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Comment by cryptonector | Hacker News Reader

> If there is another reason the light from distant galaxies turn to red, kind of a long-distance light fatigue, we couldn't really know.

It's not just red shift that demands an explanation, but also that the sky (space) is dark. If space wasn't expanding then the skies would be bright white.

Another thing is the second law of thermodynamics, which very strongly implies a lot of standard cosmology.

Of course, standard cosmology could be all wrong, but these deductions are quite solid as far as the physics that we know today.

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It's not just red shift that demands an explanation, but also that the sky (space) is dark. If space wasn't expanding then the skies would be bright white.

Another thing is the second law of thermodynamics, which very strongly implies a lot of standard cosmology.

Don't you feel that these evidences just make a case from models checked in small scales and tell the whole universe what it should do?

I don't mean the reasoning is incorrect or unwarranted, just that there could be alternative explanations that involve unknowns.

Of course, standard cosmology could be all wrong, but these deductions are quite solid as far as the physics that we know today.

When applying Occam's razor, you try to minimize entities and simplify. But what appears simpler for a physicist is sometimes surprising for the rest of us. The equations don't work as expected? No problem, let's say that the entire universe is expanding or, even better, that space is expanding. Or that all the universe was at some moment in the space of a tennis ball. Or that big bang happened everywhere at the same time...

While some explanation fits the maths, it doesn't matter if it's totally alien to common sense. Actually I find all those weird explanations strangely appealing, they give me a sense of wonder and empowerment. Cosmology feels like a superpower :)

> Don't you feel that these evidences just make a case from models checked in small scales and tell the whole universe what it should do?

Yes, we like to think that the laws of physics we can deduce locally are global. That could be wrong, for sure. But consider that we understand the physics of our solar system (which has a star), and we see other stars in the skies which... implies that at least as much of our local physics that makes stars possible also applies where we see those stars.

Why would it be bright white?

Any straight line you draw from earth to any direction in the sky will then reach a star.

Which is insanely far away and all light coming from there will have diminished in energy enough to be barely visible?

Inadequate explanation. Even if the universe was a closed box with perfectly reflecting borders, what you would actually see in the sky is something like an average brightness of the entire universe with some fluctuations.

It's not inadequate. Brightness diminishes for two reasons. One is absorption from cosmic dust (not that big of a factor), the other is that the object appears smaller, but the brightness per unit solid angle from the object does not diminish. If there were just more stars behind that you could see, the sky would have to be much brighter.

Photons do not "diminish" in energy except via red shifting (where does the energy go? into the expansion of the universe!).

Light scatters, gets absorbed and re-emitted, it's true, but if distances don't change then the universe would still be much brighter than it is today.

Any straight line you draw from earth to any direction in the sky will then reach a star.

Only if you think that there is infinite matter, all of it forming the same kind of stars, uniformly spread over the infinite space and there is no unknown form of atenuation for waves traveling millions or billions of years light.

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