Cosmologists believe that the universe started much smaller than it is now — a nearly infinitesimal point. The unfathomably rapid expansion during the first moments of our universe’s existence is called the Big Bang.
According to the Big Crunch theory for the ultimate end of the universe, the expansion of the universe will reverse, leading to a massive collapse. This would effectively be like an inverted Big Bang.
A likely outcome for our universe is that as stars continue to die, the universe will slowly cool down, eventually decaying into a cloud of particles. This cosmological outcome is called the Big Freeze.
A dramatic theory for the end of the universe, called the Big Rip, predicts that the universe will expand at an ever increasing rate. This would cause every atom in the universe to be violently torn apart.
Bosons are the fundamental particles responsible for mediating the interactions (forces) between the fermions. They include the photon, gluon, and weak bosons, as well as the Higgs boson. Some physicists have speculated the existence of the graviton; however, it is unknown if such a particle exists.