I’m not late, you’re just early: measuring the Hubble constant using ticking cosmography

Title: STRIDES: a 3.9% measurement of the Hubble constant from the strong lens system DES J0408−5354

Authors: Anwar. J. Shajib, Simon Birrer, Tommaso Treu, et al.

Institution of the first author: Department of Physics and Astronomy, University of California, Los Angeles

Status: Posted in MNRAS [open access]

The Universe is growing. But how big is it now and how fast is it growing? Much like how pediatricians use our heights as children to predict our future heights and growth rates, astronomers can measure the rate of expansion of the Universe, also known as the Hubble constant (H₀), modeling light from when the Universe was just a baby. More precisely, observations of the cosmic microwave background, the light emitted when the Universe was only 400,000 years old, give a Hubble constant of about 67 km/s/Mpc (the universe has about 14 billion years now).

But wait! The doctor across town just called. She measured a Hubble constant of 73 km/s/Mpc using a completely independent method that uses nearby stars called Cepheids. The debate over conflicting values ​​of H₀ is known in astronomy as the “Hubble tension” (see this astrobite for a full review).

It’s crucial that astronomers get to the bottom of this discrepancy, because the Hubble constant has a huge impact on our current understanding of cosmology. For example, the Hubble constant constrains dark energy patterns and neutrino masses, which are extremely low-mass particles with zero charge. One solution to the tension is to measure the Hubble constant with as many different methods as possible, so we can determine which models are incorrect. Today’s article focuses on one such technique which capitalizes on a new method involving gravitational lensing.

Using curved light to measure the Universe

Gravitational lensing occurs when light rays from a distant source are bent around a nearby massive object due to its gravitational field. The light rays from the source can end up taking different paths due to the mass occurring along the line of sight and the expansion of the universe. So for us as observers, the result is the appearance of multiple images! A cartoon of how light from a variable source is bent around a galaxy, and then appears to us as four images on Earth, is shown in Figure 1. If the source is variable, such as a quasar (extremely bright objects fed by supermassive black light holes), then the different images from the same source appear not only in different places, but also at different times, offset from each other due to the different paths taken by light (hence “time delay”). Cosmologists have found a trick: the delay is proportional to the Hubble constant and can be used to deduce it. This method of inference to measure H₀ is called delayed cosmographywhich is a fancy phrase for using measured time frames to probe the size and characteristics of the universe.

DES J0408-5354: the first multi-source system used to measure H₀

This paper uses Hubble Space Telescope (HST) imagery of a strong lens system, DES J0408-5354, shown in Figure 2. This lens system is unique in that it has multiple sources (i.e., say the quasar (ABCD images, S2 and S3 in Figure 2), where previously studied lens systems contained only a single light source. . This article also addresses one of the main problems currently facing time-lapse cosmography: the modeling of the mass distribution effecting the lensing, on which the value of the Hubble constant strongly depends. The fact that this lens system has multiple sources makes it even more difficult to model, which is why the success of this article has been so groundbreaking. To also help avoid bias affecting the modeling, the authors use a technique called blind analysis, where the authors avoided looking at the results until the end.

The authors end up measuring a constant Hubble value of H₀ = 74.2 km/s/Mpc, with an uncertainty of 3.9%, shown in Figure 3. This number is currently consistent with measurements of Cepheid stars, but the authors hope to eventually achieve 1% uncertainty with software improved modeling and more lens systems. In the modern era of precision cosmology, we are getting a clearer picture of what our universe will look like when it grows.

Astrobite edited by Evan Lewis and Wynn Jacobson-Galan

Featured image credit: Amazon