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

[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".

 

[marcus]

Ned Wright's little computer animation movie of the balloon analogy was very useful, but it was in Java and my computer won't get it now. Do other people still get the animation? If not, is there a substitute you can give a link to?

 

[marcus]

Here's Lineweaver's Figure 1 (link in my signature) You can see in the third panel that the Hubble radius 14.4 Gly is now almost out to the cosmological event horizon of around 16 Gly.
Find the "now" line in the 3rd panel, which divides the past from the future. The horizontal scale is comoving distance.

 

[bapowell]

Marcus, you are absolutely correct. I was standing on my head when I wrote my previous post.

 

[marcus]

Then we have something in common, I am often standing on my head when I write posts. And my wife calls me to help get supper ready just as I was about to read it over to catch bloopers. :w

 

[dragoneyes001]

one more clarification on observable: this is not meaning "we can see at this time from point A our location to point J the furthest we can currently "observe" and that in twenty years with new methods we'll be able to see beyond point J. but means anything which has an Observable trait in the universe regardless of our current ability to see it.

 

[Bandersnatch]

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.

Let me jump in with a less sophisticated answer:

First, one has to understand that cosmology uses the assumption of isotropy and homogenity (the cosmological principle). In plain terms, this means that we assume the universe beyond what we can observe looks more or less the same and has the same physics. It's the assumption that we are not a special dimple on something larger and weirder, in the same way as e.g., a person on Earth may assume that beyond the horizon there is just more of the same, and not some strange realm of gods with elephants standing on turtles or whatnot.

With this assumption, observations of what we can see allow us to make informed guesses about what we cannot see.

Next, let's talk about what it means to "be smaller"(or larger). Take any two points at some time t and measure their separation. Next, measure the distance between the same pair of points at some other time. If you find it to be shorter, then the distance got smaller (obviously). Now, if you can pick any two points constituting whatever object you're interested in, and find the same thing going on - no matter what distance you measure, then you have to conclude that the whole thing got smaller.
Note that you don't have to "step out" and look at it from "outside" to reach that conclusion. A microbe living in an adolescent's bloodstream could realize (if it could think) that the host is growing merely by noting that it takes longer and longer to travel between the same organs.

So, with our observable universe, we can look at how distances between various points in it change, and see that they all grow with time, and were shorter in the past. Using the cosmological principle, we conclude that this is true for all points and all distances, including those beyond our observable universe - regardless of whether the actual totality of the universe is finite or infinite.

Bottom line: the universe used to be smaller in the past, because all the things were closer together back then. It doesn't necessarily imply there being any sort of centre.

 

[Drakkith]

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.

I don't particularly like thinking of the big bang as a single event which "created space/matter". The big bang was more like a process, where the very, very dense and hot matter and radiation of the early universe expanded and cooled, eventually leading to the universe we see today.

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)

Actually, the dense plasma of the early universe is the main reason we have both an observable and unobservable, or at least it's the reason the observable part is the size it is today. Prior to about 380,000 years after the big bang, the universe was filled with a plasma, which is very, very good at absorbing electromagnetic radiation. Light couldn't propagate until the plasma cooled off and the protons and electrons suddenly combined during a process called recombination, becoming neutral and allowing light to pass through unabsorbed. What we are seeing today is light that has been allowed to propagate through the universe after recombination. What may have happened in the universe prior to this is unknown. Even if the universe existed for an infinite amount of time prior to recombination we wouldn't know it because we can't see it.

I believe the rapid expansion of inflation would only lead to severe redshifting of incoming light, not prevent us from seeing it in principle. (In the absence of the hot plasma)

 

[phinds]

What may have happened in the universe prior to this is unknown

Stephen Weinberg would seriously disagree w/ you since you have just called his entire book "The First Three Minutes" nothing but guesswork.

 

[julcab12]

Well. Guesswork is bit of an understatement. Scientific modelling, perhaps!?. Theirs a lot of things to 'play' with-- all those freedom/free parameters. Some are based on simple principle like multiple point toppled with entanglements (talking about dealing with strange constants) and others just plain dynamics-- bounce, rebound, crunch.. And finally, illusory(sort of) -- Holographic.

It might not be agreeable to most empiricist but so does most of the predictions in the past. = Happy theoretical physicist..

 

[Drakkith]

Stephen Weinberg would seriously disagree w/ you since you have just called his entire book "The First Three Minutes" nothing but guesswork.

Perhaps I had a poor choice of words then. Who am I to contradict Stephen Weinberg, after all.

 

[Chronos]

Pardon the intrusion, but, what galaxies currently 'trapped' between horizons can be expected to someday enter the observable universe? I perceive a causal disconnect here.

 

[Monsterboy]

http://m.space.com/27852-dark-energ...pr=17610706465&cmpid=514630_20141206_36802717
This article claims that the universe expansion might be slowing down ? Dark matter is the source of dark energy and once dark matter disappears , the universe might stop expanding.

I don't understand why they say universe will become a dull place in the article , if the universe stops expanding eventually all the universe will become observable right ? Stars will continue to fuse hydrogen right? We may not have galaxies though .

 

[Monsterboy]

I read that dark matter is that which is holding galaxies together , is it possible that dark matter doesn't really exist and that our understanding of gravity is incomplete ?

 

[phinds]

I read that dark matter is that which is holding galaxies together , is it possible that dark matter doesn't really exist and that our understanding of gravity is incomplete ?

Alternate theories of modified gravity (Google "MOND") are old hat. Some folks like them but they do not do a good job of explaining the universe. Google "the bullet cluster", for example.

 

[Chronos]

Of course it is possible that neither dark matter or dark energy truly exists. The problem is they remain the best explanation for the many different observations we have accumulated over the history of science. Until better explanations emerge, they will remain so.

 

[CaptDude]

Of course it is possible that neither dark matter or dark energy truly exists. The problem is they remain the best explanation for the many different observations we have accumulated over the history of science. Until better explanations emerge, they will remain so.

Its good to hear someone with your pedigree make this statement. I for one, have had a long standing problem with dark matter.

"Any intelligent fool can make things bigger and more complex... It takes a touch of genius --- and a lot of courage to move in the opposite direction." - Albert Einstein

 

[Drakkith]

Its good to hear someone with your pedigree make this statement. I for one, have had a long standing problem with dark matter.

Are you expecting something less revolutionary to come along and explain our observations?

 

[Ken G]

Good point, I think we already know it is going to be something revolutionary, so those who basically say, "dark matter is too radical for me to believe" are probably missing the boat altogether. If dark matter is wrong, it will likely be because it is not revolutionary enough to just say "hey, invisible matter could work", we might need to go even deeper to the foundations of what we think is true. Hopefully, dark matter is radical enough, too much foundation shaking can be distressing.

 

[Blackberg]

Obviously, I can understand the literal meaning of the phrase "the big bang happened everywhere at once" But I have never read a satisfactory explanation that eloquently helped me understand this concept...

How could it be otherwise? What's the alternative?

 

[bahamagreen]

No edge means just that. No edge. An edge implies a center and a center implies a preferred frame of reference and we know empirically that such does not exist.

Your statement about the ruler seems to imply that a MEASURE of distance changes over distance and/or time but that is false. The distance between things in the early universe was less than it is now, but the MEASURE of distance has not changed.

I thought he was including the relativistic length contraction... the farther we look, the faster things are receding, so their longitudinal diameters are seen as contracted, as is the space between them, as would be the meter sticks they hold out there to measure things... wouldn't the measure of distance used locally at those distant objects appear to have changed (our view would be that their meters are (were?) contracted compared to ours), right?

Also, concerning the "edge", although we would see the distant objects as approaching the edge, likewise they would see us as near the edge - but all observers would see themselves in the center. Looking to the edge (any and all directions) would be looking through concentric layers of shells each locally measured to be he same thickness, but whose thicknesses every observer would view as contracting with distance and approaching zero at the limit... this would be the case for all observers.
An edge only implies a center if the measure is constant; if the measure seen at a receding distance is smaller and approaching the limit, there is no center, all points of observation are equivalent.
This allows an infinite amount of space in a finite volume... the "curvature" is an observed contraction of measure and increase in density approaching the edge as viewed from everywhere, anywhere.

With getting to speculative, would this make the edge a singularity? Or a horizon? Or even appear like an event horizon (inside-out BH)?