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Quasars as the new cosmic standard candles

In 1929, Edwin Hubble published observations that galaxies’ distances and velocities are correlated, with the distances determined using their Cepheid stars. Harvard astronomer Henrietta Swan Leavitt had discovered that a Cepheid star varies periodically with a period that is related to its intrinsic luminosity. She calibrated the effect, and when Hubble compared those calculated values with his observed luminosities he was able to determine their distances. But even today only Cepheid stars in relatively nearby galaxies can be studied in this way.In order to extend the distance scale back to earlier times in cosmic history, astronomers have used supernovae (SN) – the explosive deaths of massive stars—which can be seen to much greater distances. By comparing the observed brightness of a SN with its intrinsic brightness, based on its classification, astronomers are able to determine its distance; comparing that with the host galaxy’s velocity (its redshift, measured spectroscopically) yields the “Hubble relation” relating the galaxy’s velocity to its distance. The most reliable supernovae for this purpose, because of their cosmic uniformity, are so-called “Type Ia” supernovae, which are thought to be “standard candles,” all having the same intrinsic brightness. However even SN become harder to study in this way as they lie farther away; to date the most distant Type Ia SN with a reliable velocity determination dates from an epoch about 3 billion years after the big bang.

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