Cosmological question - where is the earth?

[broomstar]

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.

I am sure you have known that, in special theory of relativity, time is not independent of space. We discuss everything is the frame of space-time. In general relativity, there is no gravity -- gravitation force just behave as the curvature of space-time.

[remcook]

The americans turned off the added GPS noise a couple of years ago. They can still turn it on of course...

[Dingo1]

For general information:  Most missiles have GPS systems, but also have additional systems for maintaining course as GPS can be knocked out via EMP pulse, military intervention (turning off the signals), or other means for disrupting GPS.  If the GPS position does not meet other tracking systems, missiles can disregard the GPS.  It will cause a higher ratio of near misses vs direct hits.
GPS is mainly used for surface to surface missiles.  Surface to air/Air to surface still relys for the most part on heat seeking, radar and fly by wire (laser guided)

[Astronuc]

Getting back the original question, I was about to pose it to Wright at UCLA, but one of the requirements in asking a question is to make sure it's not addressed in the responses to FAQ, so I gave it one more try.

I found part of the answer, but the answer leaves me wondering.

http://www.astro.ucla.edu/~wright/nocenter.html

According to this post - there is no center of the universe, so the answer to question, "where is the earth" is, noboby knows - or rather, it cannot be known - at present time.

We can determine its location only relative to what we can see, but not with respect to what we cannot see.  Well, yes, that's obvious, and not very satisfactory.

[engaaraa]

As to how GPS is a GR application, well, it is not. It can be conceived in an entirely newtonian fashion. However, important errors would accumulate soon enough, caused by GR effects. The corrections to this effects is built into the software.

[Dingo1]

Astronuc, part of it is a matter of trianglation, to help get a general location of our galaxy in relation to other known galaxies.  A dificult task based on the shear numbers involved.  Understand that the further away a galaxy is, the harder it is to determne it's exact distance, due to the effect of accumilating errors in the formula's.  It also is difficult in determining the exact age, as those calculations are also contain errors.  As you can tell from the recently released pictures from the Hubble telescore, we have discovers several thousand new galaxies, previously undetected.

Now a question for our staff...:

If galaxies started forming at 300 - 400 millions years after the big bang, how could we see the light generated from those galaxies, wouldn't the light generated have passed our galaxy a long long time ago??

[Astronuc]

I was searching for information on the "Local Group" or "Local Cluster" and came up with:

http://www.seds.org/messier/more/local.html

This lead me to:

http://www.anzwers.org/free/universe/localgr.html (good map of local group)

http://www.seds.org/billa/lg.html

http://www.ast.cam.ac.uk/~mike/local_more.html

http://www.astrophotographer.com/Local_cluster_plot.htm

http://www.seds.org/messier/g-group.html


and several others (and some bad links).


I would be curious to know if anyone has made observations over the last 3 or 4 decades to see how far the individual galaxies have moved - or has anyone thought of doing this?  I would expect so, but then . . .  Also, how are the other galactic clusters moving with respect to us.

Re:  "If galaxies started forming at 300 - 400 millions years after the big bang, how could we see the light generated from those galaxies, wouldn't the light generated have passed our galaxy a long long time ago??"

I have been wondering about this statement also.  And brings me to another puzzle.

If the those galaxies are that old, shouldn't they filled with old and burned out stars and thus be very dull?  Is this consistent with what is observed in the recent pictures?  What age are they using?

Another site on the age of the universe and oldest stars:

http://astro.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node74.html

and

http://astro.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node63.html

which may answer some other questions.

[engaaraa]

If galaxies started forming at 300 - 400 millions years after the big bang, how could we see the light generated from those galaxies, wouldn't the light generated have passed our galaxy a long long time ago??

Lets take an example of one galaxy observed at 400 millions years ABB. Has light from this galaxy already past us?, yes, the light that left the galaxy 399 million years ABB. The same way light from whe it will be 401 million years ABB has not reached us yet. We can the ask, but if it was not formed as a galaxy 399 million years ABB? then that light wich is past us did not come from the galaxy, but from whatever collection of energy and matter that would become the galaxy. IF we go on back to Dark Ages when there was no light, then of course we could not see it, but not because its past us, but because it was never emitted.

[Dingo1]

Eng,  That is just the point, if the light from an object took 13.5 billion years to reach us, meaning it is at least 13.5 billion light years away, at a point in time that the big bang happened 400 millions years in the past, the universe would have only been 800 million light years in diameter..hince my problem comprehending this anomity.  Are we looking at an object that is outside of our universe, or is the age of the universe much, much older than is currently calculated, by at least 13.1 billion years???

[Orstio]

Dingo:

I think you are applying the speed of light constant to the expansion of the universe.

The speed of light is a property of space-time, which is just a property of the universe.  The universe can expand at an unlimited rate (there might be a limit, it is just unknown).  The expansion is not limited by the speed of light, because it happens to the universe, not in the universe.

For example, the universe could easily have expanded 30 billion light years in only 13 billion years.

[Dingo1]

Orstio, that gets to the heart of the primary question.  Is the universe a constant, expansion or eventually contract.  General theory has the universe currently in an expansion phase at the speed of light as proposed by Stephen Hawkins.  This is based that the physics of the universe were set in the first 1/10000000 of a second, with the speed of light being the absolute speed of expansion.  I am aware that he has been in the process of revising his theory, but with his recent hospitalization as his illness has continued to progress, work has stopped, with little commentary on his new formula's

[engaaraa]

I think it can be answered by saying it is 13.5 GY away now, not before, not always, because the universe is expanding. That light had to race against the expansion of the universe itself. The record of this race is in the redshift. The camera that took the picture was an infrared camera, and the exposure had to be very long to allow the  long wavelenght (stretched proportionally to the expansion of the universe) photons to arrive at a rate of one single photon per minute. Think about that rate. The photon is arriving constantly, but it takes a full minute for it to develop enough energy in the plate to be recorded. The front end of the wave impinges on the plate a full minute before it tail!.

Moreover, the galaxies pictured in the HUDF are indeed a little closer than 13.5 GY. If I remember correctly they are 12 GY away.

[engaaraa]

About the expansion's limiting speed being the speed of light, I have seen arguments for and against. It makes sense to me though. However, the evidence suggesting that the expansion is accelerating also says that this acceleration is a recent event (cosmologically) meaning, the universe had been expanding slower in the past, possibly at a constant speed (no acceleraton).
Inflationary theory does has the universe expanding FTL at some time in its creation I beleive. Inflationary theory is the solution to the horizon problem, which although is always formally stated thermodynamically, can also be stated observationally which I think is what you are trying to do. It goes something like this:
How are two galaxies at 180 degrees from each other and both so far away from us as to date from the early universe related? Are they spacelike separated, ae they timelike separated, can they see each other? in short, are they causally related at all? after all they date from an early, small universe. The answer according to inflationary theory is yes they are causally related, the universe expanded impossibly fast to separate them as they are now, but their causal relationship remains.