I can’t prove that this is what dark energy looks like, but also I CAN’T PROVE IT ISN’T. (Credit: Gary Tonge)

WTF, Universe? Calm down already.

Why is the expansion of the Universe speeding up?

Pull yourself together, Universe. You are spiraling out of control. And by spiraling, I mean expanding. Accelerating. OUT. OF. CONTROL.

Calm the fuck down.

It is difficult to wrap our minds around how the Universe is expanding, and what the fuck that even means, but it’s true. The possibly-infinite entirety of existence is getting bigger, and has been for the past 13 billion years. But now, this expansion is speeding up. According to measurements of exploding stars, clusters of galaxies, and the leftover radiation from the Big Bang, the Universe is expanding faster and faster, and we have no fucking clue why.

We have very cleverly called this absence of a clue dark energy. Apparently, cosmologists are not nearly as inventive in naming their hypothetical entities as particle physicists, who have graced us with names like bosons, hadrons, and quarks (which come in different flavors — top, bottom, up, down, charm, and strange — and are no longer hypothetical). And all cosmology has to show for itself is dark matter and dark energy. Maybe we don’t smoke enough weed?

In this 8th edition of the “WTF, Universe?” series, I will explore some of the possible explanations for this late-time accelerated expansion. I will completely ignore the evidence for acceleration because it is basically the same as the evidence for the expansion, and what we know is not nearly so interesting as what we do not, which, it turns out, makes up around 95% of the Universe.

Faster and Faster

Hold up. 95% of the Universe? How the fuck do we know that without knowing what dark energy is?

The answer involves a lot of the weird shit covered in previous posts, so try to keep up. You can do this. Ready?

The main evidence for the acceleration comes from measuring the speed of the expansion (episode 3) as a function of distance/time (episode 1: distance = time). Einstein’s theory of general relativity (episode 4) tells us that this relation between speed and distance is affected by the global curvature of space-time and the energy density of the stuff in the Universe. (Still with us? You’re doing great!) Now, we have measured that the Universe is very flat (from the space-time triangles described in episode 5), so that leaves the energy density of stuff — the matter (primarily dark matter, whatever the fuck that is; see episode 7) that slows down the expansion, and the something else that speeds it up (like whatever drove inflation; see episode 6). So, by measuring how fast the Universe expands, we can figure out the relative amounts of this stuff, without knowing what any of it is. (Whew, we made it! *HIGH FIVE*)

Our ability to measure the energy densities of dark matter and dark energy has created a proliferation of pie charts in cosmology.

Screenshot of a google image search for “dark matter energy”. Can you count the pie charts?

Depending on what we eventually learn about dark matter and dark energy, we may never recover from this pie chart explosion, and pie chart non-proliferation treaties are impractical in this political climate. But they all say that roughly 73% of the stuff in the Universe is causing the expansion to accelerate, roughly 23% is dark matter, and the tiny bit left over is everything else. Mostly hydrogen.

The rare double pie chart really brings home how insignificant we are in the cosmos. And by “we” I mean you and everything you love and all the stars in the sky and all the gas clouds in between the galaxies…. sorry. These pie charts are dangerous!

New Thing vs. New Law

What these pie charts have in common is that they assume the accelerated expansion can be explained by a new “thing” called dark energy — but that is not the only possibility. What might save us from pie chart armageddon is if we instead need a new “law” of gravity. But general relativity is a pretty damn good law.

The simplest explanation (that fits all our observations so far) is if the new “thing” is a cosmological constant, and the simplest way to get a cosmological constant is if empty space itself has energy. But if we do the calculation, our prediction for this “vacuum energy” is different than what we measure by an unfathomable 120 orders of magnitude — that is, 1 with 120 zeros after it. Fuck.

So, theorists are exploring other options. Nothing is off the table. Perhaps our 4-dimensional space-time is embedded in a 5-dimensional “bulk” space-time, and gravity leaks out into this extra spatial dimension, making it weaker on large scales? Sure, let’s consider it. Why the fuck not?

My favorite theory — I love to hate it — is that dark energy really is the vacuum energy, and our prediction for it is only spectacularly wrong in this universe which is part of a multiverse, because otherwise we wouldn’t exist! The reasoning goes something like this:

We Exist; Therefore, Multiverse

(Or: Why Physicists are Bad at Philosophy.)

This is, of course, a bit of an over-simplification. But not much. It’s true that if the cosmological constant were marginally different, then there would never have been any galaxies or planets or people; this is also true for many of the fundamental constants in physics. The problem is this: can the fact that we exist ever explain the value of the cosmological (or another) constant, or is our existence merely a selection bias on this value?

For example, we humans could only ever live on a planet with oxygen. Does that fact explain why the Earth’s atmosphere has oxygen? Does it explain why, of all planets we could have evolved on, we evolved on the one with oxygen? Or do nitrogen-breathing aliens on some other planet ask the same questions? (These are rhetorical questions. The answers are no, no, and yes.)

So why is the expansion of the Universe accelerating? Does it point to a “dark energy” that permeates the void of the cosmos? Or do we instead need a new law of gravity? Or are there other spatial dimensions? OTHER UNIVERSES?? WHOA THERE. CALM DOWN.

My personal opinion is that it’s likely to be solved by some future theory of quantum gravity, in which the physics of the super tiny scales and the super large scales all fit together nicely in a neat little package. But don’t even get me started on how quantum mechanics is WEIRD AS FUCK.

Or maybe, do get me started? The wave function of future “WTF, Tiny Universe?” posts has yet to collapse, so I don’t know!