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Global Lensing Observations to go Beyond Einstein

Periodic Reporting for period 4 - GLOBE (Global Lensing Observations to go Beyond Einstein)

Reporting period: 2020-05-01 to 2021-10-31

Exquisite observations of the early universe have provided the strongest evidence yet that the universe we live in is very dark indeed. But a far-reaching question remains: what is the exact nature of this dark universe? It is widely believed that in order to answer this profound question, we will need to invoke some new physics that will forever change our cosmic view.

With my ERC research group, I am confronting a potentially radical new theory that has been steadily growing in favour over the past few years. Our inference that today’s Universe is filled with an invisible web of dark matter and a source of dark energy that is fueling the accelerated expansion of the Universe, could just be a consequence of our poor understanding of gravity. Using three state-of-the-art astronomical surveys, I am conducting a ground-breaking gravity experiment on some of the largest scales observed in the Universe. Our findings could show that we need to go beyond Einstein to bring about a revolution in our understanding of gravity on cosmological scales, transforming our view of the dark universe.
I co-lead the Kilo Degree Survey (KiDS), a high priority European survey that observes the night sky in only the very best weather conditions at the European Southern Observatory in Chile. The highlights from this truely exquisite data set, charted the growth of dark matter structures over time, directly testing models for how dark energy has evolved (Hildebrandt, Viola & Heymans* et al. 2017, over 600 citations, Heymans et al 2021, over 200 citations).

Our analysis found that the distribution of dark matter today was not as ‘clumpy’ or ‘condensed’ as expected based on our knowledge of the Universe right after the Big Bang. Taken at face-value, our result points towards a rather exotic evolving dark energy model or a modified gravity theory, but we need to collect more data before we can robustly draw this conclusion. Our headline publications presenting these results are in the top ten most highly cited astronomy papers for their published years (Hildebrandt, Viola & Heymans* et al. 2017, Heymans et al 2021).

These papers are just two out of 110 different publications from our project this reporting period, that range between detailed technical and instrument studies, through to developments in statistical theoretical physics. It is this broad range of skills and abilities that our team covers that will enable our long-term goal to test gravity on cosmological scales.
The Planck space mission has released exquisite observations of the early universe, providing the strongest evidence yet that the universe we live in is very dark indeed. Its precise results show that our universe is composed of 26.6% dark matter and 68.5% dark energy, while less than 5% is made up of the baryonic material that we are familiar with on Earth. With their long-standing quest to make these precision measurements essentially now concluded, cosmologists are rapidly turning their attention to a much bigger and further-reaching question: what is the exact nature of this dark universe? Our team is a world leader in a new direction being taken in Cosmology to map out the invisible dark matter and confront theories on the origins of dark energy. Interestingly the increasing precision that we have recently reported in these late-time cosmological measurements reveals tension with Planck’s initial conclusions. Is this is a sign that new data challenges lie ahead, or is it our first hint that the universe is truely exotic? The final results from the Kilo-Degree Survey have improved our precision allowing us to confront a wide range of alternative models.
A map of dark matter in the Universe published by the KiDS survey.