The Story and The Verdict

> “Nullius in Verba. (Take nobody’s word for it.)” -Motto of the Royal Society

You know the drill. New ideas come out all the time. Sometimes they’re new theories, sometimes they’re old theories with a new twist, but regardless, we need to ask the question: How good is your theory?

For whatever it’s worth, I came up with a scale for this.

The best ideas are beyond validated. They are confirmed over and over, predict new phenomena that gets verified, and don’t have any self-inconsistencies.

Well, a couple of weeks ago, a new twist on an old idea was proposed by Roger Penrose.

A little background first. Roger Penrose is a really bright guy, and — among physicists and mathematicians — is incredibly well-respected. His specialty is in the physics and mathematics of spacetime, including tilings and tessellations (below).

Penrose diagrams — mathematical ways to map infinite spacetimes onto a simple sheet of paper — are named after him. He also has a bit of fame from his work on singularities with Stephen Hawking.

Penrose has also been unafraid to put speculative ideas out there. He has been unafraid to speculate about the origins of consciousness, but at least some of his ideas were demonstrably wrong. One of his favorite speculative ideas is that of a cyclic Universe, which he’s been touting (and speaking about) for as long as I can remember.

We have a theory of the Universe, and while it doesn’t explicitly rule out cycles, the evidence strongly points towards a different story. As we’ve discussed a number of times, the first step in the Universe’s history that we can sensibly say anything about is inflation.

(Image credit: Ned Wright.)

Whatever the Universe was doing, at some point 13.7 billion years ago, some region of it was expanding exponentially fast, taking whatever shape it was and stretching it flat, across the Universe. At the same time, the tiny quantum fluctuations that happen all around us — all the time, like it or not — also got stretched across the Universe.

(Image credit: Ned Wright again.)

When inflation ended, and this energy turned into matter and radiation, the regions with positive fluctuations came out with slightly higher-than-average densities, and the regions with negative fluctuations came out with slightly lower-than-average densities.

A few hundred thousand years later, the overdense regions have grown a little bit thanks to gravity, the underdense regions have shrank a little bit more, and the leftover radiation from the big bang shows us this “fingerprint” in the form of fluctuations on the sky in the cosmic microwave background.

Now, we can use this “fingerprint” to learn about the Universe. If inflationary theory is correct, these fluctuations should follow a very specific mathematical distribution. And, according to the best data we have, they do.

But we love to put this to the most stringent tests we have. Look closely at the fluctuations in the data, and you might find some alarming features.

Did Stephen Hawking write his initials in the microwave sky? It certainly looks like it!

But, as a cosmologist, there’s no need to panic. Instead, we ask ourselves if patterns like this come about naturally from the fluctuations we expect, or whether these patterns are indicative of some physics beyond our current understanding of the Universe.

So we not only do our mathematical analyses, we also run simulations of millions and millions of Universes, the way we think it works. How frequently do we expect features like this to show up in our sky? The answer, surprisingly, is very often.

That’s good! That means our picture of the Universe — inflation + the big bang — is perfectly consistent with what we see. Now, let’s take a look at Penrose’s new idea.

He found — quite surprisingly — that there are regions of space, shaped like concentric rings, where the temperatures are much more uniform than average. In other words, the amplitude of the fluctuations are anomalously low in these concentrically-shaped regions of space.

So, you might think to ask yourself, how common are features like this? Is this in conflict with what we think the Universe is supposed to look like, or is this completely reasonable and consistent with what we expect? But Roger Penrose didn’t ask that question.

Instead, this finding was touted — by Penrose — as evidence that inflation is wrong, we live in a cyclic Universe, and these concentric circles are evidence of the Universe that existed before ours.

Fortunately, as soon as Penrose pushed this paper, cosmologists were all over it. (Thanks to Sarah and Sean for writing about this first.) What did we find?

These features are normal, and completely within the realm of what we expect. So Penrose’s idea — that inflation is wrong and we live in a cyclic Universe — is my favorite kind of invalid theory.

The kind that’s demonstrably wrong. Why is that my favorite? Because we learn from it.

That’s one of the ways that science advances: we put out speculative ideas at the limits of our understanding, we test them for validity, and if we find the idea doesn’t hold up to the data, we cast it aside. And we do it, at least, until there’s new evidence that causes us to take it up again. So I hope you not only learned a little bit about a new idea, but that you also learned a lot about what we actually know, and how we know it! Read the comments on this post…


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