For one, there were two different methods by which supernovae could occur: from accretion of matter from a companion star (L), and from a merger with another white dwarf (R). Would both of these result in the same type of supernova?

For another, these supernovae at great distances may have been occurring in very different environments from the ones we see close by today. Are we positive that the light curves we see today reflect the light curves at great distances?

For still another, it’s possible that something happened to this light during their incredible travels from great distances to our eyes. Are we sure there isn’t some new type of dust or some other light-dimming property (like photon-axion oscillations) at work here?

Image credit: NASA/Swift/P. Brown, TAMU.

As it turns out, these issues were all able to be resolved and ruled out; these things aren’t issues. But recently — and what João Carlos’ question is all about — we’ve discovered that these so-called “standard candles” may not be so standard after all. Just like the Cepheids come in different varieties, these type Ia supernovae come in different varieties too.

Imagine you had a box of candles that you thought were all identical to one another: you could light them up, put them all at different distances, and immediately, just from measuring the brightness you saw, know how far away they are. That’s the idea behind a standard candle in astronomy, and why type Ia supernovae are so powerful.

Image credit: NASA/JPL-Caltech.

But now, imagine that these candle flames aren’t all the same brightness! Suddenly, some are a little brighter and some are a little dimmer; you have two classes of candles, and while you might have more of the brighter ones close by, you might have more of the dimmer ones far away.

That’s what we think we’ve just discovered with supernovae: there are actually two separate classes of them, where one’s a little brighter in the blue/UV, and one’s a little brighter in the red/IR, and the light curves they follow are slightly different. This might mean that, at high redshifts (large distances), the supernovae themselves are actually intrinsically fainter, and not that they’re farther away.

In other words, the inference we drew — that the Universe is accelerating — might be based on a misinterpretation of the data!

Image credit: Ned Wright, based on the latest data from Betoule et al. (2014), via http://www.astro.ucla.edu/~wright/sne_cosmology.html.

If we’ve got the distances wrong for these supernovae, maybe we’ve got dark energy wrong, too! At least, that would be the big worry. The smaller worry would be that dark energy is still real, but there might be less of it than we previously thought.

So which of these worries are valid? As it turns out, only the small one, and not the big one! You see, in 1998, we only had supernova data pointing towards dark energy. But as time went on, we gained two other pieces of evidence that provided evidence that was just as strong.

Image credit: ESA and the Planck Collaboration.

Image credit: Planck Collaboration: P. A. R. Ade et al., 2013, A&A Preprint; annotations by me.

1.) The Cosmic Microwave Background. The fluctuations in the leftover glow from the Big Bang — as measured by WMAP and later, to higher precision, Planck — strongly indicated that the Universe was about 5% normal matter, 27% dark matter, and about 68% dark energy. While the microwave background doesn’t do a great job by itself of telling you what the properties of this dark energy are, it does tell you that you have about 2/3 of the Universe’s energy in a form that isn’t clumpy and massive.

For a while, this was actually a problem, as supernovae alone indicated that about 3/4 of the Universe’s energy was dark energy, so it’s possible that these new revelations about supernovae could help the data line up better.

Image credit: Zosia Rostomian, Lawrence Berkeley National Laboratory.

2.) The way galaxies cluster. In the early Universe, dark matter and normal matter — and how they do-and-do-not interact with radiation — govern how galaxies wind up clustered together in the Universe today. If you see a galaxy anywhere in the Universe, there’s this odd property that you’re more likely to have another galaxy about 500 million light years away from it than you are to have one either 400 or 600 million light years away. This is due to a phenomenon known as Baryon Acoustic Oscillations (BAO), and it’s because normal matter gets pushed out by radiation, while dark matter doesn’t.

The thing is, the Universe is expanding due to everything in it at all times, including dark energy. So as the Universe expands, that preferred scale of 500 million light years changes. Instead of a “standard candle,” BAO allows us to have a “standard ruler,” which we can also use to measure dark energy.

Image credit: NASA / JPL-Caltech.

As it turns out, the measurements from BAO are just as good at present as the measurements from supernovae, and seem to give the same results: a Universe that’s about 70% dark energy, and consistent with a cosmological constant and not domain walls, cosmic strings, or many other exotic types.

In fact, if we combine all three data sets, we find that they all point roughly towards the same picture.

Image credit: Supernova Cosmology Project, Amanullah, et al., Ap.J. (2010).

What we’ve learned from this is that the amount of dark energy and the type of dark energy we infer from supernovae may change slightly and in a subtle manner, and this may actually be good for bringing the three methods — supernovae, the CMB and BAO — into better alignment. This is one of those great moments in science where one incorrect assumption doesn’t cause us to throw all our results and conclusions out, but rather where it helps us more accurately understand a phenomenon that’s puzzled us since we first discovered it.

Dark energy is real, and thanks to this new discovery, we just might come to understand it — and its effects on the Universe — better than ever before. Thanks, João Carlos, for the opportunity to take on such an interesting discovery, and if you’ve got a question or suggestion for the next Ask Ethan, send it in here!