How we improve the detection of gravitational waves

• The first gravitational waves - which confirm, among other things, the existence of black holes - were detected five years ago, according to our partner The Conversation.

• The two instruments that “pick up” the signals of these phenomena from Earth are constantly evolving, and they will soon be joined by two more.

• The analysis of this phenomenon was carried out by Nicolas Arnaud, CNRS researcher at the physics laboratory of the two Infinites Irène Joliot-Curie of the University of Paris-Saclay.

It's not every day that we detect a new messenger capable of teaching us more about the Universe and the extraordinary stars that make it up!

This is why, when in 2016 and then 2017 gravitational waves were added to electromagnetic waves, cosmic rays and neutrinos as a means of observing the cosmos, the scientific community was enthusiastic.

And his excitement was fed by the discoveries then announced: confirmation of the existence of black holes with the first detection called GW150914, then birth of "multi-messenger astronomy" with gravitational waves thanks to the event GW170817.

VIDEO: 

Five years later, what is it?

What has happened since and what are the prospects for the next few years?

This article proposes to answer these questions from the angle of gravitational wave detectors.

A worldwide network of detectors

The first detections of gravitational waves represented the culmination of a scientific adventure of almost forty years.

A culmination of course, but also the beginning of a new phase, as scientifically exciting as the first, and rich in many technological challenges.

It is a question of making gravitational waves a tool of observation of the universe in its own right, that is to say to multiply the detections and to analyze the signals collected in a global way, to draw from them all the possible knowledge on the sources that issued them.

To date, all the detections have been carried out by giant terrestrial detectors forming a global network whose data are pooled and analyzed together by the international collaborations that have built and operate these extraordinary instruments.

The various terrestrial detectors of gravitational waves.

LIGO India is a project that should materialize in the next few years © Caltech / MIT / LIGO Lab (via The Conversation)

The two LIGO detectors are located in the United States: in Hanford, Washington, and Livingston, Louisiana.

In Cascina, in Italy near Pisa, the site of the European Gravitational Observatory (EGO), hosts the European detector Virgo.

In Japan, the KAGRA underground detector, newer than LIGO and Virgo, is preparing to join the network.

Finally, the project for a third LIGO detector located in India should materialize in the next decade.

Drive out the parasitic "noise"

Whatever the field, a detection always amounts to bringing out a “signal” from a parasitic “noise” which pollutes its measurement by covering it, in part or completely.

The reconstruction of the signal part depends on the design of the instrument and the processing of the recorded data.

But let's focus on the “noise control” part, which has underpinned the evolution of gravitational wave detectors for more than 25 years.

Everyone who has held a radio in their hands has experienced adjusting the orientation of its antenna to replace a painful crackling sound by the music played by the station being sought.

The same goes for the giant Virgo or LIGO detectors.

Their data permanently contains noise, the level of which has to be controlled, to better understand the origins and, then, to reduce it as much as possible, in stages.

It starts from the design of the switchgear by selecting its components and assembling them with care - to build the equivalent of a "(very) high fidelity" radio set.

Between data acquisition and improvements

Then, once the instrument is assembled and functional, it is a question of adjusting it as best as possible, so that it is as efficient and as stable as possible.

A detector is not frozen in time, far from it: we can - and we must - improve it to detect more and detect better.

But these actions are often invasive: major modifications to part of the detector, changes of parts, etc.

They are therefore not compatible with data taking during which the instrument must operate continuously with intervention times reduced to a minimum.

This is why LIGO and Virgo have alternated in recent years improvement phases and data collection.

Each interruption potentially causes gravitational waves to fail (since these continue to arrive from the cosmos in a random fashion) but the goal is that the progress made during a stop is such that the following data acquisition makes it possible to make up for "lost time" - by racking up many more detections than if the previous data collection had continued on the same bases.

More sensitive and stable detectors

At the end of August 2017, the two detectors LIGO and Virgo closed their 2016-2017 data collection campaign.

The shutdown that then begins will last about twenty months, until the end of March 2019. This long period is used to improve the three tools, both in terms of their sensitivity and their stability.

A more sensitive detector makes it possible to highlight weaker signals: either new sources still unknown, or sources of a type already known, but more distant - the amplitude of a gravitational wave on Earth decreases with distance from the source.

And a more stable instrument allows not only longer and more regular data taking, but also finer tuning of its operation - scientists have more leeway to study it - which further improves its sensitivity.

In short, a virtuous circle of progress that responds and is linked.

Significant results for the period 03

On April 1, 2019, a new LIGO-Virgo data acquisition begins, called “O3” (for “Observation Run 3”) and scheduled for 12 months in total.

A first six-month campaign lasts until October 1, 2019. It is followed by a one-month shutdown for the three detectors, in order to make some adjustments and tests incompatible with an intensive data-taking phase.

Then by a second six-month campaign, also started on November 1, 2019. This data acquisition will only last five months: it was interrupted on March 27, 2020 because of the progression of the global Covid pandemic -19 and associated restrictions, decided in many countries.

A summary of the progress of the Virgo detector between summer 2017 and the end of O3 data collection in spring 2020. The more sensitive a detector, the more it will be able to see distant gravitational wave sources. This property is exploited in the graph above which shows the evolution over time of the "record detection distance" of Virgo for two neutron star fusions - the same type of source as in the case of GW170817. The vertical axis uses a unit adapted for astronomy: the megaparsec which is worth approximately… 31 billion billion kilometers! In the summer of 2017, Virgo had a record detection distance of around 28 Mpc. From the start of O3 (April 2019) it rose to 50 Mpc, an increase of nearly 80%. And a year later, the 60 Mpc was reached:i.e. 20% additional increase during O3 (an impressive result given the little time available for testing when taking data) and a value ultimately more than doubled compared to 2017 © La Collaboration Virgo (via The Conversation)

Even though the O3 data taking ended prematurely, it was a great success on all fronts.

Experimentally, this is the first time that three such powerful detectors have taken data together for such a long period.

All of them have improved their sensitivity compared to previous campaigns and have succeeded in maintaining this quality over time.

Links to the most significant scientific results of the O3 period and associated French educational resources are available online.

In particular, there is information on the most recent update of the gravitational wave catalog which now contains 90 signals recorded over the period 2015-2020.

Distribution (in percentages) of the time spent by the Advanced Virgo detector in different states while taking O3 data. The most important part - in green on the circular diagram and labeled "science" - indicates the duration of the data collection: 76% of O3, or about 251 full days spread over 11 months. The second major part - the “locking”, in blue, a little over 7% of the total - counts the periods during which the detector was being adjusted to bring it into its “science” configuration. Around 7% also, we find then the gray part which includes all the moments when the detector had a problem. Finally, the remaining 10% - “commissioning”, calibration and maintenance - bring together all the activities carried out regularly on the detector, to maintain or improve it.This graph shows that the efficiency of the Virgo detector was already high during O3. The coordination between the LIGO and Virgo detectors made it possible to optimize the efficiency of the network: at least one of the three detectors recorded data almost 97% of the time and, during the second part of the data acquisition, the three instruments worked together more than half the time! This last configuration is the best to locate precisely in the sky the position of a source of gravitational waves © La Collaboration Virgo (via The Conversation)during the second part of the data collection, the three instruments worked together more than half the time! This last configuration is the best to locate precisely in the sky the position of a source of gravitational waves © La Collaboration Virgo (via The Conversation)during the second part of the data collection, the three instruments worked together more than half the time! This last configuration is the best to locate precisely in the sky the position of a source of gravitational waves © La Collaboration Virgo (via The Conversation)

Next step in mid-2022

The end of the O3 period marked the start of a new cycle with - as you have probably already understood - another improvement phase (in progress) preceding a new data taking (in the future) called… O4 !

This program is currently underway, despite complications still related to Covid-19.

Difficult to move forward when researchers, engineers or technicians are confined and, like everyone else, undergo the effects of the pandemic, when all activity requires a specific protocol to respect distancing and barrier gestures and when equipment orders are delayed by the suppliers, themselves subject to the same constraints!

Thus, access to EGO has been limited for over a year and a half and, for a long time, staff whose activity could not be done other than face-to-face had priority to come to the site - while their colleagues were invited to telecommute.

Two complementary aspects of the Advanced Virgo Plus Phase I large-scale improvement program. On the left, the fine-tuning of a component of the detector in situ while maintaining drastic cleanliness conditions;

on the right, a civil engineering operation to make room for a new piece of equipment along one of the two 3 km "arms" of the detector © photo on the left: IFAE - Barcelona (via The Conversation);

right photo: EGO / Fabozzi (via The Conversation)

The current Virgo detector development program has been named “Advanced Virgo Plus Phase I”. Without going into semantics, it is interesting, before concluding, to decipher this title to better understand what is behind it, both in terms of the ambitions and the scope of the project.

" 

 ": The current LIGO and Virgo detectors are second generation instruments - therefore "advanced".

During the 2010s, they replaced the original detectors built and used in the previous decade.

From the start of these projects in the 1990s, it seemed probable that these first instruments would not be sensitive enough to discover gravitational waves.

But they were needed to demonstrate the feasibility of this technology and pave the way for advanced detectors that would have a better chance of achieving this goal.

“ 

 ”: the improvements made to the Virgo detector between summer 2020 and spring 2021 are very substantial and have almost given rise to a new tool.

" 

 ": the program is in fact spread over more than five years and a second wave of improvements is planned after the period O4.

Even if this phase will begin at best in two years, it is now that it is being prepared: finalization of financing (several million euros), finalization of specifications, contracts with suppliers, production of components, etc.

“GRAVITATIONAL WAVES” folder

O4 data collection should start at the end of 2022;

the exact date will be known in the coming months.

We will then know if the improvements currently made on the Virgo and LIGO detectors have borne fruit!

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This analysis was written by Nicolas Arnaud, CNRS researcher at the physics laboratory of the two Infinites Irène Joliot-Curie (IJCLab) - CNRS of the University of Paris, University of Paris-Saclay.

The original article was published on The Conversation website.

Declaration of interests

Nicolas Arnaud is a CNRS researcher, currently seconded to the European Gravitational Observatory (EGO).

He mainly works for the Virgo experiment.

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