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View Full Version : What lensing effects does the expansion of space produce?



Fred Sagen
2009-Aug-17, 02:25 AM
Space is expanding.
We have deduced this from the rates at which distant objects appear to be receding.
Because space is expanding in all the three dimensions the images that we receive of distant objects are not the same size that they would be if they were at that distance and space were not expanding.
The expansion of space has a lensing effect on those images.
So is the image we receive shrunk or stretched?

DrWho
2009-Aug-17, 03:53 AM
Because space is expanding in all the three dimensions the images that we receive of distant objects are not the same size that they would be if they were at that distance and space were not expanding.
What makes you say that? Distance is distance. Objects at a certain distance will appear the same size regardless of how they got there.


The expansion of space has a lensing effect on those images.
Are you sure that you aren't confusing expansion of space with gravitational lensing?

Fred Sagen
2009-Aug-17, 04:56 AM
On the photons' journey through expanding space their separation increases unless they started out on convergent paths.

Even convergent paths may become parallel or diverge given enough travel time.


Distance is distance. Objects at a certain distance will appear the same size regardless of how they got there.
The images we receive are of the objects when they were closer than they appear.
During the lights' journey the object has receded and isn't where we perceive it to be.

AndrewJ
2009-Aug-20, 04:57 PM
On the photons' journey through expanding space their separation increases unless they started out on convergent paths.

Even convergent paths may become parallel or diverge given enough travel time.

I think that's an interesting point - MES increasing the gap between simultaneously approaching photons and thus expanding our image of the DSO somewhat. This might be measurable if MES can be measured as having redshifted the wavelength of light.



The images we receive are of the objects when they were closer than they appear.
During the lights' journey the object has receded and isn't where we perceive it to be.

Sure, we see the DSO "then" rather than "now" (although SR tells us there is no absolute measure of distance or travel time etc.) so we see the object earlier in its recession from us and thus larger. However, this is a separate point from your one about expansion between parallel photons affecting our image.

Use of the word "lensing" might be muddying the issue.

matthewota
2009-Aug-20, 05:23 PM
The photons we see are only the ones coming directly at us. I cannot see how they can be distorted by lensing that way. The light is shifted due to the Doppler effect, though

phunk
2009-Aug-20, 05:56 PM
The photons we see are only the ones coming directly at us. I cannot see how they can be distorted by lensing that way. The light is shifted due to the Doppler effect, though

But for example, if the photons came from opposite sides of a galaxy 100000ly wide, then the paths of the photons from each end of the distant galaxy, to our telescope, would be separated by an average of 50000ly. That distance would expand during the billions of years in between, it's not like the photons are gravitationally bound to each other. I'm not sure how to calculate the effect of that on our measurement though...

Jeff Root
2009-Aug-20, 06:07 PM
I'm uncomfortable with the use of the term "lensing" for this effect, but
the result is quantitatively graphed by the cyan curve (marked "DA") on
this frequently-linked little page:

http://www.atlasoftheuniverse.com/redshift.html

-- Jeff, in Minneapolis