32.7

Cosmology

Cosmology is the study of the structure and evolution of the universe. In this study, both the very large and the very small aspects of the universe are important. Astronomers, for example, study stars located at enormous distances from the earth, up to billions of light-years away. In contrast, particle physicists focus their efforts on the very small elementary particles (10−18 m or smaller) that comprise matter. The synergy between the work of astronomers and that of particle physicists has led to significant advances in our understanding of the universe. Central to that understanding is the belief that the universe is expanding, and we begin by discussing the evidence that justifies this belief.

| The Expanding Universe and the Big Bang

The idea that the universe is expanding originated with the astronomer Edwin P. Hubble (1889–1953). He found that light reaching the earth from distant galaxies is Doppler-shifted toward greater wavelengths—that is, toward the red end of the visible spectrum. As Section 24.5 discusses, this type of Doppler shift results when the observer and the source of the light are moving away from each other. The speed at which a galaxy is receding from the earth can be determined from the measured Doppler shift in wavelength. Hubble found that a galaxy located at a distance d from the earth recedes from the earth at a speed v given by

| (32.5)

where H is a constant known as the Hubble parameter. In other words, the recession speed is proportional to the distance d, so that more distant galaxies are moving away from the earth at greater speeds. Equation 32.5 is referred to as Hubble’s law.

The physics of an expanding universe.

Hubble’s picture of an expanding universe does not mean that the earth is at the center of the expansion. In fact, there is no literal center. Imagine a loaf of raisin bread expanding as it bakes. Each raisin moves away from every other raisin, without any single one acting as a center for the expansion. Galaxies in the universe behave in a similar fashion. Observers in other galaxies would see distant galaxies moving away, just as we do.

The physics of “dark energy.”

Not only is the universe expanding, it is doing so at an accelerated rate, according to recent astronomical measurements of the brightness of supernovas, or exploding stars. To account for the accelerated rate, astronomers have postulated that a kind of “dark energy” pervades the universe. The normal gravitational force between galaxies slows the rate at which they are moving away from each other. The dark energy, however, gives rise to a force that counteracts gravity and pushes galaxies apart. As yet, little is known about the properties of dark energy.
Experimental measurements by astronomers indicate that an approximate value for the Hubble parameter is

The value for the Hubble parameter is believed to be accurate within 10%. Scientists are very interested in obtaining an accurate value for H because it can be related to an age for the universe, as the next example illustrates.

| Example 8 | | | An Age for the Universe

Determine an estimate of the age of the universe using Hubble’s law. Reasoning Consider a galaxy currently located at a distance d from the earth. According to Hubble’s law, this galaxy is moving away from us at a speed of v = Hd. At an earlier time, therefore, this galaxy must have been closer. We can imagine, in fact, that in the remote past the separation distance was relatively small and that the universe originated at such a time. To estimate the age of the universe, we calculate the time it has taken the galaxy to recede to its present position. For this purpose, time is simply distance divided by speed, or t = d/v.
Solution Using Hubble’s law and the fact that a distance of 1 light-year is 9.46 × 1015 m, we estimate the age of the universe to be

The idea presented in Example 8, that our galaxy and other galaxies in the universe were very close together at some earlier instant in time, lies at the heart of the Big Bang theory. This theory postulates that the universe had a definite beginning in a cataclysmic event, sometimes called the primeval fireball. Dramatic evidence supporting the theory was discovered in 1965 by Arno A. Penzias (1933– ) and Robert W. Wilson (1936– ). Using a radio telescope, they discovered that the earth is being bathed in weak electromagnetic waves in the microwave region of the spectrum (wavelength = 7.35 cm, see Figure 24-10). They observed that the intensity of these waves is the same, no matter where in the sky they pointed their telescope, and concluded that the waves originated outside of our galaxy. This microwave background radiation, as it is called, represents radiation left over from the Big Bang and is a kind of cosmic afterglow. Subsequent measurements have confirmed the research of Penzias and Wilson and have shown that the microwave radiation is consistent with a perfect blackbody (see Sections 13.3 and 29.2) radiating at a temperature of 2.7 K, in agreement with theoretical analysis of the Big Bang. In 1978, Penzias and Wilson received a Nobel Prize for their discovery.

| The Standard Model for the Evolution of the Universe

Based on the recent experimental and theoretical research in particle physics, scientists have proposed an evolutionary sequence of events following the Big Bang. This sequence is known as the standard cosmological model and is illustrated in Figure 32-16.

Figure 32-16 According to the standard cosmological model, the universe has evolved as illustrated here. In this model, the universe is presumed to have originated with a cataclysmic event known as the Big Bang. The times shown are those following this event.

Immediately following the Big Bang, the temperature of the universe was incredibly high, about 1032 K. During this initial period, the three fundamental forces (the gravitational force, the strong nuclear force, and the electroweak force) all behaved as a single unified force. Very quickly, in about 10−43 s, the gravitational force took on a separate identity all its own, as Figure 32-16 indicates. Meanwhile, the strong nuclear force and the electroweak force continued to act as a single force, which is sometimes referred to as the GUT force. GUT stands for the grand unified theory that presumably would explain such a force. Slightly later, at about 10−35 s after the Big Bang, the GUT force separated into the strong nuclear force and the electroweak force, the universe expanding and cooling somewhat to a temperature of roughly 1028 K (see Figure 32-16). From this point on, the strong nuclear force behaved as we know it today, while the electroweak force maintained its identity. In this scenario, note that the weak nuclear force and the electromagnetic force have not yet manifested themselves as separate entities. The disappearance of the electroweak force and the appearance of the weak nuclear force and the electromagnetic force eventually occurred at approximately 10−10 s after the Big Bang, when the temperature of the expanding universe had cooled to about 1015 K.
From the Big Bang up until the strong nuclear force separated from the GUT force at a time of 10−35 s, all particles of matter were similar, and there was no distinction between quarks and leptons. After this time, quarks and leptons became distinguishable. Eventually the quarks and antiquarks formed hadrons, such as protons and neutrons and their antiparticles. By a time of 10−4 s after the Big Bang, however, the temperature had cooled to approximately 1012 K, and the hadrons had mostly disappeared. Protons and neutrons survived only as a very small fraction of the total number of particles, the majority of which were leptons such as electrons, positrons, and neutrinos. Like most of the hadrons before them, most of the electrons and positrons eventually disappeared. However, they did leave behind a relatively small number of electrons to join the small number of protons and neutrons at a time of about 3 min following the Big Bang. At this time the temperature of the expanding universe had decreased to about 109 K, and small nuclei such as that of helium began forming. Later, when the universe was about 500 000 years old and the temperature had dropped to near 3000 K, hydrogen and helium atoms began forming. As the temperature decreased further, stars and galaxies formed, and today we find a temperature of 2.7 K characterizing the cosmic background radiation of the universe.

Test your understanding of the material in Sections 32.1, 32.2, 32.3, 32.4, 32.5, 32.6 and 32.7 :

· | Biological Effects of Ionizing Radiation
· | Induced Nuclear Reactions
· | Nuclear Fission
· | Nuclear Reactors
· | Nuclear Fusion
· | Elementary Particles
· | Cosmology

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.