Please explain the statement "the big bang happened everywhere at once"

[Bandersnatch]

just a further question about the above. what he describes was a reviewed theory which was disputed or rejected. correct? so wouldn't that be actually discussing a "theory" and not simple speculation are we to never discuss subjects that have been rejected even unknowingly?

According to the guidelines:

Non-mainstream theories:
Generally, in the forums we do not allow the following:
Attempts to promote or resuscitate theories that have been discredited or superseded (e.g. Lorentz ether theory); this does not exclude discussion of those theories in a purely historical context

https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

 

[dragoneyes001]

what i meant by unknowingly is not knowing the theory has been rejected/refuted.

 

[phinds]

what i meant by unknowingly is not knowing the theory has been rejected/refuted.

What are you arguing about? Nobody got in infractions on this, it was simply pointed out that the "theory" he mentioned does not fit the forum guidelines. If one doesn't know that a theory is inappropriate to discuss here because of the guidelines, does that then make it appropriate just because one didn't know?

 

[dragoneyes001]

why would you call asking for clarification arguing?

to answer your question: does it make them wrong or in breach of the guide lines if they didn't know the theory was refuted and did not promote it as fact but inquired if it was a valid theory?

 

[phinds]

why would you call asking for clarification arguing?

Fair enough. OK, to clarity: the fact that one does not know that a theory is inappropriate makes it (marginally) OK to have posted it but one should expect to be called on it by the mods (and possibly by other nitpickers like me) and possibly to have the post deleted.

 

[dragoneyes001]

i think you posted before i edited the above post please scroll up so there is no confusion.

 

[phinds]

why would you call asking for clarification arguing?

to answer your question: does it make them wrong or in breach of the guide lines if they didn't know the theory was refuted and did not promote it as fact but inquired if it was a valid theory?

It puts them in breach of the guidelines, exactly as one would be held liable for breaking a law. Ignorance of the law is no excuse. Now, if it's clearly an innocent breach, it's not likely to elicit anything more than having it pointed out that it is inappropriate, which is exactly what happened in this case.

We do occasionally have folks who insist on continuing to promote inappropriate theories but they don't generally last long unless they are willing to see the error of their ways.

 

[dragoneyes001]

thank you that's all i was asking.

 

[phinds]

thank you that's all i was asking.

Yeah, sorry if I got a bit snippy. Greg pays me to do that because by setting such a bad example, I make all the mods look good.:p

 

[dragoneyes001]

now to get back to the subject at hand is there a layman's way of explaining the universe was in a" hot dense state"

 

[phinds]

now to get back to the subject at hand is there a layman's way of explaining the universe was in a" hot dense state"

Hm ... that seems like a very straight-forward phrase to me, although it IS a bit of an understatement. "Hot" hardly begins to describe it. Are you familiar with the CMB? At the time, about 400,000 years after the singularity, there occurred the "Surface of Last Scattering", which we now see as the CMB, which is current about about 2.7degrees C. at the time of the SLS, it was 1000+ times that much, but this is downright cold compared to things earlier on.

I refer you to Steven Weinberg's "The First Three Minutes".

 

[dragoneyes001]

I was more aiming at the Dense part since it seems to be the most contentious part.

i have read about cosmic microwave background enough to understand its relation to the early part of development.

 

[Drakkith]

I was more aiming at the Dense part since it seems to be the most contentious part.

i have read about cosmic microwave background enough to understand its relation to the early part of development.

Well, since the universe is currently expanding, then that must mean that is was denser in the past than it is now. Extrapolating backwards using known physical laws, we find that the early universe consisted of a very dense, very hot plasma. The further backwards in time we extrapolate, the denser and hotter the universe becomes. The earliest, most dense, and hottest periods reach energy and density scales beyond our current knowledge, so we know very little about them.

We expect that at the temperature and density of this very early period, the matter in the universe consisted of a quark-gluon plasma, or even something more exotic.

 

[dragoneyes001]

I think this is where things get either misinterpreted or muddled. because as I'm understanding it the universe was smaller and then expanded but as has been said that would create a center from which everything is expanding away from which if I'm not mistaken is not the case right?

 

[Drakkith]

I think this is where things get either misinterpreted or muddled. because as I'm understanding it the universe was smaller and then expanded but as has been said that would create a center from which everything is expanding away from which if I'm not mistaken is not the case right?

The observable universe was smaller, not the whole universe. When talking about the universe as a whole, it usually better to think in terms of density rather than overall size or volume. An infinite universe can still contract or expand and it will remain infinite in size. The contraction/expansion happens everywhere, and all observers would see themselves as standing still while everything moves away from or towards them.

 

[Khashishi]

Maybe the timeline is simply an open set: (0,infinity) and not a closed set [0,infinity). The limit point of the expansion simply isn't part of the model.

 

[PeterDonis]

The limit point of the expansion simply isn't part of the model.

This is correct; the initial singularity is not part of spacetime.

 

[dragoneyes001]

another question then: if the big bang is about the "observable" Universe which seems the case from the previous explanation. is the unobservable void which is being called infinite changing in relation to the observable?

 

[phinds]

another question then: if the big bang is about the "observable" Universe which seems the case from the previous explanation. is the unobservable void which is being called infinite changing in relation to the observable?

The BB is NOT about the observable universe, that's just, by definition, the part that we can see. There are things that are currently just outside the OU that will move INTO the OU at some point in the future and there are things in the OU that were not in the OU 10billion years ago.

Perhaps Marcus will jump in here. He can explain that much better than I can.

 

[Chronos]

Phinds, I disagree. We already 'see' the CMB. What do you expect to enter our observable horizon from beyond that?

 

[phinds]

Phinds, I disagree. We already 'see' the CMB. What do you expect to enter our observable horizon from beyond that?

I *THINK* that I'm quoting Marcus on this. I'll PM him and see if we can get him to chime in. I may be misrepresenting what he has said but I don't think so.

 

[bapowell]

Phinds, I disagree. We already 'see' the CMB. What do you expect to enter our observable horizon from beyond that?

More CMB.

 

[phinds]

More CMB.

That's not what I'm referring to. Marcus has shown (unless I'm badly remembering his posts) that objects (e.g. galaxies) just slightly outside the observable universe will move into the OU over time.

 

[bapowell]

That's not what I'm referring to. Marcus has shown (unless I'm badly remembering his posts) that objects (e.g. galaxies) just slightly outside the observable universe will move into the OU over time.

Sure, except for those currently outside the cosmological event horizon. Recall that the particle horizon is currently inside the event horizon, so there are galaxies -- those that are in between the two horizons -- that are not currently observable but one day will be.

 

[marcus]

another question then: if the big bang is about the "observable" Universe which seems the case from the previous explanation. is the unobservable void which is being called infinite changing in relation to the observable?

Hi dragoneyes, partly you are asking about the technical meaning of the words. In cosmology-speak, the observable universe contains all the matter from which we could have already, in principle, received some signal. So it is a constantly growing region of the universe. Not only is it expanding by ordinary distance growth but it is also enlarging as news from more and more distant matter comes in. We don't have any evidence of a "void" outside.

The BB is NOT about the observable universe, that's just, by definition, the part that we can see. There are things that are currently just outside the OU that will move INTO the OU at some point in the future and there are things in the OU that were not in the OU 10billion years ago.
...

That sums it up really really well. We are simply getting straight on what cosmologists MEAN when they say "observable universe".

 

[PeterDonis]

it is a constantly growing region of the universe. Not only is it expanding by ordinary distance growth but it is also enlarging as news from more and more distant matter comes in.

At some point, though, if the expansion of the universe continues to accelerate, the latter effect will reverse, won't it? That is, as the expansion accelerates, eventually objects that are now inside our observable universe will move outside it, because they will pass beyond the cosmological event horizon. So the OU will continue to grow in terms of distance, but it will eventually start to "shrink" in terms of how many objects are in it.

 

[marcus]

Sure, except for those currently outside the cosmological event horizon. Recall that the particle horizon is currently inside the event horizon, so there are galaxies -- those that are in between the two horizons -- that are not currently observable but one day will be.

Brian, you're the professional so I'm worried that I may be using the words "cosmological event horizon" wrong. I hope you can help me get this sorted out.
I think the distance today to the CEH is about 16 billion LY. We may be observing a galaxy that is beyond that but if we send a message to them TODAY it will never get there. They are out of causal reach. If they do something today it will never affect us, if they have a supernova we will never see it.

I think that the particle horizon distance as of today is about 46 billion LY. The matter we are currently getting CMB radiation from is now about 45 or 45.5 or something, but the actual particle horizon is a bit farther because we could, in principle have received light from slightly more distant stuff except for the opacity/glare. Or received neutrinos...etc.

I think in terms of today's distance (treated as a non-expanding label on matter, a so-called "comoving distance" label that doesn't change as the U expands) that there is a limit to the particle horizon which is as I recall something like 63 billion LY.
If some matter is, today, farther than 63 billion LY then we will never get light from it, even light that it emitted way back in the past. That LIGHT is today still outside the 16 billion LY CEH range. So it can never reach us.

The figures are just approximate. I'm using numbers for concreteness sake. Dragon eyes might want to look up "proper distance" and "comoving distance" and maybe "cosmological event horizon". I don't know if wikipedia has an entry on all or some of those.

It seems to me (and I could be wrong) that I've always heard the term CEH used to refer to that 16 billion LY distance and never to refer to the 63 billion LY comoving distance which is the limit towards which the particle horizon (currently 46 or so) is tending. Some of this stuff is in the bottom panel of Lineweaver's Figure 1 here
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg
You can see that what he calls "event horizon" is about 16
and the ultimate limit (time = "infinity") of what he calls "particle horizon" is about 63.

Of course in the long run though we will still be receiving light from all those (then dead) galaxies the light will be too redshifted for us to detect it. that's the breaks astronomy peters out

 

[CaptDude]

I think this is where things get either misinterpreted or muddled. because as I'm understanding it the universe was smaller and then expanded but as has been said that would create a center from which everything is expanding away from which if I'm not mistaken is not the case right?

The above question is one that I have been comtemplating as well, and dragon eyes phrased it exctly as I would have.

I also have another question which is as follows. (and like a lot of my questions, I know this one is not correct. But the answers will help me get to where I am trying to go): The big bang created time, space and energy/matter. Space/time was expanded through the process of inflation to a size (either finite or infinite) far larger (hence the unobservable universe) than signal emitting sources of matter. (the observable universe)

 

[marcus]

At some point, though, if the expansion of the universe continues to accelerate, the latter effect will reverse, won't it? That is, as the expansion accelerates, eventually objects that are now inside our observable universe will move outside it, because they will pass beyond the cosmological event horizon. So the OU will continue to grow in terms of distance, but it will eventually start to "shrink" in terms of how many objects are in it.

Hi Peter, the third panel of Lineweaver's figure 1 is potentially so helpful here I will try to copy it into a post. I can't think of any reason that the OU should "shrink" in terms of the amount of matter it contains. the matter could get rearranged in ways that would change the count of "how many objects". But I don't think that is what you mean. I think you mean the matter it contains, in whatever form.
To first approximation it's good to think of the matter as not moving, because its local motion is so slow compared with c that it barely counts at all. If you think of stuff at the outer limits of the observable universe today, the distances to that stuff are increasing at a rate of about 3c. Any change in the comoving distance to that stuff presumably would be negligible by comparison---a few hundred km/s (like our own galaxy's speed relative to CMB).

Comoving distance to a bit of matter essentially does not change. It is a permanent label on the matter. So the increase in the particle horizon (in comoving distance terms) is a good way to track the increasing amount of matter in the OU. I'm worried that I may not be able to reproduce Lineweaver's figure. It is also in that article by Lineweaver and Davis "Expanding Confusion" that people frequently refer to. I'll see if I can do a copy/paste.

 

[marcus]

...the universe was smaller and then expanded but as has been said that would create a center from which everything is expanding away from which if I'm not mistaken is not the case right?

Your last statement is correct. It is not the case. Expansion does not necessarily "create a center"

The above question is one that I have been comtemplating as well, and dragon eyes phrased it exctly as I would have.

You could look at some of the Cosmology FAQs or at the "balloon model" sticky thread. Or Phinds has a balloon model webpage, the link is in his signature.
The key to understanding the balloon analogy is to accept that, in the analogy, all existence is 2d and concentrated on the 2d surface of the balloon. A 2d creature living on the balloon could not point his finger in any direction off the balloon. There is no inside, there is no outside, there is no "center" of the expansion.

In terms of latitude and longitude every galaxy (white dot) painted on the balloon surface stays in the same place. They all just get farther apart. Nobody GOES anywhere.
In that universe there is no center "from which everything is expanding away".