A while back on physicsworld there was a nice little article by a cosmologist called Stephen Eales. I liked his style so I bought his book. It’s called Origins, and it tells you about planets and stars and galaxies, and then moves onto the universe. It’s a few years old but it’s still up to date, and it’s a damn good read. It’s full of those little anecdotes that brings science to life, and it is a cornucopia of knowledge. Steve is a hands-on astronomer*, and he gives due warning about the speculation in the final chapters, saying things like “the beginning of the universe is hidden in a labyrinth of misconceptions”. And I had to smile when he said theoretical physicists are well known for the prodigality with which they invent and discard theories, ”sometimes even before morning coffee”. Yep, this a guy who knows his stuff and knows what he’s talking about.
However in cosmology, not everybody does. There are some myths and mysteries in cosmology, some right up there with the Loch Ness Monster and the Grassy Knoll. One of the biggest is the size of the universe. Take a look at this NASA web page and you can read this: “We now know that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent”. You can read much the same thing on Wikipedia. And if it’s on Wikipedia, well, it’s got to be gospel, right? Uh, excuse me Mr President, that’s not entirely accurate.
So what’s the problem? The problem is that the guys who tell you the universe is infinite are the same guys who tell you about the Big Bang. Don’t get me wrong, I don’t think the Big Bang is wrong. I’m happy that the universe started out small 13.8 billion years ago and has been expanding ever since. Pourquoi? Ever heard of the stress-energy-momentum tensor? Here it is:
Yeah it’s scarey, but just pay attention to that shear stress and that pressure. The shear stress says space can be likened to ghostly gin-clear elastic, and the pressure says it can be likened to compressed elastic. Ever played with a stress ball? If you’ve got one, squeeze it down in your fist. Feel that pressure? Now let go. Notice how it expands? Think of the universe as something like that.
Anyway, what happened is that WMAP and then the Planck mission found no evidence that space was curved on the larger scale. But people who should know better fell for the non-sequitur that if space isn’t curved, it must be infinite. OK some might say ”ah, but the universe might have extrinsic curvature in a higher dimension”. Then they’ll try to sell you a universe that’s some kind of mystic doughnut. Don’t buy it. There’s no evidence for that either, see this Planck paper. And don’t fall for the myth that an infinite universe can expand, because pressure can’t cause expansion if it’s counterbalanced at all locations. Don’t get sidetracked by light being curved by the expansion of space either, because it isn’t going to curve if it’s radial. That light goes straight. And if anybody tries to bamboozle you with the cosmological constant, forget it, because that was Einstein’s greatest blunder. Gravity sucks, like his cosmology, but the universe doesn’t. Aw, it’s like Einstein is my hero, and it’s like a gravitational field is a pressure gradient, whilst space is just pressure. But moving on:
What’s going on? Why are the same people telling you the universe started out small and is infinite now? What gives? The problem is down to the nature of space and confusion between curved spacetime and curved space. On John Baez’s website you can read that gravity is ”not the curvature of space, but of spacetime. The distinction is crucial”. Fair enough. With a bit of detective work you come across Einstein’s 1920 Leyden Address along with modern papers, and you appreciate that curved spacetime relates to “inhomogeneous” space. It’s like you’re swimming in water that’s salty on the left and fresh on the right. When you look back you find that you didn’t swim straight. Your path is curved, but the water isn’t. However when you bump into the Dark Energy Detectives you read that “this straight line is now in curved space”. Huh? In the end you track it down to something called the FLRW metric which ”starts with the assumption of homogeneity and isotropy of space”. Got it. When space is homogeneous light goes straight and there is no gravity. Hence gravity didn’t collapse the universe when it was small and dense. And WMAP and Planck found that space was flat. Kushti.
Now we’re cooking, but something’s got to give. Space started out small and light goes straight. So where does it go? Einstein said space wasn’t nothing. It can be likened to elastic. Waves run through it. So what if our universe is something like a droplet of water with ripples running through it? When those ripples reach the edge, they’re going to bounce back. If space is finite there is no space beyond the edge of space, there is no beyond it. When light waves reach the edge maybe they’ll bounce back too. Like it was a mirror. Maybe you could float in front of it in your spacesuit, looking at your perfect reflection, and the reflection of galaxies too. Maybe you could jet forward, and because of the wave nature of matter, maybe you’d bounce back.
But don’t count on it, because maybe you’d get turned into inside-out mincemeat. Or should that be back to front? Or maybe a fiery death of matter-antimatter annihilation? Now that’s a mystery, and I can imagine the movie now. In space, no one can hear you scream.
Yes, it will be a science fiction movie, because the observable universe has a radius of 46 billion light years. We can never reach the edge, because the universe is expanding way too fast. But next time you stand in the garden exhaling your cigarette up into the cool night air, look to the stars. Out there, someplace, somewhere, maybe somebody can.
* Astronomy funding in the UK is under threat. At present most of the money is going into the Square Kilometre Array and the Extremely Large Telescope projects. This has forced withdrawal from existing telescopes, including “giving away” the James Clerk Maxwell Telescope after equipping it with a state-of-the-art camera.