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Author Topic: The Big Bang and the "Horizon Problem"  (Read 11505 times)

Offline johnspannenburg

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The Big Bang and the "Horizon Problem"
« Reply #25 on: 01/09/2009 05:16:00 »
OK.  A more reasoned response but still not one which answers my specific question. 
Is the ten billion the emission distance or the reception distance.  To simply state a number (especially for distant objects) is meaningless - you might as well ask the distance to yesterday , or last year etc.  The terms 'emission distance' and 'reception distance' are defined cosmological terms that specifically cater for time and the expansion of the universe.

I agree that many things do appear to be poorly presented and this hinders true understanding (perhaps for the sake of simplifying (overly) for those who have not the desire or, ultimately, the wherewithal to understand fully).

Anyway.. to explain somewhat..

The "emission distance" is the distance (if you could freeze time or somehow measure instantaneously) that would be measured between the receiving point and the emitting point - at the cosmological time of emission of the photon. 

The 'reception distance' is the distance (if you could freeze time or somehow measure instantaneously) that would be measured between the receiving point and the emitting point - at the cosmological time of reception of the photon.

Clearly they are different (ignoring local movement - or more correctly 'peculiar' movement) by the amount the universe expands between the time of emission and the time of reception.

Perhaps you can also see that the horizon problem only really seems to be a problem if the '10 billion LY' distance is the emission distance.

Maybe I will find this in my growing pile of texts on the topic.. That said, I look forward to further discussion.

Cheers.
JS
 

Offline Maniax101

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The Big Bang and the "Horizon Problem"
« Reply #26 on: 01/09/2009 13:10:23 »
John, I answered this in another post, but think of the cosmic background radiation as the surface of the big bang. So anywhere you look far enough will show you the marble. Its like an (analogy here) inverted balloon. Our boundary in space is the frontier of the big bang...
 

Offline johnspannenburg

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The Big Bang and the "Horizon Problem"
« Reply #27 on: 01/09/2009 13:40:21 »
Yes that is indeed generally how I view the situation. 
My problem comes when the horizon problem is stated and then 'inflation' is used to fix the problem.  In the (fast getting infamous) wikipedia article the problem I'm having is the 20 billion light years. Quite simply is this the 'emission' or the 'reception' distance.  Or in 'balloon' terminology - is that the distance between the emission point (A) and the reception point (B) as would have been measured over the surface of the balloon at the instant the photon was emitted? (in which case there is a horizon problem) Or is it the distance between A and B at the instant the photon is received.  If so then I do have an issue with the horizon problem.  One must specify what this 20 billion light years is.. It is certainly not.. the distance between A at the time of emission and B at the time of reception as this is simply meaningless - bit like asking how many metres it is to yesterday.

Basically if we see the cosmic background radiation some 13+ billion light years from us (in both directions) and this is the emission distance then we have two points 16+ billion light years away only a few hundred thousand years (whenever exactly the universe became transparent to photons) after the big bang and clearly some explanation (such as inflation) is required to explain that size at that age.  On the other hand if the distance are the reception distance then, it seems to me, there is no horizon problem.

Cheers.
JS
 

Offline PhysBang

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The Big Bang and the "Horizon Problem"
« Reply #28 on: 01/09/2009 15:35:04 »
OK.  A more reasoned response but still not one which answers my specific question. 
Is the ten billion the emission distance or the reception distance. 
I think the ten billion number is simply a sloppy, somewhat random choice by the author of that wikipedia article. (Welcome to the encyclopedia of the future.) But I think we could identify two points on the surface of last scattering (as it is called) that were 10 billion light years apart at the era where the background radiation was released, in the rest frame of the background radiation. That is, I think, far enough apart that the regions should be causally separated and it is a meaningful definition of distance.

When making astronomical observations, we have to be wary of distance (see "luminosity distance"), but as the surface of last scattering more-or-less defines a specific time in the rest frame of the background radiation, we can speak of specific distances in that frame on that surface.

The surface of last scattering is the collection of points where the cosmic background radiation is released. It is technically a hypersurface, in that it is all points in 3D space at a given time. Certain areas released the light a little later than others, but we can treat the surface as roughly simultaneous for our purposes.
 

Offline johnspannenburg

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The Big Bang and the "Horizon Problem"
« Reply #29 on: 20/09/2009 09:21:52 »
OK. Great - Really the first response that addresses the issue.
The summary of your answer is that it is the 'emission distance' - which, as I've discovered since grokking all the terminology, is the only answer that makes any sense.

Still it's annoying that in basically every place I've looked the way the 'horizon problem' is stated is technically insufficient (at best) or wrong..

Guess it's time to put this one to bed..
Have to think up another good question.. LOL.
 

Offline Mr. Scientist

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The Big Bang and the "Horizon Problem"
« Reply #30 on: 20/09/2009 13:15:11 »
OK. Total newbie here but I think I have an 'issue' with the horizon problem as it is stated on pretty much every website that I look at. 

Here is a direct extract from wikipedia regarding the horizon problem:

"If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years."


Before I get to that problem let me ask a question.

Assuming we could see through the Cosmic Microwave Background Radiation (CMB) basically right to the big bang (assume we can detect and view gravitational waves say and thus observe beyond the CMB) - The question is.. In which direction would I have to look to observe the universe (and in order to avoid some complication) when it was say the size of a marble or some other similar very small (non zero) size?

I believe the answer to that question leads inescapably to conclusions which undermine the whole basis for the 'horizon problem' but I would like some comments or answers to the question first.

Cheers

JS


Its all relative! :)

Best answer is the first given here.
 

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The Big Bang and the "Horizon Problem"
« Reply #30 on: 20/09/2009 13:15:11 »

 

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