Either Einstein is wrong, or dark energy exists

As scientists discovered that the universe was expanding faster and faster, they also realized that there was a lot we didn't know about the universe.

In 2023, the Euclid satellite was launched off the coast of California in the United States of America. Already that fall, the first photographs came from the room. Researchers are now eagerly awaiting more results.

– The most important reason for the Euclid satellite is that we want to understand why the universe is not only expanding, but why it is expanding faster and faster, explains Öystein Elgarwe, professor of astrophysics at the University of Oslo (UiO). .

Before, researchers had three hypotheses about what caused the accelerating expansion: Einstein's theory of space-time with modified gravity, dark energy, or that we don't understand how a vacuum — empty space — behaves.

Elgarøy is one of those ready to pounce on data from the satellite.

Gravity “must” be a brake

Until well into the 1990s, the prevailing view was that mass was the most important contribution to gravity on large scales, Elgarwe explains.

– If this is the case, then ordinary mass and dark matter dominate gravity on large scales in the universe, then we would expect the universe to expand more and more slowly, Elgaroy explains.

And here lies the big mystery: all measurements show that the universe is expanding faster and faster. Elgarwe explains that gravity is usually a force that has an attractive effect on mass. Hence this force would actually act as a brake on the expansion.

– When scientists started making measurements that showed how the universe was expanding over time, they also saw that it was expanding faster now than it was five to six billion years ago. It will expand faster over time, but it won't work if it's ordinary matter, Elgarwe says.

So what would cause the universe to expand in this way?

Einstein had a possible solution

In 1917 — before scientists knew the universe was expanding — Albert Einstein launched his first cosmological model. This solution involves the so-called theory of general relativity, which explains, among other things, how gravity works.

– At that time, Einstein and the vast majority of other scientists believed that the universe was static. It was not discovered that it was expanding until 10 to 12 years later. That's why Einstein tried to create a model of a static universe, but he couldn't do it using only attractive gravitational forces, because then the universe would collapse, Elgarwe explains.

Einstein's solution to this matter was to introduce an additional term into the equation, which is a force that resists gravity so that the universe becomes stable. This term was called the cosmological constant.

– When scientists discovered that the universe was expanding, they had no use for the cosmological constant, so they stopped including it, until they discovered that the universe was expanding faster. The researchers then found that the cosmological constant could be the force that caused the universe to expand faster, Elgarwe explains.

Looks like the problem has been solved, right?

No one can explain what the cosmological constant actually is.

In the years since, researchers have tried to understand more about the source of the cosmological constant, Elgaroy says. An important contribution could be the zero-point vibrations of all fields filling the universe, i.e. vacuum energy.

It turns out that the void, or empty space, is full of energy. Even when the temperature is zero Kelvin, which is referred to as absolute zero, which is so cold that the atoms are at rest, even there the energy is not zero.

The reason this is this is because of Heisenberg's unclear connection, says Elgarwe.

In short, the flux relationship means that you either know where the particle is, or you know its speed. This applies not only to particles but also to fields. According to Illgaroy, this means that if the field energy is zero, the rate of change of the field is infinitely high – which does not work. Therefore, the field cannot be zero. Elgarwe says the researchers tried to calculate this vacuum energy. Then they saw something strange:

A problem arises because it turns out that the energy of the vacuum is much greater than what is needed to explain the accelerating expansion of the universe, says Elgarwe. – In the most extreme way of calculating this, you get 10^120 times the critical density, when what is required is 0.7. This is a very large mismatch.

How should researchers deal with these numbers?

– One way to look at this is to say that we understand nothing about this vacuum energy. Then we have to go back to the desktop and calculate it again. Or we could say that we may have overlooked some important principles. For example, if the energy of the vacuum could actually be zero, but there must be something completely different causing the universe to expand at an increasing rate.

Dark energy

Is it possible that there is a form of energy that creates repulsive attraction? By introducing such a form of energy, researchers can scale up the equation.

“We don't know what this is, but we call it dark energy,” says Elgarwe.

On the one hand, in Einstein's equation that explains the geometry of space and time, you have a quantity that explains the geometry of space and time. On the other hand, you have all the mass and energy that exists.

– Those who think acceleration comes from dark energy get it by adding something to the equation that shows energy and mass. This thing looks like a gravitational repulsive force, but it's a kind of geometric effect in space-time, Elgarwe explains.

This “something” is often called dark energy, but no one really knows what it is.

Manipulating gravity

– Those who work with modified gravity say no, we do it by changing the volume that represents space and time and curving it in response to mass and energy, Elgarwe explains.

According to him, there are many models for both dark energy and different models of gravity. There are small differences between models.

-So far we don't have any feedback that can tell us what the correct model is. You can create good models for both explanations, and individually the models agree with the observations, Elgarwe says.

Scientists have long known that something is wrong with the models. The problem is that until now it has not been possible to make measurements that confirm or deny the three main models for why the universe is expanding faster and faster.

– And that's where Euclid comes in, because it turns out that all of these models contain numbers that aren't defined theoretically, but you can tweak them and decide to make the model fit the observations, Elgarwe explains.

In the past, scientists have measured distances in the universe and how fast they are moving away from us. In addition, measurements have been made of how the universe is cluttered in certain regions. These measurements have previously been performed separately, making it possible to create different models, adapted to what is observed.

– If you have a dark energy model and a modified gravity model, you can get them to agree on the relationship between distance and velocity for galaxies, but you can't get them to agree on how fast the mass is clumping together at the same time – “so you have to choose,” Elgarwe explains.

The solution to this is to measure everything at the same time, which is what Euclid was built to do.

This is how Euclid works

The way this works is that Euclid takes pictures of a fairly large portion of the sky, while at the same time looking further out into the universe, and thus back in time.

Through Euclid's observations, a map can be made that shows how galaxies are distributed over a large area of ​​the sky, and at the same time we can see the distribution of mass over time. The spectrum of galaxies shows how fast they are moving away from us.

Using this 3D map, researchers can see how fast the universe expanded at different times.

It may take some time to disprove Einstein

Since 1998, scientists have known that the universe is expanding faster and faster. However, it took a long time before Euclid was sent.

-We have some big theories that may turn out to be wrong. “We have to be absolutely sure of the measurements,” says Elgarwe.

It is important for researchers to trust the data they receive. Therefore, it has taken some time to develop sufficiently accurate tools – and safe enough methods – to analyze the data.

-If Einstein's general theory of relativity turns out to be not entirely correct, it should be replaced with a more accurate theory. If it turns out to be dark energy, that would be interesting too, because it's a substance we've never seen before. It's also possible that there's something fundamental we don't understand about how a vacuum works and what a vacuum actually is, Elgarwe says.

He personally believes that the two most radical possibilities are to change the general theory of relativity or that vacuum energy is the cause of the acceleration.

– What is less radical is the introduction of new matter, such as dark energy. Maybe it depends a little on who you ask, but I think this model would require as little as possible, he explains. When we have observations that we cannot explain, it is definitely new material, and it is easier to add something new than to change hundred-year-old theories, Elgarwe smiles. – This is a trick we always resort to when we don't understand something.

Maybe we'll get the answer in five years?

-If gravity is modified, then we were wrong about how the force of gravity works. If the acceleration is due to vacuum energy, then we don't understand what a vacuum is at all. Elgarwe thinks it will likely require a whole new kind of theory if we are to understand this.

Although researchers have already received images from Euclid, it may take some time before the results are released to the public. It takes time to reach firm conclusions and make sure all potential noise sources are well understood, Elgarwe explains.

– Maybe within five years, he speculates.

-What interpretation do you hope it would be?

-Personally, I hope that fixed, vacuum cosmic energy is the best. The reason why modified theories of gravity are so bad is primarily because you end up with mathematically messy theories. Vacuum energy means we have to have some radical new ideas, and I think that's an exciting prospect.

– Whatever the explanation, many new questions will arise.

The article was first published on Titan.uio.no