I have a question about the universe, inspired in part by another current topic, but probably different enough to warrant its own thread. I’ve been curious about the question for a while.
Most current cosmological theories, I believe, speak of the universe, either explicitly or implicitly, as finite, that is, having a limited number of particles, stars, etc. What is the evidence for this? In particular, given that the speed of light is finite, how can we have grounds for asserting that beyond the portion of space from which light has reached us to date, there is not endless space? Why might we not expect to see more and more stars as time goes on, indefinitely. I accept the facts the farther back you go in time, the hotter and denser the things were, and that there was a Big Bang about 30 billion years ago. But why could not the universe have come into existence infinitely extended and occupied? It would still be possible for all the galaxies to be moving away from each other, etc. What evidence is inconsistent with this infinite-universe hypothesis?
Thanks in advance for marginally decreasing my ignorance.
Two things fill my mind with ever-increasing wonder and awe: the starry
skies above me and the moral law within me. – Kant
Good question. While I personally find it easier to imagine the universe as having a limited mass, I still seems like there would be an infinity of empty space on all sides around it. And if time can be considered another dimension (?) then there would also be no beginning and no end.
I hope some good answers come up.
Tony, great question. I don’t have an answer, but I can tell you that if it is, it would explain a lot. People have been trying for years to avoid/deal with the infinities that keep coming up with the calculations trying to meld QM and GR into Quantum Gravity(or whatever they are calling it these days ;)). Not just mathematical infinities, but actual infinities. They don’t make sense if the universe is finite. I’ve wondered for a long time if physicists are holding on to that concept because it is more “comforting” to have a finite universe. OTOH, the concepts of equilibrium and entropy don’t really function in an infinite universe either, so it would really take a whole new concept of the universe to explain it. Now that I think about it, a lot of the fundamental laws wouldn’t work! Also, the calculations involving infinity are pretty difficult to work with. here is a link to a site which describes some of the ways Hawking tries to get around ininities using boundaries and Feynman’s Path Integrals.
::wishing I had pursued my study of physics instead of getting a damn job!::
Good luck!
Just make yourself comfy while I shoot nuclear particles into your heart.
(Courtesy of Wally)
Enumeration of what we have the ability to perceive is the simplest evidence available. Granted, our perception is limited. Some scientists hope to calculate the mass of the entire universe in hopes of validating/refuting the Big Crunch theory; however, the mass of the universe, and thereby the number of particles in the universe, will remain elusive.
Because it has been established that light has a finite velocity, when we perceive light from “far away” we perceive it originating from the past, that is, a perception of a particular galaxy is an image of how it was x years ago. There is no evidence, nor has it been asserted, that space is finite.
That is expected. Parts of the universe are still cooling to the point where the formation of stars is just becoming possible.
The very fact that nanoseconds prior to the postulated Big Bang can be witness is the evidence that the universe did not come into existence infinitely occupied. Rather, what is the point in defining a point in space without a relative occupied point nearby. I have never heard the assertion that space was nonexistent prior to or in the intial moments of the Big Bang; however, a detailed analysis of nothingness is irrelevant. Infinite space is not refuted, but it is meaningless under certain considerations.
If all the galaxies etc. are moving away from each other then there must have been a point when/where everything was in the same place (big bang). That implies an upper limit for the size of the universe limited by how far things could move apart since that time, i.e. a radius of the speed of light * time since big bang.
If the universe were infinitely big & the distribution of stars is more or less uniform over space and time, then in whatever direction you look there would be an infinite number of stars. If the universe was also infinitely old then in whatever direction you look you would be bombarded by an infinite number of photons. There would then be no need for daylight saving time. Since daylight saving time does exist we can discount this possibility.
There is no compelling evidence for or against a finite universe. Even the term “universe” is a little murky. If Everett’s Many Worlds Interpretation is right, here is schozophrenia with a vengeance: an infinite array of parallel universes. And Hawking’s Baby Universes Hypothesis is the same deal, but completely different.
On the other hand, if our universe is all there is, it might be infinite. Or not.
I’m a little fuzzy on this, but I’ll give my understanding anyway. Due to mass in the universe, space is curved. Matter is clumped into galaxies, galaxies into clusters, and clusters into super-clusters. On the largest scales, however, the distribution of mass in the universe is uniform. This means that the curvature of the universe is uniform on a large enough scale.
We can only see that part of the universe within 12 - 15 billion light years, because the universe began 12 - 15 billion years ago (or whatever figure is popular right now). One of the assumptions of physics is that our place in the universe is not special any way. If we assume the universe is homogenous over its whole extent, then the curvature of the universe is roughly constant.
At this point it’s easier to talk as if the universe were a 2-dimensional. Since the curvature is non-zero, the universe must be like the surface of a sphere, rather than like a flat sheet of paper. The part we see is one section of the sphere, but not necessarily the whole thing. But if the whole universe is curved the same as the section we see, it will be analogous to the surface of a sphere.
This is not to be confused with the question of whether the universe is open or closed, which is whether the universe keeps expanding forever, or halts then starts contracting. The expansion of the universe is analagous to the sphere expanding.
Anyway, the assumption here is that the universe is homogenous on a large enough scale, and observations on the part we see are consistant with this. (Last I knew. People are collecting more and more observations all the time.) Whether this assumption is correct, no one knows.
This is just plain incorrect. Link to follow.(Christ it’s hard to search for this kind of stuff on the web without running into 10,000,000 sci-fi references! ;)) Anyway, here is one.
Besides needing to be parsed ;), this is a bit dogmatic, don’t you think?
It’s been too long since my logic classes to tell you which rule this violates, but it is an incorrect assumption nonetheless according to logic and mathematical rules. Ticker, not sure if the rest of your post is a jest, but if your argument were valid, it would be compelling evidence to suggest the universe is, indeed, finite. It is not, unfortunately.
Just make yourself comfy while I shoot nuclear particles into your heart.
(Courtesy of Wally)
This is what happens when you drink Merlot and post. My intention was not to say that the universe is not finite, but that the argument was not valid. Sorry.
Here are a couple of simple explanations of why the universe isn’t infinite. These are not original; you’ll probably find this and better in a simple search…
Reason #1
If the universe were infinte, there would be stars in every direction forever. The universe having been here forever, we’d see light from each and every one of those stars, and the night sky would be as bright as day.
Reason #2
If there were stars forever, gravity would attract them together, and pretty quickly, all you’d have would be pockets of massive star matter.
Also
As you’re probably aware, the doppler effect that can be seen in all directions shows that the univers is clearly expanding, which doesn’t mesh with the presumption that it’s infinite.
Ok, I’ve studied a bit of cosmology (working on MS and PhD), and I think I can clear up a bit of the arguments here. First of all, there’s the question of what’s meant by the “Universe”. If by that you mean the “observable Universe”, then it’s definitely finite, by virtue of the light travel time-- anything farther away than (about) 15 gigalightyears, the light wouldn’t have reached us yet. If you mean more than that, i.e., all that we will eventually be able to see, there’s a few possibilities which depend on the shape of the Universe. If space is positively curved, like the surface of a sphere, (i.e., the density is higher than the critical density) then it must be finite. If the average curvature is zero or negative, as is currently commonly believed, (unfortunately, there’s no good example of a uniform negatively curved surface that can be embedded in Euclidian space; think of a saddle), then the Universe can be infinite. Even here, though, there’s the possibility of a finite Universe, depending on the topology. In the game Asteroids, the geometry was Euclidian, i.e., flat, but if you flew off one edge of the screen, you came back on the opposite edge, so the “world” was actually finite. Some think that the Universe may be the same way. We may get observational evidence for this shortly, but don’t keep your fingers crossed-- as of right now, there is no observational evidence showing the Universe to be finite or infinite.
On the other hand, lack of observations have never stopped theorists, and many theories of the Universe, as mentioned above, seem to work a heckuva lot better if the Universe is finite. You wanna argue with those, come up with a more elegant theory that doesn’t require a finite Universe.
BTW, the sig line is a coincidence, it’s the same one I’ve been using on all of my posts.
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
These points have been made by a few people. Let me explain why i think they are wrong.
Reason #1
If the universe were infinte, there would be stars in every direction forever. The universe having been here forever, we’d see light from each and every one of those stars, and the night sky would be as bright as day.
Response: This would only be true if the universe were old enough for light from many, many stars to reach us. Possibly as time goes on we will see new stars; as time goes on, ones we see now may die. But the fact that we currently see a lot of darkness does not contractict the hypothesis that the universe is infinite (spacially). Reason #2
If there were stars forever, gravity would attract them together, and pretty quickly, all you’d have would be pockets of massive star matter.
Response: Again, I am supposing that the theries that assert the universe has been around for a finite time are right. I am not asking whether the steady state theory is right. I assume that the universe is evolving. Perhaps at some point in the future there will be pockets of “star matter.” But that dioes not show it is not now infinite.
Also
As you’re probably aware, the doppler effect that can be seen in all directions shows that the univers is clearly expanding, which doesn’t mesh with the presumption that it’s infinite.**
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Response; There’s nothing inconsistent about an infinite expanding universe. Imagine the points on a line: 1,2,3 . . . They can expand to 2,4, 6 . . . and then expand to 4,8,12 . . . It would work the same way in three dimensions. Perhaps there is a problem with the limit of the speed of light, but I think that the special theory of relatively could allow that from every position, everything seems to be moving away.
tony1234
Two things fill my mind with ever-increasing wonder and awe: the starry skies above me and the moral law within me. – Kant
Thanks for your reply. There are still a few things maybe you could clear up for me.
I understand that it is currently unknown whether the curvature of the universe (space-time) is positive or negative. The evidence, as I understand it, is inconclusive. Most scientists accept and admit that the question is open (no pun intended). Nevertheless, in the general reading Iive done in cosmology, it seems conclusively assumed that the universe is finite. People write, “in the first millionth of a second, the universe expanded to the size of a [whatever].” This sort of statement clearly presumes a finite universe. Should I understand this sort of statement as refering implicity to the observable universe? or have I just been reading the wrong accounts?
You ask what I mean by “finite” or “infinite.” By these terms I am not necessarily asking whether the universe will expand forever or whether there will be a Big Crunch. I am asking whether there currently exists (whether observable or not) an infinite number of particles, stars, etc. or only a finite number.
If the universe is positively curved, is it necessarily finite in the sense of above? Because I am not familiar with the math, let me be concrete. One implication for a positively curved universe (I believe) is that is you shoot a beam of light in one direction, it will eventually return from the other. Is it possible that there are an infinite number of distinct, but contiguous portions of occupied space such that for every one, a beam shot off will return. If this is a possibility (if seems at least imaginable to me), could it be that the universe is infinite in extension, but exhibting positive curvature everywhere?
[QUOTE]
Originally posted by ticker:
**If all the galaxies etc. are moving away from each other then there must have been a point when/where everything was in the same place (big bang). That implies an upper limit for the size of the universe limited by how far things could move apart since that time, i.e. a radius of the speed of light * time since big bang.
Response: No. Just because all the galexies are moving away from each other does not imply that if you go back far enough there ust have been a (finite) point that contained them all. There is nothing inconsistent about an infinitely large expanding universe. it started off infinitely large a finite time ago and has been expanding ever since. (Potentially)
If the universe were infinitely big & the distribution of stars is more or less uniform over space and time, then in whatever direction you look there would be an infinite number of stars. If the universe was also infinitely old then in whatever direction you look you would be bombarded by an infinite number of photons. There would then be no need for daylight saving time. Since daylight saving time does exist we can discount this possibility.
Reponse: Again, this assumes that the universe is infinitely old. That is not what the evidence shows. Perhaps in time we will receive photons from newly seen stars, but the fact that the sky is not ablze today does not mean that the stars are not out there today.
As far as the observable universe goes, the strongest arguments for finite-ness in space and time are:
[ul][li]The temperature of the cosmic microwave background[/li][li]The darkness of the night sky[/li][li]The uniform density of large-scale structure[/li][li]Hubble recession[/li][li]The current hydrogen/helium/deuterium ratios[/ul][/li]
None of the infinite theories of the universe can explian all of these observations in a satisfying manner, whereas the inflationary big bang theories do a very good job of doing so.
However some of the extreme inflationary scenarios lead to a truly enormous universe, many orders of magnitude greater than what we can observe, But still finite.
The observable universe may very well be a special case of some quantum or pre-big-bang multiverse. There’s actually some hints (not yet even real evidence) that there’s more to “everything” than meets the eye. But you’ll have to wait until at least tommorrow when my mind is sharper.
Time flies like an arrow. Fruit flies like a banana.
It sems to me that the observable universe must be finite, given that we can only observe things thanks to particles that can move no faster than light, the speed of which is finite.
I’m not asking whether the universe is infinite as in “there may be a infinite number of discrete universes” in some bizarre quantum mechanic of multiverse sense. I’m asking whether the universe is infinite in the very concrete sense of whether there are now an infinite number of stars, any two of which are a finite distance apart.
tony1234
Two things fill my mind with ever-increasing wonder and awe: the starry skies above me and the moral law within me. – Kant
Glad I could be of service, tony. As to your further questions: Assuming that there’s no cosmological constant massing stuff up (which it currently appears that there is), a closed Universe (positive curvature) is synonymous with one that will re-contract (If the cosmological constant is non-zero, it might still expand forever). All such Universes are finite, and have a large-scale geometry identical to that of the surface of a sphere, except in three dimensions. The flat and negatively curved cases can both be infinite, and both expand forever, so the questions of temporal fate and spatial size are connected.
Many of the relatively-successful inflationary theories predict that the overall curvature should be exactly zero, and this seems to be consistent with the most exact measurements we’ve been able to make, but you’re right, the question is not yet definatively settled. Keep your fingers crossed for the upcoming MAP and LISA missions, and be sure to consider NASA funding at your next trip to the ballot box, and we may yet learn more. As to the description of the size of the Universe (saying, it was only the size of an orange, or whatever), you can define a characteristic length scale. For a closed Universe, it’s the radius, for an open one, it can be considered to be the approximate length scale over which it no longer looks flat (yes, there is a more precise definition, but I’d need to look it up), and for a flat Universe, it’s pretty much arbitrary, but you have to be consistent. In other words, in a flat Universe, you can’t say what the size is in absolute terms, but you can say, for example, that it doubled in size.
By the way, thanks for starting this thread. It makes me feel edumacated.
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
Correction: In my second sentence or so, that should read “assuming there’s no cosmological constant messing stuff up”, not “massing”. I usually don’t bother worrying about typos once I’ve hit “submit”, but that one could be confusing. :o
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
Thanks again for the response, but (and I don’t mean to be tedious) Im still unceratin: What are the empirical grounds for equating (as you seem to) “a universe with enough mass so that it will stop expanding after a point and then begin to contract” with “a universe with a finite total mass”? Stated in other words, what evidence contradicts the hypothesis: the universe, right now, has infinite mass spread over an infinite amount of space, and because the average density of the mass is high enough, eventually all the large masses in this infinite universe will stop moving away from each other at some point in time and start to move towards each other? I understand that such a universe may be inconsistent with a variety of cosmological theories, but what evidence supports these theories and contradicts the hypothesis? Well, I don’t necessarily need the data, but what are the reasons, if any, for rejecting such a hypothesis? Is it self-contradictory, inconsistent with some basic physical law, inconistent with some data, or a live possibility?
tony1234
Two things fill my mind with ever-increasing wonder and awe: the starry skies above me and the moral law within me. – Kant
My intention was not to say that the universe is not finite, but that the argument was not valid. Sorry.