# Inflation (cosmology)

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.

## Quotes

* ... almost any inflationary model has the property that it produces not just one big bang but ultimately an infinite number of them, at an exponentially increasing rate — and that's what we we call eternal inflation. It's eternal only into the future — we don't think it's eternal into the past — so the word "eternal" is not quite being used in its dictionary definition ...
** Alan Guth, (August 16, 2021)"Multiverse or Cyclic Universe ? Alan Guth vs Roger Penrose". YouTube. (quote at 55:27 of 1:38:28)
* 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.
** Anna Ijjas, Paul J. Steinhardt, and Abraham Loeb, (2017). "Letters: A Cosmic Controvery, The Authors Reply". Scientific American 317 (1): 5–8. ISSN 0036-8733. DOI:10.1038/scientificamerican0717-5.
* 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.
** Andrew R. Liddle and David H. Lyth, Cosmological Inflation and Large-Scale Structure. Cambridge University Press. 13 April 2000. pp. 4–5. ISBN 978-0-521-57598-0.
* I think inflation is too flexible of an idea for that to make sense.
** Paul Steinhardt, "What has been learned from BICEP2?", Talk at Strings 2014 held at Princeton University and the Institute for Advanced Study, Princeton, June 23-27, 2014.; See also slide.
* The theory of cosmic inflation offers an attractive resolution of some of the great paradoxes in cosmology: why the universe is so large, flat and uniform on large scales, and how density variations arose. Inflation has rightly dominated cosmological thinking for the past two decades, helping stimulate the development of high-precision observational programmes. The survival of simple inflationary models in the face of an impressive observational onslaught has been interpreted as convincing evidence of the correctness of the basic idea. In this paper, I review inflation, but highlight its weaknesses, explaining my reasons for believing that a more complete theory may supersede inflation without necessarily incorporating it.
** Neil Turok, (12 June 2002)"A critical review of inflation". Classical and Quantum Gravity 19: 3449–3468. DOI:10.1088/0264-9381/19/13/305.
* 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.
** Edward Witten, (June 2002)"Universe on a String". Astronomy Magazine: 42–47. (quote from p. 47)