If the vacuum energy were large and negative, it would likewise squeeze things together, collapsing the entire universe in a tiny fraction of a second. If the vacuum energy were very large and positive, life could not exist, since the huge acceleration that would result would make it impossible for individual atoms to form, much less stars and galaxies. Further, we are not really surprised to find ourselves here on Earth, rather than on the surface of Saturn or the Sun, even though the Earth is quite tiny compared to them the conditions are just more hospitable here. But we know better outside the atmosphere the temperature is very different. If we imagine some primitive physicists living in a region of Earth that was perpetually cloudy and with a very mild climate, they might expend a great deal of effort trying to predict the temperature from a theory of everything. So the idea is that the vacuum energy is a consequence of local conditions, rather than a fundamental number - much like, for example, the temperature of the Earth’s atmosphere. Still, there’s nothing to stop us from imagining other regions, just as big, which are outside what we can observe - it would be inappropriately anthropocentric to imagine that the entire universe resembles our little piece of it. The part of the universe that we observe is certainly finite, but it’s quite big - tens of billions of light-years across. It seems quite constant over our observable universe, so this scenario needs to posit the existence of regions of space far outside our observable universe, which we can’t see and which have very different conditions. So various people (I don’t know the history well, so won’t attempt to attach names to ideas) have suggested that the vacuum energy is not a constant of nature, but rather an environmental variable that can be different from place to place in the universe. But we don’t seem to have any clue how to find such a formula, or even if it exists. What everyone would like to have is a formula that predicts the correct value of the vacuum energy in terms of other measured quantities. But this possibility raises two huge questions: why is the vacuum energy much smaller than it naturally should be (by a factor of 10 -120), and why is the vacuum energy density comparable to that in matter today, even though they evolve rapidly with respect to each other as the universe expands? The leading candidate, of course, is a small vacuum energy, or cosmological constant - a tiny, persistent energy density inherent in space itself, rather than being associated with some particle or field. So my talk goes through a little flowchart of all the possibilities, similar to the approach in this paper. Our universe is accelerating and we don’t know why. ![]() ![]() I have to start increasing my speaker fees or this will get ridiculous.) ![]() (After giving a similar talk at Penn the day before, and Urbana last week, and Brandeis and Arizona the week before that. Greetings from Baltimore, where I just gave a talk on the accelerating universe at Johns Hopkins.
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