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I am not sure the validity of this claim from Wikipedia on 't Hooft–Polyakov_monopole

The "monopole problem" refers to the cosmological implications of grand unification theories (GUT). Since monopoles are generically produced in GUT during the cooling of the universe, and since they are expected to be quite massive, their existence threatens to overclose it [clarification needed]. This is considered a "problem" within the standard Big Bang theory. Cosmic inflation remedies the situation by diluting any primordial abundance of magnetic monopoles.

  1. If monopoles are massive, we may not detect monopoles -- I do not know what does it mean to say "their existence threatens to overclose it"? If we cannot detect monopoles due to monopoles are massive, then why there is a "monopole problem"? It may be that we can find monopoles only if we can reach the GUT scale?

  2. "the standard Big Bang theory. Cosmic inflation remedies the situation by diluting any primordial abundance of magnetic monopoles." What are the differences of standard Big Bang theory versus Cosmic inflation? Do the remedies of the two the same idea or no?

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  • $\begingroup$ Isn't the monopole problem is the puzzle of non-observation of GUT-predicted monopoles in the present Universe? $\endgroup$
    – SRS
    Jul 15, 2020 at 4:20

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  1. According to many GUTs, monopoles (and other possible topological defects like cosmic strings) are generally predicted to be abundantly created in the early universe. Often such objects are stable or very long-lived, so their predicted abundance contradicts our observations -- we don't see them, so even if monopoles exist, they must be very rare somehow (order of 1 per observable universe, something like that). Inflation solves the problem by diluting them to non-observable amounts (assuming inflation happened after monopoles are created, i.e. after the GUT symmetry breaking on the time scale).

  2. By Standard Big Bang cosmology (SBBC) non-inflationary cosmology is understood here (standard cosmology without inflation). Inflationary cosmology = inflation+SBBC. Any cosmological model must include SBBC, but there are models where inflation is replaced by alternatives. For example String Gas Cosmology, Bouncing Cosmology, etc. So in this sense SBBC is the necessary component for any theoretical cosmological model to have.

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  • $\begingroup$ thanks - I voted up, any good refs are welcome <3 $\endgroup$ Jul 16, 2020 at 17:07

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