It is my pleasure to introduce Dr. Eric McKenzie, associate director for the department of astronomy at the University of Maryland. He received his Ph.D. from the University of Florida, and his research has focused on galaxy evolution at specific wavelengths of redshifted light. He also helped me get some of my facts straight for the Tomorrow News Network story “The Orion War.”
This week, he will be answering questions about galaxy evolution and what the early universe was like.
James Pailly: Using redshifted light, you’ve peered back in time to see the formation and evolution of galaxies. What is redshifted light, and how are we able to see so far into the distant past?
Dr. McKenzie: The wavelengths of light that we see from a moving object get shifted to the bluer or redder end of the spectrum because of the motion. The color shift depends on whether the motion is toward us or away from us. Thus, stars and galaxies look bluer if they are moving toward us and redder if they are moving away from us.
This works just like the wavelengths of sound from a moving object. Think of the sound when cars, trains, ambulances, etc. go past you. When they come toward you, the sound is high in pitch (short wavelengths, high frequency). When they pass you and are moving away, the sound quickly switches to low pitch. If a car were driving past us at a significant fraction of the speed of light, we would see the visual color shift, as well: it would look first bluer, then redder!
Our universe is expanding: nearly all of the galaxies are moving away from us, and so their light is redshifted. When we see the most distant galaxies, they are the reddest, because they are moving away the fastest.
By studying galaxies which are billions of light years away, we see what the universe was like at a younger time. Because light travels at a finite speed, we never see anything as it looks ‘right now’. The moon is 1 1/4 light seconds away, and so we see it as it was 1 1/4 seconds ago. The sun is 8 light minutes away, and so we see it as it was 8 minutes ago. (This leads to the slightly disconcerting thought that, if the sun winked out of existence right now, we would be happily oblivious for 8 minutes while the light rays continued to travel to us.) Looking at distant galaxies gives us a picture of the early universe. We can study how galaxy ‘demographics’ have been changing over time, such as their sizes, shapes, colors, and star/gas/dust content.