Measuring the Hubble Constant: Insights from Gravitationally Lensed Supernovae.

NASA’s James Webb Space Telescope’s NIRCam image of the galaxy cluster PLCK G165.7+67.0 reveals the magnifying effect of the foreground cluster, located 3.6 billion light-years away. The zoomed-in section shows the triply imaged supernova H0pe, highlighted with white dashed circles, due to gravitational lensing. Color coding indicates different wavelengths: blue (0.9, 1.15, 1.5 microns), green (2.0, 2.77 microns), and red (3.56, 4.1, 4.44 microns). Full-resolution images are available for download from the Space Telescope Science Institute.

The Hubble constant, which describes the rate of the universe's expansion, remains a key focus for astronomers. Recently, NASA's James Webb Space Telescope has added valuable data to this ongoing research. Notably, astronomers utilized Webb's observations of Cepheid variables and Type Ia supernovae—known distance markers—to reinforce previous measurements made by the Hubble Space Telescope.

In a significant new effort, researchers led by Brenda Frye from the University of Arizona are employing an independent method: gravitationally lensed supernovae. The team discovered three distinct points of light in the direction of a densely populated galaxy cluster, a find made possible by Webb's imaging capabilities. Frye elaborates on this discovery, dubbed Supernova H0pe, and its implications for measuring the Hubble constant.

Discovery and Initial Analysis.

The investigation began with a simple question: "What are those three dots that weren’t there before?" When images from Webb’s Guaranteed Time Observations arrived, it became evident that these points corresponded to a Type Ia supernova—an explosion of a white dwarf star, often referred to as a "standard candle" due to its consistent intrinsic brightness.

The Role of Gravitational Lensing.

Gravitational lensing plays a crucial role in this research. A cluster of galaxies located between the supernova and Earth bends the light from the explosion, resulting in multiple images. This phenomenon is akin to seeing oneself reflected in a trifold mirror, where different angles provide varying views of the same scene. In this case, the three images captured by Webb were observed at different times due to the light traveling along distinct paths.

The time delays associated with these images, combined with the supernova's distance and the properties of gravitational lensing, provide a unique opportunity to calculate the Hubble constant, denoted as H0. This supernova, hence named SN H0pe, offers promising insights into the universe's expansion.

Further Investigations and Measurements.

To delve deeper into the characteristics of SN H0pe, the research team proposed additional observations through Webb's Director’s Discretionary Time program. Simultaneously, data were gathered using the MMT and Large Binocular Telescope in Arizona. Their analyses confirmed that SN H0pe is linked to a galaxy existing 3.5 billion years after the Big Bang.

Notably, SN H0pe is among the most distant Type Ia supernovae observed. A team member further measured the light's time delay by analyzing its spectrum, solidifying the supernova's classification.

Results and Implications.

The research involved multiple subgroups creating lens models to describe the galaxy cluster's mass distribution. Each model was evaluated based on its accuracy in predicting the time delays and brightness of the supernova. Results were kept blinded from the groups to eliminate bias, with the findings revealed simultaneously during a coordinated "live unblinding" event.

The calculated value for the Hubble constant from this method is approximately 75.4 kilometers per second per megaparsec, with an uncertainty of plus 8.1 or minus 5.5. This marks the second measurement of the Hubble constant through gravitational lensing and the first using a standard candle.

Conclusion.

The implications of this research are significant. The Hubble constant value aligns closely with local measurements but contrasts with values derived from the early universe. Future Webb observations are anticipated to reduce uncertainties further, enhancing our understanding of this fundamental cosmic parameter. As Frye notes, this discovery is a key step toward unraveling the mysteries of the universe's expansion.

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