Cosmological question - where is the earth?

[Astronuc]

Well the answer is pretty obvious - it's right there under your feet or what ever you happen to be sitting on (which is probably what you are doing while reading this) or lying on.

But seriously - where is the earth (and the solar system and Milky Way Galaxy) within the Universe.

Are we near the center of the universe?  Are we off-center?  How would we know?

Consider as far was we can see or detect - light travels to us from all directions under the same physical laws, and if we look in every direction and determine the most distant objects are at 15 billion light years from earth, then that leads to several thoughts:

1.  That is the position that it was 15 billion years ago.  And its not there anymore.

2.  The visible (emphasize visible) universe is at least 30 billion years in diameter (maybe ?).

3.  The boundaries must move at the speed of light - if the photons are moving out ahead of the matter (Big Bang Theory) - so if the we see an object at 15 billion light years (which was 15 billion years ago) then the light has gone twice the distance, which means the radius of the real universe is 30 billion light years, and the diameter is 60 billion lightyears - which means the universe is at least 30 billion years old.

4.  What if we are off-center.  Then we only see a fraction of the universe in the light sphere around us - and there would be a part (maybe a smaller/larger part) that we cannot see.  Could we tell the edge of the universe from the non-edge.

5.  We cannot see the edge of the universe, but at most only the edge of the part that contains matter - light traveling in a vacuum cannot be reflected (that requires a charged particle interaction, or does it).   Or is the a pure photon-photon interaction, and what is the critical angle or condition for reflection?

6.  Finally, if we conclude that we are near the center of universe, is that significant?  Of course it would be of religious significance to some, but I am looking for a more scientific significance.

Well - I would be interested in anyone's opinion.

[yale]

http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

[payloadcontroller]

Well, to make a long story short, it doesn't matter. We are, for all intents and purposes, in the center. But no place in the visible Universe is the center, because the actual center of expansion is outside of spacetime as we percieve it.

Think of blowing up a balloon. You're an ant on the surface of the balloon. You see everything around you moving away as the balloon expands, and to you, you seem at the center of the expansion.

However, the true center of expansion is INSIDE the balloon, in a place you, the ant, can't even percieve, let alone see.

That's how it works, only in many more dimensions (at least 40-odd).

[Astronuc]

Yale,  Thanks for the UCLA site.  I remember it from a few years ago, but I had forgotten about it.  Unfortunately it doesn't seem to answer the questions - but it rather provides multiple answers - depending on which theory is accepted.

PLC - I have seen references to multi-dimensional models, but unfortunately I am not up to speed on current cosmological models.

I have seen the balloon analogy, where the ant perceives the two (surface dimensions), but not the third - that is extended to hypercubes and multi-dimensional topological spaces.  In the balloon model, everything on the surface expands.

The whole subject is a limited pratical use of course, but makes for interesting diversions of the mind.

[payloadcontroller]

Quite true, astronuc. Very interesting diversions indeed.

Last I heard, the various attempts to develop a grand unified field theory required no less than 40-some dimensions, and the more complicated theories went up to several hundred.

So the balloon analogy is very simplistic, but serves as a good illustration.

[remcook]

talking about diversions....
Einstein's theory of general relativity may be seen as one of the greatest accomplishments of science, still no application has been found. Besides the insight into the nature of universe, it has brought the common person..nothing.
(My thought for the day)

oh well...what are we here for anyway?

*sinks into deep pondering*

[broomstar]

The most embarassing status of those unification theories is that, in my opinion, though they are theoretically/mathemtically beautiful, they are not expected to be tested directly by any experiment or observations in the near future -- not as lucky as General Relativity, which was tested immediately after it was proposed.

[broomstar]

And it is also not accurate to say there are "still no application has been found" of General Relativity. See:

General relativity in the global positioning system

http://www.phys.lsu.edu/mog/mog9/node9.html

[engaaraa]

Rem:
No aplication? hmmm...
What about the GPS system. It's not an aplication per se, but it would not work correctly if GR was not taken into account.
And yet, GR is not completely correct as it does not explain anything in say, chemistry, or other QM realms. In fact it makes no sense at those scales and constrains Quantum Field theories research in such a way that no progress can be made (or has not been made by very smart people for a very long time). I'm betting on QM. GR will be the theory needing corrections and not QM. Not that anybody actually understands QM in any logical way, but such is nature IMO. Not the smooth GR view at all.

[yale]

Special Relativity is a feature of our daily lives...

For example, it is the reason that gold is golden and mercury a liquid.

yale

[Dingo1]

I do have a problem with a lot of the theories and such.  The assume that the big bang shot everything out at a concise speed, leaving no central point, but what if the explosion was uneven, and there was a single point which all of the galaxies and such revolve around.  Mankind has not been around long enough to even begin to see movement that would show us if that is a possibility

[remcook]

mmm..OK... GPS ...I stand corrected then. But it remains a rather exotic subject

[payloadcontroller]

Dingo, I understand your point, but if that were true, it would show up as an anisotropy in the 3K microwave background rad. And that has been studied for decades and is determined to be very nicely isotropic.

[Astronuc]

I am still pondering all of the comments so far.  If the background radiation (3K microwave) source is 'more or less' uniformly distributed (volumetrically) then it should appear 'more or less' isotropic - right?

What are the best books or other resources on cosmological models at present?  Who are the foremost experts?

I am curious as to how GPS is an application of GR.  It is certainly a consequence of GR.  I thought that satellites in orbit exchange signals with ground facilities and each other and the synchronization and triangulation allow for location.  I also understand that the US military deliberately puts errors into the GPS system (which they know and for which they correct) in order to ensure that commerical GPS are not accurate enough for targeting purposes.  This would help prevent hostile states from using commercial GPS for missile guidance.

I am thinking of another 'gedanke Experiment' - but I will have to properly define the geometry so that the question is clear - and perhaps provide a diagram.

One of the sites mentioned seems to indicate that time is a spatially dependent variable, and perhaps it is if the local gravity field (or gravitational constant?) is spatially-dependent.

[broomstar]

Yes, there are an anisotropy in CMB, and the isotropy is to 10^(-5), as I remembered. And from the anisotropy of CMB, cosmlogists can determine many parameters of the cosmos in a surprising accuracy. I think the anisotropy of CMB gives good initial conditions for large structure evolution simulation, as far as I know.

Visit the WMAP Website for more information.
http://map.gsfc.nasa.gov/m_mm.html

On the book to read to learn about cosmology , I think it depends on your background of mathematics and physics. For an undergraduate level introductory cosmology course, the Ned Wright's cosmology tutorial
(as Yale quoted)
http://www.astro.ucla.edu/~wright/cosmolog.htm
is a good choice.

For military purpose, I heard that US is interfering the cooperation of Europe and China for the next generation GPS : Galileo.