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Lenny Susskind discusses the possible experimental implications of living in a universe that decayed via tunneling from a metastable state into a period of slow roll inflation. As the speaker was about to introduce him, he sent the speaker away, saying that he needed no introduction!

The problem is that slow roll inflation requires a large amount of fine tuning. According to standard FRW cosmology, bubble nucleation requires negative curvature. The so called inflaton potential that gives rise to this would need to be extremely flat if you want gravitationally bound structures to form in the universe. If there were lumpy on the order of one part in $10^5$, there would be too much negative curvature, things would move apart faster than the escape velocity, and there would be no structure formation.

Susskind claims that the best solution so far to this fine-tuning problem is to invoke the anthropic principle. He calculates the probability for inflation to occur given structure formation as a prior. To do so, he uses a very simple 3 parameter toy model of the potential, and finds this probability to be 90%. Based on this model, he also calculates the probability that we can make observations back to the onset of inflation – 15%.

He then discussed fossils that we could measure to establish a tunneling event. These included negative curvature, and low tensor modes due to gravity waves that would arise due some ingenious / crazy mechanism of inheritance from our ‘ancestor universe’. Near the end of the talk he mentioned another very exciting idea to detect fossils: from the inflationary scenario one would expect that there are many collisions between our bubble (the observable universe) and other such bubbles. Work has been done by others in predicting the experimental signature of such an event in the CMB! They claim that you would expect non gaussian statistics in the CMB. The paper by Chang, Kleban and Levi is on the arxiv

The talk is available via PIRSA

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