Astronomers Uncover New Clues About the Nature of Dark Energy
An international team of astronomers has presented the most compelling evidence yet suggesting that dark energy, the enigmatic force driving the accelerating expansion of our universe, may not be a constant phenomenon. Instead, it could be a dynamic force that fluctuates over cosmic time.
Implications for the Universe’s Fate
These novel measurements propose that dark energy might not condemn the universe to an ultimate “Big Rip,” where everything, from vast galaxy clusters to individual atoms, is torn apart. It’s conceivable that its expansive influence could diminish, leading to a stable universe. Alternatively, the cosmos could even reverse its expansion, culminating in a “Big Crunch” – a collapse of the universe.
Challenging the Standard Cosmological Model
The latest findings reinforce intriguing hints from prior observations, suggesting potential flaws in the standard model of cosmology, the prevailing theory describing the universe’s history and structure. These new measurements, obtained recently, are the result of the Dark Energy Spectroscopic Instrument (DESI) project, utilizing a telescope at Kitt Peak National Observatory in Arizona.
DESI Data Sparks Debate
“It’s now more than just a hint,” stated Michael Levi, a cosmologist at Lawrence Berkeley National Laboratory and director of DESI. “This discovery puts us at odds with other measurements. However, if dark energy evolves, it could reconcile these discrepancies remarkably well.”
Conflicting Findings and the Hubble Tension
The announcement was made at an American Physical Society meeting in Anaheim, California, accompanied by research papers submitted for peer review in the journal Physical Review D.
Adam Riess, an astrophysicist at Johns Hopkins University and the Space Telescope Science Institute, and a Nobel laureate for his role in discovering dark energy, commented via email: “It is reasonable to assert that this result, at face value, represents the most significant indication regarding the essence of dark energy in the roughly 25 years since its discovery.”
Interestingly, while DESI observations challenge the standard cosmological model, separate findings have bolstered it. The Atacama Cosmology Telescope team in Chile, prior to its decommissioning in 2022, released the most detailed images ever captured of the early universe, dating back to a mere 380,000 years after the Big Bang.
Their report, still awaiting peer review, appears to validate the standard model’s operation in the nascent universe. A key element within this model, the Hubble constant, quantifies the universe’s expansion rate. However, measurements of this constant over the past half-century have shown significant disagreements, a puzzle known as the Hubble tension. Some theorists have speculated that an early burst of dark energy, before atoms formed, might resolve this tension.
Ruling Out Early Dark Energy Spurt
The latest Atacama results seemingly contradict the idea of an early dark energy spurt resolving the Hubble tension. However, they do not preclude the possibility of dark energy’s nature changing later in cosmic history.
Both sets of findings have been met with considerable acclaim from other cosmologists, who also expressed a sense of cosmic uncertainty regarding their collective meaning.
Wendy Freedman, a cosmologist at the University of Chicago, remarked, “I believe few viable explanations for the Hubble tension remain at this juncture.”
Michael Turner, a theorist also at the University of Chicago and the originator of the term “dark energy,” commented, “The good news is, no cracks in the cosmic egg [the standard model]. The bad news is, no cracks in the cosmic egg.”
Dr. Turner elaborated that if a crack exists, “it hasn’t widened sufficiently for us to clearly discern the next major breakthrough in cosmology.”
The Expanding Universe: Analogy and Discovery
Astronomers often illustrate the expanding universe by comparing galaxies to raisins in rising bread dough. As the dough expands, the raisins move further apart, with more distant raisins separating at a faster rate.
In 1998, two astronomical teams measured the universe’s expansion by analyzing the luminosity of type Ia supernovae, or exploding stars. These supernovae emit a consistent amount of light, thus appearing fainter at greater distances. If the universe’s expansion were decelerating, as previously thought, light from distant supernovae should have appeared slightly brighter than predicted.
Surprisingly, both teams discovered that these supernovae were fainter than anticipated, indicating that the universe’s expansion was not slowing down, but rather accelerating.
The Mystery of Dark Energy
No known form of energy in physics can account for an accelerating expansion. The strength of conventional energy would diminish as it disperses across the expanding universe. This suggests that the energy might originate from space itself.
This dark energy exhibited characteristics reminiscent of the cosmological constant, a concept Albert Einstein introduced into his theory of gravity in 1917. Einstein’s “fudge factor” aimed to explain why the universe wasn’t collapsing under its own gravity. The cosmological constant represented a repulsive force to counterbalance gravity and stabilize the universe. However, upon learning of the universe’s expansion in 1929, Einstein famously abandoned the cosmological constant, calling it his “biggest blunder.”
Yet, quantum theory, in the 1950s, predicted that empty space possesses energy, generating a repulsive force akin to Einstein’s constant. For the last 25 years, this constant has been integrated into the standard cosmological model. This model describes a universe born 13.8 billion years ago from the Big Bang, composed of roughly 5% ordinary matter, 25% dark matter, and 70% dark energy. Crucially, the model does not define the fundamental nature of dark matter or dark energy.
If dark energy is indeed Einstein’s cosmological constant, the standard model predicts a bleak future: perpetual acceleration of the universe, leading to increasing darkness and isolation. Eventually, remote galaxies will become invisible, and all energy, life, and thought will be extinguished from the cosmos.
‘Something to Investigate’
Astronomers in the DESI collaboration are working to characterize dark energy by observing galaxies across different epochs of cosmic time. Minute variations in matter distribution in the early universe have influenced the current distances between galaxies – distances that have expanded measurably with the universe.
The most recent DESI measurement utilized a catalog of nearly 15 million galaxies and other celestial objects. On its own, this data set does not strongly suggest deviations from the standard understanding of dark energy. However, when combined with other methods of measuring cosmic expansion – such as studying supernovae and the cosmic microwave background radiation emitted shortly after the Big Bang – the data begins to diverge from the standard model’s predictions.
The discrepancy between data and theory is currently at a level of 4.2 sigma (a statistical measure of uncertainty), indicating a roughly 1 in 50,000 chance of the results being accidental. This is below the 5-sigma threshold (1 in 3.5 million chance) conventionally required by physicists to declare a definitive discovery.
Nonetheless, this inconsistency is compellingly suggestive that aspects of the cosmological model are incomplete. Scientists may need to reconsider their understanding of gravity or re-evaluate interpretations of the cosmic microwave background light. DESI astronomers believe the key may lie in the nature of dark energy itself.
Mustapha Ishak-Boushaki, a cosmologist at the University of Texas at Dallas involved in the DESI analysis, stated, “Introducing a dynamic dark energy allows for a better alignment of the puzzle pieces.”
Will Percival, a cosmologist at the University of Waterloo and a DESI spokesperson, conveyed enthusiasm about future research. “This is somewhat of a boost for the field. We now have something concrete to investigate.”
Cosmology’s Expanding Toolkit
In the 1950s, it was believed that cosmology could be explained by just two parameters: the universe’s expansion rate and its deceleration. This changed in the 1960s with the discovery of the cosmic microwave background, the afterglow of the Big Bang. Analyzing this radiation enabled scientists to study the physics of the early universe and galaxy formation. The standard cosmological model now incorporates six parameters, including the densities of both ordinary and dark matter.
As cosmology has advanced in precision, tensions have emerged between predicted and observed values for these parameters, leading to numerous theoretical extensions to the standard model. However, the latest data from the Atacama Cosmology Telescope – the clearest maps of the cosmic microwave background to date – appear to limit many of these extensions.
DESI will continue data collection for at least another year. Other telescopes, both ground-based and space-based, are also mapping the cosmos, including the Zwicky Transient Facility, the European Euclid space telescope, and NASA’s SPHEREx mission. The Vera C. Rubin Observatory in Chile is scheduled to begin capturing a “motion picture” of the night sky this summer, and NASA’s Roman Space Telescope is slated to launch in 2027.
Each of these instruments will gather light from the sky, providing complementary perspectives on the cosmos and contributing to a more complete understanding of the universe. They serve as a constant reminder of the universe’s complex and challenging nature.
Alexie Leauthaud, a cosmologist at the University of California, Santa Cruz, and another DESI spokesperson, noted, “Each data set brings unique strengths. The universe is intricate, and we are striving to disentangle numerous interconnected phenomena.”