[johnspannenburg (OP)]

It's all interesting stuff but my core question remains unanswered.
I know it's complicated and no example or analogy I give will exactly match someone's or anyone's or everyone's own mental image but
nevertheless my belief that the way the Horizon Problem is stated is incomplete / oversimplified / maybe wrong - remains.  The 'distances' referred to appear to be incompletely specified (sorry I'm an engineer - I can't help it).

I've ordered myself a copy of "Cosmology: The Science of the Universe" a book recommended and, from the looks of it, a degree level textbook.
This may get me to where want to be in my understanding of the "Horizon Problem" (maybe) and if not it will be an interesting read anyway.

Cheers.

[johnspannenburg (OP)]

OK.. having done some further research into cosmology in general I am now able to refine my question somewhat.
In the way the horizon problem is typically stated...

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."

Is the stated distance of twenty billion years the 'emission distance' or the 'reception distance' - In my opinion it must be the emission distance or else the horizon problem should not be a problem. 

Please, you must technically understand the terms used to be able to make meaningful comment.

Assuming that it is the emission distance why is this never stated? Is this implied or thought to be so obvious that it does not need stating? Or too complex that it is assumed the reader would not understand anyway?

If it is the emission distance then I understand the 'horizon problem' but not the reason for it being poorly/incorrectly/incompletely worded almost everywhere.  If it is the reception distance.. then my problem with THE problem remains - LOL.

Look forward to comments.

Cheers.
JS

[Turveyd]

Well the answers are :-

1.  We are smack bang in the middle of the universe.
2.  There is something we dont' understand about photon's and billions of years.

I suspect 2,  I also expect in time the Big Bang theory won't hold any water at all and we'll never understand this.

I also expect the universe is much much much bigger than we thing.


Big Bang is based on way to many what if's for my liking,  we don't understand things on the earth so even thinking we understand the universe is well madness to me.

[johnspannenburg (OP)]

... Groan....

Not the answers to any questions I asked.

I know I shouldn't do this as I don't want to go off topic... but if your post reflects your opinion...
Why are you even here??

Looking forward to more meaningful comment.
JS

[PhysBang]

Many things in cosmology are poorly presented, unfortunately. The wikipedia article has a number of mistakes. For example, "A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer." is simply not true. We will identify galaxies to be further away than they would be given the speed of light because of expansion.

The horizon problem is that we can identify points in the past, say on the surface that released the background radiation, that are far enough apart that, even given the past expansion of the universe, there cannot be a meaningful causal connection between these parts of the universe. Thus we expect that, aside from stipulating initial conditions, there is no reason for these regions to be similar.

Now, I'm not exactly sure why many cosmologists don't want to stipulate initial conditions, but there it is.

We can get around the horizon problem by stipulating that in the past there was a really huge expansion. But this really pushes the expansion problem to a new region. If we could eventually see far enough, we might find a new horizon problem.

[johnspannenburg (OP)]

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

[Maniax101]

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...

[johnspannenburg (OP)]

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

[PhysBang]

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.

[johnspannenburg (OP)]

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.

[Mr. Scientist]

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.