The European Space Agency’s Euclid mission was launched on 1 July 2023. Euclid will take the next 6 years to make a very high-quality 3D map of the Universe over about one-third of the entire sky, measuring more than one billion galaxies over a large range of cosmic distance and time. Euclid carries two main experiments on board. One uses infrared imaging and spectroscopy to trace out the large-scale distribution of galaxies, the other uses optical imaging to make accurate measurements of cosmological weak-lensing.
In Oxford Physics, we work primarily on the weak lensing survey. This makes very accurate measurement of the apparent shapes of more than one billion distant galaxies (with redshifts z up to around 2), with correction for the effect of the telescope and imaging detectors. From those shape measurements, we can infer statistically the amount of bending of light paths, due to the gravitational effect of dark matter, as light passes through the Universe from the distant galaxies to us. The distortion in the shapes of galaxies does not tell us directly where along the line-of-sight the most light-bending occurs. However, methods exist to estimate each galaxy’s distance from us, and by comparing the shape distortions of galaxies at differing estimated distances, we can build up a 3D picture of the distribution of dark matter. Because light from distant galaxies takes billions of years to reach us, we can also measure how the dark matter distribution has evolved over cosmic time.