Wednesday, October 5, 2011

Candles in the Sky

The physicist David Bohm used to say that physicist were not physical enough. They relied too much on mathematical equations and had no, as it were, viscerally physical sense of what they were talking about. A variant of this came to mind yesterday as I first heard and then read about the 2011 Nobel Prize for physics awarded to Saul Perlmutter, Adam Riess, and Brian Schmidt. The variant is that we interpret physical observations based on theoretical structures. But the work these gentlemen engaged in was right physical, actually. They used arrays of massively modern telescopes to observe one type of supernova activity, that associated with Ia supernovae.

The terminology here is unfortunate. We have two kinds of supernovae, 1 and 2, but these are rendered in Roman form as i and ii. In the first category, i, interests us here. Type ii are produced by large stars late in their lives. In the first category, we have three subdivisions: 1a, 1b, and 1c; the first is produced by dwarfs, the last two by massive stars. On YouTube videos we hear people talking about “one A,” but in press accounts we see Ia. But never mind. the 1a’s are all white dwarf stars to begin with, and these are always associated with another sun; each dwarf is thus one member of a binary system. Furthermore, each is the collapsed form of the bigger of the two, with immense density. Most white dwarfs are about the size of the earth but have mass equivalent to 0.6 of the sun. The 1a supernova comes into being when the white dwarf sucks the mass of its binary companion to itself. Slowly its mass increases. When it comes very close to having 1.38 solar mass, it produces an enormous nuclear explosion, the supernova of type 1a. That number, 1.38, is called the Chandrasekhar limit, named after Subrahmanyan Chandrasekar who wrote a 1931 paper titled “The Maximum Mass of Ideal White Dwarfs.” The process described above is illustrated by the fabulous graphic authored jointly by NASA, the European Space Agency, and A. Field; I bring it courtesy of Wikipedia Commons (here).

The important point here is that white dwarfs never go into nova unless they reach that mass. And knowing that mass, we can calculate their brightness at peak with great precision. It is always just about the same. For this reason whenever such a supernova appears, we know how bright it must be where it is. Measuring its observed brightness with our by now stupendous instruments, we can therefore calculate how far away it is. 1a supernovae, therefore, act as a pretty reliable standard candles, thus objects of known absolute magnitude (luminosity). Knowing their observed magnitude, we can calculate their distance from us using a simple formula.

Our Nobelists looked for and found many, many supernovae of type 1a and measured their distances from us. In the absence of any kind of theory of the cosmos, this would give us a nice, clean idea how far away the most distant galaxies—those housing the 1a’s—are from us. Instead these men were greatly surprised by their findings. The most distant galaxies turned out to be much dimmer, thus much farther away, than they had expected them to be. I emphasize that word because “the model” now comes into the picture. That model, simply, is that the universe began with a Big Bang and has been expanding for 14 billion years. The expectation was that over time, the expansion would have slowed, decelerated, owing to the gravitational pull of everything on everything. Adam Riess uses the image of throwing your car keys into the air (read Big Bang). You expect the keys eventually to lose their upward energy—and to fall back down again. Instead, these keys just seemingly kept on going up. The following little graphic shows what they expected and what they actually saw.

Not to forget. The Big Bang is behind this expectation, thus a certain energetic, one-time dynamism. The Big Bang itself is based on Edwin Hubble’s observation that the farther galaxies are from us, the more red-shifted their light actually is, thus that the peaks of the light waves are farther apart. This used to be explained by saying that space was expanding and, as it expanded, it stretched the light. Today the explanation is that some kind of energy must be causing the expansion, dark energy, dark because we cannot detect it directly. And the observed red shifts—and now the unexpected dimming out of supernovae at great distances—has been interpreted to mean that the universe is mostly just that, dark energy (as I’ve had occasion to report here).

Alternative models are not even on the back burner these days. One of these might be that light gets tired as it moves, and therefore a red shift simply means tired light; hence there is no expansion. Fritz Zwicky (1898-1974), a physicist associated with supernovae, proposed the tired light hypothesis. Astronomer Halton Arp (1927-) holds “heretical” views on the red shift as well. If the red shift doesn’t always mean what Hubble thought it did, there might never have been a Big Bang. But we like the Big Bang. In the beginning, etc. Let there be Light. And having light, we now complete the picture with energetic Darkness.

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