Einstein's 'greatest blunder' may decide how the universe ends
Believing the cosmos was static, Einstein added a cosmological constant to his equations to hold gravity in check. When Hubble showed the universe was expanding, Einstein called it the greatest blunder of his life. Since 1998, however, a version of that constant, now called dark energy, has become the leading factor in forecasting the universe's final fate.
Serious study of cosmic endings began with general relativity in 1915, whose equations admit several solutions, each implying a different future. Alexander Friedmann in 1922 and Georges Lemaître in 1927 found expanding solutions tracing back to a singular beginning, and in 1929 Hubble used Cepheid variable stars in distant galaxies to show expansion was real. Fred Hoyle's rival Steady State theory, proposed in 1948, had new matter constantly appearing, but it could not explain the cosmic microwave background found by Arno Penzias and Robert Wilson in 1965, and the Big Bang won out.
For decades the key question was geometry, set by comparing the average density of matter with a critical value. A closed universe curves like a sphere, where triangles' angles add up to more than 180 degrees; gravity would eventually halt expansion and pull everything back into a Big Crunch. An open universe curves like a saddle and expands forever. A flat one sits exactly between. The Wilkinson Microwave Anisotropy Probe found space to be flat within a 0.4 percent margin of error.
Then in 1998 distant supernovae suggested that expansion is speeding up. Dark energy now appears to make up roughly 68 percent of the universe's energy content, and matter would need to be about seventeen times denser than measured to overcome it and force a collapse. If dark energy keeps strengthening, a Big Rip could eventually tear apart even the forces that bind atoms.
The favoured outcome is the Big Freeze, or heat death. Stars would keep forming for 1 to 100 trillion years until the gas runs out; then they would burn out one by one and the universe would darken. Black holes would dominate before slowly evaporating through Hawking radiation, leaving a cold, evenly spread universe with no energy differences left to power any process, life included. Upcoming surveys by the Euclid, Nancy Grace Roman and James Webb telescopes aim to pin down what dark energy actually is.
Source: Ultimate fate of the universe
