Free Astronomy Magazine May-June 2024

18 MAY-JUNE 2024 ASTRO PUBLISHING A t the center of these side-by- side images is a special class of star used as a milepost marker for measur- ing the universe’s rate of expansion – a Cepheid variable star. The two im- ages are very pixe- lated because they are a very zoomed- in view of a distant galaxy. Each of the pixels represents one or more stars. The image from the James Webb Space Telescope is signifi- cantly sharper at near-infrared wavelengths than Hubble (which is primarily a visible-ultraviolet light telescope). By reducing the clutter with Webb’s crisper vision, the Cepheid stands out more clearly, eliminating any potential confusion. Webb was used to look at a sample of Cepheids and confirmed the accuracy of the previous Hubble observations that are fundamental to precisely measuring the universe’s expansion rate and age. [NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)] or measurement technique relies upon the previous step for calibra- tion. But some astronomers suggested that, moving outward along the “second rung,” the cosmic distance ladder might get shaky if the Cepheid measurements become less accurate with distance. Such inaccu- racies could occur because the light of a Cepheid could blend with that of an adjacent star – an effect that could become more pronounced with distance as stars crowd to- gether and become harder to distin- guish from one another. The observational challenge is that past Hubble images of these more distant Cepheid variables look more huddled and overlapping with neighboring stars at ever farther distances between us and their host galaxies, requiring careful account- ing for this effect. Intervening dust further complicates the certainty of the measurements in visible light. Webb slices though the dust and naturally isolates the Cepheids from neighboring stars because its vision is sharper than Hubble’s at infrared wavelengths. “Combining Webb and Hubble gives us the best of both worlds. We find that the Hubble measure- ments remain reliable as we climb farther along the cosmic distance ladder,” said Riess. The new Webb observations include five host galaxies of eight Type Ia su- pernovae containing a total of 1,000 Cepheids, and reach out to the far- thest galaxy where Cepheids have been well measured – NGC 5468 – at a distance of 130 million light-years. “This spans the full range where we made measurements with Hubble. So, we’ve gone to the end of the second rung of the cosmic distance ladder,” said co-author Gagandeep Anand of the Space Telescope Sci- ence Institute in Baltimore, which operates the Webb and Hubble tel- escopes for NASA. Hubble and Webb’s further confir- mation of the Hubble Tension sets up other observatories to possibly settle the mystery. NASA’s upcoming Nancy Grace Roman Space Telescope will do wide celestial surveys to study the influence of dark energy, the mysterious energy that is caus- ing the expansion of the universe to accelerate. ESA’s Euclid observatory, with NASA contributions, is pursuing a similar task. At present it’s as though the dis- tance ladder observed by Hubble and Webb has firmly set an anchor point on one shoreline of a river, and the afterglow of the big bang observed by Planck’s measurement from the beginning of the universe is set firmly on the other side. How the universe’s expansion was chang- ing in the billions of years between these two endpoints has yet to be directly observed. “We need to find out if we are missing something on how to connect the beginning of the universe and the present day,” said Riess. !