I’m Not Late, You’re Just Early: measuring the Hubble constant using time-delay cosmography

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

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

First Author’s Institution: Department of Physics and Astronomy, University of California, Los Angeles

Status: Published in MNRAS [open access]

The Universe is growing up. But how big is it now, and how fast is it growing? Just like how pediatricians use our heights as children to predict our future heights and growth rates, astronomers can measure the expansion rate of the Universe, also known as the Hubble constant (H0), by modeling light from when the Universe was just a baby. Specifically, observations of the Cosmic Microwave Background, light emitted when the Universe was just 400,000 years old, yield a Hubble constant of about 67 km/s/Mpc  (the Universe is about 14 billion years old now). 

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

It’s crucial astronomers get to the bottom of this discrepancy, because the Hubble constant impacts so much of our current understanding of cosmology. For example, the Hubble constant constrains models of dark energy and the masses of neutrinos, 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 figure out which models are incorrect. Today’s paper focuses on one such technique that capitalizes on a novel method involving gravitational lensing.

Using bending light to measure the Universe

Figure 1. An illustration of gravitational lensing. Light from a distant variable source, a quasar, is bent around an intervening mass or lens, a galaxy. The different light paths the photons from the quasar take present as four different images, which appear to us delayed in time relative to one another. The time delay can be then used to measure the expansion rate of the Universe.  [Credit: TDCOSMO Collaboration]

Gravitational lensing occurs when the light rays from a faraway 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 because of intervening mass along the line of sight and the expansion of the Universe. So to us as observers, the result is the appearance of multiple images! A cartoon of how the light from a variable source is bent around a galaxy, and subsequently appears as four images to us on Earth, is shown in Figure 1. If the source is variable, like a quasar (extremely bright objects powered by supermassive black holes), then the different images of the same source appear not only at different places, but also at different times, delayed relative to one another because of the different paths the light takes (hence “time delay”). Cosmologists have figured out a neat trick– the time delay is proportional to, and can be used to infer, the Hubble constant. This method of inference for measuring H0 is called time-delay cosmography, which is a fancy phrase for using measured time delays to probe the size and characteristics of the Universe.

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

This paper uses Hubble Space Telescope (HST) imaging of a strong lensing system, DES J0408-5354, shown in Figure 2. This lens system is unique because it has multiple sources (i.e., the quasar (images ABCD), S2, and S3 in Figure 2), where previously studied lensing systems had only contained one light source. This paper also tackles one of the main issues currently facing time-delay cosmography: modeling the distribution of the mass doing the lensing, which the value of the Hubble constant is highly dependent upon. The fact that this lens system has multiple sources makes it even more difficult to model, which is why the success of this paper was so ground-breaking. To also help prevent bias affecting the modeling, the authors use a technique called blind analysis, where the authors avoided looking at the results until the end.

Figure 2. HST image of DES J0408-5354. The main foreground galaxies acting as the “lenses” are G1 and G2, while the different images (A, B, C, D) are the same quasar appearing four different times because of gravitational lensing. S2 and S3 are additional sources. G3-G6 are nearby galaxies also contributing small lensing effects. [Figure 2 in paper]

 

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

Figure 3. Whisker plot showing different measurements of the Hubble constant. The final measured Hubble constant of this paper was H0 = 74.2 (the 5th line in the Lensing related, mass model-dependent section of this plot), which agrees with measurements from Cepheid stars, but disagrees with early Universe measurements from the CMB. [Figure 2 of Di Valentino et al. 2021]

Astrobite edited by Evan Lewis and Wynn Jacobson-Galan

Featured image credit: Amazon

About Abby Lee

I am a graduate student at UChicago, where I study cosmic distance scales and the Hubble tension. Outside of astronomy, I like to play soccer, run, and learn about fashion design!

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