(The fourth installment in the series “Alphabet of Cosmology“, in which I introduce a concept or project in cosmology.)

Let’s face it: “dark energy” is one of the dumbest names for an important concept in physics. First, it sounds like something from an action movie or superhero comic (or superhero action movie). Second, the term makes it sound like dark energy is somehow related to dark matter, which was a deliberate choice by cosmologist Michael Turner, who coined it. However, while dark matter (which as I’ve pointed out before would be better called “invisible matter”) does appear to be matter—it clusters and attracts ordinary baryonic matter gravitationally—dark energy has very different properties.

What do we mean by “dark energy”?

Since the 1920s, we’ve had strong evidence that the Universe is expanding. If the Universe contained only matter (both ordinary and dark matter) and light, then its expansion would gradually slow down, but it doesn’t. In 1998, two groups of astronomers discovered that the rate of expansion is increasing, a discovery for which three of the project leaders—Saul Permutter, Adam Riess, and Brian Schmidt—received the 2011 Nobel Prize in physics. That was a serendipitous discovery: the two separate groups of astronomers that discovered dark energy had set out to measure the deceleration of the Universe, not its acceleration. After all, galaxies exert gravitational pull on each other, so the expansion of the Universe was expected to slow down; the so-called deceleration parameter was second only to the Hubble parameter in importance in old-school observational cosmology. (The field is sufficiently fast-changing that 20 years ago nearly counts as ancient history now.)

Dark energy wasn’t entirely unexpected. Albert Einstein added a mathematical ingredient to his general theory of relativity called the cosmological constant to prevent cosmic expansion, but removed it later. Cosmologists including Steven Weinberg, Michael Turner, and Lawrence Krauss proposed that the energy of the quantum vacuum (more on that shortly!) could drive expansion of the Universe. However, they recognized the simplest calculation was wrong: it predicted far too much energy, even if their general conclusions were probably right.

Another strong (albeit indirect) piece of evidence in favor of dark energy comes from the cosmic microwave background (CMB). As I emphasized in the previous entry in the Alphabet of Cosmology, temperature fluctuations in the CMB on a variety of scales reveal the contents of the cosmos. These data revealed that dark energy comprises about 71.4% of the energy content of the Universe, per unit volume. That bizarre result led cosmologist Sean Carroll to label our cosmos “the preposterous Universe“.

This is what we know about dark energy: its general effect on the expansion of spacetime, and roughly how much of it there is compared to the other contents of the Universe. Now let’s consider what we don’t know….

Ignorance is not bliss

To understand why dark energy is weird, let’s think about how it differs from the more ordinary stuff in the Universe. As I mentioned earlier, a Universe containing only matter and radiation will slow its rate of expansion over time: as the Universe expands, the atoms, dark matter particles, and photons get more spread out, meaning the density of energy they supply to drive growth decreases. While this might sound like pressure is driving expansion, it’s not true: at the temperatures and quantities of matter particles present in an average chunk of the Universe, the pressure they contribute is effectively zero.

Dark energy, on the other hand, acts as though it has negative pressure. The figure on the right illustrates that concept: while ordinary gas pushes back when you try to compress it, dark energy pushes back less the more it is compressed. This explains why dark energy drives the acceleration of the Universe: the bigger the volume in which it’s contained, the more pressure—and energy—it supplies to expansion.
This picture is fairly clear, whatever substance dark energy turns out to be.

However, the easy view breaks down when we go a little beyond that. If you’ll forgive the inclusion of a simple equation, it might help us out:

where p is the pressure, d is the density (amount of stuff in a fixed volume), and w is known as the “equation of state”, though that’s more appropriate for the whole formula. For dark matter and ordinary matter under cosmological conditions, w is nearly zero; for light or particles (like neutrinos) moving close to the speed of light, w is 1/3. (In chemistry of physics class, you probably learned the ideal gas law; if we write d as the number of moles divided by volume, then w depends only on temperature. In other words, the ideal gas law is another version of the same equation, covering the case when temperatures and densities are relatively high.)

Dark energy, on the other hand, has a negative value for w, and a lot of effort is being expended to determine what value it should have. We have several possibilities:

• If dark energy is vacuum energy, then it’s the energy of empty space. That’s not as weird as it sounds: from quantum field theory, we know that spacetime is a cauldron of particles that only potentially exist. The properties of particles are determined through their interactions with this quantum vacuum, which is full of fields—including the Higgs field, which gives mass to many particles. (The Higgs boson is a manifestation of this field, if enough energy is present to make one.) This means the more vacuum there is, the more vacuum energy, and the more pressure. This is the simplest case, and it corresponds to a w value of -1. Since this is mathematically the same thing as Einstein’s original idea (though with the opposite sign), vacuum energy is also called the cosmological constant, since its contribution to the density of energy is constant in both space and time. Similarly, w is always -1, forever: it doesn’t change in time as the Universe expands. The problem with this simplest model is that nobody quite knows how to calculate the actual value of the cosmological constant; the most straightforward prediction from quantum field theory (first performed, as far as I know, by Steven Weinberg) leads to a number that’s far too large, compared with observation. Despite this major drawback, most cosmologists think this option is best, and I’m inclined to agree in the absence of better (simple) ideas.If you’re a fan of string theory, the landscape of string theory models includes a huge number of possible vacuum energy/cosmological constant values, several of which correspond closely to our own. I’m a little leery of that as an explanatory model, though, since it requires an anthropic argument: the Universe we inhabit is one of only a few out of the landscape where life could arise. Anthropic arguments have always struck me as feeling tautological rather than predictive: we can’t say why this Universe has the vacuum energy value it does, but we know it must have that vacuum energy because we’re here to measure it.
• If dark energy is a fluid of a fundamentally different type than any other we know about, then w is likely to be between -1/3 and -1. This idea corresponds to a veritable plethora of possible models, and I admit I don’t know much about most of them. They go under names like quintessence, and they are significantly more complicated than the vacuum energy scenario. For example, w changes with time in quintessence models: its value today is not what it was in the past, and may be different again in the future. Our current observations can’t rule that possibility out completely, though the amount of change in w seems to be fairly small for the regime where we have good data.
• Maybe w is less than -1, an option known as phantom energy. This idea has been batted around occasionally since 1998, but our best current observational data seems to rule it out. Katie Mack explained the hows and whys in a great post from last summer.

Beyond these options, there are several others that obtain w and dark energy behavior through entirely other means.

• Perhaps our beloved theory of gravity, general relativity, requires modification to fit cosmological observations. General relativity is very well tested on the scale of the Solar System and in strong-gravity cases (including binary neutron stars), but perhaps what we see as dark energy is a reflection of its breakdown on the very largest scales. A number of alternative gravitation theories are out there, many of which are fanciful or far more complicated than any well-tested theory we have. I admit, I like this idea, though I haven’t found a particular alternative gravity model that satisfies both observational data and my own high standards. (Crackpots send me their ideas about once a month, but there are plenty of reasonable people who work in this area too.)
• The cyclic universe model of Paul Steinhardt and Neil Turok describes our Universe as part of a coupled pair that interact. What we perceive as dark energy is a measure of the energy being pumped into our Universe as the bond between it and the shadow universe stretches. (I may be getting some of the details wrong; my apologies if that’s so.) I admit, I’m kind of fond of this scenario for its sheer creativity and the testable predictions it makes, but I don’t believe a word of it. I’m not sure what it predicts about the value(s) of w, either.

Those two options aren’t comprehensive! However, when we get beyond our standard, accepted physics, we end up in a Wild West of ideas, where many possibilities exist—including some that might work, but haven’t been developed yet. So, let’s talk instead about the observational side….

What we are doing about our ignorance

The test rig for the Dark Energy Camera (DECam) at Fermilab stood more than twice my height, a giant structure of concentric rings on a blue steel frame. The rings were heavily braced and bolted together on a massive scale, obviously built to hold something heavy, though by the time I saw it, its payload was gone. Altogether, the structure evoked something from a science fiction film, a gateway perhaps to another dimension, or a wormhole to a distant part of the Universe. That impression was increased by the room, a cavernous room topped with triangular segments forming a funky geometrical shape, akin to an electron micrograph of a virus. The whole structure had a lovely 1970s vibe, with a veneer of quirkiness and exciting science overlaying the blocky institutional architecture of that era. As my host, scientist Tom Diehl, told me, the roof was made of aluminum cans collected and donated by schoolchildren; while the outside was covered with a dull gray veneer, from the inside triangles appeared russet and muted gold.

As the name suggests, DECam is indeed a camera, similar in overall design to digital cameras such as one you or I might own. However, its scale is much larger: rather than being something a person uses, the camera is mounted on the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes. DECam has five lenses, the largest of which is 98 centimeters in diameter (about 39 inches); together they weigh an impressive 380 pounds. DECam’s lenses focus and correct for distortions in light from the telescope’s 4-meter-diameter mirror, ultimately directing the images onto a set of 62 chips. Together, these chips produce an image 570 million pixels in size, covering an area about 20 times the size of the full Moon in the sky.

DECam is the key technology behind the hugely ambitious Dark Energy Survey (DES), which over 5 years will take measurements of 300 million galaxies, a large number of supernovas, and any amount of other astronomical phenomena in a swath of the sky in the southern hemisphere. These observations are intended to measure the expansion rate of the Universe, characterizing dark energy. DES is one of several major international efforts underway or set to begin in the near future; the detail involved should be able to settle whether dark energy’s effects are constant in time and space, or if they vary across the Universe or change over history.

[Portions of this post appeared as a previous entry. The final section includes an edited excerpt from my book-in-progress, Back Roads, Dark Skies: A Cosmological Journey.]