Can We Determine the Center of the Universe Using Comet Orbits?

[Thermate]

What makes this different than everything else you see? If a car is moving past you it is actually very slightly further along its path than you see it due to the finite speed of light. The only difference I see is the magnitude of the difference.

I agree with that. The reason that it becomes significant in terms of such huge time and distance scales is because the current state of that part of the universe has no effect on our part, and will not for billions of years. Only events billions of years in the past have any bearing on our current state.

 

[phinds]

Only events billions of years in the past have any bearing on our current state.

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]

It's been well over a decade since I seriously thought about cosmology, so I'm a bit rusty. Something, however, seems wrong with the idea that the curvature is effectively zero.

I'm not saying you are wrong. But consider this. If we draw geodesics on a Euclidean plane, parallels remain at a constant mutual distance. Now the world lines^* of the local universal rest frames are geodesics which are growing further apart in an expanding universe. If space-time were "flat" I would expect geodesics to remain at constant mutual distance.

*I mistakenly used the term "time lines" in a previous post.

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]

The lines separate but remain forever non-intersecting, which means the geometry is always Euclidean.

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

 

[soothsayer]

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

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.

 

[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.

 

[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:

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)

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.

 

[russ_watters]

Also not to worry, you didn't sound hostile at all.

This is true. Biting off more than you can chew, yes. Hostile/aggressive, no.