20 Aug 2006
Redshift. Blueshift. Near star. Far star. Sounds like a lame attempt to start off a new Seuss book. But what is this talk from astronomers about things redshifting or blueshifting? What's it all mean? And what does it have to do with the Big Picture of our universe?
Let's start off this explanation with a small scale example we have all experienced. When a fast moving car speeds by you, you can hear a rise in the pitch of the sound as it approaches, and a lowering as it passes by and away. NASCAR fans hear this sound effect in their sleep.
This is called the Doppler effect. As the object approaches, its sound waves get smushed together, which raises the pitch. As it goes away from you, the waves in effect are stretched out and the pitch lowers.
But the same thing happens with light! As an object in space approaches, its light waves get smushed. As it moves away or recedes the wavelengths are stretched out. But we cannot hear any effect, obviously. How can we detect this?
We look at the light! There are simple ways in astronomy to examine the light of a star. We spread the light out like a rainbow, so we can see its "spectrum." We see that for a star's spectrum there are some wavelengths missing - there are thin black lines on the pretty rainbow of color. Why they are missing is for another day, but they are missing.
If we see that those missing wavelengths have moved from where they should be then we know the star is moving with respect to us. If they've moved over toward the blue end of the spectrum, we say that the star is blueshifted. Its "normal" wavelengths are shorter, more "bunched up," than when at rest so the light source - here, the star - must be moving toward us. If the missing lines have been moved toward the red end of the rainbow, then the star's light has been "redshifted," stretched out, and the star is moving away.
How far they have moved from their expected "rest wavelength" will determine how fast toward or away from us the star is traveling. You won't actually see a star get redder or bluer; even a star moving at a good clip doesn't shift its wavelengths by much. But it is a handy device for plotting the stars in space and predicting where they've been and where they're going.
However, there is another "shift" that plays a huge role in the life of our universe - the cosmological redshift.
About 80 years ago, Edwin Hubble confirmed something that was puzzling the astronomical community. By studying the light from galaxies all around us, it seemed that virtually every one was red-shifted, they were all moving away from us. Stranger, close-by galaxies were just putt-putting away from us, but the galaxies at great distances were screaming away at great speeds!
How could this be? What are the odds that essentially all galaxies were redshifted? Why wasn't there a random mix of redshift, blueshift, and no-shift? Unless!!!
Unless the entire universe was expanding! The only scenario that explains why everything is moving away from each other, and the more distant things seem to be racing away faster, while the closer objects are in no particular hurry to move away, is a cosmic kaboom.
This was the first proof of a big bang, of a beginning to the universe. But it's a little stranger than that. Get the thinking cap!
Those galaxies aren't being shot through space at those tremendous speeds. They are being carried by space. Space itself is expanding, and has been since the Creation Event.
As space expands it carries everything along with it. You, me, the dog, planets, galaxies... everything.
As the space expands, it stretches out. Light, riding on that fabric of space, gets stretched, too. Like the unfolding of an accordion, the wavelengths of light get stretched out to longer lengths. This, in effect, shifts those photons toward the red end of the spectrum. Thus, the light of essentially every galaxy out there is being redshifted. But not because of the common Doppler redshift explained above. This here is the Mother of All Redshifts - the "cosmological redshift."
The cool thing is not just that this all shows we had a beginning to the universe, although that truly is awe-inspiring. But taking all the redshift data, and crunching the bejeebers out of them, we can nail down a time when it all began, which is nearly 14 billion years ago. That's impressive, as well.
Did this shed a little light on some cosmic questions you had. Hope so. Until next time, clear skies!