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Credits: ESA/Euclid/Euclid Consortium/NASA; J.-C. Cuillandre (CEA Paris Saclay), G.Anselmi; CC BY-SA 3.0 IGO)

ERC Consolidator grant for Alessandra Silvestri: putting gravity to the test on cosmological scales

Does gravity work the same when you look at the largest scales in our universe? That’s what Leiden physicist Alessandra Silvestri will study with a 2 million euro ERC Consolidator grant. ‘We assume that it does, but we don’t actually know.’

More than a century ago, Albert Einstein formulated his theory of general relativity. A revolutionary interpretation of gravity based on the geometry of spacetime. This turned out to be one of the most successful theories in the history of physics, passing all the tests that scientists came up with. But is this theory complete? There might be more to it, says Silvestri, who aims to understand gravity on cosmological scales.

Looking at a new frontier with Euclid

‘What I do is putting the laws of gravity to the test on the largest scales,’ explains Silvestri. ‘General relativity has been well-tested for the scale of our solar system and we know it is correct. We assume it is the same on larger scales, but we never had the opportunity to confirm this, until now. With the Euclid space telescope we can test it for the first time. This telescope is like nothing we have seen before.’  

Space telescope Euclid will map dark matter and search for evidence of dark energy in the universe. Image credits: ESA

For six years, Euclid will take ultra-sharp images of billions of galaxies across more than a third of the sky to create a 3D map of the universe across space and time. ‘That is truly transformational,’ says Silvestri excitedly. ‘The first data are being collected as we speak, and in one year there will be the first internal release for us to start digging the physics out of it. ‘The timing is perfect. This grant allows me to hire five group members. That will be a much-needed workforce when the data start coming in.’

A quest for the perfect model

Silvestri is the lead of the Theory working group of the Euclid consortium. It’s her job to find a good model that explains all the observations, eventually unveiling new physics. ‘Early in my career, the approach was what you would call “trial and error”. You’d come up with candidate models and try them against the data. But that’s not very efficient. I grew more and more obsessed with constructing a broad agnostic framework guided by fundamental physics principles to derive the theory from the data. That’s especially important with the wealth of data that Euclid will deliver.’

‘Creating the perfect model is very exciting but also complex. It is not easy to infer a theory out of all those data,’ Silvestri says. ‘Euclid will also collect a lot of data on somewhat smaller cosmological scales. These are notoriously difficult to model. If there are modifications of general relativity, we expect some peculiar features to show up in this regime. Yet, the theoretical framework for this scale is lacking. With this grant, I will tackle this. I am lucky to be able to do so in Leiden, where we have a great Euclid community, covering different expertise.’ 

‘I can’t wait for the first data to come in. At the end of the five years of this ERC grant, we can hope to have a very solid framework and really start learning about the gravity of the universe.’

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