A new measurement fuels an ongoing debate about the rate at which today’s universe is expanding.
Reproduceability is key to science. A one-time “eureka!” could be the first step in a paradigm shift — or it could be a fluke. It’s the second, third, and hundredth measurements that put theories to the test.
That’s why recent measurements of the universe’s expansion have piqued interest. Even though astronomers have applied multiple methods relying on completely different physics, they’re still getting similar results: Today’s universe appears to be expanding faster than what’s expected based on measurements of the early universe. Can systematic errors explain this discrepancy? Or are new physics required?
Now Wendy Freedman (University of Chicago) and colleagues have posted a new, "middle-of-the-road" measurement on the astronomy preprint arXiv, adding a twist to the ongoing debate. The study will appear in the Astrophysical Journal.
Hubble Constant: Near vs. Far
There are two ways that astronomers can estimate the current expansion rate, also known as the Hubble constant (H0). The first is to look way back in time and space.
Exquisitely precise measurements of the cosmic microwave background (CMB), the Big Bang’s “afterglow,” provide a window into the young universe. Tiny temperature fluctuations in this background radiation correlate to density variations in a cosmos only 370,000 years old, which in turn relate to the structure of galaxies and galaxy clusters in the universe today, roughly 13.8 billion years later. Cosmologists can reproduce every last wiggle of those temperature variations using the so-called “Lambda CDM” model, a scenario where dark matter and dark energy rule the universe.
The most recent Hubble constant measurements in this vein come from the Planck satellite: 67.4±0.5 km s-1 Mpc-1. (Yes, those are weird units: here’s why.) Other, independent methods based on properties of the early universe end up with a similar number.
Astronomers can also estimate the expansion rate in the modern-day universe, by measuring the rate at which galaxies appear to fly away from our own. The trick is to find the galaxies’ correct distances. That’s where standard candles come in: Astronomers can measure the brightness of these objects and compare them to their known luminosity to calculate their distance.
Here’s where it starts to get interesting: Measurements using relatively nearby objects, such as Type Ia supernovae, Cepheid variable stars, and other standard candles result in a larger Hubble constant, with values between 73 and 76 km s-1 Mpc-1. In other words, the universe appears to be expanding faster now than what’s expected based on observations of the early universe.
And now for the twist: A new study using a new kind of standard candle finds a middle-of-the-road Hubble constant: 69.8±1.9 km s-1 Mpc-1. The result, taken on its own, agrees both with measurements of the cosmic microwave background and with nearby standard candles.
Red Giant Standard Candles
The standard candle Freedman and colleagues are using are red giant stars; specifically, red giant stars that have just made the transition from burning hydrogen to igniting helium.
“Think of it as scanning a crowd to identify the tallest person — that’s like the brightest red giant experiencing a helium flash,” says Christopher Burns (Observatories of the Carnegie Institution for Science). “If you lived in a world where you knew that the tallest person in any room would be that exact same height — as we assume that the brightest red giant’s peak brightness is the same — you could use that information to tell you how far away the tallest person is from you in any given crowd.”
Astronomers find these stars in galaxies’ outermost reaches, which means that there’s no intervening dust to affect observations. So red giant stars provide a way to measure distance that’s free of some of the systematic issues plaguing Cepheid stars and other standard candles.
Nevertheless, as Freedman and colleagues point out in their paper, the near-far discrepancy remains. Even though their study gives a lower value of the Hubble constant, it’s still on the high end compared to studies of the early universe. If, say, 67.4 were truly the correct value of the Hubble constant, then statistically speaking you’d expect at least a couple measurements to be below it.
Adam Riess (Johns Hopkins University), who has led several recent studies of Cepheid variable stars, points out that a lot depends on how the red giant stars are calibrated. Freedman and colleagues anchor their observations in the Large Magellanic Cloud, where both Cepheid variables and red giant stars reside. Here, unlike in galaxy haloes, the astronomers must account for dust. Riess thinks the large amount of dust estimated by Freedman’s team may explain why their measurement of the Hubble constant is so low.
Ultimately, current measurements won’t settle the debate. Freedman and colleagues argue that to resolve the tension, astronomers must locally measure the Hubble constant to a precision better than 1%. That’s out of reach for now, but in just a couple years, the European Space Agency’s Gaia mission will be providing trustworthy and exquisitely precise distances to a heap of red giant stars, allowing a far better calibration than what is possible now. Perhaps then we’ll resolve the Hubble constant debate for once and for all.
Further reading: Find the full back story on the cosmic controversy surrounding the Hubble constant in the June 2019 issue of Sky & Telescope.