HOLiCOW! Astronomers measuring the expansion of the universe confirm that we still don’t understand everything

The H0 Lenses in COSMOGRAIL’s Wellspring collaboration, better known as the HOLiCOW collaboration, has just released the most recent measurement of the Hubble constant (H0). This fundamental value is used throughout cosmology; it tells us how fast the universe is expanding, and it’s also used to estimate the age and size of the universe, the amount of dark matter present in the cosmos, and much more. The HOLiCOW team’s result utilizes a completely independent method of measuring the expansion to high precision using quasars that have been gravitationally lensed by massive foreground galaxies in our line of sight. Their result agrees with other measurements of this value obtained from the local universe, which hint that there may yet be some physics missing from our current model of the cosmos.

The HOLiCOW Measurements

Gravitational lensing occurs when a massive object, such as a galaxy, lies between the Earth and a very distant object. Due to the effects of General Relativity, the light from the background object is bent around the object acting as a lens. This process creates several bright images of the background object. The HOLiCOW collaboration used a combination of ground- and space-based telescopes that includes the Hubble Space Telescope, the Spitzer Space Telescope, the Subaru Telescope, the Canada-France-Hawaii Telescope, the Gemini Observatory, and the W. M. Keck Observatory to image several gravitationally-lensed quasars in pursuit of measuring the Hubble constant. In particular, the excellent wide-field data obtained with the 8.2-meter Subaru Telescope and its Suprime-Cam and MOIRCS cameras played a major role in the group’s measurement of the universe’s expansion rate.

How did they do it? Thanks to factors such as the shape of the lensing galaxies and the position of the quasars behind them, light from the distant quasars follows varying paths as it travels around the foreground galaxy to the Earth, ultimately arriving at different times. Quasars are variable; astronomers can see them flicker from month to month and year to year. By measuring each lensed image of the quasar and noting the time at which a change in brightness was observed, the HOLiCOW team measured the delays due to differing light paths. These delays are directly related to the Hubble constant. “Our method is the most simple and direct way to measure the Hubble constant as it only uses geometry and General Relativity, no other assumptions,” explains the project co-lead, Dr. Frédéric Courbin from EPFL in Switzerland, in the team’s press release of their findings. The result was a measurement of the Hubble constant to a precision of 3.8%.

The Hubble Constant

H0 is measured in units of kilometers per second per megaparsec (km/s/Mpc; a megaparsec is a distance equivalent to 3.26 million light-years). It’s calculated by a deceivingly-simple equation: H0 = v/d, where v is an object’s (i.e., a galaxy’s) motion along our line of sight and d is the distance to the object. By dividing a galaxy’s velocity by its distance from Earth, astronomers can thus measure the rate of the expansion of the universe.

The Hubble constant and its meaning are vital components of numerous cosmological calculations and models. “The Hubble constant is crucial for modern astronomy as it can help to confirm or refute whether our picture of the Universe – composed of dark energy, dark matter and normal matter – is actually correct, or if we are missing something fundamental,” says Dr. Vivien Bonvin, a HOLiCOW team member from EPFL, Switzerland.

But measuring the Hubble constant accurately is not easy. Determining velocity is relatively straightforward. However, distance is one of the most difficult values to measure in astronomy; in most cases, it can only be measured accurately for objects called “standard candles,” which are objects with very precisely known brightnesses. By measuring an object’s brightness and comparing it to the value expected, astronomers can determine the amount of dimming caused by distance to derive that distance relatively accurately. In the past, calculations of H0 have been determined using standard candles that include Cepheid variable stars and type Ia supernovae, a specific kind of supernova explosion that occurs in binary systems.

Independently Measuring H0

Independent measurements are essential in science. When a value can be confirmed via several different types of measurements, it is much more likely to be correct and accurate. The measurement of the Hubble constant obtained by the HOLiCOW team is in excellent agreement with measurements obtained in the local universe using the Hubble Space Telescope (HST).

Case closed, then — except there’s a problem, and that is the fact that our local universe measurements of the Hubble constant (including the new HOLiCOW measurement) don’t agree with measurements of H0 obtained using the cosmic microwave background (CMB) radiation left over from the Big Bang. Using the CMB, the ESA Planck satellite measured a value of H0 = 66.93±0.62 km/s/Mpc. This value fits the current cosmological models of the universe very well. But HOLiCOW’s measurement of H0 = 71.9±2.7 km/s/Mpc is in good agreement with the HST determination of H0 = 73.24±1.74 km/s/Mpc; yet, neither of these latter measurements fit as nicely into our current models of the universe as the value obtained by Planck.

Why might these values be so different, then, especially with their high precision? The answer may lie with our understanding of the cosmos — as in, it’s not yet complete. These discrepancies point toward physics that has not yet been incorporated into our current views and models of the universe.

“The expansion rate of the Universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the Universe,” explains Dr. Sherry Suyu, leader of the HOLiCOW collaboration and associated with the Max Planck Institute for Astrophysics in Germany, the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, and the Technical University of Munich. “The Hubble constant is crucial for modern astronomy as it can help to confirm or refute whether our picture of the Universe – composed of dark energy, dark matter and normal matter – is actually correct, or if we are missing something fundamental.”