If the universe is limitless and full of stars and galaxies, why don't we see them everywhere we look? Sometimes the simplest questions have deep roots. What if when we look into the night sky and see only blackness and stars, which are much less numerous than the stars in the universe, is it because we are humans and cannot see more? Why is the night sky black and devoid of light?
At first this may seem like nonsense. Of course, we have a transparent atmosphere that allows us to peer into the vast depths of space when the Sun is on the opposite side of our world. And our location in the galaxy means that only a fraction of the universe is obscured by galactic gas and dust, which usually blocks most of the light in the central regions of the Milky Way. Nevertheless, if we lived in a truly infinite universe, if the emptiness of deep space continued long enough in any direction, then wherever we looked, we would see shining points of light everywhere.
Of course, we can look into the deepest depths of empty space, where there are no stars or galaxies that can be seen with the naked eye or with ordinary telescopes, we can direct the Hubble Space Telescope to peer into this darkness for hours or even days. And then we find that the universe is full of stars and galaxies. Starlight travels millions, billions, or even tens of billions of light years and reaches our best equipment. It may take a long time to capture enough photons at such a great distance, but given the at least 170 billion galaxies present in the part of the universe that we have had the opportunity to observe, one might think that there are actually an infinite number of them.
In any case, we see clearly not infinity. Back in 1800, Heinrich Olbers realized that if the universe were truly infinite - with an infinite number of shining stars - then ultimately, wherever you looked, your eyes would hit the surface of the star. You would not see the galaxies that we see, which are mostly empty space; you would see all their stars, and the stars in the galaxies behind them, and further and further. Traveling through billions, trillions, quadrillions of light years, you would land on a star.
This is a simple mathematical fact: if you take an infinite space with a finite, nonzero density of "substance" in it, then looking at any place (and in any direction), you would definitely come to this substance through a finite distance. Assuming that the cosmos is full of stars - even if they are rarefied - but infinite and uniform in density, you will inevitably come to the star, regardless of direction.
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The same mathematical theorem tells you that eventually starlight from all directions will arrive at your place, as well as at all places in space. If our Universe were such - static, endless, with eternally shining stars - the night sky would always be bright.
What saved us from all this? Believe it or not, this is the Big Bang. The fact that the universe has not always existed and that we can only observe stars and galaxies at a certain distance - which means we receive a limited amount of light, heat and energy from them - explains why there is so little light in our night sky. Of course, there are a colossal number of points of light scattered throughout the universe. But the number that we see is limited by the speed of light and the physics of the expanding Universe. Somewhere there is a huge Universe, a bunch of stars and galaxies that we do not see, but they cannot illuminate our sky, since not so much time has passed since the Big Bang for their light to reach us.
"Wait a minute," you will notice, "the Big Bang tells us that the Universe was hotter and denser in the past, which means that radiation from this dense and hot state should be everywhere today, spread in all directions." And you'll be right: 13.8 billion years ago, the universe was so hot that neutral atoms could not form, let alone stars and galaxies. When these neutral atoms finally formed, light began to travel in a straight line and should come to our eyes from all directions all the time, no matter what we do.
And we see this light every time we turn on an old TV set to a dead channel. This “snow,” the black and white noise you see on your TV screen, comes from all sources: radio broadcasts, the Sun, black holes and all kinds of astrophysical phenomena. About 1% comes from the afterglow of the Big Bang: the cosmic microwave background. If we could see in the microwave and radio range of the electromagnetic spectrum - not only in the visible - we would notice that the night sky is almost uniform in brightness and there are no black spots anywhere.
It is a combination of two facts:
- that the universe existed for a finite time;
- and that we only see light in the visible spectrum
responsible for the darkness of the night sky. In fact, the only reason we are well adapted to seeing light is because our sun's light is in the range of thousands of degrees Kelvin, so we see everything it reflects off. In a sense, our limited senses forced us to explore the universe.
ILYA KHEL