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Why is the future different from the past? The most poignant property about the world around us is that time is always moving forward, yet, as odd as it may seem, our current theories of physics cannot account for this property. In all of fundamental physics, the future and the past are entirely similar. We select time increasing solutions by hand and call on extremely unlikely initial conditions at the big bang to justify this choice. I will address this question and describe attempts to restore the irreversibility of time as the building block of our understanding of nature.

This talk was delivered at ISSYP 2019, Perimeter's high school summer program.

Abstract: “How did our universe begin?” is possibly one of the oldest questions that have bewildered humans throughout history. As a theoretical cosmologist, our job is to find a mathematically consistent picture for early universe that could explain observations, from the largest to the smallest scales. The past thirty years have witnessed amazing progress, both in developing technology for precision cosmological observations, and in perfecting mathematical methodology to explain them. For example, ripples in cosmic geometry are now measured with the precision of one part in a million. We also have sophisticated mathematical frameworks such as general relativity and quantum theories that describe the origin of these ripples in early universe. However, with all of these extraordinary achievements, some old and new puzzles remain unsolved. For example we still have not resolved the most crucial puzzle about the origin of cosmos, namely the Big Bang Singularity problem. We will take a journey back in time to explore the fascinating realm of early universe and some of its mysteries.

Abstract: From astronomical observations, we know that the state of the early universe (just after the Big Bang) was extremely simple. This is surely an important clue about how the universe began, but what exactly is it trying to tell us? I will explain our new answer to this question: we think it is telling us that the universe before the bang is a kind of mirror image of the universe after the bang (they are related by "CPT symmetry"). (Based on recent work with Kieran Finn and Neil Turok: https://arxiv.org/pdf/1803.08928.pdf)

Abstract: There is a strong correlation between the sun rising and the rooster crowing, but to say that the one causes the other is to say more. In particular, it says that making the rooster crow early will not precipitate an early dawn, whereas making the sun rise early (for instance, by moving the rooster eastward) can lead to some early crowing. Intervening upon the natural course of events in this manner is a good way of discovering causal relations. Sometimes, however, we can't intervene, or we'd prefer not to. For instance, in trying to determine whether smoking causes lung cancer, we'd prefer not to force any would-be nonsmokers to smoke. Fortunately, there are some clever tricks that allow us to extract information about what causes what entirely from features of the observed correlations. One of these tricks was discovered by the physicist John Bell in 1964. In a groundbreaking paper, he used it to demonstrate the seeming impossibility of providing a causal explanation of certain quantum correlations. This revealed a fundamental tension between quantum theory and Einstein's theory of relativity --the two central pillars of modern physics. It is a tension that is still with us today.