Sean Carroll

But the point is, it's absolutely true.

that physicists use the word matter and mean different things.

Physicists use almost every word in multiple meanings and expect that as long as they're talking to other people who know what they're doing, they will understand the context that you're talking about.

You know, astronomers refer to any atom bigger than helium as a metal.

So oxygen is a metal and so forth, right?

But they know that, you know, they talk about the metallicity of a stellar atmosphere or something like that because they assume that you know what they mean.

Likewise, if a cosmologist talks about matter in the context of the accelerating or the expanding universe, they're distinguishing matter from radiation.

They mean any kind of radiation.

source of energy density with the property that its energy density decreases as the scale factor to the minus 3.

That is to say, the energy density goes down exactly as the volume goes up, because that's what slow-moving particles do, like

dark matter particles or stars or galaxies, okay, or black holes for that matter.

But, you know, if you ask, well, are neutrinos matter by this definition?

You know, neutrinos have mass,

but their mass is very low, so they move relatively fast, right?

Do they move slow enough to count as matter?

And the answer is, in the current universe, neutrinos are matter to cosmologists, because they're moving substantially slower than the speed of light.

But in the early universe, neutrinos were not matter.

They were radiation because they were moving close to the speed of light.

And that's just one definition of the word matter.

But the point is, you know, we invent words in natural language long before we start putting them to work.