Why does the Universe keep expanding?

Why does the Universe keep expanding?

VIKTOR T.TOTH  , PRO PART TIME PHYSISIST

The universe was born in a state of flying apart.

This flying apart continues unless something stops it.

The only something that we know of with the power of stopping the expansion is gravity. If the universe contained enough matter for matter’s self-gravity to bring the expansion to a halt, the expansion would stop.

But the universe does not have enough matter for this. So the expansion continues.

In fact, it accelerates, and that, too, has to do with gravity. This is the role played by the stuff that we call “dark energy” (really a pretty name that hides our ignorance. We are pretty sure dark energy exists, but we have no idea what it is.)

To understand what dark energy does, think of an ordinary gas first: as gravity does work on it, it contracts and its density and its pressure increase. The work (energy) done by gravity is now (stored) in the form of pressure. Dark energy, however, has negative pressure. When gravity does work on it, it therefore expands. Bubbles rising in the sea due to gravity offer a loose analogy.

Anyhow, not only does dark energy expand under its self-gravity, but the work done by gravity produces more dark energy. So as the universe expands, the density of dark energy remains the same. Meanwhile, everything else gets diluted. So after a while, dark energy remains the dominant constituent. When that happens, the expansion begins to accelerate.

So to sum up, the universe expands because no force was powerful enough to stop its expansion (the state in which it was born) and the expansion accelerates ever since dark energy, which responds to gravity as though it was repulsive, became the dominant constituent.

ΑΝΑΔΗΜΟΣΙΕΥΣΗ ΑΠΟ ΤΟ QUORA 30/6/2017