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AonSao
2009-Jan-21, 06:22 AM
I am having a hard time wrapping my head around the idea of light as a time machine. For example, Sirius is 8.6 light years away from us. This means it takes 8.6 years for light to reach us. That makes sense. However, as you go back (wayyy) further in time it gets confusing. How are we able to see light (today) from close to the big bang, if that is where we originally came from? Wouldn't the light have traveled past us long ago (if we are moving slower than C) or vice-versa (if we are somehow moving faster than C)?

My brain hurts :doh: :)

Source: TV show The Universe. Season 3 - episode 3

astromark
2009-Jan-21, 09:19 AM
AonSao; This is not the last word on this... others I am sure will contribute more for you. I will however try to explain,. When you hear the phrase 'cosmic microwave background image of the big bang' That is not an image as such. Norr is it light just reaching us from then. What is it ? Its the energy released that has cooled to almost nil. By the smallest fraction measurable we can detect that some heat is still there. In this case its in the microwave length signal and is a remnant of the BB. or so they say:)and I think I can safely say its about 13.7 billion light years away... in any and all directions.

Source: The not so reliable corners of my mind.:)

Ken G
2009-Jan-21, 09:34 AM
How are we able to see light (today) from close to the big bang, if that is where we originally came from? Wouldn't the light have traveled past us long ago (if we are moving slower than C) or vice-versa (if we are somehow moving faster than C)?This is a very standard question, and it comes from a common misconception of how the Big Bang model works. Indeed, the very phrase "Big Bang" seems to support the idea that it was an explosion that happened at a point, and if it were, then you'd be right-- the light from it would be long gone by now.

Instead, it is best to think about the universe at the time of "recombination", about 400,000 years after the "beginning". Recombination is when the hydrogen atom cooled enough to remain in its neutral form most of the time-- and that means most electrons were bound in hydrogen atoms after that epoch. That in turn means that those electrons could only absorb light at special energies-- they did not interact with most light, including most of the prevailing visible light of that epoch (which would later cool to what we now call the CMB). The reason it is good to think of that time is twofold: 1) this is the time we are really seeing when we look at the CMB, because those are the prevailing conditions that this light "came from", and 2) even more to the point, the universe at this epoch was extremely large-- possibly even infinite (we have no idea). But it was certainly large enough that there was material emitting this light at a large enough distance away from us to just be reaching us now. Also, the similar light that was emitted where we are now is currently off being observed as the CMB for some distant alien astronomers.

If you are ever confused, a good model is to imagine that the universe is a rubber picnic cloth, laid out on the ground with ants crawling all over it in different directions. The ants are the particles of light that were released during that epoch of recombination, so they end up being the CMB. The picnic sheet is being stretched, but the ants just keep marching in straight lines at the speed an ant can march-- they are oblivious to the stretching of the sheet. That is exactly what is happening, in effect, to the CMB-- and the ants reaching any point on the sheet at some "present" time came from some distant place on the sheet at the epoch of recombination.

Tzarkoth
2009-Jan-21, 03:14 PM
Looking through the wiki I found, http://en.wikipedia.org/wiki/Observable_universe which states, "This means the universe has expanded to 1292 times the size it was when the CMBR photons were released."

What we can see looks like, http://space.mit.edu/home/tegmark/wmap.html.

So from what I can gather, at the time of recombination, the radiation from the CMBR that we are seeing today was some 36 million light years away from us when it was emitted and took some 13.7 billion years to get here.

dhd40
2009-Jan-25, 07:45 PM
... Instead, it is best to think about the universe at the time of "recombination", about 400,000 years after the "beginning". Recombination is when the hydrogen atom cooled enough to remain in its neutral form most of the time-- and that means most electrons were bound in hydrogen atoms after that epoch. That in turn means that those electrons could only absorb light at special energies-- they did not interact with most light, including most of the prevailing visible light of that epoch ...(my bold)

What is the meaning of "the prevailing light of that epoch"?
If there were only hydrogen atoms, then all the light "available" must have come from hydrogen atom emissions. Therefore the hydrogen atoms could have interacted with all the light of that epoch, couldnīt they?

ngc3314
2009-Jan-25, 08:03 PM
What is the meaning of "the prevailing light of that epoch"?
If there were only hydrogen atoms, then all the light "available" must have come from hydrogen atom emissions. Therefore the hydrogen atoms could have interacted with all the light of that epoch, couldnīt they?

The radiation filling space at recombination (when stable hydrogen atoms could form and not be ionized quickly by ambient UV) was the result of processes when the plasma was fully ionized earlier on. Free-free processes generate a continuum, and for an optically thick plasma, that makes a blackbody. (The blackbody nature of the CMB does not stem from recombination itself but was established much earlier).

dhd40
2009-Jan-25, 08:27 PM
The radiation filling space at recombination (when stable hydrogen atoms could form and not be ionized quickly by ambient UV) was the result of processes when the plasma was fully ionized earlier on. Free-free processes generate a continuum, and for an optically thick plasma, that makes a blackbody. (The blackbody nature of the CMB does not stem from recombination itself but was established much earlier).

From this I understand that I should have thought of "light" as electromagnetic radiation of any frequency. Right?

speedfreek
2009-Jan-25, 09:02 PM
Photons dominated the epoch prior to recombination, but they couldn't travel very far without hitting something - they were frequently interacting with charged protons, electrons and nuclei until recombination, after which most of the atoms in the universe were neutral and photons could travel freely for the first time.

mugaliens
2009-Jan-26, 06:18 PM
AonSao; This is not the last word on this... others I am sure will contribute more for you. I will however try to explain,. When you hear the phrase 'cosmic microwave background image of the big bang' That is not an image as such. Norr is it light just reaching us from then. What is it ? Its the energy released that has cooled to almost nil. By the smallest fraction measurable we can detect that some heat is still there. In this case its in the microwave length signal and is a remnant of the BB. or so they say:)and I think I can safely say its about 13.7 billion light years away... in any and all directions.

Source: The not so reliable corners of my mind.:)

Good, astromark. I'd add that it's been bouncing around in all directions every since, reflected and refracted by gas and dust.

IsaacKuo
2009-Jan-26, 07:39 PM
How much has it been reflect/refracted, though? Not by very much, right? I think it's fair to say that it's light which is just reaching us from then.

I visualize it by imagining a 2d version of our universe. In this version, we're living in the 2d surface of a sphere. After the big bang, light was bouncing around and stuff, but at some point matter became sparse enough for light to travel more or less unblocked. That's the light of the cosmic background radiation.

From that point onward, the background light travels more or less unblocked. At any given time, some light from this era is just reaching us--the light is just reaching us from points further and further away.

Now, depending on how quickly the universe is expanding, these points may become so far away that they "wrap around" the sphere. As we currently understand it, though, there has not been enough time for the light to "wrap around" nor will there ever be enough time for the light to wrap around all the way.

George
2009-Jan-27, 03:22 PM
How much has it been reflect/refracted, though? Not by very much, right? I think that is correct.


I think it's fair to say that it's light which is just reaching us from then. Yep.


After the big bang, light was bouncing around and stuff, but at some point matter became sparse enough for light to travel more or less unblocked. There were about 1 billion particles of matter for every photon until the expansion caused the temperature to drop low enough for electrons to attach orbitally to the protons. This happened essentially all at once, but not perfectly (temp. anisotropy). [see ngc3314's post above]


From that point onward, the background light travels more or less unblocked. At any given time, some light from this era is just reaching us--the light is just reaching us from points further and further away. Yep, and the flood of light is still pouring down upon us. :)


Now, depending on how quickly the universe is expanding, these points may become so far away that they "wrap around" the sphere. As we currently understand it, though, there has not been enough time for the light to "wrap around" nor will there ever be enough time for the light to wrap around all the way. You might not want to unwrap the universe just yet. The light from the CMBR is simply that from about 13.7 billion years ago, which is only just now getting here.

George
2009-Jan-27, 03:24 PM
Is there any new information regarding the polarization that may offer insight to a time just after the Big Bang itself?

Jeff Root
2009-Jan-27, 08:04 PM
There were about 1 billion particles of matter for every photon
until the expansion caused the temperature to drop low enough
for electrons to attach orbitally to the protons.
You got the number ratio backwards. I think the number "1 billion"
is correct, but it should be a billion photons for every electron and
proton.

-- Jeff, in Minneapolis

George
2009-Jan-27, 08:22 PM
You got the number ratio backwards. I think the number "1 billion"
is correct, but it should be a billion photons for every electron and
proton. You are likely correct, though I think that is how the book I was reading ("The very first light", J. Mather) stated it. I'll try to find it.