Reading 1

In 1912 the American astronomer Vesto Slipher recorded spectra of several faint spiral nebulae and noticed something peculiar: in nearly every case the dark absorption lines were displaced toward longer wavelengths than the same lines produced in a laboratory. A decade later Edwin Hubble resolved individual Cepheid variable stars inside those nebulae. Cepheids pulse in brightness on a schedule tightly correlated with their intrinsic luminosity, so measuring a Cepheid's period reveals how much light it actually emits. Comparing that intrinsic luminosity with the star's apparent brightness yields a distance. Hubble's measurements established that the spirals were not clouds within the Milky Way at all, but independent galaxies at enormous remove.

When Hubble plotted recessional velocity against distance in 1929, the points scattered around a straight line passing near the origin. The relationship is written v = H₀d, where v is recessional velocity, d is distance, and H₀ is the Hubble constant, currently estimated near 70 kilometers per second per megaparsec. Proportionality of this kind is the mathematical signature of uniform expansion.

Modern cosmology does not interpret the wavelength displacement as ordinary Doppler motion through a fixed, static space. Instead the phenomenon is cosmological redshift: as a photon crosses intergalactic space, the space through which it travels expands, and the photon's wavelength is stretched by the same factor. Astronomers quantify the stretch as z = (λ_observed − λ_rest) / λ_rest. Because the entire spectral pattern of a given element shifts together while the spacing between lines is preserved, the identity of the element remains unmistakable even when every line has moved.

Two consequences follow directly. First, no galaxy occupies a privileged center; an observer stationed in any galaxy would record the same proportional recession of everything else. Second, running the relationship backward in time compresses all matter and radiation into a state of extraordinary density and temperature roughly 13.8 billion years ago. Observations of Type Ia supernovae in the late 1990s added a refinement that surprised nearly everyone: the expansion is not merely continuing, it is accelerating.

Compare the wavelength values described above with the Electromagnetic Spectrum and Emission Spectra of Some Elements charts in your 2024 Edition Reference Tables for Earth and Space Sciences.

Reading 2

For roughly 380,000 years after the Big Bang the universe was an opaque plasma. Free electrons scattered photons so efficiently that light could not travel any appreciable distance before being deflected; the cosmos glowed but was not transparent. As expansion cooled the plasma to about 3000 K, electrons and protons combined into neutral hydrogen atoms — an event cosmologists call recombination. Scattering ceased almost immediately, and the photons released at that moment have traveled essentially unimpeded ever since. Expansion has since stretched their wavelengths by a factor of roughly 1100, cooling that radiation to 2.725 K and displacing it out of the visible band and into the microwave region of the electromagnetic spectrum.

Arno Penzias and Robert Wilson detected this cosmic microwave background (CMB) in 1965 as a persistent hiss in a horn antenna that they could not eliminate by any adjustment to the equipment. The signal arrived with equal intensity from every direction in the sky, which no local source could explain. Later satellite missions — COBE, WMAP, and Planck — mapped the background in fine detail and confirmed that its spectrum matches the theoretical curve of a nearly perfect thermal emitter.

The CMB is not perfectly smooth. Its temperature varies from place to place by roughly one part in 100,000. These anisotropies record slight density differences present when the universe became transparent. Regions of marginally higher density exerted marginally stronger gravitational attraction, drew in additional matter, and became the seeds from which galaxies and galaxy clusters later assembled. The observed pattern of hot and cold patches matches the pattern predicted by Big Bang models to remarkable precision.

A third and entirely independent line of evidence comes from the first twenty minutes. Between roughly one second and twenty minutes after the Big Bang, temperatures were high enough for protons and neutrons to fuse. Big Bang nucleosynthesis predicts approximately 75% hydrogen and 25% helium-4 by mass, together with trace amounts of deuterium, helium-3, and lithium-7. Fusion then stopped, not because the raw material ran out, but because continued expansion dropped the temperature below the threshold required to overcome electrostatic repulsion. Measurements of the oldest and least chemically processed gas clouds match those predicted abundances closely. Every element heavier than lithium was manufactured much later, inside stars and in the explosions that end their lives.