What is the greatest secret of space

Expansion of the universe : The unsolved mystery of growing space

The astronomers no longer understand the cosmos. Basically, what they discovered around 90 years ago still applies: The huge spaces between the galaxies continue to expand. However, gravitational forces act between all matter in the cosmos. Theoretically, these should gradually slow down the expansion - at least thought the astronomers.

But in 1998 two independent research groups announced a completely different result: The expansion of space is by no means slowed down; On the contrary, it has even accelerated for around six billion years. Since then, scientists have been puzzling over what is driving the cosmos apart ever faster against the forces of attraction.

After all, astronomers have already agreed on one name for the cause of the accelerated inflation: dark energy. This media-effective baptismal name still lives up to the state of knowledge - albeit very differently than its inventor Michael Turner had hoped: The nature of dark energy is actually still completely in the dark. And this although entire armies of scientists wanted to reveal the secret of expansion of the cosmos with a multitude of creative and imaginative hypotheses.

In their need to explain, they rummaged among other things an idea by Albert Einstein that had long been believed to be out of date from the theory box. With his general theory of relativity in 1915 he had found an elegant explanation for the mysterious forces of attraction of gravity. But his equations suggested that the cosmos would have to shrink under the influence of omnipresent gravity.

Saved the idea of ​​the cosmos with a mathematical trick

A shrinking cosmos, however, contradicted Einstein's firm belief that the universe was immutable and eternal. So he arbitrarily added some kind of counterforce to gravity in his equations. With this “universal constant lambda” Einstein brought his theoretical cosmos back into exact equilibrium.

Together with the “Lambda” factor, his equations resulted in the unchangeable space-time structure, which the genius, astonishingly conservative in this regard, imagined as the only natural form of existence in the cosmos. Around ten years later, however, the American astronomer Edwin Hubble discovered that the cosmos was in fact expanding. Then Einstein removed his cosmological correction lambda from his equations. Later he is even said to have called it the “greatest donkey of my life”.

But lately the cosmological constant lambda has come back into fashion with astronomers. You even re-insert them into Einstein’s equations. This time, however, to describe a cosmos, the spaces of which even expand at an accelerated rate against the forces of gravity - as is actually observed. However, no cosmologist is completely satisfied with this mathematical device. It is considered a prime example of an “ad hoc hypothesis”, as the epistemologist Karl Popper called such arbitrary assumptions, which in his opinion were scientifically worthless.

Or is the lambda factor more than a methodical trick that theoreticians just shook off their sleeves to save the much-loved, tried and tested relativity theory? Perhaps - according to one of the most common assumptions among cosmologists - lambda indicates a physical property of the space itself. According to the laws of quantum theory, even absolutely empty space has an energy. And this “vacuum energy” could generate the pressure that accelerates the universe apart.

The worst prediction in the history of physics

Unfortunately, theory and observation are far apart: the lambda that matches the observed acceleration of the expansion of the cosmos is absurdly small. It is 121 orders of magnitude smaller than the lambda, which theoretically results from the quantum mechanical vacuum energy. Reality is missed by a factor of 121 zeros: With this, the vacuum energy as the hypothetical cause of the accelerated expansion of the cosmos has probably delivered the worst prediction that has ever been derived from a hypothesis in the history of physics. No wonder many cosmologists are looking for other explanations for the acceleration of expansion.

For example Anna Ijjas, head of the new working group “Gravitation and Cosmology” at the Max Planck Institute for Gravitational Physics in Hanover. However, she warns against counting Einstein to the scrap heap: "I am skeptical that a radical change in Einstein's theory of gravity will be necessary." But she adds: "Of course I can be wrong. We can also measure something surprising in the future. ”It would be surprising, for example, to discover that in the expansion history of the cosmos, besides gravity, another, previously unknown force played a role - and continues to play a role.

It would be a fifth basic force, in addition to the four forces with which the physicists describe the gears of the world in small and large up to now: the strong and the weak interaction force of elementary particle physics, the electromagnetic force and gravity.

But if the "Fifth Force" were more than just an ad hoc hypothesis to explain the accelerated expansion of the cosmos, why has its influence nowhere else been shown? Both in the vastness of the cosmos and in the laboratory, all bodies are conspicuous Orbits towards and around one another, which can be precisely and completely described with the laws of conventional gravity. No fifth force to be seen - nowhere. But the advocates of the fifth force have found a whole series of possible explanations for this. For example the chameleon- Hypothesis.

eRosita is supposed to track down the dark energy in space

Its name indicates a theoretically possible property of the Fifth Force, with which it would indeed be difficult to discover: Perhaps it could only develop its full effect in the almost empty, gravitation-free expanses between the galaxies. On the other hand, in environments with a lot of matter and correspondingly high gravity, it may only work over a tiny range and would therefore remain hidden. In this case, the Fifth Force would hardly be detectable in our solar system. Everywhere in our cosmic homeland we would only see the laws of gravity of the theory of relativity at work.

Recently, however, British researchers led by Dylan Sabulsky carried out an experiment with which they still hoped to track down the Fifth Force, even if it were a chameleon: In a vacuum chamber, they dropped rubidium atoms past a small metal ball. Only tiny gravitational forces acted between the individual atoms and the metal ball. A Fifth Force should have shown itself all the more clearly and steered the atoms out of their orbits.

In a press release, Ed Hinds, one of the researchers involved, described the high expectations: “It is very exciting to find out something about the evolution of the cosmos with an experiment in a London basement.” In this case, however, a fruitless attempt: the atoms remained accurate on the orbits on which they had to fly according to the laws of gravity. A fifth force did not show up.

Of course, researchers are also looking for the nature of dark energy in the accelerated expanding expanses of the universe itself. For example with the "eRosita" space telescope, which was developed by the Max Planck Institute for Extraterrestrial Physics in Garching and sent into space on July 13, 2019 from the Baikonur spaceport. "We want to search the whole sky for clusters of galaxies with eRosita and find up to 100,000", said the scientific director of the mission, Peter Predehl, the Tagesspiegel. Up to thousands of galaxies are gathered in a galaxy cluster.

However, the X-ray telescope will only be able to track down the clusters as a whole, more precisely: It will recognize the hot gas masses that fill the space between the galaxies of a cluster. Because of its high temperatures, the gas in a galaxy cluster is an extensive source of X-ray light that eRosita can collect and thus show its distribution in the cosmos.

Einstein in danger

With the X-ray telescope, the astronomers will not only recognize the geometric network that the galaxy clusters create in the cosmos. Depending on the distance from a galaxy cluster, the X-ray light from it that is now flowing into the telescope was emitted hundreds of millions or even several billion years ago.

A look with eRosita far back into the past of the cosmos will also show how the large-scale structure of the network of galaxy clusters has developed over the course of the last billions of years. In particular in the growth of the huge, almost matter-free space bubbles between the galaxy clusters, dark energy probably also left recognizable traces. For example, it could be shown that dark energy has changed over the course of billions of years.

With that a multitude of the hypotheses that creative cosmologists have worked out about the nature of dark energy would be done in one fell swoop. Even the general validity of Albert Einstein's general theory of relativity would then be in jeopardy.

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