According to the most widely held cosmological theories, the universe came into existence approximately 13.8 billion years ago in a massive explosion known as the Big Bang. Since then, the universe has been in a constant state of expansion, known to astrophysicists as the Hubble constant. For decades, astronomers have tried to measure the expansion rate, which has traditionally been done in two ways. One consists of local measurement of the expansion 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, resulting in the so-called Hubble voltage. To resolve this discrepancy, astronomers believe that an additional force (like “Early Dark Energy”) may have been present during the early Universe that we have not yet accounted for. 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 undergone a phase transition as the universe began to expand and then disappeared.
The research was conducted by Martin S. Sloth, Professor of Theoretical Cosmology at the University of Southern Denmark (SDU) and leader of the Center for Cosmology and Particle Physics Phenomenology – Universe Origins (CP3-Origins) research group; and Florian Niedermann, assistant professor of cosmology at the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm and former postdoc in Sloth’s research group. Their research is detailed in an article published in Physics Letters B on December 10, 2022.
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The Hubble voltage boils down to two methods that give different results, although both are considered reliable. Both methods are based on the Lambda Cold Dark Matter (LCDM) and the Big Bang model of cosmology (also known as the Standard Model). This model states that the early universe was dominated by radiation and matter – both baryonic (or “normal”) and dark matter. About 380,000 years after the Big Bang, radiation and normal matter were compressed into a hot, dense plasma invisible to modern telescopes (the “Cosmic Middle Ages”).
If both methods are reliable, Sloth and Niedermann argue, then perhaps the basis (not the methods) is the problem. In their paper, Sloth and Niedermann propose that dark energy underwent a phase transition during the early Universe, just before going from a hot, dense state to what we see today. As the universe expanded, the NEDE began to seethe in various places that grew and eventually collided with each other. Niedermann said that in a press release from the SDU.
“This means that dark energy underwent a phase transition in the early Universe, just as water can transition between frozen, liquid and vapor phases. Eventually, the energy bubbles collided with other bubbles, releasing energy in the process.”
This phase transition, they add, could have lasted only a very short time (about 300,000 years), or about as long as it would take for two particles to collide. Applying this NEDE, Sloth and Niedermann get the same values for the Hubble constant, regardless of the methods used. While this theory suggests that the universe behaves in ways that are inconsistent with the Standard Model, it offers a possible solution to the Hubble voltage. said sloth:
“[I]If we trust the observations and calculations, we have to accept that our current model of the universe cannot explain the data, and then we have to improve the model. Not by discarding it and its past success, but by fleshing it out and making it more detailed so it can explain the new and better data. It appears that a dark energy phase transition is the missing element in the current Standard Model to explain the differing measurements of the expansion rate of the Universe.”
The Cerro Tololo Inter-American Observatory in the Chilean Andes, home of the Dark Energy Survey (DES). Photo credit: Andreas Papadopoulos
This study is one of several trying to solve the Hubble tension by theorizing that dark energy has behaved differently over time. Accompanying these studies are efforts to observe the cosmic period that occurred just after the Big Bang (visible as CMB) and when the first stars and galaxies ended the Dark Ages (about 1 billion years later). And then there is the ongoing search for dark matter and dark energy, which make up 85% of the mass and 68% of the total energy in the observable universe.
All of these efforts are aimed at solving the Standard Model’s ultimate problems and reconciling our best cosmological theories and observations. This effort takes place on both the theoretical and observational ends of things, and utilizes next-generation telescopes — like the James Webb Space Telescope, ESA’s Euclid Observatory, and the 30-meter ground-based telescopes operating in the US will be years to come – machine learning and sophisticated supercomputers capable of simulating cosmic evolution over time.
Learning how the physical laws that govern the universe fit together is absolutely crucial to unlocking the last of its mysteries. And while we may not have figured it all out yet, we’re getting closer!
Further reading: SDU, Physikbriefe B