Question for Nereid

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marcus

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path of light through expanding space, question

this question ought to be directed generally to whomever might like to help with an explanation

if a quasar is observed with redshift 1.7 or more then at the time it emitted the light we now see the quasar was receding at speed of light or faster

question how did the light ever make it here since it was initially swept back.

I have a vague notion but it still remains paradoxical and unintuitive to some extent. Maybe I could give an explanation
or pass along one Ive heard. But I'd rather hear someone elses
take on it.

Im thinking of the teardropshape lightcones in eg. Lineweaver article "Inflation and CMB". assume you know article if not I'll give link

apologies for having initially directed this to Nereid since it is really an open question. I've tried to erase the thread so as to remove the title, but computer wont let me erase it at least for the moment
 
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Nereid

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Hi marcus, I'm glad you edited your post; I really am a newbie here, and there are plenty of members who, I'm sure, could answer your question. :smile:

In looking back through Astronomy & Cosmology, I see that there's a thread - started by you - that seems to address a similar question:
https://www.physicsforums.com/showthread.php?s=&threadid=6025

The link you gave there to the Lineweaver article "Inflation and CMB" is:
http://arxiv.org/abs/astro-ph/0305179

In what way is your question in this thread new, in the sense that the discussion in the other thread was unsatisfying?
 

marcus

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Originally posted by Nereid

The link you gave there to the Lineweaver article "Inflation and CMB" is:
http://arxiv.org/abs/astro-ph/0305179

In what way is your question in this thread new, in the sense that the discussion in the other thread was unsatisfying?
this gets to where my understanding is the shakiest and someone with additional or different intuition could help the most.

in Lineweaver Figs 1 and 4 the lightcones are teardrop shape
meaning that you see the journey of some light that is actually swept away from us at first and fights its way thru a current of expanding space and finally reaches us like a salmon swimming upstream

in the spacetime diagram the curve of the lightcone is actually the path some light could travel

the fact that some light could start towards us in a region of space that is receding from us at like 3c (three times the speed of light!) and which will recede faster (!) as it gets further, and that this light could eventually reach us

even tho it is initially swept back

this is, to me, unintuitive at a basic mechanical level. I imagine I am the light and .......hmmmm.....I was going to say I dont understand how I am going to reach the earth.....but there is a glimmer of an intuitive idea here of how it happens

is this one of these ideas you simply have to live with until it seems familiar, but never fundamentally understand, or is there
something you can say to someone that makes it seem natural right away
 

drag

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Hmm... But there's got'ta be some principal explanation
(short and clear :wink:) to this, right ?
So what is it ? :frown:
 

marcus

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Originally posted by drag
Hmm... But there's got'ta be some principal explanation
(short and clear :wink:) to this, right ?
So what is it ? :frown:
by traveling towards us, even if it is being swept back and the distance to the goal has a net increase, even so by traveling towards us the light gets into a slower flow-regime
than if it did nothing and just stayed where it was
so there is a kind of progress against the current even if
the light is being swept back and getting farther away

this is an aspect, not the whole answer
the problem of making what we know happens be conceptual
for us is a hard one and I cannot offer a "short and clear"
but I believe that there is a "short and clear" conceptual
explanation---like resolution of a bent-nail or wooden chinese puzzle---which I am just not getting
 

drag

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Greetings !
Originally posted by marcus
by traveling towards us, even if it is being swept back and the distance to the goal has a net increase, even so by traveling towards us the light gets into a slower flow-regime
than if it did nothing and just stayed where it was
"Slower flow regime" and "stay where it was" ?
Originally posted by marcus
so there is a kind of progress against the current even if
the light is being swept back and getting farther away
But, isn't is supposed to be swept back a lot faster ?
Originally posted by marcus
I believe that there is a "short and clear" conceptual
explanation---like resolution of a bent-nail or wooden
chinese puzzle---which I am just not getting
Yeah, something like that.

Let me see if I can try to make some sense of it if
I were the light. I have the quasar on the one side and
Earth on the "other side". Now space in between expands
at at say 5 c. Being the light I move to some point A,
now the space behind me expands at more than half c
and the space in front at more than 4.5 c. However,
being light - I still have to continue at c. This is
different from an SR doppler shift that I would
experience if the quasar itself was moving with respect
to Earth through space-time, but only because that
doppler shift can't reach values greater than 2 - according
to SR. O.K. it might make some sense, but the problem
I'm facing now is something like another parameter
having to do with the rate of "growth" of space-time.
No, wait a minute... O.K. this is what I just thought about -
if the light was emmited when the expansion was below
c then every time you examine the distance it has left
after that it can't axpand at faster than c... hmm...
makes sense ? If the answer's yes, what's the limmit ?
Can't we try to use it (abviously practicly quite difficult)
to conclude the average expansion rate of the Universe
(abviously it's been changing but even the change could've
been different) ?

Live long and prosper.
 

marcus

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drag,

thanks for taking a stab at it
which in itself takes some gumption
your post triggered something for me
and I found an intuitive proof that the light
can make it
however I am sleepy----its nearly midnight
so I will wait till tomorrow to talk about it
 

Nereid

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Personally, I think the first Lineweaver paper* is better, in terms of explaining what's going on.
*http://arxiv.org/abs/astro-ph/0011070

The last four paragraphs are the best, IMHO. Here they are, copy and paste (I'll come back and edit this, to get the terms as close to those in the paper as possible):
"How can photons move from the light grey superluminal region into the dark grey subluminal region? How can a swimmer make headway against a current that is faster than she can swim? It seems impossible. First consider the problem in comoving coordinates. Photons propagate towards us along our trumpet-shaped past lightcone with a comoving velocity X = -c/a. Therefore, photons move inexorably through comoving space, irrespective of their position in relation to our Hubble sphere. Their comoving velocity is always negative. Their commoving distance always decreases.

Now consider the problem in physical space. The tear-drop shape of our past lightcone means that the distance between us and the most distant photons we now see, was once increasing. The relevant quantity for understanding this behaviour is the total velocity of a photon that is heading towards us: vtot = vrec - c = HD - c = aX - c. The total velocity of distant photons is not constant because it is the sum of the distance-dependent recession velocity (vrec) and the constant peculiar velocity, c. When aX > c the distance between us and the photon increases.

Photons now reaching us from our particle horizon were emitted at the Big Bang. Since limt->0 a = [infinity] in all viable cosmological models, aX > c would have been satisfied and these photons would have initially receded from us. Similarly, the first photon we receive from any object was emitted from a region with aX > c, vtot > 0. In the early universe as time progresses, both X and a decrease. Thus, vtot of these photons evolves from positive to negative, and the teardrop shape of our physical past lightcone is ubiquitous to all times.

The swimming analogy fails because, unlike recession velocity which is smaller at smaller comoving distances, the current the swimmer has to face is the same at all comoving distances. Our swimmer has to battle an unrelenting current, while the photon constantly moves into regions with a slower “current" (slower vrec)."

(more later)
 

marcus

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Nereid, it is a real pleasure that you too see explaining this as a challenge. I feel there is some benefit to be derived from understanding because of the initial counterintuitiveness

A. one explanation is a dry mathematical one, like: "a certain integral is finite, so believe or die, lowly human!" which is OK, a lot of things are like that and we accept it

B. another explanation is what Tammy Davis and Chuck Lineweaver say---trying to be friendly and talking about someone swimming in a river----which is more intuitive but IMHO not yet intuitive enough

C. a third explanation is what I am trying to come up with right now. I will sketch it in the next post

D. and there may be still more, hopefully you are thinking up one.

Do you mind smilies? some people detest them and I wouldnt want to offend you. but sometimes I have the impulse to say :wink:
 

marcus

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Here is a quick draft of the intuitive explanation Im trying to formulate

Exclude the cosmological constant because then there really are things receding too fast to send light to us

and for most of the history of the U, expansion has not been accelerating anyway, so on average it has not been (tho in future that will change)

So assume for simplicity that expansion is not accelerating

Imagine a string of galaxies between us and it, the sender, and imagine each of these galaxies is receding from its neighbors on either side at just under c

then light can make it from any galaxy to its neighbor (this needs proof but is not awfully unituitive)

But then light can make it the whole way! Because in a finite time it can go each step of the way along the train.

See it is not so hard:wink:

this is a "draft" explanation. like to know any reaction. does it help, does it contain a flaw etc
 

marcus

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What i am imagining is the worldlines fanning out, the worldlines of 6 or 7 stationary galaxies, and arranging it so the light can fairly obviously get from one to an adjacent one,

maybe at the present moment the emitter is receding at 6 c.
So I will have maybe 6 worldlines in between so that even at the present moment in time (2003 AD) each one is receding slightly less than c from its neighbor

and at the past the emitter was receding at some other speed, whatever it is I will allow enough worldlines. If I need more than 6 I will add some, otherwise I will leave it at six.

so all along the way the light can always get from one galaxy to the next in a finite time

it may be that this merely supplements Davis/Lineweaver, whose explanation you were quoting, or it may be in some sense more visual and easily grasped, I havent decided whether or not it is a valuable aid---do you have any critical reaction?
 
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Nereid

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marcus, I think you and drag (and Dr Chinese (sp?) on the other thread) are getting there. It's good to start with the simplifying assumption a flat universe with no acceleration. Try to be concrete - what are worldlines? what are 'comoving coordinates'? what is 'physical space'? If you could ride with a photon from (X), what would you 'see'? Take a bunch of photons that we have just caught - from galaxies we now see to be at z = 0.1, 1, 2, 4, 10, 100 (maybe not a galaxy:wink: ), ... - which just happen to lie on (nearly) the same line of sight today. Does this help?
 

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