EPILOGUE
As we draw to the end of this book, it’s natural to think about how our Universe might also end. When we look out into space, on average, the light from nearly all galaxies is redshifted. They are speeding away from each other because the Universe is expanding. This discovery in the 1920s led to one of the most famous theories in the whole of science – that of the Big Bang. If you were able to rewind time on the Universe you’d see all the galaxies get closer together and all the matter get squashed down into an infinitely small space. Sound familiar? If you try and put a large amount of anything (mass, temperature, pressure) into an infinitely small space you’ll end up with a singularity.
One of the biggest misconceptions about the Big Bang theory is that it is a theory of the creation of the Universe, but it’s not. The Big Bang theory describes how the Universe went from an incredibly hot and dense state to evolve to give us the distribution and different shapes of galaxies we see today. It doesn’t explain what happens at that very first moment of ‘creation’ when time = 0. Our knowledge of physics allows us to rewind all the way back to when the Universe was just a scant 10-36 seconds old (a trillionth of a trillionth of a trillionth of a second), but before that all our known laws of physics break. The four fundamental forces of gravity, electromagnetism, the strong force (that holds atoms together) and the weak force (that governs radioactivity), behave completely differently and merge into one. To describe those moments we’d need a Grand Unified Theory (a GUT), which we don’t yet have. Similar to how Hawking needed a unified theory of quantum mechanics and general relativity to understand the entropy of a black hole, but one didn’t exist yet.
So the singularity at the beginning of the Universe is not well understood, but we know it has to be different from the singularity of black holes trapping everything beyond an event horizon, otherwise we wouldn’t all be here. For some reason, space started expanding, accelerated by something we call ‘dark energy’ but we have no idea what that actually is. The physics story is far from complete; there are far more mysteries for budding physicists to crack, standing on the shoulders of all those that have come before them that we’ve heard about throughout this book.
Just like in stars, the past 13.8 billion years of the Universe’s history have been a fight between the expansion of space outwards and the matter in the Universe causing gravity pulling inwards; it’s a fight that so far the expansion has won. But if we consider the eventual fate of the Universe, many billions of years in the future, it all depends on how much of the Universe’s energy budget went into powering the expansion and how much into making matter. If these two balance each other out then the Universe’s expansion will eventually slow, getting infinitesimally small. We have a hope of measuring this with something called the density parameter: the sum of the average density of all matter, radiation and dark energy in the Universe divided by the critical density that would perfectly balance out the expansion. If the density parameter is 1, then the expansion is perfectly balanced by the contents of the Universe and we know that eventually the Universe will reach equilibrium: a happy medium.
If the density parameter is less than one, then matter is out-gunned by the expansion, and the Universe will end in a ‘Big Rip’ scenario. The expansion will increase exponentially until it doesn’t just overpower gravity, but also the strong force binding together the particles in atoms themselves. The Universe would end up as a very sparse collection of lifeless particles.
If the density parameter is more than one, then matter outweighs the expansion. The expansion of space will start to slow before it is reeled back in and starts to contract in a ‘Big Crunch’. In this scenario all matter and energy in the Universe would be reeled back together, with pockets of the Universe becoming dense enough to form ultramassive black holes before they too are reeled down into one lone singularity. There’s something quite nice about this idea of the Universe being cyclical in nature, bringing the Universe right back to where it started. There are even some astrophysicists investigating the possibility of a ‘Big Bounce’, where the Universe cycles between Big Bang expansion and Big Crunch contraction endlessly.
To find out which of these scenarios is the eventual fate of the Universe, we can try to measure the density parameter. One of the most accurate measurements we have is from the WMAP116 satellite observing the radiation from the cosmic microwave background, an echo of radiation from the early Universe that reveals what the conditions were like back then. Combining the WMAP data with measurements of the expansion rate of the Universe using supernovae in the nearby Universe, gives a value for the density parameter of 1.02 ± 0.02. That ±0.02 is the uncertainty in the measurement, and means the value could be anywhere in the region of 1.00–1.04.
WMAP revealed that the Universe is tantalisingly close to being balanced, and yet that value errs on the side of matter winning out one day over the expansion. If the value truly is just that teensy weensy bit bigger than 1, then the ultimate fate of the Universe is a Big Crunch. All the stuff in the Universe reeled back into one final singularity: the black hole to end all black holes.
So even as you sit and read this, hurtling through space, happily shepherded around the supermassive black hole at the centre of the Milky Way with no danger of ‘falling in’, I’m sure, like me, you still can’t help but consider the inevitability of black holes. We are intrinsically tied to them in life, and in death our atoms may one day, in an unfathomably distant future, become part of the black hole at the end of the Universe. Let’s hope there’s a restaurant there too.