- #36
kimbyd
Science Advisor
Gold Member
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Thinking about this again, perhaps it isn't so much that eternal inflation describes an infinite universe as it is that the model itself is infinite by construction.
The way eternal inflation works is that as the universe inflates, each individual patch will tend to a smaller value of the potential energy, leading to slower inflation over time. However, the specific field value will fluctuate a little bit, so that some regions will end up inflating faster instead. Eternal inflation happens when the small fraction of faster-inflating regions inflate enough faster that their volume grows faster than the rest. Thus any given volume of inflating universe will tend to have parts which keep inflating forever.
The key words there are "tend to". This isn't a guarantee. For a given inflating volume, there is some probability that it will eventually stop inflating altogether. The inflation only becomes truly eternal if you allow that volume to be infinite.
This whole discussion says nothing whatsoever about whether our universe is infinite. It's just logic within the model for why they would use an infinite universe as part of it: it makes it actually eternal rather than just probably eternal. This means that we probably shouldn't take too seriously the infiniteness of that model: it's not a prediction in any sense, just a simplification.
The real consequence is that because the eternal inflation model is infinite, we have to contend with the consequences of said infinity. I believe one of those consequences is the measure problem, which makes it basically impossible to calculate relative probabilities (you can still say whether or not something has some probability of occurring, but not whether it is more or less likely than some other thing).
The way eternal inflation works is that as the universe inflates, each individual patch will tend to a smaller value of the potential energy, leading to slower inflation over time. However, the specific field value will fluctuate a little bit, so that some regions will end up inflating faster instead. Eternal inflation happens when the small fraction of faster-inflating regions inflate enough faster that their volume grows faster than the rest. Thus any given volume of inflating universe will tend to have parts which keep inflating forever.
The key words there are "tend to". This isn't a guarantee. For a given inflating volume, there is some probability that it will eventually stop inflating altogether. The inflation only becomes truly eternal if you allow that volume to be infinite.
This whole discussion says nothing whatsoever about whether our universe is infinite. It's just logic within the model for why they would use an infinite universe as part of it: it makes it actually eternal rather than just probably eternal. This means that we probably shouldn't take too seriously the infiniteness of that model: it's not a prediction in any sense, just a simplification.
The real consequence is that because the eternal inflation model is infinite, we have to contend with the consequences of said infinity. I believe one of those consequences is the measure problem, which makes it basically impossible to calculate relative probabilities (you can still say whether or not something has some probability of occurring, but not whether it is more or less likely than some other thing).