When a standard candle isn’t a standard candle


The idea of a standard candle is very important in Astronomy. The concept is simple. We understand (thanks to Sir Newton) how the inverse square law connects brightness with distance. If we know the absolute brightness of an object (like a candle) and we then measure how bright it appears, we can work out how far away it is. Using this idea with stars (which aren’t very standard in terms of brightness) we can work out things like the distance to nebulas, galaxies, and clusters of stars.

But when you’re looking at things very very far away, the candidates for standard candles become very few. You need something very bright so that you can see it, even dimly, halfway across the Universe. That’s why there’s been such an interest in a certain kind of supernova over the years. The explosion *is* very very bright. And, because it was thought to be caused by a very simple mechanism – a white dwarf star exceeding the Chandrasekhar limit of 1.4 solar masses – each explosion should be just about exactly the same. Which makes it an excellent standard candle.

In fact it’s so good that it was the first place people were able to observe the increasing inflation of the Universe. Hubble first saw that the Universe was expanding. But in the late 80’s people, using the standard candle novas saw that the rate of expansion was getting greater the further out one looked.

No one nows exactly why this should be. The best answer at the moment is the invocation of the mysterious entity of Dark Energy. Which means…, well actually no one is sure. But we have to come up with something to explain what we’re seeing and you have to admit, that’s a really cool name.

Except that perhaps we don’t have to come up with something. Because maybe things are as we thought they were. Turns out that standard candles we’ve been using aren’t all that standard. Researchers announced this morning that the explosions aren’t being caused by white dwarfs gradually exceeding the limit as thought – the explosions seem to be caused by collisions between degenerate matter objects. Which means the explosions could be much brighter than we thought (in absolute terms) which means we’re overestimating their distance from us…

“Mario Livio, a theorist at the Space Telescope Science Institute, said that while the new results and the idea of two classes of supernovas ‘muddies the water,’ they would not affect the measurements of dark energy. Most of the supernovas in those studies, he said, came from spiral galaxies, and the astronomers, moreover, were very careful to calibrate their data.

‘The main results so far will remain unchanged,’ Dr. Livio said.”

I’m glad he thinks that. But I’m not quite as optimistic. This whole idea of Dark Energy violates Occams Razor six ways to Sunday. I wouldn’t be surprised if there are other calibration errors out there.

Like what happened when we figured out that another kind of intermediate standard candle – the Cephid Variable Star – came in different forms. They can still function as a sort of candle, but you have to be very careful now.

Read the full account in the New York Times here.

The Author

Episcopal bishop, dad, astronomer, erstwhile dancer...