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About a hundred years ago, astronomers had a big problems. They had these new huge refracting (lens type) telescopes at Yerkes and Lick observatories. These wonderful instruments showed lovely views of what were then call "nebulae" (Latin for "cloud"). Unfortunately, even with these huge new scopes, astronomers couldn't tell if these nebulae were clouds of gas and dust or huge "rafts" of stars. Since the 40" Yerkes telescope was thought to be the largest lens that could be made, the nebulae question could only be solve with bigger new reflector (mirror) telescopes. The race to make the biggest and best was on (and continues today).
Things that go BANG! in the night or Hubble isn't just a telescope
In the early 1900's, Edwin Hubble was one of the finest observing astronomers in the world working at the great 40" Yerkes telescope in Wisconsin. In the 1920's and 30's, he used the new 100" Hooker telescope on Mt. Wilson in southern California to show that some of the nebulae were, indeed, made up of individual stars. But the question remained. Were these "star rafts" part of our Milky Way Galaxy or were there, indeed, other galaxies?
Gedunken experiment time. Imagine a football field. Let's make it a Canadian football field because they are 100 meters long (instead of those silly "yards" used in the US). We should use meters because we are scientists! Ok, so here we are on our Canadian football field, eh? Let's go to the imaginary hardware store and buy one of every kind of light bulb they have. Imagine setting up a row of these electric light bulbs on one goal line. Now, on a nice clear night we go to the other goal line and look back at the row of light bulbs using a pair of binoculars. It's pretty easy to tell which bulbs are brighter than others. If they look brighter they are brighter. We can only tell this because we know that all the bulbs are the same distance from us.
During the day, someone with a sense of humor takes all you light bulbs and scatters them all over the football field. That night you stand at your usual spot on the goal line and turn on all the bulbs. Now you have a problem. Does a bulb look bright because it is bright, or does it look bright because it is closer to you? For a few of the closest bulbs, you might be able to tell how far away they are because your brain can take the image from your right eye and compare it to the image from your left eye and figure out and angle between the two images (this is called Parallax) and, hence, the distance to the bulb. Your brain is really good at doing this but only out to a few meters away. Anything farther away it just guesses.
This was the problem faced by Edwin Hubble. Astronomers had measured the distance to a few close by stars using photographs taken at two different times six months apart. They knew the diameter of the orbit of Earth and could measure the parallax of the close by star by measuring how much those stars seemed to have moved relative to far away "fixed" background stars. This is the same effect as when you point you finger at an object and then close one eye, open it again, and close the other. Your finger seems to "jump" compared to the background.
So, back to our football field. If you knew that a particular bulb was 100 watts bright, you could see how bright it appeared to be and you could figure out how far away it is! The farther away the bulb, the dimmer it appears to be. Simple. What Edwin Hubble needed to tell if the "star rafts" were in our galaxy or much farther away is a known "100 watt bulb" star!
The standard light bulb
As we already know, the stars in the sky can be very bright, very dim, or something in between. This can be because the stars are bright or dim or it can be because the stars look brighter or dimmer because they are closer or farther away from us. Other than a few relatively close-by stars, it is very hard to tell why a particular star is the brightness it looks.
We also know that, if there was some way to tell how bright a star really is, we could tell how far away it is how bright it appears to be. So, what astronomers of a hundred years ago needed was a "standard star" of a known brightness.
In 1912, Henrietta Leavitt of Harvard Observatory and one of the most brilliant astronomers of the last century, discovered a type of star that periodically changed brightness over a few days. The most important feature of these "variable" stars is that the time it takes one to make a full brightness cycle is exactly related to how bright they are. Even more importantly, one of these variable stars, Delta Cephei, is close enough to measure by parallax. We have our "Standard Star"! We now call these standard stars Cepheid Variables. To figure out how far away any particular Cepheid Variable star is, all you need to do is time how many days it takes for the star to go through one bright-dim-bright cycle, look it up in Leavitt's cycle time/brightness chart, and measure how bright the star looks and you can easily calculate how far the star is away!
This is the method Edwin Hubble used to prove that the Andromeda Nebula was the Andromeda Galaxy - millions of light years away with hundreds of millions of stars!
Things that go Bang! in the night
We now know how Edwin Hubble and others figured out the distance to "near by" galaxies such as Andromeda's. Now astronomers knew those nebulae were far, far outside our Milky Way Galaxy and, by measuring how bright they look, could make a pretty good guess about how far away these galaxies truly were.
Edwin Hubble noticed something very interesting. He looked at the spectra of all
these galaxies and saw that almost all of them were "red shifted". As far as the
astronomer at that time could tell, that meant that all galaxies were moving
away from us.
If this wasn't crazy enough, Hubble measured that
the farther away a galaxy was, the more it was red shifted!!! This is crazy! Why would everyone be moving away from us? Does our galaxy smell really bad or something?
Suspecting that this red shift wasn't caused by a bad case of cosmic B.O., Edwin Hubble theorized that what we were really seeing was the history of all the galaxies in the Universe
if they all started in the same place and, for some reason,
went BANG! and the Universe "exploded".
Well, most astronomers thought this "Big Bang" idea was just as crazy as cosmic B.O.! That you could have all the matter in the entire Universe stuck together in one "cosmic egg" and that, some how, it would explode like a stick of dynamite, violated all the laws of physics known at the time. It just wouldn't work! But it did explain the red shift (mostly).
Gedunken experiment time
Imagine this Cosmic Egg of every bit of matter in the Universe. Picture it blowing up like a stick of dynamite. Obviously, everything on the outside of the egg starts moving first and fastest. If you were on a chunk of stuff from the outside of the egg you could look "out" and see absolutely nothing. If you looked "back" you could see the next layer inside the egg coming towards you but, since you were traveling faster than that layer, it would still be red shifted. Looking to the "side", you would see the other pieces from the outside of the egg moving away from you on the surface of an expanding sphere of exploded pieces. The Universe would look identical to any observer on one of these "outer" pieces.
Imagine now that you are on a blob of matter that was the next layer down in the Cosmic Egg. Now as you look "out" you will see the stuff in the first layer moving fast away from you and nothing beyond it. Again, looking "back" or to the "side" you would see much the same as what you saw from the "outside" layer.
With each "layer" going down into the Cosmic Egg you see the previous "outer" layers red shifted moving fast away from you, layers "in" from you red sifted moving slow away from you, and chunks in your layer red shifted moving at a medium speed away from you.
If you were at the center of the Cosmic Egg when it exploded, everything else would move away from you with the farther stuff moving the fastest. Since this is what Edwin Hubble saw, our galaxy must have been at the center of the Cosmic Egg!
Well, this was an unhappy situation. We never like being "special" in the Universe. How come we, of all the stuff in the Universe, were the center of the Big Bang? This is way worse than Cosmic Body Odor! Luckily there is one way out of this problem.
The Big Bang in nothing:
The problem with this explanation is that the Cosmic Egg has to be sitting there in an otherwise empty Universe. How about this idea - and I'll warn you, this is very tricky to imagine.
The trick is to imagine less than nothing!
When the Big Bang happened, there was no Cosmic Egg. There wasn't even an empty Universe full of nothing. There was less than nothing.
The Big Bang was not an explosion of existing stuff in an existing Universe. The
Big Bang started out as an explosion of "everything" that brought the Universe
(what we call "space") with it.
If you were there to see it, you would NOT see and explosion because there was no "place" an observer watch from.
The entire Universe was "inside" the explosion.
In fact, the Big Bang wasn't the explosion of matter
in the Universe (as it was with the Cosmic Egg idea);
it was the explosion of "Space", what we now call the Universe, which took the matter (mostly Hydrogen) with it.
Can you imagine that? It is a very hard thing to picture in you mind.
With this model of the Big Bang, nobody is in a special position. Every point in the Universe was at the "center" of the Big Bang. Wherever you are, the Universe looks about the same. There is no "outer edge" of the Universe and there is no "center". We don't see everything red shifting away from us because we were at the center of an explosion. We see everyone red shifting away from us because
space is expanding and taking us with it! We aren't flying away from all other galaxies; the space between the galaxies is expanding!
[Note to Editor:]
I did the "alcohol over salt" demo for my family the other day and I noticed a
change to be made to the instructions on the website. Where it says use a teaspoon
of salt and half a teaspoon of alcohol, it should be changed to a few drops of
alcohol. A half a teaspoon is WAY too much. Shows I shouldn't always trust my
memory. Trying this would have been an easy and good idea. Oh dopey me.