![]() ![]() Hubble has better visible-wavelength resolution than any ground-based telescope because it sits above the blurring effects of Earth’s atmosphere. That’s because a faster expansion rate will lead to a younger age for the universe, and a slower expansion rate will lead to an older age of the universe. Prior to Hubble’s 1990 launch and its subsequent Cepheid measurements, the expansion rate of the universe was so uncertain astronomers weren’t sure if the universe has been expanding for 10 billion or 20 billion years. “A major justification for building the Hubble Space Telescope was to solve this problem. ![]() Unfortunately, stars in galaxies are crowded together in a small space from our distant vantage point and so we often lack the resolution to separate them from their line-of-sight neighbors. They are the gold standard tool for the purpose of measuring the distances of galaxies a hundred million or more light years away, a crucial step to determine the Hubble constant. The longer the period, the intrinsically brighter they are. What’s more, they pulsate (that is, expand and contract in size) over a period of weeks that indicates their relative luminosity. “A particular class of stars, Cepheid variables, has given us the most precise measurements of distance for over a century because these stars are extraordinarily bright: They are supergiant stars, a hundred thousand times the luminosity of the Sun. At the left, NGC 5584 is seen in a composite image from Webb’s NIRCam (Near-Infrared Camera) and Hubble’s Wide Field Camera 3. The Webb data confirms the accuracy of 30 years of Hubble observations of Cepheids that were critical in establishing the bottom rung of the cosmic distance ladder for measuring the universe’s expansion rate. Webb’s sharper infrared vision allows for a Cepheid target to be more clearly isolated from surrounding stars, as seen in the right side of the diagram. Light contamination from surrounding stars may make the measurement of the brightness of a Cepheid less precise. These Cepheid variable stars are seen in crowded star fields. This diagram illustrates the combined power of the NASA’s Hubble and Webb space telescopes in nailing down precise distances to a special class of variable star that is used in calibrating the expansion rate of the universe. The brightnesses of certain stars in those galaxies tell us how far away they are and thus for how much time this light has been traveling to reach us, and the redshifts of the galaxies tell us how much the universe expanded over that time, hence telling us the expansion rate. Our sign is written into the stars in distant galaxies. “The sign cosmologists want to read is a cosmic speed limit sign that tells us how fast the universe is expanding - a number called the Hubble constant. “Did you ever struggle to see a sign that was at the edge of your vision? What does it say? What does it mean? Even with the most powerful telescopes, the ‘signs’ astronomers want to read appear so small that we struggle too. Nobel Laureate Adam Riess from the Johns Hopkins University and the Space Telescope Science Institute presents his and his colleagues’ recent work using Webb observations to improve the precision of local measurements of the Hubble constant. NASA’s James Webb Space Telescope provides new capabilities to scrutinize and refine some of the strongest observational evidence for this tension. Astronomers use Cepheid variables and Type Ia supernovae as reliable distance markers to measure the universe’s expansion rate.ĭownload the high-resolution file from the Resource Gallery.Ĭredit: NASA, ESA, CSA, and A. Among NGC 5584’s glowing stars are pulsating stars called Cepheid variables and Type Ia supernova, a special class of exploding stars. Combined observations from NASA’s NIRCam (Near-Infrared Camera) and Hubble’s WFC3 (Wide Field Camera 3) show spiral galaxy NGC 5584, which resides 72 million light-years away from Earth. However, a persistent difference called the “Hubble Tension” is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the big bang afterglow. The rate at which the universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. ![]()
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