Before the 1920s, the prevailing view among astronomers was that the cosmos was eternal and unchanging. The observable universe then consisted of a single galaxy and only a few million stars.
Edwin Hubble’s measurements of redshift showed that distant galaxies are moving away from us, establishing Hubble’s Law and revealing that the universe is expanding uniformly. Redshift, a shift toward longer wavelengths, is a consequence of the Doppler effect.
Concurrently, Albert Einstein had just published his General Theory of Relativity, which describes a finite, homogeneous universe whose geometry is shaped by gravity. Together, Hubble’s observations and Einstein’s equations laid the foundation for modern cosmology.
Corroborating evidence—such as the measured abundance of light elements and the discovery of the cosmic microwave background (CMB)—supports a coherent picture that points back 13.7 billion years to a single event now called the Big Bang.
At that moment there was no distinct time or space; all matter was concentrated in a single, hot, and highly dense point—a singularity. During the first 10-43 seconds, known as the Planck epoch, the four fundamental forces (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force) were unified into one force.
Rapid expansion, faster than the speed of light, followed in the inflationary period, enlarging the universe from sub‑atomic dimensions to roughly the size of a golf ball almost instantaneously.
After inflation, the universe cooled, allowing sub‑atomic particles to form the first light nuclei. Three seconds after the Big Bang, nucleosynthesis produced the first elements. About 300 million years later, the first stars and galaxies emerged.
While the Big Bang remains the most robust model, other theories have been proposed:
These frameworks serve as intellectual stepping stones, guiding us toward a deeper understanding of the cosmos.
Explore additional resources and related articles to expand your knowledge of cosmology.
