In July 2023 we saw the successful launch of Euclid, a space telescope on a mission to map the distribution of matter in the Universe in hopes of helping us understand the nature of dark energy and dark matter, which in terms of mass-energy density make up 95% of everything that exists. After a few teething problems which required the uploading of revised software to the craft, the 1.2 m aperture telescope is up and running and producing some remarkable images.

With the onboard visible light and near-infrared wavelength cameras and spectrometers, the Euclid team hopes to spend six years producing a 3D map of billions of galaxies out to a distance of 10 billion light years. The mission website describes the three dimensions as two in space and one in time. And, of course, the magical thing about telescopes is that, on account of the finite speed of light, they are time machines that allow us to look back in time as we peer ever deeper into space. 

The mission is named after the Greek mathematician who in ancient times formulated a system for understanding plane geometry. Most modern theories of the structure of the Universe require a four-dimensional geometry, which may actually be curved and therefore non-Euclidian.

Einstein's general theory of relativity is the cornerstone of modern cosmology. He was famously unhappy with the solutions to his equations which predicted a non-static universe, including his now infamous cosmological constant to negate the need for space-time to be either expanding or contracting. Up until the 1990s, as a consequence of Edwin Hubble's observations, which demonstrated an expanding Universe that could be extrapolated back to an infinitesimal point which we anachronistically call the Big Bang, it was expected that the expansion of the Universe was slowing, in much the same way as a pebble tossed into the air slows and eventually falls to Earth. However, it then came as something of a shock to the physics community to discover that observations of extremely distant type 1a supernovae implied that the Universe is actually accelerating in its expansion!

This surprising revelation has been backed up by measurements of baryonic acoustic oscillations, which are vast fossilized spherical shells in the distribution of galaxy clusters, produced by the last density waves to propagate through the primordial soup of the early Universe. In addition to discovering that the expansion of the Universe continues to be driven by some dark energy that permeates space, we already knew from the dynamics of galaxies in galaxy clusters, as well as the rotation curves of individual galaxies like our own Milky Way, that the visible matter of the Universe is hugely outweighed by some unseen material that we now call dark matter. One would normally expect that individual stars orbiting the centre of a galaxy would behave in a Keplerian manner, exhibiting decreasing speeds with increasing distance from the centre. However, when plotted, the stars of most spiral galaxies appear to be under the influence of a great deal of unseen mass that manifests itself as puzzlingly high orbital velocities out to great radii.

It is important to understand the difference between dark energy and dark matter. Dark matter, which is presumed to be comprised of some hitherto undiscovered particle (or even primordial black holes), is both influenced by gravity and influences the shape of space-time in accordance with general relativity, and in some sense causes attraction. Dark energy is a repulsive influence pushing as it were against the fabric of space-time like a form of negative pressure. Ironically, many physicists have re-invoked Einstein's cosmological constant in order to explain the effects of dark energy, an irony which would certainly not have been wasted on Einstein himself who, after the observations of Hubble, called the inclusion of the constant his "greatest blunder".

So whether dark energy turns out be akin to the cosmological constant, or perhaps a manifestation of the uncertainty principle of quantum mechanics that forbids empty space with zero energy (hopefully amongst the vast quantities of data expected from Euclid), there may be some clue as to the nature of the 95% of the Universe of which we don't have any understanding... to date!