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Could someone help settle a debate? The universe is infinite, or at least appears so. The cosmic background radiation was because the early universe was hot ev

Could someone help settle a debate? The universe is infinite, or at least appears so. The cosmic background radiation was because the early universe was hot everywhere. However, I’ve been telling my wife that the universe didn’t start from a single point; it has always been infinite. The concept of “smaller” is only because the matter was less dispersed than it is today. Veritasium’s video seems to support this argument. She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang. How can both views be correct? Either the universe isn’t infinite, and therefore has some kind of edge, or it is, and has always had infinite size.

varjag 3y ago
Imagine you're inflating an air balloon, first to a size of an apple then size of your head then to enormous like a weather balloon. Now consider that the surface of balloon is the space: points on the surface get further and further apart but there is no edge or boundary.
- sillysaurusx 3y ago
  How is it possible to say with any certainty that the universe was the size of an apple? That analogy also implies that space wraps around, which no experiment has ever shown to be true. This contradiction is why I have trouble. It’s a good analogy otherwise, but the details are the interesting part.
  - varjag 3y ago
    > How is it possible to say with any certainty that the universe was the size of an apple? Haven't read the paper but one likely could tell from the density. > That analogy also implies that space wraps around, which no experiment has ever shown to be true. It's an analogy meant to highlight one particular aspect, not be a perfect model of spacetime. In mathematics, there is no shortage of topological objects that are infinite but not closed otherwise.
- theferalrobot 3y ago
  Wouldn’t that be more applicable in a universe that wasn’t flat (unlike ours as best as we can measure).
  - varjag 3y ago
    There is also no air inside and nobody's actually blowing. It's an analogy.
    - theferalrobot 3y ago
      It is an analogy that is used to describe spacetime curvature though… its the wrong analogy.
      - varjag 3y ago
        No it's an analogy used to describe inflation with no boundary.
        oneshtein 3y ago
        Air balloon has clear boundary.
        consilient 3y ago
        The volume enclosed by the balloon's surface has a boundary, the surface itself does not.
theferalrobot 3y ago
You put into words a discrepancy that has annoyed the back of my mind for some time now. I had actually always learned that it went from single point to infinite instantly but grows more dispersed. But recently have been seeing a lot things in a similar vein, Kurzgesagt put out a video and even a special pin about the size of the universe at different points of time. It would seem if we know that to be the case and we know the growth rate we could then estimate the size of the universe now, but nobody suggests a finite universe with a certain size as the present universe, just the past. That would additionally suggest that there is a ‘center’ of the universe which I again have always learned as N/A since it is infinite. Really confuses me.
- LASR 3y ago
  Key word is “Observable”. The observable universe has a definite size. Shortly after the Big Bang, what is now the observable universe was the size of an apple etc. Now it is many tens of billions of light years across. The observable universe does have a center - you, the observer.
  - theferalrobot 3y ago
    Okay that distinction would make sense to me - we know everything was closer together - so what is currently the observable universe was once different sizes but we can’t speak to the grander universe as a whole as having a size in any meaningful terms. That sound correct?
    - Filligree 3y ago
      We’re reasonably sure the universe as a whole is infinite. Whether or not you want to accept that as a valid size is up to you, but I’d recommend defining “size” to include ordinals.
      - hansbo 3y ago
        Are we? How? Because the curvature is flat?
mr_mitm 3y ago
You are correct. > She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang. These articles implicitly talk about the observable universe. Or they're just wrong. It definitely didn't start from a single point, but there was a time where the observable universe was the size of an apple (or anything else that is larger than a point).
- sillysaurusx 3y ago
  Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere? In other words, the idea that the observable universe could be the size of an apple has its own contradictions. An apple is a definite size, and can fit only a certain number of atoms. However the expansion of the universe doesn’t seem to be creating more atoms; it’s only moving them around. Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning. This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
  - mr_mitm 3y ago
    > Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere? Well, sure, it's possible. I was assuming the current cosmological standard model is correct, which might in fact be wrong, or at least incomplete. Actually, I'm certain it's incomplete. Today we can trace back the universe to when it was a tiny fraction of a second old using standard physics. Everything is well understood. Before that: who knows? > An apple is a definite size, and can fit only a certain number of atoms. There were no atoms when the current observable universe was the size of an apple. It was too hot for atoms. It was more like a quark-gluon plasma. > Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning. Obviously "the size of an apple" is a phrase for laymen. We could say "10cm in diameter" using the standard definitions of the meter and second. >This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume. There is no contradiction.
theobreuerweil 3y ago
I’m not 100% on this but the way I understood this is by analogy to the 2D surface of the Earth. The surface of the Earth is finite yet has no reachable edge. The universe is the same in 3D. Like if the Earth grew then there would be more land but still no more or less “edge” of the world, and the same could hold for the universe. That said I think this is one theory rather than accepted fact?
- MrGLaDOS 3y ago
  In the case of a universe of finite size, this analogy explains how there can be such a thing as a finite space without there being boundaries provided the space is (slightly) curved. So this has less to do with an infinitely sized universe and more with the question of “What exists beyond the edge of the universe if it would be finite in size?”
  - hoseja 3y ago
    Our universe is probably a black hole in another, larger universe we cannot reach. And then turtles all the way down.
- ywain 3y ago
  > The universe is the same in 3D. This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).
  - consilient 3y ago
    > This would mean the universe has positive curvature. There are several flat compact manifolds, they're just not simply connected.
    - mr_mitm 3y ago
      In particular, almost none of them are isotropic. The assumption that the universe is isotropic is part of the cosmological principle and the foundation of modern cosmology. It's a very natural assumption to make, so the vast majority of cosmologists are quite comfortable with it, but at the end of the day it's just an assumption that could be wrong.
scotchmi_st 3y ago
The universe isn’t thought to be infinite in that sense- it’s finite but unbounded. There is a finite amount of space that did, we think, originate from an infinitesimal point. Imagine getting in a spaceship and setting off in a direction, close to the speed of light- if you travelled for long enough you’d arrive back where you started. Like someone walking on the surface of the earth. Any current cosmologists can correct me, but I believe the consensus is that the shape of the universe is a hyper-spheroid, I.e a 4D sphere.
- sillysaurusx 3y ago
  Is there an experiment which leads us to believe that the universe wraps around? I’d be very interested in any details supporting this idea.
  - scotchmi_st 3y ago
    I mean, there’s plenty of evidence for the Big Bang, and if that’s the case then the most likely shape that falls out of the maths is a hypersphere, like inflating a balloon as others have said.
    - mr_mitm 3y ago
      No, the big bang model allows for three topologies: flat, spherical or hyperbolical. A picture like this is in every cosmology textbook: https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_universe.jpg https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_u... Which one it is depends on the density. There is a critical density at which the universe is flat, and our current best measurements basically allow for all three possibilities. It's just hard to say if something is flat or actually spherical but really, really huge.
  - ben_w 3y ago
    If there were certain patterns in two different (opposite?) parts of the CMB, that would suggest it wraps. None have been found, so if it does wrap it's bigger than that.
- 3y ago
raz32dust 3y ago
"most accepted theory of physics is that the universe started from a single point": This is not true. The most accepted theory is that the universe was extremely dense, but it is not accepted that it was literally a "point" of zero volume. Our laws of physics are thought to break down beyond a certain density of matter, so we don't have any reliable predictions about it. The "dense" universe could still have been infinite even back then. Just super dense and infinite in all directions - and now all of it has expanded and it's still infinite (a bigger infinity?).
- sillysaurusx 3y ago
  But if it was dense and infinite in all directions, why do so many people use language like “it was the size of an apple”? This is the interesting contradiction. An apple is only a certain number of atoms, far fewer than the 10^80 we have today.
  - ben_w 3y ago
    I think that's supposed to refer to the currently observable bit being the size of an apple; though given how bad newspaper analogies are, this may all derive from misunderstandings one hungry physicist 60 years ago and subsequently all quoting each other.
  - aziaziazi 3y ago
    Here’s my take: - today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do. Therefore you never get to that edge, making it infinite. - before/at big bang universe is a single point. You are within this single point but can’t go outside because time does not exist and traveling need time. - when the big band happened, it grew in size very, very fast, close to speed of light. During that enlargement phase it grew to the size of an Apple and if you were within it and try to look in any direction, the further think you can see is the Apple skin. That’s the observable universe. But to see the skin you need to go there or wait for particule to come to you (eg photon). By that time the universe has already dilated and grown a lot wider. Some particules from the skin are already far away. You’ll never be able to see them : by the time needed for information to come to you, other information goes in the opposite direction.
danbruc 3y ago
The correct answer is, we don't know. We can see back until about 380,000 years when the universe became transparent enough for light to reach us, the cosmic microwave background. Everything before that is theoretical and depends on the model used, for example cosmic inflation, which has experimental support in so far as it correctly predicts features of the cosmic microwave background. Did inflation actually happen? Maybe. But even if inflation is the correct theory, go back far enough towards time zero and quantum effects become relevant to avoid the classical big bang singularity but we don't have an accepted theory of quantum gravity. So who knows what was actually going on. Observing the cosmic neutrino background could allow us the see past the cosmic microwave background but for the very first moments - if there was such a thing - we will probably have to rely on theory - which we don't have - for the foreseeable future.
- mr_mitm 3y ago
  We can "see" a little bit further than that. The observed helium abundance for example shows that our understanding of the first three minutes appears to be quite robust. Inflation covers a much different era, 10^-32s after the big bang.
yieldcrv 3y ago
Its also possible that the entire night sky lights up one day as the light from many big bangs and their universes beyond the edge of our “universe” finally reaches us so we just don’t know. we have no evidence of that but all theories would simply change with the new information and that would become standard physics.
Galaxeblaffer 3y ago
My favorite cyclic theory so far is probably Roger Penrose's conformal cyclic cosmology. In essence the universe keeps expanding exponentially which in the end becomes equivalent to a new big bang since the universe at that point will loose all notions of scale due to there being no more particles to drive scale. A much better explanation is given here https://youtu.be/FVDJJVoTx7s https://youtu.be/FVDJJVoTx7s
- denton-scratch 3y ago
  I'm a Penrose fan too. But I don't think CCC envisages any Big Bangs; CCC envisages a future heat death, where everything has turned into solitary photons. Photons have no clock, and don't experience the passing of time; distance is nothing to a photon, the moment it starts travelling, it arrives at its destination. So a universe containing only photons has no definite scale - it can re-imagine itself as being the size of a grapefruit, or any size you like. Penrose uses this idea to wish away the awkward and evidence-free idea of cosmic inflation.
ricardo81 3y ago
To add to your question and a weak attempt at answering (as a Layman)- when the universe was the size of an apple, how long would it take for a particle to travel from one side to the other? I think the answer would be forever which in a sense makes it infinite, regardless of the universe's finite size. I guess we have the luxury of removing ourselves from the universe while thinking about it, if Platonism is a thing. Reminds me of the halting problem.
- Filligree 3y ago
  Which isn’t what astronomers mean when they say it’s infinite. They mean actually infinite. There’s no trickery.
  - ricardo81 3y ago
    You say infinite like it's one thing, but there's more than one infinity.
layer8 3y ago
Language such as “it grew to the size of an apple” always refers to the currently observable universe. That portion of the universe was the size of an apple at some point in time.
raattgift 3y ago
Right now as a pulse of light hits your retina (or a radiotelescope nearby) the source of that pulse must be some event at a finite time in the past. That is the Hubble time, which is non-identical to the age of the universe because the Hubble time captures the history of the expansion of space, which is not linear. That nonlinearity makes the present Hubble time greater than the present age of the universe. In general, however, the Hubble time grows in the future and shrinks in the past. One can also think of the Hubble time at any moment as the amount of time it would take the universe to expand to its size at that moment, if the expansion were linear; this is typically how it's taught [*]. If we multiply the Hubble time by c, we have the Hubble length, which is what is being talked about with expressions like "the size of an apple". That is, if the universe was "apple-sized" the Hubble length was on the order of an attoparsec rather than the ~4.4 gigaparsecs today. Extending the Hubble length in all directions gives us a spatial surface at the Hubble time, which means that on (or inside) that surface is everything that generates something which can in principle be detected here-and-now. That's the "size" which is being talked about. Because the Hubble time is shorter in the past, so too is the "size" of the Hubble volume in the past. It is somewhat easier to think about your question in respect to the universe a bit later, like during Big Bang nucleosynthesis (BBNS) when the Hubble length was on the order of 100 parsecs and adiabatic expansion had cooled the contents of the Hubble volume (Hubble length in every direction) enough that hydrogen, helium, and lithium nuclei could form. Going back to the Hubble time we turn ~100 parsecs into ~300 light years, divde that by c and have a then-Hubble-time of 10 billionths of a second. So something ~speed-of-light (actual light, gravitational radiation, neutrinos, ultrarelativistic charged particles) emitted billionths of a second earlier and at maximum distance (~100 parsecs) would be reaching our galaxy now, billions of years later. Anything emitted then towards us but from further than that ~100 parsec boundary would not reach us at all. Anything emitted from then but travelling on longer paths (gravitational lensing, interaction with gas or dust, or simply having intrinsic mass like a neutrino) will arrive later. And of course anything emitted then might not be aimed at us, so might never get here at all. So, and I'll touch on this again below, what happens to a BBNS proton that formed about 100 parsecs from protons that ended up in the Milky Way, if it happens to get a kick in a direction away from those Milky-Way-BBNS protons? It becomes unobservable, leaving the observable universe. But what do we call wherever it ends up? And in that region, whatever we all it (the unobservable universe?), are physics the same as in the parts of the observable universe we've studied? Back to your apple-sized observable universe. We can continue a regression before BBNS, which takes us through epochs where electrons and photons and weak force counterparts are so hot and compressed they become something else (the fields of the electroweak unification), and so on. As we do so the Hubble time is shortened, and so too is the Hubble length. Eventually the Hubble time is so short that gravitational time dilation (from differing concentrations of energy and momentum) becomes a significant fraction. Thus it, and c*Hubble_time -> Hubble_length is subject to fluctuations of significant and growing fractions of the the average (mean or mode). [Ultimately this is where there is an early-universe conflict in the predictions of General Relativity (which is fine with this; we just stop splitting spacetime into spaces indexed by an averaged Hubble time {technically, the derivatives of the Robertson-Walker scale factor \a becomes un-useful}, and index on some other notion of time (or treat the whole region of spacetime as a block) and quantum field theory (which needs a clearer and quite linear notion of time to plug into its equations).] Regressing further we will calculate a lower-bound Hubble volume which is for all practical purposes infinitesimal, and thus "a single point" is a reasonable description for the observable universe at about that early age. Anything outside the then-infinitesimal Hubble volume will never reach us. Anything inside could in principle, but again lots of that wasn't aimed in our direction and/or ended up becoming "frozen" into non-massless matter (some of which will eventually radiate, like galaxies full of stars, with some of that radiation aimed at us). > didn't start from a single point; it has always been infinite We don't know what if anything is outside the Hubble volume at any time, including in the most distant past. It's strange to think that there is nothing at all one parsec beyond the present Hubble length of ~ 4.4 billion parsecs. Where does the light and cosmic rays from distant galaxies radiated radially away from us go? Does each distant galaxy have its own Hubble volume[*], just like each of us locally have? We can only decide to adopt the Copernican principle and say that some distant barred-spiral galaxy has the same view as the Milky Way does -- galaxies scattered similarly in every direction. Adopting that view motivated Guth, one of the originators of studies of cosmic inflation, to calculate that there may be something like 10^23 Hubble volumes "outside" our own. These would be 10^23 100-parsec volumes adjacent to each other all arranged around our 100-parsec BBNS volume. Each such volume may have the same early expansion history as ours, so (regressing) this would be a lot of apple-sized Hubble volumes, and ultimately a lot of effectively point-sized Hubble volumes. 10^23 is finite, of course, but I don't think we can preclude a much much larger finite number, and we can go so far as to ask questions about a literal infinite number. There will be some lower bound of Hubble volumes adjacent to, outside of, and causally disconnected from our own; that lowest number is much greater than one, however, or cosmic inflation fails to solve certain geometry problems. Additionally, zero ("our Hubble volume is it") raises the hard question of what happens even just a metre beyond the Hubble length. Is it new physics? Would there be some evidence close to but inside the Hubble length? Maybe even in galaxy clusters only tens of millions of parsecs away their radiation or morphology may show proof that they're close to an observer-independent sharp Hubble edge? JWST is already generating relevant data. Finally, from a comment of yours downthread, > An apple is only a certain number of atoms, far fewer than the 10^80 that we have today When the Hubble length was about half the length of an apple, there weren't any atoms at all -- it was too hot and dense for any of the particles you'd recognize in the Standard Model. That's why I chose the BBNS epoch (~100 parsec) instead of the deep elecroweak or GUT epoch at "apple-size" (~ 1 attoparsec). (And additionally atoms can be wholly ionized at much lower temperatures, so I write here about nuclei instead). At BBNS only hydrogen and some helium and trace lithium atomic nuclei were formed. All the carbon and so forth in an apple today was formed through stellar nucleosynthesis. There are fewer atomic nuclei today than at the end of BBNS because stellar processes fuse multiple hydrogens into carbon, oxygen and other "non-metal" (in the astronomy sense) nuclei. Also, stellar black holes have taken lots of atomic nuclei out of the picture. And also some nuclei will have taken trajectories away from us and are now further than the present day Hubble length. In cosmology we talk about the average density of baryons, compared to the averaged density of other things (among them radiation (mostly photons), relativistic and cold neutrinos, dark matter, dark energy). That is we are interested in the fraction of energy at a point that is baryonic (atomic nuclei, and more particularly protons, being the bulk of that). At earlier times, when the Hubble length is shorter, the (averaged, expected, typical) contribution of baryons at a (typical, average) point is higher. Baryonic matter has diluted with the expansion of the universe. Your 10^80 figure will come from taking this pointwise baryon density and applying it to all the points in space at the present Hubble time, cross-checking it with estimates of the masses of typical galaxies and an estimate of the galaxy count. The baryon density today can also be arrived at by comparing the baryon density at the time the cosmic microwave background radiation formed (that density left imprints in it) and the expansion history since then (which we get by various means). - -- [*] Unlike the Hubble time as the reciprocal of the Hubble constant, it is useful to think of the Hubble time at a point (in spacetime) as capturing the expansion history at that point. Since gravitation retards the expansion, every point (in spacetime) has its own Hubble time. Importantly, the Hubble time of a hydrogen nucleus peacefully floating in deep inter-galactic-cluster space will be longer than that of a hydrogen nucleus within our sun, even if they were both formed primordially. The gravitation of the sun and the Milky way and other nearby masses means the solar hydrogen has experienced less expansion. This takes us a tiny step closer to understanding the expanding universe in systems of coordinates other than the standard comoving coordinates, which is more relativistic in spirit. It reminds us that in comoving coordinates we are working with uniform densities of matter thanks to averaging, and are explicitly ignoring significant overdensities of matter (which slow down the ticking of clocks nearby).
thom 3y ago
I find it easier to picture the universe as more of a growing network of connected things, rather than an object with strict internal or external dimensions. I don’t think it (or anything) is infinite, but it might as well be because it cleverly prevents all parts from interacting with each other at all times (due to limits on information processing capacity, if you want to get quasi-religious about it).
- BrendanD 3y ago
  Greetings fellow Monad.
orra 3y ago
I believe it's an open question, the curvature of the universe, and hence whether the universe is infinite. Others have already clarified that the big bang tells us the observable universe used to be tiny. In other words, the universe was much denser. It does not tell us the absolute size of the entire universe.
- oneshtein 3y ago
  "Denser Universe" means that we were (or are) in a black hole. There is no other way around a huge mass in a small space.
photochemsyn 3y ago
Infinity is a tricky concept - a 2D creature on the surface of a 3D inflating balloon would view the surface they were on as infinite, particularly if the balloon was inflating faster than the bug could travel, such that no circumnavigation of the sphere was possible. Infinite space implies that one could travel forever in a straight line without encountering a boundary, but if we move to higher dimensions then it's possible that the universe is something like a 4D sphere of finite 4D volume bounded by a 3D 'surface' which is apparently infinite to its inhabitants. Moving to higher dimensions it might be the case that there are many universes, i.e. perhaps from a 5D perspective the multiverse is an infinite series of nested 4D universe hyperspheres, and one could go on and on like this into a infinite series of dimensions, but it's all just mathamatical metaphysics at that point as there's no observational evidence for any of that.
oneshtein 3y ago
You can use Occam's razor[0]: simpler solution with smaller set of elements has better probability to be true. a) Bing Bang model of evolution of Universe: single point, big bang, expansion of Universe, acceleration of expansion, time dilation, space-time warping. (6 extra elements) b) Steady Universe model of evolution of Universe: tired light. (1 extra element) [0]: https://en.wikipedia.org/wiki/Occam%27s_razor https://en.wikipedia.org/wiki/Occam%27s_razor
pdonis 3y ago
> How can both views be correct? They aren't. Your view is correct, at least according to our best current model of the universe. I suspect that your wife is misinterpreting the articles she is reading; I suspect they are saying only that our current observable universe, which is finite in size, started out as a single point. Which is sort of correct, but is not the same as saying the entire spatially infinite universe started out as a single point. Part of the problem here is that the "initial singularity" in the models being described (which is where the "single point" language comes from) is not actually part of the universe; it's a mathematical limit that does not actually correspond to anything physical. > Either the universe isn’t infinite, and therefore has some kind of edge The "edge" part is not correct. A spatially infinite universe would have the spatial topology of a 3-sphere, which has a finite volume bot no edge (just as the 2-sphere surface of our Earth has a finite area but no edge).
- EMM_386 3y ago
  How does something infinite "expand" though? It's not expanding into anything, because it's infinite. I always hear "space itself is expanding" but how does that work with topology? When you travel in a given direction you will arrive back where you started? Surely that "shape" or "object" or topology or whatever you want to call it has a volume, relative to something else, or it wouldn't be able to be calculated? As you can tell, I have no idea what I'm talking about but endlessly curious on this subject.
  - pdonis 3y ago
    > How does something infinite "expand" though? Infinite things don't work like our ordinary intuitions expect. > I always hear "space itself is expanding" but how does that work with topology? The topology doesn't change as the universe expands. "Expanding universe" just means that a particular metric (geometry) is imposed on the topology. If you think of the spacetime of the universe as a stack of 3-dimensional spatial slices, "expansion" just means that a given pair of points in space gets further and further apart as you advance from slice to slice into the future. That is perfectly well-defined for infinite slices, even though it's hard to imagine intuitively. The math works just fine. > When you travel in a given direction you will arrive back where you started? No. That would be the case for a spatially finite universe with the topology of a 3-sphere. But our spatially infinite universe has the topology of Euclidean 3-space. > Surely that "shape" or "object" or topology or whatever you want to call it has a volume The spatial 3-volume of the universe is infinite. > relative to something else, or it wouldn't be able to be calculated? Not at all. The universe doesn't need to be embedded in anything else to have a well-defined volume.
codethief 3y ago
> The universe is infinite, or at least appears so. The universe is certainly very big but whether it's finite or infinite is not known with certainty, see https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite_or_finite https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...

varjag 3y ago

Imagine you're inflating an air balloon, first to a size of an apple then size of your head then to enormous like a weather balloon. Now consider that the surface of balloon is the space: points on the surface get further and further apart but there is no edge or boundary.

sillysaurusx 3y ago
How is it possible to say with any certainty that the universe was the size of an apple? That analogy also implies that space wraps around, which no experiment has ever shown to be true. This contradiction is why I have trouble. It’s a good analogy otherwise, but the details are the interesting part.
- varjag 3y ago
  > How is it possible to say with any certainty that the universe was the size of an apple? Haven't read the paper but one likely could tell from the density. > That analogy also implies that space wraps around, which no experiment has ever shown to be true. It's an analogy meant to highlight one particular aspect, not be a perfect model of spacetime. In mathematics, there is no shortage of topological objects that are infinite but not closed otherwise.
theferalrobot 3y ago
Wouldn’t that be more applicable in a universe that wasn’t flat (unlike ours as best as we can measure).
- varjag 3y ago
  There is also no air inside and nobody's actually blowing. It's an analogy.
  - theferalrobot 3y ago
    It is an analogy that is used to describe spacetime curvature though… its the wrong analogy.
    - varjag 3y ago
      No it's an analogy used to describe inflation with no boundary.
      - oneshtein 3y ago
        Air balloon has clear boundary.
        consilient 3y ago
        The volume enclosed by the balloon's surface has a boundary, the surface itself does not.

theferalrobot 3y ago

You put into words a discrepancy that has annoyed the back of my mind for some time now. I had actually always learned that it went from single point to infinite instantly but grows more dispersed. But recently have been seeing a lot things in a similar vein, Kurzgesagt put out a video and even a special pin about the size of the universe at different points of time. It would seem if we know that to be the case and we know the growth rate we could then estimate the size of the universe now, but nobody suggests a finite universe with a certain size as the present universe, just the past. That would additionally suggest that there is a ‘center’ of the universe which I again have always learned as N/A since it is infinite. Really confuses me.

LASR 3y ago
Key word is “Observable”. The observable universe has a definite size. Shortly after the Big Bang, what is now the observable universe was the size of an apple etc. Now it is many tens of billions of light years across. The observable universe does have a center - you, the observer.
- theferalrobot 3y ago
  Okay that distinction would make sense to me - we know everything was closer together - so what is currently the observable universe was once different sizes but we can’t speak to the grander universe as a whole as having a size in any meaningful terms. That sound correct?
  - Filligree 3y ago
    We’re reasonably sure the universe as a whole is infinite. Whether or not you want to accept that as a valid size is up to you, but I’d recommend defining “size” to include ordinals.
    - hansbo 3y ago
      Are we? How? Because the curvature is flat?

mr_mitm 3y ago

You are correct. > She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang. These articles implicitly talk about the observable universe. Or they're just wrong. It definitely didn't start from a single point, but there was a time where the observable universe was the size of an apple (or anything else that is larger than a point).

sillysaurusx 3y ago
Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere? In other words, the idea that the observable universe could be the size of an apple has its own contradictions. An apple is a definite size, and can fit only a certain number of atoms. However the expansion of the universe doesn’t seem to be creating more atoms; it’s only moving them around. Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning. This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
- mr_mitm 3y ago
  > Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere? Well, sure, it's possible. I was assuming the current cosmological standard model is correct, which might in fact be wrong, or at least incomplete. Actually, I'm certain it's incomplete. Today we can trace back the universe to when it was a tiny fraction of a second old using standard physics. Everything is well understood. Before that: who knows? > An apple is a definite size, and can fit only a certain number of atoms. There were no atoms when the current observable universe was the size of an apple. It was too hot for atoms. It was more like a quark-gluon plasma. > Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning. Obviously "the size of an apple" is a phrase for laymen. We could say "10cm in diameter" using the standard definitions of the meter and second. >This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume. There is no contradiction.

theobreuerweil 3y ago

I’m not 100% on this but the way I understood this is by analogy to the 2D surface of the Earth. The surface of the Earth is finite yet has no reachable edge. The universe is the same in 3D. Like if the Earth grew then there would be more land but still no more or less “edge” of the world, and the same could hold for the universe. That said I think this is one theory rather than accepted fact?

MrGLaDOS 3y ago
In the case of a universe of finite size, this analogy explains how there can be such a thing as a finite space without there being boundaries provided the space is (slightly) curved. So this has less to do with an infinitely sized universe and more with the question of “What exists beyond the edge of the universe if it would be finite in size?”
- hoseja 3y ago
  Our universe is probably a black hole in another, larger universe we cannot reach. And then turtles all the way down.
ywain 3y ago
> The universe is the same in 3D. This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).
- consilient 3y ago
  > This would mean the universe has positive curvature. There are several flat compact manifolds, they're just not simply connected.
  - mr_mitm 3y ago
    In particular, almost none of them are isotropic. The assumption that the universe is isotropic is part of the cosmological principle and the foundation of modern cosmology. It's a very natural assumption to make, so the vast majority of cosmologists are quite comfortable with it, but at the end of the day it's just an assumption that could be wrong.

scotchmi_st 3y ago

The universe isn’t thought to be infinite in that sense- it’s finite but unbounded. There is a finite amount of space that did, we think, originate from an infinitesimal point. Imagine getting in a spaceship and setting off in a direction, close to the speed of light- if you travelled for long enough you’d arrive back where you started. Like someone walking on the surface of the earth. Any current cosmologists can correct me, but I believe the consensus is that the shape of the universe is a hyper-spheroid, I.e a 4D sphere.

sillysaurusx 3y ago
Is there an experiment which leads us to believe that the universe wraps around? I’d be very interested in any details supporting this idea.
- scotchmi_st 3y ago
  I mean, there’s plenty of evidence for the Big Bang, and if that’s the case then the most likely shape that falls out of the maths is a hypersphere, like inflating a balloon as others have said.
  - mr_mitm 3y ago
    No, the big bang model allows for three topologies: flat, spherical or hyperbolical. A picture like this is in every cosmology textbook: https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_universe.jpg https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_u... Which one it is depends on the density. There is a critical density at which the universe is flat, and our current best measurements basically allow for all three possibilities. It's just hard to say if something is flat or actually spherical but really, really huge.
- ben_w 3y ago
  If there were certain patterns in two different (opposite?) parts of the CMB, that would suggest it wraps. None have been found, so if it does wrap it's bigger than that.
3y ago

raz32dust 3y ago

"most accepted theory of physics is that the universe started from a single point": This is not true. The most accepted theory is that the universe was extremely dense, but it is not accepted that it was literally a "point" of zero volume. Our laws of physics are thought to break down beyond a certain density of matter, so we don't have any reliable predictions about it. The "dense" universe could still have been infinite even back then. Just super dense and infinite in all directions - and now all of it has expanded and it's still infinite (a bigger infinity?).

sillysaurusx 3y ago
But if it was dense and infinite in all directions, why do so many people use language like “it was the size of an apple”? This is the interesting contradiction. An apple is only a certain number of atoms, far fewer than the 10^80 we have today.
- ben_w 3y ago
  I think that's supposed to refer to the currently observable bit being the size of an apple; though given how bad newspaper analogies are, this may all derive from misunderstandings one hungry physicist 60 years ago and subsequently all quoting each other.
- aziaziazi 3y ago
  Here’s my take: - today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do. Therefore you never get to that edge, making it infinite. - before/at big bang universe is a single point. You are within this single point but can’t go outside because time does not exist and traveling need time. - when the big band happened, it grew in size very, very fast, close to speed of light. During that enlargement phase it grew to the size of an Apple and if you were within it and try to look in any direction, the further think you can see is the Apple skin. That’s the observable universe. But to see the skin you need to go there or wait for particule to come to you (eg photon). By that time the universe has already dilated and grown a lot wider. Some particules from the skin are already far away. You’ll never be able to see them : by the time needed for information to come to you, other information goes in the opposite direction.

danbruc 3y ago

The correct answer is, we don't know. We can see back until about 380,000 years when the universe became transparent enough for light to reach us, the cosmic microwave background. Everything before that is theoretical and depends on the model used, for example cosmic inflation, which has experimental support in so far as it correctly predicts features of the cosmic microwave background. Did inflation actually happen? Maybe. But even if inflation is the correct theory, go back far enough towards time zero and quantum effects become relevant to avoid the classical big bang singularity but we don't have an accepted theory of quantum gravity. So who knows what was actually going on. Observing the cosmic neutrino background could allow us the see past the cosmic microwave background but for the very first moments - if there was such a thing - we will probably have to rely on theory - which we don't have - for the foreseeable future.

mr_mitm 3y ago
We can "see" a little bit further than that. The observed helium abundance for example shows that our understanding of the first three minutes appears to be quite robust. Inflation covers a much different era, 10^-32s after the big bang.

yieldcrv 3y ago

Its also possible that the entire night sky lights up one day as the light from many big bangs and their universes beyond the edge of our “universe” finally reaches us so we just don’t know. we have no evidence of that but all theories would simply change with the new information and that would become standard physics.

Galaxeblaffer 3y ago

My favorite cyclic theory so far is probably Roger Penrose's conformal cyclic cosmology. In essence the universe keeps expanding exponentially which in the end becomes equivalent to a new big bang since the universe at that point will loose all notions of scale due to there being no more particles to drive scale. A much better explanation is given here https://youtu.be/FVDJJVoTx7s https://youtu.be/FVDJJVoTx7s

denton-scratch 3y ago
I'm a Penrose fan too. But I don't think CCC envisages any Big Bangs; CCC envisages a future heat death, where everything has turned into solitary photons. Photons have no clock, and don't experience the passing of time; distance is nothing to a photon, the moment it starts travelling, it arrives at its destination. So a universe containing only photons has no definite scale - it can re-imagine itself as being the size of a grapefruit, or any size you like. Penrose uses this idea to wish away the awkward and evidence-free idea of cosmic inflation.

ricardo81 3y ago

To add to your question and a weak attempt at answering (as a Layman)- when the universe was the size of an apple, how long would it take for a particle to travel from one side to the other? I think the answer would be forever which in a sense makes it infinite, regardless of the universe's finite size. I guess we have the luxury of removing ourselves from the universe while thinking about it, if Platonism is a thing. Reminds me of the halting problem.

Filligree 3y ago
Which isn’t what astronomers mean when they say it’s infinite. They mean actually infinite. There’s no trickery.
- ricardo81 3y ago
  You say infinite like it's one thing, but there's more than one infinity.

layer8 3y ago

Language such as “it grew to the size of an apple” always refers to the currently observable universe. That portion of the universe was the size of an apple at some point in time.

raattgift 3y ago

Right now as a pulse of light hits your retina (or a radiotelescope nearby) the source of that pulse must be some event at a finite time in the past. That is the Hubble time, which is non-identical to the age of the universe because the Hubble time captures the history of the expansion of space, which is not linear. That nonlinearity makes the present Hubble time greater than the present age of the universe. In general, however, the Hubble time grows in the future and shrinks in the past. One can also think of the Hubble time at any moment as the amount of time it would take the universe to expand to its size at that moment, if the expansion were linear; this is typically how it's taught [*]. If we multiply the Hubble time by c, we have the Hubble length, which is what is being talked about with expressions like "the size of an apple". That is, if the universe was "apple-sized" the Hubble length was on the order of an attoparsec rather than the ~4.4 gigaparsecs today. Extending the Hubble length in all directions gives us a spatial surface at the Hubble time, which means that on (or inside) that surface is everything that generates something which can in principle be detected here-and-now. That's the "size" which is being talked about. Because the Hubble time is shorter in the past, so too is the "size" of the Hubble volume in the past. It is somewhat easier to think about your question in respect to the universe a bit later, like during Big Bang nucleosynthesis (BBNS) when the Hubble length was on the order of 100 parsecs and adiabatic expansion had cooled the contents of the Hubble volume (Hubble length in every direction) enough that hydrogen, helium, and lithium nuclei could form. Going back to the Hubble time we turn ~100 parsecs into ~300 light years, divde that by c and have a then-Hubble-time of 10 billionths of a second. So something ~speed-of-light (actual light, gravitational radiation, neutrinos, ultrarelativistic charged particles) emitted billionths of a second earlier and at maximum distance (~100 parsecs) would be reaching our galaxy now, billions of years later. Anything emitted then towards us but from further than that ~100 parsec boundary would not reach us at all. Anything emitted from then but travelling on longer paths (gravitational lensing, interaction with gas or dust, or simply having intrinsic mass like a neutrino) will arrive later. And of course anything emitted then might not be aimed at us, so might never get here at all. So, and I'll touch on this again below, what happens to a BBNS proton that formed about 100 parsecs from protons that ended up in the Milky Way, if it happens to get a kick in a direction away from those Milky-Way-BBNS protons? It becomes unobservable, leaving the observable universe. But what do we call wherever it ends up? And in that region, whatever we all it (the unobservable universe?), are physics the same as in the parts of the observable universe we've studied? Back to your apple-sized observable universe. We can continue a regression before BBNS, which takes us through epochs where electrons and photons and weak force counterparts are so hot and compressed they become something else (the fields of the electroweak unification), and so on. As we do so the Hubble time is shortened, and so too is the Hubble length. Eventually the Hubble time is so short that gravitational time dilation (from differing concentrations of energy and momentum) becomes a significant fraction. Thus it, and c*Hubble_time -> Hubble_length is subject to fluctuations of significant and growing fractions of the the average (mean or mode). [Ultimately this is where there is an early-universe conflict in the predictions of General Relativity (which is fine with this; we just stop splitting spacetime into spaces indexed by an averaged Hubble time {technically, the derivatives of the Robertson-Walker scale factor \a becomes un-useful}, and index on some other notion of time (or treat the whole region of spacetime as a block) and quantum field theory (which needs a clearer and quite linear notion of time to plug into its equations).] Regressing further we will calculate a lower-bound Hubble volume which is for all practical purposes infinitesimal, and thus "a single point" is a reasonable description for the observable universe at about that early age. Anything outside the then-infinitesimal Hubble volume will never reach us. Anything inside could in principle, but again lots of that wasn't aimed in our direction and/or ended up becoming "frozen" into non-massless matter (some of which will eventually radiate, like galaxies full of stars, with some of that radiation aimed at us). > didn't start from a single point; it has always been infinite We don't know what if anything is outside the Hubble volume at any time, including in the most distant past. It's strange to think that there is nothing at all one parsec beyond the present Hubble length of ~ 4.4 billion parsecs. Where does the light and cosmic rays from distant galaxies radiated radially away from us go? Does each distant galaxy have its own Hubble volume[*], just like each of us locally have? We can only decide to adopt the Copernican principle and say that some distant barred-spiral galaxy has the same view as the Milky Way does -- galaxies scattered similarly in every direction. Adopting that view motivated Guth, one of the originators of studies of cosmic inflation, to calculate that there may be something like 10^23 Hubble volumes "outside" our own. These would be 10^23 100-parsec volumes adjacent to each other all arranged around our 100-parsec BBNS volume. Each such volume may have the same early expansion history as ours, so (regressing) this would be a lot of apple-sized Hubble volumes, and ultimately a lot of effectively point-sized Hubble volumes. 10^23 is finite, of course, but I don't think we can preclude a much much larger finite number, and we can go so far as to ask questions about a literal infinite number. There will be some lower bound of Hubble volumes adjacent to, outside of, and causally disconnected from our own; that lowest number is much greater than one, however, or cosmic inflation fails to solve certain geometry problems. Additionally, zero ("our Hubble volume is it") raises the hard question of what happens even just a metre beyond the Hubble length. Is it new physics? Would there be some evidence close to but inside the Hubble length? Maybe even in galaxy clusters only tens of millions of parsecs away their radiation or morphology may show proof that they're close to an observer-independent sharp Hubble edge? JWST is already generating relevant data. Finally, from a comment of yours downthread, > An apple is only a certain number of atoms, far fewer than the 10^80 that we have today When the Hubble length was about half the length of an apple, there weren't any atoms at all -- it was too hot and dense for any of the particles you'd recognize in the Standard Model. That's why I chose the BBNS epoch (~100 parsec) instead of the deep elecroweak or GUT epoch at "apple-size" (~ 1 attoparsec). (And additionally atoms can be wholly ionized at much lower temperatures, so I write here about nuclei instead). At BBNS only hydrogen and some helium and trace lithium atomic nuclei were formed. All the carbon and so forth in an apple today was formed through stellar nucleosynthesis. There are fewer atomic nuclei today than at the end of BBNS because stellar processes fuse multiple hydrogens into carbon, oxygen and other "non-metal" (in the astronomy sense) nuclei. Also, stellar black holes have taken lots of atomic nuclei out of the picture. And also some nuclei will have taken trajectories away from us and are now further than the present day Hubble length. In cosmology we talk about the average density of baryons, compared to the averaged density of other things (among them radiation (mostly photons), relativistic and cold neutrinos, dark matter, dark energy). That is we are interested in the fraction of energy at a point that is baryonic (atomic nuclei, and more particularly protons, being the bulk of that). At earlier times, when the Hubble length is shorter, the (averaged, expected, typical) contribution of baryons at a (typical, average) point is higher. Baryonic matter has diluted with the expansion of the universe. Your 10^80 figure will come from taking this pointwise baryon density and applying it to all the points in space at the present Hubble time, cross-checking it with estimates of the masses of typical galaxies and an estimate of the galaxy count. The baryon density today can also be arrived at by comparing the baryon density at the time the cosmic microwave background radiation formed (that density left imprints in it) and the expansion history since then (which we get by various means). - -- [*] Unlike the Hubble time as the reciprocal of the Hubble constant, it is useful to think of the Hubble time at a point (in spacetime) as capturing the expansion history at that point. Since gravitation retards the expansion, every point (in spacetime) has its own Hubble time. Importantly, the Hubble time of a hydrogen nucleus peacefully floating in deep inter-galactic-cluster space will be longer than that of a hydrogen nucleus within our sun, even if they were both formed primordially. The gravitation of the sun and the Milky way and other nearby masses means the solar hydrogen has experienced less expansion. This takes us a tiny step closer to understanding the expanding universe in systems of coordinates other than the standard comoving coordinates, which is more relativistic in spirit. It reminds us that in comoving coordinates we are working with uniform densities of matter thanks to averaging, and are explicitly ignoring significant overdensities of matter (which slow down the ticking of clocks nearby).

thom 3y ago

I find it easier to picture the universe as more of a growing network of connected things, rather than an object with strict internal or external dimensions. I don’t think it (or anything) is infinite, but it might as well be because it cleverly prevents all parts from interacting with each other at all times (due to limits on information processing capacity, if you want to get quasi-religious about it).

BrendanD 3y ago
Greetings fellow Monad.

orra 3y ago

I believe it's an open question, the curvature of the universe, and hence whether the universe is infinite. Others have already clarified that the big bang tells us the observable universe used to be tiny. In other words, the universe was much denser. It does not tell us the absolute size of the entire universe.

oneshtein 3y ago
"Denser Universe" means that we were (or are) in a black hole. There is no other way around a huge mass in a small space.

photochemsyn 3y ago

Infinity is a tricky concept - a 2D creature on the surface of a 3D inflating balloon would view the surface they were on as infinite, particularly if the balloon was inflating faster than the bug could travel, such that no circumnavigation of the sphere was possible. Infinite space implies that one could travel forever in a straight line without encountering a boundary, but if we move to higher dimensions then it's possible that the universe is something like a 4D sphere of finite 4D volume bounded by a 3D 'surface' which is apparently infinite to its inhabitants. Moving to higher dimensions it might be the case that there are many universes, i.e. perhaps from a 5D perspective the multiverse is an infinite series of nested 4D universe hyperspheres, and one could go on and on like this into a infinite series of dimensions, but it's all just mathamatical metaphysics at that point as there's no observational evidence for any of that.

oneshtein 3y ago

You can use Occam's razor[0]: simpler solution with smaller set of elements has better probability to be true. a) Bing Bang model of evolution of Universe: single point, big bang, expansion of Universe, acceleration of expansion, time dilation, space-time warping. (6 extra elements) b) Steady Universe model of evolution of Universe: tired light. (1 extra element) [0]: https://en.wikipedia.org/wiki/Occam%27s_razor https://en.wikipedia.org/wiki/Occam%27s_razor

pdonis 3y ago

> How can both views be correct? They aren't. Your view is correct, at least according to our best current model of the universe. I suspect that your wife is misinterpreting the articles she is reading; I suspect they are saying only that our current observable universe, which is finite in size, started out as a single point. Which is sort of correct, but is not the same as saying the entire spatially infinite universe started out as a single point. Part of the problem here is that the "initial singularity" in the models being described (which is where the "single point" language comes from) is not actually part of the universe; it's a mathematical limit that does not actually correspond to anything physical. > Either the universe isn’t infinite, and therefore has some kind of edge The "edge" part is not correct. A spatially infinite universe would have the spatial topology of a 3-sphere, which has a finite volume bot no edge (just as the 2-sphere surface of our Earth has a finite area but no edge).

EMM_386 3y ago
How does something infinite "expand" though? It's not expanding into anything, because it's infinite. I always hear "space itself is expanding" but how does that work with topology? When you travel in a given direction you will arrive back where you started? Surely that "shape" or "object" or topology or whatever you want to call it has a volume, relative to something else, or it wouldn't be able to be calculated? As you can tell, I have no idea what I'm talking about but endlessly curious on this subject.
- pdonis 3y ago
  > How does something infinite "expand" though? Infinite things don't work like our ordinary intuitions expect. > I always hear "space itself is expanding" but how does that work with topology? The topology doesn't change as the universe expands. "Expanding universe" just means that a particular metric (geometry) is imposed on the topology. If you think of the spacetime of the universe as a stack of 3-dimensional spatial slices, "expansion" just means that a given pair of points in space gets further and further apart as you advance from slice to slice into the future. That is perfectly well-defined for infinite slices, even though it's hard to imagine intuitively. The math works just fine. > When you travel in a given direction you will arrive back where you started? No. That would be the case for a spatially finite universe with the topology of a 3-sphere. But our spatially infinite universe has the topology of Euclidean 3-space. > Surely that "shape" or "object" or topology or whatever you want to call it has a volume The spatial 3-volume of the universe is infinite. > relative to something else, or it wouldn't be able to be calculated? Not at all. The universe doesn't need to be embedded in anything else to have a well-defined volume.

codethief 3y ago

> The universe is infinite, or at least appears so. The universe is certainly very big but whether it's finite or infinite is not known with certainty, see https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite_or_finite https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...