# The universe?

#### god-of-course

##### Bluegoblin.
Registered Senior Member
I basically wanted to ask: The big bang theory is principaly accept these days, but many also claim that the universe is infinite, don't these two contradict? Also could the universe be infinite in terms of space but contain a finite amount of mass? what does uniformally filled with mass mean? and is it possible that the universe is infinite in size and mass?

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god-of-course:

<i>I basically wanted to ask: The big bang theory is principaly accept these days, but many also claim that the universe is infinite, don't these two contradict?</i>

Not at all. The infinite universe is just one possible model which fits with the big bang theory.

<i>Also could the universe be infinite in terms of space but contain a finite amount of mass?</i>

Yes. In fact, that might be one reason <b>why</b> the universe is infinite - not enough mass.

<i>what does uniformally filled with mass mean?</i>

It means that whichever direction you look, and wherever you are in the universe, you'll see approximately the same amount of matter, provided that the region of space you look at is sufficiently large.

<i>and is it possible that the universe is infinite in size and mass?</i>

Not sure, but my guess is yes.

Re: Re: The universe?

Originally posted by James R
<i>Also could the universe be infinite in terms of space but contain a finite amount of mass?</i>

Yes. In fact, that might be one reason <b>why</b> the universe is infinite - not enough mass.

an infinite homogeneous universe with finite mass density would have infinite mass. i would say that the reason the universe is infinite is that it does not have a large enough mass density, not that it doesn t have enough mass.

JamesR, I know this probably sounds stupid, but are there any models, or is it even possible, that the universe is like a huge, round
ball? What I am trying to ask, is it possible that when we look back
10 billion lys for instance, we are looking in a great circle, instead
of in straight line of sight? Like all light is curved by the mass of the
universe, so the object we are looking at may actually be behind
us, for instance, instead of straight ahead. I know this is more of
a philosophical question, but wondered if there was physical model
along those lines.

Yes, 2inquisitive, there certainly are such models. They are "closed universe" models, or universes with net positive curvature.

In such a universe, if you could travel far enough in a straight line in any direction, you would eventually end up back where you started.

lethe:

Good point. I agree.

Originally posted by 2inquisitive
JamesR, I know this probably sounds stupid, but are there any models, or is it even possible, that the universe is like a huge, round
ball? What I am trying to ask, is it possible that when we look back
10 billion lys for instance, we are looking in a great circle, instead
of in straight line of sight? Like all light is curved by the mass of the
universe, so the object we are looking at may actually be behind
us, for instance, instead of straight ahead. I know this is more of
a philosophical question, but wondered if there was physical model
along those lines.

this is not a philisophical question, this is in fact an experimentally measurable question. and a good question to ask, furthermore.

yes, there is such a model. the standard Big Bang model allows for this possibility. in fact, there are basically only 2 parameter in the big bang model, one of which is the curvature. if this curvature is 0, then the universe is flat. if the curvature is positive, then the universe is like a huge 3 dimensional ball, just like you described, and if this curvature is negative, then the universe is hyperbolic.

just this year, very accurate results were released from the WMAP experiment, which put tight constraints on the value of this curvature. up to the best experimental accuracy we can muster today, the universe is flat. not a big ball.

does this mean that we know that the universe is not a big ball? no. to know for sure, we would have to measure the value of this curvature to be 0 with infinite accuracy. so all we know is that if the universe is a big ball, then it is very very very big.

note that the equations that govern the geometry of the cosmos tell us that flatness is an unstable state for the universe. therefore if there is even the slightest deviation from flatness, then the universe should not stay flat. since the universe is rather old by now, this makes it hard to explain the observed flatness. inflation, which is an extension to the big bang model, which basically says that in the very early universe, it grew exponentially for some period, was devised to solve this flatness problem, as well as a host of other problems. with inflation, the really fast exponential growth smoothed out the universe and flattened it so that its curvature would be 0 to one part in some big number like 10^60 or something, i forget.

Originally posted by 2inquisitive
is it possible that when we look back
10 billion lys .... we are looking in a great circle...? Like all light is curved by the mass of the
universe, so the object we are looking at may actually be behind

one more comment about this idea, if the circumference of this great circle is 10 billion light years, then it will take 10 billion years for that light to come back to you. so when you look in that direction, you are not looking at your own back, you are looking at the back of some distant ancestor 10 billion years in the past (which i believe is older than our whole solar system). furthermore, this picture would be complicated by the fact that the radius of the big ball universe would be changing through those 10 billion years. so it would be hard (read: impossible) for you to actually see the back of things by looking forward.

Thanks, JamesR and lethe. This clears up things a lot for me. I, of
course, had heard of both the big bang and flat model, but assumed
both had everything traveling in straight lines. I have read of inflation
and many other bits and pieces, but hadn't seen the whole perspective.

this reminds me of a slightly amusing anecdote. there is an excellent cosmology textbook by Kolb and Turner. in the first chapter or two, they review the evidence for the big bang model. one of the most compelling pieces of evidence for the big bang is the Cosmic Microwave Background Radiation, which is just the light left over from about 10 000 years after then the big bang.

this CMB fits the theoretical curve for black body radiation so well, that the measurement error bars are so small, that they are narrower than the width of the ink line used to print the theoretical curve on paper, so the experimental plot and the theoretical plot are indistinguishable. i am somewhat accustomed to seeing particle physics fits, where the error bars are sometimes so big that they are 10% the size of the whole picture. it is a rather astonishing plot.

Kolb and turner, upon mentioning this fact, make some tongue in cheek remark about this banishing any doubts of the reality of the big bang model to the farthest reaches of internet message boards. i thought of this place, and the heated debates about the truth of the big bang that sometimes crop up here, and it just put a big stupid grin on my face.

Yes, I realize we are looking into the past. That is one reason I have
trouble understanding the ever increasing rate of expansion theorized today. Things were moving faster 10 billion years ago
than they were 1000 years ago, so it seems ( I know it can't be
correct) that expansion is slowing down instead of increasing.

Originally posted by 2inquisitive
Yes, I realize we are looking into the past. That is one reason I have
trouble understanding the ever increasing rate of expansion theorized today. Things were moving faster 10 billion years ago
than they were 1000 years ago, so it seems ( I know it can't be
correct) that expansion is slowing down instead of increasing.

yes, that s what everybody thought up until very recently. everyone thought that the expansion should be slowing.

the reason we now think that the expansion is accelerating, rather than slowing, is because of some weird stuff out there called dark energy. basically, dark energy has negative pressure, which causes it to push forward gravitationally, instead of attracting gravitationally like normal matter does.

what is this dark energy stuff? well, we re not sure. it is probably the vacuum energy, but it could be other weird stuff as well. we aren t really sure what it is, but we are pretty sure at this point that it is out there.

Yes, I am familar with the hypothesis of dark energy. But it just
seems like going in circles to me that something never detected
is causing an ever increasing rate of expansion that cannot be detected,
so the dark energy is necessary to explain the ever increasing rate of
expansion!

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Surely though, if the big bang theory were to be accepted regardless of the size of the universe we would have to assume it contains a finite amount of mass right? also out side of our universe what else can we axpect to find in the cosmos? is there anything beyond?

<i>Surely though, if the big bang theory were to be accepted regardless of the size of the universe we would have to assume it contains a finite amount of mass right?</i>

No. Why do you think that?

<i>...also out side of our universe what else can we axpect to find in the cosmos? is there anything beyond?</i>

universe = everything there is

Space is mathematically finite if it is any help to you God of course.

Regards
Leeaus

Originally posted by leeaus
Space is mathematically finite if it is any help to you God of course.

Regards
Leeaus

the policy on this board is that if you want to spout crackpotisms, you must not do it when answering scientific questions. you must have a separate thread for your crackpottery.

I'm confused by the idea of a spatially finite universe. What's supposed to be on the outside of it? Nothingness? Does the universe have any external dimensions?

Also, if matter is eternal as is assumed by cosmologists then how come its not infinitely distributed yet?

(I'm not opposing the BB hypothesis, but I can't believe that this universe is finite AND all that there is, it doesn't seem logical. Or is it that my logic is at fault?).

Just being helpful Lethe. This thread questions whether or not space is finite or otherwise. The fact that space consists of three dimensions at right angles to each other means space is limited. Simply and straight forward, you would be the crackpot, particularly if you assess that such isn’t relevant to this thread.

Regards
leeaus

Canute:

<i>I'm confused by the idea of a spatially finite universe. What's supposed to be on the outside of it? Nothingness? Does the universe have any external dimensions?</i>

There is no "outside" in terms of space and time when you're talking about a universe. The universe has no external dimensions, since there's no space to scale things by "outside" the universe. However, you can define a kind of internal radius, or scale factor.

<i>Also, if matter is eternal as is assumed by cosmologists then how come its not infinitely distributed yet? </i>

Who says matter is eternal? There has only been a finite amount of time since the big bang.

<i>(I'm not opposing the BB hypothesis, but I can't believe that this universe is finite AND all that there is, it doesn't seem logical. Or is it that my logic is at fault?).</i>

It's logical enough, but not the only possibility.

Finite can still mean VERY big.

leeaus:

<i>The fact that space consists of three dimensions at right angles to each other means space is limited.</i>

In itself, your premiss is not enough to guarantee your conclusion here.