Talk:Horizon problem

Horizon problem isn't really a problem

If we examine from statistical mechanics principles what thermal equilibrium really means, we see that it is the most probable macrostate for a system (in other words, the state with highest entropy). Systems evolve towards thermal equilibrium not because nature has any sort of preference for evening out energy among all degrees of freedom, but simply because having a roughly equal partition of energy among degrees of freedom is OVERWHELMINGLY probable.

For exactly the same reason why it is overwhelmingly probable for a closed system to move toward thermal equilibrium, it is overwhelmingly probable for a completely randomly selected initial condition to be in thermal equilibrium. No causal contact is necessary. — Preceding unsigned comment added by Khashishi (talk • contribs) 23:25, 24 January 2013 (UTC)

No, this is patently not true: it is overwhelmingly probable for a completely randomly selected initial condition to be in thermal equilibrium Rentier (talk) 02:20, 18 June 2017 (UTC)

Wow, I guess we can all just go home! It's a wonder theoretical physicists didn't think of this already. Aminomancer (talk) 07:06, 26 April 2022 (UTC)

Temperature Differences at the Big Bang, & Uncertainty Principle

A good portion of this article seems to be lifted directly from http://www.absoluteastronomy.com/topics/Horizon_problem (although I suppose they could have lifted it from Wikipedia...hmm).

Anyway, the third paragraph of the "Basic Concepts" section is a bit baffling. It's not clear to me why the temperature isotropy could not have existed at the moment of the Big Bang and the subsequent temperature isotropy measured today simply be a result of that initial condition; the article does a good enough job of explaining why two areas of different temperature should not later wind up at exactly the same temperature, but what if they were the same temperature to begin with and have experienced more or less the same phenomena over the life of the universe, so they're now still mostly at the same temperature?

That third paragraph invokes the uncertainty principle, but nothing in the article demonstrates how that's relevant. The article uses the sentence '...as the uncertainty principle essentially states that there is no way for even the universe to know precisely what those properties are.' which not only fails to properly explain WHY the uncertainty principle dictates that temperatures be different at the formation of the universe, the phrase 'for the universe to know' is, while quaintly zen, not really appropriate for an article on scientific phenomena.

For now I'm going to retool the language of that particular sentence, but if anyone with a better understanding is around, the article could use a concise explanation of WHY temperatures should have been different at the moment of the Big Bang. To me, with only a basic understanding of the physics and cosmology involved, I don't see any reason why the universe in its first moments should of necessity have temperature differences. Hamiltondaniel (talk) 13:02, 22 January 2010 (UTC)

I agree that the article neither explains why initial conditions can't solve the horizon problem, nor why previous thermal equilibrium before last scattering can. These are 2 important points (commonly) missed out in explaining the horizon problem.

Firstly the initial conditions. We could solve the horizon problem by saying that the big bang had perfect initial conditions, however this arguement just leads on to other questions, such as "Why did it have such perfect conditions". Creationism uses the perfect initial conditions arguement, physics tries to answer questions, not make them!

The second important point in the horizon problem is that for a non-inflatory universe any parts of the universe that are currently separating (due to the expansion of space) faster than the speed of light have always been seperating faster than the speed of light. So different parts of the CMB not in casual contact will have NEVER been in casual contact, and could never have reached thermal equilibrium and therefore would be expected to have different temperature, which it doesn't.

Unfortunately I don't know how to put these points in the article without hacking away at it. Maybe when it comes to do a rewrite these points should be included 82.7.88.93 (talk) 10:54, 3 August 2011 (UTC) Ali 3/8/2011

I guess that in the very early universe (pre CMB) quantum fluctutations played a larger role than they do today because the universe was so much smaller. This means that even if the entire universe was created completely isotropic by the big bang (which is indeed possible and a logical result of many speculative "pre-big bang" theories), these fluctuations would have made the universe less isotropic and they would have done this to such a degree that the CMB should be less isotropic than we have measured it to be. However this requires some calculations to proof that quantum fluctuations in the very early universe really were important enough to make the universe less isotropic than the CMB suggests. Such calculations have undoubtedly been performed but it's a shame not a single source on the horizon problem that I've seen mentions them because citing them would probably answer most people's questions.195.169.213.92 (talk) 23:32, 12 June 2012 (UTC)

Where can I read up about the second important point in the horizon problem?? Why do those areas always separate faster than the speed of light?? What about areas that are now separating with a speed greater than the speed of light because of the accelerating expansion??2003:E7:2F2C:E82F:65CA:1F34:55E2:7589 (talk) 08:34, 23 November 2019 (UTC)

Poorly Written

The article makes no sense. If the universe is expanding that means at one time it was smaller, the information would've been exchanged when the universe was smaller. Information doesn't change. Why would the physics be different, if everything comes from the same source the particles should be the same. The temp and properties are the same because they all came from the same source, it's just one went to the left 13+ billion light years and one went to the right 13+ billion light years. Even though those two points are now 26+ billion light years apart and light and infomration wouldn't have had time, since the universe is expanding it stands to reason they were closer at one point. Therefore the information was exchanged back then —Preceding unsigned comment added by 63.26.200.199 (talk) 07:40, 16 November 2008 (UTC)

there is no geometrical aid in the body to understand the problem beyond that one simple image71.56.224.34 (talk) 15:57, 30 May 2021 (UTC)

The last sentence, "According to the theory, inflation was caused by a momentarily displaced quantum potential", sounds dodgy to me. As far as I know, the term "quantum potential" is used only in the context of Bohm's interpretation of (non-relativistic) quantum mechanics. It doesn't appear in the article on quantum field theory that the term links to here, and I've never seen it used in that context, either. "momentarily displaced" is also at least unclear – displaced in what sense? But I don't know enough about inflation theory to correct it. Fpahl 00:08, 22 Sep 2004 (UTC)

You're absolutely right. The truth is this is how inflation was explained to me, and I don't know enough about quantum field theory to say any more about what exactly it means while retaining an encyclopedic tone. I can do a little better here, though. It has nothing to do with Bohmian mechanics. A primitive example of a quantum potential would be the scalar potential from electromagnetism. (I say primitive because the scalar potential turns out to not actually be a potential by itself but only a "part," in a certain loose sense, of a potential.) By saying such a potential is "displaced" I mean it is "removed from its least energy configuration." I think I'll substitute "globally excited" for "displaced." –Floorsheim 01:06, 22 Sep 2004 (UTC)

The entity that the scalar potential becomes a component of in QFT is called the photon field. You can derive an effective potential from this field, but I've never seen either the former or the latter referred to as a "quantum potential". (Just to clarify things; I'm not sure whether you were arguing for using the term or just explaining what you'd meant by it.) Fpahl 08:31, 22 Sep 2004 (UTC)

Roadrunner, your new formulation seems an improvement to me. However, I hadn't heard that this "later proved problematic". Do you have a source for this? (Both for the article and out of my personal interest). BTW, please use edit summaries; they help in interpreting the history. Fpahl 08:24, 22 Sep 2004 (UTC)

I thought about this some more and realized that you probably intended this as a summary of what it says in the cosmic inflation article. If so, I think the summary is misleading – it's the tie to specific GUT fields that became problematic, not the more general ideas related to vacuum energy. Please correct me if I'm wrong; otherwise I'll be changing the sentence to reflect this view. Fpahl 16:46, 22 Sep 2004 (UTC)

I decided to remove the sentence entirely, since any precise statement would necessarily have duplicated a lot of what can be found under inflationary theory anyway.Fpahl 16:08, 8 Oct 2004 (UTC)

I can offer an explanation to momentarily displaced quantum potential (but cannot be sure that this was meant by the perosn that explained the topic Floorsheim). It is only a rough picture of course:

Image a phi^4+a*phi^2 term for the effective Lagrangian of quantum field, starting with a>0. The vacuum state, the state of lowest energy, would be phi=0. Now through interaction with other fields the a term decreases. Once a<0, the the state of lowest energy no longer is at phi=0. Not with formula, but with more complicated ones, phi=0 may be still a local maximum and the field may stay in this local maximum for some time. But when it finally transitions to the true minimum, vast amounts of energy are set free. Pjacobi 09:59, 22 Sep 2004 (UTC)

Missing Context

Anyone have any idea what the person who added that is complaining about? The article is poor, but I'm not sure context is the problem. WilyD 20:15, 22 December 2006 (UTC)

But there are still problems in this text

1) "....to be very close to isotropic, which also implies homogeneity." Does NOT imply: A ten-ringed target, as seen from the bull's eye is indeed isotropic, but it is definitely not homogeneous!

2) According to the decision of CGPM in 1983 the speed-of-light is as fixed as there are 60 seconds to the minute or 0.3048 m to the foot. Speed-of-light equals 299 972 458 m/s. Period. Today, anyone thinking in terms of a variable speed-of-light, has to turn to "time-running-slower" thinking instead. Time is indeed running slower at certain curved places, but no-one (I pray!) would suggest "to 61 seconds per minute...".

3) and maybe trickiest, I read this text thus: "Before the inflation ALL (and I mean ALL) of the universe was confined to a very limited volume of space. After the inflation the universe was spread out in infinite space (as it is today)". [In "universe" I include also all the galaxies that we cannot observe - and if we believe in homogenity they are countless]. ALL the universe confined to a limited volume imples infinite mass/energy density. Either the theory of inflation is all wrong, or maybe it just hasn't been fairly described in this article.

4) "... postulating a short 10−32 second period of exponential expansion (dubbed "inflation") in the first seconds of the history of the universe". Wasn't that inflation much earlier? The text suggests it happened around t=2-3 secs.

So I think this article requires a major overhaul. Hilmer B (talk) 16:40, 22 June 2015 (UTC)

Changes

I attempted to cleanup the article. I made multiple clarifications and additions, removed some redundancies, and removed discussions of the inflation and of the VSL models that were not related to the horizon problem (especially since a large part of the latter was directly copied to/from variable speed of light)

In two cases, I removed the clarification tag without changing the sentence. If the meaning is still unclear, please reinsert them. Rentier (talk) 02:20, 18 June 2017 (UTC)

Copernican Principle

How about a section exploring the possible solutions if we relax the Copernican Principle? — Preceding unsigned comment added by 157.14.234.194 (talk) 11:12, 6 April 2019 (UTC)

What do you mean specifically? Like, Earth in a local gravity well? Anisotropic synchronization? It seems like if the only proposal is that Earth is near the absolute spatial origin, it doesn't change the expectations. So I assume you have to be thinking of something else or additional Aminomancer (talk) 07:17, 26 April 2022 (UTC)

Reference to Rindler's paper

When Rindler's paper is first brought up in the article, [1] is cited, but [1] is not Rindler's paper. How about referring to Rindler's paper there? In fact the article seems to be completely missing a direct reference to Rindler's paper. 2601:600:877F:DD20:B179:56FE:50BC:3C01 (talk) 20:27, 11 March 2024 (UTC)

"This coordination implies that the entire sky, and thus the entire observable universe, must have been causally connected long enough for the universe to come into thermal equilibrium."

How about explaining why it is impossible for a combination of initial conditions plus local conditions to produce the same effect without causal connection?

Also, there's the use of an undefined term, "epoch". In general one should define terms before they're used. In this case there's also the question of whether epoch is meant to be an instant or some longer period of time. 2601:600:877F:DD20:B179:56FE:50BC:3C01 (talk) 20:29, 11 March 2024 (UTC)

Local conditions, epoch definition

"This coordination implies that the entire sky, and thus the entire observable universe, must have been causally connected long enough for the universe to come into thermal equilibrium."

How about explaining why it is impossible for a combination of initial conditions plus local conditions to produce the same effect without causal connection?

Also, there's the use of an undefined term, "epoch". In general one should define terms before they're used. In this case there's also the question of whether epoch is meant to be an instant or some longer period of time. 2601:600:877F:DD20:B179:56FE:50BC:3C01 (talk) 20:29, 11 March 2024 (UTC)