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

[BlindedByTheLight]

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.

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.

Thank you! It is starting to make sense now! Well, at least until someone chimes in and says, "No, no! That's all wrong!"

But let me throw one final thing at you. Your response that you described as "worse" - actually appears to be "better" to me. What I mean is, above, I wrote about how it seemed that the photon that started it's journey when only one centimeter away has been slowly crawling across that centimeter at the speed of c... while the centimeter itself was expanding. Leaving the photon to appear as if it were hoving, just a centimeter away... all this time SLOWLY traversed across that distance. But, per your "For much of the time this stuff was moving away FTL"... well, then, you are saying that the light that BEGAN it's journey a centimeter away got stretched out WAY OFF into space (distance depending on the speed and length of inflation?)

That about right?

[Hamiltonian]

well, then, you are saying that the light that BEGAN it's journey a centimeter away got stretched out WAY OFF into space (distance depending on the speed and length of inflation?)

That about right?

Yes. The stuff a few centimetres away was hoiked off into the distance so fast that no influence from it could reach us until now. But this happened post-inflation, and is still happening. There are still bits of the universe that we can't see because theyre receeding faster than the speed of light as seen from our location, so their photons are getting farther away instead of closer.

I only described this situation as "worse" because many people find the FTL aspect disturbing.

[BlindedByTheLight]

I only described this situation as "worse" because many people find the FTL aspect disturbing.

Well... obviously they're not Star Trek fans... because every time I hear FTL my female alter-ego in some alter-dimension gets wet.

Anyhoo... another question...

The initial whisking away... that was NOT due to the FTL expansion of inflation? Or it was? Or both?

[eburacum45]

I really like the explanation of all this given on this page;
http://anzwers.org/free/universe/redshift.html

you can see that there are many different values for the distance of any given object-
Luminosity Distance
Angular Diameter Distance
CoMoving Distance
and the
Light Travel Time Distance.
For some reason the last one of these is the distance usually quoted; but I think that this thread is more closely concerned with the Angular Diameter Distance, the distance that the object being observed was away from our viewpoint when it emitted the light we now see..

I would be very interested to find out what the Angular Diameter Distance of the Cosmic Microwave Background is;
a bit of wild speculation here might give an approximate answer (of course I am not a cosmologist either)

bear in mind that the universe had already been expanding for three or four hundred thousand years when the universe became transparent (some of the time at speeds faster than light), so we will be seeing stuff from the far side of the tiny universe as it existed at that time. Since the universe became transparent the diameter of the observable portion has increased by about a thousand times (and the wavelength has stretched by the same amount), so that puts the radius of the tiny, newly transparent universe at about 13 million light years.

There we are; the CMB has been travelling for 13 billion years, but because of expansion has only travelled 13 million light years ( a very rough estimate)

[buggs_moran]

(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.

It would probably have something to do with the debate on the expansion rate (Hubble constant) that continues to rage, no? I have seen numbers that range by a factor of two km/s/Mpc (50 to 100)... which could account for the large discrepancies.

[BlindedByTheLight]

There we are; the CMB has been travelling for 13 billion years, but because of expansion has only travelled 13 million light years ( a very rough estimate)

Thank you... VERY interesting. Gotta question, though. When you say 13 million light years... that is from who's frame of reference? Because, as the space between the photon and my eyes has expanded, wouldn't you count the length of that expanding space into your figure of 13 million light years?

[Hamiltonian]

The initial whisking away... that was NOT due to the FTL expansion of inflation? Or it was? Or both?

Inflation made the very early universe orders of magnitude bigger in a miniscule part of a sec. It moved stuff that was in light-travel contact far enough apart to break that contact. Then the current expansion regime took over, and much of the stuff that was out of light-travel contact started to be whisked away at FTL speeds (though much slower than the inflation episode). So both effects do the work, one very briefly and massively, one for the rest of the lifetime of the universe.

I really like the explanation of all this given on this page;
http://anzwers.org/free/universe/redshift.html

The graph at the bottom is very useful. You can see that DA (a measure of the distance at which the light was emitted) goes down towards zero again as the other distances increase. Gives a picture how really closeby stuff could emit photons that take the age of the universe to reach us.

It would probably have something to do with the debate on the expansion rate (Hubble constant) that continues to rage, no? I have seen numbers that range by a factor of two km/s/Mpc (50 to 100)... which could account for the large discrepancies.

But the current 13.7 billion year figure for the age of the universe was arrived at because the Hubble constant had been tied down more accurately, I recall.
Eboracum45's website gives a figure of 47 billion ly for the distance to the "very edge of the observable universe", which ties pretty well with the SciAm figure of 46 billion. (Now that I've read the article and not quoted a rule of thumb from memory!)
The people on your website seem to be using some other measure. Wonder what?

[Cham]

(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.

It would probably have something to do with the debate on the expansion rate (Hubble constant) that continues to rage, no? I have seen numbers that range by a factor of two km/s/Mpc (50 to 100)... which could account for the large discrepancies.

No, THAT factor of three as a theoretical origin. Here's why :

First, make the following hypothesis (this is an old scenario, actually) :

HYPOTHESIS - The universe is matter dominated without any pressure (galaxies form a gas of "dust"), and there's no Cosmological constant. No dark matter, no dark energy. Electromagnetic radiation is negligible (except for a "short" period of time close to the Big Bang).

Then, under the assumption of spatial isotropy and homogeneity, you solves the Einstein equation to find the scale factor a(t), where "t" is the cosmological time, measured by any localy stationary observer relative to the averaged distribution of matter. If you assume a spatially flat space-time (parameter k = 0), you find

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a(t) = cste * t^(2/3)                 (equ. 1)

This describes the old scenario of a dust filled universe expanding forever, with some deceleration caused by the self-attracting matter.

Now, we can show that the proper distance D(t) between any stationary observer and its cosmological horizon is, at the moment "t" :

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 D(t) = a(t) * Integrate[ 1/a(s) * ds ,   s ranging from s = 0 to s = t ]

Using expression (equ. 1) gives

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D(t) = 3 t.

That's where the 3 is coming from. So if this scenario is realist (it isn't), the cosmological horizon is today located at 45 billions of LY if the universe is 15 billions years old.

The result is extremelly scenario dependant, so is the interpretation of observationnal data.

[Hamiltonian]

That's where the 3 is coming from. So if this scenario is realist (it isn't), the cosmological horizon is today located at 45 billions of LY if the universe is 15 billions years old.

The result is extremelly scenario dependant, so is the interpretation of observationnal data.

OK, this is good, thanks very much. So (just to make sure I've got this, sorry) the actual value depends on the [matter / dark matter / dark energy] contents of the universe, and we don't have enough theoretical or observational knowledge to nail that precisely?
Is there an accepted range, or is everyone still arguing?

[Cham]

Well, just to add some fuel on the fire, let me recall that the "Hubble constant" ISN'T even a constant at all ! It's a constant over space, but not in time ! By definition, Hubble's constant is

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 H(t)  =  1/a(t)  *  da/dt          (equ. 1)

So in the classical case of the dust filled universe, we have a(t) = cste * t^(2/3), and the definition (equ. 1) gives

H(t) = 2/(3 t).

Hubble's constant is decreasing with time !


Again, this result is extremelly scenario dependent. If there's some dark matter components (I don't believe in that), or a cosmological constant (I believe in this one), then it's much more complicated.

Astronomy on the cosmological scale is a VERY tricky matter, unfortunately.


EDIT : Well just a small correction. Hubble's constant is actually the special value H(t) gets at our epoch : H0 = H( t0 ), where t0 is the actual age of the universe.