Growing up in the Midwest, I shoveled snow to earn spending money. The rate at which I cleared a sidewalk depended on two factors: how quickly I shoveled and how rapidly the snow fell. In fact, someone watching me shovel for a couple of minutes could evaluate the rate of snowfall based on how much snow covered the just-shoveled walkway. This picture––think galaxy cluster formation instead of snow accumulation––helps illustrate what astronomers have learned about the role of the mysterious, but highly fine-tuned dark energy in the universe.1
Given that we live in a big bang universe with a large component of dark matter, two processes affect how quickly galaxy clusters grow. Shortly after the big bang, the matter/energy of the universe was spread uniformly throughout space. The density of matter/energy dictates how strongly gravity causes it to clump together into stars, then galaxies, and ultimately clusters of galaxies. This density corresponds to how quickly I could shovel a walk.
In the late 1990s, astronomers studying distant supernovae found that the expansion of the universe appeared to accelerate around five billion years ago. The simplest explanation for this acceleration invokes a space-energy density (or dark energy), which causes space to expand even more rapidly as it grows. Consequently, it dilutes the mass/energy density of the universe and slows down the rate at which galaxies and clusters grow—similar to how additional snowfall impedes my progress in clearing snow from the sidewalk.
If no dark energy existed in our universe, the mass/density of the universe would be smaller to account for the number of large clusters observed in the nearby universe. With such a low density, astronomers would not expect to find any large clusters in the early universe. However, a large dark energy component means that large galaxy clusters form more slowly and earlier in the universe’s history.
Measurements of the cosmic microwave background radiation (left over from the creation event) and the distribution of galaxies provide additional evidence that dark energy exists and causes the universe to expand increasingly fast. Using the XMM-Newton X-ray telescope, a team of German astronomers found a cluster of galaxies whose light has taken almost eight billion years to reach Earth (which means the cluster existed 6 billion years after the creation of the universe). The massive size of the cluster, which shines 100 times brighter than other clusters at this distance, further buttresses dark energy theory.
Finding such a large cluster from so long ago provides strong evidence that dark energy exists and increasingly controls the expansion of the universe. While astronomers and physicists have no idea what the dark energy is, we’re not getting snowed when they say this feature must exhibit extraordinary fine-tuning in order for life in the universe to exist. Such fine-tuning seems a stretch for chance occurrence, but resonates with a purposeful creation model.