The Two Senses of “Observable Universe”

11 May 2017


From ‘Big Bang Discovery Opens Doors to the ‘Multiverse”

The observable/observed distinction

We can make a distinction between observable universes that are, in fact, observed, and observable universes that, while observable in principle, are not actually observed in fact. Thus, the set of all observable universes may be larger than the set of all universes actually observed, just as the set of all habitable planets is almost certainly larger than the set of all planets that are actually inhabited.

There are many parallels between the observable/observed and inhabitable/inhabited distinctions, and this is because this is, in each case, a modal distinction between potentiality and actuality. For a universe to be observable is for it to be potentially an object of perception, and for a universe to be observed is for it to be actually an object of perception. If “observation” is taken to include not only perception (which might be unknowing and unreflective, i.e., not self-aware) but also conception, we can revise these formulations so that some universe is potentially or actually both an object of perception and an object of thought.

But the observable/observed and inhabitable/inhabited distinctions are even more closely related than both being particular cases of potentiality vs. actuality; an observable universe is a habitable universe, and an observed universe is an inhabited universe. The universe (or a universe), then, is a generalization of a planet, so that in studying the habitable/inhabited distinction where it concerns planets, we are studying the question of observable/observed universes in miniature.

In the case of habitability (i.e., the habitable/inhabited distinction), we know the confusion that this routinely causes. With the increasing number of announcements of exoplanet discoveries, there have been an increasing number of confused accounts which imply that a planet of the right size found within a habitable zone is not just potentially habitable (arguably this formulation is redundant, and it should be sufficient to say “habitable”), but that it is, or must be, inhabited. Exoplanet scientists and astrobiologists are not guilty of this conflation, but accounts of their work in the legacy media make this conflation with regularity.

Perhaps because we see our near neighbors Venus and Mars, both smallish rocky planets like Earth, and both more-or-less in the habitable zone, we can easily understand that a planet that has the right conditions for life does not necessarily host life: these planets are habitable but not inhabited. We can bring the habitable/inhabited distinction home and understand it in human terms, but the observable/observed distinction, especially when applied to the universe entire, is likely to elude us. Moreover, the idea of an empty universe, that is to say, an entire universe without intelligent observers (observers who can both perceive the world and form a conception of what they perceive), is likely to strike many as a bit bizarre, if not absurd.

The Anthropic Cosmological Principle

Sometimes the idea that an empty universe is absurd is made explicit, or nearly so. John Wheeler is credited with saying, “A universe without an observer is not a universe at all.” In fact, Wheeler didn’t write these exact words, but the idea is pervasively present in his exposition of the anthropic cosmological principle. To give a sense of this, here is a comment on the weak anthropic principle (WAP) from Barrow and Tipler’s classic work (with a forward provided by John Wheeler):

“According to WAP, it is possible to contemplate the existence of many possible universes, each possessing different defining parameters and properties. Observers like ourselves obviously can exist only in that subset containing universes consistent with the evolution of carbon-based life.”

The Anthropic Cosmological Principle, John D. Barrow and Frank J. Tipler, Oxford: Oxford University Press, 1986, p. 19

Three interpretations are given of the strong anthropic principle:

(A) There exists one possible Universe ‘designed’ with the goal of generating and sustaining ‘observers’.

(B) Observers are necessary to bring the Universe into being.

(C) An ensemble of other different universes is necessary for the existence of our Universe.

Ibid., p. 22

As these ideas are given an extensive exposition in the text, I will not attempt to flesh them out, but I quote them here only for purposes of exhibition. It would be a considerably involved enterprise to give an exposition of the various formulations of the weak, strong, participatory, and final anthropic principles propounded by Barrow, Tipler, and Wheeler, and then to present them in comparison and contrast with what I have written here about empty universes, but I am not going to attempt that here. Some of these ideas are consistent with a range of universes, some of them empty, and some are not.

Empty, unobserved universes and scientific realism

There can only be two senses of “observable universe” if one is willing to countenance the possibility of empty, unobserved universes, which suggests a strongly realist position, and this interpretation takes to the limit of extrapolation the idea that something exists whether or not we see it (or anyone sees it). If we assume that the back side of the head of the person we are talking to continues to exist even when we do not see it (and if there is no one else looking at it), then we are assuming some degree of realism.

In the case of the person, it could be argued that the person in question is always viscerally conscious of their bodily integrity, and on this basis the back side of their head continues to be perceived, and hence continues to exist without the posit of realism. However, this argument cannot be made with inanimate objects without positing panpsychism. We assume that the back sides of houses, the insides of closets, and the contents of empty rooms continue to exist even when we are not looking at them. I can see no reason this intuitive realism should not be scaled up to entire universes that exist without being observed. This is, at least, consistent with scientific realism, even if it is not entailed by scientific realism.

The Principle of Plenitude

This kind of distinction I am making here between observable universes and observed universes immediately puts us in mind of the principle of plenitude (on which I previously wrote in Cosmology is the Principle of Plenitude Teaching by Example and Parsimony and Plenitude in Cosmology). The most obvious interpretation of the principle of plenitude in this context is that a universe that was habitable would eventually realize the potential of this habitability and would become inhabited. Perhaps this is why some advocates of the strong anthropic principle say that a universe that does not produce observers is a “failed” universe (not the kind of claim I would ever make, but one can understand something of this by saying that such a universe has failed to realize its potential). If we acknowledge the possibility of “failed” universes in this sense, then we would have empty, uninhabited universes, only we would attach a (negative) valuation to them (and presumably we would attach a positive valuation to successful universes that realize their potential and produce observers).

There is, however, another way to interpret the principle of plenitude in this context, and that is to argue that the principle of plenitude entails the realization of every possible kind of universe, and that the existence of an empty universe without observers is a potential that will eventually be realized, if it has not already been realized. Moreover, every kind of universe that can be observed by an observer that evolves within that universe constitutes another kind of universe that could exist in which the potential of such an observer is not realized. Thus if there are a plurality of observed universes, then this interpretation of the principle of plenitude suggests that there will be a plurality of observable but unobserved universes.

The Principle of Parsimony

The principle of plenitude as applied to worlds or to universes would imply densely inhabited worlds and intensively observed universes — what Frank Drake and Dava Sobel called, “an infinitely populated universe.” The principle of parsimony (often invoked as a counter to the principle of plenitude) as applied to worlds or the universe would limit us almost in a constructivistic sense to the world we inhabit — there is at least one observable universe that is, in fact, observed — though before or after the existence of this one known instance of an observer the universe would be empty and unobserved.

The intersection of the principle of plenitude and the principle of parsimony would yield at least one such-and-such (plenitude) and at most one such-and-such (parsimony), that is to say, this intersection would yield uniqueness, one and only one such-and-such — but whether this uniqueness should apply to each and every universe, or whether the universe itself ought to be considered unique, is another question.

A final reflection

It seems to me that the idea of an uninhabited planet, that is unobserved because it it uninhabited, has become a familiar and even a conventional idea of contemporary cosmology and astrobiology — it is, I think, widely assumed that we will eventually find other life in the universe, sprung from other origin of life events, but that intelligent life, and thus an observer that knows itself to be observing, is likely to be quite rare. This consensus view — if it is a consensus — encounters problems when it is extrapolated from habitable/inhabited planets to habitable/inhabited universes. Why this idea appears to transcend science (in the narrow sense) when extrapolated to the whole of the universe I am not yet prepared to say, but I will continue to think about this.

I began this post with the intention to make a simple and straight-forward distinction between observable universes and observed universes (my first draft was only three paragraphs), but as I worked on this I got myself entangled in a number of difficult questions that ended up entailing all-too-brief discussions of difficult ideas like the principle of plenitude and the principle of parsimony. This is admittedly unsatisfying, and I know that I have not done these ideas justice, but at some point I have to bring this to a close.

. . . . .

. . . . .


. . . . .

Grand Strategy Annex

. . . . .

project astrolabe logo smaller

. . . . .


2 Responses to “The Two Senses of “Observable Universe””

  1. xcalibur said

    The problem with the multiverse concept is that our powers of observation are limited. This may be one of many universes, but it’s impossible to be certain one way or the other.

    If there is a multiverse, I see no reason why there shouldn’t be empty/unobserved universes. There may be a great variety of universes with different constraints of matter/energy/space/time. For example, there may be universes full of black holes, plasma, or antimatter, and their durations may range from picoseconds to trillions of years.

    But even positing the multiverse concept doesn’t undermine the Anthropic Cosmological Principle, in my opinion. It’s simply adding another layer to the equation — you still have to ask, how could a multiverse come into existence with the capability of creating intelligent observers, unless there were an intelligent supreme being?

    As for life in our universe, I think that complexity and commonality of life are inversely proportional. I expect microbes to be very common, intelligent civilizations to be very rare, with complex macro-scale plants/animals somewhere in the middle. Again, the issue is a lack of observation and data, but this seems like a very reasonable extrapolation based on current knowledge.

    We know that life exists on Earth in great abundance and complexity, and that it has existed for nearly as long as the Earth has been habitable. We also know that the requirements of life are in abundance throughout the universe (CHNOPS, solar systems with exoplanets, liquid water, etc.) Finally, the inverse proportion rule (that I made up) applies well to life on Earth, which is currently our only reference point. Therefore, I think my hypothesis is a reasonable one.

    • geopolicraticus said

      Our powers of observation are limited, but they can be extended with science and technology. However, science and technology are also limited, so even our powers of observation augmented by science and technology yield a limited view of the world. Fortunately, if we are sufficiently clever we can keep chipping away at the limits and keep expanding our knowledge. For now, the multiverse remains unobservable, but that will not necessarily always be the case.

      Another problem is that “multiverse” means different things to different people, and we really need to clean this up so as not to talk at cross purposes. I have been meaning to write a post on the disambiguation of the multiverse concept, but the amount of research required is significant and I haven’t set aside the time to really double down and arrive at the formulations of the various positions that would be both clear and easily distinguishable.

      Best wishes,


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: