“Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe.” -Eric Linder

It's been a while since we've spoken about dark energy, and we were just talking about Einstein's greatest blunder, so let's just dive right in.

Image credit: S. Beckwith & the HUDF Working Group (STScI), HST, ESA, NASA.

This is our observable Universe, as unveiled by the Hubble Space Telescope. With hundreds of billions of galaxies stretched out some 41 billion light years in all directions, finding out about what our Universe was like in the distant past, the recent past, and what it's like today is limited only by our willingness to look. In particular, there are three great sets of observations that tell us ever so much about the Universe on the largest scales.

Image credit: Northern Galactic Cap from the SDSS-III release, via http://www.sdss3.org/

1.) The way galaxies cluster together on the largest scales. By looking at huge, tremendous surveys of galaxies, we can see how the visible matter in the Universe has clustered, clumped, and grouped together, as well as where it hasn't, and has left us great cosmic voids. By putting various ingredients into a model Universe governed by General Relativity, we can also simulate how structure should form in our Universe. Where the simulations and the observations match up, that tells us what's in our Universe.

Image credit: ESA and the Planck Collaboration.

2.) The temperature fluctuations in the cosmic microwave background. By looking at the temperature fluctuations -- the hot-and-cold spots -- in the CMB, we can know what the Universe looked like in terms of overdensities, underdensities, and how they're clustered with respect to one another all the way back at a time when the Universe was just some 380,000 years old! Because the light has had to travel for nearly the entire 13.8 billion years that the Universe has been around (it's been traveling for 99.997% of the Universe's history), we can find out information about what the Universe was like back then, but also how it's expanded since then. This pattern of fluctuations also tells us what the various combinations of ingredients are in our Universe.

Image credit: Kowalski et al., Ap.J., 2008.

3.) Direct observations of well-understood objects at various distances/redshifts in the Universe. Everything from variable stars to properties of galaxies to distant supernovae help us get a handle on this, the cosmic distance ladder. This tells us how the Universe has been expanding since as far back as we can measure until the present day.

When these three data sets are combined -- and we can combine others, too, but these three are the best data sets we have -- they tell us that there's matter in the Universe, about 31-32% of the Universe is matter (most of which is dark matter), and that there's another type of energy, dark energy, that makes up the rest.

Image credit: Planck Collaboration: P. A. R. Ade et al., 2013, A&A Preprint; annotations by me.

So, you ask, just what is dark energy, and how do we know?

In principle -- and by in principle, I mean in General Relativity -- matter, energy, topological defects, and pretty much anything else that you throw into your Universe is going to affect how your Universe expands because of two properties inherent to it: it's energy density and its pressure.

Image credit: Large Synoptic Survey Telescope, NSF, DOE, and AURA.

Because of the way the Universe is observed to expand, and because of the known way that matter (yes, even dark matter, according to General Relativity) behaves, we can infer something about the energy density and pressure of dark energy. In particular, we know that dark energy's pressure is negative, and that it's quite negative.

In General Relativity, we can relate the pressure of any component of the Universe to its energy density by the simple equation:

ρ = w P / c2,

where ρ is the energy density, P is the pressure, c is the speed of light, and w is just some number.

Image credit: A.V. Vikhlinin, R.A. Burenin, A.A. Voevodkin, M.N. Pavlinsky.

According to the best data we have right now, w is equal to -1. Now, as time goes on, we hope to constrain it better; we can say it's probably about -1.00 ± 0.08 right now, which is pretty good.

Now, here's the thing: in theory, the pressure of different things in cosmology goes in increments of 1/3. For example:

Radiation has w = +1/3, like photons and ultra-relativistic matter.

Matter, both normal and dark matter, has w = 0, or is virtually pressure-free.

Cosmic strings, or 1-dimensional topological defects, have w = -1/3. This is the border between what would cause a Universe to accelerate (more negative than this) or not.

Domain walls, or 2-dimensional topological defects, have w = -2/3.

A cosmological constant (or textures, a 3-d defect) has w = -1.

Those are the easy possibilities.

But dark energy could also be something weird. It could be a field, whose relationship between pressure and energy density changes over time. It could be coupled to something we don't understand. It could be really, really weird, and our measurements of w = -1, to the best we can see, could only be what the Universe is so far.

So we've attempted to look for a change in w over time. We've attempted to look for departures from w = -1. We've attempted to look at different models and their signatures.

Know what we've found?

Image credit: Pearson / Addison-Wesley.

The farther back into the past we look, the more and more consistent everything appears to be with the cosmological constant option.

The cosmological constant option has the theoretical bonus of being:

easily explained,

inevitable (in that it must exist, even though its value could be 0),

and requires no new physics beyond the standard model/GR to explain.

We're going to keep exploring different variants of dark energy, quintessence, scalar field-driven dark energy, etc., of course. But theoretically, there's no motivation unless we see some sort of evidence that tells us that dark energy is something more (or other) than a simple cosmological constant. And trust me, we're looking.

This doesn't mean dark energy is a cosmological constant, it just means that this is the best working hypothesis until evidence suggesting otherwise comes along, and such evidence does not exist today. That's the best we've got, so far.