Dark energy is without a doubt the weirdest thing in the universe. Despite being present in every corner of the cosmos and dominating its evolution, dark energy is so far outside our best existing theories of physics that we’re still trying to figure out if it’s something in the universe or just a basic property of space itself.
And it would be nice to know a bit more about it, because if you look ahead on a long enough timescale, dark energy will almost certainly destroy the universe and everything in it.
Here’s an analogy. Imagine you throw a baseball up in the air. The baseball and the Earth here are standing in for galaxies moving apart from each other in our expanding universe, and your throw is the expansion set off by the Big Bang.
Now, because both the baseball and the Earth have mass, gravity wants to pull them back together, no matter how hard you throw. Even if you have an inhumanly good arm and throw the ball fast enough to escape the Earth entirely, gravity will still slow the ball down at some level. Maybe the ball will fall back down; maybe it’ll coast through space forever. But what it won’t do is suddenly speed up and shoot off into the sky, defying gravity in the truest sense of the term.
But that’s what the universe is doing. After a nice long period of reasonable post-Big-Bang slowing, about five billion years ago, the expansion of the universe began inexplicably speeding up. Whatever is causing that to happen, we call “dark energy”.
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The “dark” in dark energy just refers to the fact that we can’t see dark energy and have no idea what it actually is. But it might as well be a reference to what it does to our cosmos.
A universe whose expansion is accelerating is one in which galaxies are relentlessly driven apart from each other. Space on the whole gets darker, emptier, and colder, and the lack of interactions starves galaxies of their supply of fuel for new stars. Stars die, matter decays, and eventually the cosmos is devoid of all structure, with only trace amounts of radiation wandering aimlessly through the void.
This rather bleak, lonely denouement is known as the heat death.
But it could be worse. Much worse.
The standard explanation of dark energy is a cosmological constant – an idea Einstein first proposed in 1917 as an added term in his equations of gravity to explain why the universe appeared to be static instead of collapsing upon itself. When expansion was discovered soon after, the cosmological constant term was thrown out, only to be dragged out again decades later as a possible culprit for accelerated expansion.
A cosmological constant fits the data at least as well as any other explanation, but from a theoretical viewpoint it’s a mess. It represents vacuum energy – a kind of minimum energy inherent to empty space, one whose density stays constant no matter how much the universe expands – but its magnitude is all wrong.
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The best theoretical calculations we can do from particle physics theories suggest vacuum energy should be at least 120 orders of magnitude stronger. And we know that this quantum mechanical version of vacuum energy has to be there, because it’s been detected experimentally, causing tiny metal plates to move very, very slightly via a phenomenon called the Casimir Effect. But if cosmological constant were strong enough to make the whole theoretical story fit together, it would do more than slowly isolate galaxies over time. It would violently rip apart all structure in the universe.
So we’re left with a conundrum. It would, in some ways, be easier to explain a universe with no cosmological constant than with a bizarrely small one. And there have been explanations of dark energy that don’t involve Einstein’s term at all. Some of these allow for dark energy’s density to be changing over time.
In fact, some recent observations of the expansion of the universe as deduced from very distant exploding stars and supermassive black holes have lent support for just that idea.
Unfortunately, “dynamical” dark energy comes with its own issues. If dark energy gets more powerful over time, as some of these observations have suggested, it could actually tear the universe apart.
So perhaps we should hold out hope for a heat death, that we might go gentle into that good night. At least it might remind us to take a moment to appreciate the beautiful cosmos all around us, while we still can.
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