The "Hubble tension" represents the difference between the observed rate of expansion of the universe and the expected rate of expansion of the universe. The James Webb Space Telescope improved upon previous measurements made by the Hubble Space Telescope. Despite the progress, questions remain about the rapid expansion of the universe and the underlying cosmic phenomena underlying it.
Comprehensive observations from NASA's NIRCam (Near Infrared Camera) and Hubble's WFC3 (Wide Field Camera 3) show that the spiral galaxy NGC 5584 is 72 million light-years away from Earth. Among NGC 5584's luminous stars are pulsating stars called Cepheid variables and Type Ia supernovae, a special type of exploding star. Astronomers use Cepheids and Type Ia supernovae as reliable distance markers to measure the expansion rate of the universe. Image source: NASA, ESA, CSA and A.Riess (STScI)
The rate at which the universe expands, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the universe. However, there is a persistent discrepancy known as the "Hubble tension" between the value of the constant measured using various independent distance metrics and the value predicted from the Big Bang's afterglow.
NASA's James Webb Space Telescope provides new capabilities to review and refine some of the strongest observational evidence of this tension. Nobel laureate Adam Riess from Johns Hopkins University and the Space Telescope Science Institute described recent work by him and colleagues using Webb observations to improve the accuracy of local measurements of the Hubble constant.
The challenge of measuring the universe
Have you ever had trouble seeing a sign that's at the edge of your field of vision? What does it say? What does this mean? Even with the most powerful telescopes, the "signs" astronomers want to read appear so small that we struggle.
The signature cosmologists want to decipher is the cosmic speed limit signature, which tells us how fast the universe is expanding — a number called the Hubble constant. Our constellations are written into the stars of distant galaxies. The brightness of certain stars in these galaxies tells us how far away they are from us, and therefore how long it takes for this light to reach us, while the redshift of the galaxy tells us how much the universe has expanded during that time, thus telling us the rate of expansion.
This diagram illustrates the combined ability of NASA's Hubble and Webb space telescopes to determine the precise distance to a special class of variable stars used to calibrate the expansion rate of the universe. These Cepheids appear in a crowded star field. Light pollution from surrounding stars may make measurements of Cepheids' brightness less precise. Webb's sharper infrared vision allows the Cepheid target to be more clearly isolated from surrounding stars, as shown on the right. The Webb data confirms the accuracy of Hubble's 30 years of Cepheid observations, which were critical in establishing the bottom rung of the cosmic distance ladder that measures the expansion rate of the universe. On the left, NGC 5584 appears in a composite image from Webb's NIRCam (Near Infrared Camera) and Hubble's Wide Field Camera 3. Image source: NASA, ESA, A.Riess(STScI), W.Yuan(STScI)
A special class of stars, the Cepheid variables, have given us the most precise distance measurements for more than a century because these stars are so bright: they are supergiants, a hundred thousand times more luminous than the Sun. What's more, they pulsate (i.e. expand and contract in size) over several weeks, which indicates their relative brightness. The longer the period, the brighter they are intrinsically. They are the gold standard tool for measuring distances to galaxies 100 million light-years or beyond, a key step in determining the Hubble constant. Unfortunately, from our distant vantage point, stars in galaxies are crowded into a small space, so we often lack the resolution to separate them from their line-of-sight neighbors.
Hubble's contributions and Webb's progress
One of the main reasons for building the Hubble Space Telescope was to solve this problem. Before Hubble's launch in 1990 and subsequent Cepheid measurements, the rate of expansion of the universe was so uncertain that astronomers were unsure whether the universe had been expanding for 10 or 20 billion years. This is because a faster expansion rate will result in a younger universe, while a slower expansion rate will result in an older universe. Hubble has better resolution of visible wavelengths than any ground-based telescope because it sits above the blurring effects of Earth's atmosphere. It can therefore identify individual Cepheids in galaxies more than 100 million light-years away and measure the time intervals over which they change brightness.
However, we also have to look at Cepheids in the near-infrared part of the spectrum to see the light passing through the intervening dust unscathed. (Dust absorbs and scatters blue optical light, making distant objects appear dim and tricking us into believing they are further away than they actually are). Unfortunately, Hubble's red-light vision is not as clear as blue, so the starlight we see from Cepheids is mixed with other stars in the field of view. We could interpret the average mix statistically, much like a doctor would calculate body weight by subtracting the average weight of clothing from a scale reading, but doing so would add noise to the measurement. , because some people's clothes are heavier than others.
However, keen infrared vision is one of the James Webb Space Telescope's superpowers. With its large mirrors and sensitive optics, it can easily separate Cepheid light from neighboring stars with little mixing. In the first year of operation of the Webb Universal Observation Program in 1685, we collected observations of the Cepheids discovered by Hubble in two steps along the so-called cosmic distance ladder. The first step involves observing Cepheids in galaxies at known geometric distances, which allows us to calibrate the true luminosities of Cepheids. For our purposes, that galaxy is NGC 4258. The second step is to observe Cepheid variables in the host galaxy of the recent Type Ia supernova. The combination of the first two steps transfers distance knowledge to the supernovae to calibrate their true luminosities. The third step is to observe distant supernovae where the expansion of the universe is significant, which can be measured by comparing the distance inferred from their brightness to the redshift of the supernovae's host galaxy. This series of steps is called a distance ladder.
We recently obtained the first Webb measurements from steps one and two, which allowed us to complete the distance ladder and compare with previous Hubble measurements (see figure), due to the observatory's resolution at near-infrared wavelengths. This improvement is what astronomers dream of! We observed more than 320 Cepheids in the first two steps. We confirm that early Hubble Space Telescope measurements were accurate, albeit noisy. We also observed four other supernova hosts with Webb, and we saw similar results across the entire sample.
Comparison of Cepheid variable star period-luminosity relationships for measuring distances. The red dot is from NASA's Webb, and the gray dot is from NASA's Hubble. The top panel is NGC 5584, a Type Ia supernova host, and the inset shows image markers of the same Cepheid variable as seen by each telescope. The bottom panel is NGC 4258, a galaxy with a known geometric distance, and the inset shows the difference in distance modulus between NGC 5584 and NGC 4258 measured with each telescope. The agreement between the two telescopes is very good. Image source: NASA, ESA, A. Riess (STScI) and G. Anand (STScI)
The mystery of the persistence of the Hubble tension
The results still don't explain why the universe is expanding so fast! We can predict how fast the universe is expanding by looking at its baby image (the cosmic microwave background) and then using the best models of how the universe has grown over time to tell us how fast the universe should be expanding today. The fact that current measurements of the expansion rate greatly exceed predictions is a decade-long problem known as the "Hubble tension." The most exciting possibility is that tension is some of the missing clues in our understanding of the universe.
It could indicate the existence of exotic dark energy, exotic dark matter, a revision of our understanding of gravity, or the existence of unique particles or fields. The more common explanation is that multiple measurement errors colluded in the same direction (astronomers rule out individual errors by using independent steps), so that's why it's so important to redo the measurements with higher fidelity. With Webb's confirmation of Hubble's measurements, the Webb measurements provide the strongest evidence yet that systematic errors in Hubble's Cepheid photometry do not play a significant role in the current Hubble tension. As a result, more interesting possibilities remain and the mystery of the tension deepens.
This article highlights data from a paper accepted by The Astrophysical Journal.