Future of an expanding universe

Recent observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze. The future of an expanding universe is bleak. If a cosmological constant accelerates the expansion of theuniverse, clusters of galaxies will rapidly be driven away from each other, leaving observers in different clusters unable to either reach each other or sense each other's presence in any way. Stars are expected to form normally for 1012 to 1014 years, but eventually the supply of gas needed for star formation will be exhausted. Once the last star has exhausted its fuel, stars will then cease to shine. The stellar remnants left behind are expected to disappear as their protons decay, leaving behind only black holes which themselves eventually disappear as they emit Hawking radiation. Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.