Chipping away at dark energy

On February 13, 2025, four European physicists published a manifesto about cosmology models in the journal Philosophical Transactions of the Royal Society A (bit.ly/royal-cosmos). Written as a summary of a conference at The Royal Society in April 2024, it was a challenge to the standard model of cosmology, the reigning champion among models that purport to explain how the universe got the structure that scientists see through their telescopes. Indirectly, the four scientists were also readying the cosmology community for new sets of data to be released soon from space probes over the next two years.

The meeting in 2024 was the brainchild of James Binney and Subir Sarkar at the University of Oxford, and a few other cosmologists who thought that a discussion was needed on the topic (Shaastra, July 2024; The cosmic rush). These scientists, who are among the small number of non-believers in the standard cosmological model, had wanted to brainstorm about some oddities in cosmological observations. The situation had been building up for some time. New data seemed to poke holes in some of the assumptions of the standard cosmological model, especially that it looks the same everywhere in the large scale. The afterglow of the Big Bang didn't quite appear as it should have. Analyses published in November 2024 of observations from the Dark Energy Spectroscopic Instrument (DESI) of the Mayall Telescope in Arizona have hinted, albeit at a weak statistical significance, that dark energy, a postulated entity that was causing the universe to expand faster and faster, was changing over time. In December 2024, data from supernovae seemed to support a competing model to the standard model of cosmology.

According to the standard model of cosmology, the universe started about 13.8 billion years ago from a point of supposedly 'infinite density' and then began expanding and cooling, with matter condensing to form stars, galaxies, galaxy clusters, and other such celestial entities. Cosmologists also think that the expansion of the universe is accelerating, for which they have invoked the concept of dark energy, a substance that fills the universe and pushes everything apart. Several observations about the universe are explained well by this relatively simple model, which has made it popular. P. James Peebles of Princeton University (The universe according to James Peebles) won the Nobel Prize in Physics in 2019 for his work on this model. One assumption of his model is that 95% of the universe consists of dark matter and dark energy. What we see is only 5% of what fills the universe. Cosmologists expect data from space missions to be crucial in the resolution of this puzzling aspect of the universe.