Dark Energy: Part 1

Dark matter. Dark energy. They are supposed to make up most of the universe (20% and 75% respectively), leaving just a tiny 5% to the “normal” matter and energy we humans are made of and deal with on a day-to-day basis. But what are dark matter and dark energy? Physicist Sabine Hossenfelder answers.

Let’s take “dark matter” first. Based on the amount of matter we can see either visually or via our detectors that use signals other than visible light, the stars on the edges of all spinning galaxies should get thrown into outer space. On a bigger scale, galaxies should never have formed in the first place. So what is this force that keeps stars and galaxies from flying off? The answer: gravity. Wait a minute, didn’t we just say that there isn’t enough perceivable matter to produce the needed gravitational force? Yes, but the key word is “perceivable”. The objects producing the “missing” gravity are not perceivable, so they’re called “dark matter”. Not “dark” as in sucking up light. Rather, they are transparent.

Next up: “dark energy”. If you throw something, it slows down due to friction or gravity. In vacuum and free space, it continues at the same velocity. But the universe isn’t doing either of those things: it’s actually speeding up! An increase in velocity needs either a force or something called negative pressure to produce the acceleration. Physicists decided to call that unknown entity as “dark energy”.

Einstein’s famous cosmological constant is the “simplest form of dark energy”. As the name suggests, it remains constant even as the universe expands:

“This sounds innocent, but it is a really weird property. Think about this for a moment. If you have any kind of matter or radiation in some volume of space and that volume expands, then the density of the energy and pressure will decrease just because the stuff dilutes. But dark energy doesn’t dilute! It just remains constant.”

Whoa! Doesn’t this violate the law on the conservation of energy?

“Energy just is not a conserved quantity in general relativity, if the volume of space can change with time.”

Yet another instance of how relativity upends our beliefs all the time!

“So the mathematics is all fine, don’t worry.”

Remember how we said the cosmological constant is the “simplest form of dark energy”? Let’s expand from the simple to the general case:

“Dark energy now is a generalization of the cosmological constant. This generalization allows that the energy density and pressure of dark energy can change with time and maybe also with space. In this case, dark energy is really some kind of field that fills the whole universe.”

But is dark energy something we came up with only because the universe is expanding? No, it fits with other observations like the distribution of matter, the properties of cosmic background radiation and supernovae redshifts.

And then came this recent scientific paper that questions dark energy…