. Letter of Intent .
According to the plan, in the next year or two, Europe will consider the application for creation and approve the location. The launch of ET in this case will occur in the early 2030s.
One of the options is a triangle on the border of Germany, Belgium and the Netherlands, located so that in each country there will be one corner station. It will be a symbol of a united Europe.
3r3402. News LIGO
In the meantime, LIGO announced the results of r3r3451. data processing from the previous observational cycle of O2: there were four more new mergers of black holes. Thus, for all the time, LIGO has already seen 10 mergers of black holes and one merger of neutron stars. Tomorrow all the data will be officially presented, and I will add some details to the article.
In the meantime, the detectors are being updated to increase their sensitivity, and in the spring of 2019 the launch of detectors in the new O3 annual observation cycle is scheduled. The sensitivity will be so great that it is planned to observe an average of one event per week. In the summer of 201? according to the plan, a Japanese KARGA detector will join the two LIGO detectors and the Virgo detector.
This O3 cycle will be interesting for open science, since now all potential candidates for mergers are 3r33416.
will be announced. in real time, together with an estimate of their source, which will allow all those interested to make observations in other ranges. Read more here 3r3451. .
The era of gravitational-wave astronomy is just beginning, there is a lot of interesting things ahead. Stay tuned!
I also invite you to read previous publications, where I tell you, what makes the observation
so important. neutron stars in the GW
, 3r33430. what an interesting physics r3r3451. allow us to study the fusion of black holes, and how in general LIGO can work if GW stretch the light along with the space.
References [/b]
[1]S. Hild Beyond 2nd Generation GW detectors
[2] ET Design Study
[3]3r33450. LIGO Open Science Data Center.
3r33434. 3r33434. 3r33434.
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