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  • Final Report Summary - COSMOLOGY FROM QG (What quantum gravity teaches us about the origin of the universe: Extension of early universe cosmology by using non-perturbative quantum gravity.)

Final Report Summary - COSMOLOGY FROM QG (What quantum gravity teaches us about the origin of the universe: Extension of early universe cosmology by using non-perturbative quantum gravity.)

The standard picture of cosmology posits that the Universe originated in the Big Bang, which is strictly speaking a point or region of infinite density and temperature, or in mathematical terms a singularity. The breakdown of the usual theory at the Big Bang indicates that a quantum theory of gravity is needed to extend the picture of standard cosmology and explain the origin of the Universe in physical terms. This project applied recently developed techniques from quantum gravity to cosmology in order to on the one hand provide a potential observational test for quantum gravity and on the other hand resolve theoretical issues such as infinities at the Big Bang. A common theme in the results of the project is the resolution of the Big Bang singularity by a "big bounce", i.e. the endpoint of a prior period of contraction. These results have illustrated the potential of quantum gravity to lead to a revolutionary new picture for the beginning of the Universe. The project topic is of great public interest, as has been illustrated in the wide attention the project's results have received in the media. During the project the researcher has been able to engage the public through outreach, and the results of the fellowship have increased the public visibility of the Theoretical Physics group at Imperial College, the researcher himself, and the role of EU funding in public research. In hindsight, this is now also to be seen also in the context of the future role of the United Kingdom within the European science community.

Working in the group field theory approach to quantum gravity, the project has successfully targeted several objectives. The first was the extension of exactly homogeneous models of the Universe to inhomogeneities. This was needed to describe a realistic Universe, which on the largest scales looks almost perfectly homogeneous and isotropic (i.e. the same in all directions, and at all points) but has small inhomogeneities which are manifest as galaxies and cosmic structure today. The results of the project showed how inhomogeneities can arise from quantum fluctuations in the initial state of the Universe. Recent cosmological observations such as of the cosmic microwave background have put stringent constraints on any proposed theory of the early universe, and the results obtained in this project can now be used to confront the cosmological models arising from group field theory with observation.

The second main objective was a comparison of the cosmological models derived from quantum gravity with other cosmological scenarios. In particular, the results of the project have contributed to the understanding of how the theory of loop quantum cosmology (LQC), in which the Big Bang singularity is replaced by a bounce, is related to the cosmological dynamics of a universe in group field theory. They have illustrated how group field theory also leads to a bounce in the early universe, whose details can resemble those of LQC, but this depends on a choice of initial state. There is a direct relation between initial conditions and possible phenomenology, which is the starting point for further investigations of the phenomenology of this type of bounce.

Outside of the setting of group field theory, one of the main results of the project was the development of a novel bounce scenario, called a "perfect bounce", for the early universe. This is based on the assumption of conformal symmetry in the early universe - loosely speaking, the absence of any physical scales for mass, length, etc. - and the observational fact that the Universe was dominated by radiation in its early history. The project could show in detail how these ingredients together with quantum mechanics lead to a bounce scenario in which a prior collapsing universe underwent a quantum transition into an expanding universe such as ours. Moreover, these results showed how physical phenomena before the bounce, in particular the structure of cosmological inhomogeneities, can be propagated across the apparent singularity without obstruction.

The results of the project have led to a significant further development in the connection of fundamental quantum gravity and cosmological scenarios for the early universe. This connection is a topic of wide current interest, and the last years have seen a sharp increase in general efforts towards developing alternatives to the standard paradigm of inflation, such as bounce models. The final results of this project should be seen as having developed two new approaches, the bounce scenario in group field theory and the "perfect bounce" in quantum cosmology, within this wider area of research. All these approaches face the challenge of matching observation, and some might be ruled out eventually. But more importantly, as observations will eventually reach their limit, a better grounding of cosmological scenarios within fundamental theory is essential. The two approaches developed in this project will be the subject of further ongoing research both by the researcher and other groups for the next years.

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United Kingdom
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