**Authors:** Adam G. Riess, Stefano Casertano, Wenlong Yuan, Lucas M. Macri and Dan Scolnic

**First Author’s Institution:** Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

**Status:** Accepted to ApJ [open access]

Hubble’s law tells us that all galaxies, stars and planets are moving away from each other, and the more distant the object, the faster it is moving away. We quantify this expansion as a speed per distance, which gives us a unit like **km/s** (speed) per **megaparsec** (distance). This value is known as the Hubble constant, or H_{0}.

The Hubble constant has been determined using various methods. However, two of these titan measurements disagree with each other in a way that astronomers deem significant.

The first of the measurements comes from studying the oldest electromagnetic radiation in the Universe – the cosmic microwave background (CMB). See this Astrobite for a detailed explanation of how we are able to do this. The most recent results from the CMB give us a Hubble constant of roughly 67 km/s/Mpc.

The second measurement comes from using type 1a supernovae as standard candles to calibrate distances to them (see this Astrobite for more). Essentially, by looking at these stars at various distances, we can correlate their distance with their apparent brightness. By assuming supernovae are dimmer proportional to their distance from us, we can measure the gradient of this correlation. Recent results put H_{0} at 73 km/s/Mpc.

So, one of the most prominent problems in cosmology boils down to a 6 km/s/Mpc difference. Certainly, each of these measurements have their own subtleties but there are two main things to note:

- The Hubble constant measurement from CMB and type 1a supernovae are
*independent.*They do not rely on the same measurement technique, and therefore do not have any source of error in common. This makes it harder to dismiss the tension as something which comes from a shared, inaccurate measurement. - The Hubble measurement from the CMB uses data from the
**early**universe, while the value obtained from supernovae is a**late-time**or**local**measurement. This could potentially be an interesting explanation for the tension.

### A new addition

Today’s authors stir the Hubble cauldron a bit more with 70 space-based observations of Cepheid variables in the Large Magellanic Cloud (LMC) from the Hubble Space Telescope.

A Cepheid variable is a type of star which pulsates over some period of time. Astronomer Henrietta Swan Leavitt deduced that the rate of pulsation for these stars is correlated strongly with their luminosity (see this Astrobite for more on her work and legacy). Therefore, one can know the brightness of these stars simply by observing their pulsation rate (Figure 1). Consequently, one can determine the distance to these stars just by comparing their known luminosity to the apparent brightness. Much like supernovae, this makes Cepheid variables powerful probes of the local Hubble constant. Furthermore, by studying galaxies containing both Cepheid variables and type Ia supernovae, the Cepheid-derived distances can be used to calibrate the accuracy of supernovae-derived distances, creating a robust distance ladder, which gets us to H_{0}.

Figure 1: Period-luminosity relation for the 70 Cepheid variable stars. The colours in the figure indicate the different wavelengths used for observing these Cepheids. The agreement in the slope tells us the P-L relation is not dependent on any particular wavelength. *Figure 3 in paper.*

To ensure an accurate Hubble measurement with Cepheid variables, various sources of uncertainty are considered by the authors. Among these are the differences in the telescope sensitivity to fainter, distant Cepheids compared to nearer ones, which can affect the measured brightness. Another source of error is the inclination of the LMC itself, which results in some Cepheids appearing closer or farther than average by a very small degree. After taking all sources into account, the total uncertainty in the distance measurement, and hence the Hubble constant, is 1.28%, which is the smallest error for any Cepheid variable Hubble measurement to date.

### So what’s the tension now?

Combining the LMC distances with two other distance calibrators for better constraints, the authors quote a Hubble constant of 74.03 km/s/Mpc, which is in a staggering **4.4-sigma **tension with the CMB Hubble measurement. This effectively means that the probability that the new Hubble measurement is genuine rather than a statistical fluke is above 99.999%, and therefore so is the discrepancy.

Figure 2: Various measurements of the Hubble constant colour-coded by whether they use data from the early universe (blue) or the late universe (red). At the top are potential modifications to our current cosmological model which could resolve the current tension. *Figure 4 in paper.*

Much has been said on the nature of the Hubble disagreement already, both on its nature and from pacifists looking to ease the tension (see examples here and here). More recently, gravitational waves have burst onto the scene with another independent measurement (though it is not statistically significant to fuel the flames just yet). New physics could hold the key to breaking this Hubble stalemate. For example, our universe could have a non-zero curvature, a time-dependent dark energy, or interacting dark matter. Today’s paper shows that the tension is as strong as ever, so we wait for more precise, independent measurements to help clarify the nature of our expanding universe.

Thank you, for the article. Excited to see how gravitational waves will map the landscape.