# The speed of light through space and its effects on black holes

I was curious about a couple of things that don't quite make sense to me: (and probably won't when I get the technical answer, but I'll ask it anywise.)

1. When light travels through space, is its speed effected by other bodies gravitational pull, specifically over great distances, so it LOOKS like the the universe is accelerating outward? Is the universes expansion even measured by light from distant stars?

2. Secondly, if what I've seen/heard is true about objects falling into a black hole, (that they would appear to freeze at the event horizon) wouldn't the heat and light falling into a black hole from an accretion disk make said black hole look bright?

• These should be two separate questions. – user4552 Sep 8 '13 at 0:04

1. The speed of light doesn't change. Even going straight into or away from a black hole, its speed can't change. Its energy changes though, consistent with gravitational potential. However, its speed still cannot change. Remember that for a photon, $E=h\nu$ so the frequency (and wavelength) of the light changes. Light going toward a black hole is blue shifted; as a result, scientists expect that gamma rays or x-rays near a black hole. Conversely, light traveling away from a black hole is red shifted, which may explain why we actually observe radio waves coming from areas of assumed black holes.