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Inflation in String Theory - Connecting Quantum Gravity with Observations

Periodic Reporting for period 3 - STRINGFLATION (Inflation in String Theory - Connecting Quantum Gravity with Observations)

Reporting period: 2018-10-01 to 2020-03-31

The project “STRINGFLATION” aims at predicting the energy scale of cosmological inflation and the strength of the inflationary gravitational wave signal from string theory. The results will help us to understand the structure of the very early Universe and its fundamental laws of nature a bit better. It thus advances knowledge – the primary purpose of society: The expression of our desire to know the Universe and ourselves in depth and complexity, through the extension of music, artistical expression or basic research, is the fundamental purpose and goal of all human civilization and by extension of every functioning human society – because on these three pillars we can rise above the origins of our species.
The theory of inflation predicts a background of primordial gravitational waves. If these are strong enough to be detectable as polarization patterns in the cosmic microwave background radiation, inflation happens at such a high energy scale that it ‘feels’ quantum gravity.
Therefore, we will determine the range of predictions for large-field high-scale inflation in string theory (a candidate theory of quantum gravity) driven by the mechanism of axion monodromy, which was co-discovered by the PI. Finally, we will study the distribution of inflation mechanisms among the many vacua of string theory. In combination, this gives us a first chance to make string theory testable.
The project is well on track with significant progress in WPs 1, 2, 3, 4, 5, 6, 7 and 8, while WP 9 will be addressed later-on as planned, such that all objectives are well on track. STRINGFLATION results in a total of 30 publications within the first 4.5 years. Further publications are in progress. Due to the character limit, the work is highlighted mainly by stating the most relevant publications indicating also our international collaborations:

WP2 & WP1:
arXiv:1610.05320: L. McAllister (Cornell U.), P. Schwaller, G. Servant, J. Stout (Utrecht U.) & myself;
arXiv:1705.08950: G. D’Amico (CERN), N. Kaloper (UC Davis), A. Padilla (Nottingham U.), D. Stefanyszyn (Groningen U.) & myself;
arXiv:1707.08678: Jakob Moritz, Ander Retolaza & myself;
arXiv:1805.00944: Jakob Moritz & T. van Riet;
arXiv:1809.06618: Jakob Moritz, Ander Retolaza & myself;
arXiv:1812.03999: A. Hebecker (Heidelberg U.), S. Leonhardt (Heidelberg U.), Jakob Moritz & myself;
arXiv:1902.01412: Federico Carta, Jakob Moritz & myself;
arXiv:2003.04902 & publically available software code performing the relevant classification: Federico Carta, Jakob Moritz & myself; important for WP 1 and WP 5 as well.

WP 2 & WP 4:
arXiv:1512.03768: A. Hebecker (Heidelberg U.), F. Rompineve (Heidelberg U.) & myself;
arXiv:1807.06579: Mafalda Dias, Jonathan Frazer, Ander Retolaza & myself;

arXiv:2002.02952: V. Domcke, V. Guidetti (Bologna U.), Yvette Welling & myself;

arXiv: 1810.05199: An ‘information bottleneck’ during cosmic inflation discovered by Mafalda Dias, Jonathan Frazer and myself led us to study the statistical dependence of the CMB observables of inflation on the micro-physical parameters of a given model class. Using conventional ab-initio computation schemes is prohibitively expensive for such a task. Hence, we leveraged machine learning algorithms to directly learn the stochastic map from microscopic inflation model parameters to CMB observables. This in turn allowed us to efficiently propagate the theory uncertainties of axion monodromy inflation into the predicted distributions of the CMB observables. Using this ‘theory error’ allowed us to show, that single-field axion monodromy inflation predicts a lower bound of r > 0.015 independently of the unknown underlying string theory uncertainties. Hence, this class constitutes for the first time a case where a well-motivated class of string theory models of early universe cosmology can be tested.

arXiv:1910.09568: Federico Carta, S. Giacomelli (Oxford U.), H. Hayashi (Tokai U.) & R. Savelli (INFN & Rome U.);
arXiv:2002.07816: Federico Carta & A. Mininno (IFT Madrid);
highly relevant for aspects of WP5 and WP1 as well as WP2

arXiv:1901.03657: A. Achucarro (Leiden U.), E. J. Copeland (Nottingham U.), O. Iarygina (Leiden U.), G. A. Palma (Chile U.), Dong-Gang Wang (Leiden U.) & Yvette Welling;
arXiv:1907.02020: A. Achucarro (Leiden U.) & Yvette Welling;
arXiv:1907.02951: Yvette Welling;
arXiv: 1908.06956: A. Achucarro (Leiden U.), G. A. Palma (Chile U.), Dong-Gang Wang (Leiden U.) & Yvette Welling;

arXiv:1604.05970: Mafalda Dias, Jonathan Frazer & M. C. D. Marsh (Cambridge U.);
arXiv:1606.07768: F. Pedro (IFT & UAM Madrid) & myself;
arXiv:1609.00379: Mafalda Dias, Jonathan Frazer, D. J. Mulryne (Queen Mary U.) & D. Seery (Sussex U.);
arXiv:1611.07059: F. Pedro (IFT & UAM Madrid) & myself;
arXiv:1706.03774: Mafalda Dias, Jonathan Frazer & M. C. D. Marsh (Cambridge U.);
arXiv:1710.08913: S. Hotinli (Imperial Coll.), Jonathan Frazer, A. H. Jaffe (Imperial Coll.), J. Meyers (CITA) & L. C. Price (Carnegie Mellon)

arXiv:1604.05326: Benedict Broy, D. Croone (Groningen U.) & D. Roest (Groningen U.);
arXiv:1605.00651: David Ciupke;
arXiv:1609.03570: Benedict Broy;
arXiv:1707.05830: R. Kallosh (Stanford), A. Linde (Stanford), D. Roest (Groningen U.), Y. Yamada (Stanford) & myself;
arXiv:1805.02659: Mafalda Dias, Jonathan Frazer, Ander Retolaza, M. Scalisi & myself.
The project is already at a stage where we can provide a (preliminary) qualitative answer to its ultimate objective: to determine the range of r-values describing the fractional power in inflationary gravitational waves from string theory. Our qualitative answer at this point is that parametrically small values of r (that is, values which are small even on a logarithmic scale) are disfavored in the string landscape.
Moreover, a part of our current results are already beginning to map out the spectrum of r-values among the classes of inflation models in string theory which employ structure and/or protective symmetries, and we are in the process of understanding the full impact which the crucial dynamics of backreaction from moduli stabilization has on this spectrum of r-values.
Our work for the remaining 1.5 years is therefore well on track to achieve the main objective of the project. By focusing on the backreaction dynamics of moduli stabilization on high-scale string inflation, and the distribution of the topological requirements for such models on the string landscape, we aim to establish a rough lower bound on the value of r among the high-scale string inflation models in the structured part of the landscape. This would constitute a prediction of a range of r-values for future CMB B-mode polarization experiments, and allow us to potentially disfavor string theory using future CMB data.
Exploitation of the full potential output of the initially unexpected direction and results of (arXiv:1810.05199) will enhance the results of the grant significantly. This is now possible by using the initially unused personnel funds for a recently approved grant extension until September 30, 2021, which allowed us to keep on Yvette Welling who is crucially important for this direction.
The publically available software code from (arXiv:2003.04902) also forms tangible output beyond the state of the art.
Graphic description of the project's main theme