Science Has a New Way to Gauge the Universe's Expansion Rate

The big news in cosmology for several years has been the mounting evidence that the universe is expanding faster than expected. When cosmologists extrapolate data from the early universe to predict what the cosmos should be like now, they predict a relatively slow cosmic expansion rate. When they directly measure the speed at which astronomical objects are hurtling away from us, they find that space is expanding about 9% faster than the prediction. The discrepancy may mean that something big is missing from our understanding of the cosmos.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

The issue reached a crescendo over the past year. Last March, the main group measuring cosmic expansion released their updated analysis, once again arriving at an expansion rate that far outstrips expectations. Then in July, a new measurement of cosmic expansion using objects called quasars, when combined with the other measurement, pushed past “five sigma,” a statistical level that physicists usually treat as their standard of proof of an unaccounted-for physical effect. In this case, cosmologists say there might be some extra cosmic ingredient, beyond dark matter, dark energy and everything else they already include in their equations, that speeds the universe up.

But that’s if the measurements are correct. A new line of evidence, first announced last summer, suggests that the cosmic expansion rate may fall much closer to the rate predicted by early-universe measurements and the standard theory of cosmology.

Wendy Freedman, a decorated cosmologist at the University of Chicago and Carnegie Observatories, measured the expansion rate, known as the Hubble constant, using stars that she considers cleaner probes of expansion than other objects. Using these “tip of the red giant branch” (TRGB) stars, she and her team arrived at a significantly lower Hubble rate than other observers.Although Freedman is known for her careful and innovative work, some researchers pushed back on her methods after she introduced the result last summer. They argued that her team used outdated data for part of their analysis and an unfamiliar calibration technique. The critics thought that if Freedman’s team used newer data, their Hubble value would increase and come in line with other astronomical probes.
It did not. In a paper posted online on February 5 and accepted for publication in The Astrophysical Journal, Freedman’s team described their analysis of TRGB stars in detail, summarized their consistency checks, and responded to critiques. The new paper reports an even slower cosmic expansion rate than last summer’s result, a tad closer to the early-universe rate. The more up-to-date data that critics thought would increase Freedman’s Hubble value had the opposite effect. “It made it go down,” she said.

The Trouble With Dust

The question of whether the universe expands faster than expected first cropped up in 2013, when the Planck satellite precisely mapped ancient microwaves coming from all directions in the sky. The microwaves revealed a detailed snapshot of the early universe from which the Planck team could deduce the cosmos’s precise ingredients, like the amount of dark matter. Plugging those ingredients into Albert Einstein’s gravity equations allowed the scientists to calculate the expected expansion rate of space today, which Planck’s final, full analysis pegged at 67.4 kilometers per second per megaparsec, give or take 1%. That is, when we peer into space, we should see astronomical objects receding from us 67.4 kilometers per second faster with each megaparsec of distance, just as dots on an inflating balloon separate faster the farther apart they are.
But Adam Riess, a cosmologist at Johns Hopkins University and the Nobel Prize–winning co-discoverer of dark energy, had for a few years been getting a higher value in direct measurements of the cosmic expansion rate. The trend continued; as of their latest analysis last March, Riess’s team pegged the Hubble constant at 74 kilometers per second per megaparsec, 9% higher than the 67.4 extrapolated from the early universe.

Illustration: Quanta Magazine