Why do we get a redshift?, this question used to bug me a lot. First I'll tell you an approximate answer (the approximate explanation only recovers the first term in the Taylor series expansion of the right answer, honestly)

The approximate explanation says that it is a Doppler shift. When an ambulance is moving toward you the pitch of its siren is higher than when the ambulance moves towards you. The motion of the ambulance "pushes" the wave crests of the sound closer togeter in the direction of motion and spreads them apart away from the direction of motion. This change alters the frequency of the sound and hence the pitch. A similar argument is propsed for why light from a galaxy that is moving away from us appears red.

Now here is a thought experiment to explain why this is a misconception when dealing with light. Imagine that you are travelling on a beam of light. When you get emitted from your host galaxy the universe is young, perhaps our galaxy has not even assembled itself yet. The expansion rate of the universe is low and so your "Doppler shift" will be quite low. By the time you reach my eye much time has passed. The expansion rate of the universe has increased and the redshift I measure is concurrent with that fact, not with the recession velocity of your host galaxy at the time that you were emitted. When then did your photon learn of the increase of the expansion of the universe?.
The Doppler principle can not explain this. The correct answer comes from the metric of space and time itself. The semi-classical photon is intimately tied to space itself. (from here on in the explanation becomes touchy feely and I can only recommend looking at the maths and a proper derivation of the cosmological redshift.) I believe that the true answer will not come out until we have a quantum theory of gravity and it will be discovered that the boundary conditions of the photon are connected to the geometry of spacetime. This is certanly the effect we see at the moment with the redshift but that statement might be meaningless. If you imagine that it is true than you can almost imagine that the photon gets streched as spacetime expands because the photon is connected with the idea of measurement and length and the structure of spacetime. I can't really say much more than that.

In 1929, the American astronomer Edwin Powell Hubble linked the redshift visible in the spectra of distant galaxies, to the constant expansion of the universe.

Hubble suggested that this cosmological red shift is caused by the Doppler Effect, as the space through which the light travels, is stretched out. This would hence indicate the speed of recession of these galaxies -- and, by using Hubble's Law, the distances of the galaxies.

A second mechanism for red shift is the gravitational red shift, also called the Einstein shift. It was predicted by Albert Einstein with his general relativity theory -- According to which periodic processes are slowed down in a gravitational field. This Einstein Shift is noticeable in the spectra of massive, compact stars, such as white dwarfs.

Redshift is the name of an EP released in August of 1999 on Java/Capitol records by Splashdown, a Boston-area trip-hop group.

Originally, there was to be a follow-up full LP, Blueshift, however internal problems at Java/Capitol caused the album to be permanently shelved, and for Splashdown to ultimately leave the label entirely.

This also ended up being the last official release of Splashdown anywhere. The group all went their separate ways (while reportedly staying "good friends") about two years after its release.

Track Listing

1. A Charming Spell (4:59)
2. The Archer (4:23)
3. Mayan Pilot (2:53)
5. Ironspy (4:59)
Red shift what happens when an object radiating light moves away from you really, really fast, in the same way a car's horn/clunking sounds lower when it goes away. Gorgonzola did a great writeup on Blue Shift which is the exact opposite, when the object is approaching.
Interestingly, the use of this shift by Edwin Hubble (1889-1953) was what led to the proof of the expanding universe as well as many very basic parts of Cosmology today: the age, condition, and fate of the Universe. An American astronomer had earlier noticed that the Andromeda Galaxy was receding away. Einstein had initially asserted an immobile Universe, but was corrected by Willem De Sitter.

All stars (in all directions) were moving away. Now, you can imagine one of a few things is going on. Either we are at the center of the Universe - unlikely - or the Universe is expanding. Here's the logic.

Imagine you have a big, red balloon, deflated. Draw a bunch of dots all over it. You can imagine that the distance between the dots will be on average, say, a centimeter or so. Start blowing it up. The dots will all start to spread apart. From any one dot's perspective, the other dots will appear to recede. That's what's basically going on with the Universe, and Hubble showed it because of the red shift. Also, the farthest dots will appear to recede faster, that is, really far stars are going to have really high red-shift.

Hubble, while all these crazy notions of an expanding Universe wer going on, during the 1920's and 30's, proved that there was a correlation between how far some stars were and how much they were red shifted. He convinced Einstein once and for all on the expanding Universe.

But that's not really what makes him or red-shift cool

The correlation between distance and red-shift was strong enough that Hubble said that the distance of an object could be calculated from its red-shift. The equasion goes like this:

Red Shift x Speed of Light
The Hubble Constant = Distance

The bad news is that we don't ... quite ... know what the Hubble Constant is, yet. It's somewhere between 50 and 100 kilometers per second for every megaparsec in distance, km/sec/Mpc. This means that a galaxy 1 megaparsec away will be receding from us between 50 and 100 km/sec, while another galaxy 100 megaparsecs away will be receding at 100 times this speed (bloody quick). So essentially, the Hubble constant sets the rate at which the Universe is expanding. Cool.

This brings us to Hubble's Law. v = H*r. V is how fast the object's going away. H is Hubble's (sort of) Constant. r is the velocity away from here.

Another cool thing about Hubble's Li'l Law: the inverse of the Hubble constant has units of time. By substituting in kilometers for Mpc in the Hubble constant, we find that upon inverting H we get a quantity with units of seconds (kilometers canceling out in the denominator and numerator). For a Hubble constant of 100 kilometers per second per Mpc, we get 3 x 10^7 seconds, or about 10 billion years. For H=50 kilometers per second per Mpc, the time scale is 20 billion years.

By figuring out this whole red-shift crap, we calculated the age of the Universe! Go Hubble! And you wondered why there was a big mirror (which was made at UA) in the sky named after him.

Sources: Pima Community College AST 104 Class
http://spaceboy.nasda.go.jp/note/kagaku/E/Kag13_e.html
http://astron.berkeley.edu/~mwhite/darkmatter/hubble.html

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