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doppler effect
Physics 101: Redshift and the Expanding Universe
by Pearl Tesler

Throughout our universe, light is bursting from stars, bouncing off planets, diving into black holes, wandering into nebulae, and generally going every which way. Meanwhile, a little bit of it actually shows up here on earth.

The light that does arrive here all seems to bear the same message: the universe is expanding.

How can light from the night sky tell us that the universe is growing in size? The main clue comes from something called redshift.

Redshift is light's version of a phenomenon we experience all the time with sound. Have you ever noticed how the pitch of a police siren seems to drop suddenly as the car zooms by you? As the siren approaches you, the waves of sound are squeezed together, and you hear them as being higher-pitched. After the car passes by, sound waves from the receding siren are stretched apart. You hear these stretched waves as being lower-pitched.

Follow the car
Follow the car to see a Shockwave demonstration of Doppler.

This apparent change in the pitch (or frequency) of sound is called Doppler shift. Light from distant stars and galaxies can be shifted in much the same way.

Like sound, light is a wave that can be described in terms of its frequency, the number of wave peaks that pass by each second. Just like a cosmic police car, a star zooming toward you has its light waves squeezed together. You see these light waves as having a higher frequency than normal. Since blue is at the high-frequency end of the visible spectrum, we say the light from an approaching star is shifted toward blue, or blueshifted.

Likewise, if a star is zooming away from you, any light it emits gets stretched. You see these stretched-out light waves as having a lower frequency. Since red is at the low-frequency end of the visible spectrum, we say that light from a receding star is shifted toward red, or redshifted.

Blueshift, Redshift Imagine you're moving to the left with this arrowhead. Light emitted from galaxies moving toward you would be squished, making the wavelength shorter and the light bluer. On earth, we perceive the light from galaxies moving away from us (as it appears almost all galaxies are) . . . . as being somewhat stretched, with longer wavelengths that make it look redder.

The amount of the shift depends on the speed of the star, relative to you. For a moving object to create an appreciable redshift or blueshift requires some pretty serious speeds. To get just a 1% change in the frequency of light, a star has to be moving 1,864 miles per second. For a blue lightbulb to look red, it would have to be flying away from you at 3/4 of the speed of light.

Studying light from galaxies throughout our universe, astronomers have noticed something surprising: almost all of it is redshifted. In fact, not only is it redshifted, galaxies that are farther away are more redshifted than closer ones. So it seems that not only are all the galaxies in the universe moving away from us, the farther ones are moving away from us the fastest.

On first glance, this seems to put us at ground zero of a major cosmological exodus. . .what is it, our

Expanding universe activity
Show yourself why it always seems that we're the center of the universe. (requires Shockwave Flash)

breath? In fact, we aren't really at the center of the expansion. In an expanding universe, anyone standing anywhere in the universe would see everything as moving away, or redshifted.

What puzzles astronomers most now is not that the universe is expanding, but that the rate of this expansion seems to be increasing. Using data from the Hubble Space Telescope, astronomers hope to be able to figure out the likely fate of our universe: Will it expand forever, or will the expansion reverse and cause the universe to collapse back into another Big Bang? Stay tuned.


For a more in-depth look at how astronomers can use redshift to determine speed and other properties of celestial objects, check out this presentation from NOVA's "Runaway Universe" Web site.



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