Fixed typo in equation 2 -2058453335 Assigned ID: 4403 ----- Attempting Submission 1 (Wed May 24 16:35:39 GMT 2023) ----- HST Phase I Proposal 4403 successfully submitted. Receipt: # 4403-1 ----- Attempting Submission 2 (Wed May 24 19:01:47 GMT 2023) ----- The Earth's motion with respect to the cosmic microwave background frame means that galaxies outside the Milky Way will be subject to an ever-growing parallax shift, cosmic secular parallax. A measurement of this would be the first in the field of real-time cosmology (detection of changes in the Universe on human timescales). We propose to use multi epoch HST archival data to carry out extended source astrometry of 4434 galaxies with redshift z=0.05. The long lifetime of HST has allowed an amazing dataset to build up in the HLA, with these nearby galaxies observed for between 10 and 10000 times each, over a period of up to 32 years (the mean interval between epochs for our targets is 17.1 years). This baseline, combined with the tens of thousands of resolved elements per image and many images per galaxy will allow us to attempt measurements of the apparent proper motions due to cosmic parallax. We have developed an analysis pipeline, and tests on example galaxies reveal that the precision of WCS coordinates for absolute astrometry is at the 10 mas level. This should allow us to make detection of cosmic parallax, even at the current level of accuracy for absolute astrometry. We will also attempt over the course of the project to go further and carry out differential astrometry, making use of the multiple galaxies that are present in most frames. Preparatory science for the Roman Space telescope is also our goal. PI Dr Rupert Croft carried out the first modern study of cosmic parallax (Ding and Croft 2009), and the first work on galaxy images to explore parallax (Croft 2021, which used archival HST data). He has experience as a cosmologist, working for example with galaxy data from CHFTlens (Croft et. al 2017) and quasar host galaxy data (co-I on awarded JWST Cycle 1, Unveiling Stellar Light from Host Galaxies of z~6 Quasars. Co-I Mr. Ariel Amsellem is a graduate student who will be working on the project and carried out most of the preliminary work for the proposal. He has previously worked (Shin et al 2021) on galaxies in the Dark Energy Survey, including detection and masking of ghosting and scattered-light artifacts from optical survey images (Tanoglidis et al. 2023). Large Scale Structure of the Universe 0 Regular false false false false true false SUPPORTED