Niko Šarčević on Modern Cosmology

Most of what we know about the universe actually comes from what we can’t see.
Only a tiny fraction of the cosmos is made of “normal” matter—the stuff that makes up stars, planets, and us. The rest is a mysterious combination we call dark matter and dark energy, which, although invisible to our telescopes, is absolutely crucial for how the universe expands and how structures form over billions of years.

So how do we even study something we cannot see?

One of the most powerful tools we have is weak gravitational lensing. As light from distant galaxies travels through the cosmic web, the gravity of dark matter gently stretches and shears those galaxy images. The effect on any single galaxy is tiny, almost imperceptible. But when you measure this across millions or even billions of galaxies, a pattern emerges—a subtle cosmic fingerprint that tells us how matter is distributed and how fast the universe is expanding.

This is what our guest today, Dr. Nikolina Sarcevic, works on. She is a cosmologist working at the intersection of data and theory. Nikolina is part of the LSST Dark Energy Science Collaboration, and her work focuses on understanding and modelling the systematics that can bias our measurements—things like how galaxies are intrinsically aligned, how we infer their redshift distributions, and how all of that feeds into weak lensing and dark energy constraints.

So if you’ve ever wondered how we really know that dark energy exists, or what kinds of experiments are used to learn about this invisible matter, you’re going to be thrilled. So with that, let’s go.