Measuring Hubble's Constant to 1% with Pulsating Stars

Project Information

H1PStars

Grant agreement ID: 947660

DOI

10.3030/947660

Project closed

EC signature date 26 February 2021

Start date 1 April 2021

End date 31 March 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 836 000,00

EU contribution

€ 1 836 000,00

Coordinated by

ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE
Switzerland

Periodic Reporting for period 3 - H1PStars (Measuring Hubble's Constant to 1% with Pulsating Stars)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The H1PStars project seeks to measure with unprecedented accuracy how fast the Universe is currently expanding. The cosmological quantity that measures this expansion is called the Hubble constant (H0) and is crucial to understanding how the Universe evolves in time. For example, H0 tells us the age and observable size of the Universe. However, there is a major problem with H0! The value of H0 measured using the stars and galaxies that we observe in the nearby Universe does not match the value of H0 obtained by interpreting the oldest observed radiation (the cosmic microwave background) using the most complete understanding of the fundamental forces and contents of the Universe (the LambdaCDM model). Assuming all the measurements were done correctly, this mismatch between present-day and early-Universe H0 values implies the LambdaCDM cosmology to be incomplete, that is, that we have missed some fundamental aspects of basic physics whose further inspection could lead to a major breakthrough with potentially sweeping implications across physics. For the time being, however, no solution that can satisfactorily resolve this so-called Hubble constant tension has been suggested -- at least not without causing other cosmological problems. Better measurements are therefore urgently needed to find a convincing resolution to the Hubble constant tension.

The H1PStars project addresses this need by investigating the astrophysics of pulsating stars as well as their use as "standard candles" to measure astronomical distances. Pulsating stars are fascinating objects whose brightness varies periodically due to self-excited oscillations that provide important insights for stellar astrophysics. Even more importantly, the intrinsic brightnesses of such stars are related to their pulsations via Leavitt laws, which can be calibrated using geometrically measured distances. Thus, pulsating stars are the backbone of the distance ladder used to measure the Hubble constant and also dominate its uncertainties. The H1PStars project pursues a three-pronged approach to push towards a 1% measurement of H0 by a) reinforcing the calibration of Leavitt laws based on geometric distances; b) improving the standardization of distances measured using Leavitt laws; c) developing new insights into the astrophysics of pulsating stars to ensure that the claimed accuracy of our distance measurements is supported by a solid understanding of the stars upon which they are based.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the first half of this project, we have worked on all three key objectives: a) reinforcing the calibration of pulsating stars as standard candles; b) improving the standardization of their distance estimates; c) developing a better understanding of their physics and properties. Refereed publications related to all three objectives have been published, and several more are currently in preparation. The work was disseminated at several international conferences and workshops, and a press release was published.

For a) we have extensively studied data from the ESA mission Gaia, whose third data release published an unprecedented dataset of positions, parallaxes (distances), motions, light curves, and other information for 1.8 billion stars. We worked to obtain the best Leavitt law calibrations using this data set, which required much work to understand the data and to understand how to deal with the flaws that appear at the limits of Gaia's capabilities. Additionally, we measured how the chemical composition of pulsating stars affects their distance measurements.

For b) we collected new observations with the Hubble Space Telescope (HST) and the brand-new James Webb Space Telescope. We analyzed these data along with existing HST images to quantify systematic uncertainties related to the brightness measurements of pulsating stars many tens of millions of light years distant. We also developed methods to correct for changes in the measured light that are caused by effects explained by Einstein's special relativity. Last, but not least, we measured the distance of the Whirlpool galaxy using classical Cepheids and a type-IIp supernova.

For c) we collected a very large number of observations from telescopes in Chile and the island of La Palma for the VELOCE project. These observations have allowed us to describe the pulsations (periodic expansions and contractions) measured along the line of sight in unprecedented detail. We developed novel methods to search for periodic signals in these observations and prepared the first data release of the VELOCE project.
Last, but not least, we used large computer simulations to investigate how stellar dynamics influence the properties and evolution of pulsating stars.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Using Gaia data, we achieved the most accurate calibration of classical Cepheid stars for distance measurements to date. To this end, we conducted an unprecedented search for Cepheids residing in open star clusters whose average parallaxes we determined within a typical uncertainty of 7 microarcseconds. This tiny angle corresponds to the diameter of a small coin located on the surface of the Moon as seen from the Earth. In collaboration with American colleagues, we were able to reduce the uncertainty of the previously best H0 measurement by 7%. We expect that we can further improve the accuracy of our calibration by at least a factor of two using the highly anticipated next Gaia data release.

We provided the first comprehensive investigation of relativistic effects on distance measurements based on pulsations stars. We showed that the so-called K-corrections can significantly bias distance measurements planned by other teams, requiring solutions for mitigation.

Observations collected with the Hubble and James Webb Space Telescopes have significantly improved our understanding of systematic uncertainties in the brightness measurements of Cepheids. Specifically, the JWST observations showcased the significant improvement in brightness measurements thanks to JWST's unparalleled spatial resolution and sensitivity. We expect that JWST will play a vital role in further improving the accuracy of the Hubble constant measurement.

Our dynamical simulations have drawn the first detailed picture of how the dynamics of large groups of stars affect the evolution of classical Cepheid stars. These results show that interactions among stars in binary and higher-order multiple star systems, as well as clusters, have a significant influence over the observed properties of these crucial stars.

Last, but not least, the new observations collected from Chile and La Palma have been used to develop an unprecedented reference of line-of-sight velocity curves of classical Cepheids. Thanks to a unique combination of extreme precision, temporal baseline, and phase sampling, we have identified previously unknown features of Cepheid pulsations and are able to obtain the most detailed study of Cepheid multiplicity to date.

https://esahubble.org/images/heic1611a/; Credit: NASA, ESA, A. Feild (STScI), A. Riess (STScI/JHU)