Is the universe infinite?

Nordic

If that is the only fact that supports your position on this, then I don't think you know the topic well. Don't get me wrong here, I'm just curious. The matter in the universe is expanding, yes, but not the SPACE of the universe. I think you should re-check your that though :)

afennah

Nordic, I think you'll find you have that the wrong way round. Matter is not expanding!
It is 'space' which appears to be expanding. If it was matter expanding then our galaxy would be getting bigger (which it's not). Matter is being 'carried' along with the expansion of space which is why everything we see appears to be moving away form everything else. Hey... maybe I know a little more than you think! lol.

So, my point still stands...How can the universe be infinite if it's expanding? Cheers,

darkhorror

Look at the set of numbers 1, 10, 100, 1000,... that is infinite
now look at the set 1, 5, 10, 50, 100, 500, 1000,... that is also infinite

Now there are more numbers between 1 and 1000 than there were in the first set.

mrspeedybob

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.

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.

Chalnoth

Well, we know that 6 is wrong on some level, so I'm not sure that this argument gets you anywhere.

GODISMYSHADOW

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."

darkhorror

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.

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.

Nordic

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.

goya551

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?

marcus

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.

goya551

Interesting. Has there been any insight into how gravitationally bound systems halted the expansion around them?

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.

Chalnoth

It's not just believed: this is the way General Relativity (and Newtonian gravity) works.

Chronos

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.

Deuterium2H

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.