# The observable Universe and its shape

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• revo74
There's no good answer to that. It just happens to be that way.The parameter isn't introduced, though. It's a degree of freedom that stems directly from General Relativity and minimal assumptions of homogeneity and isotropy.It may appear to be an added free parameter if you look at the first Friedmann equation:H^2 = {8 \pi G \over 3}\rho - {k c^2 \over a^2}After all, without that parameter the equation is simpler:H^2 = {8 \pi G \over 3}\rhoBut this begs the question: why should the matter/energy density be directly proportional to the square of... something?There's nof

#### revo74

Laymen here.

1. From my understanding the universe is like the surface of a balloon. The universe is expanding as a balloon grows when air in being placed inside of it. Just like a surface of a balloon if you go in any direction in a straight line you will come back to the original point.
Is this description correct, if no please elaborate.

2. When we look at other galaxies we are looking at how they were in the past as light takes a long time to travel to us. This would mean that we are looking at what that galaxy was like when the surface of the balloon was smaller. We can't observe the entire surface of the balloon in its present form because by the time light reaches us the balloon is bigger.
Is this description correct, if no please elaborate.

3. How thick is the surface of the balloon?

Laymen here.

1. From my understanding the universe is like the surface of a balloon. The universe is expanding as a balloon grows when air in being placed inside of it. Just like a surface of a balloon if you go in any direction in a straight line you will come back to the original point.
Is this description correct, if no please elaborate.
That's one possibility. There are other potential geometries.

Note that in practice the rate of expansion is too fast, so that you'd have to go faster than the speed of light to actually traverse the distance, which naturally isn't possible.

2. When we look at other galaxies we are looking at how they were in the past as light takes a long time to travel to us. This would mean that we are looking at what that galaxy was like when the surface of the balloon was smaller. We can't observe the entire surface of the balloon in its present form because by the time light reaches us the balloon is bigger.
Is this description correct, if no please elaborate.
Sure

3. How thick is the surface of the balloon?
It has no thickness at all. It's an analogy, and an imperfect one.

The universe being a surface of a balloon is only useful as an analogy when referencing how space expands. So your understanding of why we can see things that are currently much further away than we should be able to see, they were closer in the past.

The shape of the observable universe is a sphere, but that says nothing about the actual universe. That's just what happens when you are an observer, no one has any idea what it looks like on the grandest scale.

The shape of the observable universe is a sphere, but that says nothing about the actual universe. That's just what happens when you are an observer, no one has any idea what it looks like on the grandest scale.
The curvature of the observable universe is consistent with flatness, not a sphere.

The curvature of the observable universe is consistent with flatness, not a sphere.
The available evidence is consistent with either shape, but in any event the radius of curvature is much larger than the size of the observable universe. As long as the dark energy continues to behave like a cosmological constant in the future, the curvature will never have any practical impact in the future.

The available evidence is consistent with either shape, but in any event the radius of curvature is much larger than the size of the observable universe. As long as the dark energy continues to behave like a cosmological constant in the future, the curvature will never have any practical impact in the future.
Sure, but if you take flatness as your null hypothesis, there is insufficient evidence to prefer nonzero curvature.

Sure, but if you take flatness as your null hypothesis, there is insufficient evidence to prefer nonzero curvature.
It's not clear that flatness is the right null hypothesis, though.

Either way, our universe is very close to flat and there's a reasonable chance we'll never be able to measure any spatial curvature.

It's not clear that flatness is the right null hypothesis, though.
We should start with the most parsimonious model. There is not sufficient evidence to warrant introducing curvature as a free parameter.

We should start with the most parsimonious model. There is not sufficient evidence to warrant introducing curvature as a free parameter.
The parameter isn't introduced, though. It's a degree of freedom that stems directly from General Relativity and minimal assumptions of homogeneity and isotropy.

It may appear to be an added free parameter if you look at the first Friedmann equation:
$$H^2 = {8 \pi G \over 3}\rho - {k c^2 \over a^2}$$

After all, without that parameter the equation is simpler:
$$H^2 = {8 \pi G \over 3}\rho$$

But this begs the question: why should the matter/energy density be directly proportional to the square of the expansion rate? There's no fundamental reason for this to be the case. It's perfectly plausible to have a universe which is, at a given point in time, expanding much faster or much slower compared to the matter/energy density. General Relativity only predicts how this expansion will change from that initial configuration.

That's what the curvature parameter is there for: it's basically a way of describing the initial relationship between expansion and density. Because of the way GR works, that initial relationship translates into a geometric shape for the universe.

But can't this be said of any "free" parameter? For example, apply your argument to the running of the spectral index: Should we not consider it a free parameter because its absence would beg the question: why should the spectrum of perturbations be a power law?

Yes, I agree with this. But can't this be said of any "free" parameter? For example, apply you argument to the running of the spectral index: Should we not consider it a free parameter because its absence would beg the question: why should the spectrum of perturbations be a power law?
I think that's a somewhat different situation. In that case, there are a lot of competing models for what the spectrum of primordial perturbations should be. It's really difficult to do a comparison between two models that use entirely different functions for the spectrum of primordial perturbations. So the convention of using the spectral index was born: it's a practical measure to allow experimental teams to produce data that doesn't depend upon any specific model for laying down the primordial perturbations.

It was chosen largely because it's a decent first approximation to the way a lot of inflation models behave. In general I don't expect anybody really believes that the spectral index is likely to be completely accurate, but it's a fairly good way to at least get the model comparisons started.

The spatial curvature is a very different sort of thing. It's a degree of freedom that remains after making some minimal assumptions about the universe. Because it's only a single, constant parameter, there's no need to do complicated things like series expansions.

It has no thickness at all. It's an analogy, and an imperfect one.

If the universe is expanding in all directions from one central point then how could the shape of the universe not be spherical?

If the universe is expanding in all directions from one central point

It isn't.

ComplexVar89
If the universe is expanding in all directions from one central point then how could the shape of the universe not be spherical?
I recommend the link in my signature

If the universe is expanding in all directions from one central point then how could the shape of the universe not be spherical?

The Universe is, in theory, "everywhere", to imply a sphere suggests that there is a measurable "shape" or "size". It is an everywhere that has no shape, size, edge or center, these are all measurable things. The Universe theories(whether finite or infinite, thanks @) are more conceptual representations of something that is immeasurable, an unquantifiable quantity on either side the coin. I'm newly discovering that both theories work(and probably more that I know nothing of yet). They can both(probably all) be seen in a manner that seems to give the same answers.

I can understand what you mean by a "central" point though, given that Big Bang "should" have a point of origin. That's the enigmatic nature of the Universe though, with no known limit in any direction, how can there be any kind of center or edge?

The concept of an expanding Universe has nothing to do with movement or an increasing in size(whether that is what is actually happening or not), it is a relation of distant objects(relatively speaking) accelerating at an accelerated rate compared to non distant objects(relatively speaking).

Everyone feel free to correct or clarify any fine points that grab their attention here.

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The Universe is infinite, it is "everywhere", to imply a sphere suggests that there is a measurable "shape" or "size". Infinity has no shape, size, edge or center, these are all measurable things. Infinity is more a conceptual representation of something that is immeasurable than an actual measurement, an unquantifiable quantity.
This isn't known. It is clear that our universe is significantly larger than the part of it which is observable, but there's no way to know (currently) just how big it is, or whether or not it is infinite in space.

Droidriven
This isn't known. It is clear that our universe is significantly larger than the part of it which is observable, but there's no way to know (currently) just how big it is, or whether or not it is infinite in space.

Is that to say that if the Universe is not infinite then it is finite in an infinite vacuum of "empty" space that it can or may expand into at some point? Or is that another point that gets debated either way?

I get you, I just mean that we have to accept infinity as representing an unknown, immeasurable thing because we can't observe a limit. We can't say there is a definitive limit to its size(though a limit may or may not exist), nor can we say that there is a center. More a theoretical thing due to our lack of observability or measurability.

I'll edit my post to reflect it as being a general theory instead of a known definite.

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Infinity has no shape, size, edge or center, these are all measurable things.
In case the universe is infinite in space, there are still degrees of freedom regarding its curvature and its shape (means its topology). It can be flat or negatively curved. In the letter case, the topology can be somehow, it isn't determined.

Note that even being finite in space, the universe has no edge and no center.

Note that even being finite in space, the universe has no edge and no center.

Yes, a wraparound that translates to no observable, measurable or reachable "edge" or "center"

As an edge or center doesn't exist, "reachability" makes no sense in this context. If you talk about the northern pole of the Earth it would, if you talk about its surface however, it wouldn't.

This isn't known. It is clear that our universe is significantly larger than the part of it which is observable, but there's no way to know (currently) just how big it is, or whether or not it is infinite in space.

If the universe was the size of an atom at one time (a fraction after the Big Bang) and then began to expand rapidly (inflation), but at a finite speed. Then how could the universe possibly be infinite? Something of finite size that expands/grows at a finite speed cannot possible be infinite. Please explain how my reasoning can be wrong.

The universe as a whole is either finite or infinite at all times, including the time, when it came into existence.

If the universe was the size of an atom at one time (a fraction after the Big Bang) and then began to expand rapidly (inflation), but at a finite speed. Then how could the universe possibly be infinite? Something of finite size that expands/grows at a finite speed cannot possible be infinite. Please explain how my reasoning can be wrong.
The universe was never the size of an atom. The OBSERVABLE universe was somewhere around that order of magnitude in size but estimates of the total universe range from 10E25 (or thereabouts and that is a very low end estimate) bigger than the observable universe up to infinite so in any case, the OU is utterly trivial compared to the total universe.

Also, the universe did not start as a point or in a single place. I recommend the link in my signature.

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The universe was never the size of an atom. The OBSERVABLE universe was somewhere around that order of magnitude in size but estimates of the total universe range from 10E25 (or thereabouts and that is a very low end estimate) bigger than the observable universe up to infinite so in any case, the OU is utterly trivial compared to the total universe.

Also, the universe did not start as a point or in a single place. I recommend the link in my signature.

Clarify that please. How is only the observable universe believed to have been the size if an atom but the whole universe wasn't. Big Bang only occurred within the observable universe but not the whole universe? Or Big Bang included the whole universe but rather than the whole universe expanding from something the size of an atom, is Big Bang just a term to say the universe went from the size it was to the size it is now?

If only the observable was the size of an atom then why is it generally presented that the Universe, its dimensions, time and everything we call the laws of physics was birthed from a finite point at Big Bang? Is it an archaic disproven concept that is mistakingly still presented, or is it that the concept is misinterpreted?

Is it that the "space" of the Universe is finite or infinite and always has been but the matter of the Universe(would the total of matter in the Universe be what you're calling the observable universe, or do you mean the extent of what we can see from where we are is the observable universe) did not occupy all of that space at one point?

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Clarify that please. How is only the observable universe believed to have been the size if an atom but the whole universe wasn't.
Uh, because, as I said, the total universe was huge number of orders of magnitude larger than the observable universe.

Big Bang only occurred within the observable universe but not the whole universe? Or Big Bang included the whole universe but rather than the whole universe expanding from something the size of an atom, is Big Bang just a term to say the universe went from the size it was to the size it is now?
That is EXACTLY what the "big bang theory" is ... a theory of how the universe expanded from a hot dense plasma of indeterminate size to what it is now.

If only the observable was the size of an atom then why is it generally presented that the Universe, its dimensions, time and everything we call the laws of physics was birthed from a finite point at Big Bang?
finite point at big bang is pop science nonsense.

Is it an archaic disproven concept that is mistakingly still presented, or is it that the concept is misinterpreted?
It is, was, and always has been a mistaken idea and it is still vigorously promoted in pop science nonsense.

Is it that the "space" of the Universe is finite or infinite and always has been
exactly. It's one or the other but we don't know which.

but the matter of the Universe(would the total of matter in the Universe be what you're calling the observable universe, or do you mean the extent of what we can see from where we are is the observable universe) did not occupy all of that space at one point?
The observable universe has never been the whole universe.

You really should do some basic reading in cosmology and I do NOT mean pop-science nonsense.

Droidriven
Uh, because, as I said, the total universe was huge number of orders of magnitude larger than the observable universe.

That is EXACTLY what the "big bang theory" is ... a theory of how the universe expanded from a hot dense plasma of indeterminate size to what it is now.

finite point at big bang is pop science nonsense.

It is, was, and always has been a mistaken idea and it is still vigorously promoted in pop science nonsense.

exactly. It's one or the other but we don't know which.

The observable universe has never been the whole universe.

You really should do some basic reading in cosmology and I do NOT mean pop-science nonsense.
I joined this forum for the purpose of reading and getting familiar with cosmology and other areas of physics. I'm just asking questions as I go, discussion is far more enlightening than reading by itself.

tobyr65
How is only the observable universe believed to have been the size if an atom but the whole universe wasn't.

I strongly recommend reading Ned Wright's Cosmology FAQ and tutorial. The particular question you are asking is addressed here:

http://www.astro.ucla.edu/~wright/infpoint.html

Note that the "point" he refers to is assuming an idealized model that is often used in pedagogy but is not actually the one used in cosmology; in the actual models used, it would be "a very small volume".

Droidriven
I strongly recommend reading Ned Wright's Cosmology FAQ and tutorial. The particular question you are asking is addressed here:

http://www.astro.ucla.edu/~wright/infpoint.html

Note that the "point" he refers to is assuming an idealized model that is often used in pedagogy but is not actually the one used in cosmology; in the actual models used, it would be "a very small volume".

Thanks, I'm perusing it right now.

The curvature of the observable universe is consistent with flatness, not a sphere.
I was referring to the Hubble Sphere. The shape of what we can see, not in the context of it's curvature.

To me, a more germane question is: What energies, forces, mechanisms, processes etc, other than BB might be 'causing' space to expand

To me, a more germane question is: What energies, forces, mechanisms, processes etc, other than BB might be 'causing' space to expand
Why do you feel the need for other things? Basic expansion was a result of what was going on 14billion years ago and the acceleration of that expansion is caused by an as yet unknown thing we call dark energy (we understand what it does but not its fundamental nature, although there are candidates). What more do you feel is needed?

To me, a more germane enquiry might be: What energies, fields, forces, processes, mechanisms, etc. - other than remnant BB - might be 'causing' expansion of space/universe? Also, wha is the nature/source of the hypothesized cosmological constant?

To me, a more germane enquiry might be: What energies, fields, forces, processes, mechanisms, etc. - other than remnant BB - might be 'causing' expansion of space/universe?
I ask again, why do you feel there is any need for anything other than the remnant of the early universe for the basic expansion? What is it about that explanation that you think is insufficient? Quit avoiding the question.

To me, a more germane enquiry might be: What energies, fields, forces, processes, mechanisms, etc. - other than remnant BB - might be 'causing' expansion of space/universe? Also, wha is the nature/source of the hypothesized cosmological constant?
Lots, but the only evidence we have is the effects of dark energy. Dark energy could be the aggregate behavior of many forces, Occum's Razor tells us that it's probably one.

Like phinds said though: even without dark energy, the universe would still be expanding. The big bang introduced a lot of energy which pushed on everything, the only force that can bring everything back together is gravity. If the initial expansion had more energy than the escape velocity of the universe, it'll expand forever, if it's less, it'll still expand for a long time, then start falling back in.

To me, a more germane question is: What energies, forces, mechanisms, processes etc, other than BB might be 'causing' space to expand
Nothing "causes" space to expand (the present-day accelerated expansion notwithstanding). The fact that the universe is expanding is an initial condition set at the big bang -- it is not "fueled" by any force.