🚀 go-pugleaf

hatcher@msupa.pa.msu.edu wrote on 20.06.94 in <2u4mtn$ohh@msuinfo.cl.msu.edu>:

> *BLINK*  Don't follow much in the way of astronomy, do you?  You remember

Well, it seems you're not up on everything, too ...

> "origin" of the Big Bang.  If one sees that the density of galactic
> clusters increases as you go back in time (accounting for galactic
> evolution) then one can claim that space is expanding.  Now if you go back

That, however, is not how it's done, because it don't work that way. While  
the universe "out there" is certainly smaller, it gets massively distorted  
by being smeared around the larger one we have here, so no density  
increase, sorry.

This is how it *is* done: you ever heard of Mr. Hubble and his "constant"?

When you look at some far away object (say, a galaxy) in the universe, you  
can get its spectrum. You can then see that the lines in that spectrum are  
not where they ought to be, but shifted toward the red end.

This is interpreted as Doppler effect, meaning that that object is fast  
going away from us.

When you use this method to find out the velocity of objects relative to  
us, you find that the more distant it is (of course, you need some way to  
gauge distances), the faster it goes away, roughly linear with the  
distance - which means (with the help of a little math) that it looks the  
same from everywhere.

In fact, now we know this, we can turn around and use the red shift of  
extremely far away objects to get a good guess about their distance.

That's not all. The "Hubble constant", which is just the number telling  
how fast an object is when we know how far it is, has in fact the  
dimension of inverse time. The common interpretation is that the inverse  
of the constant is the age of the universe, or, to put it another way,  
since everything moves away from everything else with apparent constant  
speed, when we extrapolate back, we find a moment when everything was at  
the same place, and call this the big bang.

This, of course, makes it fairly interesting to know the exact value of  
the Hubble constant. Alas, it seems the best astronomy has so far come up  
is estimates varying by about the factor 2 - which happens to be quite  
good for astronomy, by the way.

Kai
--
Internet: kh@ms.maus.de, kai@khms.westfalen.de
Bang: major_backbone!{ms.maus.de!kh,khms.westfalen.de!kai}
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