Every now and again, a speaker communicates a point in a way that sticks in memory. I distinctly remember attending a lecture in 2004 by Sean Carroll at an American Physical Society meeting. During the lecture, Dr. Carroll expounded on the latest explanations for various cosmological measurements. After discussing potential dark matter candidates, he turned to the subject of dark energy. In dramatic fashion, the slide listing all the possible dark energy candidates (causes of) appeared. It was blank! Five years later, the situation remains the same. Yet, the experimental measurements indicating the presence of dark energy grow more compelling. Why the discrepancy between observation and experimentation?
Observations of distant Type Ia supernovae provided astronomers with the first evidence that a mysterious "substance" was causing the expansion of the universe to accelerate. The finding surprised astronomers. Theoretical calculations of any mechanism to cause the acceleration predicted values nearly 120 orders of magnitude larger than those measured. The cancellations that must occur to reconcile the predicted and measured values represent one of the most severe fine-tunings known to scientists.
Since its discovery in the mid-1990s, numerous other investigations have confirmed the existence of dark energy (a.k.a, vacuum energy or space-energy density). Analysis of the cosmic microwave background radiation and measurement of the baryon acoustic oscillations by the Sloan Digital Sky Survey both imply the existence of dark energy.
Although the data convince most scientists of dark energy's existence, they still want to know more about its characteristics. In particular, they want to know whether the amount of dark energy changes over time. Any change in dark energy influences potential models for its origins and affects how the universe develops. Recent studies of galaxy clusters help constrain the amount that dark energy has varied over the history of the universe. Cosmologists have defined an equation of state ω that characterizes how the dark energy might vary. The cosmological constant has ω = 1.
The new studies of galaxy clusters give ω = 0.991 with statistical and systematic errors of 0.045 and 0.039 respectively. This represents an improvement in accuracy by a factor of 2 and shows that the cosmological constant remains the simplest description of dark energy.
Regardless of whether dark energy is the cosmological constant or not, investigations into its properties and origin continue to provide strong evidence of fine-tuning required for this universe to support life. Such fine-tuning also points to a Designer.