Why is the Universe expanding, if gravity is a universally attractive force? Shouldn’t mutual attraction make everything collapse together, ending the cosmos in a horrible crunch of epic proportions, every living thing crying out in despair before being compressed into oblivion? (Should the writer rein in his metaphors?)

Yes, any two particles of matter mutually attract. But when we consider a large amount of matter — all the matter in a given chunk of the whole Universe — it’s not all going to collapse into a black hole, any more than all the air in a room will spontaneously collect in one corner. Think of it this way: any particle in the midst of a diffuse cloud of particles will be pulled in all directions at once, so the gravitational force will tend to average out to zero. (Now if the particles are more compressed for some reason, that’s another story! Mutual gravitation can bind them into clouds, galaxies, stars, or even black holes.)

So what drives cosmic expansion? The answer is still gravity, but we have to think of it in a slightly different way: the way of general relativity. To use the analogy from my earlier post, “The River of Spacetime“, if spacetime is a river, gravity is the current. The rate of the current’s flow depends on how much energy there is in a region of the river: the energy density. Since we know that mass and energy are intimately related, any matter particles in a region will contribute to that energy density, affecting the current, which will in turn affect the motion of the particles. (And so on.)

If you have a fairly diffuse cloud of material (whether dark matter or ordinary) in a region of spacetime, the current will either carry the particles away from or toward each other. Which direction depends on how they start: once the process begins, gravity will do the rest. In the case of our Universe, the Big Bang started the motion, so the matter in the cosmos will create current that carries particles farther from each other. That’s the expansion of the Universe.

However, the story isn’t done yet. If you place matter in a small box, it has high energy density. That implies a high potential for expansion, pushing against the sides of the box. Increasing the volume necessarily decreases the density, since there’s a fixed amount of matter inside. That dissipates the energy. This is true whether the particles collide with each other or not: averaging over the whole Universe, atoms are very widely spaced. Therefore, while atoms inside galaxies, stars, and other relatively small chunks of the cosmos collide and heat up, on the largest scales, we can safely assume atoms never hit each other. Dark matter is more dense (again on average) than normal matter, but so far as we can tell, interactions between dark matter particles are extremely rare.[1] And even though I use a box for a convenient demonstration of this principle (something I’ve done before!), the general principle doesn’t require one. Spacetime expansion is driven by energy, not by pressure on walls.

Dark energy behaves very differently: it either doesn’t dissipate at all or dissipates its energy more slowly than matter does. The first option is often called the “cosmological constant” model,[2] because it doesn’t vary in space or time; the second includes the annoyingly named “quintessence” models. So far, our cosmos seems to contain the non-dissipative variety, which means the density of dark energy stays the same as the Universe expands. To put it another way, the bigger the Universe is, the more dark energy it contains; more energy means more expansion, which means more dark energy, whichmeansmorexpansionwhichmeansmoredarkenergy. That’s cosmic acceleration.

As you hopefully gathered, matter is sufficient to drive spacetime expansion: no dark energy needed. (I left out the contribution from light, which also can drive cosmic expansion, but hasn’t played a significant part since the Universe was young.) In fact, dark energy is fundamentally different than matter (ordinary or dark), which has a diminishing effect as spacetime expands. This is why I hate the term “dark energy”, because it evokes the term “dark matter” and the two substances have very different effects. However, both matter and dark energy are present in the Universe we have, and understanding why spacetime expands requires both concepts.

Notes

I was spurred to ask and answer the question in the post title by Peter Coles, who specifically wanted science writers to be more clear on explaining dark energy: the name we give to the substance driving cosmic acceleration. Sean Carroll provided an explanation here, including how not to describe dark energy. And I confess: I’ve made the mistake Sean describes, so consider this post a correction for myself!

1. Dark matter doesn’t interact via the electromagnetic or strong force, which are the strongest forces in nature. That means any interactions between dark matter particles must be very weak (in the general sense, not necessarily the weak force).
2. The cosmological constant was first introduced by Einstein to prevent expansion, keeping the Universe the same size forever. See my earlier post and its references for more on that subject.