Dark Energy May Be Evolving
Three mostly independent catalogs of supernovae indicate that dark energy, thought to be constant, may actually be weakening.
A major recent study by the Dark Energy Spectroscopic Instrument (DESI) team, indicates that dark energy, the mysterious force causing the universe to expand at an ever-accelerating rate, may be slightly weakening over time. If confirmed, the findings could have major implications for the foundations of physics and theories describing the evolution of the universe.
No one really knows what dark energy is. First discovered in 1998, many thoughtful, elegant, rigorous, and detailed theories have failed to explain what dark energy is or to accurately predict it’s magnitude. Most physicists currently think that dark energy is a characteristic of spacetime, an intrinsic energy of space, or perhaps a pervasive field similar to the Higgs field.
It is distributed evenly throughout the universe, not only in space but also in time – in other words, its effect is not diluted as the universe expands. The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.
Dark energy has been thought to be constant in space and time, fitting into Einstein’s theories as a so-called Cosmological Constant.
The DESI team has potentially upended that assumption by unveiling a map and measurements of the cosmos with unprecedented detail and scope. The graph getting the most attention shows that three mostly independent sets of supernovae observations all point in the same direction: dark energy may have eroded over the eons. If the uncertainty between observations were random, the data would probably point in different directions.
The method relies on observations of variations formed by the passage of sound waves through the matter of the early universe. Tiny fluctuations in the early plasma moved protons and neutrons, called baryons, into a pattern of ripples and fractals that similar to what you’d see if you tossed a handful of gravel into a pond or watched wind blow over sand. As the universe expanded and cooled, atoms formed, the waves dissipated, and the fingerprints of the ripples and bubbles were left all over the universe, spreading in three dimensions. What started as small fluctuations in a plasma stew created clusters of entire galaxies. The faint pattern of ripples and bubbles left behind is referred to as Baryon Acoustic Oscillations (BAOs).
The BAO measurements are therefore a cosmic ruler. Astrophysicists can measure the size of these bubbles and determine distances to the matter responsible for the pattern in the sky. When the create a 3D map of the bubbles at near and extreme distances, they can see how fast the universe was expanding over time and model at how dark energy affected that expansion.
The DESI team playfully annotated a plot showing the “bubble” analysis with cartoon captions to the rest of us can at least get closer to understanding it.
Researchers, both those involved with the study and many independent from it, are all quick to urge caution before getting too excited. Physics as a field has extremely high thresholds for declaring a discovery or amending the laws that govern all else. In the highly controlled environments of high-energy physics labs and for astronomers with trillions of stars to sample in the night sky, there is plenty of room to describe observations with high certainty. The observations showing the evolution of dark energy could very well evaporate into statistical insignificance with more data. This study is compelling, however, and it’s a critical first step. But that three separate catalogs of exploding stars are in agreement is tantalizing.
The same physicists that were involved in the recent study, along with an army of new recruits intent on chasing reality, will now vigorously try to break the hypothesis that the DESI map and data are indicating. They will try to prove it wrong, to their best of their ability, holding to the principle that extraordinary claims require extraordinary evidence. They will ask hard questions, challenge their own assumptions, gather as much data as possible, attempt to replicate the results, and stress test the observations with new methods. The signals of dark energy evolution may vanish, or they may hold up to the cold-hearted rigor of the scientific method and redefine the mystery that dominates the universe. Either way, there will be beauty in that process too.