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Redshift
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Notes de l'éditeur
- Redshift and blueshift
- This also happens with sound, Have you heard an ambulance pass by? http://youtu.be/imoxDcn2SgoThis is caused by the Doppler shift as well, but with SOUND waves instead of light waves. But let’s go back to light waves. There is no sound in space. Why is there no sound in space? Sound UNLIKE light, requires a MEDIUM, it is vibrations in the air, or water, or whatever you are in. If there is not something for sound to move through, it will not travel. In space, nobody can hear you scream.
- SO the slogan to alien is true!
- But anyhow, how do you think astronomers determine somethings redshift? How can we tell if light is more red than it should be? The galaxies do not move past us like an ambulance or a fire truck, so we cannot compare that way. What do astronomers use to measure the wavelengths that something in space is giving off?Remember that everything has a light fingerprint. So for example helium, remember helium? Always gives off light at the SAME WAVELENGTHS. We figured out what those wavelengths are, remember? So if we see the pattern that matches helium, but with slightly different colors, we know there has been a shift.
- Here is an example.
- We are going to use this slide again later.
- Remember this guy? This is Edwin Hubble. He was the first to notice that galaxies were moving away from us, almost ALL galaxies were moving away from us, using their redshift. He used spectra to do this. He noticed that only a few galaxies were moving toward us, and they were very close to us. Why are the galaxies very close to us moving toward us? Gravity. For example, Andromeda galaxy is headed on a collision course toward us!
- Don’t worry, it won’t hit us for many billion years. And when it does, do you think stars will collide? Remember what we talked about the first day? If the Sun were a balloon, the closest balloon would be in Chicago! So most of the galaxy is empty space, and there is a lot of space between stars. Keep in mind in this picture, all of the stars around here are in the Milky Way galaxy, there are not stars floating around in space.
- So back to redshift. For velocities that are small, not close to the speed of light, redshift can be calculated by z = v/c. We can use this formula to figure out the velocities of the galaxies moving away from us. Once we have the velocity from the redshift, we can figure out how far AWAY the galaxies are to us. This is important, because if we want to figure out what the Universe looks like, we want to know not only where a galaxy is on the sky, but how far away it is, so we can construct a 3-D map. THIS IS HOW WE KNOW what the Universe looks like on the largest scales, what we are going to talk about next. We know what the Universe looks like because we can construct 3-D maps by measuring redshift, which tells us velocity, which tells us distance, through these equations.Remember that velocity is SPEED AND DIRECTION. The speed of light is about 186,000 miles per second. As of a recent 2011 estimate:H0 =73.8 ± 2.4 (km/s)/Mpc, but this is not something you will need to know.
- Do redshift activity.