Fare thee well, Allan Sandage

"I came at exactly the right time... I was 26 years old, and all the monks and priests down here were ready to retire. So I overlapped enough that I got to know them all." -Allan Sandage

As many of you have heard, Allan Sandage passed away last Saturday. Let me tell you a little bit about this man, why he stands as such a towering figure in modern cosmology, and why he should be held in even higher esteem than he normally is.

Allan Sandage was a coworker of Edwin Hubble's, and he took up one of the most pressing questions of the day after Hubble's death in 1953: What is the Age of the Universe?

This is a particularly profound consequence of the expanding Universe. Let's take a look at why.

The Universe, ever since the Big Bang, has been expanding. What determines the expansion rate? At any given time, oddly enough, it's just two things: the total energy density, and the curvature of space.

Now, energy comes in all sorts of varieties, but back in 1953, we only seriously thought about two: normal matter (protons, neutrons, and electrons) and radiation (photons and neutrinos).

And one of the things we can figure out, if we measure the Hubble constant and we determine what the densities of the various forms of energy in the Universe are, is the age of the Universe.

Now, this posed a big problem. You see, Allan Sandage's work focused on globular clusters; specifically, he was making very detailed measurements of the globular cluster Messier 3.

Image credit: NOAO.

If you know how stars work, and you observe a bunch of stars in a cluster, you can easily figure out how old the stars in that cluster are. And Sandage did just that, to unprecedented accuracy, for the stars in Messier 3.

And what he found, to his surprise and chagrin, was that the stars in there were, on average, between 8 and 10 billion years old!

Why was that a surprise? Because if your Universe has no curvature, is composed of nearly 100% normal matter, and the Hubble constant was its then-measured value, 100 km/s/Mpc, the Universe should be only 7 billion years old!

Clearly, something was wrong with this picture; you can't have a Universe where the stars in it are older than the Universe itself! So Sandage set out to measure the Hubble constant to an unprecedented accuracy, and see if the Universe weren't actually older than Hubble had thought.

Image credit: Chernin et al.

So he set out to measure Cepheids -- well-understood variable stars -- in a multitude of different galaxies. And rather than a Hubble constant of 100 km/s/Mpc, Sandage kept getting values between 50 and 60, which would nearly greatly increase the age of the Universe.

Moreover, Sandage found that there was less matter than 100% of the energy density; much less, in fact. This was particularly important, because the age of the Universe is very sensitive to how much matter is in it, if the only other ingredients are curvature and radiation.

Combining these two results, Sandage estimated the age of the Universe to be somewhere between 13 and 14 billion years. This was not only much older than previous estimates, it was the only estimate in accord with newer measurements of stars in clusters, some of which indicated ages up to 13 billion years!

But in the 1970s and 1980s, a new group of astronomers, led by Gerard de Vaucouleurs (2nd from left, above), were taking new measurements of the Hubble constant, and were getting values -- consistently -- between 85 and 100 km/s/Mpc. De Vaucouleurs, in fact, argued so vehemently for 100 -- the more extreme value -- that some astronomers still (shamefully, perhaps) use his parameter, h, which is the Hubble constant divided by 100.

Sandage was incredulous! If the Hubble constant were that high, he argued, the Universe would again be younger than many of the stars in it. Sandage was often portrayed as an old fogey -- a has-been -- who was out of touch with current observations. But his argument was sound and correct; the Universe cannot be younger than the stars in it! Something had to give.

It was in the 1990s, when the Hubble space telescope was up and running, that things finally came to a head. By the way, did you ever wonder why it was named the "Hubble" space telescope? Because its #1 science goal was, once-and-for-all, to accurately measure the Hubble constant!

As you might have suspected, everyone was wrong. The value that the Hubble space telescope's key project returned was that the Hubble constant was 72 km/s/Mpc, with a possible error of less than 10%. On its own, this would have given us an age for the Universe of about 12.3 billion years, but there was an extra bit of information we discovered in 1998. We determined that the major constituent of energy (70-75%) in our Universe was neither normal matter nor curvature, but was in fact dark energy!

The 72 km/s/Mpc and the amount of dark energy in the Universe led to our current picture of a Universe that's 13.7 billion years old. And while de Vaucouleurs was generally recognized as having been "more correct" at the time, Sandage was certainly much closer to the correct overall picture. (De Vaucouleurs passed away back in 1995.)

So rest in peace, Allan Sandage. You will forever stand as one of the pioneers of modern cosmology, and one of the key figures in determining the ages of the Universe and the stars in it. While we're saddened at having lost you, it is a happy day for cosmology that the stars in the Universe are, according to our best measurements, all younger than the Universe itself.

Goodbye, Allan Sandage. Thanks for helping us discover our Universe.