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ScienceWeek
COSMOLOGY: ON ANTHROPIC REASONING
The following points are made by M. Livio and M.J. Rees (Science 2005 309:1022):
1) Does extraterrestrial intelligent life exist? The fact that we can even ask this question relies on an important truth: The properties of our Universe have allowed complexity (of the type that characterizes humans) to emerge. Obviously, the biological details of humans and their emergence depend on contingent features of Earth and its history. However, some requirements would seem generic for any form of life: galaxies, stars, and (probably) planets had to form; nucleosynthesis in stars had to give rise to atoms such as carbon, oxygen, and iron; and these atoms had to be in a stable environment where they could combine to form the molecules of life.
2) We can imagine universes where the constants of physics and cosmology have different values. Many such "counterfactual" universes would not have allowed the chain of processes that could have led to any kind of advanced life. For instance, even a universe with the same physical laws and the same values of all physical constants but one -- a cosmological constant Lambda (the "pressure" of the physical vacuum) higher by more than an order of magnitude -- would have expanded so fast that no galaxies could have formed. Other properties that appear to have been crucial for the emergence of complexity are (i) the presence of baryons (particles such as protons and neutrons); (ii) the fact that the Universe is not infinitely smooth, allowing for the formation of structure (quantified as the amplitude of the fluctuations in the cosmic microwave background, Q); and (iii) a gravitational force that is weaker by a factor of nearly 10^(40) than the microphysical forces that act within atoms and molecules -- were gravity not so weak, there would not be such a large difference between the atomic and the cosmic scales of mass, length, and time.
3) A key challenge confronting 21st-century physics is to decide which of these dimensionless parameters such as Q and Lambda are truly fundamental -- in the sense of being explicable within the framework of an ultimate, unified theory -- and which are merely accidental. The possibility that some are accidental has certainly become viable in the context of the "eternal inflation" scenario [1-3], where there are an infinity of separate "big bangs" within an exponentially expanding substratum. Some versions of string theory allow a huge variety of vacua, each characterized by different values of (or even different dimensionality) [4]. Both these concepts entail the existence of a vast ensemble of pocket universes -- a "multiverse." If some physical constants are not fundamental, then they may take different values in different members of the ensemble. Consequently, some pocket universes may not allow complexity or intelligent life to evolve within them. Humans would clearly have to find themselves in a pocket universe that is "biophilic." Some otherwise puzzling features of our Universe may then simply be the result of the epoch in which we exist and can observe. In other words, the values of the accidental constants would have to be within the ranges that would have allowed intelligent life to develop. The process of delineating and investigating the consequences of these biophilic domains is what has become known as "anthropic reasoning".[5]
References (abridged):
1. P. J. Steinhardt, in The Very Early Universe, G. W. Gibbons, S. Hawking, S. T. C. Siklos, Eds. (Cambridge Univ. Press, Cambridge, 1983), p. 251
2. A. Vilenkin, Phys. Rev. D 27, 2848 (1983)
3. A. D. Linde, Mod. Phys. Lett. A 1, 81 (1986)
4. S. Kachru, R. Kallosh, A. Linde, S. P. Trivedi, Phys. Rev. D 68, 046005 (2003)
5. A. G. Riess et al., Astron. J. 116, 1009 (1998)
Science http://www.sciencemag.org
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COSMOLOGY: ON THE ANTHROPIC PRINCIPLE
The following points are made by Lawrence M. Krauss (Nature 2003 423:230):
1) The recognition, in the light of observational data, that Einstein's infamous cosmological constant might not be zero has changed almost everything about the way we think about the Universe, from reconsidering its origin to re-evaluating its ultimate future. But perhaps the most significant change in cosmological thinking involves a new willingness to discuss what used to be an idea that was not normally mentioned in polite company: the "anthropic principle".
2) This idea suggests that the precise values of various fundamental parameters describing our Universe might be understood only as a consequence of the fact that we exist to measure them. To paraphrase the cosmologist Andrei Linde, "If the Universe were populated everywhere by intelligent fish, they might wonder why it was full of water. Well, if it weren't, they wouldn't be around to observe it!".
3) The reason that physicists have been so reluctant to consider the anthropic principle seriously is that it goes against the grain of current attitudes. Most physicists have hoped that an ultimate physical explanation of reality would explain why the Universe must look precisely the way it does, rather than why it more often than not would not. Into the fray has entered James Bjorken. In a paper (Phys. Rev. D 2003 67:043508) entitled "Cosmology and the Standard Model", Bjorken proposes a new "scaling" approach, based on well-established notions in particle theory, for exploring how anthropically viable a small cosmological constant might be.
4) The realization that an extremely small, but non-zero, cosmological constant might exist has changed the interest of physicists in anthropic explanations of nature precisely because the value it seems to take is otherwise so inexplicable. In 1996, physicist Steven Weinberg and his colleagues Hugo Martel and Paul Shapiro argued that if the laws of physics allow different universes to exist with a cosmological constant chosen from an underlying probability distribution, then galaxies, stars and presumably astronomers might not ultimately evolve unless the cosmological constant were not much larger than the one we apparently observe today.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
The "cosmological constant" is a mathematical term introduced by Einstein into the equations of general relativity, the purpose to obtain a solution of the equations corresponding to a "static universe". The term describes a pressure (if positive) or a tension (if negative) which can cause the Universe to expand or contract even in the absence of any matter ("vacuum energy"). When the expansion of the Universe was discovered, Einstein apparently began to regard the introduction of this term as a mistake, and he described the cosmological constant as the "greatest mistake of my life". But the term has reappeared as the proposed source of apparent accelerated cosmic expansion.
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ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
The following points are made by P.J.E. Peebles (Nature 1999 398:25):
1) Contrary to expectations, the evidence is that the Universe is expanding at approximately twice the velocity required to overcome the gravitational pull of all the matter the Universe contains. The implication of this is that in the past the greater density of mass in the Universe gravitationally slowed the expansion, while in the future the expansion rate will be close to constant or perhaps increasing under the influence of a new type of matter that some call "quintessence".
2) Quintessence began as Einstein's cosmological constant, Lambda. It has negative gravitational mass: its gravity pushes things apart.
3) Particle physicists later adopted Einstein's Lambda as a good model for the gravitational effect of the active vacuum of quantum physics, although the idea is at odds with the small value of Lambda indicated by cosmology.
4) Theoretical cosmologists have noted that as the Universe expands and cools, Lambda tends to decrease. As the Universe cools, symmetries among forces are broken, particles acquire masses, and these processes tend to release an analogue of latent heat. The vacuum energy density accordingly decreases, and with it the value of Lambda. Perhaps an enormous Lambda drove an early rapid expansion that smoothed the primeval chaos to make the near uniform Universe we see today, with a decrease in Lambda over time to its current value. This is the cosmological inflation concept.
5) The author suggests that the recent great advances in detectors, telescopes, and observatories on the ground and in space have given us a rough picture of what happened as our Universe evolved from a dense, hot, and perhaps quite simple early state to its present complexity. Observations in progress are filling in the details, and that in turn is driving intense debate on how the behavior of our Universe can be understood within fundamental physics.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
Active vacuum of quantum physics: This refers to the idea that the vacuum state in quantum mechanics has a zero-point energy (minimum energy) which gives rise to vacuum fluctuations, so the vacuum state does not mean a state of nothing, but is instead an active state.
If a theory or process does not change when certain operations are performed on it, the theory or process is said to possess a symmetry with respect to those operations. For example, a circle remains unchanged under rotation or reflection, and a circle therefore has rotational and reflection symmetry. The term "symmetry breaking" refers to the deviation from exact symmetry exhibited by many physical systems, and in general, symmetry breaking encompasses both "explicit" symmetry breaking and "spontaneous" symmetry breaking. Explicit symmetry breaking is a phenomenon in which a system is not quite, but almost, the same for two configurations related by exact symmetry. Spontaneous symmetry breaking refers to a situation in which the solution of a set of physical equations fails to exhibit a symmetry possessed by the equations themselves.
In general, the term "latent heat" refers to the quantity of heat absorbed or released when a substance changes its physical phase (e.g., solid to liquid) at constant temperature.
The inflationary model, first proposed by Alan Guth in 1980, proposes that quantum fluctuations in the time period 10^(-35) to 10^(-32) seconds after time zero were quickly amplified into large density variations during the "inflationary" 10^(50) expansion of the Universe in that time frame.
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