Little is known about what dark matter and dark energy, the dominant components of the universe, really are, but the standard model of Big Bang cosmology, known as ΛCDM, incorporates how they outwardly behave. Dark energy, the model presumes, takes the form of a cosmological constant Λ, or a constant energy density per unit volume of vacuum. And dark matter is cold—that is, nonrelativistic—and interacts only via gravity and possibly the weak force.

Credit: Freddie Pagani

Observations of the universe generally agree well with the ΛCDM model, but an emerging exception is the Hubble constant H 0 , the universe’s present rate of expansion. When combined with measurements of the cosmic microwave background, a reflection of the spatial structure of the early universe, the ΛCDM model predicts that the universe today should be expanding at a rate of 67.4 ± 0.5 km/s/Mpc. But a direct measurement of H 0 based on observations of standard candles—Cepheid variable stars and type Ia supernovae—gives a different value: 74.0 ± 1.4 km/s/Mpc. Led by Sherry Suyu, the H0LiCOW (H 0 Lenses in COSMOGRAIL’s Wellspring) collaboration uses gravitationally lensed quasars to independently measure H 0 . The group’s latest result, 73.3 + 1.7 − 1.8 km/s/Mpc, agrees well with the standard-candle value. Combining the H0LiCOW and standard-candle measurements gives an H 0 of 73.8 ± 1.1 km/s/Mpc, which differs from the ΛCDM value by 5.3 standard deviations.

The challenge in any direct measurement of H 0 is in gauging the distances to faraway astronomical objects; their velocities relative to Earth, in contrast, are readily inferred from the redshifting of their radiation. Standard candles are appealing because their luminosities are known, so their distances can be calculated from how bright they appear on Earth. In H0LiCOW’s complementary measurement, the researchers studied quasars whose light is so strongly deflected by foreground galaxies that they appear as multiple distinct images, as shown in the figure. Because the light in each image traverses a path of a different length, fluctuations in the quasar’s light show up in the lensed images at different times. Measuring those time differences, which are on the order of weeks, doesn’t directly yield D d (the distance from Earth to the lens) or D ds (the distance from the lens to the quasar). But it does constrain their combination, which is enough information to calculate H 0 from the objects’ known redshifts. In 2017 the collaboration published a first result based on three lensed quasars (see Physics Today, April 2017, page 24). The current work extends the analysis to six quasars.