The universe as we know it will not last forever. Based on our current scientific understanding, the universe is continuing to expand at an accelerating rate. This means that over vast timescales, galaxies will spread apart from each other and stars will exhaust their fuel sources and burn out. Eventually, the universe will become dark, cold, and empty. But what exactly will the universe look like after its “heat death”? And could anything new emerge after the current universe dies out? Let’s explore some leading scientific theories about the far future fate of the cosmos.
The Expanding Universe
Our universe began with the Big Bang about 13.8 billion years ago. Space itself has been expanding ever since, carrying galaxies along with it. The universe is not expanding into anything outside of itself – instead, more space is being created between galaxies as expansion continues.
This expansion is accelerating due to a mysterious force called dark energy, which makes up about 68% of all the energy and matter in the universe. As dark energy pushes galaxies farther apart, the universe becomes increasingly diluted.
Key Facts About the Expanding Universe:
- The universe has expanded from an extremely hot, dense state into a vast, cool, and sparse state over billions of years.
- Expansion will continue accelerating indefinitely due to dark energy.
- Over trillions of years, galaxies beyond our Local Group will no longer be visible as they accelerate away from us.
The accelerating expansion of the universe will have stark consequences as we peer deeper into the far future.
The Stelliferous Era
For the next 10-100 trillion years, the universe will remain filled with stars and galaxies like those we currently observe. This long period of cosmic history is known as the stelliferous era.
Within galaxies, stars will continue forming from clouds of gas and dust. Stars will also continue undergoing various stages of their life cycles, ultimately dying in explosions called supernovae.
Key Facts About the Stelliferous Era:
- Star formation will continue in galaxies, though at a slowing rate over trillions of years.
- Main sequence stars like our Sun will continue fusing hydrogen into helium.
- Massive stars will exhaust their fuel quickly and end as supernovae.
- Over long time periods, galaxies not bound to the Local Group will accelerate away and become invisible.
During this era, the universe will still seem familiar to astronomers with galaxies full of shining stars. But gradually over trillions of years, the stellar landscape will change.
Degenerate Era
After 10-100 trillion years, the stelliferous era will come to an end as star formation ceases. The universe will transition into what is known as the degenerate era. With no new stars being born, existing stars begin to die out one by one.
Low mass stars like red dwarfs will have incredible lifetimes, burning for 10-100 trillion years. But eventually, even red dwarfs will die, ultimately leaving behind white dwarfs, brown dwarfs, and black dwarfs – dense stellar remnants that no longer shine.
Key Facts About the Degenerate Era:
- Star formation will end as galaxies exhaust their supplies of hydrogen gas.
- Long-lived low mass stars will be the last to shine before running out of fuel.
- White dwarfs, brown dwarfs, and black dwarfs will be the most common celestial objects.
- Ordinary matter will exist largely as stellar remnants.
The universe will become much darker during this era, with only faintly glowing white dwarfs providing any illumination. Protons may decay during this time, leaving behind only radiation and black holes.
Black Hole Era
After an unimaginably long degenerate era, black holes will dominate the universe. With no stars left to form, stellar-mass black holes will only slowly evaporate via Hawking radiation. Supermassive black holes will persist for even longer.
The black holes that do not merge together will continue orbiting and will occasionally gravitationally slingshot stars or stellar remnants outward at high speeds. But overall, large-scale structures like galaxies and clusters will disintegrate.
Key Facts About the Black Hole Era:
- Black holes ranging from stellar-mass to supermassive will outlive all stars.
- Black holes will slowly lose mass through quantum Hawking radiation.
- Galaxies and other large structures will disintegrate.
- Gravitational slingshots around black holes will fling some matter outward.
Black holes will essentially be the only survivors from the earlier universe. Their immense gravity will continue warping spacetime around them.
Dark Era
After an absolutely unfathomable 10100 years or more, even the longest-lived black holes will dissipate through Hawking radiation. The universe will transition into the so-called dark era once the last black hole completely evaporates.
At this point, the universe will be almost entirely empty. The only contents will be stray photons, neutrinos, electrons, positrons, and perhaps other more exotic elementary particles. There will be no more stars, galaxies, planets, or black holes.
Key Facts About the Dark Era:
- Black holes will completely evaporate via Hawking radiation.
- Only elementary particles like photons and leptons will remain.
- The universe will become almost indistinguishable from being empty.
- This era could last for an infinite duration.
The dark era will have essentially no structure or boundaries – just randomly distributed particles at incredibly low densities. This dismal state could theoretically last forever if the universe continues expanding exponentially.
Theories for the Ultimate Fate of the Universe
While the dark era may appear bleak, it does not necessarily represent the absolute end of the universe. Some speculative hypotheses propose possible futures that could emerge after an almost infinitely long dark era.
Big Crunch
One alternate idea is that the universe could eventually stop expanding and reverse course into a cosmic contraction leading to a “Big Crunch.” This requires the currently observed cosmic acceleration to someday reverse sign for an unknown reason. If the universe recollapses, densities and temperatures would once again climb exponentially. This might allow another Big Bang and new universe to be born.
Vacuum Metastability Event
During the dark era, a spontaneous quantum event called “vacuum metastability” becomes a possibility, however remote. This involves quantum tunnelling producing a small bubble of lower energy vacuum that expands at the speed of light. If this were to occur, it could trigger a phase transition akin to a new Big Bang.
Proposals Involving String Theory
Some proposals originating from string theory discuss the possibility of quintessence fields causing an eternal cosmic acceleration, or our three dimensional universe experiencing interactions with a higher dimensional “bulk universe.” These exotic concepts remain highly speculative at this time though.
Theories of Cosmology Beyond the Observable Universe
While we can only see a finite volume of the universe, some hypotheses like eternal inflation propose an incredibly vast or infinite universe far beyond what we can observe. The death of our local region of the universe does not preclude existence continuing elsewhere at scales beyond our cosmic horizon.
Cyclic Models and Cosmological Natural Selection
There are also models like conformal cyclic cosmology and cosmological natural selection in which universes spawn “daughter universes” with their own big bangs and expansions. These concepts present the possibility of cyclic patterns continuing indefinitely in some form.
Conclusion
The far future death of our universe is inevitable, but many uncertainties remain around the way it will unfold. Trillions of years from now, the last black holes may dissipate away into the darkness. While our current universe would end, the bigger picture for the ultimate structure and evolution of space and time remains an open question. Theories suggest our Local Universe could represent just one component in a possibly eternal multiverse with complex dynamics we have yet to comprehend. The future fate of the total cosmos likely involves physics far more exotic than we can presently imagine.