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Details of Grant 

EPSRC Reference: EP/V048422/1
Title: The Cosmological Bootstrap: a New Approach to the Primordial Universe
Principal Investigator: Pajer, Dr E
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Department: Applied Maths and Theoretical Physics
Organisation: University of Cambridge
Scheme: Standard Research - NR1
Starts: 31 July 2021 Ends: 30 July 2023 Value (£): 201,935
EPSRC Research Topic Classifications:
Mathematical Physics
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
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Summary on Grant Application Form
One of the most remarkable facts that has emerged from observations of the cosmos in the past few decades is that the distribution of stuff in the universe at intergalactic distances displays a shockingly high degree of regularity. This is true for everything we have observed: the galaxies that we see in the night sky and the Dark Matter that surrounds them; the photons in the cosmic microwave background, namely the faint afterglow of the universe' hot past, and the elusive neutrinos. Because our universe has been expanding for all of its observable history, we know for certain that this remarkable regularity must have been seeded during the first fraction of a second of the Big Bang. Our leading paradigm to describe that period in the history of the universe is called inflation and it posits that space expanded exponentially fast and small quantum fluctuations were stretched to cosmological size and eventually determined the spatial distribution of everything.

This picture of the very early universe has profound implications. First, it tells us that quantum mechanics, namely the set of physical laws that rule the atomic and subatomic scales and that enabled the digital age, is also the key to understanding the large cosmological distances of the observable universe. Second, this tells us that we can study the laws of physics at subatomic scale using cosmological surveys. Third, because the expansion of the universe is a gravitational phenomenon and since quantum mechanics must play a crucial role, we have the unique opportunity to learn about the holy grail of theoretical physics: quantum gravity.

In the past 30 years, people have made more and more elaborate models of inflation and the very early universe and they have compared them to observations. Unfortunately, despite the huge cosmological dataset at our disposal, it has become increasingly clear that there is a vast degeneracy in the space of models, which cannot be resolved by better observations. The key obstacle is that we have tried to model the evolution of the universe as time progresses, but that's not something we can observe today. All we can see is the end result of that evolution. A further obstacle is that the reliance on specific models has made it harder and harder to find general properties and patterns in the predictions of inflation.

The goal of this proposal is to devise a description of our primordial universe that does not involve time. In other words, we want to be able to predict what the possible outcomes and predictions of inflation are, without having to write down and solve all possible models. Rather, we want to rely exclusively on the pillars of our understanding of fundamental physics. These general principles, such as for example symmetries, unitarity and locality, sit at the core of our description of subatomic particles and should therefore be taken as a starting point for our understanding of the early universe.

To reach our goal of describing "time without time", we will import the approach and technology that has revolutionised our understanding of particle physics in the past two decades. This progress has not yet been transferred to the realm of cosmology, where it actually has the highest potential. Applying this approach to cosmology will allow me to predict the general, model-independent outcomes of inflation and more specifically the detailed statistical distributions that can be generated while being compatible with the laws of physics as we know them.

The output of this research will be a new understanding of how quantum mechanics works when combined with non-trivial gravitation backgrounds. It will open the door to use cosmological observations to discovering new particles beyond those that we know in the current standard model. We will be able to discover new forces at play in nature and perhaps even learn something about the perturbative regime of quantum gravity.
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