Expansion Explained please

Could someone explain how we know that the universe is expanding please, in laymans terms.A colleague and I had a discussion and we want to know how secientist know it is expanding? (basics please)Thanks
Jon

The idea behind the expanding of the universe lies in our understanding of light.Simply put the light from distant stars is shifted towards the red end of the spectrum when they are moving away from us and into the blue end of the spectrum when they are approaching us.

I can only give a very simple explanation of it. I suggest that you risk getting a half messed up idea taking it from here. Reading a few popularizations of it by real physicists and cosmologists would be a good idea. "The First Three Minutes" is good one though very old.Basically, you have to get away from the idea of an explosion. That is NOT what happened.There was a concentration of mass-energy of immense (infinite??) density. It defined all of what we think of as space. Space was full of the mass-energy. Then space expanded carrying the mass-energy with it.The surface of a ballon analogy is a not bad one. It has been described lots of times. You'll find it on the web with pictures.How do we know this?
Originally it was when we discovered that the universe was expanding. A simple reversal of that suggests a starting point.After that the cosmic background radiation which matchs what would be expected added to it.Additionally calculations of the results of the conditions produce the ratio of elements we see.There is still a lot of ongoing research trying to refine the whole thing. It is not, as far as I know, possible to predict exactly the current structure of the universe from basic prinicples. Therefore we sure don't know everything yet.

The above is exactly correct. Just to add another layer of layman to it, the same effect happens with soundwaves. A moving object giving off sound will give off a higher pitch as it travels towards you and a lower pitch as it moves away from you. This is called the Doppler Effect. You can sometimes notice this effect with passing vehicles. Since light has characteristics of both a wave and a particle, the direction of a moving source of light can be measured.

Thanks people.

OK, for a little more detail than already posted, there are actually two ways we know that the universe is expanding: 1)Redshifting of light from distant galaxies; 2) The Cosmic Microwave Background Radiation of 2.7 degrees Kelvin.First, since the speed of light is always the same (186K MPS) regardless of the speed of the emitter, its frequency/wavelength changes depending on the speed of the emitter. Thus if a light source is moving away from us we will see longer wavelengths and lower frequencies being emitted. That's what we call the Doppler effect. Sure enough when we look at galaxies far far away (the effect does not show up for stars in our galaxy nor in nearby galaxies which are actually blue shifted) we notice that their light has been shifted towards the red end of the spectrum. But it is more than just light that we observe, we actually observe absorption bands - where atoms have absorbed part of the spectrum of the light emitted from these far away galaxies. We know where these absorption bands should occur on the spectrum and if they are different, than we can measure the difference in frequency and voila come up with a velocity. The funny thing is that the farther away a galaxy is, the faster it appears to be receding from us. Ergo, we have an expanding universe.Second, is that since the expanding universe implies a hotter, denser state out of which we evolved, i.e. a big bang, the theories imply that there must be some left over radiation from this big bang that should permeate the universe. Scientists in the 40s and 50s calculated that the temperature of such a background radiation should be about 2.7 degrees above absolute zero. Sure enough, in the 60's it was discovered at 2.7 degrees, a static radio signal that permeated space whereever we looked. In the early 90's this signal was mapped more carefully and sure enough there were anamolies - enough to help explain the formation of galaxies long enough.What's really amazing is not so much this cool stuff, but how science predicted that a lot of this would show up long before it did. Science, IMHO, has done a far better job than prophecies of one kind or another.SLD

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Sld: Scientists in the 40s and 50s calculated that the temperature of such a background radiation should be about 2.7 degrees above absolute zero. Sure enough, in the 60's it was discovered at 2.7 degrees, a static radio signal that permeated space whereever we looked.

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/*DNAunion*/ That prediction's a bit too good to be true. The actual predicted temperature was 5 kelvin, not 2.7K.

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"In 1948, George Gamow and his colleagues Ralph Alpher and Robert Herman were attracted to the throught that all elements heavier than hydrogen are synthesized in the big bang. ... That year Alpher and Herman wrote, "The temperature in the universe at the present time is found to be about 5K". This predicted result is remarkably close to the value discovered by Penzias and Wilson in 1965." (Edward Harrison, Cosmology: The Science of the Universe: 2nd Edition, Cambridge University Press, 2000, p429)

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[This message has been edited by DNAunion, 11-01-2003]

(Do not bash me, I am merely an 8th-grader.)The universe at first was a single point, then in that point a huge explosion of energy caused the Big Bang. This explosion caused the universe to spread outward and it's effects are still occuring today. This causes the universe to expand. Evidence was found by many a scientist. One clue of evidence is the red-shift which occurs when light is moving away from a point and from an observer. It was discovered that light from galaxies all over were moving away from each other which means they were moving away from each other. This seemed to confirm the theory of the universe's expansion which was theorized by Edwin Hubble and prove by him. In 1948, George Gamow said that if the Big Bang had occured, then background radiation would remain and it would be a just a few degrees above absoulute zero which is about -450 F. This was confirmed by Arno Penzias and Robert Wilson when they found background radiation when searching the skies with radio astronomy. They came to realise the truth and they measured the temperature of the readiation and it was very close to Gamow's original estimate. That pretty much gives you a good idea on the evidence of the expansion of the universe.------------------
Signing Off,
ZeroFavorite Quotes:
Albert Einstein a genius?! Ha! I'll show you a real genius!

I don't understand how there is blue shift if the universe is expanding. Should everything be red shifted since it's all moving away?

There are local galaxies that are bound gravitationaly to us. All the galaxies have real motion through space as well as the "motion" of being carried along with the expansion of space. If they are close enough the real motion can be greater than that due to expansion and they may be approaching us. This produces a very few very, very nearby galaxies with a blue shift.Added by edit:
A rough analogy is an airport walkway (the horizontal beltways "people movers") If you are on the north bound one and all the people on the south bound one are south of you they are all being carried away by the belt. But if one person starts to walk/run north on the south bound belt they might be able to approach (catch up) to you. They would then be "blue shifted".Now if you want to improve the analogy you have to stop both belts. Then you start to stretch them. This moves people in the same way that moving the belt as a unit does too. However, it means that if everyone is on your belt they will all be moving away from you if they are standing "still" on the belt. If the belt streches evenly the ones farther away from you will be moving faster away from you than those close to you. Now let eveyone start to walk at random speeds up or down the belt while it steaches. At some distance far enough away from you the streching of the belt between you and that far person will be carrying them away from you faster than they can possibly run. They (and all further away) will be red shifted.However, there may be a few people close to you that happen to be walking/running fast enough toward you to overcome the streching and they will be blue shifted. Some of the close one will still be moving away because they either aren't moving fast enough toward you on the belt or they are randomly walking on the belt away form you.Did I get that clear enough? The belt is no longer "moving" in the way we see them in the airport. It is now streching instead. If you stand still on the belt you still end up "moving" away from other people standing still on the belt. In fact, everyone standing still on the belt is moving away from everyone else standing still on the belt. Of course, this will mean the belt is moving relative to the airport building which isn't what is happening with the universe.[This message has been edited by NosyNed, 11-11-2003]

In fact, if I recall correctly, the Milky Way galaxy is about to undergo collision with the Andromeda galaxy. Thus, the Andromeda galaxy would appear blue-shifted since it is approaching.------------------
Rrhain
WWJD? JWRTFM!

LOL, "about" even a cosmologist or a geologist would have to strech to get "about" to work in this case. from: M31 Andromeda
The modern values for Galactic rotation and heliocentric radial velocity yield that the Andromeda Galaxy and the Milky Way are approaching each other at about 100 km/sec.At 2900 Klyr away this means the collision is, gee let me calculate, mmm, uh, one helluva long time away.In fact I think it won't get here before the sun goes nova. Hope not anyway. All the new star formation might be hard on us.

Yes, thanks! That was a good analogy. I forgot to consider the effects of gravity.

NosyNed responds to me:

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LOL, "about" even a cosmologist or a geologist would have to strech to get "about" to work in this case.

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You're thinking on a human scale, not a cosmological one. If I recall the nubmers correctly, it's about 3 billion years.That said, the Milky Way is in the process of absorbing another dwarf galaxy (named Canis Major after the constellation it is in) even as we speak. Milky Way has a new nearest neighbourCanis Major is only about 42,000 light years from galactic center, which is closer than the Sagittarius dwarf galaxy, which the Milky Way is also absorbing.------------------
Rrhain
WWJD? JWRTFM!

As you know I am the n00b on this subject, so here is one I cannot get my head around. What was there before the Big Bang, was it just gases? if so how did the gases come about?Cheers people.Jon