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Space / Observations confirming that massive galaxy clusters existed when the universe was young indicate that the universe will continue to expand forever

Avishai Gal-Yam

If the universe were dense, the existing structures in it would develop at a relatively high speed. If massive galaxy clusters existed so long ago, the universe should contain many more clusters today than actually exist.

New observations confirm that galaxy clusters, vast systems containing millions of billions of times more matter than the Sun, existed when the universe was only about a quarter of its age today. The existence of such massive and young clusters indicates that the average density of matter in the entire universe is low, and from this it follows that the universe is expected to expand forever.

The accepted assumption among researchers today is that the universe was created in a "big bang": a huge explosion in which the universe was created when it was extremely dense, infinitely hot and spreading everywhere. The development of the universe after the big bang is determined, according to the accepted models, by the balance between two processes: the repulsion from the big bang causes the universe to expand, while the force of gravity, acting between the different parts of the universe, causes the expansion to slow down. The force of gravity between two bodies is stronger as their masses are greater and weaker as the distance between them increases. This is why the average density of the universe is what actually determines its development: if the density is high enough, the gravitational force will be strong enough to overcome the repulsion from the big bang, and the expansion of the universe will stop. In such a case, there is a possibility that the universe will begin to contract after some time.

If, on the other hand, the average density is small and the universe is relatively sparse, it can continue to expand; As it spreads, it becomes more sparse, and the distance between its various parts increases; As the distance between its parts increases, the pull of gravity becomes smaller, and is unable to stop the expansion of the universe. The density of the material which is large enough to stop the rate of expansion, but not cause contraction, is called the "critical density".

Density also affects the structure of the universe, as we know it. Shortly after its creation, the universe was very uniform - the density of matter was very similar everywhere. Today, however, the universe is not uniform at all. Most of the matter we know is grouped in orderly systems: stars, like the sun, billions of which are grouped together to form galaxies. The galaxies are also not randomly scattered in space: they are organized in groups called "galaxy clusters"; Each such cluster may contain thousands of galaxies. Even the galaxy clusters are arranged in groups called "superclusters".

The process that turned the primordial, uniform universe, created in the Big Bang, into the universe we know, containing ordered structures of matter with large and relatively empty spaces between them, stems, again, from the force of gravity. In some regions of the early universe the density of matter was slightly greater than in their surroundings. The gravitational force exerted by these concentrations of matter caused additional matter to move in their direction, which made them denser and their surroundings more sparse.

As the region contained more matter and became denser, its gravity increased and it pulled more matter towards it. Such areas became "cosmic seeds", from which grew the huge structures we see today. The growth process of such "cosmic seeds" largely depends on the density of matter.

The greater the average density of matter, the faster the resulting structures develop.

Understanding the development processes of the universe allows researchers to make use of observations in order to measure the most fundamental properties of the universe, such as the average density of matter in it. A group of researchers, led by Prof. Netta Bakol from Princeton University in the USA, analyzed the properties of galaxy clusters observed by the "Einstein" observation satellite. This satellite discovered galaxy clusters by detecting the X-ray radiation emitted by hot gas present in such clusters in large quantities. It turns out that the sample of galaxy clusters that "Einstein" discovered contains a number of clusters that are the most distant from us: in light of these clusters, billions of years were required to reach us. This fact teaches us that they existed when the age of the universe was less than half of its current age. The very existence of these clusters, the researchers claim, proves that the average density of the universe cannot be very large.

If the universe were dense, the existing structures in it - the clusters of galaxies, for example - would develop at a relatively high speed. If already such a long time ago massive clusters like those observed by "Einstein" existed, then the rapid development of the structures in the universe would have resulted in the fact that today, the universe around us, known to us from many astronomical observations, should have contained a much larger amount of large structures, like galaxy clusters, than actually appear in it. From the analysis of the observations, Prof. Beckol and her partners concluded that the average density of the universe may only reach up to about 30% of the critical density. From this it follows that there is not enough matter in the universe to stop its expansion and therefore it will continue to expand forever - one of the most fundamental predictions regarding the universe in which we live.

In new observations from the "Chandra" space observatory, published by the British astronomer Andrew Fabian and his team, a galaxy cluster even younger than those discovered by "Einstein" was discovered: this cluster existed when the age of the universe was less than a quarter of its current age. Although this cluster is less significant, its very existence, Prof. Fabian claims, confirms Prof. Bakol's conclusions.

The combination of innovative observations, with the help of which the researchers discover very distant galaxy clusters, and the understanding of the development processes of the universe, makes it possible to determine that the density of the universe in which we live is relatively low and therefore it will continue to expand forever.

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