## Is light blueshifted when it is gravitationally lensed? (Intermediate)

As an amateur, all I've heard about dark energy is that it is an explanation for why the rate of expansion seems to be increasing. A thought that occurred to me is that the further light travels, the more gravitational fields it would cross. Could it be that by magnifying it, the lensing effect blueshifts the light, reducing the redshift, so that light from closer sources has a greater average redshift, creating the impression that the rate of expansion is increasing?

Light does cross gravitational fields as it travels across the universe. The nice thing is though, that (unless it starts in the centre of a potential well) any gravity well it goes into it also has to climb out of. So the net gravitational Doppler shift from crossing these potential wells is zero.

I'm not entirely sure of this, not having the sources available, but the impression I recall of the lensing effect, is that the light from the source on the other side of the lens is magnified, as well as reflected around the source. What are the mechanics of this effect? The light is focused. What does this require of the light waves? Could it involve a compression of the waves? This is my chain of logic on this idea.

What happens in gravitational lenses is that the area of the source gets smaller (as it is focussed) so the surface brightness increases - the total brightness always remains the same.

So the frequency is not affected?

If the photons pass through the gravitational potential well they get the opposite gravitational doppler shift going in as they do going out so the net effect is zero and the frequency does not change.

Why wouldn't the photons(the area of the source) be dispersed on exiting equal to their being focused on entering?

When light rays pass near a massive object they are bent inwards so that light rays that would have otherwise spread out over a large area are all focussed onto one place. This is why the image gets brighter. This is an entirely different effect than that of photons losing and gaining energy as they go in and out of the gravitational potential well. There is a nice tutorial on gravitational lensing on this website.

This page was last updated June 27, 2015.

### About the Author

#### Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Website:  http://icg.port.ac.uk/~mastersk/