Why the night sky is so dark - even when it's full of stars
Two factors help us answer this question: how old and how big the universe is
Read the full transcript of this episode.
In the year 1823, a doctor named Wilhelm Olbers looked out at the night sky and wondered: if we're surrounded by so many stars, why is the night sky so dark?
He wasn't the first to ask the question - it's been asked and explored by Greek Monks in the 6th century, Johannes Kepler in the 17th century, and even Edgar Allen Poe in the 19th century. But for some reason, it was only once Olbers asked it that the question itself got a name: Olbers' Paradox.
Now, 300 years later, Tai Poole, the host of the CBC Podcast Tai Asks Why, has the same question, and he's on the hunt for answers.
The Universe is Expanding
There are two factors that help us understand why space is dark: how old the universe is, and how big the universe is.
One really good thing you can do is take a balloon and put a bunch of dots on it and then blow up the balloon and you start to see as you put more air into it. That's sort of what's happening in space time.- Heidi White
Back in Olbers' time, the general belief was that the universe had always existed, and that it was infinite, meaning, it never ended. But today, astrophysicists know that the universe was actually created 13.8 billion years ago in what is called the Big Bang, which is actually more like a rapid inflation than a bang.
"One really good thing you can do is take a balloon and put a bunch of dots on it and then blow up the balloon and you start to see as you put more air into it. That's sort of what's happening in space time," said Heidi White, an astrophysicist at the University of Toronto.
As the space time balloon inflates, things are moving away from each other. And it's hard to see the far reaches of our universe because we can only see as far as the speed of light, which is 300,000 kilometres per second. At that speed, the light at the far edges of our universe would take 13.8 billion years to reach us, and we can't see it yet because we're not that old yet.
"Currently even the most distant stars light in the universe hasn't had time to actually travel through space because it's been expanding to actually reach us," said White.
Because the universe is expanding, it also means that the things that emit light are being pulled away from us so fast that their light goes into the infrared, which human eyes can't detect.
"If the universe is expanding that's producing an effect that we call redshift. Redshift is this effect by which objects moving away from us, the wavelength of the light that they emit, it becomes stretched out. And what happens is that as this wavelength becomes stretched out the frequency decreases and it shifts that light to redder and redder wavelengths," White tells Tai in a recent episode of Tai Asks Why.
"At some point objects are going to be far away from us enough, that they're moving away from us fast enough, that that redshift is going to shift them right out of the part of the electromagnetic spectrum that our eyes can see."
So there might not be a star in the dark spaces in the sky because everything is being pulled apart, but even if there was a star in there, it might be in a part of the visual spectrum we can't detect.
Blocked by the Milky Way
Another aspect that is contributing to the darkness of the night sky is where our solar system is in our galaxy, The Milky Way.
"If you imagine the center of our galaxy to be the big city, we're kind of out in the suburbs… We're further out in the Milky Way," White told Tai.
"In our night sky when we see the Milky Way, we're seeing the projection of it through the disc of the Milky Way. Because we're inside the disk of the milky way."
For size reference, if our solar system was the size of a quarter, the Milky Way would be the size of the Continental United States. So when we see the streak of the Milky Way in the night sky, that's us looking across the giant expanse of the massive Galaxy, from our vantage point inside the Milky Way.
So even with trillions of galaxies like ours out there, it's difficult to see them, because everything is being blocked.
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