SOME day in the distant future, the whole universe will end. has made it her business to understand how. A cosmologist at North Carolina State University and one of the most popular scientists and science communicators on social media (her twitter account has over 350,000 followers), she has studied everything from dark matter and black holes to how the universe began, evolved and will eventually end – even inspiring a line by Irish chart-topper Hozier in which he sings “as Mack explained, there will be darkness again”.
How and when that darkness will come is the topic of her book, of Everything (Astrophysically Speaking), set for publication on 4 August by Scribner in the US and Allen Lane in the UK. It is a daunting topic, but Mack has found that focusing on such enormous cosmic questions can bring a degree of comfort in these troubled times. She spoke to 91av about her own story and that of the universe.
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Leah Crane: How did you get into cosmology?
Katie Mack: As a kid I was always taking things apart and trying to understand how they worked. At some point, I was exposed to Stephen Hawking’s writing, and that personality trait extended to the universe. I was always drawn to the weird stuff like black holes and time travel, things that didn’t have an easy, intuitive explanation. I did lots of reading about Hawking and I understood that he was called a cosmologist, so obviously I wanted to be a cosmologist too.
What big cosmological questions are you and your colleagues working on right now?
The biggest questions in cosmology right now are really around dark matter and dark energy. We have this weird situation where we’ve been able to quantify what the universe is made of to a very high degree. We can say what fraction of the universe’s energy density is matter and what is radiation, and we found out that a large proportion of the universe is made up of these invisible substances called dark matter and dark energy.
Dark matter makes up about 27 per cent of the universe, but we only know it’s there because of the gravitational force it exerts on regular matter – we can’t see it. We think that dark energy makes up even more of the universe, about 68 per cent of everything. We know even less about dark energy, only that it makes space expand so that galaxies move apart at an ever-accelerating rate.
It’s a confusing situation where we can describe the universe perfectly, except for the fact we don’t understand its two biggest components.
Surely there are some other big things that we don’t understand?
There are also questions around the beginning of the universe. We think that the big bang, which was the beginning of the universe as we know it, happened about 13.8 billion years ago, and the first tiny fractions of a second after that saw the universe expand exponentially in a process called inflation. Most cosmologists agree that it happened, but there’s no solid theory on what would have caused it.
Jumping to the far future, your upcoming book, The End of Everything (Astrophysically Speaking), is about the end of the universe. Do we know how that will happen?
There are several possibilities that I discuss in my book. The one that I think is most likely based on current data is called the heat death.
If the universe is expanding, and if its expansion continues to speed up, then space will get more and more dilute over time, which is to say there will be more and more space between each galaxy. Eventually, space gets so dilute that matter in the universe becomes less and less important. Galaxies stop colliding with each other, so they aren’t bringing in enough gas to make new stars and the old stars are burning out. Even black holes will disappear.
As time goes on and things decay, that increases entropy, which is the disorder of the universe. If you leave the universe alone for long enough and it’s decaying over time, you end up in this maximum entropy state where all that’s left is this tiny amount of background radiation known as waste heat. Once you get to maximum entropy, nothing else of importance can really occur.
Of all the possible scenarios, which is your favourite?
My favourite scenario is vacuum decay. It’s this idea that’s been around since the 1970s that our universe might not be entirely stable. It’s all based on the Higgs field, which is a field related to the Higgs boson, the particle that was discovered at the Large Hadron Collider (LHC) at CERN [the particle physics laboratory near Geneva, Switzerland] in 2012. The energy of the Higgs field determines whether the universe is in its lowest possible energy state, known as a true vacuum, or a false vacuum, which is a slightly higher energy state.
The conditions in the early universe determined which state the Higgs field would be in, and if it’s in a false vacuum state, then it could spontaneously transition to a true vacuum. That would rewrite all the laws of physics and constants of nature as we know them.
Physicists call this process vacuum decay. In the universe after vacuum decay, the new laws of physics would make it impossible for, say, molecules to exist, because the way that atoms interact with one another would be different. Space itself would be unstable, and eventually everything would collapse into a black hole.
What could make this happen?
Imagine balancing a glass right on the edge of the table. It’s fine right now, but it would prefer to be on the floor because that’s the lowest energy state, and something could happen at any time that could push it over. Similarly, it’s possible that our universe prefers a different value of the Higgs field and the slightest touch could knock it over. Like the glass, it would be more stable, but it would be broken.
There are two ways for this to happen. One would be that something disturbs the Higgs field. That would have to be an extremely high-energy event, much higher energy than we can even imagine. When the LHC first started up, there was some worry that its collisions could create a high enough energy to disturb the Higgs field, but they are nowhere near powerful enough to do that.
The other idea is that the transition could happen spontaneously through a phenomenon known as quantum tunnelling. If you have a particle on one side of a wall, quantum mechanics says it’s possible for the particle to spontaneously appear on the other side. In theory, if you put a glass on the edge of the table, all its constituent particles could align and allow it to just spontaneously quantum tunnel to the floor. It’s extremely unlikely to happen, but we can’t rule it out.
If something like this happened to our universe, a bubble of the new vacuum would spontaneously form within it: a region where we can’t exist, because our molecules would fall apart, and space itself collapses. And it would expand at roughly the speed of light. It would plough through the universe and destroy everything within it. If it got you, you wouldn’t see anything or feel anything: you’re just done. It’s this very dramatic way to destroy the universe.
Should we be worried about it?
There are several reasons not to worry about vacuum decay – for one, the false vacuum is predicted to stay stable for way longer than the current age of the universe – but physicists are paying a lot of attention to it now because our experiments do suggest that it’s possible.
What comes after the end of the universe? Is it just nothingness?
In my book, I define the end of the universe as the end of our observable universe – the volume of space that we can interact with, that has any impact on us or that we have any impact on. If everything in that region is destroyed, I rate that as the end of the universe. It doesn’t mean that there couldn’t be more space beyond that where more things continue, or another universe after ours, but for us, the end of the observable universe is the end.
I have friends who don’t want me to talk to them about space because it’s big and scary, but I personally find it somewhat comforting. How do you feel about the end of the universe?
When I was putting together this book, I interviewed a bunch of other cosmologists and astronomers about how they feel about the end of the universe: are they sad about it, or have they come to terms with the fact that we won’t go on forever?
A few people said that it was really sad. One person said that when she gives lectures about the heat death, people sometimes cry.
I haven’t really decided how I feel about it yet. I’m still kind of trying to wrap my head around it in some meaningful way. I am somebody who is not at all comfortable with the idea that I will die some day, for example. Intellectually, I know that that’s true, but it’s also terrifying. So the idea that the whole universe will die some day, that everything I love and care about will be over, is hard to wrap my head around.
“The idea that the whole universe will die some day is hard to wrap my head around”
Does thinking about things on this massive scale help you put daily troubles in perspective?
There’s something about studying the forces of nature that changes how you view everyday life. It doesn’t so much make everything insignificant, but it makes clear how little control we really have.
We live in a society with the illusion of control, and there’s a sense of security in how much we’ve altered our surroundings and built a world that suits us. But when you get to the bigger picture, we’re this tiny little speck of dust adrift in the cosmos with no say over what happens to our cosmic environment or the universe as a whole, however much we eventually come to understand it.
Studying these kinds of things, it’s not like it’s reassuring at all, but it chips away at the illusion of control in a way that lets you step back a little bit. Sometimes things are just going to happen and the universe doesn’t care about any of it.
All we can do is make the best out of what we have. There’s some amount of comfort in the fact that we’re all in this together, at the mercy of some of these bigger forces, and that’s OK.
