Winding Down: Pulsar and Astro-Timing

The End of the Dark Ages

Adam Frank for McGraw-Hill

It is hard to believe how far we have come is so short a time. Cosmology, the Science of Everything, used to be the realm of speculation and fancy. Determining how the entire Universe began and evolved is, after all, a pretty tall order. Humanity's ability to say anything reasonable about the question of cosmic origins seems like more than our little Earth-bound species should be able to hope for. For most of our history speculation and fancy were pretty much all we had when it came to understanding cosmic origins. In the last few decades, however, things have changed, and changed dramatically. We are now moving rapidly to the point where even the small details in our theory of the Universe can be verified. Take, for example, the theory of "re-ionization" and the end of the cosmic dark ages.

The best model we have for the origin and evolution of the Universe is the "Big Bang" theory. According to the Big Bang, the Universe started as an infinitely dense, infinitely hot, infinitely small bundle of space, time, matter and energy. From the initial explosion of this ball-o-everything came all the stuff we see today: galaxies, planets, people etc. The Big Bang is a pretty grand idea. As you might expect, astronomers have a lot of details to fill in to be sure that it's true. One particular detail that astronomers have long wondered about is what happens in the early stages of the expansion after the basic building blocks of matter like protons, neutrons and electrons have been formed.

For years scientists have built their Big Bang models on the idea that as the universe expanded it continually cooled allowing interesting things to happen along the way. After a few hundred thousand years, for example, the initial fireball of creation will have cooled to the point where protons and electrons are moving slowly enough to latch on to each and form the first atoms of Hydrogen.

Hydrogen formation marks a critical transition for the infant Universe. Once lots of Hydrogen exists the relation between matter and radiation changes dramatically. Some kinds of light that were locked into a tightly coupled dance with matter are suddenly freed to wander the Universe unhindered. Other kinds of light such as strong Ultraviolet photons (the stuff which gives you a sunburn) are suddenly trapped. Hydrogen atoms are like Ultraviolet (UV) sponges; they love to absorb UV light particles. After Hydrogen forms, Ultraviolet light has a hard time traveling freely though the Universe. Any UV light that is created gets absorbed almost on the spot by neighboring Hydrogen atoms. The presence of large amounts of Hydrogen means the Universe is dark (at least in terms of Ultraviolet light). In fact scientists call this period after Hydrogen formed the "Dark Ages."

What does all this have to do with how good we have gotten in our understanding of the Universe? The Universe we live in now is far more transparent to light. That means eventually the dark ages have to end. For years astronomers have had a very firm prediction for the signature that would mark the end of the cosmic dark ages. Eventually our infant Universe matures to the point where the first generation of stars form (perhaps a few hundred million years after the big bang). The light from these stars is powerful enough to rip apart Hydrogen atoms floating in space. As the Universe begins to fill with stars the amount of Hydrogen gas in space drops dramatically and UV light can, once again, sail unhindered across large stretches of the cosmic ocean. Astronomers call this the period of "re-ionization." They believe that if they look far enough out into space (which means far enough back in time) they should, eventually, see the place where the re-ionization occurs. This will be the boundary between the old dark Universe and the newer transparent one. Earlier this year two teams of scientists found just this kind of signal. The astronomers examined super distant objects called Quasars that live 90 percent of the way back to the Big Bang (that's more than 13 billion years ago!). The signals from these distant (in space and time) quasars show clear evidence that they lived just at the end of the dark ages. Very little strong UV light is seen from the Quasar showing astronomers that it had all been sopped up by encircling hungry Hydrogen atoms.

If this seems like a long convoluted story you are most certainly right. The big picture, the Big Bang, is so much easier to describe. The Universe was born in a big explosion, blah, blah, and blah… . What is amazing is how far beyond that we have now gotten in our ability to tell the story. This tale of cosmic re-ionization shows we are now getting down to, and verifying, the details our Big Bang cosmology. The birth and evolution of all that exists, is now very much a real and exact science. When you think about it, that is nothing short of amazing. We have done pretty well for ourselves.

Questions to Ponder

1) Why is there a relation between how far away an object is and how far back in time we are seeing it?
2) What is the relation between the "maturity" of a branch of science and the level of "detail" in the questions it can answer?

Check Out These Pulsar Websites

Sloan Digital Sky Survey
http://www.sdss.org/

Discovery of the Cosmic Reionization Era
http://www.astro.caltech.edu/~george/reion/

Tom's Radiative Transfer In Cosmology Page
http://cosmos.ucsd.edu/~tabel/Transfer/