There is enough luminous matter in the universe to completely illuminate the night sky even more intensely than the surface of the Sun. If you add up all the photons that come out of all the stars and galaxies and the space between them, there is enough to illuminate the entire universe. However, when we look at the night sky, this is clearly not the case.
Then why is the sky dark at night?
This question has been on the mind for a long time. We can go back as far as Thomas Digges in the middle of the 16th century, Johannes Kepler in 1610 and even a little later with Edmond Halley in the 18th century. Curiously, even the American author Edgar Allen Poe anticipated possible explanations for the darkness of the sky. But it wasn't until the 19th century, when the German amateur astronomer Heinrich Wilhelm Olbers looked into the matter, that the paradox was made famous.
Astronomer Heinrich Wilhelm Olbers
This paradox became known, unsurprisingly, as the Olbers Paradox. In simple terms, the Olbers Paradox postulates that if the universe is infinite and static, then at any given angle on Earth, the line of sight of the eye will necessarily end up on the surface of a star. An infinitely old universe means that enough time has elapsed for the light from every star that has shone to reach our eyes. When we look up at the night sky, there should be a star everywhere we look, in every piece of sky. For this reason, the whole night sky should be as bright as when we look at the Sun during the day.
The reason why the sky is dark, instead of being a bright curtain of light, is due to more recent observations and discoveries about our universe that have been made since the time of Olbers.
In the 19th century, the nature of space-time and the large-scale structure of the universe were not yet understood. Astronomers didn't even know that there were other galaxies besides our own, let alone that they were all moving away from each other. From what was known until the 19th century, it seemed very reasonable to think that the universe was infinitely old and unchanging, and in such a universe, the Olbers paradox is a real problem. We now know, however, that the universe is not infinitely old and static, the universe had a beginning with the Big Bang. This has important implications for the Olbers Paradox.
One of the reasons why our night sky is dark is precisely because the universe has a finite age, and therefore many photons have not yet had time to reach us. The photons that have reached us are precisely those that are in our observable universe. It would not be so if the heavens had always existed. The darkness of the night sky is therefore a characteristic that suggests that the Universe is not infinite.
But the Big Bang presents us with another paradox: it asserts that the early universe was flooded with photons. Everywhere, hot photons permeated space-time. At that time in our history, the cosmos was truly luminous. Given these early, warm and bright conditions, shouldn't the remnants of the Big Bang be visible everywhere you look in the sky? Shouldn't there be a curtain of light behind every star and galaxy we see? The fact is that this curtain of light is there, but the problem is that our eyes do not see it. Due to the expansion of the universe, the wavelengths of these hot, early photons have been stretched more than 1,100 times longer than their original wavelengths.
The energetic and luminous backdrop of the early universe is now filled with relatively cold microwave photons, invisible to the human eye after being stretched by the expanding fabric of our universe for more than 13 billion years. This curtain is currently being imaged by special detectors, some of which are aboard the Planck Space Telescope, the most powerful microwave telescope ever built.
The Planck telescope source: NASA
It gives us detailed views of what was once the most brilliant ball of energy ever known: the Big Bang.
This is called cosmic radiation or cosmic scatter. "The Cosmic Microwave Background or CMB is the name given to a very homogeneous electromagnetic radiation observed in all directions of the sky and whose emission peak is located in the microwave range (microwaves)." For more information on cosmic radiation, I suggest you read the wikipedia file on the subject.
Image of the fossil radiation of the Universe as observed by the Planck telescope
In conclusion, Olbers was right. So were Digges, Kepler, Halley and Poe. They thought long and hard about the idea that the night sky should be as bright as the noon sun, and it is. What they didn't realize is that the night sky is only dark for us. If they had microwave-sensitive eyes, there would never have been a paradox.
If you are passionate about the sky, check out our Fossil Radiation Table. You can let yourself imagine what it would be like to be able to observe the fossil radiation with your own eyes.