Paul Steinhardt’s “cyclic model,” a radical alternative to the big bang and inflationary cosmology, proposes that the universe’s evolution is periodic and that key events shaping its structure occurred before the bang. Peter Galison studies historic fundamental shifts in physics and what types of evidence count as truth. Having first met during their graduate-school days at Harvard, they were quick to accept Seed’s invitation to consider: Where is the line between physics and metaphysics? Is infinity unscientific? What is it, ultimately, that we want from science?
PETER GALISON: So, Paul, you’ve been thinking about the big bang and its alternatives. Where do we stand now with that?
PAUL STEINHARDT: Well, in cosmology these days, we have two competing ideas — ideas that lead us down two different paths for how the universe came to be, how it evolved, and what will happen in the future.
The standard view is that the bang is the beginning, that energy, matter, space, and time all came into existence at that moment. But if that’s the case, then we have to describe everything we see in the universe as having taken place within just 14 billion years. Furthermore, since we know that the conditions at the one-second mark were quite special, we need something remarkable to have happened within that one second.
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This has led to this idea that the universe underwent a brief period of inflation during those first few instants after the big bang. This faster-than- light expansion would have transformed the turbulent and warped universe that erupted into the smooth, uniform universe that existed by the time the universe was one second old.
But there is an alternative idea, developed in recent years, that the big bang was not the beginning. It’s a moment when a lot of matter and radiation were created, but space and time existed before, as well as after. If that’s the case, then suddenly you have a lot more time and new possibilities for setting up the conditions we know had to exist one second after the bang.
PG: One of the things that strikes me, thinking about these issues historically and philosophically, is that physics has been in a long pursuit of trying to eliminate the special, the particular. Galileo, for example, said that there is no special frame of reference, such that any constantly moving ship, as it plies its way through the Mediterranean Sea, ought to have the same mechanical physics as every other ship. Then Einstein extended this beyond mechanical things: He said electricity, and magnetism, and everything should be the same in different frames of reference. It seems to be also much of what surrounds the current cosmological debate — how to take away specialness in some way.
I mean, part of the motivation for getting away from the big-bang picture has been idea that too much had to be specified, too much had to be sort of set up.
PS: Right. The distribution of matter and energy in the universe had to have been extremely uniform throughout space. Space itself — which, according to general relativity, can curve, warp, wrinkle, and wrap itself up in all sorts of complicated ways — had to have been remarkably flat. Not perfectly uniform, or the universe would never have evolved stars and galaxies, but with tiny deviations of just the right sort.
All of this seems so unlikely that there’s no reasonable notion of how it could happen. So that is what led to the invention of inflation — an incredible stretching that results in near-perfect flatness and uniformity.
But the picture isn’t as simple as it’s often portrayed, and that’s where it gets interesting. The original idea — the way it’s often talked about in literature and textbooks, even the way we talk to students — is that inflation makes everything in the universe the same. What we’ve learned is that inflation actually divides the universe up into little sectors that are all different from one another. Some regions of space would be habitable like ours, but others would be inhabitable; still others would be habitable but would not have the same physical laws or the same distributions of matter that we see here. In fact, what we see is very likely only relevant to an infinitesimal fraction of space.
PG: In a way you could treat this multiplicity of worlds as solving one problem and creating others. In some ways you could say, okay, we have 10500 or 101,000 of what exists. They have different laws —
PS: Actually, you get an infinite number of everything. So an infinite number of patches that look like ours, but also an infinite number that would be more curved and warped than ours. And if you add some attributes from string theory, they might have different physical laws.
Because you have an infinite number of everything, you have no rigorous mathematical or statistical way of computing a probability — it’s not even a sensible question to ask. So people are in the process of trying to regulate this infinity. For example, they try to invent a rule for deciding probability that makes what we see likely. But there’s no way of deciding why that rule instead of some other one. They simply keep trying until they’ve found the answer they wanted. Some people are going down that path and are prepared to declare victory if they find something they think works.
Others take a different path. They accept the infinity of infinities and the fact that they can’t find any measure for deciding whether our circumstance is more probable or not. They’ll be satisfied with the fact that at least some patches look like what we see, and I will declare victory on that basis.
Personally, I don’t find either of these approaches acceptable, which is why I have developed an alternative picture in which the big bang is not the beginning. A big bang repeats at regular intervals of a trillion years or so, and the evolution of the universe is cyclic.
PG: It’s interesting to me how in the cyclic model — and also in inflation and in string theory — there’s this idea that you don’t want to make a whole lot of choices at the beginning that are too fine. Because that seems to violate the spirit. It goes back to even the days of Descartes.
Descartes had a sort of view that nobody holds now, but that was the principle. He said, “You start with just extension and motion and that’s all.” There are all sorts of phenomena that he tries to explain that way, but in the end he wants to explain the visible universe with this relatively simple start.
In a way, we keep that story, that attempt to have relative independence of our starting assumptions to explain the specificity we have today. In a way, it’s what drives these ideas of infinity in these different camps: People that want the landscape to have an infinity of craters, each one a different universe, and people that want infinity in space the way Descartes did, wherein there is no special region at the center of all things. It seems to me this same spirit lies behind what you and your colleagues want to do with the cyclic process. You say, “We don’t want to tune things, we don’t want to manufacture a universe that seems like it’s done with forethought.”
PS: Right. The fewer assumptions you have to make, the better — Occam’s razor. But there’s another issue, which connects to why I personally do science: science’s power to explain and to predict.
We’ve been talking about an example in which you have a complex energy landscape and an infinite number of possibilities for the universe. But we have no real explanation as to why things are the way they are, because it could have been different.
So it has no power. And without real explanatory power, it’s not interesting to me. But I’d be interested to hear how this has played out in the history of science.
