The Cosmic Microwave Background and the Light Elements

Measuring microwaves in one direction is not enough to identify a cosmic background. Astronomers compare radiation across wavelengths and across the sky. These comparisons test different properties of a proposed source.

A spectrum records how radiation intensity changes with wavelength. The CMB closely follows the blackbody shape for a temperature near 2.7 K. That shape supports an early state in which energy-exchanging interactions established thermal equilibrium. Ordinary scattered starlight does not acquire this shape merely because its path changes.

A sky map compares different directions. The CMB is nearly uniform, with small temperature differences. Foreground radiation and the effect of our motion must be accounted for before those differences are interpreted as information about the early universe.

The temperature pattern reflects several effects, including density differences, matter motion and gravity. It is not a direct density map. Nor does each coloured patch identify one future galaxy.

The spectrum and map therefore serve different purposes. The spectrum tests the radiation's thermal history. The small spatial variations constrain early conditions connected to the later growth of structure. A proposed explanation must account for both kinds of measurement.

A nuclear calculation predicts amounts, not just a list of elements. Scientists use early temperature, density and expansion conditions to calculate which light nuclei form and how much survives when reactions become too slow.

High temperature and density allowed reactions during the first minutes. Expansion then cooled and diluted matter. The reduced density made encounters less frequent, limiting further reactions. Under standard early conditions, the calculation leaves most ordinary matter as hydrogen and about one quarter of its mass as helium.

The quarter refers to mass. It does not mean that one atom in every four is helium. It also excludes dark matter and dark energy from the denominator.

Stars later made additional helium and heavier elements. To estimate the primordial helium share, astronomers study gas with few heavier elements and account for later enrichment. Agreement with the early calculation supports production before stars existed.

Deuterium is hydrogen with one proton and one neutron in its nucleus. Stellar interiors readily consume it. Its abundance in gas with little stellar processing can therefore test early nuclear reactions. Early density affects how much deuterium is converted into helium before reactions stop.

Lithium also enters these calculations. Detecting lithium does not by itself establish agreement with a predicted abundance. Each abundance requires its own comparison.

The preceding lesson distinguished the Hubble time, 1/H0, from the universe's age. H0 describes present expansion. An age calculation needs the expansion history as well.

A model follows how matter, radiation and dark energy affect expansion over time. Observations constrain the model's properties. Within the standard cosmological model, those comparisons give an age near 13.8 billion years.

The CMB contributes information through its temperature pattern. Different assumptions about composition, geometry and expansion history produce different predicted patterns. Comparing these predictions with observations constrains the properties used in the age calculation. The radiation does not directly display a cosmic age.

Old star clusters provide a different check. The universe must be at least as old as the stars it contains. Stellar ages can therefore test whether a proposed cosmic age is consistent with independently studied objects.

A reported uncertainty belongs to a particular model and set of measurements. It should not be treated as a limit covering every possible expansion history. Several observations can support the same model while remaining subject to measurement uncertainty and assumptions.

Two early events are easy to confuse because both involve matter becoming bound together. They occurred at very different times and involved different particles.

During the first minutes, nuclear reactions combined protons and neutrons into light nuclei. A helium nucleus could exist without having electrons bound around it. Matter was still too hot for neutral hydrogen atoms to remain common.

Much later, around 380,000 years after the Big Bang, cooling allowed electrons to remain bound to nuclei. Neutral atoms became common. With far fewer free electrons, radiation underwent much less scattering and could travel long distances. This transition did not create all the background photons.

Expansion continued after that transition. It lengthened the photons' wavelengths and reduced their energies. Radiation near 3000 K at neutral-atom formation is observed today as a thermal background near 2.7 K.

The two events leave different evidence. Light-element abundances test early nuclear reactions. The background spectrum tests an early thermal state, while its temperature pattern contains information about early matter and gravity.

These changes occurred throughout the expanding universe. They were not successive explosions at one site in otherwise empty space.