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Cosmic clash over Hubble constant shows no sign of abating

12 Sep 2019
Lensed quasar
Bright arcs: Hubble Stace Telescope image of a distant quasar that has been gravitationally lensed. This image was part of this latest study. (Courtesy: ESA/Hubble/NASA/Suyu et al)

A new way to measure absolute distances in the universe has allowed scientists to work out a new value for the Hubble constant, which tells us how quickly our local universe is expanding. The latest expansion rate is consistent with other direct measures obtained from relatively nearby space, but in conflict with others that rely on the universe-wide spatial features of primordial radiation. This disparity has become more pronounced in recent years and suggests that our current understanding of cosmic evolution may need an overhaul.

Evidence for the universe’s expansion emerged in the 1920s, when Edwin Hubble first observed that galaxies move away from us more quickly, the farther they are from Earth. Since then, there have been ongoing disputes about just how rapid the expansion is. While astrophysicists have measured the Hubble constant with increasing precision, a gap remains between the values of the constant obtained using two different types of observation. What is more, this discrepancy cannot be explained away by known sources of error.

Establishing the velocity of objects receding in space simply involves measuring the redshift of their emission spectra, but pinning down their distance from Earth is much more complicated. One approach is to create a “distance ladder” that starts from Earth and moves outwards in a series of steps. This usually involves calibrating the absolute brightness of far-flung supernovae by observing other supernovae in galaxies closer to us that also contain pulsating objects of known brightness called Cepheid variables. The first step, or “anchor”, is to measure the distance from Earth to nearby Cepheids.

The most precise distance ladder to date has been created by Adam Riess at the Space Telescope Science Institute in Baltimore, US, and colleagues. They used the Hubble Space Telescope to measure the distance to Cepheids lying in the Large Magellanic Cloud, some 150,000 light-years away. Their figure for the Hubble constant is 74.0±1.4 km s–1 Mpc1 is an improvement on earlier measurements of their own and on the 72±81.4 km s–1 Mpc–1 obtained by Wendy Freedman of the University of Chicago and colleagues in 2001.

Cosmic clash

However, those results clash with values based on how quickly the universe expanded shortly after the Big Bang. Measuring the length of temperature fluctuations within the cosmic microwave background (CMB) and then extrapolating forward using the standard cosmological model, researchers working with data from the European Space Agency’s Planck Satellite in 2016 reported an expansion rate of 66.9±0.6 km s–1 Mpc1. That value in turn is consistent with a figure of 67.8±1.3 km s–1 Mpc1, obtained using data from the Sloan Digital Sky Survey to measure a characteristic length scale between galaxies containing supernovae – with the length scale also set by the CMB (in this case via density fluctuations).

The latest work was carried out by Inh Jee, Sherry Suyu and Eiichiro Komatsu of the Max Planck Institute for Astrophysics in Garching, Germany, alongside colleagues in Germany, the Netherlands and the US. It provides an independent way of checking distance-ladder calculations using gravitational lenses. These are massive galaxies that can create multiple images of more distant luminous objects by bending the objects’ light rays gravitationally.

As a first step, Jee and colleagues recorded the time delay when detecting different images from a flickering quasar. Because the lens bends the light from each image along a different path through space, these delays reveal how massive the lensing galaxy is. Next, the researchers measured the velocity of stars within the lens to estimate the lens’s gravitational potential, which can be used to calculate the radius of the galaxy. By comparing this size with the apparent separation between different quasar images, they were able to work out the distance from Earth to the lens.

Robust and independent

Jee and colleagues used this approach to measure distances to two lensing galaxies. Using this information, they converted previously-measured relative distances to 740 type 1a supernovae into absolute distances. Reporting in Science, they calculate the Hubble constant to be 82.4±8.4 km s–1 Mpc1. “This is one of the first works to give a robust, independent distance to a gravitational lens,” says Suyu.

Although the figure is less precise than previous results, the researchers – working in a collaboration called H0LiCOW – have since gone on to combine their approach with an earlier type of lensing measurement based purely on timing delays. After analysing six lenses, the collaboration arrived at a Hubble constant of 73.3±1.8 km s–1 Mpc1, which is very similar to that of Riess’s group. The collaboration describes this result in a preprint on arXiv.

That figure, however, is at odds with a recent result Freedman and colleagues, who calibrated the distance to supernovae using the steady intrinsic brightness of heavy stars known as red giants. Using data from the Hubble telescope, they calculated that the Hubble constant should be 69.8±0.81.8 km s–1 Mpc1, which is midway between results from the two rival camps.

Radek Wojtak at the University of Copenhagen reckons that the tension between the two approaches means “we are getting closer to the stage” when changes to the standard model of cosmology should be considered. Such changes might include new forms of dark matter or dark energy, he says. But he cautions that researchers should continue to look for hidden errors. “The stakes are high,” he says. “We do not want to be fooled by poorly understood systematics.”

Princeton University’s Lyman Page agrees, arguing that it is still “too early to say with any certainty whether new physics is needed”. But he points out that until a few decades ago there was a 50% mismatch between different values of the Hubble constant. The current 5% discrepancy, he thinks, “is a mark of how precisely we know the universe”.

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