December 8, 2023

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Looking back fourteen billion years in time, to just after the big bang

Looking back fourteen billion years in time, to just after the big bang

In a laboratory of a futuristic building in Leiden, a young engineer wears a metal bracelet with a coiled black wire. This is to protect a fragile metal box measuring 6 x 6 x 3 centimeters from electric shock. He clicks on the box carefully. On the screen next to it, colored lines show how the box vibrates.

At first glance, the box doesn’t look special. It’s made of a shiny gray metal and has some thin wires running out of it. But with this little box, part of a new space detector, astronomers want to know something about the origin of the entire universe. They are looking for the most remote, for the oldest signs.

Astronomers know that the universe originated with the big bang. But no one knows exactly what happened. We haven’t been able to look that far yet.

The current frontiers of perception are limited by light. Because light takes time to travel, the deeper astronomers look into the universe with their telescopes, the more they look back in time. Only, the young universe was filled with an opaque soup of hot, high-pressure plasma in which light could not move freely. It wasn’t until about 380,000 years after the Big Bang that the universe cooled enough to become transparent to light. Astronomers call this light the cosmic background radiation. Looking back no further than that is not possible. At least not with the light on.

Albert Einstein

But ten billion trillion trillion trillionths of a second after the Big Bang, tiny gravitational waves are thought to have been sent out into space, ripples squeezing the fabric of space and time, lengthening and shortening the distances between all the celestial bodies that pass through them. Some fourteen billion years later, astronomers are trying to take pictures in space with the largest space detector ever built: the European Space Agency’s Laser Interferometer Space Antenna (LISA), parts of which engineers at Leiden are developing, is here at the Netherlands Institute. Space Research (SRON). . If LISA succeeds in capturing those ten billion trillion trillionth of a trillionth of a second after the Big Bang waves, astronomers will be able to look back in time further than ever before. how?

Gravitational waves arise during violent events such as the collision of black holes, and possibly also in the violent period immediately following the big bang. By calculating the conditions under which the gravitational wave arose, astronomers can study the universe in a way different from that used by light-collecting telescopes.

Detectors on Earth are already measuring gravitational waves that were transmitted much after the Big Bang. That was the first time at the end of 2015, a hundred years after Albert Einstein predicted its existence. Then the American LIGO detectors measured the gravitational wave that was emitted when two black holes collided more than a billion years ago.

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Read an interview with one of the discoverers of gravitational waves: Nobel laureate Barry Parish found bullet holes in his gravity detector

The idea that gravitational waves were also emitted shortly after the Big Bang, when the universe was still young and there were no stars or planets, stems from various physical theories. For example, according to physicists, the universe experienced a short but absurdly fast growth spurt right after the big bang. In less than a second, the universe is 10 years old60 times (sixty zeros) greater. That so-called period of inflation was so intense that space-time shook, giving rise to the primordial gravitational waves, which are still circling the Earth from all over the world today.

In addition to inflation, there is another theory from which gravitational waves were emitted in the early universe: when very small black holes suddenly appeared in the very young universe. Plasma, the primordial soup, was not evenly distributed everywhere. Some areas where the primordial soup mass was sticking together collapsed. Black holes that may have formed there also cause gravitational waves.

Gravitational waves nearly as old as the universe itself probably cannot be picked up by detectors on Earth

Giggs Nielemans astronomer

There is a problem, says astronomer Gijs Nielemans of Radboud University Nijmegen via Zoom. Nelemans is one of the leaders of the Dutch LISA consortium. “Gravitational waves that are almost as old as the universe itself probably wouldn’t be possible with detectors on Earth.” He shows that this is possible with measurements in space.

Neelemans: “This is due to the different frequencies of the waves. For example, gravitational waves received by LIGO have a high frequency between 10 and 1000 Hz.” That is, the wave crests follow one another rapidly, at a rate of 10 to 1,000 vibrations per second.”But primordial gravitational waves, when they reach Earth after their 14-billion-year journey, are completely stretched out by the expansion of the universe.”As the universe expands, the distances between The crests of the waves are getting longer and longer; the frequency is getting smaller. Primordial waves can extend for millions of miles. “We don’t know exactly how large these waves are, but many theories predict wavelengths in the LISA range, which will look at longer waves.”

A giant detector is needed to capture the long waves. It is for this reason that Lisa will go into space: in orbit around the Sun, fifty million kilometers from Earth. LISA consists of three identical satellites, each forming a triangular point. The distance between the satellites will be two and a half million kilometers. They send laser beams back and forth between them, and when a gravitational wave passes through, those beams are stretched or compressed a bit.

Those waves come from all directions. Like some kind of buzz

Giggs Neelemans astronomer

And those changes in the length of the arms would be very small, because the gravitational waves of the young universe are so weak. LISA should be able to detect a change smaller than a tenth of an atom’s width in a laser arm 2.5 million kilometers long.

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The necessary ultra-sensitivity makes LISA construction difficult. Everything has to remain very static in space to make sure that the laser arms only move by gravity and nothing else. For example, small thrusters must provide a very precise counterpressure against the pressure from the charged particles from the sun propelling the satellites and against the motions caused by the satellites themselves.

SRON metal box is important here. In the Leiden laboratory, a young engineer, Rene Wanders, opens the door to what looks like a large oven, the climate chamber. The box, which measures 6 by 6 by 3 centimeters, is a prototype of the LISA component that should read changes in the length of the laser arms: Quad image reception (Queens Park Rangers). “Here we test how the box operates between the lowest and highest temperatures that components are exposed to during and after launch: between about zero and 40 degrees Celsius.” The box is made from a piece of aluminum and a piece of titanium, a combination that expands slightly when heated.

“What LISA will pick up will be a kind of background noise that is constantly causing the laser arms to move slightly,” says Nielemans. “The waves originated in a small world that later expanded. So those waves come from all directions. Like a kind of buzz. You don’t know what each sound is saying, but you can infer, for example, from all those sounds together whether they’re from babies.” or the elderly.

Read also: The noise of gravitational waves reverberates through the universe

There is also a limit to our mind. We still have absolutely no idea what we should physically imagine in the period immediately after the big bang.

Vincent Ike Professor of theoretical astronomy

But if LISA did indeed succeed in absorbing the noise of the big bang, thus extending the outermost observational limit, that does not immediately mean that the discovery actually sheds light on the immediate post-big bang period. Not only are there limits to how far astronomers can see in the past, says Vincent Icke, but “there are limits to our brains”. He is Professor of Theoretical Astronomy at Leiden University. “We still have absolutely no idea what we should imagine physically in the period after the big bang, and certainly not mathematically at all.”

The two physical tools that describe the world around us, quantum mechanics of small particles and general relativity of large particles, collide during this period. “In quantum theory, the behavior of particles is indefinite, i.e. subject to chance. But in general relativity, the behavior of space, time and matter is absolutely constant. No one has yet succeeded in reconciling these opposing qualities. Conflict is always present, but only after the Big Bang and on Close to the horizon of black holes plays the key role, so suppose we catch gravitational waves, we still have no idea how to interpret them.

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While engineers in Leiden, among others, are developing parts of LISA, engineers in Japan, for example, are working on other ways to change the outermost control limit. They are working on the LiteBIRD satellite. Nor will it measure primordial gravitational waves directly, as LISA does. LiteBIRD searches for “fingerprints” of gravitational waves in the cosmic background radiation.

“Because gravitational waves warp space, the light traveling through space also changes,” says Rein van de Weijert, professor of astronomy and astrophysics at the University of Groningen. A light ray is a wave that can ripple in different directions: horizontally, vertically, or anything in between. Sunlight, for example, ripples in all directions, meaning it is “unpolarized”. But the cosmic background radiation has been vibrated in a certain direction by gravitational waves, or is “polarised”.

LiteBIRD is scheduled to be launched in 2028, nine years before the launch of LISA. The only question is whether the satellite will be sensitive enough to measure those changes.

Empty and dark

Astronomers and engineers around the world are building detectors to push the detection limit, “but the idea of ​​a detection limit is tricky and complex in an expanding universe,” says van de Weijert. There are different types of event horizons that are constantly changing. Engineers are pushing the “technical” limits of detection, but the area from which we can pick up signals is called event horizonis getting smaller and smaller due to the accelerating expansion of the universe.

It works like this: “Because of the accelerating expansion, distant galaxies are being pulled apart faster and faster. So we see that celestial bodies that are farther from Earth are moving away from us faster and faster. When celestial bodies eventually move away from us at the speed of light, we can no longer pick up their signals, Whether gravitational waves or light, then lie beyond the imaginary limits of perception, beyond event horizon. And then they will be invisible forever.”

And because the universe is expanding faster and faster, galaxies that are relatively close now will one day be moving away from us too quickly for us to observe. the event horizon became smaller.

Eventually, all galaxies outside the galaxy group to which the Milky Way (our galaxy) belongs will fall behind the galaxy event horizon to be withdrawn. Everything around is empty and dark. The galaxy cluster floats like an island in what appears to be a black sea. It was no longer possible to study the structure of the universe, even if you built such powerful telescopes or detectors. “The approaching limit of observation will one day mean the end of cosmology,” says Van de Weijert. But that would take billions of years.