A problem with expansion...help please

[thedeester1]

Ok some time ago someone explained the expansion of the universe to me...The expalation was that we can time the light from stars. the further away the stars are the faster they are moving away. I thought about this the other day. The further the stars are away then the older the light is. If the nearer stars light is reaching us faster then surely thats an example of slowing expansion. Ok say we look at the light from a say a far away star and its reaching us at a rate of 2...then we look at a closer star and its light is reaching us at a rate of 1 then surely the furthest star was accelerating when it emmited its light but we need to gauge our measurements on the nearest star. If the nearest stars light isnt reaching us as late as the farthest star then surely expansion is slowing?

[Nabeshin]

The motivation behind the expanding universe concept is the doppler shift, not differential timing of light signals or anything like that.

http://en.wikipedia.org/wiki/Doppler_effect

The accelerated universe comes about when you make a very careful hubble plot, you see deviations from linearity, suggesting that the expansion is not constant.

[thedeester1]

Since light is the fastest thing then thats gotta be our standard. I understand the doppler shift but how is it faster than light....Were measuring things that happened billions of years ago....After all light is the key....or is it gravity

How do we measure the doppler shift please....Do we have Gamma ray telescopes? I was just wondering cause thats the only way I can see expansion been measured....And thats gonna take some work...

[Nabeshin]

So we know, for example, that hydrogen emits light at a very precise wavelength, 656.28 nm, to be exact. The emission feature is pretty easy to recognize when you look at a spectra. So when you see the same line, but now at 600nm, you deduce that the object is moving towards you with a certain velocity (very fast, in this example!). But when you see the line at 800nm, the object is moving away. Note that these light ranges are in the red to infrared, so we certainly do not need a gamma ray telescope! The same principle could in theory be applied to ANY significant emission line, be it in the IR, UV, x-ray, or gamma ray. The reason H-alpha (the line mentioned above) is used is because it is so abundant in galaxies that even in very distant, faint galaxies it is easily recognizable.

So I'm not sure what this "faster than light" business you're getting at is. It's as simple as doing the above for a large large array of galaxies and plotting it. Where is the confusion?

[thedeester1]

Ok thank you for the help...I have further problems if you would assist. Ok I get that there are many spectrums of light now. From visible through infa, Ultra and then X-ray and Gamma. What I read though is that the speed of light is constant through a vaccuum and is slowed through mass. For example a Diamond will slow light more than a gas. I realise now that all light travels at the same speed hence no need for gamma telescopes. What I dont get is the speed of light hitting our telescopes. If it takes so long to get here from the farthest stars then maybe the light were measuring was created long before we measured it. My main problem however is the number of gas clouds it has to pass through. If were measuring the very distant light its gotta pass through a load of gas clouds. Each of which is conspiring to make stars...Although very young any of the stars in the path of the light will have mass. The more mass they have the more they slow the light. It would be possible that we can measure expansion where there is contraction due to the increased activity of the universe. As generations of stars move on?