I work with field-level inferences of the cosmic large-scale structure to study the physics of the Dark Universe. In particular, my research focuses on field-level inferences of:

  • Galaxy clustering, photometric redshifts
  • Peculiar velocities
  • Galaxy intrinsic alignment
  • Supernova clustering
  • Environments of galaxies and supernovae
Photometric redshifts
Cross-correlation of the gridded photometric galaxy coordinates before (black) and after (orange) the applica- tion of our method with the ground truth (mock data).

Next-generation photometric surveys will deliver redshifts with an uncertainty corresponding to as much as 300 Mpc or more. Reconstructing the large-scale structure from highly uncertain galaxy locations results in structures that are distorted. Here, we developed a method that mitigates such distortions, while also constraining individual galaxy redshift probability density functions. Our framework provides Markov Chain Monte Carlo realizations of the primordial and present-day large-scale structure as constrained by galaxy clustering, accounting for survey- and data-related uncertainties. Owing to the structure formation model we employ, we reconstruct the structure formation history of the large-scale structure, as well as filaments. We provide constraints on the large-scale structure on scales much smaller than the original redshift uncertainty. 

Publication: arXiv




Galaxy intrinsic alignment

Predicted shape of the FIRST J092319.1+315950 radiogalaxy (cyan) compared to its observed shape as reported by Reyes et al. 2012 (orange). The difference between the two ellipses is due to linear alignment. The background image is taken from the SIMBAD database. Credit: Eleni Tsaprazi

Galaxy shapes are not random, but align with the large-scale structure. This effect, known as intrinsic alignment, is linked to a multitude of cosmic phenomena. It provides constraints on galaxy formation and evolution, the initial conditions of the Universe, redshift-space distortions, as well as the gravitational wave background, among others. Constraints on intrinsic alignment further allow the decontamination of the weak lensing signal from intrinsic galaxy shape correlations which bias cosmological estimates. In Tsaprazi et al. 2021b, we find 4σ evidence of intrinsic alignment within a luminous red galaxy sample, by cross-correlating with three-dimensional tidal fields constrained with spectroscopic galaxy observations. Field-level approaches, like the one we present in our study, facilitate the modeling of nonlinear corrections to intrinsic alignment models and gravitational evolution, as well as joint inferences of intrinsic alignment and weak lensing.

Vlog: YouTube  

You can find a summary of common notations and concepts on galaxy intrinsic alignment in Lamman et al. 2024.





Standard candle clustering

Illustration of supernova explosions in the large-scale structure. The colormap indicates density at 4 Mpc, constrained with data from the 2M++ galaxy catalog. Credits: Eleni Tsaprazi

An endeavor that has been challenging cosmologists is to obtain an accurate picture of how the universe looks on large scales, as this picture contains invaluable information  that can help us better understand how  gravity works on the biggest scales and the origin of cosmic acceleration. To this end, galaxy surveys are employed and allow us to map the cosmic structure. However, at large distances, galaxies become too dim to detect, whereas supernovae are bright point sources that  can be detected very far, and thus uncover the locations of galaxies that would not have been resolved otherwise. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time will detect tens of thousands of supernovae per year, a detection rate which would in principle allow us to exploit supernova explosions to map the cosmic structure at large distances. In Tsaprazi et al. 2021a we studied the large-scale environments of supernovae and found that supernovae can be used to map the high-redshift universe. A byproduct of our findings was that supernovae are biased tracers of density, in contrast with traditional clustering assumptions in supernova surveys.

Vlog: YouTube





Peculiar velocities

Velocity of an observer moving relative to the background cosmic expansion. Credits: Tsaprazi & Tsagas (2020)

Recent evidence suggests that the magnitude of the local bulk flow may be inconsistent with the ΛCDM cosmology. In Tsaprazi & Tsagas 2020 we performed a study of large-scale bulk flows in the context of General Relativity and Linear Perturbation Theory to probe the full kinematics of linear peculiar velocities by accounting for relative motion effects. In doing so, we found that the relativistic energy flux of a bulk flow can contribute to its measured amplitude. On these grounds, the reported tension in the local bulk flow and the ΛCDM prediction, could be accounted for by neglected relativistic effects.