Looking up at the sky on a chilly fall night, the universe looks like a big black curtain dotted by pearls of ivory and glimmers of sparkling diamond white.
In reality, those tiny spots of light represent the actual average colour of the cosmos.
Most of the stars in the universe formed about 5 billion years ago. In the past, these stars would have appeared brighter and bluer. But as stars age, they shift from blue to yellow and eventually red. Thanks to these ageing stars, the colour of the universe has gradually shifted over time from a blueish to a reddish hue.
If you were to take in all of that, or put “the [whole] universe in a box” so that you could see all its light at once, the average of all the colours you’d be able to perceive with your human eye would look like this:
Not very, er, striking, is it? The scientists who discovered the average colour of the universe named it Cosmic Latte. Alternative titles included Cappuccino Cosmico, Skyvory, and Big Bang Buff.
Don’t let that bland colour fool you, however. If we were to take the same light, but instead of looking at it all at once (which gave us the average we saw above), put it through a prism, it would produce a rainbow of nearly all of the colours we see here on Earth, from deep violet to ruby red.
Prisms work by separating the visible light into the different colours of the electromagnetic spectrum, resulting in the characteristic rainbow of colours. A prism separating all the visible light from the universe would give us a slightly different spectrum than the one we’re used to.
To get a picture of all of that light, scientists looked at data from a large light survey of more than 200,000 galaxies. Then they built what they call “Cosmic Spectrum,” to represent the sum of all of the energy in the universe that is emitted at different wavelengths of light.
The Cosmic Spectrum looks like the graph below. You can see that some colours are more represented than others.
Which can also be visually represented this way:
This rainbow of colours is what scientists used to arrive at the average colour of the universe. That colour is getting redder as the stars of the universe age. As fewer and fewer new stars form, more stars will age and become red giants.
Eventually, Cosmic Latte might look more like Spacey Strawberry Frappuccino.