A question about light and a question about expansion for the savvy astrophysisist

Posted by everflow777

on Fri, May 1 2009 7:08 AM

Hi,

I'm am amature astonomer and familiar with basic astrophysics and some of the science and outcomes based on the fact that light travels at a constant speed. There are however, a couple things I can't seem to understand. (Ok there's more than a couple but these two I thave been bugging me for a while now)

The most distant galaxies we can see are around 13 billion light years away. It is understood that the light from these galaxies has been travelling for 13 billion years or so to get to us. My first question is how can light from galaxies have been travelling for 13 billion years since 13 billion years ago, that light source was right next to us because the universe had not expanded yet...phrased another way, the source of this light from the object NOW 13 billion ly away left 13 b ly ago. However at that time, that object was not 13 b ly away...it was right next door! What's really going on here?

Now, I'm sure many of you have heard of the analogy that the universe expanding is like a ballon that expands. It's not that galaxies are moving away from us but that the space itself is expanding. I have a feeling that the answer has something to do with this. It brings up my second question though: Like dots on a balloon, does the matter of the universe also expand as space expands? Is everything also getting bigger?

If anyone has the answers to these questions, I would surely appreciate it.

Thanks

Posted by smann

on Fri, May 1 2009 11:31 AM

I'm clearly not an expert, but as far as I know: - Matter is not also expanding. - If we measure that a distant galaxy is 13 billion LY away, that does not mean that the light has been traveling for that long. There is a good chart in this article that shows how this can occur (http://en.wikipedia.org/wiki/Expansion_of_the_universe). Look for the two blue charts on the right, talking about "isometric embedding". Basically, as the universe expands, it takes cosmic objects with it, and their distances can be greater than the distance light could have traveled while the universe was extant. Like I said, I am no expert, so reading that article hopefully explains it better than I can.

Sky-Watcher 8" Collapsible Dobsonian Telrad

Posted by DaveMitsky

on Fri, May 1 2009 12:17 PM

You should also have a look at the Wikipedia entry on the observable universe.

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

The term look-back time is defined as the time it took for light to reach us from a distant object. Due to cosmological expansion, the actual distance to said object is always greater than merely multiplying the look-back time by c, the speed of light.

http://home.att.net/~numericana/answer/cosmos.htm#lookback

There's a light travel time to redshift calculator at http://www.astro.ucla.edu/~wright/DlttCalc.html

Dave Mitsky

Chance favors the prepared mind.

Fere libenter homines id quod volunt credunt.

A man is a small thing, and the night is very large and full of wonders.

Posted by Callistonian

on Sat, May 2 2009 6:08 PM

You also said after the Big Bang there was no expansion, that is false and may be hindering your understanding. In a microsecond blast, everything, everything started outward and has since been accelerating exponentially.

Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying. Arthur C. Clark

Posted by everflow777

on Sun, May 3 2009 10:04 AM

Thanks to those who posted the links. I checked them out and they have helped a lot in understanding what's going on. I figured something like this must be happening but had never heard it discussed.It's going to take a while to absorb it but at least the essence is clear to me now....THANKS

To Callistonian: I don't know what you're talking about.

:-)