Cosmological measurements often make use of fun and interesting ideas that depart from our everyday experience. Here cosmologist Jamie McCullough explains and helps us visualize several of the concepts involved in plans to measure the distribution of dark matter in the universe and probe the growth of cosmic structure over time.
by Jamie McCullough
26 February 2025
When DESI measures the spectrum of light from a galaxy – i.e., the intensity of light as a function of wavelength or energy – we learn a lot about the physics of what is happening both inside the galaxy and in the space between the galaxy and us.
A particularly important property we can measure is a galaxy’s redshift. As galaxies recede from us in an expanding universe, their light is pulled to less energetic wavelengths, or redshifted, by the same doppler effect that makes train whistles pitch low as they travel away from us. In an expanding universe, more distant objects are redshifted more than nearby objects. As a result, if we know how the universe is expanding, we can learn the distance to the galaxy from a measurement of its redshift. With the ability to measure the spectra of as many as 5000 galaxies at a time, DESI is uniquely suited to measuring the distances to galaxies across the night sky and thereby mapping the visible universe. You can see redshift in action for a galaxy in the figure below. Here the vertical dashed line marks a bright emission line of oxygen in star-forming galaxies. The number in the upper left corner is the redshift z = v/c where v is the recession velocity and c is the speed of light.
However, the visible universe accounts for only a small fraction of the matter we know exists. The vast majority doesn’t interact with light in any known way – it’s dark matter. The spatial distribution of this dark matter is of great interest, because it drives the movement of galaxies in the universe. Mapping the dark matter — which is thought to be arranged in long filaments, i.e., in a dark cosmic web — requires more information than DESI alone can provide. If we combine the galaxy distance measurements from DESI with the measurement of galaxy shapes from imaging surveys (like, for example, the Dark Energy Survey (DES), the Kilo-Degree Survey (KiDS), and the Hyper Suprime-Cam (HSC) survey), we can map the dark structure using a method called weak gravitational lensing. This method relies on an effect from general relativity that massive objects warp the geometry of space and time. As a result, an image of a distant galaxy will be distorted as light from the galaxy passes massive objects on its way to us.
To understand how weak lensing works, we can first look at the case of a more straightforward example. In what’s called “strong lensing,” we can readily see this warping in action, as we find images of background galaxies stretched and distorted tangentially around a very massive foreground object like a galaxy cluster. This stretching is very similar to the optical effect you might see looking through the thick base of a wine glass. With a thicker piece of glass we see more refraction, just as we see more distortion behind more massive galaxy clusters. You can see this effect in the simulation below and in this other simulation, in which a moving massive galaxy passes first in front of a grid of shapes, and then in front of the Hubble Deep Field. We see that these strong lenses can stretch a background galaxy’s light into continuous rings called “Einstein rings”. They can even produce more than one image of the same background galaxy! What happens all depends on how the background and foreground objects line up.
However, the typical (weak lensing) distortion that a galaxy’s light experiences on its way to us is not as dramatic as in the above examples. Warping from large-scale structure changes galaxy shapes much less, on the order of a mere percent. As we can see in the prior simulations, the amount of distortion depends very strongly on the distance to the background object and to the lensing structure, so understanding those distances is crucial. We expect that if we measure enough of these galaxy shapes and their correlations with one another, we can trace those dark cosmic filaments dominated by dark matter. You can see a toy model of weak lensing below, where a hypothetical background grid of perfect circles becomes displaced, magnified, and sheared about the hidden structure shaded in blue.
If we measure the distances and observed shapes of these galaxies and correlate them with one another, we can devise a relationship for how alike any two shapes are as a function of their separation (𝜉±). With these weak gravitational lensing measurements and different cosmological models, we can find the one that best explains our observations and produces constraints on how clumpy our universe is (as measured by a quantity called S8) and how much matter there is in the universe (as measured by a quantity called 𝛺m).
With the incredibly precise distances we get from DESI and the increasingly precise measurements we are now getting for galaxy shapes, we can perform these lensing measurements better than ever before – making it a very promising way to probe the growth of cosmic structure over time and learn about all the contents of our universe — both the luminous and the dark!
— Edited by Joan Najita
