Dark Energy is a Misnomer

I love working at a university and being surrounded by cosmologists.  In my last post, I alluded to the idea that astronomers may one day regret calling the impulsive force pushing spacetime apart dark energy.

I was chatting with a colleague last week where we talked about this nomenclature.  It may turn out that dark energy is neither, but for very innocent reasons, the term dark energy has stuck.

The term probably came about from the discovery of dark matter.  The existence of dark matter was inferred from a variety of observations of both the microwave background radiation and the Bullet Cluster of galaxies.  The behavior of the matter we could see was influenced by something we couldn’t see (from its gravitational field).

The gravity from the dark matter was affecting the matter we could see.

Likewise, with respect to dark energy, the discovery that the acceleration of the universe was also being caused by something we couldn’t see nor detect, it seemed natural to call whatever that was dark energy.

Dark matter, dark energy, tomato, tomato…

Sean Carroll prefers the term smooth tension.

Dark energy can more accurately be thought of as a negative pressure. 

Imagine a gas inside a bottle.  Our everyday experience shows that if you left the same amount of gas in the bottle but made the bottle bigger, the pressure would go down.  If you made the bottle smaller (leaving everything else the same), the pressure would go up; the molecules in the gas exert a positive pressure.

If that gas was dark energy, the pressure would increase as the bottle got bigger, and decrease as it got smaller.

Another way, usually described by Sean Carroll (and I’m paraphrasing poorly here), imagine a cylinder where the pressure increases when you pull the piston out.

Pulling the piston actually increases the energy in the system, while pushing decreases it.

Above photo credit: goatopolis