Why Is the Night Sky Dark? The Profound Solution to Olbers’ Paradox
It may seem silly to wonder why it’s dark at night, but with so many stars in the universe, the night sky should be completely illuminated, a conundrum today known as Olbers’ paradox.
Although it’s now attributed to Heinrich Wilhelm Olbers, a 19th century German astronomer and physician, this paradox has perplexed people for centuries. Numerous well-known people have tried to unravel it, including Kepler, Lord Kelvin, and even Edgar Allan Poe, but it wasn’t until the advent of modern cosmology that we figured it out.
And the solution is deeply connected to the age and size of the universe, as well as our place in it.
Olbers’ Paradox: the Problem
It’s not hard to understand why a great thinker like Aristotle thought the universe was finite, static, timeless, and homogenous. Like us, when he looked at the night sky he saw an evenly dispersed array of stars speckled on a black background. The stars seemed to never change, to have always been there, and to be limited in number. He naturally concluded, then, that the blackness between them existed because the universe didn’t have anything to fill it with. And at the time this made sense.
Over the many centuries since then, the scientific community has hotly debated if indeed the universe is finite, static, timeless, and homogenous. It turns out that Aristotle was only right about the universe’s homogeneity. Modern cosmology has shown us that the universe is changing, had a beginning, and is quite possibly infinite.
It all started in the 1920s when Edwin Hubble made some alarming discoveries. First, as an astronomer at Mt. Wilson Observatory in California he found that the fuzzy patches in the sky were not nebulae but galaxies. The Andromeda nebula was in fact the Andromeda Galaxy, home to a trillion unknown stars spanning over 200,000 lightyears. The universe was far bigger and more mysterious than previously thought.
Second, Hubble determined that these galaxies were moving away from us by looking at their red-shift. When the source of a wave moves towards us, the wave’s frequency increases, and when it moves away, the frequency decreases. For example, when an ambulance passes, its siren sounds higher pitched due to the compression of soundwaves, and when it moves away, the siren sounds lower pitched due to its soundwaves being stretched. Likewise, when stars move towards us, their light is shifted to the blue end of the visible spectrum because blue light has a higher frequency. When they move away from us, their light waves move towards the red end of the visible spectrum, as red light has less frequency, hence the name red-shift. Using a galaxy’s red-shift, Hubble also determined that the further away a galaxy was, the faster it was moving away from us, a concept now known as Hubble’s Law. So the universe was certainly not static and might in fact be infinite.
In the nearly 100 years since then, we’ve found that the known universe has a mind-boggling amount of stars. By looking at our galaxy’s rotation we’ve figured out its mass, and by analyzing its light via spectroscopy we’ve determined that a large chunk of this mass is about 1012 stars. Thanks to the Hubble Space Telescope we’ve estimated that the observable universe has roughly the same amount of galaxies, and assuming our galaxy is of average size there must be about 1024 stars whose light should reach us. This is far more than all grains of sand on Earth.
Therefore, the observable universe has so many stars that every line of sight beginning on Earth would inevitably intersect with one. The night sky should be glowing brightly.
Olbers’ Paradox: the Solution
One proposed solution to Olbers’ paradox was that the light from all these stars is being blocked by something, but this was dismissed for various reasons. If the light was being blocked by large gas clouds, for example, then given the age of the universe, they would’ve absorbed enough energy from the blocked light to emit light of their own. So they wouldn’t be dark. And It can’t be dark matter because, as far as we know, it doesn’t interact with the electromagnetic force, meaning it “does not absorb, reflect or emit light, making it extremely hard to spot,” hence its name. So it can’t block starlight. Dark matter also exerts gravitational influence, which would affect objects or light through gravitational lensing, both of which have not been observed.
Another idea was that the stars aren’t evenly distributed. What if the galaxies formed giant clusters, leaving giant gaps in between? This was dismissed as well due to the cosmological principle. This states that over large scales the fundamental forces even out, making them the same for all observers, regardless of location. Matter, then, should be evenly distributed, which is what all observations so far have confirmed. That is, galaxy distribution is fairly homogenous. So this idea was dismissed as well.
So what’s the answer? It turns out Hubble was already headed down the correct path, but it wasn’t until nearly 70 years later that we got the last piece of the puzzle. In 1998, scientists looked at supernovae in distant galaxies to not only confirm that the universe was expanding but that it was accelerating. Beginning about 9.8 billion years ago, when the universe was roughly 4 billion years old, an anti-gravitational force likely from mysterious dark energy, began to dominate the universe, causing it to expand faster as time went on.
This provides 2 answers. First, the accelerating expansion of the universe causes galaxies to move away from us so fast that their light is stretched beyond the optical spectrum. Looking at the chart below, as it accelerates the light of a blue star, for example, gets stretched towards the red end of what the human eye can perceive and then into the infrared, beyond what we can see. In fact, according to Hubble’s law, the further away a galaxy is, the faster it’s moving away from us, meaning the furthest galaxies are hidden in the deep infrared. Therefore, the darkness of the night is partly due to the fact that quite a bit is actually there, but we just can’t see it with our eyes.
Second, galaxies far enough away are accelerating away from us so fast that they’re traveling faster than the speed of light. (Read why this doesn’t violate Einstein’s Relativity in the link below.) This means that their light will never reach us because these galaxies are beyond the boundary of the observable universe. A line of sight extending through a patch of darkness at night would eventually hit a galaxy, assuming the line could catch it. Therefore, the darkness of the night happens because most of the universe’s light will never reach us.
So, in the end, something as simple as the night sky being dark is actually a fairly profound topic that connects us to all of humanity’s struggle to understand how we fit into such a vast and complicated universe.
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