Of all the mysteries facing astronomers and cosmologists today, the “Hubble Tension” remains persistent! This term refers to the apparent inconsistency of the Universe’s expansion (aka. the Hubble Constant) when local measurements are compared to those of the Cosmic Microwave Background (CMB). Astronomers hoped that observations of the earliest galaxies in the Universe by the James Webb Space Telescope (JWST) would solve this mystery. Unfortunately, Webb confirmed that the previous measurements were correct, so the “tension” endures.
Since the JWST made its observations, numerous scientists have suggested that the existence of Early Dark Energy (EDE) might explain the Hubble Tension. In a recent study supported by NASA and the National Science Foundation (NSF), researchers from the Massachusetts Institute of Technology (MIT) suggested that EDE could resolve two cosmological mysteries. In addition to the Hubble Tension, it might explain why Webb observed as many galaxies as it did during the early Universe. According to current cosmological models, the Universe should have been much less populated at the time.
The research was led by Xuejian Shen and his colleagues from the Department of Physics and the Kavli Institute for Astrophysics and Space Research (MTK) at MIT. They were joined by researchers from the NSF AI Institute for Artificial Intelligence and Fundamental Interactions (IAIFI) at MIT, the University of Texas at Austin, and the Kavli Institute for Cosmology (KICC) and Cavendish Laboratory at the University of Cambridge. The paper detailing their findings was recently published in the Monthly Notices of the Royal Astronomical Society.
To recap, Dark Energy is the theoretical form of energy that is believed to be driving the expansion of the Universe today. The theory first emerged in the 1990s to explain observations by the venerable Hubble Space Telescope, which showed that cosmic expansion appeared to be accelerating over time. EDE is similar but is thought to have briefly appeared shortly after the Big Bang, which disappeared after influencing the expansion of the early Universe. Like Dark Energy, this force would have counteracted the gravitational pull of early galaxies and temporarily accelerated the expansion of the Universe.
The existence of this energy would also explain why measurements of the Hubble Constant are inconsistent with each other. Short of General Relativity being wrong (despite being proven repeatedly for over a century), EDE is considered the most likely solution to the Hubble Tension. Similarly, Webb’s 2023 observations uncovered a surprising number of bright galaxies just 500 million years after the Big Bang that were comparable in size to the modern Milky Way. These findings challenge conventional models of galaxy formation, which predict that galaxies take billions of years to form and grow.
For their study, the team focused on the formation of “Dark Matter Halos,” the hypothetical region that allows protogalaxies to accumulate gas and dust, leading to star formation and growth. As when said in a recent MIT News story:
“The bright galaxies that JWST saw would be like seeing a clustering of lights around big cities, whereas theory predicts something like the light around more rural settings like Yellowstone National Park. And we don’t expect that clustering of light so early on. We believe that dark matter halos are the invisible skeleton of the universe. Dark matter structures form first, and then galaxies form within these structures. So, we expect the number of bright galaxies should be proportional to the number of big dark matter halos.”
The team developed an empirical framework for early galaxy formation that incorporated the six main “cosmological parameters”—the basic mathematical terms that describe the evolution of the Universe. This includes the Hubble Constant, which describes cosmic expansion, while parameters describe density fluctuations immediately after the Big Bang, from which dark matter halos formed. The team theorized that if EDE affects early cosmic expansion, it could also affect other parameters that might explain the appearance of many large galaxies shortly thereafter.
To test their theory, the team modeled the formation of galaxies within the first few hundred million years of the Universe. This model incorporated EDE to determine how early dark matter structures evolved and gave rise to the first galaxies in the Universe. As study co-author Rohan Naidu, a postdoc with MKI, explained:
“You have these two looming open-ended puzzles. We find that in fact, early dark energy is a very elegant and sparse solution to two of the most pressing problems in cosmology. What we show is, the skeletal structure of the early universe is altered in a subtle way where the amplitude of fluctuations goes up, and you get bigger halos, and brighter galaxies that are in place at earlier times, more so than in our more vanilla models. It means things were more abundant, and more clustered in the early universe.”
“We demonstrated the potential of early dark energy as a unified solution to the two major issues faced by cosmology,” added co-author Mark Vogelsberger, an MIT professor of physics. “This might be an evidence for its existence if the observational findings of JWST get further consolidated. In the future, we can incorporate this into large cosmological simulations to see what detailed predictions we get.”