Why is it dark at night?
Unraveling ‘Dark’ Secrets of Night
‘What a silly question!’ One might think. The sun is down, that’s it. Well, the answer sounds really reasonable, but, as there has always been, is that all there is to it?
Nights on Earth are naturally dark. That’s cool. We can sleep or do other things suitable for doing in the dark. The dark sky also allows us to have a good look at the grand universe and wonder how everything came to be. The answer to ‘Why is night dark?’ is not as simple as ‘Because the sun is down.’ It links to the origin of the universe as we know it. Not simple, but beautiful.
The Paradox
If the universe was static, infinitely vast, eternal, and filled with an infinite number of stars, the sky should always be glowing when all the stars were shining. Starlights would travel in every direction and eventually would touch every corner of the universe. Every spot on the surface of every planet, including our Earth, would ultimately be basked in the starlight all the time. The night shouldn’t be dark then. The word ‘night’ should then mean something else entirely. All this would be true if only the reality wasn’t what it is and has always been: the sky is dark at night. Why? A German physician-turn-astronomer asked this question before in 1823, and the issue became known after his name as the ‘Olbers’ Paradox.’
The static universe model also proposed that the universe is homogeneous. On an immense scale, the same number of stars would fill an equal volume of the universe from any location. Imagine, according to this model, if we were to slice the universe into sections or shells, each one lightyear thick and separated by a distance of 1 billion lightyears (astronomically far), there will be an increasing number of stars in the shells farther out from the observer.
The light from the farther shells would be naturally dimmer than the light from the shell closest to the observer, but there would be more stars in the farther shells. Thus the observer would see the same amount of light from shell #2 as he would from shell #1. Every shell would contribute to a unit of light, totaling to all that could be seen by the observer. With an infinite number of shells, everything would be blindingly bright.
Indeed, nights on Earth would be as bright as days only if the assumption that the universe, being spatially and temporally infinite, and static, was correct. According to this model, there would be neither the beginning nor the end of the universe.
The universe may be infinitely vast, but it’s not infinitely old. There was a point in time where everything came into existence. This notion eventually explains the reason why the night is dark.
Light travels fast at approximately 1080 million kilometers per hour. Nothing in the universe can go faster than light. But the light still takes time to cross vast distances. One of the reasons the sky is dark at night because, since the beginning, some lights from distant stars haven’t made their way to us yet, as reasoned by Edgar Allan Poe in his 1848 essay, Eureka.
Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy — since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all. — Edgar Allan Poe, Eureka
The Big Bang Theory and the Expansion of the Universe
There are things that the static universe model can’t explain. With new technologies, scientists and astronomers have found ever more tell-tale pieces of evidence that the universe is not static. Observations have suggested that the universe is expanding. At a very fast rate, too. Something must have caused this expansion a long time ago. A big explosion, perhaps? Poe’s intuitive view of the universe with a finite age also supported this speculation because it implies that it has a beginning. Enters the Big Bang Theory.
The Big Bang theory says that the universe was a very hot place at a point in time. The hottest place in the universe (duh!) It was very dense, too. Then the bang happened. All the matter in the ‘cosmic primordial soup,’ formed shortly after the bang, started to cool down. The matter then condensed to form subatomic particles, then atoms — the building blocks of the stars — and everything else.
Near the beginning, the universe was presumably very bright but opaque because the free electrons would not allow light to travel so far. Cooling down, the universe became more and more transparent. At the same time, the universe expanded. Everything inside started to move away from one another. The expansion was so fast that the lights from the stars and galaxies formed earlier in the universe are stretched-out. This phenomenon is called the ‘cosmological redshift.’ The term ‘redshift’ comes from the fact that when the wavelength of light is stretched when the universe expands and cools down, the color of the light becomes redder as its wavelength gets longer and falls into the infrared region, causing its energy to drop following the cooling of the universe.
When we look at stars or galaxies, we are looking into their past. That is because the light takes years to travel from those objects to us. From the cosmological redshift, scientists also discovered that the farther a star or a galaxy is from us, the faster it moves away. The farthest observable celestial object is visible only in the infrared region of the electromagnetic spectrum. When the Hubble Telescope photographed the eXtreme deep field (XDF) image, it did so with infrared sensors to reveal stars and galaxies in the farthest reaches of the universe as far as we can observe with our current technology.
Another piece of evidence that supports the Big Bang Theory is the cosmic background radiation inadvertently discovered by scientists in 1965, which is the remnant radiation in the form of microwaves and radio waves spread throughout the universe. These radiations are so stretched-out much more than the lights from the furthest observable stars and galaxies. Their wavelengths are longer than those in the infrared regions. The cosmic background radiation is, in other words, the cooled version of the radiation released during the time of the Big Bang. It is the oldest observable radiation. We can also think of this radiation as the snapshot of the universe around the time the Big Bang happened, which was about 14 billion years ago.
The short answer to ‘why is it dark at night?’
The take-home message is that our night skies are dark because we can’t see lights from distant stars and galaxies as they are moving away from us so fast that their lights are stretched and become infrared lights. Not all the lights from all the stars in the universe will eventually reach us in our lifetime, too, simply because they are too far. But actually, the entire ‘observable universe’ is all lit-up at the same time by all sorts of radiations invisible to our eyes and still full of wonders waiting to be found.
