PHILOSOPHY

On Dark Energy

Astronomers discovered dark energy accelerating cosmic expansion, challenging gravity’s role and reshaping our understanding of the universe, quantum fields, and the fate of all matter.

Estimated mass-energy distribution in the universe (2013 observations): Dark energy, Dark matter, Known matter

In 1998, astronomers used the faint light of supernovae from distant galaxies to calculate the speed at which those galaxies were moving away from us due to the expansion of the universe.

They discovered that they were moving more slowly than their distance would suggest. Since the light that reached us originated when the universe was still very young, the slower-than-expected recession meant that the universe was expanding more slowly in the past. Consequently, not only is the cumulative gravitational pull of ordinary and dark matter not slowing the expansion of the universe, but something must be pushing it to expand faster than in the past.

This repulsive substance of as yet unknown nature, which counteracts gravity and expands space even more rapidly, has been called "dark energy." According to current knowledge, dark energy could eventually result in what is called the "heat death" of the universe: in several billion years, the continued, accelerated expansion of space will cool the universe until it reaches thermodynamic equilibrium. However, as long as the nature of dark energy remains unknown, along with the properties of the earliest moments of the universe, it would be more appropriate and reasonable to suspend judgment on its fate.

In the equations of general relativity, there is a quantity called the "cosmological constant" (denoted by the Greek letter Λ, meaning lambda), which is the mathematical symbol for dark energy. What we call "dark energy" is perhaps the energy of empty space, also called the "quantum vacuum." Ultimately, it all comes down to quantum fields. Indeed, all the different particles that make up matter and energy—quarks, electrons, photons, the Higgs boson—are simply localized excitations of these fields.

However, removing all particles from a given volume of space doesn't also remove the field. The field is then said to remain in its "quantum vacuum state," but within it there are still virtual particles that continually appear and annihilate in the vacuum, borrowing energy from the surrounding environment when they are created and returning it when they vanish.

However, having found a mathematical symbol for dark energy certainly doesn't mean we've understood its nature. Astronomical measurements suggest a certain numerical value for the cosmological constant, but, just like the mass of the Higgs boson in the Standard Model, we don't know why it has that value. This is a classic model calibration problem, a long-standing issue in physics, although the issue seems to be worse than that. The discrepancy between the quantum vacuum energy calculated in field theory and that observed with cosmological measurements is so large that it represents one of the unsolved problems in physics. In fact, the calculated value is 120 orders of magnitude larger than the observed one.

The best cosmological model we currently have, incorporating everything we know about dark matter and energy, is called the "Lambda-CDM" model, or λCDM. And, just as there is a loose correlation between quantum field theory and the Standard Model, there is also a similar loose correlation between general relativity and the λCDM cosmological model.

Furthermore, there is another important ingredient in the λCDM model that most cosmologists argue is necessary to explain the properties of the universe. It is called "cosmic inflation," and it provides an answer to the age-old question: how did the universe and all the matter and energy it contains come into being?