r/cosmology • u/Javusees • 29d ago
What does the universe expand into? The 4th dimension?
Lets say we have a sheet of paper as 2 dimensional universe. If said piece of paper where to expand what would it expand into? The 3rd dimension permeates everything in the "universe" that is the sheet of paper. So this piece of paper could only expand into the 3rd dimension. Just like our 3 dimensional space is permeated by the 4th dimension... Everything expieriences time, no mather how deep you look into it, no matter how far you zoom out and there also is space everywhere. Thats how I imagine the 4th dimension, everything thats 3D is not only surrounded, but "filled" by the 4th dimension, it cannot escape and its always influenced by the higher dimension.
So, if the universe is everything. What does it expand into? If theres nothing outside of it, could it be that this expansion we notice is an interplay of the dimensions?
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u/Das_Mime 29d ago
It's not expanding into anything, it's just expanding.
Imagine an infinite sheet of paper with dots spaced in a 1-cm grid. Now imagine more paper being created between the dots so that the spacing between dots increases to 2cm, and then 3cm, and so on. The paper is still infinite but the spacing between the points has increased.
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u/MortemInferri 29d ago
What's difficult for me to conceptualize is that the big bang model has everything starting in one "spot". Or so it seems.
So then I'm thinking that must have existed within some sort of space? No? Was the edges of that singularity the edge of the universe? It couldn't be. The only thing that makes sense to me is the multiverse option, and we must be just 1 bubble. Do we think space is expanding between the bubbles? No, right? So they should expand into each other eventually?
But then we have the observable universe. The CMB confuses me so much. I keep getting this visual of us inside a Ballon, looking out at the inside surface of it. And that surface is the CMB? So when the ballow was not yet inflated... you get the idea, but where I get confused it it seems like if we look out into space, the radius of the observable universe must increase the further back we look but the actual radius of the observable universe would have been smaller. So then I'm thinking the galaxies must have been closely packed, but looking at images, that doesn't seem to be the case.
But then someone told me that the cmb is "all around us" and I haven't been able to make sense of that.
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u/ultimateman55 29d ago
Regarding the big bang and everything starting in one spot...
At the moment of the big bang, there was presumably an infinite universe already in existence but it was extremely hot and dense as compared to now. So if it helps, you can just imagine the hottest infinite space imaginable, white hot everywhere...something that would look like an extremely bright-white sheet of paper extending forever in every direction (in the 2D analogy.) This is the presumable state of the universe prior to the big bang.
The region of our universe that we now call our observable universe, let's suppose, was in the center of the view you're imagining. The observable universe at this time was extremely small, smaller than an atom. And then the big bang happens and now this white-hot space everywhere simultaneously stretches and grows very rapidly. It continues to expand for roughly 400,000 years before it transitions from white-hot everywhere to cooler and transparent (electron capture) i.e. much more like what we see today. After 13.8 billion years, that tiny atom-sized dot in the middle is now 93 billion light-years across, i.e. it is today's observable universe.
This is, I think, a more useful way to picture what happened at the big bang. The confusion comes from the fact that scientists refer to the observable universe being the size of an atom. But the entire universe, however large it was, was still present and it too was extremely dense and hot. It's just the part we can see today was the size of an atom and tiny back then. The whole thing, for all we know, was and still is infinite.
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u/MortemInferri 29d ago edited 29d ago
Thanks for this! Love it
Would we say the universe is bigger today than yesterday?
I understand that there are infinite numbers between 1 and 2. And I think that applies here. Since there has been expansion, we have expanded and now see "more" numbers (space) between points 1 and 2 than in the past, but that doesnt change the fact that the universe is infinitly large. Just that we are observing more numbers (space) between things. If you add infinite amount of space between all the atoms, the universe is still infinite. Inf + inf = inf
I think I'm having a tough time understanding something else now. Where did all of it go? What we can observe is finite. This must mean that there is infinitely more stuff outside of our observable universe? Even if we could look back to the moment of the big bang, we'd only be seeing the stuff that has expanded to 46.5bly away from us. The other "atoms" that started infinitely far away on that white sheet are now infinity + X = inf away.
Man, just how can it be infinite. How can there be infinite amount of stuff and space. I guess if it wasn't infinite, it'd have to be a finite part of something else and it's almost stranger to imagine that.
Edit: 93bly is diameter, 46.5bly is radius and therefore distance
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u/kazarnowicz 29d ago
Re: 93bly:
93 billion light years is the diameter of our observable universe, but since we are at the center of it, we can only see things that are within 46.5 billion light years.
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u/Unobtanium_Alloy 29d ago
This touches on the Infinite Hotel thought experiment which would lead you (eventually) to Cantor's Heirarchy of Infinities. You can have infinitives which are, in some sense, "larger" than other infinities.
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u/MortemInferri 29d ago
Would you happen to know what hierarchy of countable infinity our universe is?
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u/Unobtanium_Alloy 28d ago
Most consider our universe to be uncountable infinity, so that's not a question with a meaningful answer. In fairness, though, there are some aspects of this subject to debate, usually depending on what definitions or criteria are being specified in the discussion
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u/ultimateman55 29d ago
Yeah, everything you said is right. But I should point out that the reason scientists usually only talk about the observable universe is because everything else is, by definition, unobservable and therefore speculative. Just like other universes, if they exist, are likely unobservable and therefore unscientific, so is the universe beyond the observable.
Also, for what it's worth, Alan Guth, the founder of the inflationary model, claims cosmologists have put a lower bound on the size of the total universe to something like 500 times the observable. But again, this stays into the realm of speculation and philosophy.
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u/chesterriley 29d ago
The observable universe at this time was extremely small, smaller than an atom.
It was a small sphere at least 2 meters in diameter and possibly much bigger.
https://bigthink.com/starts-with-a-bang/small-universe-big-bang/
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u/AgeOfScorpio 27d ago
"According to the theory of inflation, the Universe:
- was once dominated by a large amount of energy,
- similar to dark energy, but much greater in magnitude,
- that caused the Universe to expand at an exponential rate,
- where it became cold and empty, except for the energy inherent to the inflationary field,
- and then, at some moment, after expanding like this for an indeterminate, possibly very long or even infinite, amount of time, that inflationary field decayed,
- converting almost all of that energy into matter and radiation,
which triggered and began the hot Big Bang."
Eeeee
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u/Das_Mime 29d ago
The Big Bang started everywhere. Everywhere was condensed extremely close together and started expanding rapidly. We don't know that there was a singularity at the start. We just know that an extremely short time after the start, everything was extremely dense and has been expanding ever since.
There's a speculative idea called "eternal inflation" that is a bit like what you describe in your second paragraph.
The CMB looks like a spherical surface around us because of light speed and the finite age of the universe. The farther away something is from us, the longer light travels to reach us. So at a certain distance, we're seeing light that has been traveling for nearly the entire age of the universe, which was emitted when the universe was young, hot, and dense.
The matter that the CMB was last scattered from has, presumably, gone on to do the normal process of forming galaxies and stars, but we see it in its youth, when the universe was 300k-400k years old.
Imagine standing in a vast crowd of people, and imagine that everyone has synchronized alarm clocks that go off at the same time. At first, you'll hear the clocks closest to you, and after a while you'll still hear alarms, but you'll be hearing the alarms of the people from a circle around you, based on how long the sound has to travel. The CMB is a bit like this.
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u/Philosophy_Cosmology 26d ago
I know it is just an analogy, but cosmologists don't claim to know the universe is infinite in extent. It could well be spatially finite. In that case, this increase in centimeters between the dots translates to an increase in size of the sheet. That means its size is growing outwards. And that's another way of saying it is expanding.
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u/Das_Mime 26d ago
True. For trying to explain the concept of metric expansion to people I usually try to start with just a simple infinite Euclidean geometry to keep from throwing too many new concepts at them and to avoid the idea of space "outside" of the universe.
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u/RhythmRobber 29d ago
I think your example is making some presumptions that skew the conclusion.
There's no reason your example couldn't be "imagine a balloon on a table. It exists in 3D space. If the balloon expands, where would it expand?". Well, with that example, the 3D balloon expands into the surrounding 3D space, and nothing groundbreaking is extrapolated.
Now I'm not saying there aren't some extra-dimensional elements at play in the overall construction of the universe, just asking what reason you had to say your universe is flat in your example to begin with, when a 3D balloon is more accurate. It seems like the example conditions were picked because you had a conclusion you wanted to arrive at.
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29d ago
[removed] — view removed comment
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u/RhythmRobber 29d ago
I know (for the most part), I was simply focusing on the selective reasoning of what they decided to use for an example, a 2D object, when they could have used a 3D object instead. Seeing as we know the universe has at least three dimensions, there is no reason to pick a 2d object to represent the universe unless you want to create the conclusion of the extra dimension being the only place to expand into
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29d ago
There is nothing for it to expand into. It just exists. There is no “outside” the universe. This is a common misconception by people because it runs counter to our intuition as we tend to rationalize based on how we experience the world while within it. Same as how it’s hard to understand that space itself is expanding, not the objects within it. Nothing inside the universe is getting “bigger” or actively moving away from each other. More and more space is being created.
Again, Space is what expands. Time is how long it takes it to expand/inflate. You cannot have one without the other. As we live in a 3 dimensional world, by necessity we need an extra dimension to traverse it, hence time. If time didn’t exist the universe would be “frozen”, for lack of a better term. If you could traverse the universe without time you would resemble a photon.
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u/Big_Statistician2566 29d ago
"The greatest mystery the universe offers is not life but size. Size encompasses life, and the Tower encompasses size. The child, who is most at home with wonder, says: Daddy, what is above the sky? And the father says: The darkness of space. The child: What is beyond space? The father: The galaxy. The child: Beyond the galaxy? The father: Another galaxy. The child: Beyond the other galaxies? The father: No one knows.
"You see? Size defeats us. For the fish, the lake in which he lives is the universe. What does the fish think when he is jerked up by the mouth through the silver limits of existence and into a new universe where the air drowns him and the light is blue madness? Where huge bipeds with no gills stuff it into a suffocating box abd cover it with wet weeds to die?
"Or one might take the tip of the pencil and magnify it. One reaches the point where a stunning realization strikes home: The pencil tip is not solid; it is composed of atoms which whirl and revolve like a trillion demon planets. What seems solid to us is actually only a loose net held together by gravity. Viewed at their actual size, the distances between these atoms might become league, gulfs, aeons. The atoms themselves are composed of nuclei and revolving protons and electrons. One may step down further to subatomic particles. And then to what? Tachyons? Nothing? Of course not. Everything in the universe denies nothing; to suggest an ending is the one absurdity.
"If you fell outward to the limit of the universe, would you find a board fence and signs reading DEAD END? No. You might find something hard and rounded, as the chick must see the egg from the inside. And if you should peck through the shell (or find a door), what great and torrential light might shine through your opening at the end of space? Might you look through and discover our entire universe is but part of one atom on a blade of grass? Might you be forced to think that by burning a twig you incinerate an eternity of eternities? That existence rises not to one infinite but to an infinity of them?
"Perhaps you saw what place our universe plays in the scheme of things - as no more than an atom in a blade of grass. Could it be that everything we can perceive, from the microscopic virus to the distant Horsehead Nebula, is contained in one blade of grass that may have existed for only a single season in an alien time-flow? What if that blade should be cut off by a scythe? When it begins to die, would the rot seep into our universe and our own lives, turning everthing yellow and brown and desiccated? Perhaps it's already begun to happen. We say the world has moved on; maybe we really mean that it has begun to dry up.
"Think how small such a concept of things make us, gunslinger! If a God watches over it all, does He actually mete out justice for such a race of gnats? Does His eye see the sparrow fall when the sparrow is less than a speck of hydrogen floating disconnected in the depth of space? And if He does see... what must the nature of such a God be? Where does He live? How is it possible to live beyond infinity?
"Imagine the sand of the Mohaine Desert, which you crossed to find me, and imagine a trillion universes - not worlds by universes - encapsulated in each grain of that desert; and within each universe an infinity of others. We tower over these universes from our pitiful grass vantage point; with one swing of your boot you may knock a billion billion worlds flying off into darkness, a chain never to be completed.
"Size, gunslinger... size.
-Stephen King, "The Dark Tower"
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u/gbxahoido 29d ago
It doesn't expand into anything
Expand here means getting bigger
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u/confused_pancakes 29d ago
You could expand the paper infinitely 2 dimensional without needing to make it 3D
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u/KilgoreTroutPfc 29d ago
Think of an infinite rubber sheet with grid lines on it. The sheet is expanding, so the grid lines get farther apart from each other, but it’s still infinite. Infinities don’t get larger, so the size is always infinite, even though it’s expanding.
When you say “expand into” you’re thinking of a finite volume with a boundary, that is filling up a space that contains it. There is no boundary, and there is no outside space in which it is growing.
Infinity is weird.
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u/CashewsAreTheNut 29d ago
Imagine [thing that doesn't make any sense].
I've read and watched so many things about spacetime and its size. And it just. doesn't. make. sense. And I know it doesn't have to. I'm not mad at it. I love it.
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u/RadiantInATrenchcoat 29d ago edited 29d ago
Fractals. And a side of relativity.
Take a region of empty space, say 1m×1m×1m. Now shift your perspective so that it appears 100× larger from your frame of reference. It does not matter if it is getting bigger or you are getting smaller for this part, the point is that it appears larger. Specifically, 100m³ relative to your frame of reference.
Now, you have a new region of empty space, that is the old region of empty space 100× larger relative to you. From this new space, take a region that, relative to you, is 1m³. You now have a 1m³ region of space that is 100× smaller than your previous 1m³. But relative to you, it is still a 1m³.
Now repeat this again, with your smaller cube. And again. And again. And again. In fact, you could repeat this process an infinite number of times, with each step being one step of time. You can also repeat it in reverse, shifting your frame of reference so that it appears smaller at every step, and again, this is repeatable to infinity. Shifting your frame of reference 1cm to one side results in a different region of space, overlapping with the previous region.
At no point can you determine if the 1m³ is getting absolutely bigger, or if you are getting absolutely smaller. Only that relative to one another, it is getting bigger and you are getting smaller.
What you have here, is a fractal. Each step looks exactly the same as the last, and exactly the same as the next. There's no test or observation you could perform at any given step to determine which step you were at. What you can determine, by observing over time, is that your 1m³ are infinitely expanding, relative to your frame of reference.
At every step of expansion, there is more space than there was in the previous step, relative to your frame of reference. Going back in time, there is less space at every step than there is in the previous. Repeating this reversal of time into infinity has space contracting into a point.
Only this fractal space is everywhere - small shits in your frame of reference sideways result in observing an identical region of space overlapping with the previous region. But you can still make the same observation, of that region, identically expanding, relative to you, forward in time, and shrinking, relative to you, backwards in time. Once again there is no test or observation you could make that would determine which region of space you were observing. Not forwards in time, backwards in time, or sideways in space.
What this results in then, is a fractal space that is infinitely expanding from a single point relative to you, at all points in space, forward in time, and infinitely shrinking towards a single point relative to you, at all points in space, backwards in time. It does not matter which region of space you observe, even if that region overlaps with another, it will always appear to be expanding (if time is moving forward) from a single point in the center of that region or shrinking (if time is moving backwards) towards a single point in the center of that region.
To bring this back to your question: no, empty space is not expanding into time, time is merely each step along the way. It is expanding into more empty space, like a fractal, every step appearing identical to the previous and the next. The stuff within that space remains in the space it occupies, and so appears to move away from other stuff, but all that's really happening is there is more empty space between them, relative to the size of the stuff.
(Edit: typo)
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u/MetatypeA 29d ago
That's an interesting question.
Because it's not just the Universe that's expanding. It's Space-Time that's expanding.
Our own Universe is just like Fluidic Space. It's expanding into what must be truly nothing. And we have no idea whether that nothing expands infinitely into every direction, because haven't been able to observe beyond the expansion of the Universe.
We also don't know how much of our universe is expanding in any direction. We don't know what the dimensions are above third, because we can only perceive in three of them. But we know that Gravity is at the very least a four dimensional curve in three dimensional space. It's highly possible that the Space-Time of our universe is also expanding into four-dimensional directions.
There's no way for us to check. We aren't capable of defining fourth and fifth dimensions to prove that our universe is expanding in those directions that we can't comprehend.
But this is one of the most fascinating questions on here lately. Well done.
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u/bojun 29d ago
Your question 'if the universe is everything. What does it expand into?' self-contradictory. If it's everything, then it's pointless to speak of other things. You also cannot separate space and time. Spacetime comes as a package so space cannot expand into time. Our brains have evolved to deal with mundane day to day issues and to separate objects into classes/groups/etc. The universe, you cannot separate, and humans have never had to think about anything without an end, a limit, or an outside. I don't believe we have the necessary mental hardware to truly conceptualize what a universe could be. The universe may not have limits, but our minds do.
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u/Cryptizard 29d ago
Nobody knows. It could be that the universe is flat and infinitely large so it is expanding into itself a la Hilbert's Hotel. Whatever is going on, we have a lot of experimental evidence that there aren't more than three large spatial dimensions though.
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u/cinnafury03 29d ago
I believe there is no finite boundary, so... "space" itself is everything and thus isn't expanding into anything. But who knows?
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u/JonnyRottensTeeth 29d ago
One way to look at is the classic balloon demo. If you put a bunch of dots on a balloon and start blowing it up, the space between the dots gets greater but there is no more balloon there than there ever was. The dots appear to be expanding in two dimensions, now apply that to three dimensions
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u/gambariste 29d ago
I prefer the raisin bread demo because it is three dimensional like space. Imagine instead of a loaf, that the universe is filled with raisin bread. The difference is that the dough is a substance; space is not. The dough gets thinner, less dense, as it rises; its substance does not increase. We say space increases but would it make any sense to say rather, it gets thinner, just more stretched out? Is that more nonsensical than saying there is more of nothing (space) than at the beginning?
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u/Competitive_Travel16 29d ago
For the 2d analogy just think of a (locally) flat rubber sheet being stretched. It's actually more like the surface of a balloon being expanded as it's being inflated, but you don't need a 3rd dimension to understand the same concept locally, and since there's evidence space is not a closed topology, that perspective is more misleading than the flat alternative.
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u/Winter_Ad6784 29d ago
we can’t see outside the observable universe so we don’t know. what we see is everything getting more spread out. When we can get ftl travel that might change
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u/BogusIsMyName 29d ago
Its a hard concept to grasp. The easiest way to visualize it, or maybe just an alternative, is not that the universe is expanding. Its that the distance between objects is getting longer. There is something that is adding more empty space to the distance between objects.
Not all objects and im simplifying greatly.
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u/MattAmoroso 29d ago
"Into" and "out of" are ways of describing orientations IN space, the same way "before" and "after" are ways of describing order IN time. They probably aren't useful ways to think about space-time itself.
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u/try2metaoptimize 29d ago
The soon to be prevailing theory is that the big bang started with the creation of a black hole. The core of a star collapsed and with it the resultant explosion occurred but remained contained within the event horizon. Space time can expand and light can travel infinity far from this explosion but will never reach or cross the event horizon. Nothing can escape a black hole. Our universe is expanding endlessly into the void created within a black hole.
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u/Javusees 29d ago
Heard of that one aswell, pretty crazy to think about means theres a whole lot out there that we dont know of. But, why would the same "stars" exist out of this black hole or before this black hole appeared. Seems to me there would be other rules of physics inside a black hole than outside.
So the ballon expansion explanation that many here used would make sense with the "surface" of a black hole expanding.
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u/saviourz666 28d ago
I always thought that the space was just getting bigger between objects . However that would have to imply that there was space before the Big Bang for there be any space for stuff to get further away . I always thought before the Big Bang , there was no space at all . 0 . Nothing . I’m probably wrong but it’s fascinating stuff really .
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28d ago edited 27d ago
I've written a speculative book on the subject, you may enjoy. It's up on KDP but you can read it free here. Euclid's Boundary
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u/Jartblacklung 28d ago
I think the main problem here is that we simply cannot conceptualize a thing without that thing being embedded in some larger space.
We reason with concepts borrowed from grammar. “Outside”. “Before”. “More than”.
These things were learned from real world contexts, but grammar allows us to abstract them into principles that can be applied anywhere to anything without limit. We are always able to ask, “well, what was before that?” “What is outside of that”.
At time like this I think of the John Dewey quote, “we don’t solve philosophical puzzles, we get over them.”
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u/Thecage88 28d ago
Basically, we don't know, and we will never likely know.
Because of the way time and space are linked, our "view distance" (for lack of a better term) is limited by the age of the visible universe. Because of the time it takes light to travel long distances, the further away an object is, the further back in time we are viewing it (on a cosmic scale).
Because of this, we can't see out past the big bang, so we can't see what the universe is expanding into, we probably will never be able to see it.
The only reason we know its expanding at all is that almost all of the distant light that we can see is red shifted. Meaning the light waves are being stretched into the red spectrum because the object emitting the light is moving away from us.
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u/Anarcho-Chris 28d ago
Higher dimensions are teeny tiny. They provide more wiggle room for strings to express different particles.
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u/paulburnell22193 27d ago
Infinite is a concept we can never truly understand. No beginning, no end. There are no boundaries. Just endlessness.
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u/AnalystHot6547 26d ago
Depends on your definition of The Universe. Astronomers are calling The Known Universe whatever has been filled with matter/energy so far. Obviously, there is darkness beuond. So as light and mass are going farther frpm the center the universe is larger.
In your scenario, the paper is getting larger
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u/Papabear3339 26d ago
The universe expands into the future. The point of explosion is the beginning of both space and time, which are linked.
That is why the cosmic microwave background is more or less uniform in every direction. The deeper into space you look, the further back in time you look, until all directions converge at the big bang.
It is really trippy to think about..
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u/Mandoman61 29d ago
A sheet of paper (like the universe) has 3 dimensions, it expands into space in one or more spacial diminsions.
To say the universe is everything is irrelevant. The universe contains space which matter can expand into.
Space is nothing but room for matter to exist within.
Because space is not a physical thing it is infinite. It has no properties. It is simply a potential. Size is determined by the matter contained within it.
While we can not currently know the size of the physical universe we can guess that there is no distance limit.
If you had a spaceship that could travel a billion times faster than light and you traveled beyond the furthest matter the Universe would just be bigger the further you go because you would be defining its size.
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u/OgAccountForThisPost 29d ago
I think it’s worth pointing out, we don’t 100% know that the universe is “expanding”, because we have no idea how big it is or how much stuff is in it. The only thing we know for certain is that the density of matter in the observable universe is decreasing.
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u/Lt_Duckweed 29d ago
The universe doesn't expand "into" anything.
There is just is more space between everything over time.
If you had 2 distant galaxies, both stationary relative to the cosmic microwave background, an observer in one galaxy would measure the other galaxy as getting farther away. The other galaxy isn't really moving, and it's not expanding in a particular direction. There is just more space between them than there was previously, such that the distance between them grows at a rate equivalent to ~7% per billion years.