Is the cosmological constant fine-tuned to permit life?
Clarifying the question
The cosmological constant is the value of the “energy density” of the vacuum of space. Loosely, since the constant is positive, every pocket of space is growing (at a rate proportional to its size). Every square-inch is expanding to a degree that is not noticeable on small scales, but over cosmic scales it has a large affect. Notice that, as space grows, more expanding square-inches of space are added, and so the Universe's net expansion accelerates faster and faster over time. Is the cosmological constant fine-tuned for permitting life?
Cosmologists say “YES”
Steven Weinberg (Nobel laureate in high energy physics, Atheist) “…does seem to require an incredible fine-tuning. The existence of life of any kind seems to require a cancellation between different contributions to the vacuum energy, accurate to about 120 decimal places.” If not: “the universe either would go through a complete cycle of expansion and contraction before life could arise, or would expand so rapidly that no galaxies or stars could form.” [“Life in the Universe”, Scientific American, Oct. '94, 49.]
- Robert Krauss: “The problem with this from a fundamental perspective is that a cosmological constant associated in modern parlance with a nonzero vacuum energy density in the universe on a scale that would be cosmologically relevant and yet still allowed today would take a value that is over 120 orders of magnitude smaller than the naive value that one might expect based on considerations of quantum mechanics and gravity.” [The End of the Age Problem, And The Case For A Cosmological Constant Revisited (1997): Online]
- Alejandro Jenkins (Center for Th. Physics, MIT) & Gilad Perez (Yang Inst. for Th. Physics): “…the most serious fine-tuning problem in theoretical physics: the smallness of the “cosmological constant,” thanks to which our universe neither recollapsed into nothingness a fraction of a second after the big bang, nor was ripped part by an exponentially accelerating expansion. [“Looking for Life in the Multiverse”, Scientific American (Dec. 2009): Online]
- Hans Peter Nilles: “Quantum fluctuations create a vacuum energy which in turn curves the space much stronger than it is observed. Hence, the classical vacuum energy needs to be adjusted in a very accurate way in order to cancel the contributes from quantum effects. This would require a fine-tuning of the fundamental parameters of the theory to an accuracy of at least 60 digits. From the theoretical point of view we consider this is a rather unsatisfactory situation and would like to analyze alternatives leading to the observed cosmological constant in a more natural way… [the author discusses the Randall-Sundrum set-up and brane hypotheses, the conclusion reads:].. Unfortunately we have not yet found a satisfactory model where such a relation is realized and the problem of the size of the cosmological constant still has to wait for a solution.” [“Dark Energy in Extra Dimensions and String Theory: Consistency Conditions”, (Dec. 2000): Online]
- George Smoot (Nobel laureate, physics professor at University of California): “In order to make a universe as big and wonderful as it is, lasting as long as it is—we’re talking fifteen billion years and we’re talking huge distances here—in order for it to be that big, you have to make it perfectly. Otherwise, imperfections would mount up and the universe would either collapse on itself or fly apart, and so it’s actually quite a precise job. And I don’t know if you’ve had discussions with people about how critical it is that the density of the universe come out so close to the density that decides whether it’s going to keep expanding forever or collapse back, but we know it’s within one percent.” [(interview with Fred Heeren) Show Me God: What the Message from Space Is Telling Us About God (Day Star Publications, 2000) 168.]
- Steven Weinberg (Nobel laureate in high energy physics [the very early universe], Atheist): “In any case, there is one constant whose value does seem remarkably well adjusted in our favor. It is the energy density of empty space, also known as the cosmological constant. It could have any value, but from first principles one would guess that this constant should be very large, and could be positive or negative. If large and positive, the cosmological constant would act as a repulsive force that increases with distance, a force that would prevent matter from clumping together in the early universe, the process that was the first step in forming galaxies and stars and planets and people. If large and negative the cosmological constant would act as an attractive force increasing with distance, a force that would almost immediately reverse the expansion of the universe and cause it to recollapse, leaving no time for the evolution of life. In fact, astronomical observations show that the cosmological constant is quite small, very much smaller than would have been guessed from first principles.” [Facing up (Harvard University Press, 2003), 237.]
It is fine-tuned to 1 in 10 to the 120th power
- M. P. Hobson & A. N. Lasenby: “How an we calculate the energy density of the vacuum? … The simplest calculation involves summing up the quantum mechanical zero-point energies of all the fields known in Nature. This gives an answer about 120 orders of magnitude higher than the upper limits on Λ set by cosmological observations. This is probably the worst theoretical prediction in the history of physics! Nobody knows how to make sense of this result. Some physical mechanism must exist that makes the cosmological constant very small. Some physicist have thought that A mechanism must exist that makes Λ exactly equal to zero. But in the last few years there has been increasing evidence that the cosmological constant is small but non-zero. The strongest evidence comes from observations of distant Type Ia supernovae that indicate that the expansion of the universe is actually accelerating rather than decelerating. … the theoretical problem of explaining the value of the cosmological constant is one of the greatest challenges of theoretical physics.” [General Relativity: An Introduction for Physicists (Cambridge, 2006), 187.]
- Eli Michael: “Such high theoretical calculations of are a real limit to the plausibility of a non-zero cosmological constant. The above was only an example for a single field, and it is possible that the the contributions of all the different fields associated with the particles of the standard model conspire to produce a cosmological constant that is small. This argument, however, leads to the belief that the cosmological constant is exactly zero, for how could the fields conspire to cancel out all but 1 part in 10120?” [“How physically plausible is the cosmological constant?” from the University of Colorado, Boulder, (1999): online]
- Robin Collins: “Theoretically, however, the local minimum of the inflaton field could be anything from zero to ρi (see Sahni & Starobinsky 1999, sec. 7.0; Rees 2000, p. 154). The fact that the effective cosmological constant after inflation is less than ρmax requires an enormous degree of fine-tuning, for the same reason as the Higgs field mentioned –for example, neglecting other contributions to the cosmological constant, the local minimum of energy into which the inflaton field fell must be between zero and ρmax, a tiny portion of the its possible range, zero to ρi… If the cosmological constant were not fine-tuned to within one part in 1053 or even 10120 of its natural range of values, the universe would expand so rapidly that all matter would quickly disperse, and thus galaxies, stars, and even small aggregates of matter could never form.” [The Rationality of Theism (Routledge, 2003), 135.]
- Alejandro Jenkins (Center for Th. Physics, MIT) & Gilad Perez (Yang Inst. for Th. Physics): “one quantity still seems to be finely tuned to an extraordinary degree: the cosmological constant, which represents the amount of energy embodied in empty space. Quantum physics predicts that even otherwise empty space must contain energy. Einstein’s general theory of relativity requires that all forms of energy exert gravity. If this energy is positive, it causes spacetime to expand at an exponentially accelerating rate. If it is negative, the universe would recollapse in a “big crunch.” Quantum theory seems to imply that the cosmological constant should be so large—in the positive or negative direction—that space would expand too quickly for structures such as galaxies to have a chance to form or else that the universe would exist for a fraction of a second before recollapsing. One way to explain why our universe avoided such disasters could be that some other term in the equations canceled out the effects of the cosmological constant. The trouble is that this term would have to be fine-tuned with exquisite precision. A deviation in even the 100th decimal place would lead to a universe without any significant structure.” [“Looking for Life in the Multiverse”, Scientific American (Dec. 2009): Online]