Cosmology’s two biggest puzzles: 1. Hubble tension, a mismatch in measurements of how fast the Universe is expanding. 2. The observations of bright galaxies that existed when the early Universe was less populated.
Now, MIT scientists suggest that early dark energy could solve both puzzles. Dark energy is believed to be a key driver for the expansion of the Universe today. Likewise, early dark energy is a theoretical phenomenon that influences the Universe’s expansion in its first moments before disappearing entirely.
Astrophysicists reconstructed the first moments of the universe
Some physicists suggest that early dark energy could be the key to solving the Hubble tension. It could accelerate the early expansion of the Universe by an amount that would resolve the measurement mismatch.
Scientists also found that early dark energy could explain the number of bright galaxies observed in the early Universe. Their study created a model simulating galaxy formation in the Universe’s first few hundred million years.
When they added dark energy in the very early moments of the Universe, they found that the number of galaxies matched what astronomers see today.
In 2023, NASA’s JWST uncovered many bright galaxies as large as the modern Milky Way within the first 500 million years, when the Universe was just 3 percent of its current age. These bright galaxies look like clusters of lights around big cities. However, theories predict that the light appears more likely in rural settings.
Kavli postdoc Xuejian (Jacob) Shen said, “And we don’t expect that clustering of light so early on.”
Based on observations, scientists think there is either something fundamentally wrong with the physics underlying the models or there must be a missing ingredient in the early Universe that scientists still need to consider. MIT scientists explored if that missing ingredient might be early dark energy.
Physicists have proposed that early dark energy is an antigravitational force that is turned on only very early times. This force would counteract gravity’s inward pull and accelerate the early expansion of the Universe in a way that would resolve the mismatch in measurements. Early dark energy, therefore, is considered the most likely solution to the Hubble tension.
In this study, the team studied how early dark energy might affect the first galaxies in the early Universe. They focused on the formation of dark matter halos — regions of space where gravity happens to be stronger and where matter begins to accumulate.
The team created a model for early galaxy formation based on observational data. After inputting some measures of “cosmological parameters,” the model can predict the number, luminosity, and size of galaxies that should form in the early Universe.
They found that there are at least six main cosmological parameters. The Hubble constant describes how fast the Universe is expanding. Other parameters describe density fluctuations in the primordial soup immediately after the Big Bang, from which dark matter halos eventually formed.
The MIT team thought that if early dark energy helps fix the Hubble tension by changing the early expansion rate of the Universe, it might also affect other factors, like the number of bright galaxies appearing early on. To test this, they used a model of early dark energy and applied it to a galaxy formation framework to see how the first dark matter structures developed into the earliest galaxies.
Co-author Rohan Naidu, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research, said, “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 clustered in the early Universe.”
Marc Kamionkowski, professor of theoretical physics at Johns Hopkins University, who was not involved with the study, said, “A priori, I would not have expected the abundance of JWST’s early bright galaxies to have anything to do with early dark energy, but their observation that EDE pushes cosmological parameters in a direction that boosts the early-galaxy abundance is interesting. I think more work will need to be done to establish a link between early galaxies and EDE, but regardless of how things turn out, it’s a clever — and hopefully ultimately fruitful — thing to try.”
“We demonstrated the potential of early dark energy as a unified solution to the two major issues cosmology faces. This might be evidence for its existence if the observational findings of JWST get further consolidated,” Vogelsberger concludes. “In the future, we can incorporate this into large cosmological simulations to see what detailed predictions we get.”
Journal Reference:
- Xuejian Shen, Mark Vogelsberger et al. Early galaxies and early dark energy: a unified solution to the Hubble tension and puzzles of massive bright galaxies revealed by JWST. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stae1932