Wednesday, October 5, 2011

A Nobel note

This year's Nobel Prize in Physics went jointly to three researchers, "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae." Supernovae (which are massive, old, exploding stars) are actually quite good as "standard candles" in observational astronomy, because they're so bright (they can outshine an entire galaxy, in fact, if only for a short while) and they appear to have a very predictable driving mechanism (such that the assumption of distant and nearby supernovae behaving the same seems to be a valid one). So finding supernovae of differing brightness indicates they are at different distances (dimmer = farther away), and additionally, looking at the shift of light output in terms of the light's wavelength (redshift) tells us how fast the supernovae are moving relative to us. If you look at distance (plotted in the figure as bolometric magnitude, which is another way of saying brightness) versus the speed (redshift, z), you find the following relationship (from the Nobel website):
Basically, taking different cosmological models with differing predictions about the content of the universe (how much is made up of matter, for instance), you can examine your data to see which of the models fit. The Nobel Prize winners interpreted their results to mean that the universe is expanding, and that expansion is accelerating. This has led to the now familiar term "dark energy," the hypothesis that some form of unseen energy is propelling the universe ever more quickly outward from the Big Bang.

But I don't particularly want to talk about the Nobel Prize. I want to talk about some current research which argues against, not the data the Nobel Prize winners collected, but their interpretation of it.

A recent paper by Christos Tsagas of Aristotle University of Thessalonika, building on previous work and observations by others, argues that it's all in your perception (or, more exactly, in your reference frame). You might be able to observe the universe expanding outward, and at an accelerating pace, even if it wasn't actually true - if you happened to be sitting in a local region of the universe that was moving relative to the average (a phenomenon now termed "dark flow"). Our relative motion, here in the Milky Way, could make the universe appear to be expanding faster and faster, when really it's just us moving. This is demonstrated in the figure from the paper:

We all are familiar with relative motion, even if we don't know the terminology. Imagine you're in your car, stopped at a red light, and you've been fiddling with the radio. Suddenly, you're rolling backward... oh no!... until you realize that your foot is firmly planted on the brake, and it's actually the car next to you moving forward, and not you moving backward. Or imagine you're walking on a moving walkway. If you walk in the direction of the moving walkway, from someone standing at Gate B17 you look like you're walking twice as fast. But Gate B17 guy sees that you're hardly moving at all, if you're walking against the direction of the moving walkway. Now imagine this on a larger scale. What if our whole galaxy - in fact, a huge, 2.5-billion-light-year chunk of spacetime - was moving relative to the rest of the universe? It's possible that we'd see relative motion and misinterpret it as absolute motion. In the case of the accelerated expansion of the universe (the topic of this year's Nobel in Physics), we generally assume we're the stationary observer at Gate B17, but perhaps we're really the idiot trying to walk against the direction of the moving walkway.

This isn't a crazy idea, nor is it based in fantasy. It's actually a very simple argument: we're moving relative to the actual rest frame, and this skews our perception. But it's in direct competition with the interpretation that just won the Nobel Prize. So what do we do? We do science. Keep observing, keep testing, and keep refining our interpretations until we figure out which answer is ultimately correct.

Tsagas, C. (2011). Peculiar motions, accelerated expansion, and the cosmological axis Physical Review D, 84 (6) DOI: 10.1103/PhysRevD.84.063503


  1. Hi, Kelly-

    Thanks for the coverage. It reminds me of nay-sayers in decades past who claimed that we might be living in a kind of cosmic fog that continuously creates matter, preserving the steady state model. A dead model, sorry to say.

    So the question is whether the original data was taken while looking in different directions in the sky. One would think this kind of control would address the Tsagas proposal.

    Also, I have to ask.. what is the "rest frame"? Isn't that like talking about phlogiston, or the ether? I though relativity made the idea obsolete, and the universe is border-less.

  2. Fascinating. I suppose we shall see!

  3. Burk - it's a bit more technical than I've made it out to be, of course. The "rest frame" is essentially defined by the cosmological axis, as determined by the preferred directionality of the anisotropy of such things as the cosmic microwave background or the redshifts of supernovae. It's not that the rest frame is really at rest, but that we are moving relative to it, even as it moves. There is evidence (referenced in the Tsagas paper) that such anisotropy exists, which cannot be accounted for by any motion of which we are now aware (such as the relative motion of the planet, Sun, Milky Way, etc).

  4. Kelly, saw you on Burleson’s blog.

    Wow! You guys know so much!
    You mentioned a person thinking they were rolling backwards when really it was an object in front of them going forward.

    The opposite is also true:

    Once in the military, several of us were on a long trip at night. When it was my turn to drive, I was told if the car died it would not start until it cooled off.

    When I had to stop, I was to put it in neutral and pump the gas.

    At my first stop at a flashing red-light in the middle of West Texas, the ‘brake grabbed and the car jerked to a violate stop as I put it in neutral’. (The automatic gear-shift indicator light was burned out.)

    Well, I couldn’t believe my eyes seeing the red light zoom away from us. We were 50 yards away before I realized I had put it in reverse.

    Since everyone was asleep, there was a lot of “WHAT’S GOING ON?”

    I replied, “Oh—nothing” and they went back to sleep.

    By the way, later, I became a small cog in a big machine of being the tool designer for the space shuttle nose cone.

  5. Rex, it sounds like you're quite familiar with the idea of relative motion! Everybody inherently knows a little physics, even if they're not aware of it.
    Did you ever get to attend a shuttle launch while you worked on the project? It must have been exciting...


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