From an Expanding Universe to the Big Bang

Overview

How did physical nature on the cosmic scale begin? What are the dimensions of the universe? These remain the two most persistent and expansive scientific questions in the collective mind of humanity. Logically, they seem to be related to each other, as they also lead to "Why are we here?" and other introspective queries. In the first half of the twentieth century, the probing for the answer to these questions reached a high point of investigative conclusions. Larger telescopes, advances in detailed photography and experimentally sophisticated spectroscopic methods, and mathematical technique provided concrete means to explore the greatest challenge to human understanding, the universe. Through about mid-nineteenth century, astronomers had catalogued stars, methodically studied the solar system, and wondered at the unexplained sights of deep space. From there on the structure of the universe and its extent became the major thrusts of specialized astronomical disciplines, cosmology and astrophysics. The result would be momentous: not only the discovery that the universe was many times larger than traditionally thought but also expanding outward at astonishing speeds approaching that of light.

Background

Nineteenth-century English astronomers father and son William (1738-1822) and John (1792-1871) Herschel theorized beyond the acceptance of a uniform distribution of stars around the Milky Way, often simply called the Galaxy. They instituted a methodical study of the "structure of the heavens." Telescopes were reaching the size at which the cosmic fuzzy patches of light, the "star clouds and nebulae," some known since ancient times, were demanding investigation. Both the philosopher Immanuel Kant (1724-1804) and astronomer Pierre Laplace (1749-1827) had suggested these were "island universes," independent star systems beyond the Milky Way.

These anomalies prompted the senior Herschel, who favored the "island universes" idea, to focus on revamping cosmic distances by recording relative distances to the stars based on observation of apparent brightness (apparent magnitude) of starlight. His observation showed that star distributions were more concentrated in the plane of the Milky Way—that is, the Galaxy was disk-shaped, perhaps like the nebulae he saw through his large telescopes. The younger Herschel concentrated on the study of what he called—anticipating the future—"extragalactic nebulae." Catalogues of stars gave way toward the end of the century to catalogues of nebulae much advanced on that of pioneer cataloguer Charles Messier (1730-1817). New spectrographic techniques were applied, an important realization being that the visible universe had the same chemical composition as analyzed on Earth. Many nebulae catalogued were gas clouds rather than galaxies and were spectrographically analyzed as such, discrediting the "island universe" concept. But intuition still kept the idea alive with observation and photography, important in providing a permanent record of astronomical observation.

By the turn of the twentieth century the mistake of erecting big reflectors with bad design in worse locations had been learned. The best operational reflectors in 1895, the Common telescope in England and the Crossley telescope of the University of California's Lick Observatory, were modest in size (about 36-in or 91.44-cm aperture) but doing important work. The Lick launched the first detailed photographic program of the so-called spiral nebulae. They appeared to be like the close Andromeda Nebula; their smaller size logically being a matter of much greater distance from the Milky Way. Into the new century two more pieces to the galactic puzzle of the Milky Way appeared: Johannes Hartmann (1865-1936) identified interstellar gas (1904), and Edward Barnard (1857-1923) did the same for interstellar dust (1909), the latter explaining areas that seemed without stars. And about 1910 the research on star brightness/temperature relationship of Ejnar Hertzsprung (1873-1967) and Henry Russell (1877-1957) resulted in the Main Sequence diagram of star types, which brought a tempering order to stellar astronomy.

That the Lick telescope was in California was prophetic for the future of galactic astronomy. George Ellery Hale (1868-1938) of the Yerkes Observatory came west to California and chose Mt. Wilson above Los Angeles as the sight of a new solar observatory (1905). As would become clear, he was inaugurating the most important observatory complex for the investigation of the cosmos for half a century. With him came astronomer and telescope designer George Ritchey (1864-1945), who was able to put Hale's efforts for a 60-inch (152.4-cm) reflector, the largest working telescope in the world, into service by 1908 with himself nearly resolving individual stars in the Andromeda Nebula with it. Larger diameter mirrors meant seeing further into the origins of the stars—deep space—which became Hale's crusade for the future of astronomy. By late 1917 his persistence paid off with yet another reflector of unprecedented size, the 100-inch (254-cm) Hooker reflector, which would remain the largest in the world until 1948.

Here were hardware stepping stones toward solving the mysteries of the universe, but the mental ones were also at hand. The limited parallax and trigonometric methods for calculating the distances of stars from Earth gave way to the revolutionary technique based on the study of Cepheid variable, or pulsing, stars, the study by Henrietta Leavitt (1868-1921) in 1912, and the technique by Hertzsprung's determination of these stars as "standard candles" by their intensity-period relationship. Albert Einstein (1879-1955) had already published his theory of special relativity (1905), explaining the odd effects of relative motion with constant velocities approaching the speed of light. The cosmic scale was the only laboratory to validate the implications of relativity. By 1913, Einstein wrote Hale about proving an aspect of Einstein's theory of general relativity, which extended the physical parameters of relativity theory to acceleration and gravity on curved paths. That effect was the theory that strong gravitational fields could bend light—again, something only to be imagined in the massiveness of cosmic space. This was proved at Mt Wilson in 1915.

Einstein went on to theorize that the universe might have physical curvature and came up with three mathematical possibilities: positive curvature (as a circle) or a closed universe, negative curvature (as a saddle or hyperbolic surface) or an open universe, and uncurved. Einstein's ideas were much food for cosmic thought. Mt. Wilson had the telescopes to take the initial step toward determining the origins, structure, and extension of the universe. First, was the Milky Way the whole of the universe?—that could be found by solving the puzzle of whether the dense (globular) and open (galactic) clusters of stars and, more so, the diffuse spiral nebulae were internal or external to the Milky Way. Hale found the two astronomers to embark on this research—unlikely as it might seem, both from Missouri. Harlow Shapely (1885-1972) began observation with the 60-inch(152.4-cm) reflector in 1914 to investigate the globular clusters (Cepheids being found there). He was the first astronomer to apply the Cepheid variable yardstick to determine the distance to these clusters. Edwin Hubble (1889-1953) came in 1919 to be the first to work extensively with the Hooker 100-inch (254-cm) on investigation of the nebulae, also using variables as distance indicators.

Shapely completed his work by 1917. He was able to find the distances of variables in a dozen globular clusters, discovering a uniformity in large cluster star magnitudes and the size of the clusters. Finding the distances of 69 clusters, he was able to deduce that all the clusters were within the Milky Way and that the center of the Milky Way was in the proximity of these clusters in the constellation Sagittarius. Previously, the center was assumed to be our Sun and its solar system; now that was determined to be on the edge of the galaxy. This proved that the Milky Way was 10 times more extensive than long thought. Having shown this, Shapely did not wish to extend the reasoning to the nebulae and without further investigation assumed these were comparable in distance to the clusters and within the Milky Way, which was still the unique Galaxy. But data to the contrary began emerging. Ritchey had found (1917) a nova (erupting star) in a nebula in the constellation Cepheus, but instead of a brightness 10,000 times the Sun, it was so dim that he reasoned it must be very far off. He studied the Mt. Wilson photographic plates and found two more such novae, while Heber Curtis (1872-1942) did the same at Lick, finding three—others were also found. Curtis, whose research centered on spiral nebulae, became convinced these were independent galaxies to the Milky Way. A collision other than cosmic was coming.

Impact

Heading for the high profile Harvard Observatory, the sardonic Shapely left Mt. Wilson to the urbane and cosmopolitan Hubble, there being a strain of theoretical and professional attitude between them. Hubble's meticulous cataloguing of the shape of nebulae preceded inspection of the Andromeda Nebula for Cepheid indicators, becoming much anticipated by astronomers siding with Curtis that the spiral nebulae were independent galaxies. From about 1917 to 1923 the socalled "Great Debate" ensued with Curtis and other California astronomers opposing those lined up with Shapely, mostly in the East, whether there was one galaxy or many. A debate over the scale of the universe took place between Curtis and Shapely, still insistent that the Milky Way was the whole of the universe, in April 1920.

With the most powerful telescope in the world, Hubble was able to resolve stars including Cepheids not only in Andromeda but also in two seemingly smaller spirals in late 1923. In dramatic fashion at an astronomy conference on the first of the following year, a message from Hubble arrived stating that he had proved by the faintness and period of these Cepheids that the Andromeda Nebula and the two other nebula were far beyond the Milky Way—these were independent galaxies. This was the momentous birth of galactic astronomy with the boundaries of the universe dropping away and modern cosmology ceasing to be a matter of theory intrigued by relativity. Hubble himself was attuned to new cosmological theory, being acquainted with the cosmic models being expounded in the aftermath of relativity theory.

Einstein's conclusion (1917) that the universe was static and unchanging in curved space, a spherical universe, had stimulated many scientists to seek other solutions to his relativity field equations. About this same time Dutch astronomer Willem de Sitter (1872-1934) demonstrated other solutions, one implying an expanding universe of constantly decreasing curvature in time. Russian mathematician Alexander Friedmann (1888-1925) decided the universe would be the same everywhere with its size and density varying as functions of time (1922). In the meantime Hubble continued to advance outward into intergalactic space. Having to retire the limited Cepheids as distance indicators, he used the blue supergiant stars that inhabited the arms of Andromeda and other near galaxies for comparison with those in the further galaxies he was examining. When he could no longer distinguish individual stars, Hubble had to resort to even more approximate averaging of innate light brilliance in whole galaxies compared with those catalogued at 30 million light-years. In this manner, he reached a visible cosmic horizon he calculated as 1 billion light-years.

This tier upon tier of ever more remote galaxies had velocities of movement away from the Milky Way. Back in 1912 Lowell Observatory astronomer Vesto Slipher (1875-1969) had observed the radial (rotating) velocities of spiral nebulae with spectral analysis revealing a shift toward the red of the visible spectrum that seemed to indicate a Doppler shift in wavelength, as would be observed for an object moving rapidly away. Hubble noted this with all the galaxies being observed at Mt. Wilson, and he also observed that the further away the galaxy was the more the spectra was shifted to the red—that is, the further away the galaxy the faster it was receding from Earth. There had been theoretical talk about the redshift already, particularly from Belgian astronomer Georges Lemaitre (1894-1966), who interpreted the red-shift phenomena as indicating expansion of the universe (1927). When Hubble published (1929) his findings that the entire universe was uniformly expanding in every direction, his was the observational validation of a contemporary theory. The concept relation of redshift with distance, which Hubble did not interpret as Doppler shift, or velocity with distance—something not proved—was linked to him as Hubble's law and through the scientific writings of, among others, his English friends astronomers Arthur Eddington (1882-1944) and James Jeans (1877-1946).

Of course, the idea of an expanding universe led back to a fundamental question—how did it start? Eddington seemed to be the first (1932) to tackle the thought of some initial compressed cosmic state and to coin a later famous word for it: "As a scientist I simply do not believe that the present order of things started off with a bang...." He influenced Lemaitre, who advanced the idea of an unstable, exploding "primeval atom" (after 1945) that started what he called "The Big Bang," an idea still questioned. Hubble's calculations of the galaxies' initial point were not far enough back in time (today the accepted 15-20 billion years is being questioned as too much). Ultimately, the Shapely/Hubble measuring technique was inaccurate, as came to light when colleague Walter Baade (1893-1960) discovered Cepheids varied in brightness (1941-42) with implications carrying over with research using the new (1948) Hale 200-inch (508-cm) reflector at the Mt. Palomar Observatory. Hubble judged his variables dimmer than reality, resulting in a universe only half as big as that found by Bade. Still the galaxy/redshift phenomenon was valid, and some galaxies detected are moving away at near the speed of light.

The general acceptance of a curved, expanding universe was modified in 1948 by astronomer Fred Hoyle (1915-2001) and colleagues with a "steady-state" universe of uniform space and unchanging in time that has constant spontaneous creation of matter. The disposal of the concentration of matter and time requirements of a Big Bang theory attracted some astronomers, but the Big Bang came back with an intriguing hypothesis from physicist George Gamow (1904-1968) and associates in the 1940s that its initial concentration of matter would infer intense heat as well and, therefore, a low temperature blackbody heat should still survive. In 1965 the first traces of that, a 3°K blackbody temperature, was discovered pervading the background of space. And new telescopes and sensors on Earth and on space platforms continue to probe the universe for proofs of several models beyond the Big Bang origin and state of the universe.

WILLIAM J. MCPEAK