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Zenith Grant Awardee

Dr. Raphael Bousso

University of California at Berkeley

Project Title

Why is the Universe Large?

Project Summary

The universe is expanding, though for most of the 20th century, it was hard to say just how fast. In the past decade, our understanding of cosmology has improved dramatically. We know the rate of expansion with great precision, and we know that the expansion is accelerating. This has allowed us to conclude that the universe will become infinitely large. Ironically, however, most of it will be forever hidden from us. Precisely because of the ever more rapid expansion, light from distant enough galaxies can never reach us, no matter how long we wait. For all practical purposes, we live in a box. By human standards, the box is big: billions of light years. To a physicist, it is shockingly enormous, more than 1061 (a one with 61 zeros) times larger than the natural length scale handed to us by the laws of physics. Can we explain this large disparity? Why is the universe the size it is? Recent insights in string theory and theoretical cosmology promise to shine light on this mystery. I have played a role in developing some of these new tools, and I propose to apply them to this simple yet profound question.

Technical Abstract

The observable universe is large because the cosmological constant is small. The string landscape explains the value of the cosmological constant in terms of other parameters characteristic of our own vacuum, such as the time when galaxies form or when observers live. This is an example of how the landscape establishes nontrivial correlations between properties of our vacuum. But such correlations explain neither the value of the cosmological constant, nor the time of galaxy formation, from first principles. I propose to initiate a search for the origin of the very large, and very small numbers that characterize our universe, the ur-hierarchy from which others can be derived by correlation. It may be related to the structure of the underlying theory: the universe is large because there are many vacua–ultimately a consequence of pure mathematics. Another possibility is that the observed hierarchies are the minimal ones that permit complex structures like observers to exist. Novel tools like the Causal Entropic Principle can help us understand which, if any, of these possibilities apply, and how our hierarchies arise in detail.

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