# Is the universe infinite?

by QuantumJG
Tags: infinite, universe
 P: 656 Here is a different argument. Feel free to poke holes in it. I will begin with 6 postulates... 1. The universe is finiate in age 2. The universe is infinite in space 3. The universe began with a big bang at which time all mater began to exist and space began to expand. 4. The above 3 are true for all observers. 5. On a large scale the universe is homogenious. 6. General reletivity is accurate. There was a time when all the matter in the observable universe was compressed into an inch radius. (postulates 1 and 3) that time was within a fraction of a second after the big bang. This was not the only matter in existence but this super dense soup must have extended infinitly in all dirrections (postulates 2, 3, and 5). All this mater came into existence simultaniously or there would have been pressure waves which would have made the univers non-homogenious. (postulate 5). If 2 different observers were observing the big bang a fraction of a second after it happened they could disagree on wether the matter at point A in the universe was the same age as the matter at point B if points A and B were seperated by more distance then light could have traveled in the age of the universe. If the 2 observers are in wildly different frames of reverence A and B could be simultanious for 1 but seperated by billions of years for the other. (postulate 6) It would seem to me that not all 6 original postulates can be true. I'm inclined to through out #2 and suppose that the universe was not infinate at inception and therefore the entire universe at a point and could therefore come into existance simultaniously from all points of view. Sorry about my horendsous spelling. I'm on a computer without spell check.
 P: 656 Put another way, it looks like the whole universe began simultaneously but if it were infinite at inception then you have to decide what "simultaneous" means for widely separated points in space and GR makes that very difficult.
 Sci Advisor P: 4,604 Well, we know that 6 is wrong on some level, so I'm not sure that this argument gets you anywhere.
P: 40
 Quote by afennah In your comment you appear to be putting the Earth at the centre of your universe. I'm not sure that is a good idea but, your questions are relevant to what we are debating! Is the universe infinite or not. Some think yes others think no. I personally, do not believe it is infinite (Surely something infinite can't expand?... because it's infinite to start with! Ugh...Someone give me a headache tablet). I am finding the various stances on this subject fascinating. The PDF on ',Big Band misconceptions' was a good read. I await with trepidation further posts informing me that I am talking Bol*@ks. lol. Regards,
Einstein got rid of the notion of "absolute now." So by infinite Universe, you can't mean "everywhere right now," can you? I'd rather think about something finite so I won't lose my mind. Our Milky Way galaxy is othen referred to as an "island universe."
P: 137
 Quote by mrspeedybob Put another way, it looks like the whole universe began simultaneously but if it were infinite at inception then you have to decide what "simultaneous" means for widely separated points in space and GR makes that very difficult.
Very very shortly after the big bang would a lot of the GR not really work the way we have it work now? Could you have two frames of reference that were very close to each other but moving away from each other at huge speeds due to the rapid expansion of space that don't really go with what relativity would say in normal space like around the earth now? Also what about that spacetime only existed for a short time how does that work along with the finite speed of light.

Now I am sure this stuff has already been solved, I am going to have to look some of this stuff up but it seems like it may be interesting.
 P: 321 I haven't read the whole thread so excuse me if this has already been brought up, but has anyone thought about the universes expansion as possibly being "driven" by an outside force? By outside I mean literally outside of our universe....kind of like outside the membrane that may contain everything that lies inside it(stars, gas, etc). Its hard to explain but think about a bunch of bubbles clustered together. Now think of those bubbles as being individual universes. We see that when a lot of bubbles are near each other, they tend to burst and combine into larger bubbles right? What if all of these bubbles were universes, and they were "combining" to make bigger and bigger universes, such that the universe as seen from someone INSIDE one of the bubbles kept getting larger and larger because it was always combining with other bubbles? The above probably sounds insane, and drug related, but has anyone else thought of something like that? In a way, that could also work with black holes. Maybe black holes are "holes" in the bubble that is our universe, and our matter is leaving and being contributed to another bubble universe. I have no idea what I'm talking about, just speculation.
P: 9
 Quote by nlsherrill I haven't read the whole thread so excuse me if this has already been brought up, but has anyone thought about the universes expansion as possibly being "driven" by an outside force? By outside I mean literally outside of our universe....kind of like outside the membrane that may contain everything that lies inside it(stars, gas, etc). Its hard to explain but think about a bunch of bubbles clustered together. Now think of those bubbles as being individual universes. We see that when a lot of bubbles are near each other, they tend to burst and combine into larger bubbles right? What if all of these bubbles were universes, and they were "combining" to make bigger and bigger universes, such that the universe as seen from someone INSIDE one of the bubbles kept getting larger and larger because it was always combining with other bubbles? The above probably sounds insane, and drug related, but has anyone else thought of something like that? In a way, that could also work with black holes. Maybe black holes are "holes" in the bubble that is our universe, and our matter is leaving and being contributed to another bubble universe. I have no idea what I'm talking about, just speculation.
As you've already said, it's not from a scientific point of view. It's more of a philosophic statement, but I don't think that'll do us any good now will it? But anyways, I like the way you're thinking.
 P: 3 As I understand the expansion of the universe, space itself is expanding in every direction with no central point from which it is expanding. Two objects, both seemingly static in the space they exist, are receding from each other at an accelerating rate. The farther away the object, the faster the recession. For objects outside the Hubble Sphere (from Earth) they are receding faster than the speed of light and will never be observed from Earth. This easily shows the possibility of an infinite universe and a black sky at night. The speed of expansion becomes immense when we talk of large distances. But what of shorter distances such as those within our solar system? Has the expansion of space been measured closer to home?
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P: 22,356
 Quote by goya551 ... The speed of expansion becomes immense when we talk of large distances. But what of shorter distances such as those within our solar system? Has the expansion of space been measured closer to home?
Hubble Law expansion is not supposed to affect gravitationally bound systems, like planetary systems, star clusters, or galaxies. Even clusters of galaxies, if they are stable, would not be expected to show expansion.

Percentagewise, expansion is so slight too---the current Hubble rate amounts to only about 1/140 of one percent per million years. So you need a very large distance in the first place for such a small percentage to be detectable (within a reasonable time period).

To some extent it is up to you how you imagine the Hubble Law expansion of distances.
It is unintuitive because based on our earthbound experience we expect distances not to change--we expect geometry not to be dynamic---but GR says geometry is dynamic.

My own way of accommodating it is to think of a far-flung network of observers all of whom are at rest relative to the the CMB (the ancient light from the early universe era when the hot gas was more or less uniform).
Being at rest relative CMB just means that there is no doppler dipole. No motion means there's no hotspot ahead or coldspot behind---roughy the same temperature.

I think of them as all measuring the same CMB temperature, and estimating the same age of the expansion process---so they are contemporaries in that sense. So at an agreed-on moment (in their common "universe" time) the widely separated stationary observers all measure the distances between them and their neighbors. And find them increasing, percentagewise, at the rate I mentioned.

And why not. We have no right to expect that distances between stationary observers will not change. Geometry is dynamic.

It's a simple story, hardly even a story at all. You can make up your own.
 P: 3 Interesting. Has there been any insight into how gravitationally bound systems halted the expansion around them?
 P: 3 A little research shows that it is believed that the forces of a gravitationally enclosed system are simply stronger than the forces involved with expansion.
P: 4,604
 Quote by goya551 A little research shows that it is believed that the forces of a gravitationally enclosed system are simply stronger than the forces involved with expansion.
It's not just believed: this is the way General Relativity (and Newtonian gravity) works.
 PF Patron Sci Advisor P: 8,903 Gravity is much stronger than dark energy over 'short' distances, much as nuclear strong and weak forces are more powerful than gravity over really short distances. The effects of dark energy are only apparent over cosmological distances.
P: 59
 Quote by goya551 Interesting. Has there been any insight into how gravitationally bound systems halted the expansion around them?
Gravitationaly bound systems (e.g. galaxies) have a critical density high enough to prevent local expansion of space. Hence, galaxies do not expand with the Universe, but are carried along in the generalized expansion of space. Since gravity is the weakest force, yet it is strong enough to hold galaxies together...it is even more obvious why matter itself doesn't expand with space. The electro-magnetic, strong and weak forces are much stronger then gravity.

In response to an earlier post regarding why we know that the Universe has no "center" of expansion...this follows not only from observational evidence, but also is a direct consequence of the Cosmological Principle (all places are alike). A central tenant of Cosmology is that our Universe is isotropic (the Universe looks the same in all directions, from our vantage point) and homogeneous (at any given time, all places in the Universe are alike). If there existed a "center" from which the Universe is expanding from, then this would violate isotropy. Observers in different parts of the Universe would see differences (anistropies) depending on which direction they were looking.

Another way of thinking about "where" the Big Bang took place is that it took place everywhere. There is no special location.
P: 4,604
 Quote by goya551 A little research shows that it is believed that the forces of a gravitationally enclosed system are simply stronger than the forces involved with expansion.
Well, for the most part, that isn't really the case. It's the exact same force governing the expansion as governs the behavior of galaxies: gravity. In fact, you don't even need General Relativity, as you get the exact same answer on these large scales with Newtonian gravity (except that Newtonian gravity doesn't tell you how radiation responds to gravity...you do get the same expansion behavior for matter, though).

So it isn't so much a matter of local forces overriding the expansion for galaxies, clusters, and other gravitationally-bound objects, but just that the same gravity keeps those systems bound together that governs how the universe as a whole expands. The only difference is that the universe as a whole behaves like a smooth, nearly-uniform fluid that expands, while local overdensities tend to behave like orbiting systems.
P: 334
 Quote by QuantumJG Ok so me and a few of my physics (& Maths) friends were arguing this. I argued that it must be finite in size, since the universe contains a finite amount of matter and since no space is truly empty, how could the universe be finite. My friend who's a mathematician said that in her geometry subject this question was actually brought up. She said that the universe may be a 3-manifold (3D surface?) and it depends on the curvature (negative or positive) as to whether the universe is finite or not. I just want to know what the consensus is.

I personally have always considered anything infinite or boundless as impossible because anything that exists appears to require bounderies or a perimeter to delineate or give substance to it's existence. A building of infinite foors-for example would have no shape unless it has bounderies. No bounderies =no shape =no building. Unless of course we simply add floors or cause bounderies to expand forever. But bounderies of course are incompatible with infinity.

Here is part of an article dealing with the concept of infinity in relation to reality in harmony with what I just said.

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Excerpt:

Naturally in mathematics we can have infinity, numbers go on for ever, but numbers are not real, they are abstract. I cannot imagine anything 'real' that we could apply an infinite number to. The only thing I can imagine that could be really infinite is nothing, the 'nothing' I described earlier in Where did the universe come from? and we have no idea if that exists.

The concept of infinity is a puzzling one. For example: imagine a standard pack of playing cards that consists of just one of each card but two jokers. Imagine that the packs of playing cards are infinite in number (A thought exercise only of course). We therefore have more jokers than any other card in each pack, so do we have more jokers in total? You could reply that as the packs are infinite in number they can't be counted so it would be impossible to know. However, as the ratio of jokers to other cards in each pack is fixed, then at any number of packs there will always be more jokers. This would appear to indicate, that mathematically, we can have degrees of infinity. Sounds odd doesn't it? It is a valid mathematical argument though.

We have a theory for black holes that describes infinite density. See Exploding Black Holes? What does it mean, other than an unresolvable equation that occurs in mathematics? Exactly what is infinite density? Taking a rather simplistic view it could be argued that if one black hole has infinite density then nothing else can have ANY density. Clearly though in this sense we can have lots of infinite density, so the term obviously carries a meaning in mathematics that does not have the same meaning outside of it. Is the term used in the theory only because that is the way the sums work out, regardless as to its significance in the real world, or is it real?

Strictly speaking, according to Einstein's Theory of Relativity, a singularity does not contain anything that is actually infinite, only things that MOVE MATHEMATICALLY TOWARDS infinity. A black hole is formed when large stars collapse and their mass has been compressed down to a very small size and the powerful gravitational field so formed prevents anything, even light, from escaping from it. A black hole therefore forms a singularity at its centre from the concentrated mass of the collapsed star itself and from the accumulated mass that is sucked into it. A singularity's mass is therefore finite, the 'infinity' refers only to the maths.
Can we have an infinite universe for example? The answer is no, the universe is finite. Stephen Hawking in 'A Brief History of Time' (1989 page 44) describes the universe as being "finite but unbounded".

The simplest answer is that as the universe is known to be expanding, it cannot possibly be infinite. To be precise, the dictionary definition of the word universe is "all that is. The whole system of things." In this sense the universe is not expanding into anything other than itself, for whatever it is expanding into is part of the universe, there being nothing else but the universe. However, for the sake of simplicity, I am referring only to our Big Bang expanding universe as 'the universe'. (Even if you happen to disagree with the Big Bang theory, the term 'universe' will still have the same meaning here, as it refers to 'our' universe only, and does not include whatever may or may not exist outside of it.) I will try and explain a finite universe as some people understandably have problems with it.

A good place to start is to understand the very real difference between infinity and a large number.

For example, imagine an ordinary size diamond, as you would expect to find set in a typical lady's engagement ring. Now imagine a super-being armed with super-tweezers, picking out atoms from this diamond one at a time, one every second, since the creation of the universe, some 13 billion years ago. How much of the diamond would by now have been removed? The answer is you couldn't tell without looking through an electron microscope, less than a millionth of the atoms would have been removed. Try and imagine how many atoms there are in that diamond. Now try and imagine how many atoms there are in the entire universe. It is a very large number, but it is finite, and is 10 followed by 80 zeros, (maybe a few more zeros, maybe a few less), expressed as 10 to the 80th. If you want to see what it looks like.........
100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 ,000,000,000,000,000,000,000,000,000.
0r written as - One hundred million, billion, billion, billion, billion, billion, billion. billion, billion

Even this very large number would count as nothing when compared with infinity, because infinity is NOT A LARGE NUMBER be absolutely clear on this point, IT IS NOT A LARGE NUMBER, infinity is ALL THERE IS, it is NOT a number. You could keep counting (or measuring) for ever, and never reach infinity, it is only a description. Infinity describes a thing as having no end, no limit, no boundary or edge, it literally goes on FOREVER, ad infinitum.

Because infinity is not a number, large numbers are no 'nearer' to infinity than small numbers. Number 1 billion for example is no nearer to infinity than number 1, because the two, numbers and infinity, are in no way related. It is then impossible to approach infinity, a thing is either infinite and immeasurable, or finite and measurable, it cannot be part way towards infinity. Imagine running up a 'down' escalator, never moving forward. If you run for a week you are no nearer reaching the end of the escalator than if you run for a minute, you cannot get any closer to something that has no end.

An infinite universe for example would exist in every direction forever, there could be nothing else, ONLY the universe. It is then very easy to understand why our universe cannot be infinite, it is because it is expanding. It cannot be both infinite and expanding. It could be infinite OR expanding, but CANNOT possibly be both, that is a contradiction in terms, and we do know it is expanding. For an explanation of the Big Bang and why we know the universe is expanding.

http://www.thekeyboard.org.uk/What%20is%20infinity.htm
 P: 11 The universe is limited in mass and energy by the Big Bang constituents. Space goes beyond the expanding universe to a shell limited by the boundary created by God. God has no bounds and looks from beyond infinity inward.