What lies outside the universe?

Ulnarian

I realize this topic has probably been beaten to death on here but I've never had the fortune of coming across it so I made my own.

Now, if you believe the whole Big Bang theory, the universe is exanding. In order for anything to expand, there must be something for it to expand into. Now, if we define the universe as "everything", just what the heck is the universe exanding out into, nothing??

Any thoughts?

Chronos

Hi Ulnarian. Your question is often asked. We say the universe is expanding because, in this universe, objects move away from each other over time. The volume occupied by galaxies and such are otherwise meaningless. There is no space outside of space. Space can only be described in terms of the distance between physical objects.

Math Is Hard

ok, I'm a complete dum-dum at cosmology (among other things), but I always had trouble with the use of the word "expanding" in this context. In my ordinary, simplistic, everyday use of the English language, I have certain ideas that I associate with the word "expanding". For instance, if I think of a balloon expanding, one of the things I immediately think of is that it is increasing in volume and taking up more space.
But this doesn't quite work with the idea of an expanding universe, because what "space" is it "taking up" even though the objects move farther and farther apart?
What I'd really like to know is: how do you conceptualize such a thing? Or is it impossible to "picture" because it is too abstract?

JesseM

According to the Big Bang theory, the Big Bang was not an explosion in a preexisting 3-dimensional space, with matter and light expanding out into empty space from some central point--instead, matter and energy are understood to fill all of 3D space, and what's expanding is space itself. The key is to understand that the Big Bang theory is based on Einstein's theory of general relativity, which explains gravity in terms of matter/energy causing spacetime to become curved--depending on the average density of matter/energy throughout the universe, a consequence of this is that the universe as a whole can be curved, with either positive curvature, zero curvature, or negative curvature. For a closed universe with positive curvature, you can visualize it if you drop the dimensions by one--instead of curved 3-dimensional space, which is impossible for us to visualize, picture a 2D universe a la Flatland in which 2D space is actually curved into a sphere, and "expanding space" means that the sphere is blowing up like a balloon while the bits of 2D matter on the surface do not change in size. You can see that if you pasted a bunch of bits of paper on a balloon and then blew it up, each bit would see the other bits receding from it, just like what we see with other galaxies. If you play the movie backwards so that the size of the sphere approaches zero, you can seen that all the bits of matter throughout the universe get more and more squished together, approaching infinite density as the size approaches zero--this is what the big bang is supposed to be. Of course, this analogy forces you to picture the 2-dimensional surface of the sphere expanding in a higher 3rd dimension, and while it is possible that our curved 3D space is expanding in some kind of higher 4D space, mathematically there is no need for such a thing--instead of describing the curvature of a surface with reference to a higher-dimensional "embedding space", it is possible to describe curvature using purely intrinsic features that could be observed by a being confined to the surface (like whether the sum of angles of a triangle drawn on the surface is more, less, or equal to 180 degrees), and general relativity uses only such intrinsic features to describe what it means for space to be curved (see this page on differential geometry, the mathematical basis for general relativity, which talks about the difference between intrinsic and extrinsic descriptions of curvature).

For a universe with zero curvature, picture an infinite chessboard in which all the squares are growing at the same rate, while the pieces at the center of each square remain unchanged in size. If you play the movie backwards, the distance between any two squares approaches zero as you approach the moment of the big bang, which means the density of the matter on the squares (represented by the chess pieces) approaches infinity as it gets smushed together more and more tightly. A universe with negative curvature would be something like an infinite saddle-shape which is a little harder to picture expanding, but if you can picture the other two you get the basic idea. From Ned Wright's Cosmology Tutorial, a graphic showing the 2D analogues of the three types of spatial curvature, negative, zero, and positive:

Chronos

Not abstract, it is really fairly simple. Things only expand with respect to your reference frame. For example, if LA gets more distant from all other places on planet earth, over time, it suggests earth is expanding. Much the same with the universe. We can only see a finite part of the universe. All the parts of it appear to be moving away from each other.

Math Is Hard

I think my only quibble is an issue with semantics. In my immediate world, "expanding" would mean things (inside of a contained thing) moving farther and farther apart - but also, to accomodate this, this contained thing would be increasing in volume. So things in the universe can move farther and farther apart from each other while the universe itself does not increase in any kind of volume that we could measure? Only the first part of my conception of "expanding" holds.
I hope that makes sense. I am speaking only in my immediate and known 3D sensibilities.

Garth

Hi Ulnarian welcome to these Forums - if you haven't been welcomed already!

That's a very interesting question that is often not taken seriously in GR circles. It might be argued philosophically that if the universe is all that there is, then logically there cannot be a 'container' in which it is situated and into which it is expanding. Relativists would argue that such a philosophical argument is ignorant of the dynamic nature of space-time itself; for not only is space-time 'curved' by the presence of matter and energy, but also it must expand or contract; a space-time that is held static by a cosmological constant is found to be unstable.

However we can ask, "If the universe is expanding, and we are part of that universe, 'inside it', how do we measure the expansion? Might it be that our rulers expand with the universe, for they are embedded within that expanding space-time, and therefore this would render the expansion undetectable?"

One answer to this is, "In fact we do observe Hubble Red shift, the relative abundances of the elements agree with Big Bang nucleosynthesis, and the Cosmic Microwave Background (CMB) is an observation of a time when the universe was much smaller and full of hot plasma, therefore the expansion of the universe definitely has been detected.”

Consequentially we can argue that the universe has expanded relative to physical (i.e. atomic) rulers, however the wavelength of a photon is red shifted and expands with the universe, therefore measured relative to the wavelength of a photon from the CMB the universe has not expanded! Instead atoms have shrunk within a static universe.

So it comes down to, "How do you measure the expansion?" Define an atom to provide your unit of length and the universe has expanded, define the wavelength of a representative photon sampled from the CMB, say at the wavelength of its peak intensity, to be your unit of length measurement and the universe is static!

A final thought, map the 4D of a finite universe space-time onto a 3D hyper-surface and define a time by the hyper-sphere's radius. The passage of time and the expansion of the universe now become two different experiences of the same mysterious phenomenon, but now if we ask "What is the universe expanding into?" we find the answer "Its future!"

Just a thought or two..

Garth

JesseM

If the universe has finite volume then sure, you can think of the volume increasing, just like the 2D surface area of a balloon increases as you blow it up. In an infinite universe, the volume is infinite at all times--see my analogy above about the infinite chessboard where every square is growing while the pieces stay the same size.

Entropy

Nothing exists outside the universe because by definition the universe is everything.

Math Is Hard

I felt overwhelmed by your first post, JesseM, but now reading this one - something actually seems to have clicked. This particular statement "the volume is infinite at all times" makes it clearer for me. Thanks.

Chronos

I resist the notion of an infinite universe because it is unphysical. The universe is not observationally infinite. It has a finite size called the particle horizon - which envelopes us in a sphere with a radius of around 13.5 billion light years. It is the only volume of space that is, or ever will be, causally connected to our reference frame, hence, the only one physically meaningful.

setAI

I resist the notion of physical infinities- instead I embrace the original and central meaning of the concept of infinity as limitlessness-as INDEFINITE- so that Infinity really implies limitless finite forms [which strongly reinforces the idea of an emergent Multiverse]- only finity is physically real:

"...many feel that the concept of infinity is fully paradoxical and absurd in-an-of itself- but it’s only their erroneous DEFINITION of infinity- as an impossibly “large” amount of something that “goes on forever”- or a never ending counting process- but this is not the case- Infinity is a concept of LIMITLESSNESS- to understand Infinity we should look to the origins of the concept- the oldest idea of Infinity found in human culture- the six thousand year old Qabbalistic concept of AIN SOPH- got it right from the start- the Hebrew AIN SOPH was the second of a trinity of absolute concepts that crown Kether- the first sephiroth of the Tree of Life- this crown begins with AIN- which is usually translated as NOTHINGNESS- but literally means NOT- then AIN SOPH- translated as INFINITY- but literally translated it means NOT LIMIT- no limits! then finally AIN SOPH AUR- which is Infinite Light: NOT LIMIT LIGHT-

so from the very beginning of human culture we had a proper concept of infinity as a limitlessness that allows FINITE forms to be- and those finite forms are limitless- thus finity can extend INDEFINITELY- that is the crucial understanding of infinity- not a forever arching hugeness of something- but the absence of absolute limits- so really Infinity is a statement DENYING an absolute [final limit/ nothingness]- not claiming to be an absolute-"

I never understand this attitude that "we are isolated" when there is simply no evidence to support the idea that we now posess the full understanding of how our universe exists and works and evolves over trillions of years- absurd! there is simply no honest way to say that the universe we can see is causally closed to anything beyond it- it is certainly very possible- and in some models very likely that regions of spacetime are totally closed to themselves- but cannot be stated with any great certainty or confidence at all- there are so many ways in which our theories suggest possible influence of other space-time regions- from gravitons and other forces passing through higher spatial dimensions between branes to colliding branes to many-worlds QM parallel interactions to black holes/ wormholes connecting universes or even spawning them as in Smolin's CNS-

whatzzupboy

If there was a second universe what is in between us and them? Heaven? Hell? Ect?

wimms

There are 2 concepts: infinity and infinite. They are all the time confused. What you seem to refer to is observational infinity beyond which we have no means to observe, today. Despite having finite volume, which seemingly is causing your resisting, it is not real bounds. Any point at 6billion lightyears from us has exaclty the same sphere of 13 billion light years around it, us included. Ad infinitum. Thats what Big Bang is all about, isn't it.

13billion years ago, that "horison" was so much closer that things beyond it had causal relations with our frame. Similarily, its quite fatalistic to deny possibility that signal emitted somewhere beyond our horison won't ever reach our frame sometime in distant future, or even now.

wolram

My take is that space is infinite, as in, if i get in my spaceship and
head in one direction i will go on forever, without returning to the
same starting point, our universe may be governed by set rules,
but why should that preclude an infinite background? what for
instance rules out other universes existing in the same time and
background as our own, separated only by distance.

ohwilleke

The axiom that "everything that exists" and that "everything that came out of the big bang" are identical is not based upon any solid empirical evidence to my knowledge and is not in my opinion compelled by general relativity. It is an axiom about whose truth I am agnostic. The truth of this proposition is unknowable.

Suppose that 700 billion light years from Earth that there is another "universe" billions of light years across and of a corrosponding age with its own big bang. If special relativity is correct, and gravity indeed propogates at the speed of light as the consensus view holds, there would be no way we could know that this is the case. In much the same way, there is no way that a Dolphin in the Mediterranean Sea could know that Lake Victoria in Africa exists.

(Note that this is not the same as the "many worlds" intepretatio of QM which suggests that there might be multiple Earths out there, e.g.).

selfAdjoint

Perfectly correct. And just because we can know nothing about any such addenda, we have nothing whatever to say about them. And in that case, as Witgenstein advised, we should shut up about them.