Author Topic: Our place in the universe  (Read 3007 times)

Offline DanP

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Our place in the universe
« on: March 28, 2013, 11:26:03 AM »
I've heard it said, we can only "see" perhaps only 3-4% of the actual universe--most of the universe lies outside of our ability to observe it.  With that in mind, we estimate the universe to be around 4.7 billion years old, yet we have been able to observe proto-galaxies a few hundreds of thousands years old.  How is that?  Wouldn't galaxies that young be far beyond our ability to see them since they would be at the boundary of the "real" universe?  Or, are we just talking about the 4.7 billion light year radius of the universe "bubble" we can actually see?

Thanks.
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Offline bn

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Re: Our place in the universe
« Reply #1 on: March 28, 2013, 12:50:53 PM »
the universe, canonically speaking, is infinitely large. So I'm not sure where the "we can see 4% of the actual universe" comes from... perhaps they mean "baryonic matter"... as in "matter which interacts with light". the percentage is about right.

so of the matter "in our past light cone"... or rather "which we should be able to see", only 4% ish interacts with light.

the universe is 14(ish) billion years old. so we should be able to see anything within 14billion light-years from us. of course, back in the days of re-combination (when the universe cooled enough to stop being plasma) there wasn't much to see.

ummm.
not a good answer, i'm afraid.
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Offline Random Number

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Re: Our place in the universe
« Reply #2 on: March 28, 2013, 02:35:56 PM »
...yet we have been able to observe proto-galaxies a few hundreds of thousands years old.

I am not a sure if this is actually the question you are asking but I will take a stab at explaining it.
Pardon me if I misunderstood.

Well the light took a few hundred thousands of years to get to us, the photons are like a snapshot into the past as they do not change while traveling to us(other than some redshift).

Think of way back when mail took a week or so to get delivered and phones did not exist.
You could never know how the weather was where the letter was sent from.
You would have to wait until you got a letter to get any information on it and it would always be a week behind.

If we scale this up just a small bit we have the proto galaxy sending us mail in the form of light. The distance is so massive the mail has been in transit for hundreds of thousands years. And the only way we can know about distant things is by looking at this old mail they are sending us.

We know the speed of the space mail delivery as the speed of light, so if we know the distance we know how long the mail has been in transit.
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Offline DanP

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Re: Our place in the universe
« Reply #3 on: March 28, 2013, 10:08:01 PM »
Thanks for the responses.  And OOPS!, I meant 14.7 billion years--missed the "one" there.  I picked up the 4-5% from another podcast (not yours, obviously).  But you both helped me figure this out (hopefully).

Using Random's analogy--I guess when the mail was sent, the person who sent it lived a lot closer--in the next neighborhood--but by the time the letter got to me, they moved and lived in another country--too far away to see anymore.  But, I got a lot of information in the letter, regardless.

Thank you!
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Offline Lynx Cat

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Re: Our place in the universe
« Reply #4 on: April 01, 2013, 11:33:06 AM »
the universe is 14(ish) billion years old. so we should be able to see anything within 14billion light-years from us.

Wait a minute - hasn't the Universe, at some point, expanded faster than the speed of light? So that light from certain places in the early universe will never reach us because we move away from them faster than their light moves towards us?
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Offline bn

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Re: Our place in the universe
« Reply #5 on: April 01, 2013, 04:24:21 PM »
parts of the universe are almost always moving faster than the speed of light away from us. only if the universe was finite, or un-moving, or collapsing would that not be the case.

the fun thing about cosmology is that the sphere around you within which you have any hope of communicating is dynamic. sometimes it expands, sometimes it shrinks.
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