The Early Universe
First minutes of universe expansion
The Big Bang marks the origin of time, space, matter, and energy. The universe began expanding from an initial state of extreme density and temperature, and everything that exists today traces back to those first moments.
During the first fraction of a second, the four fundamental forces we know today (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force) were unified into a single force. As the universe expanded and cooled, these forces separated one by one and became distinct.
The early universe was so hot and dense that it could produce elementary particles spontaneously:
- Quarks and antiquarks formed and annihilated each other, creating what's called a quark-gluon plasma, a kind of subatomic particle soup
- Leptons also formed, including electrons and neutrinos
Within the first few minutes, the universe cooled enough for quarks to bind together and form protons and neutrons (collectively called nucleons). This set the stage for the next critical step: building the first atomic nuclei.
Formation of primordial elements
Once protons and neutrons existed, they could start fusing together. This process, Big Bang nucleosynthesis, took place within roughly the first 20 minutes after the Big Bang. After that window closed, the universe had expanded and cooled too much for nuclear fusion to continue.
Only the lightest elements could form during this brief period:
- Hydrogen (1 proton) — by far the most abundant
- Deuterium (1 proton + 1 neutron) — a heavier form of hydrogen
- Helium-3 (2 protons + 1 neutron)
- Helium-4 (2 protons + 2 neutrons) — the second most abundant element produced
- Lithium-7 (3 protons + 4 neutrons) — only trace amounts
The exact relative abundances of these elements depended on conditions like temperature and density in the early universe. Nothing heavier than lithium-7 could form because the universe was expanding and cooling too rapidly for heavier nuclei to build up. Heavier elements would have to wait for stars to form billions of years later.
Evolution of universe composition
After nucleosynthesis ended, the universe was a hot plasma of hydrogen and helium nuclei mixed with free electrons. At this stage, the temperature was still too high for electrons to attach to nuclei and form neutral atoms. Because free electrons scatter photons, the universe was essentially opaque, like a thick fog that light couldn't pass through.
About 380,000 years after the Big Bang, the universe cooled to roughly 3,000 K. That was cool enough for electrons to combine with nuclei, forming the first neutral atoms. This event is called recombination (or decoupling). With free electrons no longer scattering photons, the universe became transparent to light for the first time. The photons released at that moment are still traveling through space today as the cosmic microwave background (CMB) radiation.
After recombination, the universe entered a period called the Dark Ages, when no stars or other visible light sources existed yet. During this time, matter slowly clumped together under gravity, forming the large-scale structures known as the cosmic web. Eventually, enough matter collected in dense regions to ignite the first stars and galaxies, beginning the era of cosmic dawn and reionization.
Cosmic Expansion and Early Universe Processes
- Inflation refers to a brief period of extremely rapid, exponential expansion in the universe's earliest moments (a tiny fraction of a second after the Big Bang). Inflation helps explain why the CMB looks so uniform across the sky: regions that are now far apart were once close enough to reach the same temperature before inflation stretched them apart.
- Baryogenesis is the process that produced a slight excess of matter over antimatter in the early universe. Without this matter-antimatter asymmetry, every particle would have annihilated with its antiparticle, leaving behind only radiation and no matter at all.
- Hubble's Law describes the relationship between a galaxy's distance from us and the speed at which it's moving away. More distant galaxies recede faster, which we observe through the redshift of their light (their light waves get stretched to longer, redder wavelengths). This pattern is one of the strongest pieces of evidence that the universe is expanding.