Could a Dark Energy Phase Change Relieve the Hubble Tension?

According to the most widely-accepted cosmological theories, the Universe began roughly 13.8 billion years ago in a massive explosion known as the Big Bang. Ever since then, the Universe has been in a constant state of expansion, what astrophysicists know as the Hubble Constant. For decades, astronomers have attempted to measure the rate of expansion, which has traditionally been done in two ways. One consists of measuring expansion locally using variable stars and supernovae, while the other involves cosmological models and redshift measurements of the Cosmic Microwave Background (CMB).

Unfortunately, these two methods have produced different values over the past decade, giving rise to what is known as the Hubble Tension. To resolve this discrepancy, astronomers believe that some additional force (like “Early Dark Energy“) may have been present during the early Universe that we haven’t accounted for yet. According to a team of particle physicists, the Hubble Tension could be resolved by a “New Early Dark Energy” (NEDE) in the early Universe. This energy, they argue, would have experienced a phase transition as the Universe began to expand, then disappeared.

The research was conducted by Martin S. Sloth, a professor of theoretical cosmology at the University of Southern Denmark (SDU) and the leader of the Center for Cosmology and Particle Physics Phenomenology–Universe Origins (CP3-Origins) research group; and Florian Niedermann, an assistant professor of cosmology at the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm and a previous postdoc in Sloth’s research group. Their research is described in a paper published on December 10th, 2022, in Physics Letters B.

The Hubble Tension comes down to two methods yielding different results, even though both are considered reliable. Both methods are based on the Lambda Cold Dark Matter (LCDM) and Big Bang Model of cosmology (aka. the Standard Model). This model states that the early Universe was dominated by radiation and matter – both baryonic (or “normal”) and Dark Matter. Roughly 380,000 years after the Big Bang, the radiation and normal matter were compressed into a hot dense plasma that is invisible to modern telescopes (the “Cosmic Dark Ages“).

If both methods are reliable, Sloth and Niedermann argue, then perhaps it’s the basis (not the methods) that are the problem. In their paper, Sloth and Niedermann propose that Dark Energy underwent a phase transition during the early Universe, shortly before it went from being in a hot dense state to what we see today. As the Universe expanded, the NEDE began to bubble in various places, which grew and eventually collided with each other. Said Niedermann in an SDU press release.

“This means that the dark energy in the early universe underwent a phase transition, just as water can change phase between frozen, liquid and steam. In the process, the energy bubbles eventually collided with other bubbles and along the way released energy.”

This phase transition, they add, could have lasted a very short amount of time (ca. 300,000 years) or about as long as it would take two particles to collide. When this NEDE is applied, Sloth and Niedermann obtain the same values for the Hubble Constant, regardless of the methods used. While this theory suggests that the Universe behaves in a way that is not consistent with the Standard Model, it does offer a possible resolution to the Hubble Tension. Said Sloth:

“[I]f we trust the observations and calculations, we must accept that our current model of the Universe cannot explain the data, and then we must improve the model. Not by discarding it and its success so far, but by elaborating on it and making it more detailed so that it can explain the new and better data. It appears that a phase transition in the dark energy is the missing element in the current Standard Model to explain the differing measurements of the Universe’s expansion rate.”

This study is one of several that attempts to resolve the Hubble Tension by theorizing that Dark Energy has behaved differently over time. These studies are paralleled by efforts to observe the cosmic period that occurred shortly after the Big Bang (visible as the CMB) and when the first stars and galaxies dispelled the Dark Ages (ca. 1 billion years later). And then there’s the ongoing search for Dark Matter and Dark Energy, which account for 85% of the mass and 68% of the total energy in the observable Universe.

All of these efforts aim to resolve the last issues of the Standard Model and bring our best cosmological theories and observations into harmony. These efforts are happening at both the theoretical and observational end of things, taking advantage of next-generation telescopes – like the James Webb Space Telescope, the ESA’ Euclid Observatory, and the 30-meter ground-based telescopes that will be operational in the coming years – machine learning, and sophisticated supercomputers that can simulate cosmic evolution over time.

Learning how the physical laws that govern the Universe fit together is absolutely crucial to unlocking the last of its secrets. And while we may not have it all figured out yet, we’re getting closer!

Further Reading: SDU, Physics Letters B