New JWST data confirms, worsens the Hubble tension

One of today’s greatest cosmic puzzles concerns the expanding Universe.

For the first several billion years of our Universe’s history, the Universe’s expansion rate was decreasing and distant galaxies slowed in their recession from ours, as the matter and radiation densities dropped. However, for the past ~6 billion years, distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed toward zero. It’s because of cosmic expansion that we can see up to 46.1 billion light-years away when only 13.8 billion years have elapsed since the Big Bang.

Two major methods each give low-error, but incompatible, answers.

Taking us beyond the limits of any prior observatory, including all of the ground-based telescopes on Earth as well as Hubble, NASA’s JWST has shown us the most distant galaxies in the Universe ever discovered. If we assign 3D positions to the galaxies that have been sufficiently observed-and-measured, we can construct a visualized fly-through of the Universe, as the CEERS data from JWST enables us to do here. At greater distances, compact, star-forming galaxies are more common; at closer distances, more diffuse, quiescent galaxies are the norm.

By tracking an early, relic signal’s evolution, we measure expansion of 67 km/s/Mpc.

We can look arbitrarily far back in the Universe if our telescopes allow, and the clustering of galaxies should reveal a specific distance scale – the acoustic scale – that should evolve with time in a particular fashion, just as the acoustic “peaks and valleys” in the cosmic microwave background reveal this scale as well. The evolution of this scale, over time, is an early relic that reveals a low expansion rate of ~67 km/s/Mpc, and is consistent from CMB features to BAO features.

By starting nearby and observing increasing recession with distance, we measure 73 km/s/Mpc.

The construction of the cosmic distance ladder involves going from our Solar System to the stars to nearby galaxies to distant ones. Each “step” carries along its own uncertainties, especially the steps where the different “rungs” of the ladder connect. However, recent improvements in the SH0ES distance ladder (parallax + Cepheids + type Ia supernovae) have demonstrated how robust its results are.

This discrepancy — the “Hubble tension” — is a modern cosmic conundrum.

Modern measurement tensions from the distance ladder (red) with early signal data from the CMB and BAO (blue) shown for contrast. It is plausible that the early signal method is correct and there’s a fundamental flaw with the distance ladder; it’s plausible that there’s a small-scale error biasing the early signal method and the distance ladder is correct, or that both groups are right and some form of new physics (shown at top) is the culprit. The idea that there was an early form of dark energy is interesting, but that would imply more dark energy at early times, and that it has (mostly) since decayed away.

Many speculate an observational error on the “distance ladder” side could be the culprit.

Back in 2001, there were many different sources of error that could have biased the best distance ladder measurements of the Hubble constant, and the expansion of the Universe, to substantially higher or lower values. Thanks to the painstaking and careful work of many, that is no longer possible, as errors have been greatly reduced. New JWST work, not shown here, has reduced Cepheid-related and period-luminosity errors even further than is shown here.

We start by observing Cepheid variable stars within the Milky Way.

The Variable Star RS Puppis, with its light echoes shining through the interstellar clouds. Variable stars come in many varieties; one of them, Cepheid variables, can be measured both within our own galaxy and in galaxies up to 50–60 million light years away, with all Cepheid variables exhibiting a relationship between their period, or how long it takes them to reach maximum brightness again after fading, and their luminosity, or how intrinsically bright they are.

We accurately infer their distances by measuring astronomical parallax.

The stars that are closest to Earth will appear to shift periodically with respect to the more distant stars as the Earth moves through space in orbit around the Sun. Before the heliocentric model was established, we weren’t looking for “shifts” with a ~300,000,000 kilometer baseline over the span of ~6 months, but rather a ~12,000 kilometer baseline over the span of one night: Earth’s diameter as it rotated on its axis. The distances to the stars are so great that it wasn’t until the 1830s that the first parallax, with a 300 million km baseline, was detected. Today, we’ve measured the parallax of over 1 billion stars with ESA’s Gaia mission.

Then we measure Cepheids in nearby, well-measured galaxies.

The top two panels show two nearby, Cepheid-rich galaxies: NGC 4258 (left) and NGC 5584 (right), with JWST’s field-of-view overlaid atop them. The bottom panels show JWST views, with individually identified Cepheid variables highlighted in each image.

Finally, we measure type Ia supernovae within those galaxies and beyond, linking these cosmic “rungs” together.

As recently as 2019, there were only 19 published galaxies that contained distances as measured by Cepheid variable stars that also were observed to have type Ia supernovae occur in them. We now have distance measurements from individual stars in galaxies that also hosted at least one type Ia supernova in 42 galaxies, 35 of which have excellent Hubble imagery. Those 35 galaxies are shown here. On the other hand, fewer than a dozen have identified individual tRGB stars within them.

Could an error in Cepheids be biasing our measured expansion rate?

Many different classes of objects and measurements are used to determine the relationship between distance to an object and its apparent speed of recession that we infer from its light’s relative redshift with respect to us. As you can see, from the very nearby Universe (lower left) to distant locations more than 10 billion light-years away (upper right), this very consistent redshift-distance relation continues to hold.

By measuring Cepheids in nearby galaxies, JWST probes this possibility.

This nearby spiral galaxy, NGC 4258 (also known as Messier 106), is just ~20 million light-years away but contains many known Cepheids that are similar to Cepheids found in the Milky Way. This is an important galaxy for calibrating the cosmic distance ladder.

Observing galaxy NGC 4258, JWST found no photometric bias for Cepheids.

This image shows several Cepheid variable stars with different periods within nearby galaxy NGC 4258: an important galaxy for Cepheid and distance calibrations. The bottom 6 rows show the same stars as measured by both Hubble (grey labels) and JWST (purple labels) at various wavelengths. The superior resolution in JWST images reduces prior Hubble errors by significant, substantial amounts while validating and remaining consistent with prior results.

Instead, it confirmed and enhanced previous Hubble Space Telescope findings.

This composite image shows the barred spiral galaxy NGC 5584 with supernova SN 2007af shining brightly within it. Nearby galaxies with identifiable Cepheid variable stars and that have hosted at least one type Ia supernova within them are incredibly important to the cosmic distance ladder method of measuring the expanding Universe.

Cepheids in NGC 5584, which also had a (2007-era) type Ia supernova, also reveal no bias.

This graph shows the relationship between the magnitude of the brightness of Cepheid variable stars (y-axis) versus their period of variability (x-axis) in galaxies NGC 5584 (top) and NGC 4258 (bottom). The new JWST data is shown in red, while the old Hubble data is shown in grey. The errors and uncertainties of this relation in both galaxies are greatly reduced, primarily owing to JWST’s superior resolution over Hubble’s.

The period-luminosity relation, a key calibrator of Cepheids, is now more precise than ever.

By enabling a better understanding of Cepheid variables in nearby galaxies NGC 4258 and NGC 5584, JWST has reduced the uncertainties in their distances even further. The lowest points on the graph show the estimate for the distance to NGC 5584 from the expansion rates inferred from the distance ladder (left side) and what’s expected from the early relic method (right side). The mismatch is significant and compelling, and the uncertainties are tiny compared to the differences between the two methods.

With superior resolution, JWST has reduced any uncertainties down to their smallest values ever.

Standard candles (left) and standard rulers (right) are two different techniques astronomers used to measure the expansion of space at various times/distances in the past. Based on how quantities like luminosity or angular size change with distance, we can infer the expansion history of the Universe. Standard candles involve looking at objects whose intrinsic brightness is known at all cosmic distances, while standard rulers involve looking at features such as the average separation distance between any two galaxies (imprinted from baryon acoustic oscillations during the early stages of the Big Bang) that evolves as the Universe expands.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.