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Message: <blockquote data-quote="freyar" data-source="post: 6682846" data-attributes="member: 40227">There's lots and lots to say on this subject, but I'll try to avoid writing a book and just stick to the idea of vacuum energy.Here's the background for the question: The universe is expanding, and through the mid '90s, everyone expected that the expansion should be slowing down due to the gravitational attraction of all the stuff in the universe.  Then two different research groups independently showed that the expansion is actually speeding up.  This requires some kind of unusual physics to explain, and the simplest explanation is that there's a cosmological constant term in the Einstein equations of general relativity.  This term is also how vacuum energy, or the energy of empty space, would appear in the Einstein equations.  Vacuum energy is interesting because, as space expands, the density remains constant, so the total energy goes up.  This property makes the expansion of space speed up.  Of course, there are other ways to get the universal expansion to accelerate (which as a whole are given the name dark energy), and I'm happy to talk about those, but I'm keeping to vacuum energy here.The problem with vacuum energy (known as the cosmological constant problem), as fuindordm indicates, is that rough estimates of it from particle physics calculations give a ginormous number, around 120 orders of magnitude too big (that is, 1 followed by 120 zeros times the actual number).  Until the discoveries of the accelerating expansion, most physicists believed there must be an unknown physical principle that sets the cosmological constant to exactly zero, since making something so much smaller than it "should" be but not actually zero seems like quite a tall task (indeed, many people who believe in other forms of dark energy at least implicitly believe the cosmological constant is zero even now).  This latter, more difficult situation is the one we ended up with.  How do you make something so much smaller than it "wants" to be?So, what ideas do we have (especially from the last decade or so) from string theory?  One possibility is a "string inspired" idea that uses extra dimensions.  The essentials are that all the normal matter (and probably dark matter too, I'm not that sure of those details) are stuck to objects called branes (short for membranes), which have the usual 3 spatial dimensions.  But there are also 2 extra dimensions perpendicular to the branes.  There can be a large vacuum energy on the branes, but, instead of causing our space to expand faster and faster, it causes the extra dimensions to curve (into a shape like an American football or rugby ball).  There are some people who really really love this idea, but not a lot of people are sold on it (at least, not a lot of people have worked on it).  Part of the reason is that it seems like it shouldn't work in the end for some technical reasons.  In fact, I saw a paper the other day arguing that this idea is fatally flawed.The main development on the cosmological constant problem in recent times came up in 2003 and involves the (weak) anthropic principle.  The anthropic principle says that, if a universe can be observed, it must be capable of supporting intelligent life that can observe it.  The point is that the vacuum energy must be incredibly tiny for stars and galaxies (and presumably therefore life) to form. In fact, the maximum possible value for the vacuum energy is not much higher (maybe a factor of 100) than that observed, and Steven Weinberg (a Nobel-winning particle physicist) actually predicted the discovery of the cosmological constant based on these anthropic arguments back in the 1980s.  Where string theory comes in is to provide a mechanism to work.  You see, for the anthropic principle to make sense as a physical principle, you need a universe where there are lots of different regions of different effective vacuum energies; this is often called a multiverse.  To get this, you need a theory with a lot of different states of different vacuum energies and a way for the universe to transition in between them.  About 12 years ago, string theorists working on problems about moduli fields (see the Ant-man answer above) discovered that string theory apparently satisfies both of these properties.  In other words, it looks like string theory automatically gives you a multiverse.  Since then, there's been a lot of work on understanding what the probability is that we'd live in a part of the multiverse with our value of the vacuum energy.  This is kind of tricky, since it's hard to come up with a mathematically rigorous definition of probability that applies to chunks of an infinite universe.This idea is pretty polarizing.  A lot of physicists think the use of the anthropic principle is a cop out.  Some of them have argued that there are technical (read as "highly mathematical") reasons to think that the calculations suggesting a string theory multiverse are subtly wrong.  On the other hand, as witnessed by the amount of work in this area, a lot of people think this use of the anthropic principle makes sense and seems to fall out of the mathematics of string theory.  The divide is partly, but not entirely, by age, with younger physicists a bit more in favor of anthropic arguments.  Full disclosure: I think anthropic arguments are perfectly legitimate and have worked on multiple parts of this story.  I'm also fairly sensitive to the technical questions raised by "anthropic opponents" and am perfectly willing to admit that there could be a subtle reason some of the calculations don't work the way they appear to. In any case, I think if you polled string theorists, you'd find a pretty significant split on whether the multiverse is a good/correct solution to the cosmological constant problem or not.  And that's where we stand on vacuum energy (leaving aside other models of dark energy).</blockquote>

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