Within the big bang theory, how can I conceptually understand the needing for an inflationary expansion? Why the CMB should be different than the observed one without inflation? Really the concept please.

PS in case the requirement is due to the cosmological principle. In this case I understand that if I have to see the same CMB every where ( say outside our observable universe or even inside but being at midway ) then inflation is necessary. To clarify what I mean here: with the same thinking, strictly applying the cosmological principle tell me that the universe is spatially infinite ( especially if curvature is , as it seems, quite unity) .

  • $\begingroup$ This question is quite broad. What about the Wikipedia page is not clear to you? $\endgroup$ – Javier Nov 3 '17 at 15:39
  • $\begingroup$ @javier I think the answer is as I suggested in the question. ...basically without inflation there should be no universe outside the observable one... something like that. I do not understand why it is said two far away point can't be at same T... $\endgroup$ – Alchimista Nov 3 '17 at 15:49
  • $\begingroup$ Flatness problem & Horizon problem $\endgroup$ – Andrei Geanta Nov 3 '17 at 15:55
  • $\begingroup$ Thanks I will try again to get the point about the two problems $\endgroup$ – Alchimista Nov 3 '17 at 15:57
  • $\begingroup$ @Arthur. Ok . I am always stuck to the same problem. It is assumed that two opposite patches of the CMB had to be in contact at big bang? In other words:it is assumed that all universe (observable and beyond) originated from the same minuscule speck? Because if it so, without inflation there should be nothing out there. ... $\endgroup$ – Alchimista Nov 3 '17 at 19:29

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