Dark energy in the early universe could solve two major problems in cosmology

A new study by an international team of physicists proposes that a mysterious force known as “early dark energy” could solve two of the biggest puzzles in cosmology, and fill in some major gaps in our understanding of how the early universe evolved.

The two puzzles in question are the “Hubble tension,” which refers to a mismatch in measurements of how fast the universe is expanding; and the observation of numerous early, bright galaxies, at a time when the early universe should have been much less populated.

Now, the team of researchers, which includes Cavendish astrophysicists, has found that both puzzles could be resolved if the early universe had one extra, fleeting ingredient: early dark energy.

The teams’ results, reported in the Monthly Notices of the Royal Astronomical Society, are the first to show that if early dark energy existed in the infant universe, it would have influenced the formation of galaxies, and the expansion of the universe, enough to explain both the Hubble tension and the observations of early bright galaxies.

Dark energy is an unknown form of energy that physicists suspect is driving the expansion of the universe today. Early dark energy is a similar, hypothesized phenomenon that may have had only a brief appearance, influencing the expansion of the universe in its first moments, before disappearing entirely.

“By introducing Early Dark Energy (EDE), we can solve the Hubble tension and also accommodate for the high density of galaxies in the first billion years of cosmic time, recently discovered by the James Webb Space Telescope,” explained Dr Sandro Tacchella, assistant professor at the Cavendish Laboratory and co-author of the study.

EDE is considered to be a sort of anti-gravitational force that was acting only in the early universe. 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 is therefore considered a possible solution to the Hubble tension.

The current, standard galaxy formation models predict that the universe should have taken its time in forming the first galaxies. It would have taken about a billion years for primordial gas to coalesce into galaxies as large and bright as the Milky Way.

However, in 2023, the James Webb Space Telescope (JWST) made a startling observation. With an ability to peer farther back in time than any observatory to date, the telescope uncovered a surprising number of bright galaxies as bright as the modern Milky Way within the first 500 million years, when the universe was just 3 percent of its current age.

For physicists, the observations imply that either there is something fundamentally wrong with the physics underlying the models, or there is a missing ingredient in the early universe that scientists have not accounted for. The international team of physicists explored the possibility of the latter, and whether the missing ingredient might be early dark energy.

“What we show is, EDE alters the structure of the early universe in a subtle way where the amplitude of fluctuations goes up, and you get bigger dark matter halos and brighter galaxies in earlier times, more so than in our more vanilla models,” said co-author Dr Rohan Naidu, postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “It means things were more abundant, and more clustered in the early universe.”

Dark matter halos, regions of space where gravity happens to be stronger and where matter begins to accumulate, seem to form the ‘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,” Kavli postdoc Dr Xuejian (Jacob) Shen explains.

The model developed by the team for early galaxy formation, the same one that happens to resolve the Hubble tension, predicts the number, luminosity, and size of galaxies that should form in the early universe, given some measures of ‘cosmological parameters.’ Cosmological parameters are the basic ingredients, or mathematical terms, that describe the evolution of the universe.

Physicists have determined that there are at least six main cosmological parameters, one of which is the Hubble constant — a term that describes the universe’s rate of expansion. Other parameters describe density fluctuations in the primordial soup, immediately after the Big Bang, from which dark matter halos eventually form.

The team reasoned that if early dark energy affects the universe’s early expansion rate, in a way that resolves the Hubble tension, then it could affect the balance of the other cosmological parameters, in a way that might increase the number of bright galaxies that appear at early times.

“We do indeed observe that EDE changes the cosmological parameters, leading to early galaxy formation and therefore, without major modifications, we can solve two challenges without creating disagreements with other observations,” said Tacchella. 

“A priori, I would not have expected the abundance of JWST’s early bright galaxies to have anything to do with EDE, but their observation that EDE pushes cosmological parameters in a direction that boosts the early-galaxy abundance is interesting,” said Marc Kamionkowski, professor of theoretical physics at Johns Hopkins University, who was not involved with the study. “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 a fruitful—thing to try.”

“Although these results are fascinating, it is important to keep in mind that even the EDE model is just a simple model that is consistent with the data, and more detailed observations need to be collected to further constrain this model and possibly rule it out in the future. One possible way to do this is through better and more complete observations of early galaxies probing the first few hundred million years after the Big Bang,” concludes Tacchella.

Reprinted with permission of MIT News, this story was adapted by the Cavendish Laboratory. Read the original article. 

Reference: Dr Xuejian Shen 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. 2024. DOI:10.1093/mnras/stae1932

Lead Author: Dr Xuejian (Jacob) Shen, MIT Kavli Institute

Co-Authors: Professor Mark Vogelsberger, MIT; Professor Michael Boylan-Kolchin, University of Texas at Austin; Assistant Professor Sandro Tacchella, Cavendish Laboratory; Dr Rohan Naidu, MIT Kavli Institute.

Main Image:  This image taken by the James Webb Space Telescope (JWST) focuses on a region studied by the JWST Advanced Deep Extragalactic Survey (JADES) team. Researchers used JWST's NIRCam instrument to observe this field in nine different infrared wavelength ranges to discover some of the most distant galaxies currently known to us. In this image blue represents light at 1.15 microns (115W), green is 2.0 microns (200W), and red is 4.44 microns (444W).

 Credit: NASA, ESA, CSA, B. Robertson (UC Santa Cruz), S. Tacchella (University of Cambridge, B. Johnson (Center for Astrophysics, Harvard & Smithsonian), M. Rieke (Univ. of Arizona), D. Eisenstein (Center for Astrophysics, Harvard & Smithsonian).