A new study from University of Chicago researchers suggests that dark energy, the mysterious force driving the universe's accelerated expansion, may not be a constant as long believed. Instead, data indicates it could be a dynamic phenomenon that has been weakening over the past several billion years, a finding that could reshape our understanding of the cosmos.
The research, published in the journal Physical Review D, combines data from major astronomical surveys to challenge a core principle of the standard model of cosmology. If correct, this evolving nature of dark energy could help resolve long-standing discrepancies in measurements of cosmic expansion.
Key Takeaways
- Researchers propose that dark energy is not a constant force but is evolving and may be weakening over time.
- The study combines data from the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI).
- Findings suggest dark energy's density has decreased by about 10% in the last few billion years.
- This new model could resolve the "Hubble Tension," a major puzzle in modern cosmology.
Revisiting the Cosmological Constant
For decades, the scientific community has worked with the understanding that the universe is expanding at an accelerating rate. This acceleration is attributed to dark energy, which is thought to make up roughly 70% of the universe's total energy density. The prevailing theory, part of the Lambda Cold Dark Matter (ΛCDM) model, treats dark energy as a cosmological constant—an unchanging force inherent to the fabric of space itself.
However, this model has faced challenges. A significant issue known as the "Hubble Tension" has emerged from conflicting measurements. Observations of the early universe, such as the Cosmic Microwave Background (CMB), suggest one expansion rate, while measurements of the more recent universe suggest a faster rate. This discrepancy has led scientists to question whether the cosmological constant is truly constant.
What is the Hubble Tension?
The Hubble Tension refers to the disagreement between the value of the Hubble Constant (the rate of universe expansion) as measured from the early universe (via the CMB) and the value measured from the local, or modern, universe (via objects like supernovae). This persistent conflict suggests that our standard model of cosmology might be incomplete.
Evidence from New Data
The new research, led by Anowar J. Shajib and Joshua A. Frieman, analyzed a comprehensive set of cosmological data. By combining information from the Dark Energy Survey and the Dark Energy Spectroscopic Instrument, they found that models where dark energy evolves over time provide a better fit for the observations than the standard model does.
"The evolving dark energy 'model' used in most previous data analyses is just a mathematical formula that isn't constrained to behave as physical models do," the researchers explained in a statement. "In our paper, we directly compare physics-based models for evolving dark energy to the data and find that these models describe the current data better than the standard, non-evolving dark energy model."
According to their analysis, the density of dark energy has decreased by approximately 10% over the last several billion years. While this is a much smaller change compared to the density of matter and other forms of energy, it is a significant deviation from the idea of a constant force.
A Model Based on Axions
To explain this dynamic behavior, Shajib and Frieman developed models based on theoretical physics. Their work proposes that dark energy could be related to a type of hypothetical particle known as an axion. These ultra-light particles, first theorized in the 1970s, have also been suggested as a candidate for dark matter.
In their scenario, a field of these axion-like particles would have remained constant for the first several billion years of the universe's history. Then, as the universe continued to expand and cool, this field began to evolve, causing its energy density to slowly decrease over time.
The Ultimate Fate of the Universe
The nature of dark energy is critical to predicting the long-term future of the cosmos. Different theories lead to vastly different outcomes:
- The Big Rip: If dark energy's repulsive force grew stronger over time, the expansion would accelerate uncontrollably, eventually tearing apart galaxies, stars, and even atoms.
- The Big Crunch: If dark energy weakened significantly or became attractive, gravity would eventually halt the expansion and cause the universe to collapse back in on itself.
- The Big Freeze: If the expansion continues, but perhaps at a slowing rate of acceleration, the universe will grow colder and darker as stars burn out and galaxies drift ever farther apart.
The model proposed by Shajib and Frieman suggests a future that avoids the two most violent extremes. A weakening dark energy implies that the acceleration of the universe will slow down. This points toward a Big Freeze scenario, where the universe continues to expand for trillions of years, ultimately becoming a vast, cold, and empty space.
"It's a bit embarrassing that we have little to no clue what 70% of the universe is," said Frieman. "And whatever it is, it will determine the future evolution of the Universe."
The Path Forward for Cosmology
While this study provides compelling evidence for a dynamic dark energy, the scientific community acknowledges that more data is needed for a definitive conclusion. The standard ΛCDM model has been remarkably successful for decades, and any proposed replacement must clear a high bar of evidence.
Future astronomical surveys are poised to provide the necessary clarity. The ongoing Dark Energy Spectroscopic Instrument (DESI) project and the upcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will map the universe with unprecedented precision. These powerful instruments will measure the distribution of galaxies and the history of cosmic expansion in great detail.
As the researchers noted, "Near-future surveys such as DESI and the Vera Rubin Observatory... will be able to definitively tell us whether these models are correct or if, instead, dark energy really is constant." The results from these observatories will be crucial in determining whether cosmology is on the verge of a major paradigm shift.