Physicists have a competent understanding of the universe’s origins. This is in part due to the fact that the history of the universe is preserved in the stars and cosmic microwave background radiation. Light travels at a fixed speed. When light from an object 12 billion light-years away reaches the Earth, astronomers are observing that object as it was 12 billion years in the past. Yet the exact fate of the universe remains uncertain. Unlike the past, the future of the universe is not imprinted in the stars. Scientists must extrapolate the physics of today to make predictions about the future.
One problem with using the physics of today to make predictions about the future is largely philosophical rather than scientific. In particular, prior experience cannot determine what will happen tomorrow. Knowing that the sun has risen one-hundred times in the past does not guarantee that the sun will rise tomorrow. This thorn in the flesh is attributed to the philosopher David Hume, and is known as the problem of induction. Although physicists tend to ignore philosophy, the belief that the laws of physics of today hold true tomorrow is an assumption in science that no empirical evidence can suffice.
Philosophy aside, there are competing, scientific theories about how the universe will end. Most physicists believe that the fate of the universe lies in the hands of the cosmological constant—a mysterious repulsive force that increases with distance. Physicists used to believe that the expansion of the universe would gradually wane and collapse as a corollary of gravity. In contrast to gravity, the expansion of the universe seems to be speeding up rather than slowing down.
This theory is known as “the big freeze.” If dark energy overcomes the effects of gravity, the universe will expand forever. In this scenario, galaxies gradually drift away from each other. The galaxies drift so far apart that light emitted by galaxies never reaches another galaxy. This being the case, much of the evidence for the big bang will be obliterated. Billions of years into the future, the cosmological constant will become so powerful that it rips galaxies and their atomic constituents apart.
Another theory known as “the big crunch” predicts that the universe will collapse into a contracting fireball. Physicists used to believe that whether the universe reverts back into a state of oblivion was in the hands of gravity. Now, the fate of the universe rests in the hands of the cosmological constant. Yet the cosmological constant may not be a constant that stays fixed with the flow of time. It could be that the cosmological constant is a variable that fluctuates overtime. If the cosmological constant is unstable, it could decay and allow the universe to collapse. Although physicists have largely abandoned the big crunch theory, the collapse of the universe remains a real possibility.
Another scenario speculates that the laws of physics can change and shift the dynamics of the universe. In particular, if the universe exists in a false vacuum state, the constants of physics can sporadically change from one state to another state. In this scenario, the laws of physics change almost instantaneously, transcribe across the universe and obliterate everything embedded in the cosmos.
Apocalyptic scenarios about how the universe will end is known as eschatology and used to be based upon mythology and speculation. Remarkably, eschatology in now a real, sub-branch of physics and cosmology. Yet it could be possible that vital information about how the universe will end was obliterated in the past; in the same way light from distant galaxies will extend the horizon of aloof observers confined in another galaxy. If this is the case, current cosmological scenarios about how the universe will end may be just as mythological as any other story peddled by humans.
By Nathan Cranford