PDA

View Full Version : Did the lights just go on? (CMB vs.size of Universe question)



Tog
2011-Oct-12, 12:42 PM
I've been having a really hard time reconciling the idea that we're not in the center of the universe with the idea that it looks the same in every direction, and that the Cosmic Microwave Background is the same distance everywhere. The other day, I think I finally got it, but I want to see if this is really the case.

First the things I think I know:

The universe is 13.7 billion years old.
Because of expansion, the actual diameter is much bigger than 13.7 billion light-years.
Because of the speed of light, we can only see back 13.7 billion light-years (max). Anything further than that and the light will not have had time to reach us.
The CMB is is the border between the clear universe we now have and the opaque one we had until 400,000 years ago.

If I have those right, then is this correct?

Since we can only see 13.7 BLY from where we are, the CMB does not represent the edge of the universe (or 400 KLY short of it), but rather it's like a small bubble suspended in a larger bubble. The reason the CMB will look the same to everyone no matter where they are is that their smaller bubble of visibility is centered on them wherever they happen to be.

Did the light really go on for me?

Grey
2011-Oct-12, 01:32 PM
Did the light really go on for me?I think you've pretty much got it. Point 3 is a little iffy, though. The CMB that we observe has been travelling for very close to 13.7 billion years to get to us. However, those photons did not start out 13.7 billion light years from us (or where we would eventually be). They started that trip much, much closer, but because of expansion, it's taken them a long time to get here. Think of running on a treadmill going the wrong way.

BigDon
2011-Oct-12, 04:20 PM
Your point four, shouldn't that read as the Universe was opaque for the first 400,000 years?

Tog
2011-Oct-13, 08:00 AM
I think you've pretty much got it. Point 3 is a little iffy, though. The CMB that we observe has been travelling for very close to 13.7 billion years to get to us. However, those photons did not start out 13.7 billion light years from us (or where we would eventually be). They started that trip much, much closer, but because of expansion, it's taken them a long time to get here. Think of running on a treadmill going the wrong way.
Ahh good point. That makes sense. Thanks.


Your point four, shouldn't that read as the Universe was opaque for the first 400,000 years?
I'm blaming that on that on the fact that I was typing on the work computer instead of mine. The keyboard doesn't feel right.
Nice catch.

Strange
2011-Oct-13, 08:43 AM
I keep coming back to point 4. The word "border" makes it sound like a location in space, whereas it is really a location in time. Would "transition" be a better word?

astromark
2011-Oct-13, 09:03 AM
Yes TOG you can rest easy.. with only a little adjustment and a tweak here and...and YES.

You are with the illuminated.. Unfortunately so many people do never want for that understanding..

Its pleasing to see that as interested in astronomy as we are. We can want for such understanding.

Now to go down the road to the local school and bang the table a bit.... :)

Grey
2011-Oct-13, 12:34 PM
I keep coming back to point 4. The word "border" makes it sound like a location in space, whereas it is really a location in time. Would "transition" be a better word?I found a very nice analogy in a lecture by Charles Lineweaver: the surface of last screaming (http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html). Not only does it provide an excellent way to visualize what's going on, it's somehow very entertaining to imagine.

Strange
2011-Oct-13, 01:18 PM
I found a very nice analogy in a lecture by Charles Lineweaver: the surface of last screaming (http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html). Not only does it provide an excellent way to visualize what's going on, it's somehow very entertaining to imagine.

Nice. I had heard of that before but not seen the original source.

Border seems like a fine word.

chornedsnorkack
2011-Oct-14, 01:26 PM
Remember that the relic radiation does have temperature fluctuations, apart from the dipole caused by peculiar motion of Sun and Milky Way.

Are the hot spots in relic radiation closer than 13,7 milliards years and cold spots further, or vice versa?

Grey
2011-Oct-14, 03:47 PM
Are the hot spots in relic radiation closer than 13,7 milliards years and cold spots further, or vice versa?Neither, I think. They just represent the earliest fluctuations in density that we can observe. The hot spots were a little denser, and so warmer, and the cool spots were a little less dense. But they happened at the same time.

chornedsnorkack
2011-Oct-14, 08:27 PM
So the hot spots were the denser?

If we watch a specific hot spot of relic radiation for some time, like a few hundred million years, should we see it evolve into a new galactic supercluster?

Cougar
2011-Oct-15, 03:30 AM
If we watch a specific hot spot of relic radiation for some time, like a few hundred million years, should we see it evolve into a new galactic supercluster?

I've waited awhile to see how someone more knowledgeable than me might answer this. My patience has worn thin. :razz:

Just where are the material correspondences, in their many evolutionary configurations, to the minute background overdensities? The very early universe was one of many collisions and interactions. So I imagine there is a lot of mass moving around and ending up somewhere else.

But Mr. Snorkack's hypothetical would seem to have time going in the wrong direction. :think: "Watching a specific hot spot of relic radiation for some time" is just going to see that hot spot recede, with weakening signal, according to the surface of last screaming. :whistle:

But of course, time goes on everywhere in the universe. That CMB hot spot has evolved through time just as our hot spot did. In fact, residents of that hot spot are just now viewing our region of space as relic radiation, as we view theirs.

Tobin Dax
2011-Oct-15, 04:17 AM
I agree with you, Cougar, FWIW. To actually watch a (current) hot spot evolve into a supercluster would take a billion years, give or take. But it wouldn't show up in the CMB (or CSB?) because the source of that radiation is continuously increasing in distance. We'd have to watch the point in space where the hot spot is now.

Tog, congrats on the new lightbulb. :) I'm not sure that looking at #3 and #4 as separate points is useful. We are stuck at the center of our bubble of visiblilty no matter which one it is (or even if it's both).

The "Surface of Last Screaming" analogy shows that well, too. I'll have to remember that one.