Could dark matter be light

[Zula110100100]

I was wondering two hings. First, does light's energy equate to mass and result in the bending of space time? The fact that it does follow the curve of space-time as well as mass-energy equivalence principle lead me to believe it would. Secondly, was that energy accounted for in calculating the mass of the universe(or whatever we did to figure there was dark matter)? Because that seems like it would be quite a few photons of really small energy that would add up to a lot of energy centered around where the most photon-emitting objects were.

and sort of extra: Thirdly, could the effects of gravity not cause enough loss of energy in photons traveling for such great distances to shorten their wavelengths and cause redshift, I didn't think we expected a constant energy level in anything else, why light, it should lose energy over time like anything else... eventually causing redshift and the farther away the object the more it would be red-shifted?

 

[phinds]

I
and sort of extra: Thirdly, could the effects of gravity not cause enough loss of energy in photons traveling for such great distances to shorten their wavelengths and cause redshift, I didn't think we expected a constant energy level in anything else, why light, it should lose energy over time like anything else... eventually causing redshift and the farther away the object the more it would be red-shifted?

This point of view was put forth quite some time ago but was totally debunked; just doesn't work. Google "tired light".

 

[Chalnoth]

I was wondering two hings. First, does light's energy equate to mass and result in the bending of space time? The fact that it does follow the curve of space-time as well as mass-energy equivalence principle lead me to believe it would. Secondly, was that energy accounted for in calculating the mass of the universe(or whatever we did to figure there was dark matter)? Because that seems like it would be quite a few photons of really small energy that would add up to a lot of energy centered around where the most photon-emitting objects were.

Yeah, the only problem there is that we can observe photons. They make up about 0.001% of the energy density of the universe, far too little to explain dark matter.

and sort of extra: Thirdly, could the effects of gravity not cause enough loss of energy in photons traveling for such great distances to shorten their wavelengths and cause redshift, I didn't think we expected a constant energy level in anything else, why light, it should lose energy over time like anything else... eventually causing redshift and the farther away the object the more it would be red-shifted?

Lengthen their wavelengths (shortening their wavelengths would entail giving them more energy). But yes, this is exactly what happens. This is the normal redshift. As phinds mentions, some have proposed some ways that light might lose extra energy, in addition to the normal redshift. But these "tired light" scenarios have failed to agree with experiment.