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As i see it, light behaves in certain ways, as the Double Slit experiement shows, So when light comes into contact with dark matter, it becomes both a wave and a particle, the wave is bent around the dark matter, which we see as gravitational lensing, but the particle will go through the dark matter, but will also leave our frame of reference, follow the particle through dark matter and its speed would not change staying at $c$, but only relative to the the space it travels through, it would eventually pass through dark matter, we would see this first as a red or blue shift as it slowly emerges (from our perspective), and continues on it way, Never knowing it had passed through any medium, where as the wave had passed around the dark matter thousands, millions, or billions of years before, depending upon the mass of the dark matter.

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This isn't the way light behaves. There isn't a wave and a particle that follow separate paths. –  Peter Shor Nov 17 '12 at 23:28
Related: physics.stackexchange.com/q/2110/2451 and links therein. –  Qmechanic May 4 '13 at 18:57
This seems to be assuming dark matter in order to disprove the Doppler shift. Confusing. –  iamnotmaynard May 2 at 22:55

3 Answers 3

Light interacts with dark matter only through gravitation. That is why we try to map dark matter distribution in the universe by statistically studying the small distortions in the images of background galaxies (weak lensing regime).

Whole redshift-sliced 3D maps of dark matter have been traced for some small regions of the sky, by means of weak lensing distortion. In all cases, the observed properties of the background objects are in a statistical sense the same (apart from shape distortion) for objects observed through, and not through, dark matter clumps.

That is one of the proofs for dark matter not interacting with photons.

Another proof is in the strong lensing regime of multiple lensed quasars. The Hubble parameter can be derived by measuring time delays between flux changes of a background quasar, as detected in two lensed images. That time delays happen because the photons travel through different paths, both through the dark matter halo of the lens galaxy and the Universe. But the measured line waveleght redshifts agree perfectly with the optical paths derived from the geometry of the lens, and no additional frequency shift has ever been observed. As expected from GR, photons leaving a dark matter clump behind recover the colour they had when they entered it.

That is, dark matter affects light only by means of the metric tensor, but doesn't interact with the photons. Otherwise it would had been already observed.

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After reading your question again, I definitively don't know what your question is... –  Eduardo Guerras Valera Nov 17 '12 at 22:22
Come on @MarkEmery, Eduardo has given you quite a nice answer (+1), even though reading your question it is hard to guess what you are up to. –  Dilaton Nov 18 '12 at 11:48
@Mark Emery, dark matter is as "folded space" as normal matter, and that is why we observe different images of the same background quasar when a galactic dark matter halo happens to be in the line of sight. The phenomenon is studied with real data since 1979 with the discovery of the first lensed quasar Q0957+561. Dark matter DOES slow down the photons as you well say, but that is indeed the way all lenses work (glass lenses too). You are right, but there is no othe way we could be observing lensed images... –  Eduardo Guerras Valera Nov 18 '12 at 13:07
And for the images to reach us, the photons must have already crossed the halos. Otherwise we wouldn`t see the background images at all! And there is no statistical differences in surface flux or redshift between images that reach us after travelling across dark matter and the others. That is, there is lot of empirical evidence of photons having crossed dark matter and not have suffered any scattering or interaction further than the gravitational deflection. It is not dogma, but just overwhelming empirical evidence. –  Eduardo Guerras Valera Nov 18 '12 at 13:14
The magnification of lensed images is a consequence of their bigger apparent size, but mean surface flux is unchanged, as well as the frequency of the light. Dark matter has no cross-section for photons, or even for baryonic matter, it is collisionless and only produces curvature. That is why many still are searching for alternative gravity theories. Because there is observational evidences far beyond any doubt, that photons do cross dark matter and do reach us unscattered. –  Eduardo Guerras Valera Nov 18 '12 at 13:25

This is another version of the "tired light" hypothesis. This doesn't work for a variety of reasons, among which are the facts that there are no absorption or emission peaks evident in light, there is no blurring of the light from distant objects (stars look like points, not globs), and there appears to be no dependence on the angular view from the sky when looking for redshifts (while dark matter DOES appear to be clumped).

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light passing through dark matter would be similar to light passing through fiber optics, there is no reason for "blurring",dark matter is folded space, which would from our perspective focus light into a very narrow beam. –  Mark Emery Nov 21 '12 at 20:14
@MarkEmery: scattering is scattering. And fiber optics certainly do blur the light--the index of refraction depends on the wavelength of the light passing through it. If the interaction with dark matter is independent of wavelength, then it would be quite unique. –  Jerry Schirmer Nov 21 '12 at 20:31
As you say scattering is scattering, and there is no blurring of light from distant objects, but there should be,it is dark matter that refocuses these objects. –  Mark Emery Nov 21 '12 at 20:51
@MarkEmery: what? Are you talking about gravitational lensing? That is one of the few effects that is NOT frequency-dependent (so long as the frequency of the light is much less than the curvature of the relevant region of space). If you have light bouncing off of/through dark matter, you have to explain why there is no dispersion of the light--i.e., answer why is the DM/matter interaction frequency-independent. –  Jerry Schirmer Nov 21 '12 at 21:00
Our universe is thought to be made of 85% dark matter, plus galaxy clusters, alot of gravitational lensing going on, but still we see "no blurring" from distant objects,what we are really seeing is the photon particle that as travelled through the D/M, which as not scattered, the wave that went around the dark matter has G/L as been lost in the maze of D/M, we only see the odd effect that as not yet been completely distorted, Once the photon leaves D/M it continues as a wave and a particle.but each time on coming in contact with D/M, the photon wave goes around, and the particle goes through. –  Mark Emery Nov 22 '12 at 1:49

Dark matter--as the term is understood--does not interact electromagnetically. Light only interacts electromagnetically.

So, no it could not.

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Sorry we do not yet fully understand Dark matter, you are only stateing speculation yourself. "So,no it could not", is what i expect from one with no ability to see other than what they are told. Not long ago it was thought that light could not be a wave and a particle, but we move on. –  Mark Emery Nov 17 '12 at 18:45
This question is for those who can think outside the box, –  Mark Emery Nov 17 '12 at 18:56
I'm not speculating. Dark matter is known to not interact electromagnetically because if it did we would be able to see it. Essentially nothing else is known about the stuff, but that is an observational fact. –  dmckee Nov 17 '12 at 19:09
We can not see it, because once the particle enters dark matter it leaves "our space time", dark matter is higher dimensional, even though the "particle" would still be travelling at c within this medium, –  Mark Emery Nov 17 '12 at 22:12
Light interacts gravitationally too, as is explained in the other answer. –  Dilaton Nov 18 '12 at 0:00

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