New Scientist, UK: It seems like most of what is between Earth and the sun is two other planets – other than that, there isn't much else that we can see. But is space actually completely empty? Not really.
There are a few senses in which we can think of space-time as being teeming with stuff. One is quantum-mechanical in nature. Quantum field theory, the tool we use to study particle physics, says particles flicker in and out of existence, even in a vacuum. And they aren't something big like a star suddenly appearing and then disappearing.
There is another way in which the universe is fundamentally full of things. For almost 80 years, we have been getting to know an all-pervasive type of light that we scientists call the cosmic microwave background radiation or CMB.
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Like many things in science, the CMB was first detected by accident. The first hint was from Andrew McKellar's 1941 observations of the region around a star. He noticed that rather than being a temperature of absolute zero on a Kelvin scale, which is what you might expect from empty space, it was about 2.3 Kelvin or -271°C.
Then, in the 1960s, Arno Penzias and Robert Wilson were taking some measurements using a radio telescope when they noticed a background noise in the signal that wouldn't go away. The structure of the signal meant that its wavelength could be associated with a temperature. They found the temperature to be about 3.5 Kelvin, in effect rediscovering McKellar's original measurement.
In the decades since that moment, we have launched multiple space telescopes to measure this radio signal more closely, and the CMB has become an incredibly important tool in observational cosmology.
