Why Is the Sky Black? The Science Behind Black Skies

The Centaurus constellation twinkles in a black sky

We’re fortunate to live under a sky of many colors. During the day, it’s a shade of bright and cheerful blue. At sunrise and sunset, it glows vivid red, orange, yellow, and sometimes even pink. And at night, it’s a deep, velvety black with endless pinpoints of stars.

But have you ever paused to wonder why the night sky is dark? Just about all the light on Earth comes from the sun, which is a star. So if the night sky is filled with stars, it seems like it should be lit up like it is during the day.

As you know, that’s not the case. Here’s a look at why the night sky is black.

Examining Assumptions About the Universe

Illustration of nebulas and galaxies in outer space

Historically, scientists have assumed a few things about our universe. These three assumptions make up the Steady State Theory:

  • It’s infinite
  • It’s static (meaning it doesn’t expand or contract)
  • At a large scale, it’s homogeneous (meaning it looks more or less the same throughout)

These might all sound like reasonable assumptions. But if all three were true, any line of sight (looking from Earth up at the night sky) would land directly on a star. Think of it this way — imagine walking through an infinite forest. If the trees around you actually stretch on forever (and they’re evenly distributed), every possible line of sight would land on a tree.

Now, back to the stars. If the Earth is surrounded by infinite stars distributed evenly, all we would see when looking up at the night sky would be stars (with no space in between). That means that the night sky would be full of light. So, if all three assumptions of the Steady State Theory were true, it would look nothing like the night sky we know.

This is where Olbers’s paradox (also called the dark night sky paradox) comes in. According to Olbers’s paradox, if the universe were infinitely old, infinitely large, and filled with an infinite number of stars, the night sky would be full of light. Because the night sky is dark, at least one of the assumptions of the Steady State Theory must be false.

This paradox is named for Heinrich Wilhelm Olbers, a German astronomer. Olbers first described the paradox in 1823, but he was not the first astronomer to consider or write about the problem — it was simply attributed to him over time.

According to Edward R. Harrison, an astronomer, cosmologist, and author of the 1987 book Darkness at Night: A Riddle of the Universe, the first person to describe the paradox was Thomas Digges. Digges was an English astronomer born in the year 1546. Since then, many astronomers and cosmologists have studied and come up with their own solutions for this age-old puzzle.

What Makes the Night Sky Black? Possible Answers

A blue-green galaxy and a field of stars

Nobody knows the exact reason why the night sky is black, so as of yet, we can’t definitively answer Olbers’s paradox. But there are plenty of explanations that have been offered over the years. Take a look and see which one you find most plausible.

The Universe Is Finite

This is a solution that was proposed in 1610 by renowned German astronomer Johannes Kepler. Kepler believed that the universe was finite — that after a certain point, there was only blank space. In a finite universe, there’s still black space between the stars and galaxies when you look up from Earth.

But there’s one problem with this theory. As other scientists have pointed out, a finite universe would collapse in on itself. There’s a powerful gravitational pull between stars and planets. And unless there’s a continuous expansion to balance it out, that gravitational pull will make the universe implode.

The Big Bang Model

Infographic illustrating the Big Bang Theory

This commonly accepted solution to Olbers’s paradox states that the universe is constantly expanding. As that expansion occurs, the visible light from distant stars increases dramatically in wavelength — to the point that it’s no longer visible to the human eye. This is a process called redshift.

If you’re already familiar with the spectrum of visible light, you know that as you go from violet to red, the wavelengths of light get longer. Beyond red light is infrared light — it has even longer wavelengths than red does. The name “redshift” came about because the light from these stars shifts in the direction of red light (and beyond). Here’s a simple illustration of what redshift looks like in the universe:

Illustration of how redshift works in space

This shifted light forms what is sometimes called the cosmic microwave background. These longer wavelengths often fall into the microwave range, but they look black to the human eye. This “background” creates what we see as black space between stars and galaxies.

The Universe Isn’t Infinitely Old

You might not think that poetry and science often interact. But in 1848, famed poet Edgar Allan Poe offered a reasonable solution to Olbers’s paradox. He believed that although the universe might be infinitely large, it wasn’t infinitely old.

The speed of light is finite, so Poe thought that there simply hadn’t been enough time for the light of distant stars to reach Earth. As a result, the night sky didn’t appear to be lit up.

Some of today’s research supports this theory. The most widely accepted age of the universe is around 13.8 billion years (although a recently published paper claimed it’s nearly twice as old).

So, how did they calculate that? They were able to very carefully study fluctuations that occurred in the cosmic microwave background mentioned above. They then used this information and Einstein’s Theory of General Relativity to estimate the universe’s age.

This means that the part of the universe we can actually observe is around 13.8 billion years old. There may well be an infinite universe that we simply can’t see because its light hasn’t reached us — just like Poe hypothesized.

Hydrogen Gas Is Absorbing Light

The Hubble Space Telescope orbits Earth

In his writings about the paradox, Olbers put forth a couple of theories that could possibly explain it. One of these was that clouds of hydrogen gas were absorbing light and energy from the galaxies spread across the universe.

For roughly 200 years, scientists dismissed this explanation. But in 2016, new research suggested that it may be true. This research, conducted using the Hubble Space Telescope, indicated that while there were enough galaxies in the universe (10 times more than previously thought) to fill the night sky with light, large clouds of hydrogen gas between those galaxies and Earth absorb enough light to keep the night sky dark.

What’s the Real Answer?

A deep black sky with countless tiny stars

We can’t know with absolute certainty what the answer to Olbers’s paradox is. But currently, a significant proportion of experts believe that an ever-expanding universe is the cause of our dark night sky.

As you saw above, research and critical thought surrounding Olbers’s paradox is nothing new. This problem has puzzled the scientific community for centuries. But through careful study and examination, scientists and ordinary people alike have gained a better understanding of our universe.

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