**Title:** The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration

**Authors:** Edvard Mörtsell, Ariel Goodbar, Joel Johansson, Schail Dhawan

**First Author’s Institution: **Oskar Klem Centre, Department of Physics, Stockholm University

**Status:** Submitted to arXiv [24 May 2021]

The Hubble constant (H_{0}), which is used to describe the expansion rate of the Universe, can be found around every corner in the field of cosmology. It helps us with everything from determining the age of the Universe to helping us test theoretical cosmological models. What a superstar! But recently, the Hubble constant has started to gain a reputation as a troublemaker for one crucial reason: independent methods used to calculate the Hubble constant are producing wildly different results (Figure 1). One of the greatest discrepancies comes from the value of the Hubble constant measured from Type Ia supernova (SNIa), which produces a value of H_{0} = 73.2 ± 1.3, and the cosmic microwave background (CMB) with a value of H_{0} = 67.4 ± 0.5,*—*a 4.1 standard deviation difference! A discrepancy this big has astronomers scratching their heads: are astronomers simply mismeasuring the size of the Universe, or are there possibly fundamental new physics at play here just waiting to be discovered? Since these values should be identical in theory, the extreme difference between these values has set the astronomical community on edge, and kicked off a race to solve this *Hubble tension**. *

## Investigating the SNIa Hubble Constant

So what could be the issue? Well, let’s first dissect the SNIa method used to calculate the Hubble constant. According to the deceivingly simple Hubble law, the two ingredients we need to estimate the Hubble constant is the speed that a supernova is traveling away from us, as well as its distance.The first key value, a supernova’s velocity, can be found by calculating its redshift, since we know what the light spectrum of an SNIa should look like. To find its distance, astronomers rely on a tool known as the cosmic distance ladder. Basically, astronomers confidently measure the distances to nearby objects, and then they use these measurements to climb up the rungs of the ladder to make measurements of increasingly distant objects. One such step in this ladder comes from the distances to Cepheid variable stars. Measuring these distances relies on precise calculations of their magnitudes, which can be found through their color-luminosity relation. So, in order to measure the distances to supernovae, astronomers indirectly depend on these calculations of distances to Cepheids.

As it turns out, our current color-luminosity relationship may have one fatal flaw: it assumes that there is one universal color-luminosity relationship for every Cepheid. But why did this turn out to be a possibly disastrous simplification? Well, as Cepheids live in dusty homes known as galaxies, galactic dust can absorb a Cepheid’s light and redden its color. Prior studies have demonstrated that even within our own galaxy, the Milky Way, galactic reddening varies widely from place to place. If astronomers accidentally over-correct for dust extinction, the local distance ladder will change, and therefore the inferred value of the Hubble constant will change as well. So, the authors of today’s paper decided to test if the Hubble tension could be solved by toying with the color-luminosity relationship. To do this, they relied on data to derive a specific relationship for each galaxy individually.

While several methods were tested, one of particular interest was based on so-called Wesenheit magnitudes. This technique was used to correct a Cepheid’s color by considering both dust extinction and variations in its intrinsic color. After these corrections were made to the Cepheid magnitudes, exciting new values for the Hubble constant were found. The Wesenheit recalibrated Hubble constant found a value that reduces the discrepancy between Hubble constant measurements to only a two standard deviation difference. Exciting! You can see for yourself in Figure 2.

So, does this mean the Hubble tension is solved? Not quite. As we have seen, the SNIa value of the Hubble constant directly depends on a well-calibrated cosmic distance ladder. However, imprecise measurements of Cepheid magnitudes may have made our distance ladder less reliable than astronomers may have originally believed. Through a little fine tuning of these magnitudes by accounting for things such as dust absorption and intrinsic variations in color, our authors were able to narrow the gap between the Hubble constant measured by SNIa and the CMB. While more research is still needed, exploring the potential error in our distance ladder from these Cepheid magnitudes is a compelling new avenue for cosmologists to study to finally resolve the Hubble tension. Who knows, maybe all the Hubble constant needed was a little dusting off!

*Astrobite edited by Mark Popinchalk*

*and W. Vivyan Yan*

*Featured image credit: NASA, ESA, Hubble Heritage Team*

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