Certain questions about our existence on Earth are so fundamental that they have been incorporated into religious mythologies. These questions not only concern the origin of the Earth and the evolution of life but also extend to the origin of the universe and to the nature of space and time. Did the universe have a beginning and will it ever end? What existed before the universe formed? Does the universe have limits and what exists beyond those limits?
The universe started like a bubble in a stream. At first it was not there, and suddenly it formed and expanded rapidly as though it were exploding. Science has its share of practical jokers that immediately referred to the start of the expansion of the universe as the Big Bang. From the very beginning the universe had all of the mass and energy it contains today. As a result, its pressure and temperature, say 10^-32 sec after the Big Bang, were so high that matter existed in its most fundamental form as “quark soup.” AS the universe expanded and cooled, the quarks combined to form more familiar nuclear particles that ultimately became organized into nuclei of hydrogen and helium.
Formation of atomic nuclei began about 13.8 sec after the Big Bang when the temperature of the universe had decreased to approximately 3 billion degrees Kelvin. This process continued for about 30 min, but did not go beyond helium because the nuclear reactions could not bridge a gap in the stabilities of the nuclei of lithium, beryllium, and boron. At that time the universe was an intensely hot and rapidly expanding fireball.
Some 700,000 years later, when the temperature had decreased to about 3 thousand degrees Kelvin, electrons became attached to the nuclei of hydrogen and helium. Matter and radiation were thereby separated from each other, and the universe became transparent to light. Subsequently, matter began to be organized into stars, galaxies, and galactic clusters as the universe continued to expand to the present time.
But how do we know all this? The answer is that the expansion of the universe can be seen in the “red shift” of spectral lines of light emitted by distant galaxies, and it can be “heard” as the cosmic microwave radiation, which is the remnant of the fireball, that still fills the universe. In addition, the properties of the universe immediately after the Big Bang were similar to those of atomic nuclei. Therefore, a very fruitful collaboration has developed among nuclear physicists and cosmologists that has enabled them to reconstruct the history of the universe back to about 10^-32 sec after the Big Bang. These studies have shown that the forces we recognize at low temperature are, at least in part unified at extremely high temperatures and densities. There is hope that a Grand Unified Theory (GUT) will eventually emerge that may permit us to approach even closer to understanding the start of the universe.
What about the future? Will the universe continue to expand forever? The answer is that the future of the universe can be predicted only if we know the total amount of matter it contains. The matter that is detectable at the present time is not sufficient to permit gravity to overcome the expansion. If expansion continues, the universe will become colder and emptier with no prospect of an end. However, a large fraction of the mass of the universe is hidden from view in the form of gas and dust in interstellar and intergalactic space, and in the bodies of stars that no longer emit light. In addition, we still cannot rule out the possibility that neutrinos have mass even when they are at rest. IF the mass of the universe is sufficient to slow the expansion and ultimately to reverse it, then the universe will eventually contract until it disappears again in the stream of time.
Since the universe had a beginning and is still expanding, it cannot be infinite in size. However, the edge of the universe cannot be seen with telescopes because it takes too long for the light to reach us. As the universe expands, space expands within it. In other words, it seems to be impossible to exceed the physical limits of the universe. We are trapped in our expanding bubble. If other universes exist, we cannot communicate with them.
Now that we have seen the big picture, let us review certain events in the history of the standard model of cosmology to show that progress in Science is sometimes accidental.
In 1929 the American astronomer Edwin Hubble reported that eighteen galaxies in the Virgo cluster are receding from Earth at different rates that increase with their distances from Earth. He calculated the recessional velocities of these galaxies by means of the Doppler effect from observed increases of the wavelengths of characteristic spectral lines of light they emit. This red shift is related to the recessional velocities by an equation derived in 1842 by Johann Christian Doppler in Prague:
wavelength of moving source/wavelength of a stationary source = 1 + recessional velocity/speed of light.
Hubble’s estimates of the distances to the galaxies were based on the properties of the Cepheid Variables studied previously by H.S. Leavitt and H. Shapley at Harvard University. The Ceipheid Variables are bright stars in the constellation Cepheus whose period of variation depends on their absolute luminosity, which is the total radiant energy emitted by an astronomical body. Hubble found such variable stars in the galaxies he was studying and determined their absolute luminosities from their periods. The intensity of light emitted by a star decreases as the square of the distance increases. Therefore, the distance to a star can be determined from a comparison of its absolute and its apparent luminosity, where the latter is defined as the radiant power received by the telescope per square centimeter. IN this way, Hubble determined the recessional velocities and distances of the galaxies in the Virgo cluster and expressed their relationship as:
v = Hd
where v is the recessional velocity in km/sec, d is the distance in 10 million light years, and H is the Hubble constant.
The Hubble constant can be used to place a limit on the age of the universe. If two objects are moving apart with velocity v, the time (t) required for them to become separated by a distance (d) is:
t = d/v = 1/H
The initial results indicated that the Hubble constant had a value of 170 km/sec/10 million light years, which corresponds to an expansion time of less than 2 billion years. This result was very awkward because age determinations based on radioactivity had established that the Earth is older than this date. Eventually Walter Baade discovered an error in the calibration of the Cepheid Variables, and the value of the Hubble constant was revised. The presently accepted value is 15 km/sec/ 10 million years, which indicates and expansion time for the universe of less than 20 billion years. This date is compatible with independent estimates of its age based on consideration of nucleosynthesis and the evolution of stars. By combining all three methods, the universe age was refined to approximately 15 billion years.
The Big Bang theory of cosmology was not accepted for many years for a variety of reasons. The turning point came in 1964 when Arno Penzias and Robert Wilson discovered a microwave background radiation that corresponds to a blackbody temperature of about 3 degrees Kelvin. The discovery of this radiation was accidental, even though its existence had been predicted twenty years earlier by George Gamow and his colleagues Ralph Alpher, and Robert Herman. Because they were unaware of Gamow’s work, Penzias and Wilson were skeptical about the phenomenon they had discovered and took great care to eliminate all extraneous sources of the background radiation. For example, they noticed that two pigeons had been nesting in the throat of the antenna they were using at Holmdel, New Jersey, however, the intensity of the background radiation remained constant and independent of time in the course of a year.
Word of this phenomenon reached a group of astrophysicists at nearby Princeton University who were working on models of the early universe under the guidance of Robert Dicke. Eventually, Penzias called Dicke, and it was agreed that they would publish two companion letters in the Astrophysical Journal. Penzias and Wilson announced the discovery, and Dicke and his colleagues explained the cosmological significance of the microwave background radiation. In 1978 Penzias and Wilson shared the Nobel Prize in physics for their discovery.
The radiation discovered is a remnant of the radiation that filled the universe for about 700,000 years when its temperature was greater than about 3 thousand Kelvin. During this early period, matter consisted of a mixture of nuclear particles and photons in thermal equilibrium with each other. Under these conditions the energy of radiation at a specific wavelength is inversely proportional to the absolute temperature. According to an equation derived by Max Planck at the start of the 20th century, the energy of blackbody radiation at a particular temperature increases rapidly with increasing wavelength to a maximum and then decreases at longer wavelengths. Radiation in thermal equilibrium with matter has the same properties as radiation inside a black box with opaque walls. Therefore, the energy distribution of radiation in the early universe is related to the wavelength and to the absolute temperature by Planck’s equation. The wavelength near which most of the energy of blackbody radiation is concentrated is approximately equal to 0.29/T, where T is the temperature in Kelvin.
The original measurement of Penzias and Wilson was at a wavelength of 7.35 cm, which is much greater than the typical wavelength of radiation at 3 Kelvin. Since 1965, many additional measurements at different wavelengths have confirmed that the cosmic background radiation does fit Planck’s formula for blackbody radiation. The characteristic temperature of this radiation is about 3 Kelvin, indicating that the typical wavelength of photons has increased by a factor of about 1000 because of the expansion of the universe since its temperature was 3000 K