Conformal Cyclic Cosmology: Is the end of the universe the beginning?
It is generally accepted in cosmology that around 14 billion years ago, all that existed was a singularity. A single, dense, hot point. This underwent a brief period of inflation before a subsequent expansion into the universe we know today. This process is described as the Big Bang — incidentally, a pejorative term coined by the astronomer Fred Hoyle in 1949 to describe the seemingly paradoxical appearance of reality from an explosion.
At the time, Hoyle favoured the steady-state model of cosmology, advocating for a universe that had been around forever, gradually creating more matter to fill the gaps as it expands. This model of a constant-density, eternal universe is now rejected by cosmologists. There is substantial modern evidence that suggests a finite-aged universe that started with the Big Bang and a fixed amount of matter that continually spreads out with the expansion of space.
But what if it were possible to reconcile the notions of an eternal universe with the Big Bang? Is it possible for us to have our cosmological cake and eat it too?
Our universe started in a surprisingly ordered — low entropy — state, which allowed it to fill itself with stars, galaxies, and life. As time moves on, however, the universe slowly decays into disorder — this is characterised by increasing entropy, a property that describes how energy in the universe becomes progressively less useful for doing stuff like forming stars, making vegan pizza, and existing.
Eventually, as the universe becomes more disordered, structures will decay, stars will die, and even black holes will evaporate. The increasing entropy of the universe will ultimately lead to the heat death of the universe. At this point, there won’t be much left apart from cold, dark space and the occasional photon travelling forever into the void.
It seems like a bleak picture for the universe. Compared to its apparent beginning, the ending is a damp squib. A slow fizzling out, with no possibility of anything else interesting happening. Unless, of course, you consider dark, cold, nothingness to be interesting, in which case, great (though shame you won't be there to enjoy it).
The story of our ending, however, might not be fully complete.
The theoretical physicist Roger Penrose advocates for another theory called Conformal Cyclic Cosmology (CCC). In his theory, as the universe tends towards its final state of cold, disordered emptiness, the notion of time starts to lose meaning. Even more, the large end state of the universe starts to look like a very small starting point. In a way that may be somewhat hard to grasp, the end of the universe starts to look a lot like the Big Bang. It’s a counterintuitive notion and, of course, like many good theories of nature, it requires a mathematical description to fully understand.
Yet the conclusion of CCC is clear, the final state of a universe could be the beginning of another.
If CCC is correct, our universe is one of an infinite lineage of universes, and will ultimately become the progenitor to the next in the chain. The Big Bang is both the start of our universe and the death of an older universe that came before (though one might argue that the notion of ‘older’ and ‘younger’ cease to be applicable over the aeons of time that Penrose describes between these universes).
Nevertheless, in CCC our universe is neither the beginning nor the end. It is simply one fluctuation in a state of eternal universal existence. Hoyle’s theory, whilst not completely correct, is somewhat redeemed after all. The universe is eternal, just not in the way he thought. Like the universe itself, perhaps our theories of reality come full circle too.
It’s an oddly comforting notion that we might just be one of many universes. It might make us feel small to realise we’re occupying an even less significant position than we once thought, but the stars in the sky have told us that from the beginning. An infinite cycle of repeating universes tells us that even the end of time isn’t truly an end but the beginning of a whole new aeon of stars, galaxies, and life.
Like any good physical theory, CCC makes some testable predictions. One of the most profound is that we should be able to see the remnants of dying black holes from the previous universe. The radiation left over from the evaporation of supermassive black holes might travel across the boundary from the end of one universe into the next. It’s just a case of spotting the temperature fluctuations when we observe the radiation in the sky.
Whilst it is still not completely clear if these distant swan songs are out there and whether they originated in a previous universe it’s an astounding possibility and one for which there is some contested evidence.
Intuitively the cyclicity of our cosmos feels right. Maybe because cycles and oscillations are everywhere: from life to astronomy, the seasons, mythology, economics and even down to the most fundamental quantum fields that underpin the particles in our world. The fact the universe itself might be another example of one of nature’s favourite hallmarks is an endearing idea.
Yet, the universe has persistently proven itself to be far stranger than we could ever imagine. Maybe space is like a phoenix in an eternal cycle of life and death. Maybe time repeats itself in an infinite recurrence of universe after universe. Or perhaps this is the only one after all, with a small, hot beginning and a cold, dark finale. Though, as Hoyle would tell you, when it looks like we’ve finally understood the cosmos the reality is often that we’re only just beginning.