Free Astronomy Magazine May-June 2024

17 MAY-JUNE 2024 ASTRO PUBLISHING T his image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the farthest galaxy in which Hubble has identified Cepheid variable stars. These are important milepost markers for measuring the expansion rate of the universe. The distance calculated from Cepheids has been cross-corre- lated with a type Ia supernova in the galaxy. Type Ia supernovae are so bright they are used to measure cosmic distances far beyond the range of the Cepheids, extending measurements of the universe’s expansion rate deeper into space. [NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)] W hen you are trying to solve one of the biggest conundrums in cosmol- ogy, you should triple check your homework. The puzzle, called the “Hubble Tension,” is that the cur- rent rate of the expansion of the universe is faster than what as- tronomers expect it to be, based on the universe’s initial conditions and our present understanding of the universe’s evolution. Scientists using NASA’s Hubble Space Telescope and many other tel- escopes consistently find a number that does not match predictions based on observations from ESA’s (European Space Agency’s) Planck mission. Does resolving this discrep- ancy require new physics? Or is it a result of measurement errors be- tween the two different methods used to determine the rate of ex- pansion of space? Hubble has been measuring the cur- rent rate of the universe’s expansion for 30 years, and astronomers want to eliminate any lingering doubt about its accuracy. Now, Hubble and NASA’s James Webb Space Telescope have tag-teamed to produce de- finitive measurements, furthering the case that something else – not measurement errors – is influencing the expansion rate. “With measurement errors negated, what remains is the real and exciting possibility we have misunderstood the universe,” said Adam Riess, a physicist at Johns Hopkins University in Baltimore. Riess holds a Nobel Prize for co-discovering the fact that the universe’s expansion is accelerat- ing, due to a mysterious phenome- non now called “dark energy.” As a crosscheck, an initial Webb ob- servation in 2023 confirmed that Hubble measurements of the ex- panding universe were accurate. However, hoping to relieve the Hub- ble Tension, some scientists specu- lated that unseen errors in the measurement may grow and be- come visible as we look deeper into the universe. In particular, stellar crowding could affect brightness measurements of more distant stars in a systematic way. The SH0ES (Supernova H0 for the Equation of State of Dark Energy) team, led by Riess, obtained addi- tional observations with Webb of objects that are critical cosmic mile- post markers, known as Cepheid variable stars, which now can be correlated with the Hubble data. “We’ve now spanned the whole range of what Hubble observed, and we can rule out a measurement error as the cause of the Hubble Tension with very high confidence,” Riess said. The team’s first few Webb observa- tions in 2023 were successful in showing Hubble was on the right track in firmly establishing the fi- delity of the first rungs of the so- called cosmic distance ladder. Astronomers use various methods to measure relative distances in the universe, depending upon the ob- ject being observed. Collectively these techniques are known as the cosmic distance ladder – each rung