Search for Milli-lenses to discriminate between dark matter models

Project Information

SMILE

Grant agreement ID: 101040021

DOI

10.3030/101040021

EC signature date 27 January 2022

Start date 1 July 2022

End date 30 June 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 486 875,00

EU contribution

€ 1 486 875,00

Coordinated by

IDRYMA TECHNOLOGIAS KAI EREVNAS
Greece

Periodic Reporting for period 1 - SMILE (Search for Milli-lenses to discriminate between dark matter models)

Reporting period: 2022-07-01 to 2024-12-31

Summary of the context and overall objectives of the project

One of the most compelling mysteries in both cosmology and particle physics is the nature of Dark Matter (DM). We investigate this problem using strong gravitational lensing of active galaxies on the key but poorly-explored milliarcsecond scales. Gravitational lensed images with angular separation on milliarcsecond scales probe gravitational lens systems where the lens is a compact object with a mass between ~ a million and ~ a billion solar masses. This mass range is particularly critical for the widely accepted Lambda-CDM cosmological model, which predicts many more DM halos in this mass range than are currently observed. The most direct way to explore these small angular scales is through the high-resolution imaging capabilities of radio Very Long Baseline Interferometry (VLBI). In the SMILE project, we use VLBI data of a large sample of active galaxies (~ 5000 sources) to search for gravitational lens systems on milliarcsecond scales. Since no gravitational lenses on milliarcsecond scales have yet been found, confirming any candidate from this search as a true gravitational lens would be a first and major discovery. A null result instead will allow us to infer a new constraint on the abundance of compact objects in the mass range of interest, with over an order of magnitude better precision than in previous studies, and tighter than the number sub-galactic scales halos predicted by Lambda-CDM. Such a constraint will help discriminate between DM models that predict different numbers and density profiles for halos in this mass range. It could also help to constrain a possible contribution of primordial black holes as a DM component.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We have developed a semi-analytical model to estimate the milli-lensing optical depth as a function of the background source’s redshift, taking into account various dark matter (DM) models. We therefore used the model to predict the expected number of milli-lenses detectable within SMILE, when different DM scenarios are considered (Loudas, Pavlidou, Casadio, and Tassis, 2022, A&A 668, A166). Our results confirmed the discriminating power of SMILE, which will be able to rule out viable DM models regardless of whether the search for milli-lens systems yields a null or positive result.

Additionally, we have made significant progress in defining and testing an observational strategy and analysis method to distinguish genuine milli-lens systems from other sources with similar morphology but different intrinsic nature. This has been accomplished through a dedicated pilot study (Pötzl F. et al., submitted http://arxiv.org/abs/2409.15229).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The science of gravitational lensing at milli-arcsecond scales remains largely unexplored, despite its relevance in probing a range of mass — from ~ a million to ~ a billion of solar masses — which is particularly critical for the widely accepted Lambda-CDM cosmological model. Predicting the number density of milli-lenses within a given volume of the Universe, and considering different DM halos mass functions and density profiles for the relevant mass range as we did in Loudas, Pavlidou, Casadio, and Tassis, 2022, A&A 668, A166, represents a significant advancement beyond the current state of the art.

Representation of a gravitational lens system at milliarcsecond scales.