Cosmic expansion

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Stars were the main business of astronomy in the early 1900s, but a few quirky investigators were trying to understand the spiral nebulae, which look like little pinwheels on astronomical photographs In the early decades of the twentieth century, Vesto Melvin Slipher worked at the Lowell Observatory in Arizona, a facility established by Percival Lowell, Lowell, scion of Boston industrialists, was fascinated with the idea of studying life on Mars. He used the vast wealth spun out of dark satanic mills on the Merrimack River in Lowell, Massachusetts to build his own observatory near Flagstaff, Arizona, to see what the Martian civilization was up to. Although this sounds as if Lowell was a man whose imagination was running wild, in the late 1800s there was serious discussion of intelligent life with an advanced civilization actively cultivating the planet Mars. Now that we've sent TV cameras, chemistry labs, and gamma-ray spectrometers to the surface of Mars there's less room for speculation. Though there are intriguing signs of water erosion on that planet, and possible microscopic structures in Martian rocks that look like living things, there is no trace of the system of irrigation canals that Lowell wanted to inspect. Instead, Mars looks like Tucson before the developers arrived.1

At the time when Einstein was formulating his theory of gravity, the spiral nebulae were thought to be pan of our own Milky Way system, and perhaps solar systems in formation, so studying spirals was a reasonable par of the Lowell Observatory's mission. In the new spirit of astrophysics, Slipher undertook heroic efforts with his small telescopes and inefficient photographic plates to obtain spectra of these spiral nebulae. In 1912, he succeeded in getting a spectrum of M31, the Andromeda nebula, and then worked diligently to compile spectra of several more of these enigmatic objects. His spectra of some spiral nebulae resembled the spectra of stars, with the same absorption lines that mark the spectrum of the sun. This identification allowed Slipher to measure the velocity of each nebula from the shift in its spectrum lines. Except, for M31 and its satellite M32, almost all the spirals he measured were moving away from us, and many were moving at velocities that were much higher than had been measured for any Milky Way star. Slipher may have thought his measurements were part of learning whether the spiral nebulae are little solar systems in formation. But Arthur Stanley Ed-dington thought the velocities of the spiral nebulae might be a central clue to cosmology based on general relativity, and included Slipher's as yet unpublished velocities for 41 galaxies, 36 of which were recession velocities, and the largest of which was 1800 kilometers per second, in his 1923 textbook The Mathematical Theory of Relativity. Somebody was thinking about the spiral nebulae in connection with general relativity and the possible expansion of the universe! As Eddington put it, "The great preponderance of positive (receding) velocities is very striking,'"1

A galaxy spectrum exhibits familiar absorption or emission lines at an unfamiliar location, shifted toward longer, redder wavelengths, Slipher's heroic collection of 41 galaxy spectra provided half the key to understanding the nature of the expanding universe. The other half came from work by Henrietta Leavitt at the Harvard College Observatory, In Harvard's hierarchical, patriarchic system, the director assigned tasks—and a remarkable group of women carried them out. Harvard had a station in the southern hemisphere at Arequipa, Peru, and it produced a formidable stack of photographs of the Magellanic Clouds to be measured. Henrietta Swan Leavitt sifted through these plates to find the variable stars in the Magellanic Clouds. Scrupulous comparison of one night's data with the next, showed that there were many bright variable stars in the

Wavelength (Angstroms)

Figure 5. f. Galaxy redshifts. The rerishift of a galaxy can be measured fnom the change in wavelength of emission or absorption lines in its spectrum. Cosmic expansion stretches the entire spectrum to the red_ Here are two galaxies, one at fow redshift, and another at a higher redshift. The spectra are similar, just stretched to the red. Courtesy of Barbara Carter, Harvard-Smithsonian Center for Astrophysics.

Wavelength (Angstroms)

Figure 5. f. Galaxy redshifts. The rerishift of a galaxy can be measured fnom the change in wavelength of emission or absorption lines in its spectrum. Cosmic expansion stretches the entire spectrum to the red_ Here are two galaxies, one at fow redshift, and another at a higher redshift. The spectra are similar, just stretched to the red. Courtesy of Barbara Carter, Harvard-Smithsonian Center for Astrophysics.

Magellanic Clouds, rhythmically growing brighter and dimmer in a regular, periodic way. This type of variable was known from work in our own galaxy: they are called cepheid variables. Cepheids are yellow giants that pulse with periods ranging from a few days to a few months.

In our own galaxy, some cepheids are nearby and some are far away, so it is hard to know the true brightness of a star without some other piece of evidence. A flashlight shining in your eye appears brighter than a lighthouse, or even brighter than a supernova—but it's just closer. In the Magellanic Clouds, the whole system is far enough away from us that all the stars in the cloud are very nearly at the same distance. This means that objects that appear bright really are bright and objects that appear dim are truly intrinsically dim. Henrietta Leavitt used this simple fact to learn something very useful about cepheids.

By 1908, Leavitt found that "the brighter variables have the longer periods."1 The bright cepheids are physically larger, and their vibrations take longer, much as a big bell sounds a deep note, while dimmer cepheids are smaller, and have quicker pulsations, like a small bell ringing a higher note. This relation between the period and the luminosity was like being able to read the label on a distant light bulb.

You could tell which were the stellar equivalent of 100-watt lamps and which were only 40 watts by measuring something that did not. depend on the distance: the period of vibration. The stars were bright (a cepheid with a 30-day period is about 10,000 times as bright as the sun) and the periods were in the convenient range from days to weeks, so cepheids became very useful for gauging the distances of stellar systems. Suppose you found a cepheid in a spiral nebula. If it had the same period as one in the Large Magellanic Cloud, then it presumably had the same intrinsic brightness. By measuring the apparent brightness and applying the inverse square law, you could figure out the distance to the spiral. That would tell whether they were in the Milky Way or not. But in 1920, nobody had done that yet.

In 1920, the National Academy of Sciences sponsored a debate on the nature of the spiral nebulae. Heber D. Curtis argued that the spirals were distant and not part of our Milky Way system. I larlow Shapley, from Mount Wilson, argued against this "island universe" hypothesis. He asserted that the evidence favored the spiral nebulae being part of the Milky Way galaxy. One of Shapley's best arguments concerned the sudden eruption of stars in some of the best-studied spirals. For example, on 20 August 1885, Hart wig at the Dorpat Observatory in Estonia reported a bright new star in the center of M31 that reached 6th magnitude, bright enough to see with a small pair of binoculars. Other novae had been sighted in spiral nebulae. Shapley argued, quite sensibly, that if these stars were like the novae that had been spotted in the nearby regions of the Milky Way, since they appeared to be so bright, it meant the spirals must be nearby and part of our own galaxy.

Otherwise, Shapley noted, if the spiral nebulae were outside the Milky Way, these new stars would have to be ridiculously bright, 100 million times brighter than the sun. It would offend Occam's razor to imagine that there were more types of novae than required by present knowledge. Shapley couldn't imagine "super" novae and considered this "out of the question." Good rhetoric. But not necessarily good science.

On the other side, Cunis advanced a number of reasons why the nebulae might be outside the Milky Way, and he demurely countered the problem of the bright novae by saying, "the dispersion of novae in spirals and in our galaxy may reach [a factor of 10,000] ... a division into two classes is not impossible.'"4

Scientific debates are a sure sign that the data are just not good enough. In other fields, debates or adversary proceedings like a trial may be the best way to find what we will accept as the truth, or at least a verdict. In scientific research, there's a debate only when there isn't decisive evidence, so that a healthy dose of opinion is required to make sense of the available facts. The truth is out there, all right, but we don't yet grasp it. Since the truth is patiently waiting for us to cast off moss-covered errors and illusions, fallible humans have time to blunder their way forward to the real story. The right tools help.

Harlow Shapley left Mount Wilson to become the director of the Harvard College Observatory, During his long and vigorous career, Shapley had a famous round desk, with wedges reserved for observât ory business, scientific research, current correspondence, and manuscripts, and he would rotate the appropriate segment for each topic before him during a working day. He had long since retired when I met him in 1970, a small bent man, 85 years old, in a blue suit. The occasion was the tour of inspection by Harvard's Board of Overseers' Visiting Committee, a distinguished group of outsiders who come every few years to take the temperature of Harvard's astronomy department.

As an undergraduate at Harvard, I did a junior project on the Crab Nebula, the remnant of a supernova in our galaxy observed in a.i> 1054. That had been fun, though at the time I had no idea I could contribute anything to this field. As a collcge senior, 1 worked on ultraviolet observations of the sun, using data from a satellite project led by Leo Goldberg, director of the I larvard Collcge Observatory. Leo was also chairman of the astronomy department and every year he sent around a deftly worded note to all the students. He encouraged us to submit our senior thesis work for something called the Bowdoin Prize.

"The Prize Committee deplore the continuing paucity of entries in the natural sciences."

After I looked up "paucity," I entered rny thesis cm ultraviolet observations of the sun. Careful inquiry made to the plural committee revealed that, though the prize essay had a strict word limit, pictures (proverbs to the contrary) did not count! I amplified my prose with many illustrations and nudged out entries on symbolism in Joyce to win a prize for "useful and polite literature (in the English language)." Since then I have tried to be both useful and polite. But more the one than the other.

I picked up the prize check on the ninth floor of I larvard's administration building, rode the elevator down to the fourth floor, and endorsed it to pay back a student loan. I still recall the sensation of feeling light as the elevator accelerated downward, and leaden as it stopped. Years later, my mother said, "You should have bought an oriental rug."

As a senior who had written a prize-winning essay, I was trotted out as part of the dog-and-pony show we presented for the nabobs of the visiting committee. As a reward, I was invited to the lunch the observatory had catered. As at other ceremonial occasions, the central participants sat together, while the less significant sat on the periphery. Shapley was seated next to me, in the outermost circle. I wanted to ask him about his discovery of our place in the galaxy and his memory of the debate with Curtis. Alas, he was not interested in anything but his shrimp cocktail, and that small dish took all his attention. Still, it is good to touch the past. After all, Shapley knew George Ellery Hale, and that's the main line of apostolic succession all the way back to Galileo.

1 lale's way of "contributing to the best of our ability" to solution

Figure 5.2. The 100-inch telescope at Mount Wilson This telHcnpt was the largest in the world for thirty years after it went into operation in November 1917. Edwin Hubble used the 100-inch to find and measure cepheids ill nearby spirals and to obtain galaxy redshifts. Although Mount Wilson is no longer a dark Sin, the telescope is still ill use. Courtesy of The Observatories of die Carnegie Insutution of Washington.

Figure 5.2. The 100-inch telescope at Mount Wilson This telHcnpt was the largest in the world for thirty years after it went into operation in November 1917. Edwin Hubble used the 100-inch to find and measure cepheids ill nearby spirals and to obtain galaxy redshifts. Although Mount Wilson is no longer a dark Sin, the telescope is still ill use. Courtesy of The Observatories of die Carnegie Insutution of Washington.

of the problems raised by Einstein's difficult theory was a practical one. He built the 100-inch telescope at Mount Wilson, near Pasadena, California. It was the largest telescope in the world from its completion after World War I to the construction of the 200-inch telescope at Palomar after World War II. Mount Wilson is a wonderful site with clear nights and steady air that Hale had been developing for astronomy since 1904. The 100-inch telescope was built in the engineering style of the Titanic—iron and rivets and big electrical switches and snapping, sparking relays that evoke the most stimulating moments of Frankenstein movies in the middle of a quiet observing night. Having prudently avoided all contact with icebergs, the 100-inch, unlike the Titanic, is still in use. However, the relentless growth of the little village of Los Angeles, which had a population of about 150,000 when Mount Wilson was established for astronomy, has made the skies today much too bright for studying faint objects with this telescope.

In the 1920s, this telescope was precisely the right tool to end the debate alxjut distances to the spiral nebulae. And Edwin Powell Hubble, one-lirne Missouri lawyer, sometime boxer, Rhodes Scholar, artillery captain, Anglophile, pipe smoker, fly fisherman, and agile social climber, was exactly the right person in the right place at the right time to find the decisive data. Hubble worked at the Mount Wilson Observatory at 813 Santa Barbara Street in Pasadena. He used the 100-inch telescope to look for variable stars in spiral nebulae. He found them.

By repeatedly photographing NGC 6822, M33, and M31 and assiduously comparing one image to the next, just as Henrietta Lea-vitt had done for the Magellanic Clouds, Hubble identified cepheid variable stars in these systems. The cepheids in M31 were about 100 times fainter than the cepheid stars with the same period that Henrietta Leavitt had seen in the Magellanic: Clouds. The apparent brightness of a star declines as the inverse square of the distance. For the same stars to be 1/100 as bright, the cepheids in M31 had to be about 10 times as far away. A present-day distance of 165,000 light-years to the Large Magellanic Cloud puts M31 nearly 2 million light-years away. Hubbies discovery of cepheids in these galaxies, reported in the period 1925-1929, showed that these stellar systems were definitely not a solar system in formation as Slipher had surmised, or some odd swirl at the outskirts of our own Milky Way as Shapley had argued. The Andromeda nebula and, by extension, the other spirals were immense and remote stellar systems—galaxies, which are equivalent to the entire Milky Way.

There isn't just one big central galaxy, with us in it and a void around. Luminous stuff in the universe is made up of galaxies, large and small but on the scale of a billion suns, separated by millions of light-years. I tubble also drew attention to the presence of bright, novae—like the 1885 event in M31—in remote systems, as an example of "that mysterious class of exceptional novae which attain luminosities that are respectable fractions of the total luminosities of the systems in which they appear.If the galaxies were distant, these were no ordinary novae. These were the objects that Zwicky and Baade would later call the supernovae. Curtis had been right—a

figure 5.3 Hubble observing at the 100-inch telescope. Hubble, dad in jodhpurs and wearing cavalry boots, is perched or a bentwoad chair at the Newtonian focus of the 100-inch telescope in 1923 He is holding the controls of the plateholder, Which needed constant guiding during the exposure of a photographic plate to compensate for small errors in the telescope drive mechanism. Courtesy of The Observatories of the Carnegie Institution of Washington.

division of the novae into two classes was not impossible. In fact, it was required: there were ordinary novae seen in our galaxy and in the nearest spirals, and much brighter objects, the supernovae, erupting in the distant spiral nebulae.

V. M. Slipher measured velocities of galaxies from the shift of the absorption lines in their spectra. Hubble measured distances to a handful of galaxies using cepheids, then used those to calibrate

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