Where did light from 15 billion light years away start...?

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Where did light from 15 billion light years away start...?

Post #1by BlindedByTheLight » 24.10.2005, 03:01

Got a question I've always wondered... the universe is roughly 15 billion years old...

...and light that reaches us from 15 billion light years away is about the farthest we can see, right? (because light that takes any longer to reach us wouldn't have had time to reach us yet, right?)...

...and, if you buy the Bang Bang theory, the universe has been expanding from an extremely dense "explosion" 15 billion years ago...

(can anyone guess where I'm going? I'm sure you can)...

So, by my reckoning, that light beam that took 15 billion years to reach us... must have been right next to us when it first started it's journey 15 billion years ago and, thus, should have only taken... a split second to reach us, not fifteen billion years. Seems like a paradox of sorts.

Okay... what's my flaw?
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Post #2by Malenfant » 24.10.2005, 03:15

Well if we saw light that was from 15 billion light years away, we'd have to revise how old the universe is because currently we think it's 13.7 billion years old :).

I'm kinda curious as to the answer here though. As well as to this question - what happens if we manage to see something that is 13.7 billion lightyears away? If we look back far enough, shouldn't we actually see the big bang itself?

I suspect this is all answered somehow by Inflation theory, but I don't know how...

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Post #3by julesstoop » 24.10.2005, 06:18

For one...
There was only light after the universe cooled doen to about 3000 K, before that the plasma was too hot to let light pass: it couldn't travel farther than a very short distance.
After a while (100.000 yrs?) the universe cooled down enough. You could represent this moment in time as a wall that's surrounding us presently at a great distance. We can still 'see' it in the form of 2,7 K background radiation.

I realise this isn't a full answer, but I hope it's helpful nevertheless.
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Post #4by BlindedByTheLight » 24.10.2005, 07:48

julesstoop wrote:After a while (100.000 yrs?) the universe cooled down enough. You could represent this moment in time as a wall that's surrounding us presently at a great distance.


Thank you for the reply (and the interesting information) but that doesn't actually answer my question. Barring inflation, after 100,000 years, the furthest light would be travelling would be 100,000 light years - which would then take roughly 100,000 years to reach my eyes. But, as noted, I have been told that some of the light originating during that time took over 13 billion years to reach my eyes.

Of course, I'm sure someone will correct some specific mis-assumptions I've made in that last paragraph... but, to anyone who replies, please do not lose sight of my initial question. In other words, posts like this often get RAPIDLY filled up with debates about side points and the like and the initial question never gets answered.

So I'd be in your debt if any corrections also came with info on the initial point... :)

Thanks
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Post #5by Hamiltonian » 24.10.2005, 09:08

There's more to the Universe than the observable Universe. So there was material in the early Universe that was far enough away for its light to take 13 billion years to reach us across the expanding Universe, so that we are seeing it for the first time now.
And it's at present farther away than 13 billion light years, because space has expanded around the travelling photons. The photons crossed all of the intervening space when it was smaller than it is now.
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Re: Where did light from 15 billion light years away start..

Post #6by neo albireo » 24.10.2005, 10:21

BlindedByTheLight wrote:So, by my reckoning, that light beam that took 15 billion years to reach us... must have been right next to us when it first started it's journey 15 billion years ago...


After those 100'000 (or was it 300'000) years or so it took the universe to cool down a bit, a photon could move freely in the universe, but as the universe expanded incredibly fast, while the photon tried to reach us, the distance grew bigger and bigger, making it more and more difficult for the photon to reach us.
I'm not an astronomer, I'm sure there will be better answers coming.
Just one more thought: I think that question is linked to the question "why is the night dark?"

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Post #7by buggs_moran » 24.10.2005, 15:05

Ok. Your light particle left it's host 13.7 billion years ago. However, that host and everything else continued to move apart with the expansion of the universe for the past 13.7 billion years. So now the host is 78 billion light years away (from us anyway, not that we are the center of the universe or anything), giving the diameter of the whole universe around 156 BLY across. Big, huh? But, then diameter implies a center, of which there is none, sooo....ow my head. We can only see for as long (in time) as there has been light, which is 13.7 billion years. So we are limited to a shell of light that old, even though the universe is vastly, if not infinitely larger than what we see...

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Post #8by Hamiltonian » 24.10.2005, 15:33

Well, the material that now appears at the edge of the observable universe would have been closer than 13.7 billion light years to start with. Remember that the intervening space was steadily expanding. The photon travelling towards us would be like a caterpillar moving along an elastic rope: the distance ahead would keep stretching.
Infact, when the photon set out, it would be travelling though space that was expanding away from us FTL (no violation of special relativity. the emitting object is at rest in space. space expands.) But the rate of expansion decreased, and the distance we can see (Hubble distance) increased, and eventually the photon got inside the Hubble distance and started propagating in our direction.
Today, when the photon arrives, the emitting object is about 41 billion light years away, which is the limit of the observable universe.

(I don't understand from your diagram, buggsmoran, where you get your figures.)
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Post #9by Buzz » 24.10.2005, 18:51

Have a look at this: http://www.astro.ucla.edu/~wright/cosmolog.htm
What I understand from this is that velocity is measured as units of distance per second, and these units themselves expand. The number of "space units" per second that a photon travels remains the same, but the distance covered per second is growing!

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Post #10by buggs_moran » 24.10.2005, 18:53

I understand that space was expanding at the same time as light was traversing it. Sorry I do not have calculations. As usual, as a teacher, I have a tendancy to regurgitate many times since time is limited (heh...). Since my background isn't astrophysics I will quote the following article which I based my information on. I had read it some time ago and obviously paraphrased it unsuccessfully... :D


http://www.space.com/scienceastronomy/mystery_monday_040524.html
Robert Roy Brit and Neil Cornish wrote: Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."

All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger."
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Post #11by Tanketai » 24.10.2005, 18:58

Hamiltonian wrote:Infact, when the photon set out, it would be travelling though space that was expanding away from us FTL (no violation of special relativity.


Well, that doesn't sound like FTL at all. The speed the photon is travelling will always be lightspeed, so it couldn't travel FTL. We ARE talking about a time when the the Universe was rapidly changing; the medium in which light travels (space itself) was changing in density (structure?), so the speed of light has changed considerably from then.
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Post #12by Hamiltonian » 24.10.2005, 21:34

Tanketai wrote:
Hamiltonian wrote:Infact, when the photon set out, it would be travelling though space that was expanding away from us FTL (no violation of special relativity.

Well, that doesn't sound like FTL at all. The speed the photon is travelling will always be lightspeed, so it couldn't travel FTL.
If you look at the bit you've quoted again you'll see I wrote that the photon was passing through "space that was expanding away from us" faster than light. The photon moves at lightspeed towards us. But the expansion of space beyond the Hubble distance carries stuff away from us faster than light. So the photon makes no headway towards us.
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Post #13by BlindedByTheLight » 24.10.2005, 21:41

Hamiltonian wrote:There's more to the Universe than the observable Universe. So there was material in the early Universe that was far enough away for its light to take 13 billion years to reach us across the expanding Universe, so that we are seeing it for the first time now.
And it's at present farther away than 13 billion light years, because space has expanded around the travelling photons. The photons crossed all of the intervening space when it was smaller than it is now.

Thanks Hamiltonian... I'm going to quote yours since it seems to be the most complete answer... however, your answer still brings me back to my original question. You wrote:

So there was material in the early Universe that was far enough away for its light to take 13 billion years to reach us across the expanding Universe


When you say that, 13 billion years ago, this material was "far enough away" to take "13 billion years to reach us" how far away do you mean? Because I was under the impression that, 13 billion years ago, that material had to be - essentially - right next to us.
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Post #14by Hamiltonian » 24.10.2005, 21:49

buggs_moran wrote:Since my background isn't astrophysics I will quote the following article which I based my information on. I had read it some time ago and obviously paraphrased it unsuccessfully...
Ah, okay. I was going mad trying to work out from the diagram where the numbers came from. It seemed like I was missing something.

There's a conflict between your source and mine: Scientific American, March 2005, Lineweaver and Davis' article "Misconceptions About The Big Bang".
If space were not expanding, the most distant object we could see would now be 14 billion light-years from us, the distance light could have travelled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light years.
(I remembered the factor of three, which is where my 41 billion came from.)

This could be the largest ever conflict between data sources ever reported since the universe began.
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Post #15by BlindedByTheLight » 24.10.2005, 22:07

Hamiltonian wrote:There's a conflict between your source and mine: Scientific American, March 2005, Lineweaver and Davis' article "Misconceptions About The Big Bang".


LOL... then perhaps it's time for a working astrophysicst to chime in? :)
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Post #16by Buzz » 24.10.2005, 22:23

I thought the link I posted was from Lineweaver too, but it wasn't. So here is another one: http://arxiv.org/find/grp_physics/1/AND+au:+lineweaver+ti:+inflation/0/1/0/all/0/1
You are right that the distance of the photon to the location we are now was very small, but it is not a contradiction. It's hard to explain...
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Post #17by Hamiltonian » 24.10.2005, 22:24

BlindedByTheLight wrote:When you say that, 13 billion years ago, this material was "far enough away" to take "13 billion years to reach us" how far away do you mean? Because I was under the impression that, 13 billion years ago, that material had to be - essentially - right next to us.
Well, at 10e-32 seconds after the Big Bang (so, just after the Inflation Era), the universe observable today was a few cm radius. So the "edge" was out of lightspeed contact with the "centre".

Before inflation, the stuff that currently makes up the whole observable universe was within light-travel time of itself, but inflation then shoved most of that stuff beyond the prevailing Hubble distance. Its only now coming back into lightspeed contact with our part of the universe. So I shouldn't have said "seeing it for the first time now" in my original post. I wasnt thinking of the pre-inflation universe as a place where "seeing" happened.

So the answer to your question is "a few centimetres away" if your talking immediately post-inflation. "Right next door and in contact" if your talking pre-inflation.

BlindedByTheLight wrote:I'm going to quote yours since it seems to be the most complete answer
Please don't quote the distance to edge of the currently observable universe today, unless we can get that figure cleared up.
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Post #18by BlindedByTheLight » 24.10.2005, 22:36

Hamilton... are you, then, essentially saying that the light that is just now reaching us after 13.7 billion years was a few centimenters away ROUGLY at the the start of its journey but that, because of expanding space - which was expanding almost as fast as the light was moving - it took 13.7 billion years to reach us?

Or, using actual numbers, the light was moving at the speed of c but space was expanding at, say, 0.9999999999999 (+ some more 9's) so it took so long for light to reach us?

If that's the case, though, it seems as if the light would have been kinda "hovering" just a few centimeters away - like the catepillar on a moving walkway... plugging away toward us at c while space moved away below it.

(Also, inflation... not sure exactly what that is... other than some kind of rapid expansion - but don't know any details)
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Post #19by Hamiltonian » 24.10.2005, 23:00

BlindedByTheLight wrote:Hamilton... are you, then, essentially saying that the light that is just now reaching us after 13.7 billion years was a few centimenters away ROUGLY at the the start of its journey but that, because of expanding space - which was expanding almost as fast as the light was moving - it took 13.7 billion years to reach us?
Yes.

BlindedByTheLight wrote:Or, using actual numbers, the light was moving at the speed of c but space was expanding at, say, 0.9999999999999 (+ some more 9's) so it took so long for light to reach us?
Worse. For much of the time this stuff was moving away from us faster than the speed of light. That is, the space with the photons in it was moving away from us faster than the photons were moving towards us.

BlindedByTheLight wrote:If that's the case, though, it seems as if the light would have been kinda "hovering" just a few centimeters away - like the catepillar on a moving walkway... plugging away toward us at c while space moved away below it.
Its the concept that stuff so close together could be moving apart so quickly that makes this mind-boggling, I agree.

BlindedByTheLight wrote:(Also, inflation... not sure exactly what that is... other than some kind of rapid expansion - but don't know any details)
Hah. Now you really really really need that cosmologist.
A very very rapid expansion of the early universe (much faster than the speed of light), at around 10e-35s after the big bang. First theorised by Alan Guth in 1980. Fixes a number of apparent conflicts between theory and the appearance of the observable universe. More recent observations are making things a bit complicated for inflation to explain. Opinion seems to vary on whether inflation theory is failing or just needs a bit of a rewrite.
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Post #20by Hamiltonian » 24.10.2005, 23:09

Important revision. But of course, there are no photons arriving from the few-cm universe 13.7 billion years ago. The first photons to actually get out and about in the universe were formed at the 100,000 year mark, when radiation decoupled and the cosmic microwave background was formed.
So if we're talking about the few-cm universe, for "photons" in my last post you'd have to substitute some lightspeed influence that could propagate through the early universe. If such a thing existed, then it would have the same problem reaching us that the "photons" in my post had.
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