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Textbooks on cosmology describe the phenomenon of cosmic inflation in terms of the existence of a scalar field (or many scalar fields, as the answer and comments pointed out), called inflaton. Such as choice without motivation seems ad hoc. Is there a reason why inflaton should be a scalar field (instead of a fermionic or vector field) apart from making the theory simple?

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    $\begingroup$ Much about inflation is ad hoc ... $\endgroup$ – John Rennie Jun 27 '18 at 4:22
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    $\begingroup$ This might have something to do with the potential. I.e. in slow roll inflation, you want to have a potential of a certain shape, being flat at the start, so it 'rolls slowly' and running into a potential minimum to oscillate and start the reheating phase, producing standard model particles. It might be that you cannot achieve this with fermions so easily. $\endgroup$ – DomDoe Jun 29 '18 at 10:26
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There are hundreds of inflationary models, and some of them indeed use vector fields (I'm sure you can come up with spinor models as well). But then you can ask yourself - why describe inflation with "100" scalar fields, when you can just as well describe it with only one? Of course you should keep in mind those other possibilities, but lean towards the simplest ones.

Good textbooks make it clear that inflation is a paradigm, not a theory, and focus on general aspects of inflation that are common to all possible models.

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    $\begingroup$ Just a note to OP: People do work on multiple field inflation too arxiv.org/abs/astro-ph/0702187 $\endgroup$ – Avantgarde Jun 28 '18 at 22:44
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    $\begingroup$ My question was not "Why one or many scalar fields?" but "Why scalar fields? Why are scalars fields preferred? Just for simplicity?" @Avantgarde $\endgroup$ – SRS Jun 29 '18 at 10:03
  • $\begingroup$ @SRS I know. My comment was merely a side note to the answer. $\endgroup$ – Avantgarde Jun 29 '18 at 14:52

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