10 Cosmology
Follow how the universe expanded from a hot, dense early state, how astronomers study its history, and what may shape its future.
Expansion and cosmic history
Cosmology studies the universe as a whole: its origin, large-scale structure, evolution, and possible future. Modern cosmology combines observations, such as galaxy redshifts and the cosmic microwave background, with physical theories, especially general relativity.
On very large scales, the average distance between faraway, unbound galaxies increases with time. This does not mean galaxies are moving outward from a central point into empty space; rather, space itself expands. Gravitationally bound systems, including the Solar System and the Milky Way, do not expand along with the universe.
Astronomers use to observe expansion. As light travels through expanding space, its wavelength stretches, shifting spectral lines toward the red end of the spectrum. More distant galaxies generally have greater redshifts. Because light takes time to reach us, observing more distant objects also means seeing them as they were further in the past.
Takeaway: Expansion describes the changing distances between unbound galaxies as space grows, and distant light reveals earlier stages of cosmic history.
The Big Bang and early expansion
The describes the observable universe’s evolution from an extremely hot, dense early state about 13.8 billion years ago. The expansion occurred throughout space; it was not an explosion at one location. The model explains several independent observations, including cosmic expansion, the abundance of light elements, and the cosmic microwave background. It describes early development but does not establish what, if anything, preceded that state or what caused it.
Many versions of the model include , a brief period of extraordinarily rapid expansion early in cosmic history. may help explain the universe’s large-scale uniformity and the small variations that later grew into cosmic structure. Its physical cause remains unknown.
Takeaway: The explains the universe’s development from an early hot, dense state, while important questions about the earliest moments remain open.
The first light and cosmic structure
For its first several hundred thousand years, the universe was a hot plasma. Photons repeatedly scattered off free electrons, so light could not travel far. About 380,000 years after the Big Bang, the universe cooled enough for electrons and protons to form neutral atoms. Light could then travel freely through space.
That ancient light is the . Today it appears as a nearly uniform microwave glow at about 2.7 kelvin. As the universe expanded, the light’s original wavelengths stretched.
The CMB is not perfectly uniform. Tiny temperature differences correspond to slight density differences in the young universe. Gravity amplified these variations over time, helping matter gather into stars, galaxies, and larger structures. The CMB therefore offers a snapshot of the early universe and clues about its contents and evolution.
Takeaway: The CMB connects the universe’s early conditions to the later growth of cosmic structure.
From primordial elements to galaxies
The universe’s history includes several major transitions:
First minutes: As the universe expanded and cooled, nuclear reactions produced most primordial hydrogen and helium, along with small amounts of other light elements.
About 380,000 years: Atoms formed, the universe became transparent, and the CMB began traveling through space.
Hundreds of millions of years onward: The first stars and galaxies formed. Their light gradually changed the surrounding gas, while gravity assembled larger structures.
Recent billions of years: Expansion, which had been slowing under gravity, began to accelerate. Observations of distant supernovae helped reveal this acceleration.
These stages link the early universe’s conditions to the formation of the structures observed today.
and the universe’s possible future
is the name given to whatever drives the universe’s accelerated expansion. Its nature is not known. One leading explanation is the , a constant energy density associated with space itself. Other possibilities propose that changes over time or that our theory of gravity needs revision.
If remains nearly constant, the universe is expected to keep expanding. Over immense timescales, galaxies beyond our gravitationally bound neighborhood would become increasingly distant, and the universe would grow colder and more dilute. This projected future is often called the or heat death.
The is not certain. The universe’s ultimate fate depends on the still-unknown nature and long-term behavior of . Some speculative models allow for future collapse or more extreme expansion, but current observations do not establish that either will occur.
Takeaway: Accelerated expansion is observed, but its cause and the universe’s ultimate fate remain unresolved.