Ultimate fate of the universe

The ultimate fate of the universe is a topic in physical cosmology that explores possible scenarios for the evolution and end of the universe based on scientific hypotheses. The subject became a valid cosmological question after Edwin Hubble's observations in the 1930s–1950s showed that galaxies are moving away from each other, leading to the Big Bang theory. This theory, which holds that the universe began in an extremely dense state roughly 13.787 billion years ago and has been expanding and becoming less dense ever since, was confirmed by the discovery of cosmic microwave background radiation in 1965, a prediction the competing Steady State theory could not explain.

The scientific basis for these inquiries emerged with Albert Einstein’s 1915 theory of general relativity, which describes the universe on the largest scale. Solutions to its equations, proposed by Alexander Friedmann in 1922 and Georges Lemaître in 1927, suggested an expanding universe from an initial singularity. A key parameter in determining the universe’s fate is the density parameter, omega, which compares the average matter density to a critical value. This selects one of three possible geometries: flat (parallel lines remain parallel), open, or closed. For much of the 20th century, cosmologists aimed to measure this parameter, or the rate of expansion deceleration, to predict the end.

However, starting in 1998, observations of distant supernovas indicated the universe’s expansion is accelerating, not decelerating. This led to the concept of dark energy—a catch-all term for a field with negative pressure, often modeled as a positive cosmological constant. Einstein had originally introduced such a constant to keep the universe static, later calling it his "greatest blunder." Today, cosmologists consider factors like the average motions of galaxies, the universe’s shape, and the amounts of dark matter and dark energy. Upcoming surveys from space telescopes are expected to clarify whether dark energy is a constant energy intrinsic to space, a time-varying quantum field, or something else. There is a strong consensus that the universe is flat and will continue expanding forever.