Approximately four thousand years ago, the Babylonians were skilled astronomers who could predict with remarkable accuracy the visible motions of the Moon, stars, planets, and the Sun across the sky. They were even able to calculate solar and lunar eclipses. However, the first civilization to interpret these observations in the form of a theoretical model was that of ancient Greece.
By the fourth century AD, the idea had emerged that the stars were embedded in a vast celestial sphere rotating around the Earth every 24 hours, while the planets, the Sun, and the Moon moved within a medium known as ether, located between the Earth and the stars. This model was further developed over the following centuries and reached its culmination in the second century with the magnificent geocentric system devised by Claudius Ptolemy.
According to Ptolemy, celestial motions had to be perfect, and perfection could only be expressed through circular motion. Therefore, stars and planets were described as heavenly bodies moving along circular paths. However, to account for the complex motion of the planets—particularly their apparent periodic backward motion—additional circular motions known as epicycles had to be introduced. In this way, planets were modeled as moving along intricate circular paths around a stationary Earth.
Despite its complexity, the Ptolemaic system described the apparent motions of the planets with great accuracy. For this reason, when Copernicus proposed the heliocentric system in the sixteenth century, his model did not appear observationally superior to that of Ptolemy. Copernicus suggested that the Earth rotates about its own axis and, together with the other planets, moves in a circular orbit around the Sun. Nevertheless, the available observational evidence of the time continued to favor the geocentric model.
There were also practical reasons why Copernicus’s theory was not readily accepted. One of the greatest astronomers of the sixteenth century, Tycho Brahe, understood that if the Earth moved around the Sun, the relative positions of the stars should change when observed from different points along Earth’s orbit. This phenomenon is known as parallax. However, at that time, there was no observational evidence of stellar parallax. This led to two possible conclusions: either the Earth was indeed stationary, or the stars were located at unimaginably vast distances.
Only in the early seventeenth century, with the invention of the telescope, did this issue begin to be resolved. Galileo Galilei discovered moons orbiting the planet Jupiter using a telescope. This discovery dealt a serious blow to the Earth-centered view of the universe. If moons could orbit another planet, why could planets not orbit the Sun?
At the same time, Tycho Brahe’s assistant Johannes Kepler found the key to establishing the heliocentric model. He demonstrated that planets move around the Sun not in perfect circles, but in ellipses. Later, Isaac Newton showed that this elliptical motion could be explained by his inverse-square law of gravitation.
The long absence of observable stellar parallax indicated that the stars were located at enormous distances from the Sun. The universe began to appear as an infinite sea of stars. With the aid of his telescope, Galileo was able to resolve thousands of stars invisible to the naked eye. Newton, in turn, proposed that the universe was an infinite and eternal system of stars, each resembling our own Sun.
In the nineteenth century, the astronomer and mathematician Friedrich Bessel finally succeeded in measuring stellar distances using the method of parallax. It was determined that the nearest star (other than the Sun) lies at a distance of approximately 25 trillion miles from the Sun (by comparison, the Sun is only 93 million miles from Earth).
Most of the stars visible in the night sky are located within a bright band known as the Milky Way. Immanuel Kant and other thinkers suggested that the Milky Way was in fact an “island universe,” or a galaxy, and that other galaxies must exist beyond it.
Astronomers also observed faint, diffuse objects in the night sky known as nebulae. Some suspected that these objects might be distant galaxies. This hypothesis was finally confirmed in the 1920s by the American astronomer Edwin Hubble, who demonstrated that some of these nebulae were indeed independent galaxies comparable in size to the Milky Way.
Hubble made an even more remarkable discovery: galaxies are moving away from us, and the speed of this recession is proportional to their distance. This observation found a natural explanation within Albert Einstein’s General Theory of Relativity—the universe is expanding.
In fact, Einstein could have predicted the expansion of the universe as early as 1915, when he first formulated his theory. However, he introduced an additional term into his equations—the cosmological constant—to counterbalance gravity and maintain a static universe. After the discovery that the universe is indeed expanding, Einstein reportedly described the introduction of the cosmological constant as “the greatest mistake of my life.”
In 1917, the Russian mathematician and meteorologist Alexander Friedmann showed that Einstein’s equations allow solutions describing an expanding universe. According to this model, the universe originated approximately ten billion years ago, and galaxies have been moving apart ever since. The English astronomer Fred Hoyle mockingly referred to this model as the “Big Bang,” and the name ultimately became established in scientific literature.
As an alternative to the Big Bang model, Bondi, Gold, and Hoyle proposed the Steady State theory. According to this theory, the universe expands but remains unchanged over time, requiring the continuous creation of matter to maintain a constant density.
This debate was decisively settled in 1965, when Arno Penzias and Robert Wilson discovered the cosmic microwave background radiation. This radiation was interpreted as the weakened thermal remnant of the early stages of the Big Bang and dealt a decisive blow to the Steady State theory.
Subsequent calculations demonstrated that the relative abundances of hydrogen and helium produced during the Big Bang are in excellent agreement with observations. The predicted abundances of other light elements were also found to match observed values.
From the 1970s onward, the Hot Big Bang model became widely accepted in cosmology. Today, scientists are probing even deeper, investigating how galaxies formed, what the universe is made of, the role of dark matter and black holes, the geometric structure of the universe, and the true nature of the cosmological constant.
Cosmic microwave background radiation plays a central role in these investigations. In 1992, NASA’s COBE satellite detected the first temperature anisotropies in this background radiation. These tiny fluctuations were interpreted as the primordial “seeds” from which galaxies later formed.
Since the 1980s, interest in the physics of the early universe has grown rapidly. The Hubble Space Telescope and other modern satellite missions continue to provide increasingly precise images of the universe, turning cosmology into one of the most dynamic fields of modern physics.