During graduate school I used a telescope in southern Arizona. The long, winding mountain road to the ridge where this telescope sat passed a twenty-foot boulder that had fallen off its perch and slid down the mountain. The somewhat weathered (but easily discernible) trough it formed during its fall caught my eye. Because a rock this size falls infrequently I was fortunate to witness the consequences of its slide before they were completely weathered away. It would have been even more improbable to see the rock as it fell, since the boulder spends the vast majority of the time either on top of the mountain or at the bottom. Picturing the universe as that boulder, cosmologists witness an amazing "timing coincidence": they see the rock barreling down the mountain.
Throughout the universe's history, different components have dominated the cosmic energy budget even though the total energy of the universe remained unchanged. The diagram below (using different colors) shows these different epochs. During the earliest periods (a fraction of a second after the big bang), whatever caused the inflationary expansion dominated the energy budget. After inflation but before the generation of the cosmic microwave background (380,000 years after the beginning), electromagnetic radiation dominated the energy budget. For the next several billion years matter dominated. And now the universe is transitioning to an epoch where the space-energy density dominates. As illustrated in the diagram, any randomly chosen time likely falls into one of the epochs where a single form of energy dominates. However, our observations place us right in the middle of a transition period—much like seeing the rock rushing down the mountain.
A recent Astrophysical Journal article offers one explanation for this coincidence.1 "Any random time" implies that no constraints exist for when observers could measure the cosmic energy budget. It's like saying that observers can measure the energy at any time in the universe's history. However, in reality, observers need a planet on which to live. Additionally, observers require that sufficient time must pass in order for the numerous planetary transformations to occur. The article's authors argue that the only time period when planets capable of hosting observers exist is during the transition from a matter-dominated universe to a space-energy-dominated universe.
Before the transition occurs, not enough elements heavier than helium will have been produced to make habitable planets. After the transition finishes, star formation will have ceased, meaning that no habitable planets can form.
Coincidentally, the time period that permits observers corresponds with the time period where those observers can measure all the energy available in the universe. After the transition, matter will be too thinly dispersed to accurately measure the energy density. Before the transition, the space-energy density was too small to be detected.
Not only do we (the observers) get to witness the proverbial boulder rolling downhill, but we also see the rock clearly enough to precisely measure its size, history, and composition. Such a cosmic coincidence seems consistent with the notion of a Creator who has deliberately timed observers' arrival and equipped them for spectacular discoveries.
1. Charles H. Lineweaver and Chas A. Egan, "The Cosmic Coincidence as a Temporal Selection Effect Produced by the Age Distribution of Terrestrial Planets in the Universe," Astrophysical Journal 671 (December 10, 2007): 853-60.