We see stars all around in space, in fact there are innumerable stars out there. No matter in which direction we look, we always see some or the other star clusters or galaxies. By this logic our sky should be appearing bright at any given time. But it isn’t.
So, why doesn’t the combined light from all of the stars add up to make the whole sky bright all the time?
Heinrich Wilhelm Olbers, a German astronomer asked this question in 1823. Presently it is called Olbers’ paradox after him. Though he was not the first person to ask this question, it’s already been asked for centuries before.
Olbers’ paradox goes like this: If the size of the universe is infinite and this infinite universe is littered with stars all across, than we will undoubtedly observe a star toward any direction we look. Thus the night sky should be splendidly bright instead of dark, yet it isn’t.
While proposing the paradox, Olbers had the idea of universe not being infinite, static and timeless. It couldn’t be all three, though couple of these is possible.
In the 1920s, Edwin Hubble changed the way how we were perceiving the Universe at that time. He discovered that our Universe is not static. In fact, the galaxies are speeding away from us in all directions.
This marked the inception of the theory of the Big Bang, which states that the Universe was once a single point in time and space, and then expanded quickly and is still expanding. This proves that our Universe is not static or timeless.
Presently, the Universe is roughly about 13.7 billion years old and it has been expanding for the whole time. This means that the stars in our Universe only had 13.7 billion years to born and die.
The light from the stars that do exist requires time to reach us. We only see the light that has had enough time since the big bang to travel from its origin to us.
Furthermore, as the Universe expanded, the wavelengths of the initial visible light were expanded out and shifted towards the wide end of the electromagnetic spectrum and they became microwaves.
This is called Cosmic Microwave Background Radiation and it can be detected in every possible direction.
If fact in every direction we look, we’re actually seeing a place bright as a star. Yet, we can’t see the light only because the expansion of our Universe has stretched out the light to longer wavelengths which is not visible to human eyes.
If we could look upon the Universe with microwave detecting instruments, we’d see this brightness in every possible direction.
In the end, the continuous expansion and evolution of Universe that too with a finite age, are the real cause why we can not see the visible light all around us and the night sky looks dark.
Originally published at https://theabsolutecosmos.blogspot.com.