This article is taken from the journal Les Indispensables de Sciences et Avenir n°209 dated April/June 2022.

In every cubic centimeter of vacuum, there is a certain amount of energy specific to space, just by the fact it exists. At least that’s what the current Standard Model of cosmology suggests. An energy, of course, very, very little: not enough to boil a drop of rain in a cubic kilometer of empty space… Galactic Traveler’s Guide, “Space Is Big”, So great that this energy, equally present throughout its vast volume, is slightly more than twice the energy contained in matter, whether black or ordinary. The effect of this so-called “dark” energy – the only reason we don’t know its nature – is to accelerate cosmic expansion.

This strange idea took hold in 1998, when two teams led by Australian Brian Schmidt and American Saul Perlmutter announced that a supernova that exploded about 8 billion years ago appeared less bright and therefore farther away than expected. Until then, astronomers believed that the expansion of the universe had slowed steadily since the Big Bang, due to the gravitational attraction exerted on each other by galaxies – as well as decreasing the speed of the thrown object as it moved away from Earth. She goes.

gravitational attraction vs repulsive gravity

This article is taken from the journal Les Indispensables de Sciences et Avenir n°209 dated April/June 2022.

In every cubic centimeter of vacuum, there is a certain amount of energy specific to space, just by the fact it exists. At least that’s what the current Standard Model of cosmology suggests. An energy, of course, very, very little: not enough to boil a drop of rain in a cubic kilometer of empty space… Galactic Traveler’s Guide, “Space Is Big”, So great that this energy, equally present throughout its vast volume, is slightly more than twice the energy contained in matter, whether black or ordinary. The effect of this so-called “dark” energy – the only reason we don’t know its nature – is to accelerate cosmic expansion.

This strange idea took hold in 1998, when two teams led by Australian Brian Schmidt and American Saul Perlmutter announced that a supernova that exploded about 8 billion years ago appeared less bright and therefore farther away than expected. Until then, astronomers believed that the expansion of the universe had slowed steadily since the Big Bang, due to the gravitational attraction exerted on each other by galaxies – as well as decreasing the speed of the thrown object as it moved away from Earth. She goes.

gravitational attraction vs repulsive gravity

However, studies on supernovae seem to imply that during the last third of cosmic history, the expansion of the universe stopped slowing down and, conversely, … began to accelerate! As if a ball launched in the air was suddenly blown into space by some mysterious force. The most immediate explanation – already propounded by Einstein – One exerts repulsive gravity, to acknowledge the presence in space of a constant “vacuum energy”. According to this hypothesis, by the time the universe was sufficiently dense (roughly, twice as small as it is today), the gravitational attraction exerted by matter on itself slowed, as it should, expansion. But as the universe grew, its contents diminished. Thus it was only a matter of time before the density of matter fell below – constant, therefore – of this “vacuum energy”, and repulsive gravity took over.

Furthermore, several observations from the 1990s showed that on very large scales, space is “flat” (which simply means that it obeys the laws of Euclidean geometry). In fact, the cosmic radiation background – the microwave image of the young universe that lines the bottom of the sky – will be distorted, as if by the effect of a sort of magnifying glass, by the volume of space that would be separated if the latter were curved. Was. However, this is not the case… The problem, until Schmidt and Perlmutter declared, is that by enumerating all the normal matter that exists in the universe and all the dark matter whose existence we suspected, we never obtained any further spatialities of the universe. More than a third of the mass required to be flat. Miracle: What constitutes the missing two-thirds was the correct value in the vacuum energy determined to explain the cosmic acceleration!

Since then, many observations have confirmed the existence of this dark energy … without understanding its nature. “there are many hypothesesAstrophysicist Sandrine Kodis at CNRS explains. One possibility is that it is real ‘vacuum energy’ – but that hardly convinces anyone now.” This is because if, as we said, a model that assumes a constant energy constant for vacuum gives excellent empirical results, theoretical physics fails to explain its value: in particle physics, “vacuum The energy” must be either infinite, or more likely strictly zero, but nothing reasonable given that it is actually 0.000000000000006 joules per cubic centimeter!

That’s why many scientists like to imagine that the vacuum itself has no energy, but contains another field whose behavior would be Almost of vacuum energy: “A kind of fluid that would enter space and have a slightly different equation of state”, adds Sandrine Kodis. By the “equation of state”, astrophysicists express, in broad outline, the way a substance thins as cosmic space expands. So, for example, the internal energy of each cubic centimeter of space doesn’t dilute at all with expansion (when you add cubic centimeters to the universe, you add the same amount of vacuum energy). But the other possibility, an energy field—sometimes called a “quintessence”—that would be much less dilute, would play a similar role. “Needless to say, at this point in time, theorists are having a field day and regularly coming up with models, almost all of them more bizarre than the other!”The researcher laughs.

“Years pass and the problems remain”

How to decide? The current cosmological model is content to determine a constant energy, sufficient to agree at best with all observations. But we can refine these. It turns out that the way large structures – galaxies and clusters – grow over time is very sensitive to the exact recipe of the universe’s contents. And that mapping the deep universe on a large scale makes it possible to study its evolution in detail. Will the study of this evolution reveal more fickle dark energy behavior than the current model?

This would probably allow us to infer its nature. “This is for example the purpose of the Euclid mission. But there are others, such as large sensors carried from Earth.Sandrine excites Kodis. Missions are all the more necessary that have appeared, stress for many years in cosmological models regarding the rate of expansion, or contrasts between different measures of the amount of matter in the universe.”

These contradictions may simply be the result of biases generated by the measurement method. “But the years go by and the problems remainSandrine worries about Kodis. So the model can be very simple.” Some theorists think that the apparent acceleration of expansion may simply be a local phenomenon, due to the fact that we live in an unusually sparse bubble of the universe that will expand slightly faster than the rest. : As we observe these distant regions. , of normal densities, as they once were, would lead to the illusion that the universe has only recently begun to accelerate … an idea which has however given unsatisfactory results only ‘up to here’. In any case, the key lies in these maps of the future of the deep universe. “The whole community is very excited to have the data coming in the next few years!”, The finale of Sandrine Kodis.

René Cuillier. By