Measuring Hubble's Constant to 1% with Pulsating Stars

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What's up with Hubble's constant (H0)? Recent H0 measurements have shown that the Universe is expanding faster than cosmology predicts, indicating a possible cosmological crisis. To wit, H0 measured to 1.9% precision in today's Universe using a cosmic distance ladder composed of classical Cepheids and type-Ia supernovae differs by 8.9% (4.2 sigma) from H0 predicted by cosmology based on observations of the Cosmic Microwave Background emitted 13.8 billion years ago. However, it remains unclear whether new physics must be invoked to reconcile cosmology with today's H0, or whether today's H0 is subject to as yet unknown or underestimated systematic errors. An unbiased 1% measurement of H0 is required to understand whether physics is on the brink of a major breakthrough. So, how solid is our distance ladder? To answer this, my research seeks to a) mitigate biases that can shift the center value of reported H0 measurements, b) quantify relevant systematic uncertainties, and c) reinforce the foundation of the distance ladder through a solid astrophysical understanding of pulsating stars, in particular, classical Cepheids. These steps must be taken now to achieve an unbiased 1% H0 measurement and to ensure the legacy of today's distance ladder for future space-borne facilities and ground-based extremely large telescopes. Imminent data releases of the ESA mission Gaia and precise time series spectroscopy will provide unprecedented opportunity to calibrate the distance ladder and unravel the structure and evolution of Cepheids through their variability. The H1PStars project will leverage my expertise in the astrophysics of classical Cepheids and the calibration of the distance ladder to support precision cosmology via accurate stellar physics, and vice versa. Thanks to this fresh perspective, my team and I will either reconcile the tension in H0 or confidently establish a need to revise cosmology.