I develop field-level frameworks that employ the cosmic large-scale structure as a probe of the physics of the Dark Universe. My research focuses on:

 

Supernovae

Simulated supernovae (orange) in a reconstruction of the cosmic large-scale structure around the Coma cluster (Abell 1656). The purple points are galaxies in the real Universe, whereas the white ones are simulated galaxies that follow the same clustering pattern and from which we simulated supernovae.

One of the biggest puzzles in cosmology today is the Hubble tension — a mismatch in the measured expansion rate of the universe, called the Hubble constant (H₀), depending on how and where it’s measured. It has been suggested that the motions of galaxies induced by gravity (called peculiar velocities) in our cosmic neighbourhood might be throwing off these measurements. In this study, we take a closer look at that idea. We simulate the positions of type Ia supernovae — cosmic mileposts used to measure distances — using realistic galaxy data from the local Universe. We develop and validate a Bayesian Hierarchical Model which allows us to infer H₀ from SNeIa accounting for peculiar velocities self-consistently, while constraining the gravitational impact of the local Universe on the Hubble constant. We found that the effect of peculiar velocities is not sufficient to reconcile the Hubble tension alone. Our study paves the way for the use of SNeIa for inferences of the Hubble constant in the very nearby Universe.

Publication: MNRAS

 

 

 

 

 

 

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

 

 

 

 

  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. 

Manuscript: arXiv

 

 

 

Intrinsic alignment

Galaxy shapes are not random, but align with the large-scale structure. This effect, known as galaxy 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. 

Galaxy shapes and orientations traced over a portion of JWST’s NIRCam image of Abell 2744. Credit: Lamman et al. 2024

In Lamman et al. 2024, we created a guide to intrinsic alignment formalisms, models, estimators and useful references. My colleagues prepared a presentation of the guide in Shaun Hotchkiss’ Cosmology Talks on YouTube!

 

 

 

 

 

 

 

 

 

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

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.

 

 

 

 

 

 

 

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.