Can stop time when space stops expanding

How does the universe end?


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5. How does the universe end?

The universe is in motion and has been expanding since the Big Bang. If one ignores the hypothetical dark energy, then two forces determine the fate of the universe: The momentum of the Big Bang drives it apart, the gravity of the matter contained in it slows its expansion. How it continues depends on which force wins. If the universe expands forever, it faces the big freeze, a modern variant of heat death: Then the matter is lost in ever wider space. All structures fall apart. If, on the other hand, the force of gravity is stronger, the universe will one day collapse again. Then the big crunch threatens. It is the exact opposite of the Big Freeze: the universe is getting closer and closer. Everything is squeezed together like in a cosmic scrap press until it is as hot and dense as it was at the time of the Big Bang. Perhaps it will pop again and a new cycle of the universe will begin. Then the cosmologists speak of the big bounce.

But there is still this strange dark energy that cosmologists have only recently taken into account. Like an anti-gravity force, it drives the universe apart. And because its energy density, i.e. the energy per room volume, is constant, the more the room expands, the greater the dark energy. If this continues, the universe will expand forever and ever faster. In an extreme variant of this scenario, the dark energy could one day become overpowering. It will tear everything apart, not just galaxies and stars, but also molecules and atoms, until only radiation and indecomposable particles are left. Cosmologists speak of the Big Rip, an aggravated form of the Big Freeze. Much of it is still based on speculation.

It is also conceivable that the universe does not die of natural causes, but has an accident beforehand. It could collide with a neighboring universe. Princeton University physicist Paul Steinhardt believes this happens regularly. According to him, a second universe in higher spatial dimensions exists very close to us. What we call the Big Bang is therefore only the inside view of a collision with our twin universe. The next collision in the distant future will shake the universe again so that its current life cycle ends and a new one begins. Paul Steinhardt enjoys a good reputation among his specialist colleagues, but this worldview has so far convinced few of them. After all, it has the advantage that it does without the mysterious dark energy.