Unless you are blind, it’s obvious to the rest of us that there is quite a difference between daytime and night-time. At night, the sky is dark! Why? Well, it’s obviously dark at night because the Sun isn’t shining in our night sky – wrong answer. In fact, if you stop to think about it, the night sky should be as bright as the daytime sky. That it’s not means there’s a paradox, usually referred to as Olbers’ Paradox (after German astronomer Heinrich Wilhelm Olbers). Resolutions have been around for yonks; I introduce another one here.
Olbers’ Paradox: Why is the night sky dark? In our vast Universe, assuming it is static and infinite in space and time, with an infinite number of stars (or galaxies or some sort of radiating objects) in it, it would seem that no matter in what direction you looked, sooner or later your line of sight would intersect or intercept a star or galaxy or some sort of radiating object. Thus, the night sky should be as close to infinitely bright as makes no odds. Of course you know and I know that’s not the case.
Perhaps there are massive interstellar clouds of gas and dust which, like window blinds blocking out sunlight, block out the stars, galaxies and related. Trouble is, you can only absorb radiation for just so long before the substance doing the absorption can’t absorb any more before reradiating it. Your window blinds eventually feel the heat and you will too when you touch them. However, there’s one exception to that rule and the point of this revisitation – see below.
The most logical resolution to the paradox is that there’s only a finite number of radiating objects out there and thus our line of sight could extend out to infinity and not intersect or intercept any radiating objects.
Then too, our Universe is expanding and therefore many astronomical objects (distant galaxies mainly) exhibit Doppler Effect red-shifts. This is just the Doppler Effect for an emitter of light that’s receding from us as opposed to the more familiar shift in sound to lower frequencies such as a train blowing its whistle drops in pitch as it passes and recedes away from us: same principle. Anyway, their visible light is red-shifted into the infrared which we can’t see and so therefore our line of sight assumes there’s nothing there. Two problems: firstly that just substitutes heat for light so the sky should be hot; secondly, radiation at shorter wavelengths than visible light, like ultraviolet, should be red-shifted down into the visible range.
Since our Universe is expanding, maybe the light from vastly distant objects hasn’t had time yet to reach our eyeballs and telescopes, so we can’t yet intersect or intercept their light. In fact, over time, because our Universe is expanding, distant cosmic objects are constantly slipping beyond and over our observable horizon, just like a ship at sea sailing away from us disappears over the observable horizon.
The Exception to the Rule: Here’s my new explanation, which is a supplement, not a substitute, in explaining the paradox. It’s a variation on those massive interstellar clouds of gas and dust which block out the stars, galaxies and related. In our Universe there are two kinds of astronomical objects. There are cosmic faucets like stars and anything else that gives off or reflects electromagnetic (EM) waves. That’s the cosmic “In Tray”. Then there are cosmic sinks and drains that absorb electromagnetic waves – Black Holes, the cosmic “Out Tray”.
It would seem to me that over the course of 13.7 billion years, an awful lot of EM (light, IR, UV, radio, microwave, gamma-ray, etc.) photons, not to mention neutrinos and cosmic rays, would have gobbled up and removed from the Universe’s inventory by being sucked into and forever residing in the insides of Black Holes. Since all astronomical observations, hence conclusions about the state of the Universe, rely on the detection of that which is emitted or reflected by cosmic faucets, then it stands to reason that in order to arrive at valid conclusions, what cosmic sinks and drains remove from the Big EM Picture must be taken into account. But is it? I’ve never read any account where the removal of EM photons from the Universe’s inventory has been considered. Maybe I haven’t read widely enough – maybe.
One example that springs to mind of the possible significance of Black Holes is the minor temperature variations in the cosmic microwave background radiation (CMBR), that leftover heat from the Big Bang event 13.7 billion years ago – perhaps those slightly cooler spots in the CMBR are due to large Black Holes between our measuring devices and the CMBR that is sucking up those CMBR microwaves before they reach our measuring telescope or space probe or high altitude balloon. I seem to recall cosmologist George Gamow back in the 1940’s making a theoretical prediction that the (then undetected) CMBR would be somewhere between 5 to 7 degrees Kelvin, instead of the roughly 2.7 degrees Kelvin that eventuated. Perhaps, the overall cooler CMBR was due to Black Holes sucking up lots of those CMBR photons over all those billions of years.
So another reason why the night sky is dark is because in many cases when you look outwards, you intersect or intercept a Black Hole in your line of sight that’s between you and some sort of cosmic faucet further on.
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