Fig. 2-0: Galaxy Cluster Abell 2218, about 2.1 billion light years from Earth, which is composed of thousands of individual galaxies. The large mass of the cluster has also been used by astronomers as a gravitational lens that magnifies and makes visible even more distant galaxies, which are distorted into the long thin arcs that are visible in the image. (NASA, ESA, Richard Ellis (Caltech), and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France). Acknowledgment: NASA, A. Fruchter, and the ERO Team (STScI and ST-ECF))
Earth is a minor member of a system of planets in orbit around a star we call the sun. The sun is one of about 400 billion stars that make up our Milky Way. The light from this myriad of stars allows observers in neighboring galaxies to define our galaxy’s spiral form. The galaxy is the basic unit into which universe matter is subdivided.
Like its billions of fellow galaxies, ours is speeding out on the wings of a great explosion that gave birth to the universe. That these major pieces of the universe are flying away from each other is revealed by a shift toward the red of the “bar code” of spectral lines of the elements in the light reaching us from distant galaxies. The close correlation between the magnitude of this shift and the distance of the galaxy from Earth tells us that about 13.7 billion years ago all the galaxies must have been in one place at the same time. The catastrophic beginning of the universe is still heralded by a dull glow of background light. This glow is the remnant of the great flash that occurred when the debris from the explosion cooled to the point where the electrons could be captured into orbits around the hydrogen and helium nuclei. The Big Bang was the impulse from which everything else in the universe has been derived. Contained in the galaxies lying within the range of our telescopes are about 400 billion billion stars. A sizable number of these stars are thought to have planetary systems.
Careful examination of the data from galaxies shows that a vast amount of matter is not accounted for in what we can see. This “dark matter” is not visible to us and is poorly understood, but it makes up almost six times as much total mass as the atomic matter that makes up stars, planets and life. Careful measurement of retreating galaxies shows that they are speeding up with time and that the universe will not ultimately contract into a “big crunch.” To explain this phenomenon requires “dark energy” that has a repulsive force that counters the effect of gravity. Some 76% of the universe is believed by physicists to be made of dark energy, relegating the material that we know and understand to only 4% of what was created in the Big Bang. While the beginning event is well established, great mysteries concerning the contents and operation of the universe remain to be understood.
Where the universe comes from and how it got here are the essential first questions as we delve to the starting point of Earth’s history, deep in time even long before the formation of the Milky Way. Was there a beginning? When and how did it happen? In this chapter we will see that there was indeed a spectacular beginning to the universe—the origin of everything we can observe—and we can even determine when it happened. From there, everything else unfolds.
The universe as we know it began about 13.7 billion years ago with an explosion that astronomers refer to as the Big Bang. All the matter in the universe still rides forth on the wings of this blast. Speculations as to the nature of this cosmic event constitute the forefront of a field called cosmology. What went on before this explosion is a matter that is not currently subject to scientific investigation, because every observable phenomenon in the universe (that we know of) dates from the Big Bang. No physical record of prior events remains.
To state that we know the age of our universe and its mode of origin is rather bold. Is this fanciful thinking or is there evidence? While it is remarkable that we have detailed knowledge of the beginning of the universe, the observations astronomers have made provide compelling support for the Big Bang theory of universe origin. On a scale of reliability that goes from 0 (idle speculation) to 10 (proven fact), this theory gets a 9.9!
Fig. 2-1: Schematic illustration of one aspect of Olber’s paradox. If the universe is infinite in space and time, then looking out into space from Earth, every line of sight ultimately hits a star or distant galaxy. If the line did not intersect light within the box, it could simply be extended further until there was an intersection. Therefore, the night sky should be filled with light, but it isn’t.
Before presenting this evidence, let us consider a seeming paradox that confronted astronomers before the concept of an expanding universe was proposed. This paradox was articulated by Heinrich Olbers in 1826. To state it simply, no one was able to explain the fact that the nighttime sky is dark. The black background between the stars seemed to demand either that the universe has a finite extent or that the light from the most distant stars is being intercepted by dark matter in the voids of space. To understand this, one has only to envision a universe of infinite extent made up of luminous objects separated by empty voids. In such a universe, no matter where we looked we would see the light from some distant star (Fig. 2-1). The sky would be blindingly bright! The obvious alternative is that the universe is finite. In a finite universe, we could look between the stars into the black void beyond. Another possibility is, of course, that there are clouds of nonluminous matter floating in the voids between stars and that these clouds block the light from the more distant stars from our view.
The first alternative appeared unacceptable, because in a universe of finite extent there would be nothing to hold the stars apart. The mutual star-to-star gravitational attraction would lead to an unbalanced pull toward the “middle” of the universe. It would be as if we secured a series of balls on a great three-dimensional latticework and then connected each ball to each of the other balls with a stretched rubber band. While the balls near the center of the latticework would be pulled more or less equally from all directions, those near the edge of the lattice would be pulled toward the inside. If by magic we suddenly removed the latticework, leaving only the balls and stretched rubber bands, there would be a massive implosion as the balls streaked toward the lattice’s center. Only if the lattice were infinite in extent would nothing happen. In this case the pull on every ball would be exactly balanced. The universe has no latticework to hold the stars apart, yet there they are. Hence, the finite universe explanation for the dark sky must be rejected as inadequate.
The second explanation—that the light from very distant stars is intercepted by dark clouds of dust and gas along its path to the Earth—is also unacceptable. In this situation the light from stars at intermediate distances would also be affected. We should see a glow of scattered light similar to that in the night sky over a great city or from headlights approaching through fog. No such glow is seen! So this explanation must also be rejected.
More than a hundred years passed before this cosmological puzzle was solved. In 1927 a Belgian astronomer, Georges Lemaître, proposed that the universe began with the explosion of a cosmic “egg.” This clever concept neatly explained the long-standing paradox in that the force of the explosion prevents the gravitational pull from drawing the matter toward the center of the universe. It would be as if a bomb were to blow the balls on our lattice away, overpowering the pull of the rubber bands. In the absence of observational evidence, the Lemaître hypothesis would have received relatively little attention. Within two years of its publication, however, Edwin Hubble reported observations that turned the attention of the scientific world toward the concept of an expanding universe. Hubble reported a shift toward the red in the spectra of light reaching us from the stars in very distant galaxies. The simplest explanation for such a shift was that these distant galaxies were speeding away from ours at incredible speeds.
The light coming from the sun consists of a spectrum of frequencies. As these light rays pass into and out of raindrops, they are bent. Each frequency is bent at a slightly different angle separating the bundle of mixed light into a rainbow of individual color components. Each of these frequencies leaves a different imprint on our retina. We see them as colors.
Isaac Newton in the seventeenth century did a number of experiments with light, forming rainbows by passing sunlight through a glass prism. Light rays passing through such a prism are bent according to frequency. As shown in Figure 2-2, the red light (that with the lowest frequency detectable by our eyes) is bent the least, and violet light (that with the highest frequency detectable by our eyes) is bent the most. What we see as white light is actually a combination of all the colors in the visible spectrum.
Astronomers have long used prisms (and more recently diffraction gratings) in their telescopes as a means of examining the color composition of the light from distant galaxies. Rather than producing a continuous spectrum, the light from stars is broken up by dark bands that mar the otherwise smooth transition from red to orange, to yellow, to green, to blue, and to violet. The dark bands are produced by the absorption of certain frequencies of light by the element-containing halo of gas surrounding the star that is producing the light. A packet of light can interact with an atom only if it has just the right energy to lift one of the atom’s electrons from one of its permitted energy levels to another. While transparent to some frequencies of light, the excitation of elements in the gas absorbs other frequencies so that specific wavelengths of light do not get through. Early spectra identified only the most prominent lines (Fig. 2-3). When examined in detail, thousands of lines became apparent. Most do not completely blacken the rainbow; they produce a weakening of the intensity of the light at that frequency. This weakening is the result of partial absorption of the departing light by the star’s “atmosphere,” depending on the abundance of the element.
Fig. 2-2: Light rays passing through a prism are bent according to frequency. The red light (that with the lowest frequency detectable by our eyes) is bent the least, and violet light (that with the highest frequency detectable by our eyes) is bent the most.
Fig. 2-3: A portion of the spectrum of the sun, called the Fraunhofer spectrum after its discoverer, compared to portions of emission spectra of hydrogen and sodium. The C and F lines of the Fraunhofer spectrum are created by hydrogen in the sun’s atmosphere; the dark D lines in the yellow are the most prominent lines of the sodium spectrum. Other lines are produced by absorption by other elements. See also color plate 3.
Astronomers originally took an interest in these bands because they offered a means of making chemical analyses of the star’s halo of gas. Unlike Earth, whose atmosphere has a composition that bears no relationship to that of its crust or interior, the composition of a star’s atmosphere is close to that of its bulk. Each partially darkened line in the spectrum represents a single element. Using laboratory arcs as a means of calibration, astronomers were able to estimate the relative abundances of the elements making up the atmospheres of neighboring stars. Since all stars contain at least some of all the elements, the characteristic lines become a fixed “bar code,” with spacings and relative intensities controlled by the fundamental characteristics of atoms (see Fig. 2-3).
Fig. 2-4: Schematic illustration of how the spectral lines of distant stars shift toward the red (the right-hand ends of the spectra). The bottom spectra represents a nearby star in our galaxy and the spectral lines correspond with the wavelengths we could observe for the elements on Earth. For nearby galaxies, which appear large in a telescope, the spectra are shifted slightly toward the red. For distant galaxies, which appear very small in the telescope, the red shift is much greater. Arrows indicate the extent of the red shift for each spectrum.
As bigger and better telescopes become available, astronomers were able to extend their chemical analyses to more distant objects. It was here that the great discovery came. When astronomers looked at very distant objects, they found that the characteristic “bar code” shifted with respect to the rainbow background. For example, patterns of lines that were in the blue part of a spectrum taken from the sun would instead be found in the green part of the spectrum for the light from a distant galaxy; a line that appeared in the yellow part of the sun’s spectra would appear in the orange part of the distant galaxy’s spectrum, and so forth. The “bar code”—the spacing and relative intensity of lines—remained the same. But it looked as if someone had lifted the whole set of dark lines off the background rainbow, moved it toward the red end, and then replaced it. More startling was the finding that the more distant the object, the greater the shift toward red (see Fig. 2-4).
We can understand how this occurs if we first grasp what we will refer to as the “train-whistle concept” (physicists call it the Doppler shift). Those who have indulged in train watching may remember that most engineers of express trains blow their whistles as they roar through local stations. Anyone standing on the platform experiences a strange sensation as the train passes. The pitch of the whistle suddenly drops! It drops for exactly the same reason that the lines in the spectra for distant galaxies shift. Since the whistle situation is a bit easier to comprehend, we will consider it first.
Sound travels through the air at a velocity of 1,236 km/hr. If the train passes through the station at 123 km/hr, then the frequency of the sound impulses on the listener’s ears would be 10 percent higher as the train approaches and 10 percent lower after it has passed by. This phenomenon is easily understood if we substitute for the train’s whistle a beeper that gives off one beep each second. Were an observer to count the beeps from a train stopped down the tracks, he would get 60 each minute. Were he to count the beeps from a train speeding toward him at 123 km/hr, he would hear 66 each minute. Were he to count the beeps from a train speeding away from him at 123 km/hr, he would hear only 54 beeps per minute. The ear counts the frequency of sound waves hitting our eardrum. When the source of the sound is receding, each beat is further away and has to travel further to reach the eardrum, so the ear detects a lower frequency and sends to the brain a lower pitch.
If a source of light is receding, the “pitch” of its light is also lowered. However, as light travels at a staggering 1,080 million km/hr, the frequency of light reaching us from a speeding train is not significantly changed, because to have an effect the speed of retreat has to be a significant fraction of the speed of transmission. So if we observe a shift toward red in the spectrum of the light reaching us from a distant galaxy corresponding to a 10% reduction in frequency, the galaxy must be speeding away from us at the amazing speed of 108 million miles an hour!
As stated above, the more rapidly a galaxy is retreating, the larger will be the shift of its light toward the red. Examples of spectra observed from a series of galaxies are shown in Figure 2-5. The great discovery that followed the discovery of the red shift was that those galaxies that exhibit the greatest red shift are also the farthest away. This discovery required a series of developments that gradually led to a robust distance scale.
Fig. 2-5: Galaxies and their light spectra: On the left are shown photos of five galaxies taken with the Hale Observatory telescope. As these objects are probably similar in size, Virgo must be located much closer to Earth than Hydra. Also shown, on the right, are schematic light spectra from the galaxies, compared to spectral lines as measured on Earth. The horizontal white arrows show the displacement of an easily identified pair of dark lines from its position in a light spectrum for the Sum (or for a laboratory arc). The recession velocities corresponding to these arrow lengths are given. As can be seen, the more distant the object, the greater is recession velocity. (Images courtesy of California Institute of Technology)
Fig. 2-6: Illustration of how geometry can be used to determine the distance to far away objects. The surveyor observes the distant rock out in the ocean from the ends of a baseline on the shoreline of known length and notes the angles between baseline and the lines of sight. He can then compute the distance through trigonometry.
Distances are far more difficult to measure than velocities—sufficiently difficult that it is beyond our task here to try to grasp exactly how it is done. A few paragraphs will suffice to show the general principles.
As do all surveying schemes, measurements out into space start with a baseline (Fig. 2-6). If a surveyor wants to measure the distance of an object that he cannot easily reach (like a rock out in a lake), he sets up a baseline on shore and measures its length. He then observes the rock from both ends of the baseline and notes the angle between the line of sight and the baseline. Simple trigonometry allows him to calculate the distance to the rock.
As can be seen from Figure 2-7, the ranges of distance confronting the astronomer are staggering! The astronomer boldly starts by using Earth’s orbit about the sun as his baseline. By making observations of objects in the sky from the extremes of the orbit, the astronomer can use the triangulation method to measure the distance of “rocks” out in space. Even with this seemingly gigantic baseline, this proves to be a very tough task. The baseline is 3 × 108 km long. Even the nearest star is 4 × 1013 km away. Thus, it is akin to measuring the distance of a rock 10 km off the coast using a baseline only 1 cm long!
Fig. 2-7: Distance scales. Astronomers must cope with distances ranging over 19 orders of magnitude.
Through the use of a very accurate technique called parallax, the distance to a few thousand of our nearest neighbor stars can be determined using Earth’s orbit as a baseline. This method, however, is limited to a very, very small portion of our own galaxy.
The baseline was greatly extended by showing that our own sun is rushing through our galaxy at a rather large rate of speed—6 × 108 km/yr. In this way an ever-growing baseline far longer than Earth’s orbit affords has been established. It would be as if a surveyor were driving along a shore road in a truck, periodically taking sightings on a distant island. From the truck’s velocity and the elapsed time, he could determine the length of his growing baseline. In a related but more complicated way (called statistical parallax), astronomers have been able to measure the distances of stars out to about 3 × 1015 km. Even so, all these stars reside in our own galaxy.
Finding the distance to galaxies beyond our own posed a problem so formidable that the trigonometric approach had to be abandoned. Nature, however, provided an alternative approach, which astronomers hit upon and exploited. Some of the stars in our galaxy show regular pulsations in their luminosity. Hence, they are more like lighthouses than headlights. These stars show a range of blinking rates. The important characteristic is that stars that blink at the same rate have the same luminosity. It’s as if the Coast Guard decided to have all its lighthouses use “light bulb” strengths related to their turning time. For example, all lighthouses with 100,000-watt bulbs would turn once each minute; those with 200,000-watt bulbs would turn twice a minute, and so forth. The variation in intensity of these stars is large—almost a factor of 10, so they are quite easy to spot even in galaxies other than our own (Fig. 2-8).
The astronomers jumped on this relationship and reasoned that blinking stars visible in nearby galaxies probably followed the same rules: from the blinking rate the luminosity of the star could be estimated. By comparing that luminosity at the source with the intensity of light seen from Earth, the distance of the star, and hence of its hosting galaxy, could be determined. This “headlight” method is a quantitative version of how we intuitively judge the distance of an approaching car on a dark highway. As automobile headlights have similar luminosities, we judge the distance of an oncoming vehicle by the brightness of its headlights. The distances to the nearby galaxies could be estimated by the differences between the intensities of light received from these distant blinkers and the intensity of light received from one of its cousins in our own galaxy, whose distance had been determined by trigonometry. Knowing the distances of these nearby galaxies, the astronomer could then from trigonometry determine their diameters. A “map” showing the Milky Way and its neighboring galaxies and clouds of gas and dust is shown in Figure 2-9.
Fig. 2-8: Illustration of the change in magnitude (intensity of light) of a pulsating star in a nearby galaxy. The squares at the top are blowups at three different times of the small region in the galaxy identified by the small box in the lower image where one of these stars, centered in the boxes, can be identified. (Courtesy of NASA; http://apod.nasa.gov/apod/ap960110.html. Credit: NASA, HST, W. Freedman (CIW), R. Kennicutt (U. Arizona), J. Mould (NU))
Unfortunately, the galaxies that show significant red shifts are so far away that even our biggest telescopes cannot resolve individual stars, though much more distant galaxies have been able to be measured by this method using the Hubble telescope, which avoids the interference of Earth’s atmosphere by residing in space. For the most distant galaxies, however, the whole galaxy appears only a bit larger than a nearby star. Thus, no individual pulsing stars can be identified and the lighthouse method is not applicable.
The last step out into space is then taken by using the size of the galaxy itself. More often than not, galaxies are found in clusters. Astronomers have carefully studied the sizes of the galaxies in nearby clusters. As with the sizes of people (and cars), they follow some simple rules. The assumption is made that the galaxies in very distant clusters have a similar spectrum of sizes and brightness as the ones in “nearby” clusters. For example, the distance of a car can be estimated not only from the brightness of its headlights but also how far apart they seem to us. While there is some variation, we will not be far off with this method. Like the automobile driver, the astronomer infers the distance of these clusters by the sizes of their individual galaxies.
Fig. 2-9: Illustrative map showing the Milky Way and its nearest neighbor galaxies and clouds of gas and dust. The Andromeda Galaxy, with one trillion stars, is 2.5 million light years from the Milky Way. (Courtesy of NASA/CXC/M.Weiss; http://chandra.harvard.edu/resources/illustrations/milkyWay.html)
Recently, astronomers have improved on this approach by observing a certain class of supernova explosions in distant galaxies. Such events occur in any given galaxy roughly once each century. Thus, each decade about one in every ten galaxies is lit by such a strong flash of light that it can be observed. These flashes are thought to be excellent headlights.
Having determined both velocity and distance for galaxies throughout the universe, astronomers can then make a graph with the distance of the galactic cluster on one axis and the velocity at which it is receding from us on the other axis. As shown in Figure 2-10, when the observations for various galactic clusters are plotted on such a graph, the points form a linear array. A factor of 10 increase in distance is matched by very nearly a factor of 10 increase in recession velocity. What is the significance of this striking relationship?
The significance of the distance-vs.-red shift relationship is that it is what would be expected for galaxies that were once all together at the same time and place. Consider, for example, a birthday party where everyone leaves at exactly the same time. Some people walk home at 4 km/hr, others cycle at 10 km/hr, others drive at 50 km/hr, and one takes a helicopter at 500 km/hr. Imagine that they all travel for an hour in a straight line but heading off in different directions. After one hour, the walkers are 4 km away, the cyclists 10 km away and so on. Plotting the speed of their retreat vs. their distance from the party house would produce a straight line, and the slope of the line gives the time they left the party. It is also true that plotting the distance from any one of the groups would lead to the same result, so everyone who was at the party would produce a graph with the same slope, because they were all at the party and they all left together. The farther two groups are from one another, the faster they must be moving apart. The same thing happens in three-dimensional space. If we turn time around and move the various galaxies backward at the rates they are observed to be retreating, all come together at the same time! The actual date can be obtained from the distance (from our galaxy) and the rate of recession (from ours) of any distant galaxy.
Fig. 2-10: Relationship between galactic distance and galactic recession velocity. Each point represents a distant galaxy (or cluster of different galaxies). Since the distances range over a factor of 100, a logarithmic rather than linear scale is used.
So this simple diagram both shows us that all the galaxies were together at one point in time and gives us that time, which is the age of formation of the universe. Since one axis is cm and the other is cm/sec, the slope is time (or 1/time). The ratio of distance to recession velocity yields an age for the universe. The result is that the matter in the universe is flying outward on the wings of an explosion that occurred about 13.7 billion years ago.
In Figure 2-11 the evolution of the distance-velocity relationship is depicted. Were we to have lived only 5 billion years after the Big Bang, the line depicting the velocity-distance trend would have been about three times steeper than the one we obtain today. This is because the retreat velocity for any given galaxy remains nearly the same, while the galaxy’s distance from us increases.
A natural question that arises from this reasoning is, Where is the center of the universe? A train analogy in Figure 2-12 shows why the velocity/distance relationship tells us nothing about this. Observers on night train A speeding along one track see a light mounted on top of train B speeding along another track. They also hear train B’s whistle. They know this train left the central station at the same time their train did. From the strength of its light they determine the distance of train B. From the pitch of its whistle they know that it is moving away from them and the exact speed of this recession. Knowing only this much, the observers could not determine where the central station is located. Neither can astronomers locate the center of the universe.
Fig. 2-11: Evolution of the distance-velocity relationship. Each of the four galaxies a, b, c, and d moves away from us at a different velocity. These velocities have remained nearly constant through time. However, as the universe becomes older, the distance separating these galaxies from us increases. Some 15 billion years after the explosion, they are three times as far away as they were 5 billion years after it.
Additional support for the Big Bang was provided by the discovery that the universe has a nonvisible background glow. To understand this glow it is necessary to realize that all objects above absolute zero emit radiation that is diagnostic of their temperature (see Fig. 2-13 for a description of the various temperature scales). This radiation, called blackbody radiation, can be used to estimate the temperature of distant objects. The wavelength of emitted radiation decreases as the temperature increases. At very low temperatures, the radiation is not visible. But as the temperature gets above a few hundred degrees C, the wavelengths start to enter the visible range, and the object glows a dull red. At higher and higher temperatures the object turns orange, then white hot, and so on. It is not a single wavelength of radiation that is emitted, but a characteristic pattern that is completely diagnostic of the temperature of the object. This is apparent on the dark coils of an electric stove, for example. As the coils get hotter, the radiation emitted changes wavelength. At first the coils remain dark because the emission is in the infrared, where our eyes are not sensitive. Then they glow a dull red as some of the radiation reaches the visible range. If they get too hot they are almost white, as many colors of the visible spectrum are emitted. Measuring the detailed pattern of radiation from a distance can tell us the temperature of the object. For example, Earth gives off radiation characteristic of its surface temperature of about 288°K. This radiation is centered in the infrared range. The sun gives off radiation characteristic of its surface temperature of 5700°K. This radiation is centered in the visible range.
Fig. 2-12: The train analogy. Passengers on night train A sight the beacon of night train B, which left the central station at the same time they did. From its brightness they determine the distance between the two trains. They also hear the whistle. From its pitch they determine the speed at which the two trains are moving apart. However, unless they have some other information (e.g., the direction in which their train is going and the speed at which it moves along the track), there is no way for them to determine where the central station is located. Of the infinite number of possibilities, three are depicted here.
Fig. 2-13: Description of the three temperature scales. Absolute zero on the Kelvin scale is the temperature of no molecular motion. In everyday life in the United States used the Fahrenheit scale is used. The centigrade scale is used by most other countries in the world.
That’s the background. The surprising data was obtained by physicists Robert Wilson and Arno Penzias of the Bell Laboratory in New Jersey, who for other reasons were experimenting with an detector that was very sensitive to very long wavelength radiation—electromagnetic waves in the 0.1- to 100-cm range (i.e., microwaves) When they happened to turn their instrument toward the heavens, they found that although no visible light can be observed in the dark voids between stars and galaxies, there is a nonvisible glow. Looking at the detailed pattern of the radiation, they were able to show that this universal glow was the same as that emitted by an object whose temperature is 2.73° above absolute zero. After the Wilson and Penzias discovery, subsequent work, including very precise measurements from satellites, has demonstrated with great precision that the relative intensities of the various wavelengths of radiation in this range are consistent with this very cold glow (fig. 2-14).
Fig. 2-14: The cosmic microwave background of the Universe determined by the far infrared absolute spectrophotomer on the COBE (Cosmic Background Explorer) satellite. The universe glows with radiation in the microwave range. By measuring the intensity of this glow at many wavelengths, it can be seen that the spectrum corresponds exactly with blackbody radiation from a material with a temperature of 2.725 + 0.002°K. The temperature corresponds extraordinarily well with the Big Bang theory. (Courtesy of NASA;http://lambda.gsfc.nasa.gov/product/cobe/firas_overview.cfm)
What is the source of the universal blackbody radiation? Shortly after the Big Bang, a great flash of light appeared when the protons and electrons in the expanding universe cloud cooled to the point where they could combine into neutral atoms. At that time the universe was only about 100,000 years old and the gas had a temperature of about 4,000°K. The reason why this light, which was given off from a gas at 4,000°K, appears now to have been given off by an object about 1,500 times cooler (that is, one with a temperature of 2.76°K) has to do with the expansion of the universe since that time. While the computation of the magnitude of this “cooling” is too complex to be described here, to the physicist it is exactly as expected. Hence, the discovery of the afterglow of the Big Bang is taken by physicists as a strong confirmation of the Big Bang hypothesis.
As we shall learn in the next chapter, universe matter right after the Big Bang consisted almost entirely only two elements, hydrogen (H) and helium (He). By careful modeling of the Big Bang, physicists have been able to calculate what the proportions of hydrogen and helium should have been from the atomic reactions that took place. The calculated proportion of 10:1 corresponds with the observed H/He ratio in the universe.
The combination of these different and independent lines of evidence—the velocity/distance relationship of galaxies, the background radiation of the universe, and the chemical composition of the universe all combine to support the Big Bang hypothesis of the universe’s origin.
The universe has been expanding from the beginning, counterbalanced by an inevitable gravitational force. This led to the idea that gravity might be strong enough to gradually slow the expansion to zero, and then a grand contraction would occur, leading to a “big crunch” and even a possible oscillation of the universe. Could this be shown from observations? The launch of the Hubble Space Telescope in 1998 provided the necessary data, as well as the surprising and completely unexpected result—that the expansion has been accelerating. Theorists have striven to come up with possible explanations for this result, and the various ideas come under the name of dark energy. Dark energy is not a minor phenomenon; to explain the observations, it must make up some 70% of the universe! Furthermore, it has the opposite effect to what we perceive as matter and energy and exerts an expanding force on the universe that is able to overcome gravitational attraction.
A further problem is that all the visible matter in the universe is insufficient to account for the mass that is required to explain various cosmological observations. The remaining, invisible mass is referred to as dark matter. Dark matter is also not trivial, but makes up about six times as much mass as “normal” matter. Dark matter is not in stars, planets, or back holes. Physicists know what it isn’t, but are not sure what it is!
All our discussion in the remainder of this book will be referring to what we call “normal” matter and energy. This world that we can see and discuss turns out to be only ~4% of the composition of the universe (Fig. 2-15). As we continue with our discussion of what is known, and the questions about what is known, it is also useful to reflect on the fact that the unknown remains far greater than the known.
Fig. 2-15: Pie chart illustrating the composition of the universe. All the matter that we can directly observe and will be discussing in this book makes up only 4% of the universe.
About 100,000 years after the Big Bang, when the expanding matter had cooled to the point where the heretofore free electrons could become entrapped in orbits around the positively charged nuclei, helium and hydrogen gas formed. This gas was lighted only by the afterglow of the Big Bang. At this point the universe was a dull place indeed. No galaxies, no stars, no planets, and no life were to be found. There were only molecules of gas in a rapidly expanding cloud.
Then, for reasons as yet not entirely understood, the cloud began to break up into a myriad of clusters. Once formed, these clusters remained as stable units bound by their mutual gravitation. Each of these clusters in turn evolved into one or more galaxies. Within these galaxies the gas further subdivided to form many billions of brightly burning stars. The universe was no longer dark!
While these early stars are by now either dead or lost among their younger counterparts, we can be quite sure that they had no Earthlike planets. The reason is that Earthlike planets cannot be formed from hydrogen or helium. Elements not present in the young universe are required. Thus, the next step in our journey to habitability will be to see where and how the remaining ninety elements formed.
Human beings have always been interested in looking for knowledge and inspiration in the heavens. Natural curiosity and questions such as, “What is the spectrum of the sun and how does it compare to stars?” and “How far away are the stars?” led to unexpected discoveries. Distant galaxies have their element “bar code” of spectral lines shifted to the red, requiring that they be moving away from us at great rates of speed. Most surprisingly, the speed of retreat correlates with the distance, suggesting a common origin at the same time and place 13.7 billion years ago. This inference from direct observation obtained unexpected support from another observation based on curiosity, the answer to the question, “Does the universe emit any background radiation?” The blackbody radiation then turned out to be striking confirmation of the Big Bang. Subsequent understanding of nuclear physics led to another confirming prediction in the H/He ratio. All of these combine to make the Big Bang one of our fundamental pieces of knowledge of where we come from, and when it happened.
In the last decade, further observations from the Hubble Space Telescope show us that all the matter that we can observe makes up but a small fraction of the universe. Much remains to be discovered in our exploration of the universe.
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William J. Kaufman III. 1979. Galaxies and Quasars. New York: W. H. Freeman & Co.
Joseph Silk. 2001. The Big Bang, 3rd ed. New York: W. H. Freeman & Co.
Steven Weinberg. 1977. The First Three Minutes. New York: Bantam Books.
Richard Panek. 2011. The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality. Boston: Houghton Mifflin Harcourt.
