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ArgoNavis
2006-Mar-17, 09:55 PM
Wilkinson Microwave Anisotropy Probe data has been found to support the inflationary epoch of the early Universe, when the Universe expanded from the size of a pea to the size of a grapefruit (correct me if I am wrong).

I understand that this is the reason that the Universe does not have a dimension of 27.4 billion light years (ie the logic here is that the Universe is 13.7 billion year old, so space, with matter embedded in it, has expanded 13.7 billion light years in all directions). Rather the dimensions of the Univese are larger by a significant fraction because of the compounding effect of the early expansion. I have seen numerous figures for the actual dimensions of the Universe, from about 50 billion light years across to 156 billion light years across.

Is that actual figure roughly known?

I am sure the actual picture is somewhat more complex than the way I put it above - anyone care to provide more details?

muchos

Disinfo Agent
2006-Mar-17, 11:25 PM
Does this help?

How big is the universe? (http://curious.astro.cornell.edu/question.php?number=72)

trinitree88
2006-Mar-18, 12:00 AM
Wilkinson Microwave Anisotropy Probe data has been found to support the inflationary epoch of the early Universe, when the Universe expanded from the size of a pea to the size of a grapefruit (correct me if I am wrong).

I understand that this is the reason that the Universe does not have a dimension of 27.4 billion light years (ie the logic here is that the Universe is 13.7 billion year old, so space, with matter embedded in it, has expanded 13.7 billion light years in all directions). Rather the dimensions of the Univese are larger by a significant fraction because of the compounding effect of the early expansion. I have seen numerous figures for the actual dimensions of the Universe, from about 50 billion light years across to 156 billion light years across.

Is that actual figure roughly known?


I am sure the actual picture is somewhat more complex than the way I put it above - anyone care to provide more details?

muchos

Excellent question ArgoNavis. If you buy into the sequence of standard candles, they follow logically to a common value...13.7 B. We constantly check and refine the methodology, and several independent lines seem to agree.
Try looking up parallax,Cepheid variables, Fisher-Tully relationships, type1a supernovae,...but it's not quite lock-step stuff. Pete.

GOURDHEAD
2006-Mar-18, 03:59 AM
Inflation (hyper expansion) lasted from about 10^-43 seconds to 10^-35 seconds so some believe. Neither the accuracy of this interval of time nor the accuracy of the rate of expansion during the inflation period nor whether it was constant nor whether it has been constant since is known (by me). Using a liberal estimate of one light year (the consensus is that it was much smaller, say between the volume of a baseball and that of a standard basketball for the radius of the universe at the beginning of the post inflationary period and a subsequent rate of expansion that caused the affected objects to be separated at less than the speed of light, the average expansion rate would have to have been quite near the speed of light (maybe 10^-10c less than c) to allow part of the universe to remain invisible (unobservable) to those who look today. Then there's the problem of the scale factor at which local gravity overcomes the expansion forces in order to keep quarks, protons, atoms, me, planets, etc., from being stretched to pieces. I would be astonished if any one knows how to credibly guess at the size of the universe. A value I have seen is a "sphere" of 75 billion light years radius.

ArgoNavis
2006-Mar-18, 09:59 AM
I would be astonished if any one knows how to credibly guess at the size of the universe. A value I have seen is a "sphere" of 75 billion light years radius.


I suspected, given the diversity of values, that they are just guessing. 75 sounds good but.

kzb
2006-Mar-21, 06:26 PM
If you measure an object as being 13.7 billion LY distant NOW, is it still there, or has it gone further away during the time the light has taken to reach us, due to expansion of universe?

Jeff Root
2006-Mar-21, 06:52 PM
If you measure an object as being 13.7 billion LY distant NOW,
is it still there, or has it gone further away during the time the
light has taken to reach us, due to expansion of universe?
The wording is so tricky that you actually mucked up your
question! :) You meant:

If you NOW measure an object as being 13.7 billion LY distant...

You see things as they were when the light left them. Obviously
if a galaxy is six billion light-years away, then we see it as it was
six billion years ago, and that galaxy and ours have moved apart
considerably in that time.

The only thing that we can see 13.7 billion light-years away is the
plasma which emitted the cosmic background radiation, and I resist
calling that plasma "an object". :)

-- Jeff, in Minneapolis

joema
2006-Mar-22, 04:15 AM
...The only thing that we can see 13.7 billion light-years away is the plasma which emitted the cosmic background radiation, and I resist calling that plasma "an object". :)...
I don't know about 13.7 billion light years away, but the most distant quasar is about 13.0 billion light years away, and that's definitely an object. Here's a picture: http://news.bbc.co.uk/2/hi/science/nature/2646301.stm

Re ArgoNavis' question, this can be confusing because of several different ways of expressing cosmological distances:

Emission distance -- the distance the source was when the light we SEE NOW left the source (there and then).

co-moving radial distance is the tape measure distance if we could somehow at this cosmic instant measure the distance to the object when the light we see arrived here. Hubble's Law uses this.

Distance based on "lookback time". What is typically used in popular-level cosmology discussions. It's product of light speed multiplied by travel time of those photons across the continuously increasing distance of an expanding universe.

"Lookback distance" can be non-intuitive because of popular misconceptions about the expanding universe. Remember, the universe is NOT expanding from objects flying outward through fixed space like particles from an explosion. If that were so, light from ancient nearby objects would have long reached earth's vicinity -- relativity says you can't out run a photon, nor does a rapidly receding object slow down emitted light. Rather objects are receding because the fabric of space itself is expanding, increasing the distances between objects.

This "stretching" of space interposes additional distance during light's travel period. THIS explains why light emitted 13 billion years ago when a rapidly receding object was much closer takes 13 billion years to reach earth -- because the intervening spatial fabric expanded during the travel period, interposing additional travel time. Thus light from our relatively "local" early universe can still take many billions of years to reach us.

These relate to the size of the universe. E.g, consider an object with a lookback distance of 13.2 billion light years. The emission distance would be about 4 billion light years, but the co-moving radial distance about 28 billion light years. However I've also seen estimates of about 78 billion light years for diameter of the universe based on co-moving radial distance.

So the most distant object we can see is actually at this instant about 28 billion light years away, although we see it as it was about 13 billion years ago.

It seems this "lookback distance" -- about 13-14 billion light years -- defines the size of the observable universe, although the physical distance to those objects at this instant is about double that.

Whether beyond that observable region the universe is infinite or finite isn't known. There are strong scientific opinions both for and against.

Cougar
2006-Mar-22, 03:13 PM
Well, there's the visible universe, the part that is visible to us, and then there's the REST of the story. Of course, the extent and characteristics of anything outside of our light horizon is pretty speculative, but according to calculations performed by Alan Guth, the originator of the theory of inflation (which has just recently gotten additional observational support)....
"...the entire universe is expected to be at least 10^23 times larger than the observed universe! ...if the inflationary theory is correct, then the observed universe is only a minute speck in a universe that is many orders of magnitude larger."
Many orders of magnitude is an understatement.

I recall reading in The Secret Melody [1995] by Trinh Xuan Thuan that light from galaxies that were outside of our light horizon last year is now coming into view, although I'm not sure how this works or where these newly observable galaxies are supposed to appear, because the further we look out into the depths of space, we're looking BACK in time.....

the_bullet
2006-Mar-22, 03:29 PM
I recall reading in The Secret Melody [1995] by Trinh Xuan Thuan that light from galaxies that were outside of our light horizon last year is now coming into view, although I'm not sure how this works or where these newly observable galaxies are supposed to appear, because the further we look out into the depths of space, we're looking BACK in time.....
I thought that it was in fact the opposite that was true. Due to the accelerating expansion wouldn't galaxies currently in our "horizon" be soon beyond our horizon, and anything currently beyond our horizon will be beyond it forever ?

kzb
2006-Mar-22, 06:30 PM
<<This "stretching" of space interposes additional distance during light's travel period. THIS explains why light emitted 13 billion years ago when a rapidly receding object was much closer takes 13 billion years to reach earth -- because the intervening spatial fabric expanded during the travel period, interposing additional travel time. Thus light from our relatively "local" early universe can still take many billions of years to reach us. >>

Joema
This seems to imply the speed of light between two objects is not constant! If the object was 4 billion LY distant when it emitted the light, then surely the flight time of that light is 4 billion years. Otherwise the speed of light would not be c, but c-recession speed ?

The expansion of space shows up as the red-shift, which has effectively stretched out the light waves on their way here?

Bullet wrote:

<<I thought that it was in fact the opposite that was true. Due to the accelerating expansion wouldn't galaxies currently in our "horizon" be soon beyond our horizon, and anything currently beyond our horizon will be beyond it forever ?>>

Again, that would mean we could observe something accelerating past light speed. Is that allowed now?

joema
2006-Mar-22, 09:24 PM
...This seems to imply the speed of light between two objects is not constant! If the object was 4 billion LY distant when it emitted the light, then surely the flight time of that light is 4 billion years. Otherwise the speed of light would not be c, but c-recession speed ?

The expansion of space shows up as the red-shift, which has effectively stretched out the light waves on their way here?..
Object recession doesn't affect light speed, but it red shifts the frequency.

The speed of light between objects is constant, regardless of object speed. However there's another effect involved -- universal expansion.

Imagine two cases:

Case 1: A spaceship zooms by the earth at a constant 99% the speed of light, headed for Alpha Centauri. One year later, it emits a pulse of light. That light will take about 1 year to reach earth, despite the ship's high outbound speed.

Case 2: Same as the above, BUT rapid universal expansion is taking place. During the 1 yr travel time to earth, the fabric of space has expanded so the DISTANCE the light must travel is now TWO light years. It therefore takes two years for the light to reach earth.

This is why in the early universe, light could be emitted from an object only 4 billion light years away, yet it could take 13 billion years to reach earth. The universe was rapidly expanding, interposing additional distance during the light's travel period.

GOURDHEAD
2006-Mar-23, 03:57 AM
The curently accepted value for the "Hubble constant" is between 50 and 100
km/second/megaparsec, and is believed to be on the increase for the last 5 or so bilion years. So the Hubble constant is a variable, and we do not know how it has varied. If we assume the radius of the physical universe was one light year at the instant inflation ended and that the expansion immediately became less than one light year/year, and I think most do assume this, how large would the post-inflationary rate of expansion have to be and what is the minimum rate of its change (are step functions permitted) allowed in order for the physical universe existing today to be larger than the observable universe (light from each point available to every other point)? Does this suggest that the size of the universe immediately after the conclusion of the inflation epoch was much larger than one light year or that the paradigm is in need of even more bells and whistles thus further offending Occam? How shall we improve our guesstimates?

kzb
2006-Mar-23, 01:12 PM
<<Case 2: Same as the above, BUT rapid universal expansion is taking place. During the 1 yr travel time to earth, the fabric of space has expanded so the DISTANCE the light must travel is now TWO light years. It therefore takes two years for the light to reach earth.>>

Joema
I don't think this is correct !?
I thought the whole point of relativity is there is NO "fabric of space" ?
I also thought a fundamental principle is that light cannot be observed, by any observer, to travel at any velocity other than c (excluding density of medium effects etc).
Surely the whole point of the red-shift theories is that, in your example, the light would take 1 year to reach earth, if the distance at the time it was emitted was 1 light year. It would however be red-shifted to double the wavelength if the distance increased during its travel to 2 LY.

Any "expansion of the space" is fundamentally indistinguishable from (in your example) alpha centauri speeding away from us. I thought this was a major axiom of relativity ?

joema
2006-Mar-23, 02:50 PM
...I thought the whole point of relativity is there is NO "fabric of space" ?...
Actually it's the opposite: Einstein found there IS a fabric of space which we call spactime. It can be bent and distorted by gravity, and is stretched by universal expansion.

Here's a Cornell astronomer's explanation of why in the early universe light from a "nearby" object (say 4 billion light years back then) can take 12 billion years to reach us: http://curious.astro.cornell.edu/question.php?number=70

"It takes light 12 billion years to get to us from such a galaxy, in which time the universe itself has expanded to its current size. When the light was emitted from the galaxy, it was much closer to us than it is now, and space has been expanding ever since."

"After that distant galaxy emitted the light that we are seeing today, that light had to travel across a rapidly expanding universe before it could get to us. Like an ant trying to get to the other side of a stretching rubber band, the steady progression of the light in our direction is retarded by the stretching of space between us. So even though the galaxy was much closer than 12 billion light years at the time of emission, we're not seeing it until 12 billion years later. So we say that its distance today is 12 billion light years."

Cougar
2006-Mar-23, 04:13 PM
Due to the accelerating expansion wouldn't galaxies currently in our "horizon" be soon beyond our horizon, and anything currently beyond our horizon will be beyond it forever ?Sounds right to me. But this concept of "horizon" rather throws me. I tend to imagine a sphere around us expanding at the speed of light, with the edge of the sphere being our light horizon. But such a view apparently does not account for the finite speed of light and is therefore inaccurate. When we look in any direction, the most distant objects we can see are also the most far back in time, when the entire universe was much smaller than it is today. As technology allows us to see further, our "horizon" becomes closer and closer to the big bang - or at least to the era of recombination. This seems opposite to the (apparently) naive picture of what our light horizon is.

Relativity is certainly fundamental to an accurate model of the universe and our place within it. But it sure is mindbending....

kzb
2006-Mar-23, 06:43 PM
My brain hurts. I actually have a different interpretation of the Cornell astronomers words.

<<So even though the galaxy was much closer than 12 billion light years at the time of emission, we're not seeing it until 12 billion years later. So we say that its distance today is 12 billion light years.>>

That means to me that the galaxy is actually where we see it to be NOW, i.e. 12 billion LY distant.

However, that doesn't tie in with what I've been thinking about re spaceships flying off to alpha centauri.

If a ship is travelling away at 99.99...% c, and one light year away from earth it sends a pulse of light, it seems to me that pulse should take 1 year to reach us. (It will be red-shifted due to the ship velocity.) However, in that one year, the ship has travelled another light year away. This means we see the position of the ship as it was when it emitted the light, not as it is "now". Logically this would mean the observable limit of the universe is currently twice as far away as we see it now.

However, isn't this all interconnected? To us, the Lorentz contraction would shorten distance along the direction of travel for highly red-shifted galaxies. Approaching the observable universe limit, galaxies would become apparently squashed and also nearer together along the direction away from us.

I'm sure I read somewhere that there is no fabric to expand. I think the astronomer is saying the speed of recession is kind of equivalent to a rubber band, in that if you picture that you get the current distance correct. Actually I just can't explain what I'm trying to say, as you may have noticed.

This is my angle on the edge of the universe question: when the big bang occured, it was a point and therefore all parts could see each other. Since nothing can exceed c, no matter lost touch with other matter in the universe, and that would mean today, all parts of the universe are observable, at least in principle.

Then someone came up with inflation, where the universe was allowed to momentarily expand faster than light. This means matter lost touch and we could be in a bubble of observable universe within a bigger universe, with no way of knowing how big the universe is outside our bubble.

However, wouldn't current observations argue against the latter view? After all, we can currently observe the microwave background, which is taken as a highly red-shifted view of the plasma after the big bang. If the latter view of a bubble in a larger universe were correct, wouldn't we observe galaxies right out to the observable limit and no microwave background?

Jeff Root
2006-Mar-23, 07:20 PM
The distance most often used (by far, in anything written for
non-specialists) is the distance the light travels to reach us.
If a galaxy "is" eight billion light-years away according to a
press release, that means the light traveled a distance of eight
billion light-years to get here. It took eight billion years to
do that. When the light was emitted, that galaxy and the Milky
Way might have been seven billion light-years apart, and now,
eight billion years later, the two are ten billion light-years
apart.

Three different figures, from shortest to longest:

1) How far apart the galaxies were when the light was emitted
2) How far the light traveled
3) How far apart the galaxies are now

The angular size of an object and the amount it is dimmed by
distance are determined by the distance the light has traveled.
In other words, it looks as far away as the light has traveled.

-- Jeff, in Minneapolis

phunk
2006-Mar-23, 07:22 PM
If the latter view of a bubble in a larger universe were correct, wouldn't we observe galaxies right out to the observable limit and no microwave background?

No, the microwave background didn't come from the edge of the universe, it came from everywhere in the universe. No matter where the observable limit is, the CMB was emitted there too.

joema
2006-Mar-23, 08:21 PM
...I'm sure I read somewhere that there is no fabric to expand. I think the astronomer is saying the speed of recession is kind of equivalent to a rubber band, in that if you picture that you get the current distance correct...
Here are a few more quotes to help explain this:

Astronomy Professor Adam Hill:
http://www.mhhe.com/physsci/astronomy/uspeak/feb_02_uspeak.mhtml

"Einstein, in all his genius, recognized that Space was not just emptiness but had its own separate reality. Space and time are a kind of "fabric" of reality and like any fabric they can stretch..."

"...the Big Bang, a cataclysmic explosion that initiated the expansion of all that is. I am not kidding about the "all" here. Space was part of the explosion too. The big bang was not an explosion into space; it was an explosion of space. There was no Universe outside the Big Bang then or now. When we talk about the expansion of the Universe we are talking about all the space that ever existed. That is why we have to give up the idea that there is an "edge" where space just runs out. There cannot be anything outside of the Universe's space. It is existence! That is all. Imagine the Universe to be the stretched rubber of an inflated balloon. As the balloon is blown up, space expands but there is no edge. "Ah", you say "what about the inside and outside of the balloon?" My answer is there is no such thing. Existence is the skin of the balloon. The inside and outside are "fictions" we use to visualize the whole picture but they are not real. In the real world there is no inside or outside of space and time."

Paraphrasing physicist Brian Greene and his book "The Fabric of the Cosmos", http://www.amazon.com/gp/product/0375727205/qid=1143145044/sr=2-1/ref=pd_bbs_b_2_1/102-3377975-7027369?s=books&v=glance&n=283155,

http://www.roycecarlton.com/speakers/greene_bio.html
"Whereas Einstein's general relativity shows that the fabric of space can stretch in time (resulting in our expanding universe)..."

http://pancake.uchicago.edu/~carroll/cfcp/primer/expanding.html
"It is best to think of space itself stretching, so that the amount of space between any two distant galaxies is increasing....the universe is not (so far as we know) expanding "into" anything; it's just that the amount of space in our single universe is growing with time."

http://www.astronomynotes.com/cosmolgy/s3.htm
"with the expanding universe, space itself is expanding in three dimensions---the whole coordinate system is expanding. Our universe is NOT expanding ``into'' anything ``beyond''."

http://helios.gsfc.nasa.gov/qa_sp_ex.html
"in our expanding Universe space itself is expanding"

Ken G
2006-Mar-25, 01:33 PM
An important point to recognize, which will save some confusion, is the difference between a theory and a pedagogy. A theory is a description of reality, to some level of precision, which makes predictions that can be tested. As it deals with the results of experiments, it is independent of the coordinate system you adopt to describe the reference frames of the source and observer. However, a pedagogy, is a way of understanding a theory, and it is not unique because it does depend on the chosen coordinate system and on other arbitrary conventions. Cosmologists are rarely careful about making this distinction, unfortunately. An example of the right way to do this is in the quote from ~carroll at uchicago, who said "It is best to think...". An example of the way to fail to do this is in the other quotes, such as the last one, "in our expanding Universe space itself is expanding". That last statement is simply not a statement of reality, as it is not a unique perspective. But it is a perfectly workable picture of reality, i.e., it is the favored pedagogy for describing the standard theory of cosmology, even though it says more than does the standard theory. Personally, I rather like the alternate pedagogy where time has been speeding up and the lengths of rulers have been shrinking since the early times. This latter bit quite naturally explains why things can be farther away now (being measured by small rulers) than when they emitted the light (being measured by larger rulers back then), and why the lookback distance is in between the two (being measured by a range of rulers appropriate to the everchanging age).

kzb
2006-Mar-28, 12:17 PM
I think someone once said, there's only 6 people in the world who truly understand relativity. I know I'm not one of them, but are any of those 6 on these forums I ask myself.

I think Ken G has a good point. The way I see it, some things become kind of equivalent at cosmological scales. Time and distance at this level are probably just different ways of looking at the same thing.

Getting back to the original question, why is it so difficult to come up with a reasonable estimate of the size of the universe "horizon" ? We know the likely range of Hubble's constant, we know c to a high degree of precision and we can even estimate how Hubble's constant has changed over time.

On another thread, Ken G said the Lorentz contraction does indeed make things closer together (to us, that is) towards the horizon, BUT it is not an infinite progression, there IS a limit. So again, I would have thought that the more mathematically able would be able to calculate that the distance of that limit, or at least give a range within which it would fall.

BTW, how do we know the microwave background is the "echo of the big bang", and not due to an apparently compacted dense layer (Lorentz contraction), of highly redshifted galaxies (and/or other matter) approaching the horizon?

Ken G
2006-Mar-28, 03:10 PM
On another thread, Ken G said the Lorentz contraction does indeed make things closer together (to us, that is) towards the horizon, BUT it is not an infinite progression, there IS a limit. So again, I would have thought that the more mathematically able would be able to calculate that the distance of that limit, or at least give a range within which it would fall.

It certainly has been done, given the currently favored values of the various parameters. I think Ned Wright has a calculator for doing this, if you check around his web pages. It takes a real expert to be sure it is being done correctly, so it's best to look it up.



BTW, how do we know the microwave background is the "echo of the big bang", and not due to an apparently compacted dense layer (Lorentz contraction), of highly redshifted galaxies (and/or other matter) approaching the horizon?
The background is a perfect blackbody, and this is how we know it is not starlight or light from galactic accretion disks. It was thermalized by very homogeneous material spread all over the universe in all directions-- a 3000 K electron/proton gas. That fits perfectly into a pre-galaxy formation universe as in the Big Bang theory.

five_distinct
2006-Mar-28, 05:08 PM
Hm, maybe we won't know how big it is until we can no longer see the CMB.

kzb
2006-Mar-28, 05:47 PM
Have you seen the "..cannonball.." thread? That's a relevant question here also.

If we fire a cannonball at 90% c, will it eventually see itself at rest with respect to its local galaxies, or will it always see them flashing (sic) past ?

From your reasoning, space would be added between us and the cannonball, due to the expansion of the universe, IN ADDITION to that due to its own velocity.

This would mean that it never comes to equilibrium with its surroundings, always seeing them flashing past. It's velocity relative to its immediate surroundings remains at 90%c indefinitely.

Are you comfortable with this? I'm not, because it means the cannonball is apparently accelerating relative to us, with no input of energy. That can't be right.

five_distinct
2006-Mar-28, 09:23 PM
It's not actually accellerating though...It only seems to be accellerating because of the fact that we're expanding away from it, and it's expanding away from us.

kzb
2006-Mar-29, 05:30 PM
Ken G thank you for mentioning Ned Wright. Had a look at his web pages last night and I have to say they are the best I've seen on cosmology.

Anyone else not seen it, much recommended:

http://www.astro.ucla.edu/~wright/cosmology_faq.html#MX

ngeo
2006-Mar-29, 11:31 PM
In a question about the possibility of the CMB not being an echo of the Big Bang but emitted from objects at the horizon, Ken G wrote: “The background is a perfect blackbody, and this is how we know it is not starlight or light from galactic accretion disks. It was thermalized by very homogeneous material spread all over the universe in all directions-- a 3000 K electron/proton gas. That fits perfectly into a pre-galaxy formation universe as in the Big Bang theory.” What rules out the possibility that the 3000K electron/proton gas could be protostellar clouds at the horizon?

trinitree88
2006-Mar-29, 11:54 PM
[QUOTE=kzb]I think someone once said, there's only 6 people in the world who truly understand relativity. I know I'm not one of them, but are any of those 6 on these forums I ask myself.

KZB...that reference has to do with an old quote about Einstein's Special Theory of Relativity. When it first came out in 1905...it was estimated only about 6 people understood its ramifications immediately.
Today, there are tens of thousands of people who have a basic understanding....and some thousands who have a thorough understanding of both Special and General Relativity....several are on these boards...Grey, KenG, Tim Thompson...and many more. Ask away your questions. ;) Pete.

ngeo
2006-Mar-30, 01:15 AM
Ken G, googling "protostellar cloud hydrogen plasma" I found an old BAUT thread in which my above question was discussed. (Imagine my surprise at my own forgetfulness.) The thread diverged, and trinitree88 mentioned a talk given about it that I did not find. However your final question was "where would the electrons and protons come from?" The belated answer is that the electrons and protons are forms of rotating space (i.e. creations of an energetic field - that would be the universe, and maybe off topic). So there is no need to answer my question above unless you have a different view.

kzb
2006-Mar-30, 11:44 AM
Trinitree88 -There might well be 1000's of people who THINK they have a thorough understanding, but do they....!

<<...question about the possibility of the CMB not being an echo of the Big Bang but emitted from objects at the horizon...>>

Before I looked at the above web site, I was going to bang off a retort to the effect that your instruments must be pretty damn good to tell the difference. I'd have thought that a thick layer of populations of stars of differing luminosity, protostellar gas clouds, all at different, but high, red-shifts, would give a pretty convincing impersonation of black-body radiation at this range.

However, there is a page on the web site I mentioned above that actually DOES go into the difference and answers this very question. It seems there IS a detectable difference in the spectrum

ngeo
2006-Mar-31, 02:08 AM
kzb I used the edit "find" feature on this computer's toolbar to look for 'protostellar clouds' in Ned Wright's tutorial, and didn't find it. If you know the page dealing with detectable difference in CMB spectrum vs. protostellar clouds, perhaps you can post it.

kzb
2006-Mar-31, 11:54 AM
ngeo, you are right, in that the section I was thinking of does not specifically mention protostellar clouds. I' ve pasted in the text below sans diagrams. I'd think protostellar clouds would give many of the same difficulties as stars, as a source of the CMB ?

Can the CMBR be redshifted starlight? NO!
The CMB radiation is such a perfect fit to a blackbody that it cannot be made by stars. The reason for this is that stars are at best only pretty good blackbodies, and the usual absorption lines and band edges make them pretty bad blackbodies. In order for a star to radiate it must have a temperature gradient in its outer regions, as shown below:




Because of this temperature gradient, the light we see is a mixture of radiation from the hotter lower levels (blue) and the cooler outer levels (red). When blackbodies with these temperatures are mixed, the resulting purple curve is close to but not exactly equal to the blackbody curve (black). Of course a real star has lines as shown by the G2V (solar-like) star spectrum in green. So the purple curve is already an idealized star in which the absorption of radiation is independent of the frequency. In order to show how badly this purple curve fits the FIRAS observations of the CMB spectrum, the graph below plots just the difference between the purple curve and the blackbody, along with the FIRAS data points.



The errorbars have been multiplied by 20 so they will be visible, and the purple curve misses these enlarged errorbars by a large factor. In fact, the purple curve can by very well approximated by a kind of distorted spectrum known as a Kompaneets or Sunyaev-Zel'dovich distortion with a parameter y = 0.0062. The actual fitted value for y from the FIRAS data is y = -0.000001 +/- 0.000006, so even an idealized star with no absorption lines is 1000 times too "non-black" to fit the CMB observations.

One can easily have stars visible from different redshifts which will smooth out the absorption lines, but these stars will appear as different temperature blackbodies and it is the mixing of different apparent temperatures that causes the deviation of the purple curve. Hence more mixing will make things worse.

The gray dashed curve on the plot shows how much the fit can be improved by adjusting the emissivity or sky coverage of the stars. Normally the emissivity is 1-R where R is the reflectance, and thus the emissivity usually has to be less than 1. But the best fit emissivity is 1.09. So this would be unphysical unless the redshift is caused by tired light. In this case the apparent emissivity is (1+z)3 >> 1, and one can lower this down to an apparent emissivity of 1.09 by only partly covering the sky with stars. This requires some fine tuning, since for a typical stellar temperature of 4000 K a redshift of 1+z = 1466 is needed which gives an apparent emissivity of 3,150,662,696 that needs to be canceled by a sky coverage of 0.00000000034511 in this model. But the errors in the CMB fit are still 300 times larger than the FIRAS data allow, so this model can also be ruled out.

How does the Big Bang produce a nearly perfect blackbody CMB? In the Big Bang model there are no temperature gradients because the Universe is homogeneous. While the temperature varies with time, this variation is exactly canceled by the redshift so the apparent temperature of radiation from redshift z is given by T(z)/(1+z) which is equal to the CMB temperature To for all redshifts which contribute to the CMB.

Ken G
2006-Apr-01, 03:42 PM
In the Big Bang model there are no temperature gradients because the Universe is homogeneous. While the temperature varies with time, this variation is exactly canceled by the redshift so the apparent temperature of radiation from redshift z is given by T(z)/(1+z) which is equal to the CMB temperature To for all redshifts which contribute to the CMB.
(And indeed, the reason this relation holds is that the temperature is itself being set by the redshift. A redshifted thermal radiation field is simply a colder thermal radiation field, and that's why the temperature variation is there in the first place.)

Nonkers
2006-Apr-02, 09:42 AM
Some creationists argue for a 6000 light-year universe. Moon and Spencer for a few light years http://www.asa3.org/ASA/PSCF/1988/PSCF3-88Phillips.html
Walter van der Kamp for 60 light days. And Neville Jones for a universe light-minutes or even light-seconds in diameter!
http://www.geocentric-universe.com/page49.htm

All have been comprehensively refuted (I hope!).

kzb
2006-Apr-03, 11:49 AM
Was there not something about multiple images of the same galaxies? A couple of years ago the idea was reported that the universe is a lot smaller than current theories. The size is an illusion caused by light going round in circles, so we see multiple images of the same galaxies. I seem to remember a few candidate images being put forward to support this idea at the time.

Ken G
2006-Apr-03, 11:55 AM
It is an idea that circulates from time to time, but has no support, and it certainly would by now unless the wrapping happens at scales not much different from what is already the whole observable universe.