December 13, 2006

The Collapse of Dark Energy?

The Collapse of Dark Energy?

‘Dark energy’ is only an inference. There is not a single particle or track in a bubble chamber to prove that it exists.

Louise Riofrio, in an email to Open Source, December 8, 2006.
Nova art

Artist’s rendering of a nova explosion [adriangonsalves / Flickr]

Dark energy has nothing to do with football, but the discovery of a massive supernova led Gregg Easterbrook to devote a portion of his Tuesday Morning Quarterback column to the possibility that dark energy might not exist after all. I happened to catch Easterbrook’s departure from his football beat, and got curious. Current theory asserts that most of the universe is made up of dark energy. If dark energy doesn’t exist, the theoretical consequence is the collapse of the universe, which seems like a pretty big deal.

The new supernova that got Easterbrook speculating apparently violates a Nobel Prize-winning mathematical proof that supernovas of this type are always a uniform mass and brightness. To fly rapidly down the chain of logic here: math says supernovas are all the same brightness (standard candles); because they’re the same, we can use them to measure distance; because we can measure this distance, we can measure acceleration; because we see acceleration, we think there’s dark energy. Dark energy is an inference about something we observe, but can’t otherwise explain, like Newton (or perhaps Aristotle) trying to explain why things fall down.

A quick search for “dark energy” and “supernova” turned up an entry on researcher Louise Riofrio’s blog A Babe in the Universe. I emailed her for a fuller explanation of the stakes:

If supernovae are not standard candles, then the whole case for an ‘accelerating universe’ would fall apart. Red shifts are the only evidence for acceleration. The other supports are highly dependent on priors: Simulations of large-scale structure and interpretations of the cosmic microwave background. These latter two tell nothing at all about acceleration.

Louise Riofrio, in an email to Open Source, December 8, 2006.

I also emailed Harvard Professor Robert Kirshner, author of The Extravagant Universe, who conveniently happens to be the faculty member affiliated with my dormitory. He assured me that that the accelerating universe model is not on the verge of collapse. According to Kirshner, in the unlikely event that astrophysicists had accidentally included a few extra-bright supernovas in previous calculations, it would not affect the support for dark energy.

Even if there is one supernova (out of about 200 that have been studied well enough to see this) that doesn’t behave like the others, as long as you can recognize it as an oddball, and set it aside, it will not affect the reliability of your measurements of the history of cosmic expansion that lead to the inference of dark energy. The only way these [massive supernova] guys would mess it up would be if you didn’t recognize them as different and mixed them in with the others. Even then, these events cannot be the explanation for the observations that lead to the idea of an accelerating universe driven by dark energy because they are brighter than average… If these guys are mistakenly included, then there must be even MORE acceleration than we have inferred so far.

Professor Robert Kirshner, in an email to Open Source, December 8, 2006.

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