Could the concept of 'Early' Dark Energy provide a solution to the enigma surrounding the expansion of the universe?

Scientists have known for over a century that the universe has been expanding since the big bang, an event that occurred 13.8 billion years ago and marked the beginning of everything.

However, a puzzling issue remains unsolved: the rate at which the universe is expanding. This uncertainty arises from the difference between the rate calculated from the cosmic microwave background radiation, a remnant of the big bang, and the faster expansion rate determined from supernova observations. This difference in expansion rates is known as the 'Hubble Tension,' referring to the Hubble Constant, which quantifies the universe's rate of expansion.

Scientists have long believed that the ongoing expansion of the universe is driven by a mysterious force known as dark energy, which seems to have started reversing the universe's deceleration about 7 or 8 billion years after the big bang.

What Is Dark Energy?

"Dark energy is a theoretical form of energy in the universe that, based on our current understanding, constitutes roughly 70 percent of the total energy in the universe," explains Glenn Starkman, a distinguished professor and co-chair of the physics department at Case Western Reserve University.

"The key evidence for dark energy's existence comes from the accelerating expansion of the universe, which has been ongoing for several billion years," Starkman says. "To cause such an expansion requires an energy source that doesn't dilute much, if at all, as the universe grows. This rules out most energy sources, like ordinary matter or dark matter, both of which become less dense as the universe expands. The simplest model of dark energy is that it represents the constant energy density tied to empty space. Therefore, as space expands, dark energy's density remains unchanged."

However, there are still many unanswered questions about dark energy, including why it didn't exist from the beginning. Even with dark energy's inclusion in the standard model, it doesn't explain the discrepancy between two measurements of cosmic expansion.

What about Early Dark Energy?

Recent studies, set to be published soon, present exciting findings. These studies, based on data collected by the Atacama Cosmology Telescope (ACT) between 2013 and 2016, may offer clues to solving a longstanding puzzle. Researchers have uncovered hints of an early form of dark energy, which existed in the universe's first 300,000 years. This discovery was first highlighted in a Nature article by Davide Castelvecchi, detailing two separate papers—one from the ACT team and another from a different group that included astrophysicist Vivian Poulin of the University of Montpellier, along with Tristian L. Smith and Alexa Bartlett from Swarthmore College.

The concept of early dark energy was first proposed by Vivian Poulin, a former postdoctoral researcher at Johns Hopkins University, alongside Tristian L. Smith and others. This idea emerged as a potential solution to address some unresolved questions in cosmology.

"Early dark energy represents a different kind of dark energy, distinct from the one responsible for the current acceleration of the universe's expansion," says Starkman. EDE, as it is known, would have significantly impacted the universe when it was much smaller and hotter—about 10,000 times smaller than it is today. This concept has been suggested to help resolve the contradictions surrounding the universe's expansion rate over time.

As explained in the Nature article, early dark energy wouldn't have been powerful enough to directly accelerate the universe's expansion billions of years later. However, it would have had an indirect effect by influencing the rate at which the plasma from the early universe cooled down. This would, in turn, alter how measurements of the cosmic microwave background are interpreted—particularly those related to the age and expansion rate of the universe based on the travel of sound waves through the plasma before it turned into gas—leading to a faster expansion rate in alignment with current astronomical calculations.

Early dark energy presents a challenging theoretical puzzle, but as theoretical physicist Mark Kamionkowski from Johns Hopkins University, one of the authors of the 2018 early dark energy paper, told Nature, "it's the only model we can get to work."

The Final Answer Remains Uncertain

The two studies could strengthen the argument for early dark energy, but one of the researchers involved admits he is not fully convinced and stresses the need for more research to reach a definitive conclusion.

"I have always been skeptical about early dark energy models due to issues they face in reconciling with precise measurements of the universe's large-scale distribution of galaxies and matter ('large-scale structure,' or LSS)," explains J. Colin Hill, assistant professor of physics at Columbia University and co-author of the ACT team's study, in an email. (Hill's doubts are further detailed in a 2020 paper he co-authored, as well as a later paper and another by different researchers addressing similar concerns.)

"The conclusion from the three papers referenced above is that early dark energy models that match the CMB data and the Riess et al. H0 data lead to predictions for LSS that contradict the data from these surveys," Hill writes in the email. "Therefore, we concluded that an alternative theoretical model is likely required, or at least some adjustments to the early dark energy scenario."

In the latest study that Hill and his ACT colleagues recently released, they chose not to include LSS data in their analysis. Instead, their focus was primarily on CMB data. "The primary objective was to examine whether the Planck and ACT CMB data provided consistent results within the context of early dark energy. We discovered that they produced somewhat different outcomes, which is a significant puzzle we are now diligently trying to understand. From my point of view, the LSS issue related to the early dark energy scenario remains unresolved," says Hill.

"Furthermore, the Planck data on its own, which is still the most accurate dataset in cosmology, does not show any preference for early dark energy," Hill notes. "Therefore, even though the ACT data hints at early dark energy, I remain cautious about whether this model could truly be the final explanation. We'll need more data to determine that for sure."

If it truly existed, early dark energy would have been analogous to the force believed to be driving the current accelerated expansion of the universe. However, it would require a major revision of the theoretical model to fit into this context.

"The key distinction is that early dark energy must have only played a role for a short time in the history of the cosmos and then 'disappeared'," Hill explains. "To accomplish this, we create particle physics models involving a new field (specifically, an axion-like field) that briefly accelerates the universe's expansion before recombination, then rapidly fades away and becomes irrelevant."

"In contrast, the current leading theory for standard dark energy is that it is simply a cosmological constant, likely originating from vacuum energy," Hill continues. "This type of energy doesn't change over time. However, it's possible that standard dark energy could arise from some fundamental field we have yet to comprehend. If that's the case, it may evolve over time and might share some similarities with the early dark energy model discussed earlier."

Hill emphasizes that to address these questions more accurately, further data will be required. He remains hopeful that answers will be uncovered in the next decade. Fortunately, many advanced experiments are set to come online shortly. He specifically mentions observatories such as the Simons Observatory, which will focus on studying the cosmic microwave background (CMB), as well as the Rubin Observatory, and the Euclid and Roman space telescopes, which are set to provide fresh insights into large-scale structure (LSS). "It should be very exciting to see what we find," Hill adds.

Hill offers a YouTube video where he discusses the concept of early dark energy.

Starkman stresses the need for caution with such "extraordinary" claims, unless the evidence is substantial and undeniable. He points out that there is also evidence contradicting the theory of early dark energy (EDE). The existing results highlight growing discrepancies between two experimental data sets related to cosmic microwave background observations—one from the European Space Agency's Planck satellite, which operated in the early 2010s, and the other from the current Atacama Cosmology Telescope. The former does not support EDE, while the latter now seems to. Such inconsistencies between experiments are common and frustrating. It may be tempting to think that additional data from the Atacama Cosmology Telescope could resolve the issue, but merely adding more ACT data won’t clarify why Planck’s data doesn’t support EDE. This situation likely requires a reassessment of one of these experiments in order to provide a clear resolution.

Wendy Freedman, a professor of astronomy and astrophysics at the University of Chicago, who has contributed to studies on cosmic expansion, believes that exploring alternative models is crucial.

The Lambda Cold Dark Matter (LCDM) Model

Freedman, the author of an article published on Sept. 17, 2021, in The Astrophysical Journal, writes in an email: "We currently follow the lambda cold dark matter (LCDM) model as our standard cosmological model. In this framework, about one-third of the universe's total matter and energy density is attributed to matter, predominantly dark matter, while the remaining two-thirds come from dark energy."

Freedman continues, stating, "At present, we still do not understand the exact nature of dark matter or dark energy. Nevertheless, LCDM fits extremely well with a broad range of experiments and observations. Given our current level of understanding, it is vital to continue testing the standard model. The apparent discrepancy between the Hubble constant values obtained from CMB measurements and certain local measurements could be pointing towards new physics. This is why I emphasize the need to explore models beyond lambda CDM."

However, Freedman adds a crucial note: "Alternatively, an unknown systematic error could be responsible for the observed discrepancy. Thus, it is also essential to reduce the uncertainties in the present Hubble constant measurements."