The Big Freeze is a potential long-term fate of the universe in which cosmic expansion continues indefinitely, driven by dark energy or a cosmological constant. As the universe expands, galaxies move farther apart, and the supply of usable energy becomes increasingly diluted.
Over trillions of years, stars will burn out, galaxies will cease forming new stars, and existing stellar remnants—white dwarfs, neutron stars, and black holes—will slowly cool. The universe will become progressively colder, darker, and more diffuse, with most matter spread thinly across vast cosmic distances.
Eventually, processes like proton decay (if it occurs) and black hole evaporation via Hawking radiation could further reduce matter into low-energy radiation. The universe would approach thermodynamic equilibrium, a state of maximum entropy, leaving a cold, dark, and nearly empty cosmos.
The Big Freeze scenario is consistent with observations of accelerated expansion and the dominance of dark energy. It contrasts with catastrophic outcomes like the Big Rip or Big Crunch, representing a gradual, inevitable decline rather than an abrupt destruction or collapse.
In essence, the Big Freeze envisions a universe where time stretches on endlessly, but life and structure eventually fade away into darkness and cold.