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Kevin Webb
2007-Nov-03, 12:12 AM
when any type of detecion device is pointed towards regions of space that are expanding is anything detected ?

where is one such region that is known to be a place of spatial expansion ?

is the only evidence of spatial expansion indirect eg galaxies moving apart ?

Cougar
2007-Nov-03, 03:06 AM
is the only evidence of spatial expansion indirect eg galaxies moving apart ?

Your first two questions kind of boil down to this one. The short answer: No.

I expect there is more evidence than I know about -- I'm just an interested amateur. But here is a pretty compelling bit of evidence showing that an event occurring at a redshift of 0.361 appears to be playing out more slowly than normal from our vantage point. I believe there's only one explanation for this: time dilation.


A Definitive Measurement of Time Dilation in the Spectral Evolution of the Moderate-Redshift Type Ia Supernova 1997ex (http://adsabs.harvard.edu/abs/2005ApJ...626L..11F) -- Ryan J. Foley, Alexei V. Filippenko, Douglas C. Leonard, Adam G. Riess, Peter Nugent, and Saul Perlmutter

A couple quotes in the Abstract:

...In an expanding universe where 1+z represents the factor by which space has expanded between the emission and detection of light, the amount of aging in the supernova rest frame should be a factor of 1/(1+z) smaller than the observed-frame aging....

....[Our] result is inconsistent with no time dilation with a significance level of 99.0%, providing evidence against "tired light" and other hypotheses in which no time dilation is expected.... The result is within 1 sigma of the aging expected from a universe in which redshift is produced by cosmic expansion.

Fortunate
2007-Nov-03, 04:42 AM
when any type of detecion device is pointed towards regions of space that are expanding is anything detected ? where is one such region that is known to be a place of spatial expansion ?

Space is expanding all over. With the exception of points that are bound to each other (by gravity, for instance), every point in space is expanding away from every other point. At least this is the picture we have.


is the only evidence of spatial expansion indirect eg galaxies moving apart ?

Then I'm not sure what you mean by "indirect". I would think that observing galaxies moving apart would be extremely "direct". Unfortunately, the objects of interest are so far away from us that it would take a very long time to "see them move" with respect to each other. We are better off trying to ascertain their motion relative to us than their motion relative to each other.

One of our most compelling pieces of evidence for expansion is the fact that the spectrum of the light from distant objects is "redshifted", that is, the measured wavelengths are longer. We take that as evidence that the objects are receding from us. The further away the objects are, the more the light seems to be redshifted, so the faster the objects seem to be receding. Most of these redshifts don't seem as if they could be caused by objects moving through space - the objects would have to be moving too fast. The redshifts seem to indicate that the universe is expanding.

Time dilation, discussed in the paper cited by Cougar, is another indication of expansion. According to special relativity, if a galaxy or a galaxy cluster, for instance, is moving very rapidly with respect to us, the observed time for events to unfold in that galaxy or cluster should seem longer to us. This is referred to as "time dilation." We actually observe time dilation in some cases. Again, the observed time dilations are much too great to be caused by objects moving through space, so these time dilations also seem to indicate expansion. They also correlate numerically with the redshifts in the way our theories predict, so the results relative to these two indicators mesh consistently.

Good night. I'm going home.

Kevin Webb
2007-Nov-06, 04:20 AM
Space is expanding all over. With the exception of points that are bound to each other (by gravity, for instance), every point in space is expanding away from every other point. At least this is the picture we have. when it is said that space is expanding, what is space made of ?

GOURDHEAD
2007-Nov-06, 02:05 PM
when it is said that space is expanding, what is space made of ?Placeholders. Domains in which distance can be measured with rulers and coordinate systems can be applied to define locations. A very fundamental attribute of existence. At the level of fundamental attributes, the composition of protons, neutrons, electrons, quarks, etc., is equally difficult to define.

Has anyone been able to quantize the expansion of space or the rate of increase of the expansion of space, or to define the boundaries between regions susceptible to expansion and those prevented from expanding by "gravitational binding"? Has anyone postulated about there being a hard limiting threshold to maximum density such that as the big bang proceeded from "a point source" (vanishingly small volume), mass was only introduced at a rate that was limited by the threshold (definition of the threshold limiting influences is left as an exercise for the student) and this process continued over a relatively large segment of time (maybe even to this day)? Would such a process be driven more by a "mass birthing potential" or a "space expansion potential", or both?

Kevin Webb
2007-Nov-06, 07:56 PM
okay the question "what is space made of ?" is, i admit, a bit embarrassing, however GOURDHEAD's response made me think a bit more...

when it is said space is expanding, it is likened to that if a very special (that is had no gravitational ability and would be influenced by the expansion of space completely) rule (measuring rod) was placed in a region where no mutual gravitation comes into play, this rule would stretch. from a region out side the expansion zone, a wavelength for example is stretched (red shifted) because 1cm of space is now 2cm of space. from within the expansion zone 1cm is still 1cm because everything, including the measuring device has stretched.

am i getting closer to understanding expanding space ?

Fortunate
2007-Nov-06, 08:52 PM
If you figure it out, please tell me.:)

Fortunate
2007-Nov-06, 08:58 PM
Here is my understanding. I am no expert. We say space is expanding because all objects, except for some that are bound to each other, seem to be getting further away from each other. Wow, that was really short.

worzel
2007-Nov-06, 11:34 PM
I thought it was "just" a co-ordinate system choice. I say "just" because it is a very useful one for cosmology because it doesn't give a special status to any particular galaxy.