Roman will study how the universe’s web of matter has evolved by measuring how gravity subtly bends the path of light across vast distances—a phenomenon called gravitational lensing. 

This effect occurs because anything that has mass warps space-time, the underlying fabric of the universe. Extremely massive things like clusters of galaxies warp space-time so much that they can create strong gravitational lensing by bending the path light travels when it passes by them. That can smear or duplicate images of distant galaxies, or if the alignment is just right it can magnify them like a natural telescope.

Roman will be sensitive enough to study weak lensing, the same effect on a much smaller scale. The observations will help scientists create a far more detailed map of the distribution of matter, both seen and unseen, throughout the universe.

Dark Matter

This strange substance is perplexing because it’s invisible and only interacts with normal matter gravitationally; we can only see dark matter indirectly by watching how it affects things we can see. Roman’s weak lensing measurements will reveal the presence of clumps of dark matter that create distortions that are much too subtle to see in any single galaxy. Astronomers need a wide-eyed telescope like Roman to “see” where dark matter is via tiny changes it creates in the measured shapes of hundreds of millions of galaxies.

By mapping both normal and dark matter across an enormous swath of cosmic history, Roman will reveal how structures have grown under gravity from shortly after the big bang to today. Mapping dark matter more precisely than ever before could help astronomers figure out what it’s made of.

Dark Energy

Creating a 3D matter map will also aid the dark energy hunt because dark matter acts as drag, countering the universe’s expansion; its gravity pulls, and dark energy pushes. Astronomers will study how each force influenced cosmic evolution, which could help them figure out how and why dark energy is speeding up the universe’s expansion.

Weak lensing also reveals how the stretching of space-time influences the paths that light travels. Roman’s observations will offer clues about dark energy’s strength and behavior both through how it influences cosmic growth and leaves an imprint on the universe’s geometry. 

Since weak lensing measures how structure evolves over time, it provides insights that are highly complementary to other methods, like exploding stars and ancient cosmic sound waves. Together, these techniques will offer a more complete picture of cosmic expansion and dark energy’s impact. 

Telescopic Teamwork

Combining Roman’s observations with those from other key telescopes will help astronomers learn much more than they could from any of them individually. 

For example, Roman and ESA’s Euclid space telescope will conduct weak lensing observations in different types of light—visible wavelengths for Euclid and infrared wavelengths for Roman. Either approach introduces small errors due to inherent detector properties that astronomers need to correct for, so each mission can use the other’s data to cross-check their corrections. 

Pairing Roman data with observations from the ground-based Vera C. Rubin Observatory will allow scientists to try to detect the same objects in both sets of images. That’s important because ground-based observations aren’t always sharp enough to distinguish multiple, close sources as separate objects. Sometimes they blur together, which makes weak lensing effects much harder to see. By comparing Roman and Rubin images, scientists may be able to “deblend” objects Rubin sees and inch closer to achieving Roman-like quality over Rubin’s much greater sky coverage.

A key element in using weak lensing to study dark energy is measuring not just the shape but also the distance to each galaxy, since that influences the degree of warping. It’s sort of like how the magnification changes if you hold a magnifying glass over a page and move it up and down. Combining Rubin’s visible observations with Roman’s infrared measurements will give astronomers very accurate distance measurements, helping them correctly interpret weak lensing signals.