THE HUBBLE TENSION
THE UNIVERSE IS EXPANDING. AND WE CAN MEASURE THE SPEED OF ITS EXPANSION. THE TROUBLE IS, DEPENDING ON HOW WE MEASURE THAT SPEED, WE KEEP GETTING DIFFERENT RESULTS
The first anomaly concerns the speed at which the Universe is expanding. Astronomers determine this in two ways and herein lies the problem: the two methods yield different values.
The obvious method is to observe galaxies (the basic building blocks of the Universe) in the nearby Universe and measure how fast they’re moving away f rom us. They’re scattering like pieces of cosmic shrapnel in the aftermath of the Big Bang, the titanic explosion in which the Universe was born 13.82 billion years ago.
A second way of determining the expansion rate is to deduce it in the early Universe and extrapolate it to today. The primordial expansion rate is encoded in the cosmic background radiation, the ‘afterglow’ of the Big Bang, which is still around us today and accounts for 99.9 per cent of the photons, or particles of light, in the Universe.
The problem is that the expansion rate measured locally is eight per cent greater than deduced by extrapolating from the early Universe. There are those who believe there is a cosmic ingredient we’ve overlooked which has speeded things up over the past 13.82 billion years. But extraordinary claims require extraordinary proof. Does the evidence stack up?
The f irst thing to say is that measurements of the cosmic background radiation by the European Space Agency’s ( ESA) Planck satellite have ushered in the age of precision cosmology and are considered the gold standard of ast ronomical observations. Thanks to Planck, not only have we learnt that the Universe is 13.82 billion years old, but also that it consists of 68.3 per cent dark energy, 26.8 per cent dark matter and 4.9 per cent ordinary atoms.
Given astronomers’ faith in Planck, all attention has focused on the local expansion measurements. These, however, are fraught with difficulties.
The expansion rate is characterised by the so- called Hubble constant. In 1929, the American astronomer Edwin Hubble discovered that the further away a galaxy is from us, the faster it’s moving away. The Hubble constant ( H0 ) connects these two quantities, so that a galaxy that’s D megaparsecs more distant than another galaxy (where one megaparsec, or Mpc is equal to 3.26 million light-years) is receding
at H0 x D kilometres per second faster. The speed a galaxy is moving away is encoded in its spectrum – the way the intensity of its light varies in frequency. Specifically, the velocity of a galaxy can be deduced from the shift in pitch, or frequency, of its light, an effect similar to the shift in pitch of the sound of the siren on a passing police car. The difficult thing is determining the distance to a galaxy.
For ‘ nearby’ galaxies, astronomers observe Cepheid variables, extremely luminous pulsating stars whose pulse rates are related to their intrinsic luminosity. It means that, if astronomers see a Cepheid that’s four times as faint as another, they know it’s twice as far away – and not simply a fainter star at the same distance.
Cepheid variables are known as ‘standard candles.' Astronomers use them to establish the first rung on the cosmic distance ladder, in the process determining the distance to galaxies that contain Type Ia supernovae. These detonations of a super- dense white dwarf in a binary star system are believed to all be of similar luminosity. This makes them an even more luminous type of standard candle than a Cepheid and enables the determination of distances to even more remote galaxies.
For 25 years, the Hubble constant was determined from Cepheids observed by NASA’s Hubble Space Telescope. But there were concerns. When observing Cepheids at great distances, even Hubble’s sharp eyesight couldn’t be sure of picking out a lone Cepheid. There was always the chance of a Cepheid being smeared together with a star close to the line of sight, causing astronomers to overestimate the Cepheid’s brightness.
Enter NASA’s James Webb Space Telescope ( JWST). Launched on Christmas Day 2021, the 6.5m (23f t)
infrared telescope has sharper vision than Hubble. Using it, a team led by Nobel Prize- winner Prof Adam Riess of Johns Hopkins University in Baltimore determined that Hubble’s estimate of the Hubble constant was correct. The eight-percent discrepancy between expansion rates remains.
This ‘ Hubble tension’ could still be a mirage. There could be unrecognised measurement errors. Prof Joseph Silk of the University of Oxford suspects so. “I admire the detailed, painstaking attempts at calibration by Prof Adam Riess and his colleagues,” he says. “However, I’m still not completely convinced.”
Silk points out that the stellar environments in which Type Ia supernovae are born have changed over time. This change potentially makes these supernovae in distant galaxies a different luminosity to those in nearby galaxies (more distant galaxies allow us to see further back
in time because of the finite speed of
light). “And perhaps more worrying,” says Silk, “is that an alternative approach to determining the distance scale, led by Prof Wendy Freedman
and colleagues, systematically finds
a lower value of the Hubble constant than the supernova method.”
Freedman, of the University of Chicago, and her colleagues look for giant stars at the tip of the red giant branch. Here, stars make an abrupt transition from burning hydrogen in their cores to burning helium and have a remarkably consistent luminosity. “They’re the best understood distance indicator in astronomy,” says Silk. Freedman’s group has yet to publish its own results from using the JWST, but Silk suspects they’ll shrink – or perhaps even remove – the Hubble tension.
But say the Hubble tension is real, what missing ingredient could have sped up the expansion of the Universe? “I wish I knew,” says theorist Prof Marc Kamionkowski of Johns Hopkins University. “It’s really puzzling.”
Prof Ian McCarthy of Liverpool John Moores University adds: “The implication is that something about the Standard Model of Cosmology is incorrect. That would be very exciting.”
Riess has a theory. “Dark energy or dark matter could have more exotic properties than the most ‘ vanilla’ assumptions we make for them in the Standard Model,” he says. The Standard Model is the Big Bang + dark matter + dark energy + an ‘inflation’, a period of super-fast expansion in the first split-second of the Universe.
The vanilla assumption about dark energy is that it’s a so-called ‘cosmological constant’ that maintains a constant
"DARK ENERGY OR DARK MATTER COULD HAVE MORE EXOTIC PROPERTIES THAN THE MOST ‘VANILLA’ ASSUMPTIONS"
energy density as the Universe expands. It means the dark energy and its effect grows as space grows. Although unimportant early in cosmic history, eventually it dominated the Universe, putting a rocket under cosmic expansion. But, given we don’t understand the nature of dark energy, it’s entirely plausible that its energy density has evolved with time.
According to Kamionkowski, the energy density of dark energy may have increased recently, boosting cosmic expansion. Alternatively, there might have been early dark energy, which
boosted cosmic expansion in the first
few hundred thousand years of cosmic history. By overlooking it, astronomers would have underestimated the cosmic expansion rate they deduced from the cosmic background radiation. “Early dark energy is promising, but not quite as easy to get to work as we initially thought,” he says. It’s testable, however,
because it would have left a fingerprint
on the way the cosmic background radiation varies over small regions of the sky. How dark energy evolved with time is due to be tested by a host of experiments, such as ESA’s Euclid satellite, currently in orbit, and the Vera C Rubin Observatory, which is being built in Chile.
Another way the Universe could have sped up is if Einstein’s theory of gravity breaks down on the largest scales. Is the gravity that’s trying to slow the expansion of the Universe weaker than expected? Silk, however, remains sceptical of all the proposals. “To date, all attempts to introduce new physics ingredients to fully resolve the Hubble tension have failed,” he says.