“Some say the world will end in fire,

Some say in ice.

From what I’ve tasted of desire

I hold with those who favor fire.

But if it had to perish twice,

I think I know enough of hate

To say that for destruction ice

Is also great

And would suffice.” -Robert Frost Every once in a while, some Earth-shattering discoveries come along that forever change our view of the Universe. Back in the late 1990s, observations of distant supernovae made it clear that the Universe wasn’t only expanding, but that distant galaxies were actually speeding up as they moved away from us, a Nobel Prize-worthy discovery that told us the fate of our Universe. But among your questions and suggestions this week was one from João Carlos, who pointed out a new study, and asked: I read this on Eurekalert! and thought you should, too. I can’t wait to see [y]our comments about this. The “this” in question was from a University of Arizona press release — a place where I was a postdoc just a few years ago — that said the following: Image credit: screenshot from http://uanews.org/story/accelerating-universe-not-so-fast. This is potentially a very, very big deal for our understanding of all there is, and how our Universe will end. Let’s go back nearly 100 years to a lesson we should have learned, and then come forward to today to see why. Image credit: European Southern Observatory (ESO), via http://www.eso.org/public/images/eso1424a/. Back in 1923, Edwin Hubble was looking at these obscure, faint “spiral nebulae” in the sky, studying novae occurring in them and trying to add to our knowledge of just what these objects were. Some people contended that they were proto-stars within the Milky Way, while others believed them to be island Universes, millions of light years beyond our own galaxy, consisting of billions of stars apiece. While observing the great nebula in Andromeda on October 6th of that year, he saw a nova go off, then a second, and then a third. And then something unprecedented happened: a fourth nova went off in the same location as the first. Image credit: Edwin Hubble / Carnegie Observatories, viahttps://obs.carnegiescience.edu/PAST/m31var. Novae do sometimes repeat, but it usually takes hundreds or thousands of years for them to do so, as they occur only when enough fuel builds up on the surface of a collapsed star to ignite. Of all the novae we’ve ever discovered, even the most rapidly replenishing takes many years to go off again. The idea that one would repeat in only a few hours? Absurd. But there was something we knew about that could go from very bright to dim to bright again in just a few hours: a variable star! (Hence, his crossing out of “N” for nova and excitedly writing “VAR!”)

Images credit: ESA / Hubble, of the star RS Puppis, via https://forums.robertsspaceindustries.com/discussion/217069/suggestion-light-echo-visual-effects.

The incredible work of Henrietta Leavitt taught us that some stars in the Universe — Cepheid variable stars — get brighter-and-dimmer with a certain period, and that period is related to their intrinsic brightness. This is important, because it means that if you measure the period (something easy to do), then you know the intrinsic brightness of the thing you’re measuring. And since you can easily measure the apparent brightness, then you can immediately know how far away that object is, because the brightness/distance relationship is something we’ve known for hundreds of years! Image credit: E. Siegel. Now, Hubble used this knowledge of variable stars and the fact that we could find them in these spiral nebulae (now known to be galaxies) to measure their distances from us. He then combined their known redshift with these distances to derive Hubble’s Law and figure out the rate of expansion of the Universe. Remarkable, right? But unfortunately, we often gloss over something about this discovery: Hubble’s conclusions for what that expansion rate actually was were totally wrong! Image credit: E. Hubble, 1929. The problem, you see, was that the Cepheid variable stars that Hubble measured in these galaxies were intrinsically different than the Cepheids that Henrietta Leavitt measured. As it turned out, Cepheids come in two different classes, something Hubble didn’t know at the time. While Hubble’s Law still held, his initial estimates for distances were far too low, and so his estimates for the expansion rate of the Universe were far too high. In time, we got it right, and while the overall conclusions — that the Universe was expanding and that these spiral nebulae were galaxies far beyond our own — didn’t change, the details of the expansion definitely did! And now, we come to the present day. Image credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team. Far brighter than Cepheids, supernovae can often outshine — albeit briefly — the entire galaxy that hosts it! Instead of millions of light years away, they can be seen, under the right circumstances, more than ten billion light years distant, allowing us to probe farther and farther into the Universe. In addition, a special type of supernova, type Ia supernovae, arises from a runaway fusion reaction taking place inside a white dwarf. When these reactions occur, the entire star is destroyed, but more importantly, the light curve of the supernova, or how it brightens and then dims over time, is well-known, and has some universal properties. Image credit: S. Blondin and Max Stritzinger. By the late 1990s, enough supernova data had been collected at large enough distances that two independent teams — the High-z Supernova Search Team and the Supernova Cosmology Project — both announced that based on this data, the Universe’s expansion was accelerating, and that there was some form of dark energy dominating the Universe. Like many people, I was skeptical of this, as if supernovae weren’t as well-understood as we’d thought, these entire conclusions would go away.

Images credit: NASA / CXC / M. Weiss.