Past: Originated from the Big Bang about 14 billion years ago.
Present: Universe is expanding at an accelerated rate.
Future: Uncertainty about how the universe will end; three main scenarios are proposed.
Three Scenarios for the Universe's End
The Big Freeze
Mechanism: If the force pulling galaxies apart (repulsive force) is strong enough to stretch the gravitational force (rubber band) until it loses elasticity.
Outcome:
Universe expands but stops accelerating.
Galaxies separate, becoming lonely objects in space.
Emitted light redshifts to long wavelengths, low energy.
Gas too thin to form new stars.
Leads to a dark, cold universe (also called Heat Death or Big Chill).
The Big Rip
Mechanism: If the repulsive force is stronger enough to stretch the rubber band past its limit, causing it to tear.
Outcome:
Continued acceleration of the universe's expansion.
Overcomes gravitational and all other forces holding matter together.
Matter breaks down into smaller pieces, leading to destruction of atoms and subatomic particles.
The Big Crunch
Mechanism: If the gravitational force overcomes the expansion force, reversing it.
Outcome:
Galaxies and stars rush towards each other, colliding and clumping together.
Temperatures rise as space tightens.
Compression leads to a very dense, hot, compact state similar to pre-Big Bang.
Additional Theory: The Big Bounce
Concept: A universe that expands and contracts repeatedly through cycles of Big Bangs and Big Crunches.
Characteristics: Each bounce erases the previous history.
Determining the Future Scenario
The ultimate fate depends on:
Shape of the universe.
Amount of dark energy it contains.
Changes in expansion rate.
Current observations suggest a trend towards the Big Freeze.
Timeframe
Estimated time before the Big Freeze occurs: about 10^100 years (extremely far future).