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Q&A: Cosmology

I understand you can't think of the big bang as beginning at some point in space (although I did read that our visible horizon did begin at one point). However, there still appear to be inconsistencies about this area. If the galaxy that's 12.3 billion light years away today is moving away from us at near the speed of light (from the Hubble constant calculation), it would have been very close to us 12 billion years ago. The light we're seeing now is 12.3 billion years old and at the time of emission we were only 1 billion light years away at that time or closer; so, it took the light 11 billion years to get to us after emission, when we were only 1 billion light years away (or closer) to start with?

This tells me that the old theory of tired light is more reasonable and that the lost energy from the light is going into dark energy or dark matter as the photons pass so much dark matter in such a large transit across space. Does that make sense?

Secondly, why are there no dust clouds (the ones that block light in a galaxy to cause those dark lanes) in between the galaxies? I would think they could be found because they would block deep field areas sufficiently to become detectable. Has anyone found inter-galactic dust lanes? It seems it would have to be there for galaxies to form. Then you'd wonder where that came from, the Big Bang.

Yes, you are right about the galaxy that is 12.3 billion light years away from us, it would have been much closer to us 12 billion years ago. And in fact 12 billion years ago everything in the universe was much much closer together. The idea of the Big Bang came from just this kind of reasoning - if all the objects in the universe are expanding away from each other, then earlier they must have been closer together. Well, if you take that to its logical conclusion, at some point in the past that distant galaxy you're talking about must have been right next to us! And if you go back even earlier, the universe must have been extremely dense with collisions left and right between all the particles. If you go back even earlier, the universe is one point of infinite density, at least this is what we think now (and of course, we may not be right!).

For your second paragraph - there are 2 ways for light to lose energy or get "tired" as you say. First, if it collides with a particle it will lose energy in the collision. The energy the photon loses is gained by the particle it ran into, and the particle gets a bit hotter or vibrates. The other way a photon loses energy is if space itself is expanding as it travels through it (as we believe is happening) in which case the wavelength of the light gets longer, and longer wavelength light has less energy. As far as I know, there is no energy from photons being transmitted to dark energy or dark matter, although this dark energy is such mysterious stuff that it might be doing just about anything! Please see more information at

Lastly, there are huge clouds of dust and gas in between galaxies, your thinking is correct. We have an illustration and discussion of hot intergalactic gas at: If you look toward the bottom of the page and click on "More Images of Hot Intergalactic Gas" there is a dust and gas image at the top of the page. Dust is observed in the infrared wavelengths though, so the new Spitzer Space Telescope should have beautiful images in the near future (if they don't already), please check out their web pages too:

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