I've read up on number 2--the big freeze but can't really wrap my head around the physics of this. Would anyone care to explain like I'm 5 the theory behind the big freeze aka, heat death of the universe?
Yeah, this is the most popular theory at the moment I think, I'm no astrophysicist but I'll explain what I know.
Historically, the "Big Crunch" was what most scientists believed. This involved a "Big Bang", an expansion of the universe, then after a while the force of gravity would slow the expansion down. Eventually the force of gravity would overcome the initial force of expansion, causing the universe to begin contracting. This contraction would accelerate, ending in a spectacular "Big Crunch", with all the matter in the universe trying to occupy a single point. Some then speculated that another "Big Bang" would occur, and the cycle would continue.
However, in 1998 the Hubble Telescope observed some distant supernovae which showed that, a few billion years ago, the universe was actually expanding more slowly than it is today. Many more experiments have shown this to be true - The universe seems to not only be still expanding, the expansion is actually accelerating. Now this threw the "Big Crunch" theory out of the window, and caused scientists to make new theories involving something called "dark energy". Although no-one really knows what dark energy is, most scientists believe it's a mysterious energy which repels matter, overcoming the force of gravity (which attracts matter to other matter), and causing the accelerating expansion of the universe.
With these discoveries, it seems that the "Big Freeze" or heat-death of the universe is a likely scenario. In this theory, all matter keeps getting further and further apart, and eventually reaches maximum entropy, where all the energy is spread evenly throughout the whole universe. In such a state, there would be no heat or light, as there wouldn't be enough concentrated energy to sustain the processes. It would be very dark and cold.
However there is the "Big Rip" theory, which I admittedly know very little about, which predicts that the accelerating expansion of the universe will cause it to tear itself apart as a result of dark energy becoming stronger over time. The idea (I think) is that as dark energy gets stronger, it begins to tear apart smaller and smaller objects, until it is strong enough to tear apart subatomic particles, in essence destroying all matter in the universe.
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