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When Hubble determined the distance to Andromeda he based his estimation on Cepheids. However, the result was less than half the current value. What was the cause of this error and could another adjustment happen in the future?

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The recalibration was made by German astronomer Walter Baade. The Wikipedia article on Baade says:

During World War II, he took advantage of wartime blackout conditions (which reduced light pollution), to resolve stars in the centre of the Andromeda Galaxy for the first time. These observations led him to define distinct "populations" for stars (Population I and Population II). The same observations led him to discover that there are two types of Cepheid variable stars. Using this discovery he recalculated the size of the known universe, doubling the previous calculation made by Edwin Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the International Astronomical Union in Rome.

We now have a much better understanding of Cepheid variables than Hubble did, so it is unlikely that this particular element of the cosmic distance ladder will be significantly adjusted again. However, other elements are less certain and could indeed be adjusted in the future. There is, for example, considerable uncertainty in the value of the Hubble constant, since different ways of measuring it give different values.

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  • $\begingroup$ Thank you for the excellent answer $\endgroup$ Nov 4, 2023 at 23:38

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