In two recently published papers, members of the Waterloo Centre for Astrophysics at the University of Waterloo presented a new measurement of the Hubble parameter using data from the Dark Energy Spectroscopic Instrument (DESI). DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
The Hubble parameter defines how fast the Universe is expanding and can be directly linked to the time elapsed since the Big Bang. The first measurement was made by Edwin Hubble in 1929, by comparing the distances to local galaxies to the speed they are moving away from us. Since then, methods used have become more refined, and our knowledge of this parameter has improved significantly, but the measurements don’t always agree.
One method uses local objects like the original method of Hubble, while the other uses the overall properties of the Universe as measured by the cosmic microwave background. They give different results to a level that has many astronomers worried.
This drove Waterloo scientists to create a new method based on energy densities in the Universe, rather than distances. As space expands, the density of matter decreases, so measuring densities can therefore measure the expansion rate. The method is particularly robust, but it requires a link between the observed density of the material seen in the universe to the total density, including the material we cannot see, like dark matter.
"To measure the density of all material in the universe, we looked at how galaxies are distributed in the Universe,” says Dr. Alex Krolewski, the lead author of the paper measuring the Hubble Parameter. “We were able to decompose the pattern into two components, one for the material we can see, and one for the material we cannot. The relative amplitude of each component lets us measure the ratio of the amounts of the different types of material in the Universe."
Waterloo Centre for Astrophysics PhD student, Andrea Crespi, led an additional paper exploring how to measure the dark matter density.
"The relative contribution of matter that we can see and dark matter depends on how each component evolves over cosmic time,” says Crespi. “By modelling this separation throughout the history of the Universe as probed by DESI, we were able to measure their relative amplitude with high robustness—an essential ingredient for determining the Hubble parameter.”
While the measurements made do not solve the tension between other measurements, new data from the DESI and Euclid experiments over the next 10 years will increase the precision of the new measurement, leading to stronger constraints.
"It is very satisfying to be contributing to a field with such a long history,” says Dr. Will Percival, co-author on the papers, Director of the Waterloo Centre for Astrophysics, and co-Spokesperson for DESI. “With this method, we have sacrificed precision for robustness, but that is exactly what we need to move forward and reconcile previous results.”
The papers, “Baryon fraction from the BAO amplitude: a consistent approach to parameterizing perturbation growth,” and “A measurement of H0 from DESI DR1 using energy densities,” are published online via arXiv.