Section 9: Further Studies of Dark Energy
The problems posed by dark energy are fundamental and quite serious. While the observational programs to improve our measurements are clearly worth pursuing, we can't be sure that they will lead to a deeper understanding. We may need a better idea about gravity even more than precise determinations of cosmic expansion's history, and it seems likely that a truly new idea will seem outrageous at first. Unfortunately, the fact that an idea is outrageous does not necessarily mean that it is a good one. Separating the wild speculations from the useful new ideas is a tricky task, but better observations will help us to weed out some of the impossible ideas and let us see which of the remaining ones best fit the data.
Figure 21: The future scale of the universe depends on the nature of dark energy.
Source: © NASA, CXC, M. Weiss. More info
One way to approach dark energy is to try to pin down its equation of state. So far, our measurements of the cosmic equation of state are completely consistent with the cosmological constant, but perhaps some variation will show up when we do more precise measurements. Any deviation from a constant energy density would show that the cosmological constant idea was not right, and that we require something more complicated to match the facts for our universe.
Another way to think about cosmic acceleration is to ask whether it is telling us something new about general relativity. After all, though Einstein's theory is a beautiful one and has passed every experimental test to which it has been subjected, it dates back to 1917. Possibly there is some aspect of gravity that he did not get exactly right or that would show up only on cosmic length scales. So far, we have no evidence that Einstein's geometrical theory needs amendment, but physicists are always alert for possible cracks in the foundation of our understanding. They want to use astrophysical measurements to test whether general relativity needs to be modified.
Identifying galaxy clusters
To date, the predictions that best test whether general relativity is the right theory of gravity to account for an accelerating universe are those that predict how matter will clump and structure will grow in an expanding universe. By comparing clusters of galaxies, which are the largest aggregations of matter we know of, at large and small distances, astronomers have been able to put some useful limits on this aspect of gravity. Again, everything so far agrees with general relativity: Only more precise measurements could detect a small deviation from that picture.
For more precise measurements, we need better samples of galaxy clusters. Finding such clusters in optical images is tricky because some of the galaxies are in the foreground and others are in the background, making the more distant clusters more and more difficult to distinguish. Since the whole measurement depends on comparing the numbers of distant and nearby clusters, this is a dangerous sort of bias.
Figure 22: X-ray images of galaxy clusters observed with the Chandra X-Ray Observatory.
Source: © NASA, CXC, MSFC, M. Bonamente et al. More info
A better way to find galaxy clusters relies on their emission of x-rays. The gravitational well formed by 1014 solar masses of (mostly dark) matter in a galaxy cluster means that the gas (mostly hydrogen) that falls into the cluster or that is exhaled from the galaxies can gain a lot of energy in the process. It must be very hot to have enough pressure to keep from collapsing to the center of the cluster. The temperature for the gas in clusters is about 107 K, and the emission from such a hot ionized gas occurs principally in the x-ray part of the spectrum.
X-ray telescopes can search the sky for large sources of x-ray emission, and in some cases we can identify the sources with galaxy clusters. However, the inverse square law applies to x-rays just as it does to optical light. So, the more distant clusters are fainter, and the sample becomes more incomplete and harder to interpret as you observe fainter clusters. Although our best data so far comes from the x-ray selected samples, astronomers have a better technique.
Seeking patterns in the CMB
The signature of hot gas in a cluster of galaxies includes more than emitted x-rays. The fast-moving electrons in the gas can sometimes collide with photons from the cosmic microwave background. These collisions kick up the low-energy radio photons to higher energies. The interactions show up as distinct patterns in the CMB in the neighborhood of a cluster.
The map of the CMB usually shows only slight temperature variations from point to point, but the lack of low-energy photons reveals itself as a large cold spot in the map. If you tune your radio receiver to a slightly higher energy, you'll see a bright area that contains the extra photons kicked up to higher energy by the collision with the fast electrons in the cluster. In 1969, Physicist Yakov Zel'dovich and his student Rashid Sunyaev, now a distinguished astrophysicist, worked out the theory of this pattern. It is only recently that the Sunyaev-Zel'dovich effect has become a practical way to find clusters of galaxies.
Figure 23: The Sunyaev-Zel'dovich effect allows astronomers to find the signature of galaxy clusters in the CMB.
Source: © NASA, CXC, M. Weiss. More info
Using the South Pole Telescope, which observes the CMB from Antarctica, astronomers have started to prepare a new, large, and uniform catalog of clusters. One surprising feature of the Sunyaev-Zel'dovich measurements is that the distinctive signature of a cluster does not depend on its distance. So, this method should work just as well at finding distant clusters as at finding nearby ones. This seems likely to be the best way to measure the growth of structure in the universe in the years ahead, and to test whether Einstein's theory of gravity is enough to account for all the facts in our accelerating universe.
Future searches for dark energy
Future measurements will include better samples of supernovae to measure the expansion history of the universe, but astronomers are also developing new ways of monitoring the properties of the universe. These include looking directly at the clustering of galaxies and, less directly, at the gravitational lensing that clumped matter produces.
As shown in Units 3 and 10, mass in the universe curves space. This means that the mass associated with galaxies can act like lenses that distort and magnify the scene behind them. We see this effect in galaxy clusters, which form long, thin arcs of light by bending the light from galaxies behind them. Weaker lenses can distort the shape of background galaxies. By carefully measuring how images of galaxies are warped at low and high redshifts, we can construct another picture of the way in which mass has grown more concentrated over time. This will give us a further clue to whether the growth of structure in the universe we live in matches or contradicts the predictions of general relativity.
Figure 24: The Joint Dark Energy Mission will make precise measurements of the effects of dark energy from space.
Source: © NASA, GSFC. More info
Measuring the properties of the universe is the only path we have for learning the properties of dark energy and testing whether Einstein's gravity theory gives us the whole story for assembling galaxies and clusters out of a very subtly rippled past. What's missing is an experimental test on the laboratory scale, or even the solar system scale, that would show us the presence and properties of dark energy. Unlike the case of dark matter, where it seems that we have the technology to detect the phenomenon and are on the brink of doing so, nobody has a clue about how to make a laboratory experiment to detect dark energy. Just as Einstein had to rely on astronomy to test his theory of general relativity, our only "laboratory" for measuring dark energy seems to be the universe itself.