Shrinking Universe: Visible Universe & Universal Center of Mass

[Jonny_trigonometry]

The universe can't be fully seen from any vantage point within. We must define what we see (with electromagnetic radiation) as not the universe, but the visible universe. This is merely a portion of the universe itself. Most matter in the universe is embedded in space that is moving away from our region of space faster than light speed (mainly, matter very far away). This is around 14 billion light years from our viewpoint, roughly along any radial line from the Earth (provided Earth is defined as our viewpoint). In a way, you could call it the "event horizon" of the gravitational potential well that we are within (in theory). If we looked at our position in the universe as being in a potential well, then the universe isn't expanding (or at least the visible universe), it is all falling toward a center, but not nessisarily according to the laws of gravity since the even horizon is so large. It is possibly a much weaker force, and perhaps compliments gravity like how magnatism compliments electricity (which would be a dream come true for a TOE). The point is, picture our planet, better yet our solar system, better yet our galaxy and neighboring galaxies as all reletively at rest respect to each other (but not fully as it would imply that they are all at absolute zero). They are all influencing each other and dynamically orbiting each other very slowly, and chaotically. Now just think of speeding up their rate of time, and viewing them as particles with vibration about an equilibrium, (the center of mass of the system). Suppose this whole system is influenced by the center of mass of a much more massive system, say, the center of mass of the entire universe (assuming the universe is finite), or maybe just a humungusly massive black hole; then like systems that are closer to the center are being pulled harder, and systems further away are being pulled less. A region closer to the center is moving with a higher velocity toward the center than we are, therefore it appears to be moving away from us, and a region further away from the center than we are is moving toward the center at a velocity lower than ours, therefore it appears to be moving away from us. Therefore, from our region of space (which contains our local galaxies), it would appear as if everything (like systems in other regions of the universe) is moving away from us. Not just that, but also fits the model that things further away will be moving faster away than things not as far away. frames at our radius would have no red shift caused by the curvature of space, but there would still be redshift and blueshift from movement within space. You may wonder, why we haven't reached the center already, well in general reletivity, there is more space in a region more toward the center of a black hole than there is in a region further away. Who knows how much we all (and our surroundings) have shrunk in the time it took to read this? Keep in mind, there must be a universal center of mass if the universe is finite, if infinate, i don't see why this can't work givin that it's a huge mass that our observable universe is stuck inside (one of an infinate). I'm just throwing seeds.

 

[Chronos]

So what point are you trying to make? Saying the universe is only as big as you can observe is old news.

 

[SpaceTiger]

Keep in mind, there must be a universal center of mass if the universe is finite,

You're thinking in Newtonian terms. In GR, a finite universe can be isotropic and have no center. Take, for example, a simple closed geometry (spacetime hypersphere). All points would be equivalent, just like the surface of a normal sphere, so there would be no center of mass (in space).

If you consider a flat universe and only consider the parts of the universe that are observable, then the observer will always be at the center, whether he/she is on Earth or in the middle of the Virgo cluster.

 

[Janus]

Suppose this whole system is influenced by the center of mass of a much more massive system, say, the center of mass of the entire universe (assuming the universe is finite), or maybe just a humungusly massive black hole; then like systems that are closer to the center are being pulled harder, and systems further away are being pulled less. A region closer to the center is moving with a higher velocity toward the center than we are, therefore it appears to be moving away from us, and a region further away from the center than we are is moving toward the center at a velocity lower than ours, therefore it appears to be moving away from us. Therefore, from our region of space (which contains our local galaxies), it would appear as if everything (like systems in other regions of the universe) is moving away from us.

The problem is that this only works if you only consider objects that are on a line either pointing directly to or directly away from the center. Objects that are at the same radius from the center as we are would be seen as moving towards us as the circumference we share contracts.

 

[Tapsnap]

Objects that are at the same radius from the center as we are would be seen as moving towards us as the circumference we share contracts.
__________________But what if as matter moved towards the center it was also shrinking and light closer to the center was slower? Then those objects on the same radius would also appear to be moving away.

 

[Chronos]

And that is nonsense. Propose a single experiment that supports your point.

 

[chroot]

1) The limit of the visible universe is called the "particle horizon," and is 47 billion light-years away -- not 14 billion light-years. The visible universe was smaller in the past, so light didn't take as long as it does now to cross it.

2) A finite distribution of matter does not need to have a center-of-mass, given that it has non-Euclidean geometry.

3) Personal theories are not welcome here on physicsforums.com. This thread is treading on thin ice.

- Warren

 

[Jonny_trigonometry]

Sorry people. Thanks for all your viewpoints, I couldn't have let go of that idea without you!

"2) A finite distribution of matter does not need to have a center-of-mass, given that it has non-Euclidean geometry."

Is this because non-euclidian space can be arranged in a way that it's boarders (or edges) can be connected together?

here's another question to show my stupidity: What evidence is there for a universe where if you travel in a straight line long enough, you'll end up at the same place you started?

I guess my biggest problem is with non-euclidian geometry. Aren't there a couple different kinds? One kind uses extra dimensions, and the other uses fractional dimensions? Has anyone come up with a fractional dimensional theory of the geometry of the universe yet?

As you may have already concluded, I don't care if I'm viewed as a complete moron...

 

[SpaceTiger]

Alright, I feel the need to post my thoughts on these issues, as its an extremely common question among amateurs (indeed, I used to frequently ask it myself). I'm not a theoretical cosmologist, but I think this question can be answered realitively pedagogically. The following will assume that the popular theories are right, but some of the answer may change if that turns out not be the case.

Point 1: The observable universe is flat. This means that when you restrict your question to that which we can see, there's no crazy looping around, it's just a large region of three-dimensional space with boundaries at a given distance (tens of billions of light years). These boundaries are not walls or anything like that, they're just the limits of what we can see with our telescopes. The center of the observable universe is, by definition, the location of the observer. That is, we're the center of the observable universe. This isn't saying anything deep or philosophical, it's just saying that if you define a region from which light could have reached us during the age of the universe, it's going to be symmetric about our observation point. Likewise, if someone lives in Andromeda, they will be at the center of their observable universe as well.

Point 2: The entire universe, including everything we think exists, is much larger than the observable universe and we don't know its overall topology (flat, hyperbolic, etc.). We may never know. If inflation is correct, then any information about the state of the universe prior to inflation, including the topology, would be have been smeared out by the rapid expansion. The universe would basically be covering its tracks.

Point 3: The observable universe is not the same as the contents of our light cone. That is, it's possible that the pre-inflationary universe was closed and we interacted with everything that exists, but the rapid expansion during inflation would have wiped out any information about these interactions and the only things that could be deterministically studied are the things in our observable universe. This is why scientists are usually only concerned those few tens of billions of light years in our observable universe.

Most of the other things you'll hear about these issues are speculation and/or fringe theory. I'm sure I've omitted some important things, but that's the basic gist of what we currently know.

 

[X-43D]

As of 2005, does anyone have any idea of how the "graceful exit" problem (the fine-tuning) could be solved in inflation?

I also read that the accelerating expansion of the universe is an illusion as an effect of general time dilation.

 

[Jonny_trigonometry]

Thanks, this is like a free course in cosmology! You've cleared a lot of things up Space Tiger.

I don't understand why the observable universe is considered flat though, since it is bent all over the place by the presence of matter. I understand that it would appear flat because the light is being bent along with space, even though it may not actually be flat (like flatlanders in a crumpled up 2D space). I think my problem is that I just don't know the terminology, and a flat universe allows for all this.

If a photon is a "light packet" with energy hf, is it a composion of waves with varying wave numbers just like matter waves? if so, then could redshift be the dispersion of the wave group (photon) as it traves through space? The longer the distance (invariant of the path taken) the more dispersion?

 

[chroot]

The observable universe is not flat everywhere, because, as you point out, matter warps spacetime locally. When a cosmologist says the universe is flat, it means the universe is flat globally, in the largest scales.

In the same way, your tabletop is "flat" in the large-scale sense, but, if you look at it with a microscope, it's really not flat at all, only flat on average.

- Warren

 

[SpaceTiger]

I don't understand why the observable universe is considered flat though, since it is bent all over the place by the presence of matter.

You're right, I'm being a little sloppy in my terminology. Spacetime is not flat locally, but the overall shape is flat, as chroot already described.

If a photon is a "light packet" with energy hf, is it a composion of waves with varying wave numbers just like matter waves? if so, then could redshift be the dispersion of the wave group (photon) as it traves through space? The longer the distance (invariant of the path taken) the more dispersion?

Perhaps http://www.astro.uwo.ca/~chunter/p407/timedelay/node10.html reference will clear that up a bit.

 

[Jonny_trigonometry]

How many events can occur in a cubic centimeter during a single moment of time?

 

[turbo]

How many events can occur in a cubic centimeter during a single moment of time?

LOTS! Quantum theory says that the "quantum vacuum" at absolute zero is teeming with pairs of virtual particles, that arise and self-annihilate in time periods short enough to avoid violating the Heisenberg uncertainty principle. These virtual pairs are likely to locally exclude the existence of other virtual pairs, and they may represent the "ground state" of our universe, and may also define the small-scale structure of the universe. People working in the field of loop quantum gravity hope to unify quantum theory with General Relativity, and they will have to contend with these concepts before they can formulate a dynamic (or even kinetic) theory of quantum gravity. Curved space-time is foreign to the quantum world.

 

[sruthisupriya]

People working in the field of loop quantum gravity hope to unify quantum theory with General Relativity,

What is loop quantum gravity?

 

[turbo]

What is loop quantum gravity?

Got Google?

http://en.wikipedia.org/wiki/Loop_quantum_gravity

 

[Garth]

Spacetime is not flat locally, but the overall shape is flat, as chroot already described.

Just a small point; it is space that is generally thought of as globally flat. In the standard model space-time is only 'flat' if the universe is empty, then are all the components of the Riemannian zero. (It is also possible to produce a contrived model in which a 'Dark Energy' component with equation of state w = -1, and exactly the correct density, cancels out the contribution of a possible cosmological constant.)

Garth