It is sometimes objected that, because our observations are limited to a single universe (e.g. a Hubble volume), then the existence of 'other universes' cannot be observed, and so their existence cannot be considered a proper scientific hypothesis. Even taking into account the fact that future observers will see a larger particle horizon, and so have access to a bigger volume of space, most regions of the multiverse (at least in the eternal inflation model) can never be observed, even in principle. While this may indeed preclude direct confirmation of the multiverse hypothesis, it does not rule out the possibility that it may be tested indirectly. Almost all scientists and philosophers accept the general principle that the prediction of unobservable entities is an acceptable scientific hypothesis if those entities stem from a theory that has other testable consequences. At this stage, string/M-theory does not have any clear-cut experimental predictions, but one may imagine that a future elaboration of the theory would produce testable consequences. These theories are not idle speculations, but emerge from carefully considered theoretical models with some empirical justification.
A test of the multiverse hypothesis may be attained by combining it with biophilic selection. This leads to statistical predictions about the observed values of physical parameters . If we inhabit a typical biophilic region of the multiverse, we would expect any biologically relevant adjustable parameters to assume typical values. If one considers a vast parameter space of possible universes, there will be one or more biophilic patches - or subsets -of the space, and a typical biophilic universe would not lie close to the centre of such a patch (i.e. it would not be optimally biophilic). In other words, there is no a priori reason why the laws of physics should be more bio-friendly than is strictly necessary for observers to arise. If, therefore, we discovered that some parameter (such as the amount of dark energy) assumed a value in a tiny subset located deep inside the biophilic parameter range, this would be evidence against it being a random variable that had been anthropically selected.
There is a hidden assumption in the foregoing reasoning, which is that life originates only once in each universe. If life happens many times, then optimally biophilic universes may contain many more observers than minimally biophilic universes, and this weighting factor must be taken into account when considering a randomly chosen observer. We must now distinguish between biophilicity in relation to laws and biophilicity in relation to contingency. Regarding the latter, it is possible that life is indeed 'a damned close-run thing' (to paraphrase Lord Wellington) - a statistical fluke, unique in the observable Universe, arising from a highly improbable molecular accident.
If, however, we discover a second genesis of life - an independent origin on a nearby planet - then this would imply that the Universe is teeming with life and is at least near-optimally biophilic in relation to contingency.
But even in the absence of any data concerning multiple geneses, we may still consider biophilicity in relation to the laws of the Universe. At first glance, there is little reason to suppose that the Universe is minimally bio-philic in this respect. Take the much-cited example of carbon abundance. The existence of carbon as a long-lived element depends on the ratio of electromagnetic to strong nuclear forces, which determines the stability of the nucleus. But nuclei much heavier than carbon are stable, so the life-giving element lies comfortably within the stability range. The electromagnetic force could be substantially stronger, without threatening the stability of carbon. Now, it is true that, if it were stronger, then the specific nuclear resonance responsible for abundant carbon would be inoperable, but it is not clear how serious this would be. Life could arise in a universe where carbon was merely a trace element, or abundant carbon could occur because of different nuclear resonances. Of course, if it could be shown that other, heavier, elements are essential for life, this objection would disappear. (The prediction that much heavier elements are essential for life could be an interesting prediction of the multiverse theory.)
A simpler example is the amount of dark energy in the Universe. Once again, the observed value is comfortably in the middle of the biologically acceptable parameter range. Theory predicts that the density of dark energy (A) should be vastly greater than the observed value, so we might expect in the multiverse explanation that the observed value would be near the top end of the biologically permissible parameter range. But A could be an order of magnitude bigger without threatening the existence of galaxies and stars, and hence life . On the face of it, therefore, the observed Universe is not minimally biophilic, and many scientists seem to think it is actually optimally biophilic.
Another consideration concerns the very existence of physical laws. In those versions of the multiverse in which even the appearance of law is attributed to anthropic selection, there is clearly a problem about minimal biophilicity. The multiverse explanation would lead us to expect that we live in a universe that has the minimal degree of order consistent with the existence of observers. Departures from order, or lawfulness, that are not biologically threatening should therefore be permitted. To take a simple example, consider the law of conservation of electric charge. The charge on the electron could happily fluctuate by, say, one part in 106 without disrupting biochemistry. In fact, measurement of the anomalous magnetic moment of the electron fixes the electric charge to eleven significant figures -a stability far in excess of that needed to ensure the viability of living organisms. So either the electric charge is fixed by a law of nature, in which case the multiverse cannot be invoked to explain this particular aspect of cosmic order, or there is some deep linkage between the charge on the electron and some aspect of physics upon which the existence of life depends far more sensitively. But it is hard to see what this might be.
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