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The Expanding Universe

On occasion, I will take a few weeks to explore a theme in astronomy on these pages. In 1997 that theme was the solar system. This time, for the next few weeks, I invite you to take a look at the Universe In Motion.

We think of the skies as static, unchanging. The only motions we see easily are the rising and setting of the Sun, Moon and stars, but that is a reflection of our own Earth's rotation. The stars themselves don't seem to move at all among themselves, and it takes a keen eye to discern the motion of the planets, which takes days or weeks to become obvious.

Yet objects in the sky are in constant motion, and some move with an incredible intrinsic velocity. Usually, these objects are so far away that the distance itself shrinks the apparent motion, the way distant mountains hardly seem to move at all even though you may be driving past them at 100 kilometers an hour. Sometimes it does take many years to perceive the movement of heavenly bodies, and sometimes it happens in the blink of an eye. Every Monday, we'll take a look at some of these celestial travelers.


Week of March 15, 1999

In 1781, French astronomer Charles Messier (pronounced mez yay) was an established comet hunter. He would pry the skies with his small telescope looking for the faint, fuzzy blurs that meant that another celestial visitor was on its way. The problem was, there were lots of fuzzy things in the sky, very few of which were actually comets. These objects were an annoyance to him, so he decided to keep a catalog of objects that were not comets, so that he would not mistake them for comets in the future. The Messier Catalog became a staple of observers who wished to avoid those irritating objects.

Among those objects in the catalog were a group called ``spiral nebulae'' (nebula means cloud). They were faint and fuzzy like comets, but showed a spiral pattern when seen with large enough telescopes. No one was sure just what they were. Some theorized that they were enormous collections of stars, vast in extent, while others believed they were spinning disks of gas much closer to home. The argument raged for a long time, and at one point there was even a highly publicized debate over whether our own Milky Way Galaxy was a small part of a vastly larger Universe, or was indeed the Universe itself.

Proof came in 1924 when astronomers at Mt. Wilson, using the world's largest telescope at the time, discovered that these ``nebulae'' were actually composed of stars. They were at such a great distance that the individual stars blurred together into a nebulous fog. Some of these stars turned out to be a special kind of variable star; a star that allows its distance to be measured if only you can accurately measure how fast it changes brightness. When the distances were measured, astronomers were astonished to find that the galaxies - as the nebulae were later called- were very far away, hundreds of thousands of light years away.

But the big surprise was still ahead. Edwin Hubble was an astronomer at Mt. Wilson in 1920. He took spectra of the galaxies, and reported an amazing finding: by measuring the Doppler shift of the galaxies, he discovered that the overwhelming majority of these galaxies had a red shift, and not a blue one. That means they were rushing away from us. A few years later, H. R. Robertson reported that the more distant the galaxy, the higher the redshift, and therefore the faster the galaxy was receding from us.

I don't know if these men realized what they had found at the time, but their discoveries fundamentally changed the way we view the Universe. The interpretation of these findings is that the Universe is expanding. It's not really that the galaxies are moving away from us; it's space itself that is expanding, carrying the galaxies along with it like raisins in a loaf of rising bread. We are not at the center, as you might naively think from the observations; every galaxy would see every other galaxy rushing away from it, and the farther the galaxy the faster the recession.

We have made dozens of observations that appear to verify this, and the idea that the Universe was created in a single moment and has been expanding ever since is now used as the standard model. We've all heard the term ``Big Bang'', but that's a misnomer: nothing really exploded. Certainly it was hot, and certainly it is expanding, but an explosion implies something blown outward into something else, like shrapnel into the air. But remember, it was space that was created, and there isn't anything into which it expands. The Universe is all there is.

This is the final Snack in the Universe in Motion series, and it's an appropriate way to end: with the entire Universe set into motion. The concept of an expanding Universe boggles the minds of nearly everyone (and I am no exception). But there is something I have learned over many years of studying not just astronomy, but physics, chemistry and even the way people behave: things are not always as they seem. We go about our daily lives not realizing that the way we see things is through a personal filter, and we expect all things to behave in some way we can understand. As I have said before, though, the Universe is under no obligation to perform to our expectations. Time flows more slowly for someone moving relative to someone else. On very tiny scales, the Universe bubbles and froths with vast amounts of energy. Whole stars can explode, and their cinders can wrap space around them so tightly the stars disappear forever. There are countless examples of how the Universe is a weird, bizarre and just plain surprising place. The wonder in it is not that it is so unknowable, but that it is knowable at all. And all this, because an astronomer catalogued objects that were annoying him!


Here are some links you might find useful dealing with this topic:

  • Messier's Catalog can be found online with pictures of all the objects in it. This is a nice way to browse some of the most beautiful and brightest objects in the sky.

  • If you like those, then you can get even more images and info about specific objects in the catalog from The Web Nebulae. This page and the one linked above are brought to you by Bill Arnett, one of the hardest working people in the astronomy WWW, and you should spend as much time as you can perusing his work!

  • Baffled by the idea of cosmic expansion? Try poking around the USENET news group sci.astro's excellent FAQ. Ned Wright's Cosmology FAQ is also a fantastic place to answer any questions you have about black holes, the Universe and everything. You can try the sci.physics FAQ too!



    ©2008 Phil Plait. All Rights Reserved.

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