Picture of galaxy from when expansion only 600 million years old

[michel123456]

Michel - what do you think the CMBR is?

 

I don't think my post was un-intelligible (but then I wouldn't - its very hard not to understand yourself no matter how confused one's witterings are). I really think we might be able to get the nub of why we disagree - I think your ideas of the actuality of the CMBR are incorrect. The fact that you used the phrase "smaller CMBR", and I think meant just that, makes me sure of it. Sisy's diagram is quite understandable.

 

There is no 'present time" in any of those diagrams. What each observer sees is in the past. Once you talk about distance, you talk about time as well. There is no distance without time, and there is no time without distance. And I still consider there is a problem in Sisyphus's diagram. It may be understandable (i did understand what he meant, until I compared with Spyman's).

 

_look: 2 observators in the same present cannot observe each other: they can only observe each other's past. It is a basic of spacetime. The Alien waving hand on UDFy-38135539 cannot see us: we are in his future. He is waving to another planet in his past. And in order to see planet earth, another distant galaxy must be in our future: they can see us (supposition), we can't see them.

 

Don't ask me what I think CMBR is. We won't agree. For me it is a horizon: each observator no matter his place, no matter his time, will observe the same CMBR., or in other words, CMBR is relative to the observator. For all of you it is an event that happened at a specific time after the BB. It is an absolute event that happened "everywhere" at the same time. And I am trying desesperatly to show the problems that arise from considering CMBR as an absolute event. Shoot me.

 

You disagree with the "small CMBR". It is not my invention.

from post #46 (sketch missing, too bad)

 

 

Here. In the early universe, light has only had time to travel from A to Earth (making that the perceived location of the CMBR), and everything is close together. In the present universe, light has had time to travel from D to Earth, and everything is farther apart. Where is the problem?

 

from post #56

(..)This radiation has continued to travel as the universe has expanded. The radiation that was given off by the area which will later contain sol and earth has been travelling for 13.ish billion years away from us. the radiation we can currently detect has been travelling for 13.ish billion years towards us. 1 billion years ago - an observer near sol would have seen the radiation that had travelled for 12ish billion years, it would have been less red-shifted and coming from a nearer point of the universe.(...)

 

Emphasis mine.

I understand that, following BBT, CMBR appears small (small radius) for objects close to it (in time), and large for objects far away from it(in time).

Edited November 5, 2010 by michel123456

[Sisyphus]

There is no 'present time" in any of those diagrams.

 

Yes, there is. This seems to be the basis for many of your issues: some kind of weird aversion to the idea of simultaneity.

[Martin]

Don't ask me what I think CMBR is. We won't agree. For me it is a horizon: each observator no matter his place, no matter his time, will observe the same CMBR., or in other words, CMBR is relative to the observator. For all of you it is an event that happened at a specific time after the BB. It is an absolute event that happened "everywhere" at the same time. And I am trying desperatly to show the problems that arise from considering CMBR as an absolute event. Shoot me.

 

In normal cosmology talk, the CMBR is not a horizon. It is the background radiation filling the whole universe (approximately uniformly) that arose from a definite event that occurred some 380,000 years after the start of expansion, universe time---the measure of time built into the standard model used by cosmologists.

 

This diagram looks OK to me except that the usual term for the red circle is "surface of last scattering". The CMBR is radiation. It is not a place. It is not an event.

It arose from an event, which we can date reasonable confidence---we can say when (in standard model time) it occurred.

And we can say where the matter now is which gave rise to the radiation which we now see, so in that sense there is a place (at present, at this moment in universe time). The place where the matter is whose light we are now receiving as CMBR, that place is called "surface of last scattering". That's what's shown in the diagram, with the approximately correct radius.

 

This diagram depicts the present moment (in standard cosmology model, or universe time).

The distance shown is in a measure called "proper distance" which is used a lot by cosmologists. It has another name. Proper distance at the present moment equals what is called "comoving" distance, but let's just call it proper distance. The proper distance is what you would measure by radar, or timing light signals, or using meter sticks, if you could STOP EXPANSION at the moment in question. Then you would have enough time to measure and distances wouldn't be changing on you. The diagram shows that our surface of last scattering is about 45 billion LY from us, which is about right. Estimates can vary.

 

I'm curious to know if the attachment diagram from Sisyphus' post will come out, so I'll post this and see.

 

As far as I can see in this discussion to the best of my understanding Michel is not using standard professional cosmo ideas and other people are pretty much right and trying to straighten him out. I don;t want to argue, Sisyphus is staff and can do whatever moderation is needed, if any. So I will try to stay out. Wikipedia has stuff on ordinary cosmology, like Friedmann equations, FRW model (Friedmann Robertson Walker model). It's probably OK. If anybody wants. Wikipedia probably has stuff on "surface of last scattering" and CMB.

Edited November 6, 2010 by Martin

[michel123456]

Sorry Martin for not using the right terminology.

I am seriously trying to find a grip somewhere, a base you would accept for you to consider my arguments, at least a little bit.

 

Insisting on Sisyphus, basis on standard cosmology terms:

On the left; the Earth. (call it "E")

On the right, the Observer 45 billions LY away (call it "O").

E & O are on mutual simultanate present time, as stated by Sisyphus who made the diagram.

 

The red circle around the Earth is supposed to represent CMBR from Earth's perpective.

Michel is asking: how is it possible that O stands on E's CMBR? If O stands on Earth's CMBR, then O is in the past of E, not in the present of E, as originally stated.

Edited November 6, 2010 by michel123456

[Martin]

Sorry Martin for not using the right terminology.

I am seriously trying to find a grip somewhere, a base you would accept for you to consider my arguments, at least a little bit.

 

Insisting on Sisyphus, basis on standard cosmology terms:

On the left; the Earth. (call it "E")

On the right, the Observer 45 billions LY away (call it "O").

E & O are on mutual simultanate present time, as stated by Sisyphus who made the diagram.

 

The red circle around the Earth is supposed to represent CMBR from Earth's perpective.

Michel is asking: how is it possible that O stands on E's CMBR? If O stands on Earth's CMBR, then O is in the past of E, not in the present of E, as originally stated.

 

I don't think your terminology problem is that you have the same ideas as the others but merely call them by different names. I think your underlying set of ideas is different.

When there is a long argument that doesn't get anywhere, it often arises from some language problem---mismatched terminology, incongruent concepts.

 

How about this? You totally stop arguing and make a determined effort to understand what I'm trying to communicate.

 

The CMB is not something in the past, or something somewhere else, it is a soup of photons all around us that we swim through. CMB photons, on a per cubic meter basis, are vastly more numerous than any other species of matter or radiation.

 

If you move fast enough in some direction relative to that soup, you will experience a doppler hotspot in the direction of motion. Out away from sources of microwave noise I guess a few meters per second should be detectable, certainly one km/s would be detectable with current instruments.

 

Cosmology (the professional topic) comes with an ideal universe-wide concept of rest, that we can know approximately. An observer is at rest if he can detect no doppler dipole in the surrounding soup of CMB radiation. The ideal is precise out to about 1/1000 of a percent as I recall. It is limited by the failure of the CMB to be perfectly uniform.

 

Cosmology also comes with an idea of the universe-wide present moment----universe time basically corresponds to the temperature of the soup, a measured anywhere in the universe by an observer who is at rest.

There will be minor adjustments depending on the observer's depth in grav. potential----think of him as outside of any major concentration of matter. All the stationary observers who measure the same CMB soup temperature belong to the same instant of universe time.

 

You can also say that universe time corresponds to the age of the universe as measured by all the stationary observers all over. Practically that is approximate because there is uncertainty---we say about 13.7 billion years. If somebody in another galaxy also says about 13.7 (after converting units) then he and I are approximately in synch, belong to the same moment, approximate epoch.

 

So for practical purposes in cosmology we can talk about NOW.

 

OK. WHAT SHALL WE CALL THE MATTER WHICH. BACK THEN, EMITTED THE CMB SOUP WHICH IS AROUND US NOW?

 

And what shall we call the PLACE WHERE THAT MATTER NOW IS? AT THIS MOMENT.

 

That matter will by now have condensed into galaxies and stars and planets, just like our matter has. So it will not look like the hot gas that it once was. That matter, in the present moment, is what Sisyphus depicted in red, as a huge sphere around the earth. It will now consist of galaxies, cold rubble, dustclouds, the usual stuff. It will have a certain thickness. Basically a hollow shell consisting of the usual stuff we see around us. It was once glowing hot gas. But somewhere in that shell there is no doubt a planet with astronomers on it who study the CMB soup. Sisyphus put a DOT on the shell for THEM as they are in the present moment.

 

And he also drew the same kind of huge spherical shell around THEM. Which is the matter, as it is now, which, back then, emitted the radiation which is REACHING THEM AT THIS MOMENT. It is a symmetric situation. And obviously we are on that shell. Do you find anything confusing so far? If not, maybe I should get back out of this thread. Truly, several people in thread already can explain this quite well, judging from their comments so far.

[michel123456]

That matter, in the present moment, is what Sisyphus depicted in red, as a huge sphere around the earth. It will now consist of galaxies, cold rubble, dustclouds, the usual stuff. It will have a certain thickness. Basically a hollow shell consisting of the usual stuff we see around us. It was once glowing hot gas. But somewhere in that shell there is no doubt a planet with astronomers on it who study the CMB soup. Sisyphus put a DOT on the shell for THEM as they are in the present moment.

 

Emphasis mine on our point of divergence. This point is in the past, not in the present time.

I quit.

[Martin]

Then hopefully that part of the discussion is over. In case anyone else might be interested in the galaxy that was the main thread topic, you might want to check its recession rates (both then, when it emitted the light, and also now) or its distances from us then, and now.

 

I suspect that many here (several, anyway) have used the "cosmo calculator" at Wright's website, but in case you haven't and want to, google "cosmo calculator".

 

Then type in 8.6 for the redshift, over on the left. When the calculator comes up it has a "3" in the redshift box, as an example. You need to replace that 3 with whatever redshift you want. And press "general".

 

For redshift 8.6, should say the presentday distance is 30.384 Gly. For convenience I will round off---and say 30.4 billion light years, the distance NOW.

 

You may also want the distance back THEN when the light we are now getting was emitted by the galaxy. In the case we're considering that corresponds to "angular size distance", which the calculator will say is 3.16...Gly. In other words the distance THEN was 3.2 billion light years.

 

That is what you would have measured if you were alive back in year 600 million, and could have stopped expansion---frozen the expansion process---and measured by any normal means like timing a light signal or a series of radar ranging, or a humongous steel tape measure.

 

=====================

 

There is another calculator called "cosmos calculator" (try googling that) which if you first input the same three parameters that Wright uses----over on the left put in .27, .73, 71, and then whatever redshift---gives roughly the same answer. That one also gives the recession rates then, and now.

 

As a check, I will tell you what I get from that one, when I put in the usual parameters (.27, .73, 71) and then redshift 8.6. Then you can see if you get the same thing---and so are using the calculator right.

 

I get that the distance NOW is 30.38 billion lightyears and the rate of recession is 2.2 times the speed of light.

I get that the distance THEN was 3.16 billion lightyears and the rate of recession, back then, was 3.55 times the speed of light.

 

You can see that as long as it is primed with the standard 3 parameters---same was what the other calculator uses without needing to be told---this one outputs essentially the same distances.

The main difference is that it also gives the recession rates.

Edited November 7, 2010 by Martin