For decades, scientists have disagreed about a fundamental question: how quickly is the Universe expanding? But this year, astronomer Wendy Freedman announced results that could help to put the controversy to rest.
The long-standing puzzle has been that two methods to measure the cosmic expansion rate, known as the Hubble constant, give stubbornly different answers. Studies using fluctuations in the cosmic microwave background — the afterglow of the Big Bang — suggest that for every megaparsec (Mpc; or 3.2 million light years) farther out one looks, galaxies rush away 67 kilometres per second faster. But when scientists, including Freedman, measured the recession rate of far galaxies and estimated their distance, they got a larger Hubble constant: variously 72–74 km s−1 Mpc−1.
The method for estimating the distance of galaxies is crucial. It relies on observing the brightness of supernovae (exploding stars) in those galaxies. To calibrate how a supernova’s apparent brightness relates to its distance, researchers rely on comparisons to ‘standard candles’: well-studied stars found relatively nearby, in the Milky Way’s cosmic neighbourhood.
Analyses led by Adam Riess at Johns Hopkins University in Baltimore, Maryland, use stars known as Cepheids and give the higher Hubble constant. So Freedman, at the University of Chicago in Illinois, worked with collaborators to perfect two other types of standard candle that would serve as a cross-check. “She really started building up this independent path,” says Kristen McQuinn, an astronomer at Rutgers University in New Brunswick, New Jersey, and at the Space Telescope Science Institute in Baltimore, Maryland. That is courageous, says McQuinn, because it requires a lot of telescope observations and painstaking analysis.
This year, Freedman revealed her findings, which used the James Webb Space Telescope (JWST). As she announced at conferences in April, and in a preprint posted in August, when she combined the two newer standard candles with supernova data, both analyses put the Universe’s expansion rate within the error margin of the 67 km s−1 Mpc−1 cosmic microwave background results (W. L. Freedman et al. Preprint at arXiv https://doi.org/nq36; 2024).
But the puzzle is far from solved. Freedman found that the Cepheid-based technique still mysteriously gives a value higher than she found with the other standard candles. Then, later in August, Riess published his own team’s analyses — arguing that all three standard candles show a higher Hubble constant (A. G. Riess et al. Preprint at arXiv https://doi.org/nq4g; 2024).
The reason for that difference seems to be that scientists don’t agree on which sets of galaxies to include in their analyses. But there is hope that more JWST observations, or different methods, such as using gravitational waves, might sort out the issue.
If so, it will be one more fundamental aspect of the Universe that Freedman has helped to illuminate. She first established her name in the 1990s, when she led what Riess calls a “transformational” study with the Hubble Space Telescope, called the Key Project. At that time, different groups gave measurements of the Hubble constant that disagreed by a factor of two — and the age of the Universe itself could only be estimated as between 10 billion and 20 billion years. Using Cepheids and supernovae, Freedman’s work vastly improved distance estimation and, combined with Nobel-winning discoveries by Riess and others, improved estimates for the Universe’s age.
Freedman has also led some of the effort to develop what will be the world’s most powerful telescope, in Chile — as director of the Carnegie Observatories, in Pasadena, California, from 2003 to 2014. Much of the Giant Magellan Telescope is built, but it still needs US$1.6 billion for completion. The US National Science Foundation will decide soon whether to fund it.
Even after the Hubble constant is conclusively settled, Freedman won’t be ready to retire. “No way,” she says. “I feel like I have one more big thing left in me. I have a few ideas kicking around in my head, and I have enough energy and enthusiasm to continue. I love what I do.”
