Cosmic Clocks and Cosmic Chronometers: Constraining the Hubble Parameter Now and Then

Abstract: Constraining the expansion history of the Universe with new and complementary approaches is crucial now that we are in the age of precision cosmology, where managing systematic effects and increasing the accuracy of the measurements is essential. In this talk, I will present how the ages of the oldest objects in our Universe can shed light on its expansion history, both at high redshift and in the local Universe. At high redshift, the Hubble parameter can be directly measured by analyzing the differential aging of massive, passive galaxies through the "cosmic chronometers" (CCs) approach. I will show two applications of the method, the first at z~1.5 using data from the VANDELS survey, the second at z~0.5 making use of high-quality VLT/MUSE spectroscopic data for three galaxy clusters. After a careful selection process of the CCs sample, we derive their physical properties by performing full spectrum fitting (FSF) on their spectra and photometry. From the observed differential aging of these galaxies across redshift, we obtain new, direct measurements of the Hubble parameter. In the local Universe, the oldest stars and globular clusters behave instead as "cosmic clocks", naturally placing a lower limit on the current age of the Universe and, in turn, an upper limit on the Hubble constant (H0). While traditionally their age estimates have been confined to redshift z=0, the advent of the James Webb Space Telescope has enabled observations of globular clusters in lensed fields at redshifts far beyond z=0, opening the door to extending the measurement of cosmic clocks to earlier epochs. Here I will present both applications of the cosmic clocks approach: the first at z=0, where we analyse a sample of globular clusters in the Milky Way, the second is an exploratory study on the candidate stellar clusters in the Sparkler galaxy, located at redshift z = 1.378 and strongly lensed by the galaxy cluster SMACS J0723.3-7327. Using the same FSF method, we derive the age of these clusters without making any cosmological assumptions, enabling us to set a lower limit on the age of the Universe and constrain H0. As current and future surveys expand the pool of massive, passive galaxies and lensed globular clusters, cosmic chronometers and cosmic clocks offer a powerful tool for constraining the expansion history of the Universe across an unprecedented range of epochs.