Observations suggest that the expansion of the
universe will continue forever. If so, then a popular theory is that 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.
[1]
If
dark energy—represented by the
cosmological constant, a
constant energy density filling space homogeneously,
[2] or
scalar fields, such as
quintessence or
moduli,
dynamic quantities whose energy density can vary in time and space—accelerates the expansion of the universe, then the space between clusters of
galaxies will grow at an increasing rate.
Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies.
[3] Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed for
star formation will be exhausted. And as existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker, one star at a time.
[4][5] According to theories that predict
proton decay, the
stellar remnants left behind will disappear, leaving behind only
black holes, which themselves eventually disappear as they emit
Hawking radiation.
[6] 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 https://en.wikipedia.org/wiki/Timeline_of_the_far_future
https://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Dark_Era_and_Photon_Age