The Milky Way galaxy: The microwave version
This lovely image is of our home, the Milky Way galaxy, but in a way our mortal eyes cannot perceive it. It doesn’t show stars, but the stuff that makes (or will make) up stars and planets and whatnot — clouds of gas and dust.
Our eyes can see only a tiny fraction of light — the visible spectrum, ROYGBIV (rainbow colors) — but the universe also glows in other kinds of light: gamma ray, X-ray, ultraviolet, infra-red, microwave and radio. And each frequency tells us something different.
The atmosphere blocks some of those frequencies (fortunately for life in Earth), so to view the universe in this exotic light astronomers have to depend on telescopes out in space. The European Space Agency, for example, launched the Planck Surveyor telescope to capture images in the microwave range, like this one here.
Microwave imaging gives us two important sets of information about the Milky Way and the universe we are in.
First, the huge clouds of gas and dust in the galaxy (which are mostly invisible to our eyes) are what eventually turn into stars and planets (and all the stuff that ends up on planets). In the photo, those clouds are all those wispy bluish-white and pinkish-white tendrils stretching out from the center (the galactic equator).
Putting it another way, that’s what we looked like about 5 to 6 billion years ago, before the Sun, the Earth (and the rest of the solar system) condensed out of a cloud of dust and gas. Needless to say, everything on the Earth was once in that same cloud, including the atoms that make up you and me.
So studying present-day dust clouds can help us understand the clouds that became us a long time ago.
Secondly, astronomers are also keenly interested in the background “behind” the Milky Way, because that’s the radiation left over from the Big Bang — the beginning of the universe. In the image, it’s colored magenta and orange. (Those are “false colors,” since microwave light doesn’t really have color.)
Our best estimates now put the Big Bang around 13 to 14 billion years ago. At the time, the universe was much smaller, denser and hotter than it is now. It was so dense, in fact, that light could not travel very far at all. It wasn’t until matter spread out far enough to become transparent that light from the Big Bang could get through. That happened about 380,000 years after the Big Bang.
So those microwaves are really, really frakkin’ old. And the pattern of magentas and oranges can help astronomers learn more about the early universe, before there were even stars and planets around.
The Cosmic Background Radiation is one of the main sources of evidence for the Big Bang. In 1948 physicists George Gamow, Ralph Alpher, and Robert Herman calculated that the expansion of the universe after the Big Bang would have shifted the original radiation from gamma rays to microwaves of a specific frequency (corresponding to a temperature). Other theoretical physicists at Princeton revisited the prediction in the mid-1960s and started to build a detector to test their predictions. (One of those guys was my freshman year physics lecturer, David T. Wilkinson, one of the best teachers I ever had.)
Meanwhile, down the road at Bell Labs, two researchers looking for something entirely different had already built such a detector. No matter how they fiddled with the contraption, there was a constant “noise” corresponding to a temperature of 3.5 K that they couldn’t get rid of. So, they phoned the physics guys at Princeton and asked their advice. Entirely by accident, the Bell researchers, Arno Penzias and Robert Woodrow Wilson, had detected exactly the kind of temperature that the theoretical guys had anticipated!
Penzias and Wilson later got a Nobel Prize in Physics for their accidental discovery. The Princeton physicists only got the simple satisfaction of knowing they were right.
The image above is a high resolution version of what Penzias and Wilson found with their ground-based “telescope.” In a way, it’s a portrait of what the Big Bang looks like 13 to 14 billion years after the fact. Studying the “afterglow” can tell us more about how the early universe behaved, so we can better understand how it is now.
Remember, a picture is worth a thousand words.