One theory says that the universe, after the period of expansion, will shrink and eventually implode into a singularity.
Another theory claims that the universe is destined to expand forever and it will not end with a bang but with a whimper. Earth, the sun and all stars will gradually pass away and some 10,000 trillion trillion trillion trillion trillion trillion trillion trillion years from now the universe will be an increasingly diffuse soup of electrons, positrons, neutrinos and radiation. It is not sure what would follow this "dark era". The universe would actually not end, but any given physical processes in it would cease to be important.
These theories might be valid only based on the assumption that recognized laws of physics will continue to operate over the very long-term future.