¶In June, ESA selected three ‘deep fields’ for their upcoming Euclid mission. These deep fields, or regions of the sky unusually free of Milky Way stars, will be a partial focus of Euclid’s 2022 mission to map the structure, gravitational distortion, and distance-redshift relationships of galaxies in order to study dark energy and dark matter. Dark energy (also known as Einstein’s Cosmological Constant and self-proclaimed “greatest blunder”) is the non-zero energy density of space itself that we think is causing the accelerating expansion of the Universe. (Since the expansion started accelerating ~10 billion years ago, Euclid will focus on galaxies whose redshift is within that window.) The nature of dark energy is intimately tied to the eventual fate of the Universe—whether it will collapse, expand at a moderate pace, or expand so fast that it rips literally everything apart—which is something that keeps us all up at night. Dark matter, Euclid’s other focus through a survey of gravitational lensing, has nothing to do with the ill-named dark energy. Dark matter is matter that seems to interact only through gravity and must exist to explain gravitational lensing, the rotation of galaxies, and many other observations (cf. an entirely scientific and accurate survey of possible dark matter explanations). It’s estimated that about 71% of the mass-energy of the universe is dark energy, 24% is dark matter, and only about 5% is the “normal” matter that we observe. Related: Cosmology by Nobel laureate Brian Schmidt and From the Big Bang to Dark Energy are both excellent free online courses about these subjects. (Special thanks to our intern Emma Drake for helping write this and other entries this summer—she’s off to Berkeley to study astrophysics!) | 
