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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?

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  • $\begingroup$ These should be two separate questions. $\endgroup$ – Ben Crowell Sep 8 '13 at 0:04
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  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.

  2. It won't appear hot or bright, because that would require the light to escape from the black hole to reach our eyes. All the light we see as evidence of black holes comes from the accretion disk, or from Hawking radiation. The Hawking radiation sort of appears to come out of the black hole, but according to Hawking comes from right outside the black hole. The spectrum of the Hawking radiation turns out to be equal to blackbody radiation, so what we see from the black hole itself is limited by the temperature of the black hole, and then red shifted on its way to us.

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if your hypothesis is true, then most objects in the sky are nearer than they appear, similar to how swimming pools look shallower than they are because light actual slows in water. the problem is this says nothing about how the distances are changing. actual observations show that light from faraway objects have had their frequency decreased over the time those photons spent traveling to us more so than light from nearer objects. the reasonable conclusion is that space itself expands. further, there is no evidence that light slows from gravity, but there is plenty for light following the curvature of vacuum space (around a mass) at the speed of light.

at the event horizon space is curved so that light is trapped in orbit (still at speed of light!). note that the glowing accretion disk is not at the event horizon but at a much higher orbit, otherwise it wouldn't be detectable by telescopes. even Sagg A at the center of our galaxy produces detectable radio waves. the best we can conclude is the narrowing of the region in which the black hole actually resides.

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