Astrophysics Part V: The End of the Universe

Andromeda Galaxy

So we have considered the evolution of stars, and evidence supporting the big bang, however, we have not yet covered the end of our universe.  This topic has, like many  phenomena of astrophysics been debated over for many years.  There are multiple theories, but all of them depend on the mass of the universe.  The universe could be either

• Closed
• Open
• Flat

Possible Fates of the Universe

A closed universe means that gravity will stop the universe from expanding, causing it to contract, this would result in a massive contraction of the universe in a big crunch, from which the universe could begin again.  The density of the universe would have to exceed the critical density, ρ > ρ_c.

An open universe means that gravity would be too weak to stop the expansion.  The universe would expand forever, the density of the universe would be less than the critical density thus ρ < ρ_c.  The universe will end in either a big rip or big freeze.  In the big rip, acceleration of the expansion of the universe caused by the presence of dark energy would become too large, and would supersede the strong nuclear force, the electromagnetic and gravitational forces, resulting in the reduction of all matter to elementary particles.  However, the possibility of a big freeze is also possible.  As the universe expands, the concentration of dust and gas clouds would decrease, and the fuel needed for star growth would decrease as well.  This would result in less stars from forming.  Furthermore, the extreme redshift of electromagnetic waves, would leave the waves with little energy.  Meanwhile, the distance between galaxies would be rapidly increasing.  The universe would be unable to sustain life, as Earth would become too cold.

Lastly, the universe may be flat.  The density is equal to the critical value, consequently the universe will only begin to contract after an infinite amount of time. This means that the universe’s density must equal critical density, ρ = ρ_c.  The fates of the universe would be the same as an open universe.