Narrowing the Constraints on Dark Energy

Narrowing the Constraints on Dark Energy

Scientists almost universally acknowledge the enormous “problem” represented by the extreme fine-tuning inherent in dark energy (a.k.a. space-energy density or vacuum density). It even has its own name—the “cosmological constant problem.” In a nutshell, various ways to predict its value theoretically differ from the observationally determined value by 45 to 120 orders of magnitude. (See the technical but readable article by Sean Carroll at Living Reviews1).

Because of this discrepancy, the majority of scientists believed the space-energy density was exactly zero. They reasoned that it was much easier to conceive of ways that the space-energy density would exactly cancel to zero rather than to a small but nonzero value that permitted a universe where life could exist. However, the universe did not accommodate that belief, as definitively demonstrated by Type Ia supernovae observations in the late 1990s. These bright, distant beacons provided clear evidence that “dark energy” existed and had a value far below theoretical predictions.

Because of the “disturbing” design implications, a number of alternative explanations were offered, one being that the properties of this particular class of supernovae changed over time in such a way as to mimic the signature of a nonzero dark energy. However, recent results from the Hubble Space Telescope strongly rule out this and most other alternative explanations.

The unique aspect of these observations is the unprecedented number of large redshift supernovae included in the sample—over 23 with redshifts placing them within the first 4 billion years of the universe’s history2. The team found no difference in the spectral properties of these ancient supernovae with more recent—within the last 10 billion years—samples. Using this new sample, the team reduced the uncertainty in the measured Hubble constant during the period more than 10 billion years ago from 50% to less than 20%. Consequently, the evidence for a cosmic “jerk” (the transition from past deceleration to the present acceleration) is much stronger.

The results also indicate that the dark energy is consistent with a constant value in the first 4 billion years, matching previous conclusions for the last 10 billion years. This provides the first meaningful constraint on the dark energy in the former epoch. As the number of Type Ia supernovae increases and scientists better understand the small ways in which they differ, these probes will be used to provide even tighter constraints on the nature of the exquisitely fine-tuned dark energy.