1. The gravitational coupling constantâi.e., the force of gravity, determines what kinds of stars are possible in the universe. If the gravitational force were slightly stronger, star formation would proceed more efficiently and all Stars would be more massive than our sun by at least 1.4 times. These large stars are important in that they alone manufacture elements heavier than iron, and they alone disperse elements heavier than beryllium to the interstellar medium. Such elements are essential for the formation of planets as well as of living things in any form. However, these Stars burn too rapidly and too unevenly to maintain life-supporting conditions on surrounding planets. Stars as small as our sun are necessary for that.
On the other hand, if the gravitational force were slightly weaker, all stars would have less than 0.8 times the mass of the sun. Though such stars burn long and evenly enough to maintain life-supporting planets, no heavy elements essential for building such planets or life would exist.
2. The strong nuclear force coupling constant holds together the particles in the nucleus of an atom. If the strong nuclear force were slightly weaker, multi-proton nuclei would not hold together. Hydrogen would be the only element in the universe.
If this force were slightly stronger, not only would hydrogen be rare in the universe, but the supply of the various life-essential elements heavier than iron (elements resulting from the fission of very heavy elements) would be insufficient. Either way, life would be impossible.a
3. The weak nuclear force coupling constant affects the behavior of leptons. Leptons form a whole class of elementary particles (e.g. neutrinos, electrons, and photons) that do not participate in strong nuclear reactions. The most familiar weak interaction effect is radioactivity, in particular, the beta decay reaction:
neutron à proton + electron + neutrino
The availability of neutrons as the universe cools through temperatures appropriate for nuclear fusion determines the amount of helium produced during the first few minutes of the big bang. If the weak nuclear force coupling constant were slightly larger, neutrons would decay more readily, and therefore would be less available. Hence, little or no helium would be produced from the big bang. Without the necessary helium, heavy elements sufficient for the constructing of life would not be made by the nuclear furnaces inside stars. On the other hand, if this constant were slightly smaller, the big bang would burn most or all of the hydrogen into helium, with a subsequent over-abundance of heavy elements made by stars, and again life would not be possible.
A second, possibly more delicate, balance occurs for supernovae. It appears that an outward surge of neutrinos determines whether or not a supernova is able to eject its heavy elements into outer space. If the weak nuclear force coupling constant were slightly larger, neutrinos would pass through a supernova's envelop without disturbing it. Hence, the heavy elements produced by the supernova would remain in the core. If the constant were slightly smaller, the neutrinos would not be capable of blowing away the envelop. Again, the heavy elements essential for life would remain trapped forever within the cores of supernovae.
4. The electromagnetic coupling constant binds electrons to protons in atoms. The characteristics of the orbits of electrons about atoms determines to what degree atoms will bond together to form molecules. If the electromagnetic coupling constant were slightly smaller, no electrons would be held in orbits about nuclei. If it were slightly larger, an atom could not "share" an electron orbit with other atoms. Either way, molecules, and hence life, would be impossible.
5. The ratio of electron to proton mass also determines the characteristics of (he orbits of electrons about nuclei. A proton is 1836 times more massive than an electron. if the electron to proton mass ratio were slightly larger or slightly smaller, again, molecules would not form, and life would be impossible.
6. The age of the universe governs what kinds of stars exist. It takes about three billion years for the first stars to form. It takes another ten or twelve billion years for supernovae to spew out enough heavy elements to make possible stars like our sun, stars capable of spawning rocky planets. Yet another few billion years is necessary for solar-type stars to stabilize sufficiently to support advanced life on any of its planets. Hence, if the universe were just a couple of billion years younger, no environment suitable for life would exist. However, if the universe were about ten (or more) billion years older than it is, there would be no solar-type stars in a stable burning phase in the right part of a galaxy. In other words, the window of time during which life is possible in the universe is relatively narrow.
7. The expansion rate of the universe determines what kinds of stars, if any, form in the universe. If the rate of expansion were slightly less, the whole universe would have recollapsed before any solar-type stars could have settled into a stable burning phase. If the universe were expanding slightly more rapidly, no galaxies (and hence no stars) would condense from the general expansion. How critical is this expansion rate? According to Alan Guth,6it must be fine-tuned to an accuracy of one part in 1055. Guth, however, suggests that his inflationary model, given certain values for the four fundamental forces of physics, may provide a natural explanation for the critical expansion rate.
8. The entropy level of the universe affects the condensation of massive systems. The universe contains 100,000,000 photons for every baryon. This makes the universe extremely entropic, i.e. a very efficient radiator and a very poor engine. If the entropy level for the universe were slightly larger, no galactic systems would form (and therefore no stars). If the entropy level were slightly smaller, the galactic systems that formed would effectively trap radiation and prevent any fragmentation of the Systems into stars Either way the universe would be devoid of stars and, thus, of life. (Some models for the universe relate this coincidence to a dependence of entropy upon the gravitational coupling constant.7, 8.)
9. The mass of the universe (actually mass + energy, since E = mc2) determines how much nuclear burning takes place as the universe cools from the hot big bang. If the mass were slightly larger, too much deuterium (hydrogen atoms with nuclei containing both a proton and a neutron) would form during the cooling of the big bang. Deuterium is a powerful catalyst for subsequent nuclear burning in Stars. This extra deuterium would cause stars to burn much too rapidly to sustain life on any possible planet.
On the other hand, if the mass of the universe were slightly smaller, no helium would be generated during the cooling of the big bang. Without helium, stars cannot produce the heavy elements necessary for life. Thus, we see a reason why the universe is as big as it is. If it were any smaller (or larger), not even one planet like the earth would be possible.
10. The uniformity of the universe determines its stellar components. Our universe has a high degree of uniformity. Such uniformity is considered to arise most probably from a brief period of inflationary expansion near the time of the origin of the universe. If the inflation (or some other mechanism) had not smoothed the universe to the degree we see, the universe would have developed into a plethora of black holes separated by virtually empty space.
On the other hand, if the universe were smoothed beyond this degree, stars, star clusters, and galaxies may never have formed at all. Either way, the resultant universe would be incapable of supporting life.
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On the other hand, if the gravitational force were slightly weaker, all stars would have less than 0.8 times the mass of the sun. Though such stars burn long and evenly enough to maintain life-supporting planets, no heavy elements essential for building such planets or life would exist.
2. The strong nuclear force coupling constant holds together the particles in the nucleus of an atom. If the strong nuclear force were slightly weaker, multi-proton nuclei would not hold together. Hydrogen would be the only element in the universe.
If this force were slightly stronger, not only would hydrogen be rare in the universe, but the supply of the various life-essential elements heavier than iron (elements resulting from the fission of very heavy elements) would be insufficient. Either way, life would be impossible.a
3. The weak nuclear force coupling constant affects the behavior of leptons. Leptons form a whole class of elementary particles (e.g. neutrinos, electrons, and photons) that do not participate in strong nuclear reactions. The most familiar weak interaction effect is radioactivity, in particular, the beta decay reaction:
neutron à proton + electron + neutrino
The availability of neutrons as the universe cools through temperatures appropriate for nuclear fusion determines the amount of helium produced during the first few minutes of the big bang. If the weak nuclear force coupling constant were slightly larger, neutrons would decay more readily, and therefore would be less available. Hence, little or no helium would be produced from the big bang. Without the necessary helium, heavy elements sufficient for the constructing of life would not be made by the nuclear furnaces inside stars. On the other hand, if this constant were slightly smaller, the big bang would burn most or all of the hydrogen into helium, with a subsequent over-abundance of heavy elements made by stars, and again life would not be possible.
A second, possibly more delicate, balance occurs for supernovae. It appears that an outward surge of neutrinos determines whether or not a supernova is able to eject its heavy elements into outer space. If the weak nuclear force coupling constant were slightly larger, neutrinos would pass through a supernova's envelop without disturbing it. Hence, the heavy elements produced by the supernova would remain in the core. If the constant were slightly smaller, the neutrinos would not be capable of blowing away the envelop. Again, the heavy elements essential for life would remain trapped forever within the cores of supernovae.
4. The electromagnetic coupling constant binds electrons to protons in atoms. The characteristics of the orbits of electrons about atoms determines to what degree atoms will bond together to form molecules. If the electromagnetic coupling constant were slightly smaller, no electrons would be held in orbits about nuclei. If it were slightly larger, an atom could not "share" an electron orbit with other atoms. Either way, molecules, and hence life, would be impossible.
5. The ratio of electron to proton mass also determines the characteristics of (he orbits of electrons about nuclei. A proton is 1836 times more massive than an electron. if the electron to proton mass ratio were slightly larger or slightly smaller, again, molecules would not form, and life would be impossible.
6. The age of the universe governs what kinds of stars exist. It takes about three billion years for the first stars to form. It takes another ten or twelve billion years for supernovae to spew out enough heavy elements to make possible stars like our sun, stars capable of spawning rocky planets. Yet another few billion years is necessary for solar-type stars to stabilize sufficiently to support advanced life on any of its planets. Hence, if the universe were just a couple of billion years younger, no environment suitable for life would exist. However, if the universe were about ten (or more) billion years older than it is, there would be no solar-type stars in a stable burning phase in the right part of a galaxy. In other words, the window of time during which life is possible in the universe is relatively narrow.
7. The expansion rate of the universe determines what kinds of stars, if any, form in the universe. If the rate of expansion were slightly less, the whole universe would have recollapsed before any solar-type stars could have settled into a stable burning phase. If the universe were expanding slightly more rapidly, no galaxies (and hence no stars) would condense from the general expansion. How critical is this expansion rate? According to Alan Guth,6it must be fine-tuned to an accuracy of one part in 1055. Guth, however, suggests that his inflationary model, given certain values for the four fundamental forces of physics, may provide a natural explanation for the critical expansion rate.
8. The entropy level of the universe affects the condensation of massive systems. The universe contains 100,000,000 photons for every baryon. This makes the universe extremely entropic, i.e. a very efficient radiator and a very poor engine. If the entropy level for the universe were slightly larger, no galactic systems would form (and therefore no stars). If the entropy level were slightly smaller, the galactic systems that formed would effectively trap radiation and prevent any fragmentation of the Systems into stars Either way the universe would be devoid of stars and, thus, of life. (Some models for the universe relate this coincidence to a dependence of entropy upon the gravitational coupling constant.7, 8.)
9. The mass of the universe (actually mass + energy, since E = mc2) determines how much nuclear burning takes place as the universe cools from the hot big bang. If the mass were slightly larger, too much deuterium (hydrogen atoms with nuclei containing both a proton and a neutron) would form during the cooling of the big bang. Deuterium is a powerful catalyst for subsequent nuclear burning in Stars. This extra deuterium would cause stars to burn much too rapidly to sustain life on any possible planet.
On the other hand, if the mass of the universe were slightly smaller, no helium would be generated during the cooling of the big bang. Without helium, stars cannot produce the heavy elements necessary for life. Thus, we see a reason why the universe is as big as it is. If it were any smaller (or larger), not even one planet like the earth would be possible.
10. The uniformity of the universe determines its stellar components. Our universe has a high degree of uniformity. Such uniformity is considered to arise most probably from a brief period of inflationary expansion near the time of the origin of the universe. If the inflation (or some other mechanism) had not smoothed the universe to the degree we see, the universe would have developed into a plethora of black holes separated by virtually empty space.
On the other hand, if the universe were smoothed beyond this degree, stars, star clusters, and galaxies may never have formed at all. Either way, the resultant universe would be incapable of supporting life.
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