Question: What do you think will happen in the future to our universe?

Laurence Perreault Levasseur answered on 18 Jun 2014:

In the relatively near future – something like about 5.4 billion years (from the point of view of the whole Universe! from our human point of view is in an unimaginably long time!), the sun will run out of hydrogen to burn in its center, and will start to become a red giant. Doing so, it’s going to become bigger and bigger, and planet Earth will be destroyed by the sun in the process (it’ll literally be ‘eaten’ by the sun: it’ll be so big the Earth will be inside it!). By that time, chances are that we will have mastered the technology to colonize other planets of the solar system, and even gone to other stars, so there is no need to worry too much!

On an even longer timescale, however, things don’t look as good. Depending on how much dark energy there is in the Universe, in 100 billion years all galaxies beyond the local group will have disappeared beyond the cosmic light horizon (that’s because of the expansion of the Universe, by that time everything pass that will be expanding away from us faster than the speed of light, so we will never see it again). That’s only about 47 galaxies! (Recall that the Universe right now is only 14 billion years old, and it’s already pretty old!! So all these numbers are INSANELY big, many times the current age of the full Universe!!!)

In about 450 billion years, all those galaxies (in particular, our Milky Way and Andromeda) will have collided into one gigantic galaxy. Between 10^12 and 10^14 years from now (that’s one trillion to 100 trillion years), there won’t be enough hydrogen left in the galaxy to ensure normal star formation, so no new stars will be formed and remaining stars will slowly burn their fuel and die.

In about 110 to 120 trillion years, all remaining stars will have exhausted their fuel, and the only remaining objects will be star remnants (white dwarfs, neutron stars and black holes) and brown dwarves. Sometimes collision between those things will make new stars, but we can estimate that about 100 stars will be shining in the galaxy.

In 1 quadrillion years, the sun will have cooled down to 5 degrees above absolute zero. That’s -268 Celsius!!!

By 10^30 years from now (there is no name for such a large number), all stars remnants will either have been ejected from the galaxy or collapsed into the central black hole of the galaxy. Between 10^36 to 10^43 years, the protons of all atoms in the Universe will (probably) decay, and the only remaining objects will be black holes. This is the Black Hole Era.

Black holes will eventually evaporate through Hawking radiation, and by 1.7×10^106 years from now, they will have disappeared. This is the heat death of the Universe.

By this time, things look pretty dreadful, but the next one is great, because by 10^{10^{56}} (that is an INSANE number, by the way) random quantum fluctuations will have created a new big bang, and so everything will start all over again!!

Ok, so last note, these estimates are based on what we currently understand of physics, and so with such big extrapolations, chances are they are completely wrong (and the further in the future, the higher the chances we got it wrong), so I wouldn’t worry too much about it, we literally have many times the age of the Universe to figure it out! (but it’s important to do research to understand those things better!!)