Both Brian Greene (The Hidden Reality) and Will Kinney (An Infinity of Worlds) describe an eternally inflating multiverse that surrounds bubble universes of non-inflating space. We reside in one such bubble universe. We measure our bubble's size out to the cosmic horizon and find a value of 46 billion light-years, but most cosmologists believe its size goes beyond that. Both Greene and Kinney say the space inside any bubble universe is in fact infinite, but only from the perspective of an observer inside the bubble. From an observer outside (in the eternally inflating space), the bubble is clearly finite. In fact bubbles can collide, which requires they have an edge. Collisions are something cosmologists are actively investigating by examining our own bubble's CMB radiation map.

This infinite/finite logical conflict is apparently resolved by using different perspectives of time, but neither author adequately explains it (at least in a way I can understand). Can anyone shed light on how two observers would come to such different conclusions?

  • $\begingroup$ As Nobel laureate John Mather has mentioned, a universe can [by definition] expand only into itself, at least if you're referring to the spatial expansion required by General Relativity. Kinney's work isn't (without a link) available to me, but Greene's might be interpreted as describing a temporally endless subdivision of the space separating such gravitationally-bound objects as galaxies. $\endgroup$
    – Edouard
    Oct 5, 2022 at 3:05
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    $\begingroup$ Greene seems to explain this well (see figure 3.8b and accompanying text), and Kinney has a good picture (figure 7.2). Without a better idea of what you don't understand, it's hard to know how to answer. $\endgroup$
    – benrg
    Oct 5, 2022 at 6:26
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    $\begingroup$ This is just to warn readers that in these discussions the word 'universe' is used very loosely. Its use often rests on the assumption that whatever physical theory the authors are grappling with is able to furnish a complete description of a universe, but we really don't know that. $\endgroup$ Oct 5, 2022 at 16:57

2 Answers 2


Greene's book contains this picture (figure 3.8b):

It's schematic (these discrete squares don't really exist), but it's meant to illustrate how the universe can be finite with respect to one notion of time and infinite with respect to another. If you take the "current time" to be the vertical axis, then the inflationary bubble only spans a finite number of squares at each time, and it has a boundary. If you take the "current time" to be the value written in each square (which is supposed to represent the energy density of either the inflaton field or the matter that it decays into), then at any particular time other than 100, only the interior of the inflationary bubble exists, and the rest of the universe is in the past. Both of these time coordinates respect causality (the past light cone at any point only contains points at later times, for either notion of time), so they are both equally valid.

The value-in-the-square time coordinate makes sense if you want to talk about events inside the bubble, because all of the squares with the same value in them are essentially interchangeable: the universe at any particular value of that time coordinate is homogeneous and isotropic. (It looks as though the square at the vertex of the V is special, but that's just because this model with squares is not very accurate.) The value-in-the-square time is what's called "cosmological time" in the standard cosmological model.

If you want to talk about events outside the bubble, then the value-in-the-square time is useless since it is ill-behaved (degenerate/singular) in the region where all squares say 100. The vertical-axis time doesn't have that problem, but it is less useful for events inside the bubble since it doesn't respect the symmetry of the bubble.

You could say, as Greene does, that the value-in-the-square time is the time for an inside observer and the vertical-axis time is the time for an outside observer, but I dislike that language, since it suggests that you are somehow obliged to choose particular coordinates based on your location. The reality is that you can use any coordinate system that covers the region of the universe that you're interested in. Some coordinates are more convenient than others, but none is ever wrong.

Also, Greene says

Trixie, floating [in the 100 region], is observing the formation of a nearby bubble universe. Focusing her inflaton-meter on the growing bubble, she is able to directly track its changing inflaton field value.

which makes no sense: it's impossible to see the bubble from outside, since it expands at the speed of light, and no information about it can propagate faster than that. You can't even know that a bubble exists there, since it starts from an uncaused random fluctuation. The only way to measure those numbers is to wait for the bubble to engulf you, after which you can never leave. So really Trixie ought to be considered an inside observer, despite her choice of coordinates.

  • $\begingroup$ Although it's usually considered uncouth to mention voting, I feel I should say that I posted one of the two favorable votes that Benrg's answer (a critique of illustrated commentary by Columbia University's professor of physics, Brian Greene) has so far received. My own (problematic) answer had been posted a couple of hours before, but, even so, I'm still wondering whether bubble walls can have reflections, like whichever one "Trixie" may've seen. I'm (almost) sure that Benrg is aware that the speed-of-light limit applies to travellers, but not to expansion rates. $\endgroup$
    – Edouard
    Aug 5 at 18:12

I'll take Benrg's word for the identical nature of the conclusions (relevant to the OP's question) that can be drawn from the two diagrams, although I can only see one of them (Greene's) myself.

Assuming that those conclusions differ from each other not in terms of which author derived them, but in terms of infinity coexisting with finitude, I think that the "difference" might be found in the effect of pi, whose possibly infinite array of figures is (in Greene's diagram) represented by a combination of the tiny curvature at the downward end of the half-diamond whose 2D outline cuts across varying portions of the identically-shaped (and identically-sized) numbered boxes providing a stylized representation of the interior of the bubble, which the outside observer sees as part of a single sphere.

To understand this, it's essential to understand that the irrational number pi (which is irrational because it cannot be expressed exactly as a ratio of two integers) is a mathematical constant, as discussed at https://en.wikipedia.org/wiki/Pi . Although abbreviated versions of pi (the ratio of the circumference of a circle to its diameter) have, since the advent of computers, been carried out to "trillions of digits", that inherent lack of completion which it represents can be rather plausibly used to approximately represent infinity (which, itself, is not a number, but a concept).

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    $\begingroup$ This answer is better suited to numerologystackexchange.com $\endgroup$
    – D. Halsey
    Aug 5 at 13:23
  • $\begingroup$ @D.Halsey --hysterical (or, as you'd probabalistically say, historical)! $\endgroup$
    – Edouard
    Aug 5 at 16:16
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    $\begingroup$ The possibility for the universe to be infinite and finite is entirely a matter of its spatial volume (or length). It has nothing to do with the way space is subdivided. Benrg's answer is exactly correct; you can also find discussions of the point in published academic works, e.g. around figure 2 of this chapter. $\endgroup$
    – Sten
    Aug 5 at 16:33
  • $\begingroup$ @Sten -Thanks; I'll have a look, although, language-bound as I am, it's difficult to see how anything infinite would not also be finite, unless its divisibility would not be possible (which would hardly seem to be the case, in a universe or multiverse comprised--however momentarily--of space and particles). $\endgroup$
    – Edouard
    Aug 5 at 16:41
  • $\begingroup$ I've been called away for something: I'll comment further tomorrow, Sten. Thanks for your patience. $\endgroup$
    – Edouard
    Aug 5 at 18:29

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