An Uneasy Alliance with My Bete Noire, Dark Energy

Image Credit: NASA via

Sorry to keep freaking you out, readers. I can’t help it! I’m obsessed with Dark Energy.

What’s not to like? (And by “like” I mean “be terrified about.”)

Here’s Matthew R. Francis, writing in Slate:

Even the name “dark energy” is a placeholder for our ignorance, representing the fact of cosmic acceleration without indicating its identity. Astronomers have measured the acceleration rate and determined that dark energy constitutes more than two-thirds of the energy content of the cosmos, but its identity has defeated theoretical physicists.

Okay, fine. So it’s unknown, like a lot of things. Life’s full of mysteries! But hardly any of them are this big…or this sneaky:

Despite dark energy’s magnitude, astronomers didn’t know about its existence until recently because its effects are subtle. It doesn’t noticeably affect the planets in the solar system or the motion of stars in the galaxy. That subtlety enshrouds it in mystery: Scientists are busily determining what dark energy does, and they have yet to reach any consensus on what dark energy is.

So what are the theories being posited? Can we find something reassuring there? Come on, scientists!

The most popular candidate is vacuum energy: the energy from quantum fluctuations in empty space.

Hmmm….vacuum energy? Not sure I like the sound of that…

This idea was first kicked around in the 1970s and ’80s, long before cosmic acceleration was discovered. Physicists recognized that the same stew of quantum processes that determine the properties of electrons and other particles would grant energy to empty space. From general relativity, any energy has a gravitational effect. In this case, the energy would serve to accelerate cosmic expansion.

The effect of vacuum energy can easily be incorporated into Einstein’s theory as something known as a “cosmological constant,” an extra gravitational factor not created by matter.

Okay, I’m breathing a little easier. Something that’s been around for a while. Einstein was onto it. Sort of a math thing, really.

The problem: The amount of energy predicted by quantum theory is much larger than the observed amount of dark energy, something researchers knew even before the discovery of cosmic acceleration.

D’oh! Dark Energy dances away, laughing its head off.

Dark energy has made physicists face the possibility that general relativity breaks down on the scale of the universe. In that case, new rules in gravity could kick in on distances larger than galaxies, driving cosmic expansion without needing any new substance or quantum effects.

General relativity breaks down!? New rules in gravity??? Come on, Matthew R. Francis. Tell us how crazy this is!

The idea isn’t crazy.

Ugh. Moving on…

Just like Earth has a surface and interior, so-called braneworld models depict the universe we know as a surface or “brane” (short for membrane), with one or more extra dimensions that we can’t access directly… In some braneworld models, dark energy is the result of gravitational tension between our universe and a neighboring brane.

A neighboring brane??? I think I’ll stick to the vacuum! What do these branes do (other than haunt my dreams)???

One version – the cyclic model – predicts that dark energy will dissipate at some point in the future as the branes reach a critical distance from each other, after which point cosmic expansion will reverse.

Reverse and then what? Reverse and then what?


On me?

Leaving what?

Oh good lord, Dark Energy. Haven’t you done enough damage already?

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