Quasars and the Great Cosmic Time Machine

Time travel has always fascinated me. I'm a huge history buff, and I'd dearly love to stand in the Roman forum two thousand years ago, or peek into the court of Elizabeth the First, just to see what things looked like back then. I don't have any fantasies about interacting with anybody or changing history, I just want to know what really happened, what important details have been left out of our picture of history

Although I can't travel back in Earth's history, as an astronomer, I have the means to look back in time to study the history of the rest of the universe. How does that work? It comes from the fact that light does not travel at infinite speed.

The fastest it can travel is about 186,000 miles per second, which is not too shabby (it could travel around the entire Earth eight times in one second), but still limited. And when it comes to astronomical distances, you begin to notice the time it takes for light to get places.

Take the moon for example. It takes about two-and-a-half seconds to send a beam of light to the moon and back (round trip), creating a small but noticeable lag in conversation when the astronauts were wandering around up there. The sun is about eight minutes of light travel time away (that's 93 million miles), so we see the sun in the sky as it appeared eight minutes ago. If the sun were to stop shining or explode, we wouldn't know it for eight minutes.

Not a lot changes about the sun in eight minutes, but as soon as you get out of our solar system, the time lag created by the speed of light starts to get important. The distances become so large that astronomers give up trying to measure things in miles, and start to talk about light travel time instead. That's why a light year (confusingly enough to introductory astronomy students) is not a measure of time, but distance. It's the distance light travels in one year, about 6 trillion miles. Even the nearest stars are a few light-years away, and many of the brightest stars in our sky are several hundred.

When you look up into the night sky, there's a pretty good chance that some of the stars you see have burned out already, maybe even exploded in a supernova. Betelgeuse, the brightest star in the constellation Orion, is a prime candidate for going supernova any day now. It may already have done so. It's quite possible that waves of high-energy light from Betelgeuse's supernova explosion are hurtling, at this very moment, through space on their way to us. They'll put on a spectacular show when they get here, but it will take them almost 800 years to reach Earth.

Step out to even our nearest galactic neighbors, and you're immediately forced to deal with a substantial time-lag. The nearest large galaxy to us, Andromeda, is two million light years away. In this case, it's almost guaranteed that the brightest stars we see in Andromeda no longer exist. And it's in this realm of external galaxies that the time-light lag really starts to get intriguing.

In a real sense, we can look back in time and watch the universe form. When you get out to distances approaching a billion light years, things start to look noticeably different. To me, this is the best proof we have that the universe really did have a beginning.

By and large, very distant objects are more active, more violent, than things we find closer to home. A great example of this are the quasars. Almost a hundred years ago, during the dawn of radio technology, astronomers turned some of the first primitive radio antennae to space, curious to find what objects, if any, naturally produced radio waves.

Astronomers were expecting to detect radio waves from the sun and Jupiter (both of which are quite bright in radio), but they didn't expect the dozens of other sources, scattered around the sky, that didn't seem to have any known star or galaxy associated with them. As radio telescopes got more sensitive, dozens turned into hundreds of mysterious, radio-bright points in the sky. Not really knowing what to classify these radio stars as, astronomers named them "quasi-stellar radio sources," shortened to "quasars."

When the giant Hale telescope at Mount Palomar was finally turned to the sky after World War II (which had understandably distracted most of the science and technology experts in the world for almost a decade), one of the astronomer's first priorities was to solve the mystery of quasars. As it turned out, this happened in short order. Tiny, dim points of light were found at the exact location of the mysterious radio sources, and red-shifts were summarily measured to estimate the distance to the quasars.

For the first time, people got a hint of how big the universe really was. Quasars turned out to be billions of light years away, the farthest objects yet detected at that time. That meant they must be intrinsically very bright, to be detected by radio (and now visible light) from so far away. The numbers were impressive. A typical quasar was thousands of times as bright as our own Milky Way galaxy, but there was a twist. Most of the radiation from quasars didn't match the kind of light that stars naturally produced. Something else must be supplying these beasts with energy, something even more powerful and efficient than starlight.

We now have a pretty good idea what the power source of a quasar is, and our telescopes have even given us a glimpse into the hearts of a few of their central engines. Wherever we can see inside, we find a super-massive black hole, millions or billions of times the mass of our sun. Black holes are bottomless pits of gravity with the power to suck in all forms of matter and energy. So how does something that sucks up energy supply the power to light up a quasar? The answer is that all the energy is coming from material that hasn't fallen into the black hole yet, but is trapped in its gravitational field.

Hot clouds of gas and dust, even entire stars are ripped apart by the black hole's strong tidal pull and spun into a vast disk, in some cases light years across, around the black hole. The intense heating and acceleration release monstrous amounts of energy, much of which is directed into jets of radio waves that stream out of the quasar's core. The more matter falling down the black hole, the more energy is released and shot out into space.

That's where we get back to the light-time lag. Astronomers noticed almost immediately that the farther away a quasar was, the more energy it produced. And there are no quasars in our local group of galaxies. For some reason, quasars are objects that are only seen very far away (and therefore very far back in time). Intriguingly, there are galaxies that look something like quasars at medium distances away from us. They have similar cores to quasars (big black holes producing lots of radio waves), but they're nowhere near as active and violent as the distant objects.

This pattern (no quasars nearby, quasar-like galaxies a bit farther out, then quasars getting brighter the farther out you go) led astronomers to a lovely conclusion. Perhaps quasars were baby galaxies. We still don't understand how the very first galaxies formed, but it might go something like this: in the early universe, after things cooled down from the Big Bang, matter began to clump together, attracted by gravity. Somehow, in the center of these first conglomerations that would someday become galaxies, a big black hole formed. Whether you need to make massive stars first (which then go supernova and form black holes) or if you can somehow form a black hole directly from the collapse of the matter is unknown.

But in a way, the black hole acts as a "seed" for the rest of the galaxy, pulling more and more matter in and organizing it into a spinning disk. At first, lots of material gets sucked down, creating the violent radiation that makes a quasar. Over time, things calm down as the material finds stable orbits around the black hole and less stuff actually falls in. Stars and planets form out of the spinning matter, especially in the cooler, peaceful outer reaches of the disk. In the end, you're left with a fully-formed adult galaxy. The big black hole is still there, but without a lot matter falling in, it doesn't cause much trouble.

Finally, it's important to remember that the great cosmic time machine runs both ways. We can look out into the universe and back into the past, but so can anyone else out there. The distant quasars we observe have probably matured into galaxies much like our own, and perhaps there are beings living in them and studying the distant (to them) Milky Way galaxy. Might we look like a quasar to them? There's some good evidence that we just might.

For a long time now we've known about a radio source in the center of our galaxy, relatively weak, but very much there. Recent radio mapping, as well as infrared observations, have revealed a spinning disk of material around a compact, hot center. Preliminary estimates of its mass are around a million solar masses, all packed into an area not much bigger than our solar system. You guessed it: money has it that it's a giant black hole, just about big enough to have powered a quasar long ago.