I remember that time in 2012 when it had gotten into everyone’s minds that somehow the world was going to suddenly come to an end. The Mayan calendar would terminate in December, and it was obviously a clear indication they had predicted that an apocalyptic disaster would befall the planet. Maybe they just ran out of ink… Not to alarm anyone, but this time the world really is coming to an end, and the science’s there to prove it. Please, don’t panic.
In the early 20th century, astronomers began to look at the absorption spectra of stars in nearby galaxies, and they determined something rather peculiar. They noticed that the absorption lines had the same general pattern as stars in our own galaxy, but were shifted by the same relative amount towards the red end of the spectrum. These absorption lines were caused by the elements in the star’s atmosphere physically absorbing some of the wavelengths in the electromagnetic spectrum, which meant that when we measured the light emitted from the star, some of the colours were missing.
This ‘red-shift’ phenomena was accredited to the Doppler effect, which is when the frequency of a wave changes for an observer moving relative to its source. This effect can be witnessed regularly in everyday life. You might have noticed that when an ambulance drives past you, its siren increases in pitch as it drives towards you (the sound waves are bunched together and therefore have a higher frequency) and decreases when it passes (the sound waves are spread apart and have a lower frequency when they reach you).
It was implied from this observation that these nearby galaxies were gradually moving away from us. From further measurements of other galaxies around our own, we found that nearly all the galaxies were red-shifted. What was even more surprising was the fact that galaxies further away from us seemed to be moving at an even faster rate – all galaxies surrounding us are constantly expanding outwards with ever-increasing speed. It seems like no one likes us that much. This then soon led to the conclusion that the universe originated from a dense singularity in an immense expulsion of mass and energy, an event which we now know as the Big Bang.
Ever since this realisation, the ultimate fate of the universe has become a valid cosmological question. Many theories have been put forth which attempt to predict the universe’s demise. Each of them are as bad as the other, but there’s no harm in knowing how we’ll finally go, right?
The Big Freeze
In this particularly chilling scenario, the universe continues with its unhealthy growth, cooling down exponentially as it expands at a slower rate, asymptotically approaching absolute zero temperature. Stars run out of fuel and cease to shine, leaving the universe a dark, frozen wasteland. The light from any remaining stars is unable to reach any other object due to the accelerated expansion between them. Many black holes emerge from these deceased stars, dominating the universe, but they too fade away through Hawking radiation. Ultimately the universe will reach thermodynamic equilibrium, also known as maximum entropy, in which case it will no longer be able to perform any work. I know, it’s scary. The universe could get fired.
At the moment, this scenario is considered by scientists to be most representative of what will eventually occur. I must admit, it’s not a very dramatic exit.
The Big Rip
Again, we witness the universe expanding at an accelerating rate. This scenario relies on a certain property of dark energy (phantom energy in this case [which is an extremely cool name]) to be true, and which we currently know very little about. We do know, however, that it is accelerating the universe’s expansion. Essentially if this acceleration continues without limit, with the dark energy overwhelming all other forces, all the matter in the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, will be ripped apart. At a finite time in the future, all distances in the universe will be infinite. That is, of course, if those two things still exist.
The Big Crunch
In this final scenario, we assume that the average density of the universe is great enough to stop its expansion through the mutual gravitational attraction of all its matter. It then contracts inwards and begins to collapse on itself, ultimately ending at a black hole – like singularity, a reverse process of the formation of the very same universe. In some ways I find this model rather elegant; the universe pops into existence, ballooning outwards before losing air and shrinking back to whence it came. It’s certainly better than simply existing as pure nothingness.
Some scientists believe that another Big Bang can occur after the universe has collapsed into the singularity. This then creates a continuous cycle of explosion and collapse, Bang and Crunch, which is known as conformal cyclic cosmology, or the Big Bounce. The universe we know could just be one iteration in this infinite cycle of universe recycling.
I just hope there really is a restaurant at the end of the universe. I’m gonna be peckish.