The Big Bang or the Big Bounce? with Surjeet Rajendran

In this episode of the Believe Show, we hear from Surjeet Rajendran, a physics professor at Johns Hopkins University about his belief that the Big Bounce is a better theory of the universe than the Big Bang preceded by no space/time at all.

I’ve known Surjeet for over a decade. Today’s topic stems from a conversation we had recently. When Surjeet talked about the Big Bounce, I said, "What? Are you serious?" That’s why I wanted to have him on this show.

The below dialogue is less than half of the full episode, so if you want to hear it all, click here or play the embedded video below.

Senia: Within your field of particle physics, what do you believe that many other physicists do not not agree with?

Surjeet: We know that our universe right now is expanding. That's something that we have known a long time ago.

If you look at that observation, it makes you ask the question, "Let me turn the clock back and see how the universe must have been." If I turn the clock back, the universe gets smaller and smaller and smaller. Ultimately, it gets very, very small. What happens at that point?

Senia: What I learned when I was a kid is that the Big Bang happened, and everything exploded, and from that, everything started expanding.

Surjeet: Right. That's exactly what people say. They'll say there's something called a Big Bang. The fancy way of saying it is that there was what is called a singularity, a point of infinite density from which everything emerged. But what happened prior to the Big Bang?

Senia: That's funny. I've actually never thought about it. You certainly don't study in school what happened before the Big Bang.

Surjeet: Yes. Some people would even make the claim that, "That is not a well-defined question, because the laws of nature didn't exist then." When we hear a singularity, what it means is that our current laws don't work anymore. Then we can ask, "We hit a point where our theory broke down." What happened before that? One very reasonable possibility would have been that if you look back in time, the universe was shrinking, it got to some size, and then re-expanded.

That's what's called the Big Bounce. The universe was shrinking, then crunching, and then hitting a minimum size of some kind, and then bouncing and re-emerging.

Senia: A lot of physicists, and certainly mostly people, believe in the Big Bang, right?

Surjeet: Yes.

Senia: Your belief is different?

Surjeet: It's a question of what one means by the Big Bang. There is a sense in which I believe in the Big Bang, which is to say that there was a point of very large energy density from which we emerged.

Some people think of the Big Bang as a point before which there was no space-time at all. There is a pretty significant belief in the physics community that before that, even space-time did not exist. That is the version of the Big Bang that I don't believe.

Remember, for a physicist, "nothing" is very interesting. When we think of nothing, we call it a vacuum. A vacuum still has space and time. It just has no particles. But when these guys are talking about nothing, they're talking about not even space and time existing.

Senia: What does that mean, space and time don't exist?

Surjeet: It's a very weird field. We don't know what it means. There's a strange equation, the Wheeler-DeWitt equation, that predicts various things. One of the things it can do is produce a universe from nothing, literally nothing.

I don't think it makes a whole lot of sense.

From the physics perspective, if I go back in time, I get to a situation where the densities get very large. General relativity has broken down. I don't see any reason why we have to give up on the notion of space-time itself at that point.

We can ask, "What are the possible behaviors that that phase of matter could have had?" One possibility is that the universe gets to the high density and just remains that way for a long time, even forever. Or the universe could have gone to that high density and then started back toward lower and lower density.

Senia: What would cause that? What would have caused everything to go from expanding to contracting and then to expanding again?

Surjeet: People skeptical of the bounce might say, "Let's look at what a bounce requires. A bounce requires matter to go towards each other as the universe is crunching, and then it has to re-expand."

How can matter affected by gravitational forces re-expand? How can matter bounce out? It has to fight off gravitational attraction.

That is what many people believe is impossible, and I think is actually very reasonable. I have multiple reasons for this.

First, if you think about a bounce, what is the hard part of a bounce? The hard part of a bounce is not matter going towards each other. That's super easy. It is very common in our world that gravity attracts stuff. So the fact that matter wants to go towards each other is a very normal phenomenon that we see all the time.

The hard part of a bounce always is the re-expansion. How did the matter emerge from this high density state? I don't know how it exactly happened, but it happened, in the sense that whether I believe in my bounce or something else, it is certainly the case that when we go back in time, we came from a point of very large density, and we emerged from it. Right?

Senia: You're making a logic-based argument?

Surjeet: Yes. Is it reasonable that this phenomenon happened? I would say it is reasonable that the phenomenon happened, because we exist. I may not have the equations for it, but we clearly got out somehow.

Secondly, this also ties in with another problem in physics. It is funny because everyone believes this phenomenon, called black hole evaporation.

Senia: What is black hole evaporation?

Surjeet: The conventional picture is that nothing ever gets out of black holes. But Stephen Hawking showed that picture is not consistent with quantum mechanics. Black holes have to evaporate away.

A black hole forms and then evaporates. It produces radiation and then disappears.

Everybody believes this. If I look at how this black hole process happens, it's very much like a bouncing universe. I take a bunch of particles. I put them all together. Gravity gets so strong. They form a black hole. We don't have the equations to describe the next step very well, but everyone believes that that black hole doesn't remain a black hole. It produces particles that get out.

Getting People to Consider the Bouncing Universe

If you put these two facts together, the fact that we emerged from a point of very high density, and the fact that black holes evaporate, there are extremely good reasons to believe that matter can emerge from strong gravitational forces. Once matter emerges from strong gravitational forces, there is nothing wrong with the the Big Bounce.

We had a paper a few years ago about how a bounce could be achieved. In fact, it was even calculable, without even relying on unknown theories. It's a very technical paper, and people didn't spend the time to understand it. I'm trying to motivate people to understand it more. The hard part is that people need to believe something before they spend time on it. So, I need to flip beliefs. I have found two direct questions useful.

1. What do you think happened in the Big Bang? We got out of there, right? So, however we got out means that we were able to beat strong gravity.
2. If you’re like most physicists, you believe that as gravity gets strong, a black hole forms, and then the black hole evaporates. So why do you think shrinking and re-expanding is not possible with the entire universe?

Senia: What is one thing you would like us to know about physics, even though we're not physicists?

Surjeet: I do physics because it's a lot of fun. It's an incredible privilege to be able to think about these totally crazy things. People often view physics as very mathematical or technical. But in reality, what’s most interesting in physics is coming up with simple questions that get you to deep things. Thinking of something simple like, "Where did I come from?" can lead to all kinds of amazing stuff.

Photo by Sammie Chaffin on Unsplash

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