5 Foundations of Cosmology
Explore how cosmology developed and how observations of galaxies, radiation, supernovae, and matter reveal the universe’s structure and changing history.
What studies
connects observations of stars and galaxies with physical theories about gravity, matter, space, and time. It describes the universe on the largest scales, including its contents, structure, history, and evolution. Modern describes a universe that has changed over time and continues to expand.
From static models to an expanding universe
Many cultures developed accounts of the sky and the universe. Influential ancient Greek models placed a stationary Earth at the center of nested celestial spheres. Later observations and mathematical models shifted Earth from the center of the planetary system; the heliocentric model is notably associated with Nicolaus Copernicus.
In the early twentieth century, Albert Einstein’s general theory of relativity provided a new theory of gravity and a way to describe the universe on the largest scales. Its equations allowed for a changing universe, although Einstein initially favored a static one. In the 1920s, Alexander Friedmann and Georges Lemaître developed expanding-universe models.
Astronomers also found that some fuzzy objects called “spiral nebulae” were separate galaxies far beyond the Milky Way. Edwin Hubble’s distance measurements helped establish this in 1923. In 1929, he presented evidence that more distant galaxies generally recede faster than nearer ones. These findings overturned the idea of a static, Milky-Way-sized cosmos.
The Big Bang and cosmic expansion
The describes the as having been much hotter and denser in the past, followed by expansion and cooling over time. It does not describe matter exploding from one point into pre-existing empty space: it describes the expansion of space itself from an early hot, dense state.
Several independent observations support the model, including cosmic expansion, the cosmic microwave background, and the observed amounts of light elements.
The and its contents
The is the region from which light or other signals have had time to reach us since the early universe became transparent. It is not necessarily the entire universe, and what lies beyond the observable region is unknown. Because light takes time to travel, distant objects are seen as they were in the past; looking farther away also means looking farther back in cosmic history.
The universe is about 13.8 billion years old, while the is about 93 billion light-years across. This is possible because space expanded while the light was traveling: regions that emitted the oldest light are now much farther away than light could travel in a static universe. A measures distance, not time.
Galaxies are large systems of stars, gas, and dust bound by gravity and embedded in . Stars form from gas clouds and produce light and elements through nuclear processes. Some massive stars end their lives as black holes, regions where gravity is so strong that not even light can escape from within the event horizon. These objects are important parts of cosmic structure, but expansion is most clearly measured across very large distances between galaxies.
Redshifts reveal expansion
A galaxy’s spectrum contains recognizable lines associated with particular atoms. In distant galaxies, these lines are often shifted toward longer, redder wavelengths. This occurs because the expansion of space stretches light as it travels. Comparing galaxy distances with redshifts reveals that, on large scales, more distant galaxies generally recede faster. This relationship is described by the .
Expansion does not mean that every galaxy moves away from every other galaxy in the same way. Nearby galaxies can move toward one another under gravity, and systems such as stars, galaxies, and galaxy groups can remain gravitationally bound while the universe expands on larger scales. Nor does expansion have a single center in space: observers in distant galaxies would also see the large-scale universe expanding around them.
The cosmic microwave background
The is faint radiation that fills space in every direction. It is the cooled remnant of light from the early universe, released when the universe had expanded and cooled enough for atoms to form and light to travel freely.
The CMB’s nearly uniform temperature and tiny variations support the picture of an early hot, dense universe. Those variations also provide clues about how later structures formed.
Supernovae and accelerated expansion
Astronomers compare the measured brightness and redshift of distant Type Ia supernovae with those of nearer ones to trace how the expansion rate changed over time. Observations in the late 1990s showed that the universe’s expansion is accelerating today, rather than continuing to slow under gravity alone.
is the term for the unknown cause of this acceleration. It names an observed effect, not a complete explanation of its physical nature.
and cosmic structure
Galaxies and galaxy clusters behave as if they contain more matter than can be accounted for by stars, gas, and dust alone. Evidence for this unseen component includes the motions of stars and galaxies, as well as —the bending of light from more distant objects by gravity. This unseen component is called , and its gravitational influence helps explain how galaxies and larger structures formed.
and are different ideas. acts gravitationally like matter, while is associated with the accelerated expansion of the universe.
A changing cosmic picture
Together, galaxy redshifts, the CMB, supernova observations, and evidence for form a broad account of cosmic history. The universe expanded from an earlier hot, dense state, cooled, and developed galaxies and stars from small early variations in matter. Its expansion has changed over time and is accelerating in the present era.
Scientists continue to test this picture by measuring cosmic distances, mapping matter, and observing galaxies at different stages of their history.