Red shift what happens when an object radiating light moves away from you really, really fast, in the same way a car's horn/clunk
ing 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 shift
ed. 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
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.
Pima Community College
AST 104 Class