Well.. actually. They think the universe will keep expanding forever right now.. But its so out on the fringe it isn't really 'knowable' right now. As far as we know, the universe could be escapeable, we could store enough energy for the million or so years it might take for a new one to form. It is too early to decide it must end.
However. There are too many things that can happen to a single planet to wipe out all life. Everything from big rocks, to rogue black holes, to seemingly small things like pandemics.
The only way to really ensure the continuation of humanity is to populate other planets. Then other galaxies. More and more distant.
Of course... Over time we'll become many different sub-strains of humanity.. In time, we won't recognise/be genetically compatable with eachother.. But by then, that shouldn't matter.
However. There are too many things that can happen to a single planet to wipe out all life. Everything from big rocks, to rogue black holes, to seemingly small things like pandemics.
The only way to really ensure the continuation of humanity is to populate other planets. Then other galaxies. More and more distant.
Of course... Over time we'll become many different sub-strains of humanity.. In time, we won't recognise/be genetically compatable with eachother.. But by then, that shouldn't matter.