In December 1997, the Hubble Space Telescope detected a tremendous flash that happened 10,000 million years ago: it was a supernova, named 1997ff, that exploded in an elliptical galaxy. It was the most distant supernova ever observed and the most important piece of evidence that our universe is stranger than we could imagine. He showed that two-thirds of everything in the universe is in the form of “dark energy” that works, on large scales, as if it were negative gravity.
Its existence was announced to the world in 1998 by two groups of astronomers: the Supernova Cosmology Project (SCP), led by Saul Perlmutter of the Lawrence Berkeley National Laboratory, and the High-Z Supernova Search Team (High-Z), led by Brian Schmidt. from the Australian National University. They both earned the 2011 Nobel Prize.
What these two groups did was look for supernovae in distant galaxies, measure their distance and thus determine the value of one of the most important numbers in cosmology, the deceleration parameter , which accounts for the change in the expansion speed of the universe . Unsurprisingly, all cosmologists were convinced that the expansion was slowing down, as befits any explosion worth its salt. But what they found took them totally by surprise. Consistently, all the supernovae they found in distant galaxies appeared 20% fainter than expected. This couldn’t be. Making good on the Sherlock Holmes adage “once the impossible is ruled out, what remains, however unlikely, is true,” both groups came to the same conclusion: the universe had expanded 10% faster in the last few thousand years. millions of years . That meant there must be “something” exerting some sort of repulsion, pushing galaxies away faster than expected.
an accelerated universe
But to ensure that everything that was believed until then about the expansion of the universe was wrong needed incontrovertible proof. And it came on the heels of supernova 1997ff : it was twice as bright as any explanation that didn’t involve an accelerating expansion of the universe predicted. The conclusion was unappealable: in the universe there is something that overcomes the force of gravity –which tends to slow it down– and is accelerating it. This uncovered the box of thunder and cosmologists began to rub their hands: they had a new toy to play with. And since they are some characters with a rather feverish imagination, they decided to end the monopoly of gravity and introduced a surprising and almost impossible to believe entity: dark energy . The issue was of such magnitude that, in the words of Princeton cosmologist Paul Steinhardt, “the dark energy revolution is as important as the Copernican revolution.”
Of course, not everyone agreed. Important astronomers such as Joseph Silk said that “I hope it disappears”; the great Michael Rowan-Robinson of Imperial College London was even more blunt: “I am skeptical about cosmological repulsion.” We can’t blame him: nobody knew for sure what it was or where it came from that phantasmagorical energy that does not reside in any object in the universe, but in space itself.
The two possibilities of dark energy
The bets on what dark energy is are divided in two. The first points to a cosmological constant , immutable in time. It is reminiscent of a term introduced around 1917 by Albert Einstein because his general relativity equations resulted in an expanding universe, and the universe was then thought to be static. The second option is that dark energy does change over time and has been given a very Aristotelian name, quintessence .
Since the end of the 20th century, theoretical physicists have tried to settle between the two, but they have not seen the light at the end of the tunnel. In fact, since the beginning of this century the explanations have been multiplying, although the idea that dark energy is hidden in the form of a cosmological constant remains the most popular option. And if so, physics faces a huge problem: any current theory predicts that it has a value that is 10 60 – a 1 followed by 60 zeros – greater than the one observed by astronomers. It is the most disastrous forecast in science and one of the fundamental problems of physics in the 21st century. To get an idea of what it means: imagine that we experimentally measure the size of a proton and find that it is on the order of a billionth of a meter. Now, according to our best theories, it actually had to measure a hundred billion sextillion meters. That is, the proton should be vastly larger than the visible universe! Clearly, something is woefully wrong with the theory.
What is dark energy? If the previous problem is solved and it ends up being a cosmological constant, answering the question would take us directly to the physics of the very small, to quantum mechanics. According to her, the vacuum -understood as the absence of matter and energy- does not exist. It is actually a hotbed of particles that appear and disappear in less time than a breath lasts. This “ vacuum energy ” has a visible effect on the universe, giving it that extra push.
But if dark energy changes over time, if it is quintessential (fathered by Rahul Dave of the University of Pennsylvania in 1998), then we are dealing with a form of energy associated with a mysterious quantum field that spans the observable universe that arises of the abstruse quantum field theory. According to her, particles are nothing more than vibrations in a quantum field, a modification of the structure of the space in which we live.
Everything that surrounds dark energy is mysterious. Perhaps no one has expressed the sentiment of much of the physics community better than Steven Weinberg: “It is difficult for physicists to attack this problem without knowing what needs to be explained.”
