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Cosmic ruler and cosmic ladder

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If you have a question about this talk, please contact Tommaso Giannantonio.

The Baryon Acoustic Oscillation feature is a key observable in cosmology, corresponding to the maximum distance sound waves of baryonic matter can travel in the early Universe. It leaves a measurable imprint in the clustering of galaxies at late times, which can be used as a “standard ruler”. I will discuss how this feature can be used in combination with other observations to construct a cosmic distance ladder which strongly constrains the expansion history of the Universe. While the traditional way to build a cosmic distance ladder is to calibrate locally and extend it out to high redshift, the BAO feature allows us to calibrate at z~1000 and extend the ladder inwards. In combination with other observations (standard candles and/or standard clocks) it is even possible to measure the expansion history of the Universe, constrain its spatial curvature, and determine the length of the standard ruler in a way that is model independent. The better than 1% measurement of the BAO location from galaxy surveys requires a better than 1% modeling for its correct cosmological interpretation. With this approach one can trade off precision for robustness. In fact it is possible to use only very weak assumptions of symmetry and a smooth expansion history, and one does not even need assuming General Relativity. Since the length is set early on in the Universe’s history, the conclusions are independent of late-time physics such as Dark Energy properties or late modifications to gravity. Even without assuming GR, the Universe looks very much like the standard LCDM model.

This talk is part of the Cosmology Lunch series.

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