The End of Everything (Astrophysically Speaking) by Katie Mack

Reviewed by Terence Jagger

Katie Mack is an American astrophysicist, but her writing is very informal and almost journalistic or chatty – which is great for a subject like this, because, let’s face it, most of us (specialists in Anglo-Saxon history like me, for example) are not going to understand more than the bare bones.  She is interested in the end of everything, that is, how the entire cosmos might come to an end (if indeed it will) – not just our solar system, or our galaxy, the Milky Way, but the whole boiling.  It’s quite a question, and I was amazed to find out that the possible answers, in broad terms, are reasonably well defined and not too hard (relatively speaking, of course, they are pretty mind-bending really) to get some sort of grasp on. It’s a surprisingly fun and easy read, and if you have any interest in such things, I would certainly recommend it.

The five scenarios she examines are punchily named Big Crunch, Heat Death, the Big Rip, Vacuum Decay and the Bounce.  The good news is that four of them are unimaginably far off in the future, and the fifth – though it could happen before I get to the end of this sentence – is staggeringly unlikely but even if it did occur, is the one of which we would have literally no warning. (Phew, made it.) It would literally be life as normal one instant, then the rest is silence.

The universe is roughly 14 billion years old, though our earth is much younger.  And we are pretty sure how it all started, in a singularity and a big bang.  And its been expanding ever since, possibly at an ever increasing speed.  And we are very confident of the answer to the first “end of” question – how will the world end?  This has been

… the subject of speculation and debate among poets and philosophers throughout history.  Of course, now, thanks to science, we know the answer: it’s fire. Definitely, fire.  In about five billion years, the Sun will swell to its giant red phase, engulf the orbit of Mercury and perhaps Venus, and leave the Earth a charred, lifeless, magma-covered rock.  Even this sterile smouldering remnant is likely fated to eventually spiral into the Sun’s outer layers and disperse its atoms in the churning atmosphere of the dying star.

So we’ll all be long gone by then. But that isn’t thinking big enough for Mack, she wants to know about the end of everything.

Big Crunch  This is the idea that the expanding universe will one day reverse and bounce back on itself, just as the ball you throw up into the air slows, stops (for an infinitesimal fragment of time), and then falls back to you.  To know if this might happen, we need to know how much stuff there is – and therefore whether it generates enough gravity to pull everything back together one day – and that’s quite hard.  And another fascinating point is that if it did start we (not that there would be any life in our corner of the cosmos) wouldn’t be able to tell for a very long time, because the speed of light limits what we can see. Long after the shrinking has started, we would still see the universe expanding.  But the best understanding at the moment is that, no, we won’t collapse, but that the universe will keep expanding and expanding.  So no Big Crunch (tho’ the alternative is not much more alluring).

The alternative is Heat Death.  Everything expands, gets further and further apart, and gets colder and colder – and even the fundamental particles decay.  We achieve maximum entropy (disorder) at about 10-40 degrees above absolute zero – and then nothing can ever happen again, ever. But that’s tens of billions of years away, if that is indeed the answer.

Big Rip This is really tough to grasp, and has to do with dark energy (put it in the too hard tray and skip to the next bit). There is an outlandish chance that space itself will expand catastrophically – I think that’s right – and that means that everything falls apart – the planets float away from the sun, the stars from their galaxies, and the constituents even of the very atoms that make up all matter break up.  It would be extremely fast once it happened, though “we” would have some warning as the universe changes and loses shape, but there’s no point losing sleep over this – the earliest possible date is about 20 billion years off.

Vacuum Decay This really weird too, the idea that our physics is extremely finely balanced and could, in effect, be upset – vacuum decay – by a sufficiently large disruption – probably one, reassuringly, much much bigger than any cosmic event we know about, and if it did occur, it would likely be hundreds of billions of years off.  But this is the one you will never know about, because the collapse will happen at the speed of light, so the information that something is happening will arrive at the same instant as the thing itself. So no time to understand, or to worry – just instantaneous annihilation.

Bounce  “On September 14, 2015, at 9.50am .. you were, for the briefest moment, just a little bit taller.” You only grew by one millionth of the width of a proton – and that was soon reversed – but that’s when a gravitational wave, the first we have ever recorded, washed through everything on Earth (and a lot wider, of course). An intriguing possibility arising from the existence of gravitational waves – and the extraordinary weakness of gravity – and our failure to integrate it into a unified model with the other fundamental forces – is that gravity is leaking, and that we are in an “ekpyrotic” universe (and no, I don’t quite follow why either).  This is a cyclic universe, of continual expansion and shrinkage, in which our universe meets another universe, simultaneously obliterating them both, starting off a new cycle.

Mack accepts the futility of trying to know the end of the universe – even if we did finally know it perfectly, that knowledge would be destroyed by the end of the universe itself, or earlier.  But as she explains, there is no alternative, for her at least, to wanting to know:

We’ll continue on, making new paths through the woods to see what we might find …Someday … the cosmos itself will come to an end. In the meantime, we have the entire universe to explore, pushing our creativity to its limits to find new ways of knowing our cosmic home. We can learn and create extraordinary things, and we can share them with each other. And as long as we are thinking creatures, we will never stop asking “What comes next?”.

John Donne, Elizabethan and Jacobean poet and preacher extraordinaire, apparently hoped that the end of the world would come in his lifetime, so great was his fear of death.  That’s not a comfort this book can offer you, but it is fascinating and exciting, even if totally irrelevant to our quotidian existence. Have fun!

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Katie Mack, The End of Everything (Astrophysically Speaking) (Allen Lane, 2020). 978-0241372333,  210pp., hardback.

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