Age versus Size of the Universe

Naty1

Marcus:

I have no problem with the assumption that matter is uniformly distributed within the observable universe. .... However, it seems speculative to assume that this local density and uniformity is repeated over the whole universe.

I am not so sure as Marcus about the last sentence....the 'speculative' part.... I note that each day we receive new cosmic background radiation,CMBR, from which at the time of its origin the early seeds of the universe were present, and those signals appear with the same characteristics as yesterday's, and the day before and the day before that and.....so forth. So from what Marcus has taught me I conclude as the observable universe grows each day, it exhibits the same radiation characteristics as the prior day and that at least suggests that there is nothing unusual just beyond yesterday's observations. So I expect tomorrows CMBR
will be just like todays.

Chronos:

It could even be infinite.

Another way to express this, I think, is that an infinite universe cannot so far be ruled out.

timmdeeg

Quote by Johninch

The consensus is that the universe is flat or nearly flat. If it's flat, it will not have an edge or boundary, but the problem for me is that it would be infinitely large, according to the generally accepted model of a flat universe.

If it's nearly flat, it will be huge and presumably this is the reason for the discussion about it's size, because nobody can say how huge it is. .

Let's assume that the universe is exactly flat and obeys the FRW model. Flat then means locally flat because nothing else we are able to measure. However the model says that an arbitrary oberver will agree to that regarding his local position in the universe. Thus globally viewed the curvature is constant, well in accordance with the cosmological principle one of the consequences of which is that there is no center and no boundary.

Still, even being exactly flat leaves the global structure of the universe, it's topology, open. In case the universe has a compact topology, e.g. a 3-torus which is flat locally, then it's size is finite. Some date indicate this possibility.

If it's nearly flat, we know less.

marcus

Hi John, I'm more of a cosmology-watcher than a participant. I follow developments as best I can, and love the subject. This is my personal perspective. To answer the part I highlighted in blue I would say that cosmology is a mathematical science i.e. it aims at the simplest best-fit mathematical model. And GR is our law of gravity and our law of geometry (why triangles add up to approximately but not exactly 180 and how matter affects this) that we base the cosmic model on.

Someday GR will be improved but for now it is the accepted dynamics of geometry that has been tested repeatedly in many ways and passed all the tests and has proven remarkably accurate.

So the key thing to realize is that cosmology is an APPLICATION of the accepted law of gravity/geometry. The aim is not merely an ad hoc model that fits the observational data- it is to find the simplest bestfit model which is a solution of the GR equation.
The simplest best-fit model which accords with the currently accepted dynamics of geometry and matter.

Quote by Johninch

I consider that the universe is everything which exists. The universe includes all baryonic matter, dark matter, dark energy, dirac space and anti-matter. There is nothing beyond the universe because nothing does not exist.

I have no problem with the assumption that matter is uniformly distributed within the observable universe. If we assume that the total universe is several orders of magnitude larger than this, then a locally uniform distribution of matter would seem to be a reasonably likely local phenomenon. However, it seems speculative to assume that this local density and uniformity is repeated over the whole universe.

I interpret your remarks concerning the irregularities in the CMBR as meaning that although we interpret those irregularities to result in the formation of matter and galaxies, we nevertheless assume that the expansion of the universe has always proceeded at an equal rate for all galaxies everywhere. This would give us the even distribution. Please check: I previously understood that some galaxies are receding much faster than others, which would give the universe a non-uniform distribution and a non-spherical shape. But now I understand that the difference in the calculated speed of distant galaxies is only (mainly) due to distance and time, right?

You apparently misinterpreted my post concerning the boundary. I was referring to the boundary of the universe and not the boundary of the part occupied by matter, baryonic or otherwise. That is to say, outside my imagined boundary there is not anything. The imagined situation is the following:

I am magically and immediately transferred from here to a planet on the edge of a galaxy where I can only see the local milky way plus other galaxies on one side of the sky only. All of those galaxies appear to be receding at a very fast and accelerating rate. On the other side of the sky I do not see anything at all. It is totally dark to my eyes and to all of my very advanced instruments. I fully understand BBT. What do I conclude?

Here are my answers:
-My planet is on that boundary of the galaxy farthest from the rest of the universe.
-My galaxy is at a boundary of the universe, beyond which there is no more universe.
-As I do not observe anything in the “dark” direction, I have to assume that there is absolutely nothing there.
-To make sure, I send a rocket up. It performs “normally” but the onboard instruments do not detect anything (except itself).

What’s wrong with that? Is there a reason why the imagined planet cannot logically exist?

.

I suppose that LOGICALLY you can send a rocket probe into non-existence and you can imagine a region of nothingness and a region of existence and a boundary between them. I can't think of a LOGICAL reason not to imagine a rocket going thru the boundary and its onboard sensors not detecting anything except its own existence.

But I don't think that one can MODEL that situation with a SOLUTION OF THE GR equation. A region of non-existence is not a feature of our currently accepted dynamics of geometry and matter, which we are pretty much stuck on using as the best available so far.

Now to respond to the blue highlight paragraph. I think the assumption of approximate uniformity is a reasonable working assumption because
1) it is simple
2) so far no evidence to the contrary has withstood scrutiny

People regularly review this assumption to see how it is holding up, and people challenge it now and then. It is a serviceable working assumption, I would not think of it as an article of faith. As long as the model continues to fit the data people will keep on using it---until something better shows up.

marcus

Hi Naty, in post #18 it sounded as if you were quoting me but I think you were actually quoting John, so i was a bit confused and didn't see how to respond.

Johninch

Quote by marcus

I suppose that LOGICALLY you can send a rocket probe into non-existence and you can imagine a region of nothingness and a region of existence and a boundary between them. I can't think of a LOGICAL reason not to imagine a rocket going thru the boundary and its onboard sensors not detecting anything except its own existence.

Thank you Marcus for your patient comments.

However, in the part of your reply quoted above, you are being too generous towards my logic!

My thought experiment of sending a rocket in the opposite direction to all of the known universe does not involve going through a boundary into nothingness. That's impossible. The rocket is part of the universe and is enlarging it.

This is my way of illustrating how the universe could have a boundary. The universe does not need any space to expand into, it creates its own space by expanding. This is what you told me already, so I am sure you agree.

With my thought experiment I am merely wanting to show that it's possible to be at the boundary of the universe without implying that there is anything beyond. So far I have read everywhere that the universe does not have a boundary. So if my experiment is possible, then it can have a boundary.

.

Naty1

Marcus:

Hi Naty, in post #18 it sounded as if you were quoting me but I think you were actually quoting John.....,

from #18:

So from what Marcus has taught me I conclude as the observable universe grows each day, it exhibits the same radiation characteristics as the prior day....

DUDE!! I was throwing you a COMPLIMENT..... from several years of prior discussions.
Relax and enjoy; Bask in the glory!! [LOL]

marcus

Thanks for the compliment, Naty! What puzzled me in your post #18 was that it looked like you were quoting me, saying "it seems speculative" when the words in quotes were actually from somebody else. I knew I had not said what you had me down as saying, so I went back looking for those words and found they were from Johninch.

Quote by Naty1

Marcus:

Quote by Johninch

I have no problem with the assumption that matter is uniformly distributed within the observable universe. If we assume that the total universe is several orders of magnitude larger than this, then a locally uniform distribution of matter would seem to be a reasonably likely local phenomenon. However, it seems speculative to assume that this local density and uniformity is repeated over the whole universe...

I am not so sure as Marcus about the last sentence....the 'speculative' part.... I note that each day we receive new cosmic background radiation,...
... So I expect tomorrows CMBR will be just like todays.

Chronos:

[actual quote from Chronos]

Another way to express this, I think, is that an infinite universe cannot so far be ruled out.

You give a cogent reason for not regarding the cosmological assumption of uniformity as all that speculative. I guess now that I think of it the only problem was your post should have begun:
=======
Johninch:

......

I am not so sure as Johninch about the last sentence...
=======

What you were saying made a lot of sense, you just had the people's names confused. I see now how I should have responded.

marcus

I wouldn't call the cosmological principle (the basic uniformity assumption) speculation, exactly. It think it comes under the heading of Occam's Razor---the idea is that in a mathematical science you should use the SIMPLEST model that fits.

You keep the model simple until you actually get evidence that you need to make it more complicated. And you keep watching out for such evidence!

I would call it speculative to start imagining some boundary off somewhere too far away for us to see, when we have no indication of a boundary. Some day we might, but we haven't yet seen any suggestion of one that withstood scrutiny.

I would likewise call it speculative to start assuming there is a center of the universe far away in some direction (but we just don't know in what direction to point in :-)

Those would be unnecessary complications for which there is no justification. But we should always keep an eye out for any asymmetries in the statistics that might suggest NON-uniformity. And people do watch for that, and periodically sound the alert. What I've seen happen is that the alleged evidence is carefully examined---and so far has been found wanting, and dismissed.

Naty1

Marcus

Thanks for the compliment, Naty! What puzzled me in your post #18 was that it looked like you were quoting me, saying "it seems speculative" when the words in quotes were actually from somebody else.

Listen up: At my age I'm increasingly likely to misquote, and if I do happen to get a quote correct, to mis-attribute the source. get used to it!!

Just be glad I threw the compliment at the correct person!!

Johninch

Thanks everybody for your help on my questions. In the meantime I have been reading more about the cosmological principle, which doesn’t allow a boundary to the universe. Therefore my example of half the sky not containing anything is simply false. Why this cosmological principle has to be true, I don’t understand yet, but I accept it as current theory.

To conclude, could somebody give me definitive answers to the following questions (please note that I am talking about the total universe and not just the observable universe):

1) Does the cosmological principle imply that the universe is infinite? This seems to follow from it not having any boundary. Why then do I read about the universe having dimensions like radius and a certain number of galaxies outside of the observable portion?

2) How do I get from the finite BB event to an infinite universe? How can the universe expand when it is already infinite and boundless? I already know that expansion means distances increasing. Perhaps it has to be explained to me what infinite means.

Please note too, that a) I never suggested that the universe had or has a center, b) I never suggested that my imagined boundary could be crossed and c) I always understood that the universe could double back on itself, so that there would be no boundary in certain directions.

I am really not wanting to challenge the cosmological principle, I just want to understand what current theory is saying.

.

Naty1

1) Does the cosmological principle imply that the universe is infinite?

No. We discuss various sizes regardless of how big the cosmos might be overall; just as you can discuss distances in your local home neighborhood without knowing the size of the earth.

Best bet is to read here about the cosmological principle:

http://en.wikipedia.org/wiki/Cosmological_principle

2) How do I get from the finite BB event to an infinite universe?

The big bang has no defined size nor does the universe.

This might be a good time to reread the posts above as Chronos,for one, already answered the question about the 'size of the universe':

...We do, however, have solid reasons to believe the entirety of the universe is far larger than the little patch within our particle horizon. It could even be infinite, although the only thing we can deduce with any degree of reliability is a minimum size.....

edit: I see I also answered it already: " Nobody knows the size of the universe..." and so have others....
It does take a while to absorb such information and to then try to put the pieces together.....

Johninch

My question: Does the cosmological principle imply that the universe is infinite?
Your answer:

Quote by Naty1

No. We discuss various sizes regardless of how big the cosmos might be overall

Wiki clearly states: "The cosmological principle implies that at a sufficiently large scale, the universe is homogeneous; different places will appear similar to one another."

Well then, in whichever direction and however far I travel, I will see more of the same. What’s the difference between that and infinity?

My question: How do I get from the finite BB event to an infinite universe?
Your answer:

The big bang has no defined size nor does the universe.

I am not asking for the size of the BB nor of the universe, I just want to know, as a first step, whether we are saying that the universe is finite or infinite.

It does take a while to absorb such information and to then try to put the pieces together.....

Yes, well, it’s certainly difficult for me.

Basically we are stuck on the point, whether BBT is compatible with an infinite universe. My reading is that it is not.

If I am right that the universe must be finite, I want to understand why it is not possible to stand somewhere and look in some direction and see few or no galaxies. We don’t have to call it a boundary or edge if that jars.

.

Naty1

"The cosmological principle implies that at a sufficiently large scale, the universe is homogeneous; different places will appear similar to one another."

Well then, in whichever direction and however far I travel, I will see more of the same. What’s the difference between that and infinity?

How about travel on the surface of the earth.....

I just want to know, as a first step, whether we are saying that the universe is finite or infinite.

This has been answered several times in this thread by different people.

..whether BBT is compatible with an infinite universe. My reading is that it is not.

A big bang IS compatible...but it is also compatible with a finite one.

The size of the Universe is unknown; it may be infinite.

http://en.wikipedia.org/wiki/Size_of...re.2C_and_laws

There are probably more than 100 billion (1011) galaxies in the observable Universe.

That's a LOT of 'different places appearing similar to one another'....meaning coming across a LOT of galaxies, not that each looks exactly the same.....

marcus

Quote by Johninch

My question: Does the cosmological principle imply that the universe is infinite?
...
...

Wiki clearly states: "The cosmological principle implies that at a sufficiently large scale, the universe is homogeneous; different places will appear similar to one another."

Well then, in whichever direction and however far I travel, I will see more of the same. What’s the difference between that and infinity?

...

Big difference!

It is the easiest thing in the world for a spatially finite universe to satisfy homogeneity. To make it simple imagine that space is one-dimensional, with one dimensional galaxies, and the 1D creatures find that if they travel long enough and far enough in one of the two possible directions they find themselves back where they started.
(They discover they live in a finite 1D space with "ring" geometry.)

And then think of the analogous thing in 2D. Space is 2D with 2D galaxies scattered about, and the 2D creatures discover (by exploring and measuring triangles and stuff) that they live in a finite "balloon surface" geometry.

You can't take geometry for granted, you can't assume it is standard Greek Euclidean, you have to find out empirically, by measuring, what the geometry you live in really is. Cosmology is the business of finding out the largescale geometry. Just like the 1D and 2D creatures had to do in the examples.

Now think of the analogous thing in 3D. Imagine space is 3D with 3D galaxies scattered about more or less homogeneously. And suppose we 3D creatures start measuring very large scale triangles to find if there is a very slight deviation from 180 degrees. If there is a consistent pattern of getting very slightly MORE than 180, this will indicate that we live in a finite 3D analog of the "balloon surface" geometry.

So far the measurements have not been conclusive but there are some recent measurements that lean in the direction of that kind of spatial finiteness.

Maybe you should click on the "balloon" link I keep in my signature, and watch the movie a few times. Think about the experience of being in one of those 2D galaxies you see in the movie, with the other galaxies receding from it. finite volume, but no boundary anywhere.

Johninch

Quote by marcus

Big difference!

It is the easiest thing in the world for a spatially finite universe to satisfy homogeneity....
Maybe you should click on the "balloon" link I keep in my signature

I did and I also read the rest of Ned Wright's tutorial. The animated sphere has a boundary, so it doesn't help.

You have to see that these pictorial illustrations are doomed to failure, because you can't draw a finite universe without boundaries. It's impossible - you may come to the edge of the paper or let the ink fade out, or draw anykind of complex geometrical shape but you can't demonstrate absence of a boundary with an illustration. And if it can't be shown on paper, this is an indication of the problem.

The oft repeated analogy of the balloon's surface is not translatable to 3D in my mind.

I also want to emphasize that I came off the idea of a crossable boundary some time back. I hope I made it clear that I am only imagining lack of galaxies and other observables from a vantage point within the universe. The problem of the word boundary is that it implies something on the other side and that is not what I mean.

In the meantime I have done more searching and have found a couple of very interesting threads in this forum on this very subject in 2006 and 2007. I have the impression that several posters treating the subject of BBT, expansion, finite/infinite universe, boundary, and so on from a logical point of view were not satisfied before those threads were locked.

I thank everybody for their efforts and I will for sure keep my eyes open for further insights into BBT. As far as I am concerned, the thread can be closed. "Locked" is so agressive!

.

marcus

Quote by Johninch

I did and I also read the rest of Ned Wright's tutorial. The animated sphere has a boundary, so it doesn't help.
...

The point of the animated sphere analogy is that it has no boundary. All existence is concentrated on the 2D edgeless surface. The 2D creatures living there cannot point their fingers in any direction that is not on the sphere.
There is no "inside" or "outside" to the sphere.
You have to concentrate in order to get the help from the analogy.

You as a 3D creature cannot point your finger in a 4th spatial direction. In fact we may live in a 3D analog of the 2D sphere. But so far we have no evidence of any "inside" or "outside" or any boundary. No more would 2D creatures living in a universe in which all of existence was concentrated on that animated sphere.

About locking this thread, I am not a moderator or mentor, so I don't decide about those things. Personally I see no reason to lock. If you are dissatisfied or bored you can always drop out. But at this point it's possible someone else might be reading and have questions or comments they want to make. I'm happy with the thread so I think I will hang around a while longer, and see.

Plus you might conceivably change your mind and want to discuss some more. Who knows?

Naty1

Johnich

...I came off the idea of a crossable boundary some time back..... I am only imagining lack of galaxies and other observables from a vantage point within the universe.....

Well one CAN imagine being able to look out waaaaaay beyond the current 46bly sphere from which we receive the most distant light. why not....if Einstein could imagine catching up to light we can imagine an 'empty' portion of the universe.......

I think I already posted about the fact that each day we receive new CMBR...and it's like yesterdays. No surprises there. Each additional bit looks like the prior homogeneity with small fluctuations....

There may in fact be a 'lack of galaxies' out, say 100 times or 1,000 or 10,000 times further than we can observe today. Nobody can prove that one way or another. But then you should have some reason for such a hypothesis. I can't think of any, we have no such model, but such things don't make it impossible.

It's fun to speculate, to do thought experiments, and then see if they 'pan out' or not. Or whether you can even justify a thought one way or another. The great Richard Feynman noted something to the effect that 'a successful physicist succeeds by making just about every imaginable mistake before arriving at a correct solution.' [I don't mind being shown to be wrong here; I am used to it because my wife does it every day!!]

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marcus Recognitions: Gold Member Science Advisor	Hi Naty, in post #18 it sounded as if you were quoting me but I think you were actually quoting John, so i was a bit confused and didn't see how to respond.