Galaxy clusters


Galaxies often form small groups or unite into gigantic clusters and superclusters containing thousands of galaxies. The immense gravitational fields of these galaxies act upon one another; they interact and sometimes even consume each other, stripping away individual stars and gas nebulae. This process creates gas bridges where new stars are born.

It also happens that one galaxy passes right through another. For instance, our Milky Way is currently approaching the Andromeda Galaxy. In about 4 to 5 billion years, a major collision will likely occur. However, contrary to what one might expect, this will happen without a grand uproar. The distances between individual stars are so vast that they will not collide, even if our galaxy moves completely through the Andromeda Galaxy. cf. Chapter on Galaxies with a Peculiar Appearance.
Galaxies and galaxy groups / Galaxies with a peculiar appearance.

Much like the planets in our solar system, galaxies also orbit one another, moving at speeds of up to thousands of kilometers per second.
However, there is a striking difference here::
The orbital velocity of the planets depends on their distance from the Sun. Gravity and centrifugal force balance each other out in each case. Orbital speeds decrease with increasing distance (Kepler's Law).

In contrast, the orbital velocity of the stars within a galaxy, or that of its orbiting dwarf galaxies, remains nearly constant. The orbital velocity is independent of how far the star or the orbiting dwarf galaxy is located from the core of the central galaxy.

For the galaxies to orbit each other at this speed, a greater mass is required as an 'engine' than these galaxies themselves possess.

The question arises: Where does this 'greater engine power' or mass come from?
There must be something additional that allows the galaxies to orbit one another at this speed.
Astrophysics refers to this force/mass as Dark Matter..
Dark matter is an invisible form of matter in space that emits or reflects no light. It is not directly visible, but it makes itself known through its gravitational pull.
Without dark matter, galaxies would fly apart. Dark matter acts like invisible 'glue' and holds galaxies and stars together like a 'skeleton'.

• Wikipedia Infos:
  Wikipedia: Dark Matter
  Wikipedia: Kepler's laws of planetary motion

Since the Big Bang, the universe has been expanding. Due to the mutual gravitational pull of all masses, this expansion should theoretically slow down and eventually come to a standstill. One might even assume that the fragments of the cosmic expansion would draw closer together again because of their mutual gravity, ultimately reuniting—meaning the sky would, in fact, fall on our heads.

In 1998, however, two research groups from the USA independently published precise measurements of the expansion of the universe. They were able to prove that the expansion of the universe is not slowing down, but on the contrary, is accelerating!"
The researchers Saul Perlmutter, Adam Riess, and Brian Schmidt were even awarded the Nobel Prize in Physics in 2011 for this discovery.

Yes, everything is flying apart faster than anyone thought. To this day, it remains impossible to explain why this is the case. And whenever science does not know why something behaves the way it is observed, it invents a new term:
Die Dark Energy was born.
Dark energy is therefore the term used for an effect that explains the accelerated expansion of the universe.

Scientists have also discovered that in its early phase, the universe drifted apart much slower than previously assumed. Consequently, the universe is older than previously thought.
In the meantime, there is a plethora of highly complex theories attempting to explain dark energy. In practice, however, virtually nothing is known about dark energy to this day.
New data is eagerly anticipated—for instance, from the European space probe Euclid, which launched on July 1, 2023.
It remains exciting ...

• Wikipedia Infos:
  Link zu Wikipedia: Dark Energy
  Euclid (space telescope)
  Brian P. Schmidt
  Saul Perlmutter
  Adam Riess
  

Since we now know that dark energy prevents the universe from collapsing, what alternative scenarios are conceivable?
Two scenarios are currently being discussed within the scientific community:

• The Big Freeze

  This scenario is currently considered the most likely by scientists.

  • The Principle
    Driven by dark energy, the universe expands forever.
  • The Process
    Galaxies drift so far apart that light can no longer travel from one to another. Aging stars run out of fuel, and no new gas remains to form subsequent generations of stars.
  • The End
    After trillions of years, the last stars fade away. All that remains are dead stellar remnants and black holes, which will eventually evaporate. The universe becomes an infinitely vast, dark, and absolutely cold space where nothing ever happens again (also known as the "Heat Death" of the universe).
• The Big Rip

  This scenario occurs if dark energy becomes increasingly stronger and more dominant over time.

  • The Principle
    The repulsive force of dark energy eventually overcomes all other fundamental forces of nature.
  • The Process
    First, galaxy clusters are driven apart. Later, individual galaxies are affected, followed by solar systems (planets fly away from their host stars), and ultimately stars and planets themselves are disrupted.
  • The End
    In the final minutes of the universe, the force becomes so immense that it tears apart even atoms and atomic nuclei. At this exact moment, space and time cease to exist as we know them.