1.3 Matter in the Universe
The final line of evidence for the hot big bang is the observed relative abundances of the light elements. Calculations of the conditions in the early Universe predict that the Universe should contain about 75% hydrogen, 25% helium-4, about 0.01% deuterium and helium-3, and trace amounts of lithium. This is indeed what is observed in the Universe at large. Further helium and other elements, such as carbon, oxygen and others are made in the cores of stars during stellar nucleosynthesis, then dispersed into the wider Universe when stars die.
The material in the Universe that we can actually see, or detect directly by virtue of the electromagnetic radiation that it emits or absorbs, comprises components such as galaxies, stars, planets, gas and dust. These visible components are composed of so-called baryonic matter, built from the familiar atoms, such as hydrogen and helium, which are composed of protons, neutrons and electrons. However, baryonic matter comprises only around 5% of the total mass-energy density of the Universe implied by the critical density; we can characterise it by the baryonic matter density parameter, .
Non-baryonic matter is referred to as dark matter, and it is not composed of these familiar constituents. Although dark matter does not interact with electromagnetic radiation, it does possess mass, so it interacts via the force of gravity, and appears to comprise a further 25% of the total mass-energy density of the Universe. It may be characterised by the dark matter density parameter, . The combined matter density parameter may be written as .
The astute reader will have noted that the 5% of baryonic matter and the 25% of non-baryonic dark matter still leaves a large amount of the Universe’s critical density unaccounted for, if the geometry of the Universe really is flat. Dark energy is the name given to this missing component of the Universe’s critical density budget. It accounts for around 70% of the current total mass-energy density of the Universe and may be characterised in terms of the cosmological constant, represented by the symbol . This plays a key role in the evolution of the Universe, as you will see in Section 4 of this course. The main observational evidence for dark energy is the observed accelerating expansion of the Universe referred to earlier. The dark energy density parameter associated with the cosmological constant drives this acceleration, and we may write .
As Figure 5 shows, the proportions of baryonic matter, dark matter and dark energy in the Universe were quite different soon after the big bang at the time when the Cosmic Microwave Background was produced. Although neutrinos and photons are still present in vast numbers in the Universe today, they contribute a negligible amount to the overall matter-energy density of the Universe because their densities have become significantly diluted as the Universe has expanded. In contrast, the contribution of dark energy was initially negligible, but today is the dominant component of the Universe: unlike any form of matter or radiation, it does not become more dilute as space expands.