finding center of universe

phinds

Now there's a point of view you don't see very often.

Chronos

Distance is a very messy thing in cosmology. Figuring out where things are at in the universe relative to one another can be very confusing. For example, the most distant observable thing in the universe is the CMB at z~1100. When those photons were emitted, the source of the CMB was a mere 42 million light years from our current position in the universe. At the same time, photons from a galaxy at z~3 were 5.7 BILLION light years distant when emitted by that galaxy. At seems rather illogical that a foreground galaxy at z~3 can emit photons at better than 10 times the distance of the background CMB, but, that is the way it is with expansion. It also provides us with seemingly exotic concepts like luminosity distance and angular diameter distance. If all this does not confuse you, I've done a poor job explaining it.

soothsayer

The lines separate but remain forever non-intersecting, which means the geometry is always Euclidean. If you take a plane and expand it, the geometry is still Euclidean, even during the expansion. The metric for describing spacetime intervals does not change. This is because the expansion is not due to an "open" cosmic geometry, but due to dark energy. If the universe were open, Ω < 1, or closed, Ω > 1, then the value for Ω would actually change over time, thus the geometry would change over time, but Ω = 1 is constant in time.

Thermate

There are non-Euclidean geometries in which parallel geodesics don't intersect.

soothsayer

Here is the deal with these geometries and parallel lines.

Ω > 1, Closed => Elliptical space: parallel lines intersect, the value of Ω changes, but is always > 1. Space contracts, leading to "Big Crunch"

Ω = 1, Flat => Euclidean space: Parellel lines do not intersect, and parallel lines only come in unique pairs. Geometry does not change. Space expands, but decelerates, so that space is constant after an infinite amount of time.

Ω < 1, Open => Hyperbolic space: Parallel lines do not intersect, but to contrast with Euclidean geometry, there are infinitely many unique, parallel, non intersecting lines. I believe all parallel lines diverge. Geometry changes, space expands at an increasing rate forever.

We live in Ω = 1 with Dark Energy. The difference between this and an Ω<1 universe is that there are not infinitely many unique parallel lines ones can draw in a moment in time, even with expansion due to D.E.

soothsayer

By "unique" parallel lines, I mean this: In Euclidean geometry, we can draw many lines that are parallel to one another, but they are simply translations of one another, which doesn't mean much. If we took a eucliden geodesic and rotated it by ANY angle theta, it will intersect once, at some point. In Hyperbolic geometry, if we have two geodesics of some finite separation, we can rotate one of those geodesics by some finite angle theta such that the lines STILL do not even intersect. In fact, there are infinitely many such lines, as you can easily see, which are all UNIQUE lines. This is the difference between flat space with dark energy and open space--both have expanding geodesics through time, but geodesics also diverge in slices of constant time in a hyperbolic universe.

Inflation complicates the picture. In the typical description of inflation, the universe started as a point, of Ω > 1, and as it expanded, instead of quickly reaching a maximum and re-collapsing, it reached a point where Dark Energy was 27 orders of magnitude stronger than it is today, and rapidly pushed the universe to be flat, that is, Ω is so close to one, we don't notice any difference, and after that very early inflationary period, the geometry of space has not changed any measurable amount. I don't know what effect this would have on the horizon of the observable universe.

Alfang

I Hate it when people use the term " we know" or "the fact that" when citing theories, as long as they are still theories, we don't know anything ( sorta)

TheTechNoir

Why is it that the majority of times that someone brings something up that is covered in the cosmology FAQ and they are told that it is not a meaningful question and are recommended to read the FAQ instead of doing so, they respond with another question that is covered in the FAQ?

Alfang

Well, as a participant of several forums, If you don't have new people join and ask the same questions again and again, the forum dies. there are only so many questions to be asked and if we don't welcome the newcomers in and answer the same questions over and over this forum goes away. A simple FAQ could be written to cover ALL subjects and eliminate any and all questions. All forums should be about personal interactions and sharing of knowledge and community and not of just pure information gathering.
This should be especially true in a forum like this that discourages and even forbids challenging common thoughts and presenting new ideas to how things in the universe work.
I have some new ideas that might change the world, But I can't present them here, less I get banned from here, are you all so uptight that we can't have new people ask redundant questions?

Alfang

If you don't wanna respond to a question that is covered in a FAQ, don't, someone like me will, and I will welcome them and give them the respect they deserve for just being curious

TheTechNoir

I think you misunderstood what I meant... The situation I am referring to is similar to when someone asks a question, someone posts a link to answer the exact question, then they respond with an obvious ignorance to the content of the link. I just mean why ask questions when you are going to ignore what your response advises you of or tells you of and then restate the same question that has already been answered for you in a previous post in the same thread.

TheTechNoir

I respond frequently to questions that are covered in the FAQ. Any way I apologize I derailed the thread here. And there is a sub-forum for your new ideas that might change the world.

Alfang

TheTechNoir My bad, so sorry, I think your right, I did misunderstand. And I hope I didn't sound hostile, that was not my intent at all. I am a bit uncomfortable here as I'm the dumbest one in the room. I was speaking to the whole crowd and not you. You can see I'm new here, 5 or 6 posts so far, I'm still trying to feel my way around. I see the moderators love to lock threads over petty things and I don't want that either. I don't mean to derail this either. peace........

russ_watters

Starting off in a new forum by criticizing the way they do things is not a good way to start, but there is a bigger problem here: This post reflects a severe misunderstanding of how science works. You say "still theories" as if there is something better an idea could be in science. There isn't. Theories are as good as ideas get and when something is solid enough to be a theory it means we do know an awful lot about it.

Alfang

Russ, so sorry, I didn't mean to come off like that. I realize theories are the best that they are and are the tools and guidelines we use with great success. My simple point was that if they were Facts, they would be laws. Again sorry, I didn't come here to be "The Bigger Problem"

russ_watters

I know that was your point. It is wrong. That is not how science works. I suggest you start by reading this or another description of the scientific method: http://en.wikipedia.org/wiki/Scientific_method

phinds

As Russ said, you are missing the point here. Do you think "Newton's LAW of Gravity" is a fact? Back then, it was thought that "facts" could be pinned down, but today's scientific community is more cautious and doesn't call ANYTHING a law.

Einstein's theory of general relativity has been verified to something like 10 decimal places but is not called a law. In fact, it is KNOWN to be wrong (doens't work at the quantum level).