13.3 The far future of galaxies and structures

Evolution and Fate of Galaxies and Structures

Galaxies aren't static objects. They merge, consume each other, and slowly burn through their fuel over cosmic timescales. As the universe ages and expands, these structures will become increasingly isolated, cool, and dark. This section traces that trajectory from the galaxy mergers happening now to the final fate of matter itself.

Evolution of galaxies over time

Galaxies evolve primarily through mergers and the gradual depletion of star-forming material. Both processes push galaxies toward a quieter, redder future.

• Mergers and interactions will continue to reshape galaxies. Smaller galaxies get absorbed by larger ones. The Milky Way and Andromeda, for instance, are on a collision course and will merge in roughly 4.5 billion years. The result of major mergers like this is typically a giant elliptical galaxy, a large, smooth blob of stars with little organized structure.
• Star formation will decline over the next trillion years as galaxies exhaust their reserves of gas and dust. Without raw material, fewer new stars can form. The universe's peak era of star formation actually occurred about 10 billion years ago, so we're already on the downslope.
• Galaxies will redden as their stellar populations age. Short-lived, hot blue stars (blue supergiants) will die off first, leaving behind long-lived, cool red dwarfs that burn their fuel so slowly they can shine for trillions of years. The overall color of galaxies will shift from blue-white to deep red.
• Dark energy and accelerating expansion will push distant galaxy clusters beyond our reach. Galaxies outside the Local Group will recede faster and faster. Over time, they'll cross the cosmic horizon, the distance beyond which light can never reach us. The observable universe will effectively shrink, even as the actual universe grows.

Concept of island universes

The term "island universes" here doesn't refer to the old historical usage (where it meant individual galaxies). In this context, it describes the fate of gravitationally bound galaxy groups that become completely isolated from each other by cosmic expansion.

• Galaxy clusters that are gravitationally bound, like our Local Group (containing the Milky Way, Andromeda, and a few dozen smaller galaxies), will hold together against the expansion. Gravity within these groups is strong enough to resist the stretching of space.
• The space between clusters, however, will expand faster than light can cross it. Clusters separated by billions of light-years today will become permanently unreachable from one another.
• Within each island universe, galaxies will still interact. Mergers, tidal stripping, and gas exchange will continue to reshape the member galaxies. But there will be no new material or information arriving from outside.
• The accelerating expansion means light emitted from one island universe will never reach another. Each bound group becomes a self-contained cosmos, with no way to observe or communicate with anything beyond it. An astronomer in the far future, born into one of these island universes, would see nothing beyond their own cluster and might reasonably conclude that nothing else exists.

Ultimate fate of celestial objects

On truly enormous timescales, even the stars and remnants within these island universes will wind down.

Stellar evolution plays out by mass:

1. Low-mass stars (like red dwarfs) will exhaust their hydrogen and become white dwarfs, which then slowly radiate away their residual heat over trillions of years, eventually becoming cold, dark black dwarfs.
2. High-mass stars will end in supernovae, leaving behind neutron stars or black holes depending on the mass of the collapsing core.

After the last stars die, the universe will be populated almost entirely by stellar remnants: white dwarfs cooling toward invisibility, neutron stars, and black holes. This transition takes place over roughly $10^{15}$ years (a quadrillion years, far longer than the current age of the universe at ~$1.4 \times 10^{10}$ years).

Black holes evaporate. Through Hawking radiation, black holes slowly lose mass by emitting particles. The process is extraordinarily slow. A stellar-mass black hole would take around $10^{67}$ years to evaporate, and a supermassive black hole could persist for $10^{100}$ years or longer. But given enough time, even they vanish.

Proton decay remains an open question. Some grand unified theories predict that protons are not truly stable and will eventually decay into lighter subatomic particles (positrons and pions, which further decay into photons and leptons). If proton decay occurs, it would happen on timescales of $10^{34}$ to $10^{41}$ years, meaning all ordinary matter, every atom, would eventually disintegrate. This has not been experimentally confirmed, but it's a real theoretical possibility.

Heat death is the projected endpoint. If these processes play out, the universe approaches maximum entropy: energy is spread perfectly evenly, temperature is uniform and near absolute zero, and no gradients exist to drive any physical process. No work can be done, no structures can form, and nothing changes. The universe becomes a vast, cold, empty expanse, effectively frozen in time.