Faster velocity than the speed of light?

sydfremmer

A related topic that I have a difficulty understand is the expansion of the universy. I read that the universe is expanding such that some galaxies move away from each other faster than the speed of light. If this is the case, does it not mean that some galaxies are moving away from us faster than c?

Naty1

It is difficult to understand. Our everyday notion of distance doesn’t work on cosmological scales, so everyday intuition is useless.
Yes, that is possible in curved spacetime. But it is a different concept than in your original post which is in flat space-time. At the Hubble radius, the recession velocity is c; beyond the Hubble radius galaxies move away from us at greater than c. [We also move away from such observers at greater than c.]

The 'distance' measure used in FRW cosmology model of our universe is most commonly the proper distance, is based on the FRW metric with a rate of change as measured by co-moving observers; this velocity exceeds c for sufficiently large distances.

All this ‘superluminal’ velocity at great distances tells us is how one of many different possible definitions of distance changes; Other metrics that use different co-ordinates may not contain any apparent superluminal recession.

For more, checkout the Balloon Analogy by pHinds from these forums: http://www.phinds.com/balloonanalogy/

phinds

Not in the sense you mean. They are RECEEDING from us at FTL but that is not the same as MOVING. Seems weird, I know. Google "metric expansion". Nothing is moving, it's just that the distance is increasing because space is expanding.

Fredrik

It does. Apparently most of the galaxies that can be seen from Earth are "moving" away from us faster than the speed of light (in the sense that the derivative of the distance with respect to time is >c). But this is a result of expansion of space, and doesn't have a lot to do with motion through space. It's still impossible for a massive particle near such a galaxy to move faster than c relative to that galaxy.

Edit: I agree that "moving" is a misleading word, so I added the quotes in the first sentence, and a clarification.

Naty1

Here is a great paper, but rather long and detailed:

Expanding Confusion:
common misconceptions of cosmological horizons
and the superluminal expansion of the universe
Tamara M. Davis, Charles H. Lineweaver
http://arxiv.org/pdf/astro-ph/0310808.pdf

There is an abbreviated version that used to be available in Scientific American, but I haven't seen it lately. [edit: I found my link to UCLA but it not longer works.]

An alternative: Ned Wright....
http://www.astro.ucla.edu/~wright/co...y_faq.html#FTL

sydfremmer

Thanks again :)

Now I can sleep

Fredrik

Marcus has a working link in his signature. http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf

Naty1

Alas, that link no longer seems to work either....
I double checked Marcus signature and that link IS the one he uses....

The abstract is here:

http://www.scientificamerican.com/ar...ut-the-2005-03

at Scientific American the article now costs $7.95 unless one has a subscription.

Fredrik

That link is working fine for me. I tried it before I posted it, and I just checked it again.

Naty1

Curious...I keep getting an error message #404?? via a google chrome browser.....at least six attempts now....
ah, the mystery of 'the cloud'....

"That link is working fine for me. "
now that's just plain 'showing off' [LOL].....

Fredrik

What the... NOW I'm getting a 404 error as well. Maybe they reorganized some time after I posted the link last night, and my browser just got the article from the cache when I tried today. Ah, now the web site of that entire university appears to be down. They must be making some major changes right now. Maybe the link will work again when they're done, and if not, maybe we can figure out its new URL.

phinds

This is a physics forum. Let's leave the supernatural out of it. Physical laws do just fine.

Fredrik

Uhm...did you copy and paste the abbreviated text for my link instead of just clicking on it? The link in my post is working fine, but if I click the one in your post, I get the 404 error.

I linked to You linked to

FeynmanIsCool

haha I was waiting for that!