In England, also in 1948, an alternative theory emerged called the steady-state universe. Different versions of it were proposed by English mathematician and astronomer Fred Hoyle and by the team of British mathematician and cosmologist Hermann Bondi and British astronomer Thomas Gold, but the key idea was that although the universe was expanding, its average properties did not change with time. As the universe expanded, the density of matter would be expected to diminish, but new hydrogen atoms were created that formed clouds of gas that condensed into new stars and galaxies. The number of new hydrogen atoms required per year was so tiny that one could not hope to observe this process directly. However, there were predictable observational consequences that should allow one to distinguish between a steady-state universe or a big-bang universe. (The term big bang was coined by Hoyle as a mildly pejorative characterization of the rival theory in a radio talk in 1949.)

For example, in a big-bang universe, when one looks at galaxies that are far away, one also sees them as they were in the remote past (because of the travel time of the light). Thus, one might expect that distant galaxies are less-evolved or that they contain more young stars. But in a steady-state universe, one would see galaxies at all possible stages of evolutionary development at even the farthest distances. The density of galaxies in space should also diminish with time in a big-bang universe. Therefore, galaxies at great distances should be more densely crowded together than nearby galaxies are. But in a steady-state universe, the average density of galaxies should be about the same everywhere and at every time. In the 1950s the Cambridge radio astronomer Martin Ryle showed that there were more radio galaxies at great distances than there were nearby, thus showing that the universe had evolved over time, a result that could not be explained in steady-state theory.

The discovery of quasars (quasi-stellar radio sources) in the early 1960s also told heavily against the steady-state theory. Quasars were first identified as strong radio sources that in visible light appear to be identified with small starlike objects. Further, they have large redshifts, which implies that they are very far away. From their distance and their apparent luminosity, it was inferred that they emit copious amounts of energy; a single quasar might be brighter than a whole galaxy. There was no room for such objects in a steady-state universe, in which the contents of any region of space (seen as it is now or as it was long ago) should be roughly similar. The quasars were a clear sign that the universe was evolving.

Steady-state theory never had a large following, and its supporters were centred in Britain. Nevertheless, having a competing theory forced the big-bang cosmologists to strengthen their arguments and to collect supporting data. A key question centred on the abundances and origins of the chemical elements. In steady-state theory, it was essential that all the elements could be synthesized in stars. By contrast, in the aßγ paper, Alpher and Gamow tried to show that all the elements could be made in the big bang. Of course, in a more reasonable view, big-bang theorists had to accept that some element formation does take place in stars, but they were keen to show that the stars could not account for all of it. In particular, the stars could not be the source of most of the light elements. For example, it was impossible to see how during the lifetime of a galaxy the stars could build up the helium content to 30 percent.

One obstacle for big-bang theory was the absence of any stable isotopes at atomic mass 5 or 8. In 1952 Austrian-born American astrophysicist Edwin Salpeter proposed that three alpha particles (helium nuclei) can come together to produce carbon-12 and that this happens often enough to resolve the mass-gap problem in the interiors of stars. However, conditions in the early universe were not right for bridging the mass gap in this way, so the mass-gap problem was seen as favouring steady-state theory. Hoyle adopted Salpeter’s proposal in 1953. In 1957 Hoyle, with American astronomers William Fowler, Margaret Burbidge, and Geoffrey Burbidge (or B²FH, as their paper was later called), gave an impressive and detailed account of the abundances of most elements in terms of conditions appropriate to stellar interiors. Although the B²FH paper was not explicitly a steady-state theory, it was often seen as favouring that model, as it had not made use of temperature and pressure conditions appropriate to the big bang. But in papers of 1964 (with English astrophysicist Roger Tayler) and 1967 (with Fowler and American physicist Robert Wagoner), Hoyle concluded that the lighter elements could be built up satisfactorily only in conditions like those of the big bang. Hoyle himself continued to favour supermassive objects as the origin of the elements over the big bang, but most astronomers saw this work as vindicating big-bang theory. In defending a failed cosmological theory, Hoyle had done an enormous amount of good work of lasting value on nucleosynthesis.

When good estimates of the cosmic abundance of deuterium and other light elements became available, big-bang theory proved capable of detailed explanation of the cosmic abundances of all the light elements. In current scenarios, hydrogen (H) and its heavier isotope, deuterium (²H), most of the two helium isotopes (³He and ⁴He), and lithium (⁷Li) were produced shortly after the big bang. Given whatever one assumes about the present-day density of matter in the universe, one can calculate what sort of cosmic abundances should have resulted from the big bang. It is regarded as a triumph of the big-bang model that the present-day abundances of these elements can all be explained from one set of initial conditions. According to current thinking, most of the heavier elements were then built up in stars, neutron star mergers, and supernova explosions.

The cosmic microwave background proves the theory

In 1965 American astronomers Arno Penzias and Robert W. Wilson were working at Bell Laboratories on a 6-metre (20-foot) horn antenna. The original purpose of the antenna was to detect reflected signals from high-altitude balloons, with the goal of applying the technology to communications satellites, but Penzias and Wilson had adapted it for doing radio astronomy. They detected a constant, persistent signal, corresponding to an excess temperature of 3.3 K (−269.9 °C [−453.7 °F]). After eliminating every source of circuit noise they could think of, and even shooing a pair of pigeons that had been roosting (and leaving behind “white dielectric material”) in the horn, they found that the signal remained and that it was constant, no matter in which direction the telescope was pointed. At nearby Princeton University, they consulted with American physicist Robert Dicke, who was studying oscillatory models of the universe with hot phases and who was therefore not surprised by what they had found. About the same time, astrophysicist James Peebles, Dicke’s former student, also published a paper predicting the existence of a universal background radiation at a temperature of 10 K (−263 °C [−441 °F]), apparently completely unaware of Alpher and Herman’s earlier prediction. Suddenly the pieces fell together. The cosmic microwave background (CMB) was accepted as the third major piece of evidence in support of the big-bang theory. In the early stages of the expansion, when atoms were all still completely ionized, the universe was opaque to electromagnetic radiation. But when the universe cooled enough to allow the formation of neutral atoms, it suddenly became transparent to electromagnetic radiation (just as light can travel through air). At this “decoupling time,” the electromagnetic radiation was of very high energy and very short wavelengths. With the continued expansion of space, wavelengths were stretched until they reached their current microwave lengths (from about a millimetre to tens of centimetres in wavelength). Thus, every bit of empty space acts as a source of radio waves—a phenomenon predicted (twice!) by big-bang theory but for which steady-state theory had no ready explanation. For most cosmologists, this marked the end of the steady-state theory, even though Hoyle and his collaborators continued to tweak and adjust the theory to try to meet objections.

By the mid-1960s, big-bang theory had become the standard cosmology, underpinned by the observed expansion, the measured abundances of the light elements, and the presence of the cosmic microwave background. Of course, the theory was eventually to acquire many different forms and refinements.

Dark matter

Over the course of the 20th century, it became clear that there is much more to the universe than meets the eye. On the basis of early estimates of the mass density of the Milky Way, English physicist and mathematician James Jeans suggested in 1922 that the galaxy might contain three times as many dark stars as visible ones. In 1933 Fritz Zwicky, by studying the dynamics of clusters of galaxies, concluded that there is not enough visible matter in the galaxies to hold the clusters together gravitationally. He also pointed out that the measured quantity of luminous matter was far below the value that would be necessary for critical density—i.e., to produce a universe with an expansion that would gradually slow to a halt at infinity—but he speculated that the dark matter could conceivably be enough to make up the difference.

Jeans’s and Zwicky’s comments did not attract a lot of attention, and dark matter became a central issue only in the 1970s. In 1974 Peebles, Jeremiah Ostriker, and Amos Yahil in the United States and Jaan Einasto, Ants Kaasik, and Enn Saar in Soviet Estonia concluded, on the basis of studies of galactic dynamics, that 90–95 percent of the universe must be in the form of dark matter. American astronomer Vera Rubin published a paper in 1978 studying the rotational velocities of stars in galaxies as a function of their distances from the galactic centre. Rotational velocities were found to be nearly constant over a fairly large radial distance, though predictions based on the distribution of visible matter implied that they would decrease with distance. Rubin’s discoveries were interpreted as evidence for the presence of substantial amounts of dark matter in the haloes around galaxies. About the same time, radio astronomers, using a spectral line of hydrogen at 21-cm wavelength, obtained a similar result in the outer parts of galaxies where there is little starlight. Present-day thinking is that the universe is very close to flat (Euclidean) in its geometry, which implies that it is close to critical density. However, the nucleosynthesis calculations show agreement with the present-day abundances of the light elements only if one supposes that ordinary baryonic matter (i.e., matter made of protons and neutrons) accounts for no more than about 5 percent of the critical density.

Candidates for dark matter in the form of ordinary baryonic matter include black holes, Jupiter-sized planets, and brown dwarfs (starlike objects that are too small to ignite nuclear reactions in their interiors). Some of the new grand unified theories (GUTs) of particle physics predict the existence of large quantities of exotic fundamental particles, called weakly interacting massive particles (WIMPs). The 1998 discovery that neutrinos have mass (they had been considered perfectly massless since Austrian-born physicist Wolfgang Pauli’s prediction of them in 1930) provides a small part of the answer. But the nature of the bulk of dark matter is still unknown.