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Re: CMBR and neutron stars

Subject: Re: CMBR and neutron stars
From: "George Dishman"
Date: Sun, 7 Aug 2005 09:55:59 +0100
Newsgroups: sci.astro, sci.physics.relativity
"N:dlzc D:aol T:com (dlzc)" <N: dlzc1 D:cox T:net@xxxxxxxxxx> wrote in 
message news:jNbJe.275166$Qo.182521@xxxxxxxxxxxxx
>
> I don't have the applet required to play it.  I found another site that 
> had it in Quicktime.

It used Real Player but as long as you got
the idea, a bulk material can switch from
opaque to transparent.

> Hydrogen absorbs light.

Hydrogen also emits light. By the laws of
thermodynamics, there must be a precise
relationship between the two.

> We find its signature everywhere we look, when we look at point sources. 
> But we don't find it with the CMBR.  And it has been exposed longer, 
> assuming the Universe filling gas didn't coalesce.

If the gas is in equilibrium, the energy
absorbed must be balanced by that emitted.
It is therefore exactly at the crossover
between producing absorption and emission
lines. Given the slight anisotropy, perhaps
it will be possible some time soon to detect
a line redshifted down but it has taken
sensitive space-borne missions just to
measure the anisotropy in broad bands so
looking for individual lines would require
another step up in design.

>>>> At some point the CMBR had redshifted enough so it could
>>>> not be absorbed at all by hydrogen, and since then the
>>>> universe has been transparent to it (except for isolated
>>>> objects we call stars).
>>>
>>> Let me ask a related question.  The CMBRM has been
>>> described as opaque and isothermal.  Presumably "opaque"
>>> could be defined as no emissions detectable from beyond a
>>> certain place (watch my terms).
>>
>> "a certain place" may give the wrong impression
>> although obviously we are "here".
>
> We can't see beyond the CMBRM, so it is opaque.

That is correct, it rapidly absorbs any light
that passes through it.

> If we were on the inside of an event horizon, we could not see beyond that

That is not correct. Light does not pass out
across the horizon but it does fall in. If
you could hover just inside the event horizon
you would be bombarded by high energy photons
falling in.

> Universe's Big Bang, and see specular images of the container Universe. 
> The Big Bang (aka. the inside of an event horizon) is opaque *without* 
> invoking space-filling hydrogen plasma.

The event horizon is not opaque, light passes
through it without being absorbed.

>> It is perhaps
>> better to consider opaque in this case in terms
>> of the mean free path of a photon being greater
>> than the time from emission to the time of
>> complete transparency multiplied by the speed of
>> light regardless of where the photon is emitted
>> since the CMBRM was almost homogenous throughout
>> the universe.
>>
>>> So let me ask this question about the Universe that contains ours...
>>
>> There is no "Universe that contains ours" in the
>> Big Bang model so it is not meaningful to ask the
>> question.
>
> The Schwarzchild solution to GR for a black hole, describes another 
> Universe inside the black hole, with internal time starting where external 
> space leaves off.

I'm not sure about that, it seems to depend on
the coordinate system you use but I know too
little of GR to comment sensibly.

> Since the Big Bang model is commonly dressed in the clothes of GR, are you 
> sure 'There is no "Universe that contains ours" in the Big Bang model so 
> it is not meaningful to ask the question'?

I am fairly sure that the picture of the event
horizon of a black hole to which you are
referring is related to an isolated mass in a
lower density environment while the big bang
scenario has uniform density throughout. You
can get a similar effect I believe in a uniform
density regime:

http://scienceworld.wolfram.com/physics/SchwarzschildBlackHoleConstantDensity.html

but in this case it would be more like the
horizon that is expected to be produced by
cosmic acceleration and is a limitation on
visibility _within_ the universe.

However, that needn't mean our universe isn't
embedded in something different. The idea I
find most understandable to illustrate the
possibility is that of Alan Guth where he
talks of multiple universes embedded in
'false vacuum' which is in a permanent state
of inflation. That is not exclusive though,
just indicative of the feasibility.

> You say imagine the Landolt reaction in reverse...

Purely to suggest the change from opaque to
transparent could be due to a phase change in
the hydrogen/helium mix (plasma to gas) rather
than requiring the material to coalesce into
structures leaving vacuum between as you were
suggesting. That could come later.

> imagine that our Big Bang is the inside of the event horizon of the 
> (probably really big) black hole that contains our Universe.
>
> So sure are you?

I am sure that the H/He mix was present because
we see the radiation from it in the form of the
CMBR, it is predicted by nucleosynthesis and we
can see the mix in primeval stars. I can be sure
it was opaque for over 300k years from lab
experiments and WMAP. I can't be sure what we
would have seen had it not existed, but then we
wouldn't be here to see anything. Beyond that,
I would just point you at the faq entry and say
that I wouldn't presume to be any more sure than
Philip Gibbs, and since it is 8 years old,
perhaps even that might be slightly outdated.

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

George



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