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In physical cosmology, cosmic inflation, cosmological inflation, or just inflation is a theory of exponential expansion of space in the early universe. The basic concept of inflation is empirically likely according to most astrophysicists.
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- Our point is that we should be talking about the contemporary version of inflation, warts and all, not some defunct relic. Logically, if the outcome of inflation is highly sensitive to initial conditions that are not yet understood, as the respondents concede, the outcome cannot be determined. And if inflation produces a multiverse in which, to quote a previous statement from one of the responding authors (Guth), “anything that can happen will happen”—it makes no sense whatsoever to talk about predictions. Unlike the Standard Model, even after fixing all the parameters, any inflationary model gives an infinite diversity of outcomes with none preferred over any other. This makes inflation immune from any observational test.
- The definition of inflation is extraordinarily simple: it is any period of the Universe's evolution during which the scale factor, describing the size of the Universe, is accelerating. This leads to a very rapid expansion of the Universe, though perhaps a better way of thinking of this is that the characteristic scale of the Universe, given by the Hubble length, is shrinking relative to any fixed scale caught up in the rapid expansion. In that sense, inflation is actually akin to zooming in on a small part of the initial Universe.
- I think inflation is too flexible of an idea for that to make sense.
- As for inflation, scientists believe it occurred in the early universe at a temperature far above the energy of particle accelerators and relatively close to the energy at which quantum gravity becomes important. We do not yet have a convincing, detailed model of how and why inflation transpired because the models of particle physics that we have are not adequate at the huge energies of inflation. Understanding inflation requires a much better knowledge of particle physics than we have now, and possibly a full knowledge of string theory and quantum gravity.