The picture begins with some small thing sometimes called the “cosmic egg” but no one really knows how big it was. The thing exploded and out of that explosion came all the matter, all the energy, and all the space that exists today. For years astronomers have worked hard to figure out when the explosion occurred. That determination turns out to be quite difficult, but a consensus seems to be forming around a date about 13 billion years ago. There are many details still to be worked out involving understanding why the bang happened and connections of the Big Bang to the production of the atomic and sub-atomic particles which were produced, but the general picture is well understood.
I’d like to concentrate on four major discoveries that have led us to the Big Bang picture and add how we believe the new University of Texas Hobby-Eberly Telescope will help solve some related problems.
I can still remember reading an article on Olber’s Paradox when I was in junior high school. That experience long ago began my fascination with astronomy. The fact that the sky is dark at night gives us some insight into how the universe is built. This amazing realization goes back to Thomas Digges in 1576 and was also discussed by Johann Kepler in 1610 and Edmund Halley, of comet fame, who lectured on this paradox at a 1721 meeting of the Royal Society chaired by Isaac Newton. This became known as Olber’s Paradox and is named for Heinrich Wilhelm Olber, an Austrian astronomer who wrote on the paradox in 1826.
To understand this concept adjust your mind’s eye to zoom out from the earth until the galaxies or even the cluster of galaxies become little points of light. Imagine a universe in which these points of light can move but in which there is no overall expansion or contraction. Further imagine that these points of light extend infinitely far from the earth and are distributed uniformly throughout the infinite universe. Near the earth, the points of light are bright because they are nearby, but there are only a few of them. Farther away, the points of light are individually dimmer but there are more of them. If space is divided into huge spherical shells, like the layers of an onion, with the earth at the center then the amount of light reaching the earth from each shell should be the same as from every other shell. Nearby shells contain a few, individually bright clusters of galaxies; more distant shells have more, individually dimmer, clusters of galaxies, and the growing number and the dimming effect of distance exactly cancel each other out so that each shell contributes the same amount of light. The light from each shell may be feeble but with an infinite universe and infinitely many shells the light will add up. It turns out that the limit will not be an infinitely bright sky but a sky as bright as the surface or a star. The fact that this has not happened gives us some insight into how the universe is built.
In the early 1800s it was not clear what was wrong so the word “paradox” was applied. It now turns out that about 90 percent of the reason that the sky is dark at night is that the universe isn’t infinitely old. The reason that solves most of the problem is that in order to contribute to the brightness of the sky tonight as seen from the earth, light from the shell five billion light years away needs to have begun its journey five billion years ago in order to get here tonight. Since the universe is only 13 billion years old, there aren’t any shells from which the light could have left more than 13 billion years ago. It amazes me that in 1848 Edgar Allen Poe suggested, without any real evidence, that a finite age may solve Olber’s Paradox. The remaining 10 percent of the explanation for the darkness of the sky at night is that the universe, as a whole, isn’t fixed but it expands.
In 1917, the year that Albert Einstein was putting the finishing touches on the general theory of relativity as it applies to cosmology, Vesto Slipher at Lowell Observatory measured the radial velocity of the 25 nearest and brightest galaxies. Radial velocity is measured by the Doppler effect, which shifts light to different colors if the object emitting the light or the object receiving the light is moving toward or away from each other. If they move toward each other, the light shifts toward blue colors while moving apart shifts the light to red colors. Since radio waves are light with longer wavelengths than the eye can detect, the Doppler effect is used by police radar guns to measure how fast cars are travelling. If a car shifts the wavelength (color) too much, a ticket is issued.
To Slipher’s great surprise 21 of his sample of 25 galaxies were moving away or “red shifting.” But Slipher did not know how to measure the distances to his 25 galaxies. In 1929, Edwin Hubble used Cepheid Variable stars, which are very luminous and vary their luminosity in a distinctive, recognizable way, as standard candles. Since the luminosity of these stars is known, their brightness (or dimness) can be measured, and since the rate at which distance dims a star is also known, they can be used to measure the distances to their host galaxies. To everyone’s astonishment, Hubble found that the recessional velocity (amount of red shift) and the galaxies distances are directly correlated. The greater the distance, the greater the recessional velocity.
Throughout the 1930s and 1940s, aided by the huge new 100-inch telescope on Mount Wilson above Pasadena, Ca., Hubble and others measured more and more distant galaxies and refined and extended the relation between distance and recessional velocity. A graph of distance on one side and recessional velocity on the other side is a straight line with a lot of scatter, due to uncertainty primarily in the measurement of distances to galaxies. The universe is expanding! The expansion rate is the same in all directions and we seem to be at the center of the expansion. I emphasize that the expansion of the universe is not a theory, not speculation; it is an observed fact which any successful theory of the birth of the universe must cope with.
The relation between the recessional velocity of galaxies and their distances from us is called Hubble’s Law, and the slope of the straight line is called the Hubble Constant. It was realized immediately that Hubble’s Law was consistent with a Big Bang and that, in spite of appearance, the Earth is not at the center of the universe. Picture a strange kind of car race in which all the cars are lined up at the starting line and the race is conducted on a long straight highway. Each driver is told to drive at a certain speed once the gun fires and to keep the car at that speed. We even can include a car whose driver is told to drive at zero and others told to back up at certain speeds. The gun fires and the cars take off. An hour later, the cars are spread out. The one farthest from the starting line is the one being driven the fastest. An observer standing at the starting line, who plots the car’s distances and recessional velocities, will get Hubble’s Law.
But picture yourself standing on the roof of any arbitrary car. Cars ahead of you are being driven faster than you are. Cars behind you are being driven slower than you. Cars far ahead of you are being driven far faster. Plot the other cars’ distances from you and their recessional velocity from you, you will get Hubble’s Law. Every driver will see the other cars recede from him. Moreover, if all the cars are being driven at constant speeds and if we were all together at the start, then any car’s distance from me and its recessional velocity from me gives me how long the race has been going on. If we were together at the start of the race, a car 10 miles ahead (or behind) me receding from me at 10 miles per hour must have been racing for one hour. Any arbitrarily chosen car will give the same answer.
To figure out how old the universe is, we measure a galaxy’s distance from us and its recessional velocity from us, and if we both came out of the “cosmic egg” at the start, then the age of the universe must be how long it would take that galaxy to get that far from us moving at that speed. We get about 13 billion years for the age. The uncertainty is due to the difficulty in measuring the distances accurately, to uncertainty in whether the galaxies have slowed down a bit from their initial speeds, and to complications caused by the effect of gravity on the light which travels from the distant galaxies as described in the General Theory of Relativity.
The Big Bang is not the only plausible theory for the origin of the universe that is consistent with Hubble’s Law. A brilliant “rival” called the Steady State Theory was proposed. In spite of the fact that we now believe the Big Bang picture to be correct and the Steady State Theory picture to be wrong, the latter was enormously influential in sorting out how stars produce the chemical elements we see today. That particular study will be discussed later as a specific contribution that the Hobby-Eberly telescope is expected to make. The remaining two discoveries leading to the Big Bang picture are particularly influential in deciding between the Big Bang and Steady State pictures.
Three Degree Background Radiation
Arno Penzias and Bob Wilson, working for the Bell Laboratories, discovered in 1964 a faint radio signal seen in all directions whose strength was the same as the radio signals from a hypothetical perfect radiator heated to three degrees above absolute zero. Although it was known that disturbed galaxies, supernova remnants, the sun, the Earth’s atmosphere, and lightning emit radio waves, it was clear that this background radiation signal was not coming from an object but from the sky itself. This radio signal turns out to be from the birth of the universe, and it is entirely consistent with the Big Bang and not predicted in the Steady State Theory.
When the universe was very young, a few minutes after its birth, it was expanding, very dense and very hot. As Steven Weinberg has described in his beautiful and very readable book, The First Three Minutes,it is possible to describe in amazing detail the second-by-second expansion, the drop in the temperature and density as the universe expands, and the composition of the universe. It is clear that the Big Bang forms mostly hydrogen and helium, which are the simplest chemical elements. Radiation (light and radio waves, and so forth) cannot move far. The universe is quite foggy (opaque) mostly due to the free electrons. It is too hot for the electrons to combine with the protons and produce neutral hydrogen atoms. About one million years after its birth, the universe cooled to 3,000 degrees Kelvin at which temperature electrons combined with the protons; neutral hydrogen atoms formed and the opacity dropped abruptly.
Turning the picture around, we jump to the present. Suppose that an astronomer wishes to examine the past. A telescope is a time machine, and the light to see the Andromeda galaxy, M31, left there 2.2 million years ago in order to arrive tonight since that galaxy is 2.2 million light years away. Distant quasars, 10 billion light-years away, are seen as they were 10 billion years ago. If you wish to see farther into the past, just look farther away. Of course very distant objects are quite faint and require large telescopes in order to be seen.
If we look far enough away, we should be able to observe the birth of the universe. When we do so, we see this wall of fog one million years after the universe formed. Attempts to look farther (earlier) are frustrated by the very high opacity of the universe at times earlier than one million years after its birth. The temperature of the wall is 3,000 degrees K (where K, Kelvin, are on the absolute temperature scale whose zero is -454 degrees Fahrenheit). Remember Hubble’s Law, however. Objects this distant are receding from us at very high speeds. At this distance, wavelengths of light are redshifted to 1,000 times greater wavelengths. So the light signals characteristic of a perfect reflector at 3,000¸ K are shifted to 1,000 times greater radio wavelengths characteristic of a perfect radiator of three degrees K. Subsequent studies of the characteristics of these radio signals show them to be exactly consistent with those a Big Bang would be expected to produce.
Helium in Oldest Stars
The proposal of the Steady State Theory stimulated the study of the role of stars in producing the chemical elements heavier than helium. The most important producer of the energy that stars shine into space is the thermonuclear (fusion) reaction that converts hydrogen into helium in a star’s core. Additional nuclear reactions in stars later in their lives produce carbon, nitrogen, oxygen, iron and so forth. In fact, all the chemical elements except for the initial hydrogen and helium and a bit of lithium are produced in stars. The detailed calculations of the composition of the universe after the Big Bang show that about 74 percent of the mass of the matter is hydrogen, about 26 percent is helium and a tiny fraction of 1 percent is lithium. What we know about the Big Bang fixes these fractions to an accuracy of a percent or two. If this is correct, then the oldest stars formed out of gas were 74 percent hydrogen and 26 percent helium. As stars live and die, more helium is produced.
Later generations of stars form from gas that is polluted by the ejecta of previous generations. High mass stars live the shortest times — a few million years. Low mass stars can live billions of years and the lowest mass stars are capable of living 20 billion years or so. The universe is only 13 billion years old. With a few exceptions, these stars produce helium in their cores which isn’t ejected and made visible until the star approaches death. If the star isn’t near death, then what we see is the star’s surface showing the star’s original composition. If the Big Bang is correct, then no star should be able to be found which shows a surface composition less than 26 percent helium. In spite of much searching, no star which shows less than 26 percent helium has ever been found, exactly in accord with the Big Bang’s prediction.
In order to see farther, astronomers need to see fainter. In the last 50 years, the cheapest way to do this was to improve the efficiency of the devices we use to detect light. We have gone from photographic films, which typically register less than 1 percent of the light which strikes them, through a variety of electronic light detectors with increasing efficiency to the current Charge-Coupled Devices which approach 100 percent efficiencies. Now the only way to see farther objects is to build bigger telescopes. Using the technology of the 1930s, the famous 200-inch telescope at Mount Palomar is about as far as you can go. Computers have made the current huge telescopes, which dwarf the 200-inch, possible. Computers are used to design the telescope. Computers can predict nearly exactly how the telescope’s steel structure will perform.
As a result, the Hobby-Eberly Telescope and the 2.7-meter Harlan Smith Telescope weigh about the same, even though the former has a mirror with 10 times the area of the latter. These huge modern telescopes have segmented mirrors. They are too large to manufacture and transport in one piece. The Hobby-Eberly Telescope’s mirror is composed of ninety-one hexagonal pieces, each of which must be kept aligned with the others to an extremely high precision. Keeping the mirrors aligned, the telescope pointed at its target precisely, the light detectors functioning and so forth, requires a set of computers running all the time the telescope is operating. These days a telescope’s control room looks exactly like a computer center. This kills the romance of the old days where one spent all night in the dome with the telescope doing everything by hand, but it does relieve the agony of the last few hours of a long winter night where the cold and fatigue mounted to painful levels.
In spite of the power of modern computers, huge telescopes are expensive. The two largest single telescopes, the Keck telescopes on the island of Hawaii, are 394 inches across and have nearly four times the light-gathering power of the 200-inch. The third largest telescope in the world is the Hobby-Eberly Telescope, with a mirror 362 inches across and 3.3 times the light-gathering power of the 200-inch. In order for us to consider such an ambitious project, we had to design a telescope which specialized in certain astronomical tasks.
Professors Dan Weedman and Larry Ramsey at Penn State University came up with a clever design which can’t see the whole sky and specializes in astronomical spectroscopy, but costs about 15 to 20 percent of the cost of a Keck Telescope. Nevertheless, the telescope costs $13.5 million. So Penn State and UT Austin formed a partnership and sought additional partners. The Hobby-Eberly Telescope consortium consists of the two founding partners, plus Stanford University, the University of Goettingen, and the University of Munich. Each partner’s time on the telescope is determined by each partner’s financial contributions to the capital and operating costs. Texas is the majority partner (52 percent of the time) and McDonald Observatory operates the telescope for the consortium. Penn State has 31 percent of the time; Stanford has 7 percent and the two German partners have 5 percent each.
Let me concentrate on one study by Texas astronomers using the Hobby-Eberly Telescope. Each chemical element absorbs and emits light at a characteristic set of frequencies (or colors). These are called spectral lines. Sodium has a strong pair of spectral lines in the yellow part of the spectrum giving the peculiar color characteristic of sodium streetlights. By studying the strength of the spectral lines in stars, Texas astronomers are among the world’s leaders in determining the composition of stars.
Depending on the age of the star being studied, it was born out of gas clouds polluted by differing amounts of the ejecta of previous stars. The more recently the star was born, the more pollution. The amount of each chemical pollutant gives considerable insight into how many and what kind of stars lived and died in the area where the star being studied was born. One can think of this as a kind of cosmic archaeology. Previous studies have been restricted to stars in the sun’s neighborhood. The Hobby-Eberly Telescope allows the studies to be extended to the halo of the Milky Way Galaxy and to the brightest stars of nearby galaxies. Since the processes by which galaxies formed influences the star formation rates, these studies give us insight into the process by which galaxies form.
The University of Texas at Austin has a high-ranking astronomy program and a majority share of a huge new telescope. These are certainly exciting times.