According to modern cosmologists, the expanding universe in its very beginnings had three possible fates. (1) The force of gravity could have overwhelmed the force of expansion, and the universe could have rapidly collapsed back on itself, before any stars and galaxies could have formed. (2) The force of expansion could have overwhelmed the force of gravity, so that the universe would have expanded too rapidly for stars and galaxies to form. (3) The forces of gravity and expansion could have been adjusted very carefully so that the universe expanded at just the right speed for stars and galaxies to form and persist over billions of years.
The fate of the universe therefore depends upon a critical average density of matter. According to cosmologists, the critical density is 5 atoms per cubic meter. If the density is more than 5 atoms per cubic meter, gravity will be strong enough to cause the universe to collapse. If the density is much less than 5 atoms per cubic meter, the universe will expand too rapidly for stars and galaxies to form.
The cosmic number omega is the ratio between the critical density and the actual density (Rees 2000, pp. 72–90). If the critical density and the actual density are equal, then the ratio is 1, and hence Ω (omega) = 1. This allows a slowly expanding universe in which stars and galaxies can form, as is the case with our universe. But in our universe, the actual density of visible matter is far less than the critical density. If all visible matter, in the form of stars, galaxies and gas clouds, is taken into account, the actual density is only .04 of the critical density. But observations of the movement of the visible matter have convinced scientists that there must exist another form of matter in the universe, called dark matter. For example, spiral galaxies are shaped like rotating pinwheels, with two or more curving “arms” of stars streaming from a bright central core. When astronomers look at spiral galaxies, they see that they do not contain enough ordinary visible matter to keep the arms curving as closely as they do toward the centers of such galaxies. According to the current laws of gravity, the arms should be less curved. For the galaxies to maintain their observed shapes, they should have ten times more matter than they visibly have. This means there is some “missing matter.” What form does it take? Some astrophysicists suggest the dark matter may be made of neutrinos, strange particles generated during the Big Bang with very small mass, or myriads of black holes of extremely great mass. “It’s embarrassing,” said Rees (2000, p. 82), “that more than ninety per cent of the universe remains unaccounted for—even worse when we realize that the dark matter could be made up of entities with masses ranged from 10-33 grams (neutrinos) up to 1039 grams (heavy black holes), an uncertainty of more than seventy powers of ten.”
When the dark matter is added to the visible matter, the actual density of matter in the universe becomes about .30 of the critical density. For this to be the situation now, after billions of years of expansion, the ratio of the actual density of matter in the universe to the critical density had to be extremely close to unity (i.e., one to one). Rees (2000, p. 88) stated,“Our universe was initiated with a very finely-tuned impetus, almost exactly enough to balance the decelerating tendency of gravity. It’s like sitting at the bottom of a well and throwing a stone up so that it just comes to a halt exactly at the top—the required precision is astonishing: at one second after the Big Bang, Ω cannot have differed from unity by more than one part in a million billion (one in 1015) in order that the universe should now, after ten billion years, be still expanding and with a value of Ω that has certainly not departed wildly from unity.”
λ (lambda): levity in addition to Gravity?
If gravity were the only force operating in connection with the expansion of the universe, then astronomers should detect that the rate of expansion is decreasing. Gravity should be slowing down the rate at which all the material objects in the universe are moving away from each other. In short, we should observe deceleration of the expansion. The force of gravity depends on the total density of matter. The more density, the more gravity. The more gravity, the more deceleration. Depending on the exact density of matter in the universe, the rate of deceleration could be faster or slower. But there should be some deceleration, as the force of gravity counteracts the expansion. Instead, scientists have noted an apparent acceleration in the rate of expansion. This was somewhat unexpected, as it indicates that in addition to gravity there may be another fundamental natural force that is repulsive, rather than attractive. In other words, there may be antigravity in addition to gravity.
The antigravity force was discovered by scientists who were hoping to find the total amount of dark matter in the universe (Rees 2000, pp. 91–95). The visible matter in the universe contributes only .04 of the critical density. The critical density is the exact amount of matter necessary for a Big Bang expanding universe to exist for long periods of time with relatively stable stars and galaxies. There must be enough matter to slow therate of expansion so that all the matter in the universe does not quickly disperse into a featureless gas. But there must not be so much matter as to thoroughly overcome the expansion, causing the universe to quickly recollapse into a black hole. Because the visible matter in the universe is distributed in ways not possible according to the laws of gravity, scientists have inferred the existence of clumps of dark matter, which although invisible possess gravitational force. Taking into account the gravitational force of these clumps of invisible dark matter allows cosmologists to explain the distribution of visible matter. But when the clumped dark matter is added to the visible matter, the total amount of matter is still only.30 of the critical density. Some scientists have proposed that the present state of our universe would most easily be explained if the actual density of matter in the universe very closely approached the critical density, so that their ratio (Ω) was one to one (Ω = 1). But that would require that there should be some more dark matter in the universe. Therefore, some scientists have proposed that there might be large amounts of extra dark matter evenly distributed throughout the universe. Unlike the clumped dark matter, this evenly distributed dark matter would not exert noticeable gravitational force on individual galaxies. And it would therefore not show its influence in the form of anomalies in the distribution of matter in and among galaxies. However, the evenly distributed dark matter might be slowing down the overall expansion of the universe.
To test their ideas, scientists measured the red shifts of a particular type of supernova: “A distinctive type of supernovae, technically known as a ‘Type 1a’, signals a sudden nuclear explosion in the center of a dying star, when its burnt-out core gets above a particular threshold of mass and becomes unstable,” stated Rees (2000, p. 93). “It is, in effect, a nuclear bomb with a standard yield. . . . What is important is that Type 1a supernovae can be regarded as ‘standard candles’, bright enough to be detected at great distances. From how bright they appear, it should be possible to infer reliable distances, and thereby (by measuring the red shift as well) to relate the expansion speed and distance at a past epoch. Cosmologists hoped that such measurements would distinguish between a small slowdown-rate (expected if the dark matter has all been accounted for) or the larger rate expected if—as many theorists suspected—there was enough extra dark matter to make up the full ‘critical density.’” Two groups of researchers were surprised to find that their measurements of these supernova red shifts showed no deceleration effect at all. Instead, their measurements showed the rate of the expansion of the universe was actually increasing. This meant two things. First, there was not any significant amount of extra dark matter. Second, in order to explain the increase in the rate of the universe’s expansion, scientists had to propose a kind of antigravitational force.