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I have just recently started learning about inflationary cosmology so I don't know much about it. But from my understanding, the theory says the big bang was started by the expansion of a small region by negative pressure in the backbone universe that all the other pocket universes expand in.

If this understanding is correct (please correct me if I'm wrong) then could a pocket universe randomly be birthed out of negative pressure in our universe to give us a pocket universe, within our universe?

If this can't happen why? There something different in the composition of our universe and that of the pocket universe backbone?

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    $\begingroup$ Did you see a pocket universe anywhere? You haven't even seen theory that says such things. Theory, that's the math that describes the things that have been observed really well. Math that is portrayed to describe things that haven't been observed, that's not theory, that's just "intellectual nonsense". $\endgroup$
    – CuriousOne
    Commented Jun 26, 2016 at 1:08
  • $\begingroup$ @CuriousOne I think I understand what you meant but I have to disagree with the last bit. There are examples of math (to not call it a theory) being used to describe things never seen before which were later proven to describe reality. The best example that comes to my mind is Pontecorvo's work about neutrino's oscillations in 1958. Basically he invented and solved the problem decades earlier people observed it. I am pretty sure people at that time thought that was just nonsese or useless math. $\endgroup$
    – Diracology
    Commented Jun 26, 2016 at 1:55
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    $\begingroup$ @Diracology: Neutrino oscillations were intellectual nonsense right until they were observed. There is nothing wrong with being conservative on this point because, see, if they hadn't been observed, one could have found good mathematical arguments for why Pontecorvo was wrong. We can always fix these things in hindsight, but we can never be too careful to approve them in foresight... Einstein and the cosmological constant should be the most dire warning for why that's just not a winning strategy. $\endgroup$
    – CuriousOne
    Commented Jun 26, 2016 at 2:01

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I don't know if negative pressure (but see my added edit below) , more importantly there is a theory of inflation, and some good evidence for it. It was caused by a yet unknown inflation field, with its parameters somewhat matching what the cosmic microwave background (CMB) measurements show. [edit added: The field is a quantum field that rolled from a high energy state to a lower equilibrium one, and released a lot of energy. The equivalent equation of state for the energy momentum tensor for it does have the positive energy density and a negative pressure]

As for eternal inflation, it is speculation that if it could happen once it could happen multiple times in different places. Nobody knows and there is no evidence, pure speculation.

Cosmology matches well the CMB measurements. Including the inflation parameters. Look it up in Wikipedia or many online lectures. Sean Carroll is pure speculation to sell books.

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Pocket universes have arisen in different theories. Just to name you two, one is alan Gut's inflationary theory idea that Eternal inflation produces pocket universes with all physically allowed vacua and histories. Another is that from sean carroll, who claims that inside every black hole there is an entirely new universe.

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  • $\begingroup$ A scientific theory is a large, well defined and largely confirmed body of explanation for actually observed phenomena. What you are talking about are, at most, models and hypotheses. Cosmology, specifically, is defined by the $\Lambda CDM$ model, which, at this time, says absolutely nothing about pocket universe or eternal inflation, neither of which have been observed (and neither of which are even remotely physically observable within the proposed parameter spaces of the more outlandish models that contain them). $\endgroup$
    – CuriousOne
    Commented Jun 26, 2016 at 1:29
  • $\begingroup$ I beg to disagree $\endgroup$
    – user65081
    Commented Jun 26, 2016 at 1:55
  • $\begingroup$ You are disagreeing with the definition of "scientific theory" and "model"? Why? The folks who are proposing these models know full well what they are and what they are not. Cosmology, at this point, is a bunch of models, it's not a theory. $\endgroup$
    – CuriousOne
    Commented Jun 26, 2016 at 2:03
  • $\begingroup$ I disagree with @curiousone. Cosmology is as much a theory as the standard model of particle physics is. The word model is not a distinguishing factor. What is true is that cosmology still has unanswered questions. Standard model also has unanswered questions. They both represent our best theories of one and the other,mwhile knowing there is more to know. They are both consistent theory, with cosmology not toally clear on inflation but still good evidence of it. A model is to play around with math. Inflation got the Nobel prize. Eternal inflation is just speculation. $\endgroup$
    – Bob Bee
    Commented Jun 26, 2016 at 2:41
  • $\begingroup$ @BobBee: The standard model of particle physics is, as the name says, a model. For one thing, it explains nothing (it's structurally not even designed to explain anything, it's just a fit to the observations) but it gives us a mysterious and not so small number of "parameters", which very few people believe to be fundamental. A useful theory has no or, at most, very few parameters. There is very little positive evidence for inflation if we don't forcefully exclude extensions for general relativity like Einstein-Cartan. Inflation has not been awarded a Nobel, at least not in this universe. $\endgroup$
    – CuriousOne
    Commented Jun 26, 2016 at 2:53

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