Two competing theories quickly emerged to explain the expanding universe phenomenon: the Steady State theory and the Big Bang theory. The first postulated that the universe is self-sustaining, and claimed that what we see today is what will be seen in billions of years time; galaxies move apart, but the total picture remains the same. According to the Steady State theory, the stars that are moving outwards are being replaced by new ones that are perpetually being formed out of cosmic dust. This dust is being created from atoms, the theory went, that themselves are being continuously created from cosmic energy. (We cannot observe this creation, because the theory predicts that it need only occur at a rate of about one atom per 500 cubic meters, every 1000 years, which is far too slow to be detected.)
In contrast, the Big Bang explanation stated that the universe began with an explosion, and that everything within has been moving apart since that moment, with the intervening space expanding and cooling as it proceeds. In this theory, everything was created during, or shortly after, “the bang,” rather than slowly and continuously, atom by atom.
Some of the discussions reported in the press when these theories were announced, shortly after the second world war provided a stimulating intellectual alternative to the dreary task of becoming formally educated. At that time, I favoured the Steady State theory because I couldn’t understand how everything could come from nothing in one big burst. Creation seemed slightly more feasible if it happened very slowly, and I thought that perhaps the atoms being created came from energy released as the matter in receding galaxies became stretched further and further apart. The Steady State theory also seemed more appealing philosophically, because if the universe continued forever, one would never have a beginning to explain.
However, three observations eventually dismissed the Steady State theory. First, if the universe did start with a bang, then there should still be some trace evidence of this explosion to be found. And there is; it has been heard since radio astronomy first began. Everywhere one searches, in addition to the electromagnetic information received from stars and galaxies, there is a constant hiss of background radiation. This hiss comes from energy that remains (unconverted into matter) from the originating Big Bang.
Second, the universe does not remain the same over time, as the Steady State theory requires; it changes as it becomes older. This was discovered when quasar locations were established upon a four-dimensional map of the universe. This showed them to exist only at great distances from our system, signifying that they were present billions of years ago but no longer exist today. Astronomers also find (by searching far and near in distance, and so effectively far and near in time) that galaxies tend to change their shape as they age.
Third, scientists have calculated the variety and abundance of chemical elements that should have been formed following the Big Bang, and their calculations predict exactly the ratios that are found to exist in space. Furthermore, the particular mixture predicted by the Big Bang theory (and corroborated by observation) is quite different from the mixture that the Steady State theory predicts.
Various other kinds of evidence support the Big Bang theory, and it rules the roost today. The creating Bang is calculated to have occurred about 13.7 billion years ago.
While no cosmologist doubts that our universe is expanding, there has been much debate about whether it will continue to expand forever. Should this be the case, our universe will end up as a diffuse, dark, frigid, dead junkyard, with no energy differences left to power change. If, however, the attractive gravitational forces are strong enough, the universe’s current expansion will be slowed down, stopped, then reversed. This reversal would cause everything to be pulled tighter and tighter together, and it would all eventually be gathered into one gigantic black hole, presumably to continue shrinking until it reverted back to whence it came.
This uncertainty about the universe’s future may have been resolved by measurements taken over the last five or so years. Measurements of the brightness and red shift of distant supernovae (see next section) yield the recession speed of the universe when it was young. Comparisons of that speed with the recession speed shown by nearby galaxies reveals that the universe’s rate of expansion is increasing, not slowing down. (It is currently thought that the slightly repulsive gravitational force of the “vacuum energy” of empty space may be causing this acceleration.)
- See Fred Hoyle, The Nature of the Universe (Oxford: Basil Blackwell, 1950).
- Gases cool when they expand and heat up when compressed (as is readily noted when using a hand pump to inflate a bicycle tire). Refrigerators exploit this property, using a pump to compress a gas outside the refrigerator (usually in tubes on the back, where excess heat dissipates into the environment) and allowing the gas to expand, and therefore cool, inside (usually in tubes surrounding the freezer box).
- Since light from the most distant galaxies we can observe takes over thirteen billion years to reach us, the Big Bang must have occurred before then.
- Matter and energy are different aspects of the same thing (as E = mc2 informs us), and one can be turned into the other. (The symbols E, m, and c, stand for energy, mass, and the speed of light, respectively. Since c is so very large, and is multiplied by itself in this equation, a tiny piece of matter is equivalent to a very large amount of energy. Thus, it takes a very large amount of energy to produce a speck of matter.)
- This radiation has since been accurately measured (by instruments on the COBE, or Cosmic Background Explorer, spacecraft) to be energy at three degrees above absolute zero. Calculations of the temperature changes which residual radiation would undergo over time following the Big Bang predict precisely this temperature.
Another COBE experiment mapped the universe’s very early energy distribution, and found small ripples that could have been the variations that led to the formation of galaxies and galactic clusters. (If the originating Big Bang radiation was perfectly uniformly distributed, the specks of matter that formed from it [via E = mc2, or rather, m = E/c2] would also have been perfectly uniformly distributed, and gravitational pulls on each speck would have balanced on every side. In such a case, there would have been no gravitationally caused condensation, and therefore no stars, galaxies, planets, life, or us.)
Recent measurements of the polarization of cosmic background radiation provides additional evidence of the veracity of the Big Bang theory.
- Quasars are enormously bright objects located toward the edge of our universe, and look similar to stars (hence their name—“quasi-stella”). Quasars existed only within the first few billion years or so of our universe’s formation. They depended upon the presence of supermassive black holes (gigantic agglomerations of matter about one hundred million times more massive than our sun). Each quasar emitted massive amounts of energy (the light we are seeing now, billions of years later—typically about three times more radiation than is currently emitted by the sum total of all of the stars in our galaxy). Electromagnetic radiations from quasars were produced by electrically charged matter (i.e., gases, stars, star clusters and even galaxies) spinning around the black hole before being swallowed. (Once inside a black hole’s boundary—the “event horizon”—nothing can escape, not even light; hence the name, black hole.)
- One intriguing argument against the idea that the universe could have existed forever, as required by the Steady State theory, is that we have not been overrun by visiting aliens, either directly or by way of von Neumann probes. (These are devices that technologically competent life forms will be able to construct that explore planetary systems and use what they find to replicate themselves many-fold, before moving on again.) Of course, this argument fails if we are the only intelligent beings in the universe.
For more about self-replicating machines, see John von Neumann (A. W. Burks, ed.), Theory of Self-Reproducing Automata (Urbana: University of Illinois Press, 1966).
- See Donald Goldsmith, The Runaway Universe: The Race to Find the Future of the Cosmos (Perseus Publishing, 2000).
- Also known as “dark energy”—named (as is dark matter) because it cannot be seen.
Space is not a void as most assume; it is filled with a form of energy called dark or vacuum energy. This energy exerts a very weak negative gravitational force that builds in magnitude as the intervening space increases. Its weak nature explains why its repulsive force needs trans-universe distances to have any affect. Vacuum energy may owe its existence to a dynamical quantum field (similar to an electro-magnetic field) called “quintessence,” or it may be an inert property of empty space (accounted for by the cosmological constant), a possibility first proposed by Einstein.
Dark energy accounts for about 65% of the universe’s mass. Normal matter, of which we and everything we see are made, amounts to only 4%. Dark matter (see earlier) accounts for the rest.