Does the Universe need humankind?
15 Dec, 2006 07:28 pm
If a tree falls in the forest, and nobody is there to hear, does
it make a sound? This age-old question is analogous to one that
cosmologists have been asking - would the Universe exist if the
laws of physics precluded the evolution of intelligent observers? New calculations performed by cosmologists at Oxford University
and Case Western Reserve University indicate that the answer to
this question depends critically on how exactly one counts the
number of "intelligent observers" in a hypothetical universe.
For instance, humankind (or any other observer) could not have emerged in a Universe that lasted less than a few hundred million years before ending in a Big Crunch. Many scientists argue that the anthropic principle can shed light on properties of our Universe that cannot be otherwise explained.
For example, imagine sitting in a reception room, waiting to interview for a job for which you have applied. While sitting, you wonder how many other candidates have been invited for an
interview. From a probabilistic point of view, the fact that you have been invited suggests that the number of shortlisted candidates is large. That is your probability of having being shortlisted is larger if the number of shortlisted candidates is larger. In cosmology, one the n tries to infer the characteristics of the advertised job from the knowledge that at least one candidate (humankind) has shown up in the cosmic interview.
One popular application of the anthropic principle in cosmology is to the value of the cosmological constant, the dark energy that seems to be powering the observed accelerated expansion of the Universe. Nobody presently understands why the cosmological constant we observe is extremely small compared to theoretical expectations. However, it has been argued that this is precisely what one ought to see if we were only one sample out of a much bigger collection of possible Universes, each of them exhibiting different values for fundamental constants of Nature (including the cosmological constant) - this is called the Multiverse scenario [1,2].
The presence of intelligent observers in our Universe (us), would then necessarily restrict the possible values of such constants. In other words, the nature of our Universe is determined by the existence and, more particularly, by the number of intelligent beings that exists in it.
In an article published last November in Physical Review Letters , Glenn Starkman (Case Western Reserve University) and myself argue that the implications of the existence of humankind for the properties of the universe cannot actually be determined. Our extremely limited knowledge about the number and distribution of other intelligent observers in the cosmos (if they exist) represents an almost insormontable obstacle to any conclusion one would like to draw from the anthropic principle.
In particular, in our work we considered an alternative way of counting observers - dropping
the idea of observers altogether, and counting instead the maximum number of observations that advanced civilizations could in principle carry out, given the fact the maximum amount of energy any observer can collect and exploit is limited by the laws of physics. This in turn limits the amount of (energy-consuming) observations any observer can ever hope to be able to make. We call this probabilistic scheme "MANO", for "Maximum Allowed Number of Observations".
We focused on the value of the cosmological constant in our Universe and found it to be enormously higher than one would have expected. According to Glenn Starkman "It would seem that the mere fact that we observe the Universe does not make it more likely to have the largest possible value of the cosmological constant consistent with our existence".
Clearly, if the anthropic principle is to be used in a scientific way, we need to figure out better ways of computing the probabilities involved, especially regarding the emergence of intelligent observers, as our MANO example shows.
This study fits into a vigorous debate among cosmologists about the significance to be attributed to our existence in what is seen as a Universe suspiciously supportive for life. Much work remains to be done before science will be able to give a final answer (if ever) to the profound question of why we are here and why the Universe we inhabit is the way it is.
 S. Weinberg, "The Cosmological Constant Problem", Rev.Mod. Phys 61 (1989) 1.
 J. Garriga and A.Vilenkin, "On likely values of the cosmological constant", Phys.Rev. D 61 (2000) 083502. arXiv:astro-ph/9908115.
 G. Starkman and R. Trotta, "Why anthropic reasoning cannot predict Lambda", Phys.Rev.Lett. 97 (2006) 201301. ArXiv: astro-ph/0607227 A slighlty expanded version can be found at astro-ph/0610330