Elena Kopteva

The discovery that the universe is expanding at an accelerating rate — a finding that led to the idea of dark energy — remains one of the deepest mysteries in modern cosmology.

Together with my colleagues, I work to understand this question using Stephani-class cosmologies — smooth, non-rotating universes filled with a perfect fluid whose pressure changes from place to place. In these inhomogeneous models, the observed cosmological acceleration can be reproduced without the need for dark energy.

In our work, we treat these models as diagnostic labs rather than literal global cosmologies. We develop results such as a closed-form redshift–magnitude relation, and we continue to test these models by comparing them with observations of supernovae, baryon acoustic oscillations, and structure growth, to see which versions remain consistent with the universe we observe.

They are surrounded by cosmic environments — dark-energy–like fields, radiation, matter flows, and evolving cosmological backgrounds — all of which can subtly shape what we observe.

My colleagues and I build and analyze exact general-relativistic solutions that describe black holes within these environments. We study two broad cases: (a) static, radially varying anisotropic media, such as dark-energy–like fluids or networks of topological defects; and (b) time-dependent cosmological backgrounds modeled by dust, radiation, or both. To construct such models, we use generalized Lemaître–Tolman–Bondi (LTB) spacetimes.

Within an effective-medium framework, we create horizon maps and baseline models that serve as starting points for adding cosmic structure, testing perturbations, and linking analytical results with numerical simulations.