Does dark energy exist? Considering that three scientists were awarded the Nobel Prize for discovering the accelerated expansion of the universe, presumably caused by dark energy, one would think this question was settled amongst astronomers. However, different models can explain the acceleration without invoking dark energy; so scientists continue to explore how to test whether dark energy actually exists. One particularly compelling signature of dark energy, which is thought to comprise over 70 percent of the universe, recently received validation.
Testing for Dark Energy
The dynamic behavior of space changes the wavelength of light on its way to astronomers’ telescopes. The most pronounced (and well-known) change results from the heavens being “spread out” as the universe ages. Because space is expanding, a photon (unit of light) traveling through space has its wavelength stretched and made longer. Astronomers use this effect to measure the expansion history of the universe and to determine the distance to the farthest objects.
The gravitational attraction of large objects causes a more subtle change to a photon’s wavelength that allows astronomers to determine whether dark energy exists. Most galaxies in the universe reside in clusters. The Local Cluster includes the Milky Way and Andromeda galaxies, the smaller Triangulum galaxy, and dozens of dwarf galaxies. Other galaxy clusters include thousands of galaxies. As a photon travels into one of these clusters, it gains energy as it moves deeper into the gravitational well caused by all the mass. And, as expected, the photon loses energy while it travels out of the cluster’s gravitational well.
In a static universe or one dominated by mass, the energy gained by the photon as it enters the well matches the energy lost when leaving the well. Thus, the photon has the same wavelength before and after. In a universe undergoing accelerated expansion, the depth of the well shrinks as the acceleration spreads the mass over a larger volume. So the photon gains more energy entering the cluster well than it loses on the way out, causing its wavelength to decrease (shorter wavelengths mean greater energy). In a similar fashion, the wavelength of the photon will increase as it travels through void regions with very little mass. Scientists refer to this feature as the late integrated Sachs-Wolfe effect (ISW).
The cosmic microwave background (CMB—faint radiation from the big bang) provides a good tool for measuring the ISW because the fluctuations in temperature measure the mass density distribution in the early universe. Although the CMB was emitted at a known wavelength, local density fluctuations changed the wavelengths. So astronomers must figure out some way to correct for this effect. Fortunately, they can also measure the distribution of mass on large scales using maps of galaxies and galaxy clusters like those provided by the Sloan Digital Sky Survey. They can then use these maps to look for the ISW.
Various research groups had used different maps of the large-scale structure of the universe to indicate detections of the ISW and then tried to integrate all the measurements into a single result. Previous analyses showed the existence of dark energy with a confidence level of 99.999 percent. However, some people raised objections to the analysis.
More recently, researchers addressed the criticisms by reevaluating the claims for dark energy. They included the most up-to-date CMB and large-scale structure data. The new analysis affirmed the previous conclusions and demonstrated that the dark energy results hold, independent of different processing routines.1
Why It Matters
Given the astronomy community’s general confidence in the existence of dark energy, one might ask why it’s necessary to perform all these detailed tests. Scientists continue testing for two main reasons. First, the additional tests help eliminate alternative explanations for the measured acceleration of the universe’s expansion. Similar testing over the past 100 years helped establish big bang cosmology by showing the deficiencies of steady-state, oscillating universe, and other alternative cosmologies.
Second, more-detailed measurements help scientists build a more-complete model. In the case of dark energy, beyond the fact that it accelerates the expansion of the universe, scientists currently have little idea what the dark energy is or how it works. In the case of big bang cosmology, such testing eventually revealed the need for an epoch of inflation to account for all the observations.
These more-detailed models help us understand how God brought this universe into existence, how it has matured (or, dare I say, evolved), and how it currently exhibits all the characteristics necessary to support life. Dark energy plays a pivotal role, so let’s keep the tests coming.