As a theoretical cosmologist, you would think I'd welcome the chance to answer these questions - but it isn't clear this is an inquiry that physics can answer, says Chanda Prescod-Weinstein.
THERE are two questions that I somewhat dread when communicating science to the public. And, unfortunately, they are rather common, because they are interesting to everyone, myself included.
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The first is: “What was there before the beginning of the universe?” The second: “What is space-time?” You might think that I would actually relish the opportunity to answer such queries because they go to the heart of what theoretical cosmologists like me do. It is literally in the job description, which I often tell people is to try to understand the origin and evolution of space-time and everything inside of it.
Despite this clear and rather monumental mandate, I actually try to avoid answering the question of how the universe began or what was there before, because it isn’t clear that this is an enquiry physics can answer. Let me refine what I mean a bit. First, consider a definition of “the universe”. Technically speaking, I mean space-time, which is the place where everything we know exists.
It is the only place we know; the only cosmic home we can ever have, as far as we are aware. We have already run into my other problem: trying to define space-time for audiences. It feels like it should be simple. We are used to thinking of a place as existing in terms of “space”: a physical location independent of time, the basis of Isaac Newton’s view of the cosmos.
But Albert Einstein’s relativity showed us that, in order to properly account for the finite speed of light, we could no longer think in these terms. Instead, space and time must be one phenomenon, where the different components mix. This feels like a rather dramatic conclusion to come to, given that the only new ingredient that we added to classical, Newtonian physics was a finite speed of light.
But this is a lesson in how one change to the physical laws can radically transform our mechanical understanding of the cosmos. The finite speed of light suggests that no matter what speed someone is moving, they have to measure the same speed of light as someone else moving at a different speed. When we sit down to work out the equations that describe this, the only way to make each person measure the same speed of light is to conclude that time flows differently for people moving at different speeds relative to one another. This outcome implies that space and time can’t be separated from one another.
Combining space and time into one inseparable entity is so outside the realm of everyday life that for most people, if not all, this will feel unintuitive and therefore wrong. But one of the lessons we must learn as empirical thinkers is that our intuition is a helpful, but imperfect, guide, and the universe is bigger than our experiences. And by the universe, I now specifically mean space-time, which is 100 quadrillion times bigger than Earth – almost unfathomably large.
You might say that I literally just explained that space-time and space aren’t the same thing, yet here I am comparing space-time to Earth. You see why I dread this conversation a bit? It is partly because the questions I get in response are smart. And this is a good point, that it can feel a bit like I’m comparing apples and oranges. But to be clear, I haven’t suggested that we can no longer measure distances. The number I quoted above is a ratio based on the size of the observable universe: it is what we can see.
And here is what is interesting about the size of the observable universe: it stretches 46.5 billion light years in all directions. So you would imagine this implies that some of the light we might see could have been travelling to us at the speed of light for 46.5 billion years and that 46.5 billion years is the age of the universe. In fact, the universe is only about 13.8 billion years old.
We are tempted to arrive at the first, wrong answer when we ignore the fact that space-time is expanding, carrying light along with it and stretching it out. The expansion and changing size of space-time is a dynamical effect that is produced through the mixing of space and time together. It is in this sense that I compare the size of Earth and the universe itself.
But now we are talking about an age, that raises the obvious question of the universe’s birth. Look, I’m happy to discuss what happened when the universe was 10−36 seconds old. This was the era of inflation, when space-time expanded exponentially. Inflation is so far the best explanation we have for why, at large scales, our universe roughly looks the same in every direction, from every location. So it isn’t like I am unwilling to discuss the almost beginning of space-time. But the answer is that our equations become completely dysfunctional at time zero. We have no idea what came before.
Chanda Prescod-Weinstein is an assistant professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her research in theoretical physics focuses on cosmology, neutron stars and particles beyond the standard model.