Red Shift.

[peter]

Can I ask a few questions about the nature of red shift in cosmology and what it might tell us. As far as I get it, this idea of an expanding universe is based upon the red shift observed when examining the light from any and every galaxy in the visible universe, explainable only by the conclusion that they are all moving away from each other, like dots on the surface of an expanding balloon, except in 3d.further to this, I believe that it is not that the galaxies themselves that are moving apart through space, but rather the space itself beween them that is expanding. So far so good.

Now, if this is essentially correct (ie my understanding) can anyone tell me what can be determined from this. Can, for example, minute changes of the extent of this red shift over time, be used to tell us if the universe is expanding at an accelerating rate, a constant one or even a decelerating rate. Does it give us a value for the rate of expansion at any time or is it all just relative. If the red shift value was falling over time this would imply a decelerating rate of expansion of the universe, and could we extrapolate this to a conclusion that at some point the universe would definitely go into a collapsing phase (accompanied by an observable blue shift as this gained pace). How much of this stuff is known as we speak?

[Hashi Lebwohl]

Here is the main article on redshift, which also includes sections on expansion and estimates of the age of the universe, as well as explanations as to how galaxies may appear to be moving away from us at faster-than-light velocities.

[Zarathustra]

The expansion of the universe is accelerating, due to "dark energy," I believe. I don't think we learned this from the red shift, which we've known about since Hubble (the scientist, not the telescope), whereas the acceleration is a recent discovery.

[Fist and Faith]

I've never understood how we know there is redshift. If we see something that is not moving, relative to us, we see its color. Then, if it moves away from us, we can see the redshift. How do we know a galaxy is moving away from us, rather than it is just sitting still and is the color it is?

[Vraith]

I've never understood how we know there is redshift. If we see something that is not moving, relative to us, we see its color. Then, if it moves away from us, we can see the redshift. How do we know a galaxy is moving away from us, rather than it is just sitting still and is the color it is?

Been a long time since I looked at that, but part of figuring it out is spectral emissions.
For instance, almost all stars are mostly hydrogen. Hydrogen can only release energy at specific wavelengths. But when we examine the wavelengths that strike our instruments, they're redder or bluer.

[wayfriend]

Exactly. We have some notion of what a "normal" galaxy looks like, as it's spectrum is determined by the most common kinds of atoms found in stars.

When we see a spectrum that's similar but shifted a bit, we know it's due to red shift or blue shift.

Of course, it could actually be caused by stars which contain an unusual and implausible combination of atoms which coincidentally produce a similar but shifted spectrum. But we consider this less likely.

[Fist and Faith]

Ah. I guess that makes sense.