How can the universe end?
Probably heat death.
As I understand it, there are basically two possibilities:
Big Crunch. The universe collapses under gravity to a singularity, possibly followed by another Big Bang and reformed universe.
Heat death. The universe keeps expanding and cooling forever, eventually becoming very dark and very cold.
The Big Crunch seems unlikely now on the basis of our observations. It seems that the expansion of the universe is accelerating.
The Universe can end with maximum entropy knows as the heat death.
The supernova experiments of 1997 and 1998 provided empirical evidence that the rate of the expansion of the universe is increasing. This means that the universe will continue to expand until maximum entropy is reached.
At maximum entropy there will be no black holes left all the mass, gravity and energy stored in the black holes will have dissipated. All matter will have been turned into energy. The energy will have reached maximum disorder. All energy will be at a very low level approaching absolute zero. This is sometimes referred to as the heat death of the universe. What it means is that the universe will be dead because there is no useable energy left in the universe.
The supernova experiments indicate that no only did the present universe have a beginning it will have an ending. This means that matter and energy are not eternal, nor self existent. Something besides matter and energy must exist and material realism is a failed scientific philosophy.
The fact that the present universe is expanding means the universe is an open system. The fact that the universe is an open system means that something besides natural cause as science knows it must exist.