I remember sitting on the long teeter-totter, near the ground. My older, larger brother on the far end shifted his weight all the way back, causing my end of the seesaw to quickly rise. A second later I hung on tight to avoid getting thrown off the end. While my brother and I spent hours repeating this cycle for fun, a similar mechanism may provide important insights into the dark energy pervading the universe and its fine-tuning to support life.
Dark energy comprises 73 percent of the universe and drives its expansion. This hypothesized energy does not dilute away as the universe expands; hence, it retains a persistent “density.” From a physics perspective, the small value of the dark energy density presents a problem. (See sidebar below, Cosmological Constant Problem)
This problem comes in two parts. First, none of the components contributing to the dark energy has the same energy scale as the measured value. So, a combination of the contributors, some positive and some negative, must add together to equal the measured value. Second, all the contributors are much larger than the measured value. Thus, the combination entails an exquisite, but not quite perfect,
cancellation of extremely large values to get an incredibly small one.
The recent discovery of the Higgs boson illuminates a potential solution to the first part of the problem using a tool called the seesaw mechanism.
Consider some quantity of matter with mass that has components that arise from two different mass scales. In general, the measured values of the masses need not correspond to the actual mass scales. Instead, they will usually be a mixture of the two. If the two mass scales differ in value significantly, then the mixing can cause one measured value to increase dramatically while the other will decrease in a similar fashion (like a seesaw).
Such a scenario explains the small masses of neutrinos quite well. The two mass scales for neutrino contributions arise from couplings to the electroweak scale (100 GeV/c2) and from a hypothesized coupling occurring at the grand unified scale (~1015 GeV/c2). The mixing of these two scales gives masses for the known neutrinos (10-9 GeV/c2) and predicts another neutrino with mass beyond the grand unified scale (which currently exceeds detection abilities).
Two cosmologists applied this method in exploring the properties of dark energy in light of the recent discovery of the Higgs boson. Measurements now establish the existence of the Higgs, which determines the energy scale of the scalar field responsible for it. Grand unified theories set the other energy scale where physicists expect undiscovered scalar fields to exist. Mixing between the scalar fields at these two different energy scales naturally produces another scalar field with the right energy to explain the dark energy pervading the universe.1
This research advancement warrants a couple of comments. First, as acknowledged by the scientists involved, this study posits the existence of unknown (and undetected) particles and fields at energies far beyond scientists’ ability to probe with particle accelerators. Moreover, the existence of those postulated quantities does not appear to have any measurable consequences in laboratories. Consequently, observations of the universe as a whole provide the only mechanism to test the validity of such models.
Second, the models provide a natural explanation for the first part of the cosmological constant problem by naturally producing contributions to the dark energy of the right energy scale (without requiring summing of different contributors). However, the models still don’t explain why the known contributors cancel so exquisitely. Research like this, while still speculative, demonstrates solutions to known problems and gives direction to future explorations. It also illuminates the beauty and elegance of this universe as well as the remarkable fact that humans are here to try to understand its mysteries.
Cosmological Constant Problem
Scientists’ measured value of the dark energy density is around 10-8 ergs/cm3. They can also calculate a predicted value based on the expected contributions arising from the best particle physics and cosmological models. Contributions on the energy scales of the Higgs potential and the Planck scale lead to calculations of the dark energy density at 10112 ergs/cm3. Scientists refer to the difference between the expected and measured values (10112 / 10-8 = 10120 or 120 orders of magnitude) as the “cosmological constant problem.” (For clarity, this discrepancy does not mean that a change in the cosmological constant value of one part in 10120 leads to an uninhabitable universe.)