In a follow-up article I will review the results and implications of the most recent discoveries about dark energy.
Dark energy refers to the as-yet unobserved energy (called “dark” in that we cannot see it directly) and unknown (we’re still “in the dark” as to its actual nature) that has been proposed to explain the accelerated expansion rate of the universe.
For the past quarter-century, we have also known that this expansion is accelerating….But what is the mysterious “dark energy” that drives this acceleration?
The simplest explanation involves what Einstein dubbed a “cosmological constant” (Λ) and implies that dark energy is a constant energy inherent to spacetime itself.
An enormous mismatch exists between the observed value of dark energy, when expressed as the cosmological constant in Einstein’s field equations of general relativity, and the prediction for dark energy considered as due to the vacuum energy calculated from quantum field theory.
One of the most amazing accomplishments in humanity’s quest for scientific understanding has been our success in comprehending the origin and subsequent unfolding of the entire observable universe. Even though humans have lived under the “starry heavens” since our earliest days, scientific comprehension of the universe lingered beyond our reach until the last century.
Technological advancements that enabled us to construct and use telescopes with enough light-gathering power to resolve distant galaxies became available to astronomers in the 1920s. Evidence of intelligent design comes with the additional factor of discoverability afforded us by our “privileged” location in the history of the universe. As Richards and Gonzalez write in The Privileged Planet,
…earthlike planets are exquisitely fit not only to sustain life but to provide the best platform to discover the hidden wonders of the cosmos.
Discovery proceeded apace in the early 1900s when spectroscopic instruments, coupled with a developing quantum theory of atoms, allowed for quantitative measurements of specific wavelengths of light and accurate interpretation of individual photons speeding towards us from distant stars.
New Opportunities for Refinement
Each advance in astronomical instrumentation has offered new opportunities to refine our understanding of the cosmos and our place in it. Over the last few years, an ambitious mapping of millions of galaxies throughout space and time has aimed to shed light on the dark energy that is surmised to supply the motive force for the accelerated expansion of our universe. In a follow-up article I will review the results and implications of the most recent discoveries about dark energy.
Dark energy refers to the as-yet unobserved energy (called “dark” in that we cannot see it directly) and unknown (we’re still “in the dark” as to its actual nature) that has been proposed to explain the accelerated expansion rate of the universe. This unexpected acceleration of how fast distant galaxies are moving apart was first discovered in 1998 through observations of Type 1a supernovae in multiple galaxies. The 2011 Nobel Prize in Physics recognized the significance of this cosmic discovery.
Since the first observational evidence of the expansion of the universe was discovered by Edwin Hubble and other astronomers, it was reasoned that the rate of expansion would show slowing over cosmic time, due to the mutual gravitational attraction of all the matter in the universe. Careful measurements of the expansion rate for about the first half of the history of the universe agreed with this prediction, but then the unexpected showed up. As often happens, nature has a way of sidestepping our assumptions and often gives us a more elegant universe to consider, as I describe in Canceled Science:
Measurements suggest that the expansion rate indeed began to slow initially, but then the rate began to accelerate. Imagine if you threw a ball straight up into the air, and after slowing for a bit in its upward trajectory, it suddenly began to pick up speed and accelerated faster and faster out of sight. You might be surprised, to say the least.
An Enormous Mismatch
A factor known as the cosmological constant, first introduced by Einstein in his general relativity equations to attempt to cancel out an unexpected prediction of an expanding (or contracting) universe, was reinstated by modern cosmologists to explain the surprising acceleration of expansion discovered in 1998.
For nearly a century, we have known that our Universe is expanding. For the past quarter-century, we have also known that this expansion is accelerating….But what is the mysterious “dark energy” that drives this acceleration? The simplest explanation involves what Einstein dubbed a “cosmological constant” (Λ) and implies that dark energy is a constant energy inherent to spacetime itself.
An enormous mismatch exists between the observed value of dark energy, when expressed as the cosmological constant in Einstein’s field equations of general relativity, and the prediction for dark energy considered as due to the vacuum energy calculated from quantum field theory. The mismatch has been described as
…arguably the most severe theoretical problem in high-energy physics today, as measured by both the difference between observations and theoretical predictions, and by the lack of convincing theoretical ideas which address it. Geraint F. Lewis and Luke A Barnes, A Fortunate Universe: Life in a Finely Tuned Cosmos (Cambridge: Cambridge University Press, 2016), 159, citing a university cosmology text.
A non-zero cosmological constant is consistent with the observed acceleration of the expansion rate of the universe. Its value can be estimated from general relativity theory and measured values of the density and expansion rate of the universe, yielding a numerical energy density of space (cosmological constant) of 3.3 GeV/m3. (For comparison, this is approximately equal to the mass-energy of three or four hydrogen atoms per cubic meter of space.)
Measurement and Theory
Theoretical predictions of the quantum fluctuations of the vacuum of space also give a non-zero value of vacuum energy (meaning that every cubic meter of empty space actually possesses energy). Calculating the value of this energy per cubic meter and comparing it to the measured value stated above shows that the theoretical prediction is about 120 orders of magnitude larger! To reconcile measurement and theory, then, seems to require some unknown additional term in the theory that would carefully cancel out (to 120 decimal places) the far-too-large prediction of quantum field theory. This is an extremely tall order, and would only work if the hypothesized term was “fine-tuned to unimaginable precision.”
The nature of dark energy may be a mystery, but it brings to light one of the most powerful pieces of evidence for just how special our universe is to support life.
