A known physical scale used to measure cosmic distances by comparing its apparent size.A standard ruler is a physical object or scale whose intrinsic size is known, allowing astronomers to measure cosmic distances by comparing its apparent angular size. This is a fundamental tool in cosmology, as it provides a way to map the universe's expansion history and test models of dark energy. The concept is central to studies of galaxy rotation, large-scale structure, and the homogeneity of the universe on vast scales.
The concept of using known-size objects to measure distances dates back to early astronomy, but the modern cosmological standard ruler—baryon acoustic oscillations—was first proposed in the 1990s by multiple groups, including Eisenstein, Hu, and Tegmark, building on earlier work by Peebles and Yu.
The method relies on the simple geometric relation between an object's true size, its distance from the observer, and its apparent angular size. If you know the true size of a 'ruler'—such as a galaxy, a cluster, or a feature in the cosmic microwave background—you can calculate its distance by measuring how large it appears in the sky. In practice, astronomers use standard rulers like baryon acoustic oscillations (the regular spacing of galaxies imprinted by sound waves in the early universe) or the turnoff point in stellar populations.
Standard rulers are used in large galaxy surveys like the Sloan Digital Sky Survey and the Dark Energy Spectroscopic Instrument to measure the expansion rate of the universe and constrain dark energy properties.
The 'standard ruler' of baryon acoustic oscillations has a characteristic length of about 150 megaparsecs—roughly 490 million light-years—which is the distance sound waves traveled in the plasma of the early universe before it cooled.
