One of those sleepy Saturday's I suppose. An interesting thing about science, that screws with the inevitable progression of science concept, is how often you'll encounter a scientific concept that literally points giant flaming arrows at a scientific concept centuries before anyone finds a new and different route. One of those is Olber's Paradox, which results in a headscratcher as follows:

If stars are distributed relatively evenly throughout the Universe, then if I imagine a sphere around the Earth big enough to encompass X stars, a sphere twice the radius should have 8x stars, and thrice the radius should contain 27x stars and 4x the radius 64x etc. Now two stars, same brightness, one twice the distance of the other from me is as quarter as bright, thrice the distance 1/9th the brightness and so on. So a big shell of stars 1000 light years from me would appear a four times brighter than one 2000 light years from me, for individual stars, but there would be 4 times more of those stars at 2000 light years. So if you picture an onion, with us at the center, each layer of that onion should be, to the guy at the center, equally bright. More layers of onion, more total brightness, all the way out to infinity.

So here's the Paradox. Why isn't the sky at night time brighter? Why is the sky dark at night? Any black space between stars you see should be filled with other stars, it doesn't matter how far away individually they are there should always be a star there at the end of any direction you stare. And this was first postulated long before Hubble and Einstein and co. Before Olber himself even, Kepler himself was apparently troubled by it.

Now in an infinite, steady state Universe this is an ironclad Paradox, the entire night sky should be one big sheet of radiation and there's no avoiding it, even dust or objects in the way doesn't matter because in a vacuum all light absorbed is re-emitted just at a different frequency, basic blackbody radiation. Kepler argued that this meant the Universe was finite, but that itself does not work, and we'll get to why. First a bit of background. Back in 1576, when Kepler was a kid and Copernicus had proposed a heliocentric Universe a couple decades earlier, Thomas Digges suggested that stars might not be pasted onto some shell somewhere but be distributed throughout a huge, possibly infinite volume of space. This concept really stuck in the minds of a lot of people at the time. But the opposition immediately dismantled the concept because of the Olber Paradox (not called that yet, as Olber was from the 19th century), basic math says what we said above, an infinite number of stars over infinite space, no matter how far away some are results in a big even white shell for a night time sky.

So Kepler's answer, the Universe is large but finite in size. Otherwise, as he said "The whole celestial vault would be as luminous as the Sun".

No problem you say, but Newton obliterated that. Here's why. In a finite Universe stars pull at each other and will collapse into a big ball. Newton's physics demanded, no option, that a Universe be infinite in size, so that there was always something further away pulling stuff apart.

Its an interesting note on history, much as many think Columbus was trying to convince people the world was round not flat, when in reality they already thought so and would just telling him, truthfully, that it was round but larger than he thought, people have a notion about Copernicus and the like that they were trying to show conclusive evidence of how the Universe was to people who simply shook a bible at them. Not at all. A dark night time sky literally disproves Newton's Laws of gravity, its ironclad, like 2+2=4. If the Universe is infinite, then the night time sky is as bright as the sun, if it isn't then the Universe collapses on itself. This paradox, thus far unnamed, drove mad even Newton's strongest supporters who were many, because his laws so perfectly and elegantly explained planetary motion. And that's how things stood for quite a while, with many trying to cobble together explanations as ludicrously contrived and convenient as retrograde epicircles to explain a geocentric model were.

I know some of you are jumping ahead now, Newton was right you say, but wrong about the Universe itself, it is not infinite in size or age, furthermore light takes time to travel to places. Which is true, and sounds like a good explanation, but is also totally wrong, it doesn't solve Olber's Paradox but we'll come to that later.

So comes along de Chesuax in 1744, he built the whole mathematical construct out to several trillion light years out. And yes, they did know what a light year was by then, Roemer and Cassini had already worked that out, though they were off a bit, before Newton even published his works. de Chesuax showed that equally bright to the Sun was wrong, that the night sky should be much brighter then the sun, then handwaved the suggestion that maybe some interstellar medium of some sort absorbed it.

Everyone knew of this problem, but rarely did anyone want to tackle it, because they were very wedded to the Steady State model by then and the answer was clear, that model was impossible. So virtually no one tried to formally explain it, it was embarrassing. Probably why it was named for Olber, a very minor astronomer who came on the scene very late.

So absorption, dust gets in the way, was the handwave until Herschel (son of the guy who discovered Uranus) killed that idea of gas or debris absorbing the light. He showed that abosrbtion didn't work and that in de Chesuax's model the Earth should be vaporized in mere hours by the proposed amount of light, as the dust or whatever absorbed light it would heat up and emit light itself, merely delaying the problem.

This in many ways was the death knell for a Steady State Universe, but there was no alternative but the old Geocentric model now thoroughly in the dustbin of history. It was handwaved into the Victorian Universe, a universe assumed infinite in size from a practical standpoint but not actually quite infinite, just 'close enough', and for those who raising an eyebrow at that yes, 'close enough' and 'kinda sorta' are indeed considered bad arguments when dealing with infinity, but handwaves are common and that's a normal one in spite of being always bogus.

At last, in 1901, a few years BE, Before Einstein, comes Lord Kelvin, who does an exhaustive and long overdue analysis of Olber Paradox, now named such. He pointed out that stars do not shine forever (though his reasoning was utterly wrong) and that light took time to get from A to B (though his reasoning was wrong) but he did raise relevant issues that would be shelved for nearly a century, because Einstein, Hubble, and the others soon arrived and the Olber Paradox was shelved, even though none of the new theories actually justified doing so. Kelvin, who quite accurately (for the wrong reasons) said Sun-type stars lived about 10 billion years calculated that for us to see a star on every path would require looking out 10^23 light years, that stars only live 10^10 years, and that the light simply hadn't reached us yet. Problem being that he had no reason to think the Universe wasn't 10^23 year older or more, except the Bible, nor any reason to think that if a star died it wouldn't be replaced. So this explanation was discarded. Doubly troubling is that as we did begin to be able to see further away, and further back in time, those distant dim galaxies were typically much brighter then the nearer, older ones. More quasars, more supermassive stars, though this wasn't known just yet, but Kelvin's cooling stars (pre nuclear theory) would have been brighter in the past, those stars, in those models, gave off light because they were hot, and in doing so got colder and dimmer with time.

Now the modern stuff, at last!, come 1950 there were two competing theories, an expanding universe of finite age and an expanding universe of infinite age. It was in the infinite aged one that a resolution is available for Olber's Paradox, not the finite one, incidentally. That's because in that version of the Universe they decided that the laws of the Universe probably changed with time. Bondi, the main proponent who addressed this, decided we could simply toss out conservation of matter and assume a small amount of hydrogen was constantly being created everywhere all the time, about 1 atom a lungful every billion or so years. Hard to measure that but across hundreds of cubic light years more than enough to make new stars.

If you're snorting in derision, by the way, at the handwave of matter appearing from nowhere, remember the competing theory claimed the exact same thing, that huge amounts of matter simply emerged during some 'Ludicrous Big Bang'. Matters also does pop out of nowhere all the time, it just usually pops right back into nowhere.

Bondi claimed (wrongly but it was very popular) that the red shift of light, from an expanding Universe, meant we were surrounded by light just not visible light. A couple years later we discovered the CMB.

Here's the analogy. You're standing in a forest and someone's tell you it goes on forever in every direction. You look around and see trees everywhere you look but notice gaps with no trees. You pull out binoculars and observe trees in those gaps but see more gaps, with no trees in them. You say to the other person, "This forest is not infinite, I can see empty spaces." He smiles and says "Well, the space you're looking at isn't flat, after a dozen or so miles the forest disappears beneath the Horizon."

Ignoring the obvious, that a spherical planet forest is not 'infinite' and were it so there's be no horizon anyway, this makes sense except near the horizon as those tree trunks start disappearing their leaves get in the way and probably make it a perfectly opaque line of site. But ignoring that, this is essentially what the red shift is telling us, the expanding universe is curved, not flat.

Here's the thing, it's not just that light hasn't reached us yet and much of it is redshifted, were that so something like the CMB wouldn't make logical sense either, because the CMB is a big ancient sphere of light all round us. It's that many objects are constantly leaving the Observable Universe itself. We don't see the whole Universe, what we see is the Observable Universe, those things that were near enough when they sent light for it to have reached us. Now the Universe is expanding, and so is the Observable Universe centered around us. Difference being that the Expanding Universe has the same amount of matter in it while the Observable Universe, while expanding, is hemorrhaging matter.

Stuff disappears, its still in the Universe but not the Observable universe, light from it will never reach us, as the Universe expands galaxies get further away until eventually they disappear, no photon from them ever again to reach this world. The light from them hasn't just not reached us yet, it never will. Even as the Observable Universe expands, as more time passes for light to reach us. The density of stars is plummeting because not only is the Universe expanding, and the observable Universe too, but the Observable Universe it expanding in size but decreasing in mass. And this process is occurring faster then light is being produced.

So there we go, the answer to Olber's Paradox. The Night Sky not only isn't as bright as the sun because the Universe is young, and would get brighter, but the reverse, the night sky is getting darker with time. Things get further away, whole galaxies disappear forever, and to top it off those galaxies near us are dimmer because they're older, slowly running out of fuel and the truly massively bright items like quasars and regular supernovas.

Fun thoughts maybe, I think its amusing to me because we have this modern outlook that the rise of science was very black and white, the smart people versus the morons. And its so easy to forget that so many of those theories had that 'big gaping hole' in them which rendered them logically impossible, that's an unpleasant thought so we rewrite it so that Christopher Columbus was arguing the world was round to a lot of Flat Earthers. It always makes me wonder if someone a century form now will be laughing at us dumb 21st century scientists, concocting crazy theories, but I don't wonder long because 1) It serves no purpose and 2) I'm sure the answer is "Yes".

Or maybe its just a boring Saturday and I'm so unused to having free time to kill I've forgotten how to But assuming anyone was masochistic enough to read this to the end, I've presumably helped you kill some time, or given you a headache, or both.