John: So let’s continue with our discussion of Hawking. Specifically, let’s look at his contributions to the big bang theory.

Maggie: That is such a great show!

John: Not the sitcom… I want to discuss the actual theory.

Maggie: I know. I was just being funny. But come on, how good is Jim Parsons as Sheldon?

John: He is so good. He is one of Hollywood’s greatest actors.

Maggie: I know! But I guess we are off track. Let’s move on.

**The Steady-State Model**

John: One of Hawking’s early theories helped advance the big bang model of the universe and put to rest the steady state model.

Maggie: Whoa, whoa, whoa… What is the steady state model?

John: It was a popular theory or model of the universe, which at the time competed with the big bang theory.

Maggie: What time are we talking about?

John: The middle of the 20^{th}century, when Hawking was just beginning his cosmological work.

Maggie: Got it. Can you tell me more about this theory before we discuss the big bang theory?

John: I thought you would never ask. The steady state theory suggests that the universe is eternal—it doesn’t have a beginning or an end. But it also recognized that the universe is expanding, as Edwin Hubble had already observed in 1929 when he formulated Hubble’s law. And because I know you are going to ask, Hubble’s law states that galaxies are moving away from the earth at speeds proportional to their distance. We don’t need to fully understand this, we only need to know that Hubble proved that galaxies are moving away (or receding), hence the universe is expanding. The steady state theory accepted this point.

Maggie: Wait, wait, wait, I have a question. If the universe is eternal and the universe is also expanding, wouldn’t the galaxies and even the planets be so far away from each other that nothing would be visible or noticeable? Come on, if they have been moving away from each other for ever, we wouldn’t even know they exist. Right?

John: So right, and such a good question.

In order to account for the density of matter in the universe, and the fact that the density of matter does not decrease while the universe expands, the model suggests that matter is continuously being created. In other words, new matter is popping up all the time. That is why the universe can continue to have the same density, but at the same time also expand.

Maggie: I don’t really understand the density of matter idea, but this is what I do get. In order to explain why we can still see other galaxies and planets, the steady state theory says that new matter is being created to fill the emptiness left by the expansion of the universe.

John: What a wonderful summary, and close enough for our purposes. You are so smart. I really enjoy talking about these ideas with you.

Maggie: Whatever… Moving on now… I am not buying it. I don’t know why, but in my gut this theory just seems funny.

John: Don’t discount your intuition, because Stephen Hawking didn’t buy it either. Rather, he bought into the big bang theory and helped prove that theory, making it the accepted model of the universe. We are going to discuss the big bang model soon, but before we do, do you want to hear something funny?

Maggie: Sure.

John: The biggest proponent of the steady state theory was someone we previously discussed: the astronomer Fred Hoyle. If you recall, he was the professor that Hawking wanted as his thesis supervisor, but didn’t get. He was assigned Dennis Sciama instead.

Maggie: Oh, that’s right. And then Hawking helped disprove the steady state theory. Funny.

And just think what might have happened if Hawking had been assigned Hoyle. We may not have ever had Stephen Hawking as we know him.

John: I know, it weird to think about. You never know where life is going to lead. But on to the next idea.

Maggie: I can’t wait. This is fun.

**Black Holes and Singularities**

John: When Hawking was a graduate student he attended a lecture by Roger Penrose.

Maggie: I know, the mathematician turned physicist. We discussed him earlier.

John: Right. Good memory. Penrose had been working on the idea of black holes.

Maggie: I don’t know what that means. It has always seemed so weird to me.

John: It is weird. When we use this phrase, we are talking about “gravitationally collapsed stars,” which I will explain. I think scientists just got tired of saying “gravitationally collapsed stars” in their papers and lectures and decided to shorten it to “black holes.” It’s just easier to say.

Maggie: Okay.

John: As we talk about this, remember our main principle: we don’t need to understand these concepts perfectly; we only need to understand them sufficiently for our purpose here. And although we will talk more about black holes in a later conversation, we just need to understand a little about them right now. Okay.

Maggie: I got it.

John: When a big star comes to the end of its life cycle and starts to run out of energy, its gravity takes over, causing it to collapse into itself. The gravity literally causes the matter in the star to shrink and shrink and shrink and shrink, and it gets smaller and smaller and smaller. As it gets smaller, the matter gets more and more concentrated, more and more dense. At some point, it becomes so small and so dense that space and time as we understand them cease to exist. And it is at this point that the gravitational pressure is so great that even light cannot escape it, which is why a gravitationally collapsed star appears dark to us, and why we can call them black holes.

And here is a term to consider.

Once the star collapses beyond a certain point, the gravitational pressure becomes so intense that the matter is said to be “infinitely dense.” This is a mathematical concept that Roger Penrose came up with, which is really hard for people like us to understand. Penrose called this area of infinite density a “Singularity.”

Maggie: A singularity? That is an odd thing to call it.

John: I know, but you know mathematicians. Nothing can be easy.

Why don’t you see if you can summarize what we have said so far.

Maggie: A black hole occurs when matter from a collapsing star becomes so small and so dense, and its gravity becomes so powerful, that even light can’t escape its gravitational force. And when the matter becomes dense enough, space and time cease to exist as we know it. The area in a black hole where matter is so collapsed that it is referred to as “infinitely dense” is called a “singularity.”

John: Really nice job.

Think about it like this. Let’s imagine that you have a foam basketball. Can you squish it together?

Maggie: Yes, I can use my hands to do that.

John: And what would the energy in your hands represent?

Maggie: Gravity.

John: Good. So why don’t you start to squish it together. What is happening?

Maggie: It is slowly getting smaller and smaller. And I suppose if I was strong enough I could squish it until it was so small I could not even see it, like a collapsing star.

John: Great. By using your gravity-hands you are collapsing the basketball until the matter is so concentrated and dense that only math can describe it. It is at that point that space-time ceases to exist as we understand it. In other words, the curvature of space (remember Einstein) disappears. That’s a singularity.

Maggie: I like that analogy. So what did Hawking do with this?

**The Big Bang Theory**

John: Hawking took Penrose’s theory about collapsing stars and applied it to the universe.

Maggie: You are going to explain that, right?

John: Of course.

We already said that scientists recognized that the universe is expanding. Hubble observed this in 1929. But even before him, others had suggested such a theory, like Georges Lemaitre, a Catholic priest. In 1927, he concluded that the universe was expanding in order to explain what is known as the redshifts of spiral nebulae. (Don’t worry about understanding that. We are not going to use it here.)

Well, Hawking thought a lot about the expanding universe and came up with a genius thought experiment. What would happen, he wondered, if we reversed the expanding universe? What would happen if we thought about the universe as collapsing instead of expanding?

And right at his fingertips he had a theory of collapsing stars: Penrose’s theory about black holes. So he took that theory, expanded it, and applied it to the entire universe.

Hawking theorized that, if we turn the clock backwards, all matter in the universe would eventually collapse into itself, just like a black hole. Due to the pull of gravity, the universe would eventually collapse until it became a singularity. And, as we saw above, space and time would cease to exist in this infinitely dense point. In other words, by turning time backwards and applying Penrose’s theory, Hawking concluded that the universe must have begun with a singularity.

This is all fine in theory, and a really fun thought experiment, but did it work mathematically, Hawking further wondered? And here he added to his genius idea. Hawking then took Einstein’s theory of relativity, and the really hard math needed to apply that theory, and proved that the universe certainly could have begun with a singularity.

Does this make any sense?

Maggie: Yes. Let me see if I can summarize it to show you that it makes sense.

Hawking got the idea that the universe began with a singularity by applying Penrose’s idea to the entire universe. Then he applied Einstein’s theory of general relativity, and some really hard math, to prove that our expanding universe could have really begun with a singularity.

John: Wonderful summary. And here is an idea that will blow your mind. Since time and space do not exist in a singularity, we can’t say the universe began in any space or at any time. Rather, space and time began with the expansion of the singularity at the big bang.

Maggie: Thinking about that is kind of like being in the matrix. Weird.

John: It is mind-bending.

Maggie: And I bet Hawking’s theory really made Hoyle mad.

John: Well, I am sure it didn’t make him happy.

Maggie: But what a cool idea. And it is really cool how he came up with the idea by using Penrose’s theory. Like you said earlier, scientists build on the ideas of other scientists.

John: It just goes to show that you never know where your next genius idea will come from. So we should continue to listen and learn and learn and listen.

Maggie: Are you promoting yourself?

John: Well, I didn’t think so, but now that you say that, maybe I was. Sorry.

Maggie: No need to be. I really like these discussions.

Great discussion, I’ll be anxious to read the discussion on dark matter.

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