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davefoc · 11th September 2009, 04:34 PM

Does the expansion of the universe perpendicular to the direction from a light source, affect the number of photons detected by the observer?

OK, the universe is expanding from us in all directions. But the theory is that the universe is expanding isotropically. So it is expanding in all directions from the photon source and the photons as they travel through space.

Does that mean that the photons reaching an observer will have been thinned out by the expansion of space through which the photon is traveling?

If this thinning takes place do the estimates of distant supernovae that are based on intensity need to take into account that the intensity of light from the event will not only be reduced by the distance to the event (inverse square law) but by the expansion of space perpendicular to the path of the light?

I am not talking here about the expansion of space in the direction of the light beam. That increases the distance from the source which reduces the intensity of the received light.

Brian-M · 11th September 2009, 09:40 PM

If the expansion of space causes a beam of light to be spread-out over a larger area (by increasing the space between the individial photons in the beam) then yes, that would reduce the number of photons detected by the observer.

(I think that's what you were asking.)

I have no idea if astronomers account for that in their calculations, but I'd be surprised if they didn't.

edd · 12th September 2009, 03:45 AM

There's two distance scales you want to read up on:
* the luminosity distance
* the angular diameter distance

The first is basically a measure of how faint something looks, and the second is a measure of how small it looks.

http://arxiv.org/pdf/astro-ph/9905116 covers them and others in detail. The luminosity distance is derivable by noting how the surface brightness changes combined with how the angular size changes. Really what you're asking is if the effects from the angular size distance is included, I think, so your answer is yes.

11th September 2009, 04:34 PM	#1
davefoc Philosopher Join Date: Jun 2002 Location: orange country, california Posts: 7,333	The inverse square law and the expanding univers Does the expansion of the universe perpendicular to the direction from a light source, affect the number of photons detected by the observer? OK, the universe is expanding from us in all directions. But the theory is that the universe is expanding isotropically. So it is expanding in all directions from the photon source and the photons as they travel through space. Does that mean that the photons reaching an observer will have been thinned out by the expansion of space through which the photon is traveling? If this thinning takes place do the estimates of distant supernovae that are based on intensity need to take into account that the intensity of light from the event will not only be reduced by the distance to the event (inverse square law) but by the expansion of space perpendicular to the path of the light? I am not talking here about the expansion of space in the direction of the light beam. That increases the distance from the source which reduces the intensity of the received light.
	__________________ The way of truth is along the path of intellectual sincerity. -- Henry S. Pritchett Perfection is the enemy of good enough -- Russian proverb

12th September 2009, 03:45 AM	#3
edd Graduate Poster Join Date: Nov 2007 Posts: 1,615	There's two distance scales you want to read up on: * the luminosity distance * the angular diameter distance The first is basically a measure of how faint something looks, and the second is a measure of how small it looks. http://arxiv.org/pdf/astro-ph/9905116 covers them and others in detail. The luminosity distance is derivable by noting how the surface brightness changes combined with how the angular size changes. Really what you're asking is if the effects from the angular size distance is included, I think, so your answer is yes.
edd Graduate Poster Join Date: Nov 2007 Posts: 1,615	__________________ When I look up at the night sky and think about the billions of stars out there, I think to myself: I'm amazing. - Peter Serafinowicz

11th September 2009, 09:40 PM	#2
Brian-M Daydreamer Join Date: Jul 2008 Location: Downunder Posts: 4,386	If the expansion of space causes a beam of light to be spread-out over a larger area (by increasing the space between the individial photons in the beam) then yes, that would reduce the number of photons detected by the observer. (I think that's what you were asking.) I have no idea if astronomers account for that in their calculations, but I'd be surprised if they didn't.