Stupid physics question #29

[Wavicle]

Okay, I apologize in advance for asking what must be a rather obvious question to everyone out there, but...

Suppose that the age of the universe is 14BY and the most distant object we can see is 13B lightyears away (I picked these because they seem to be close to the actual values while still being round numbers). This means we're looking at an object as it was when the universe were only 1B years.

But it seems like the most distant we could have been from the object at that time is just under 2B lightyears. So how is it that it took 11B years for the light to reach us?

The only explanation I can think of is that the expansion rate of the universe is really high so it really did take 11B years for the light to reach us. I can't think of a reason for the expansion speed to not be this except that it just "sounds" obscenely fast. I guess the expansion speed would show up in the red shift, and I know we judge how far something is by how red shifted it is. Is the red-shift in this case high enough to suggest a speed which seems like it'd be in the upper quartile of c?

Can someone fill in this missing piece here, or did I essentially answer my own question?

 

[drkitten]

But it seems like the most distant we could have been from the object at that time is just under 2B lightyears. So how is it that it took 11B years for the light to reach us?

Because space itself is expanding,

Is the red-shift in this case high enough to suggest a speed which seems like it'd be in the upper quartile of c?

I'm not quite sure what this means. But there are, in fact, objects -- quasars, mostly -- that have a strong enough red shift that their apparent velocity (as seen from earth) is greater than that of light. But that's, as I said, because space itself is expanding. Use the standard "inflating balloon" model. Two stationary dots on a balloon will move further apart from each other, not from any intrinsic velocity, but because the balloon itself is "adding space" in between the dots. If the universe adds space fast enough, the apparent velocity will be arbitrarily fast.

 

[Yllanes]

A more detailed answer (and many additional interesting questions) can be found in Ned Wright's excellent Cosmology FAQ.

 

[Wavicle]

A more detailed answer (and many additional interesting questions) can be found in Ned Wright's excellent Cosmology FAQ.

Ahhh, interesting site, thanks for the link.

So if I'm reading that right, my mistake was in my understanding of red shift of distant objects. The majority of the doppler shift of the light of emitted by distant objects is due to stretching of space along its path, causing the wave to get "pulled" longer. There can still be some redshift caused by velocity away from us, but I imagine over large distances this is completely overwhelmed by the cosmological redshift. Blueshift would always be caused by an object moving towards us faster than the universe is pulling everything apart.

Am I getting warmer, or am I still missing it?

 

[Yllanes]

Am I getting warmer, or am I still missing it?

No, you are pretty close, as you see it was not such a stupid question after all. If you want to get a more solid understanding you can read the cosmology tutorial in the site I linked.

It's important to realise that a redshift is not always caused by a Doppler effect.