Section 6: The Concept of Dark Energy
It's a good thing that we have such confidence in the concordance of the measurements described in this unit, because the overall picture they indicate is completely astonishing. If we take the measurements seriously—and we should—they point to a universe dominated by something that acts to make the universe speed up, partially balanced by matter that makes galaxies clump together. However, two large and deep mysteries surround this view.
Figure 15: The abundance of light elements indicates that most of the universe is not protons, neutrons, or electrons.
Source: © NASA, WMAP Science Team. More info
One is that most of the matter in the universe cannot take the form that makes up galaxies, stars, and people: the familiar elements of the periodic table and their subatomic constituents of protons, neutrons, and electrons. Based on our understanding of the way light elements such as helium would form in the hot Big Bang, we know that this nuclear cooking can agree with the data only if most of the universe consists of something that is not protons, neutrons, or electrons. We call this dark matter, but, as we saw in Unit 10, we don't know what it is.
The other mystery stems from the fact that the cosmological constant is not the only candidate for the component of the universe that makes expansion speed up. If we try to estimate the energy density of the cosmological constant from basic quantum mechanical principles, we get a terrible quantitative disagreement with the observations. The computed number is at least 1060 times too large. Before the discovery of cosmic acceleration, physicists took this disagreement to mean that somehow nature covers up for our ignorance, and the real value is zero, but now we know that can't be right.
A dense web of evidence tells us that the energy associated with gravity acting in empty space is not exactly zero, but it isn't the gigantic value computed from theory, either. Clearly, something is missing. That something is a deeper understanding of how to bridge the two great pillars of modern physics: quantum mechanics and general relativity. If we had a good physical theory for that combination, presumably the value of the cosmological constant would be something we could predict. Whether that hope is valid or vain remains to be seen. In the meantime, we need a language for talking about the agent that causes cosmic acceleration.
Dark energy, dark matter, and the cosmological constant
To cover the possibility that the thing that causes the acceleration is not the cosmological constant, we use a more general term and call it dark energy. For example, dark energy might change over time, whereas the cosmological constant would not. To express our combination of confidence that dark energy is real and our ignorance of its precise properties, we describe those properties by talking about dark energy's equation of state—the relation between the pressure of a gas and its density.
The cosmological constant doesn't act like any gas we have ever used to squirt paint from an aerosol can. We're used to pressure going down as the gas expands. If dark energy is really a constant energy density, as it would be if it were identical to the cosmological constant, then the vacuum energy in each cubic meter would remain the same as the universe expands. But if dark energy behaves slightly differently from the cosmological constant, that energy density could go up or down; this would have important, and possibly observable, consequences for the history of cosmic expansion.
Taking our best estimates for the fraction of the gravitating matter that is dark matter and the fraction associated with the glowing matter we see, and assigning the convergence value to dark energy yields the amazing pie chart diagram for the universe that we encountered in Unit 10. To our credit, the diagram is full. However, it is full of ignorance: We don't know what dark matter is, and we don't know what dark energy is. In science, this is not such a bad situation to be in. It leaves plenty of room for future discovery.
Figure 16 : The composition of the universe, with 96 percent invisible and unfamiliar.
We have some ideas of how to proceed in learning more about the nature of both dark matter and dark energy. As we saw in Unit 10, physicists have embarked on several efforts to determine the identity of dark matter, using beams in accelerators, detectors in underground laboratories, and instruments in space. All of this is quite speculative, but very exciting. We could well be on the threshold of making a crucial measurement about dark matter.
Our expectations for learning more about dark energy are more modest. Larger, more sophisticated, and more certain surveys of supernovae, galaxy clustering, and other tracers of the way the universe has changed over time provide the most promising path forward.