Great Discoveries in Science: The Big Bang Theory

In Maori mythology, Rangi (sky) and Papa (Earth) were intertwined until their son Tane, god of the forest, forced them apart.

T he science of the universe and how it has developed over time is called cosmology. Throughout history, humans have asked cosmological questions. How did Earth form? How did the universe form? What forces have shaped the universe over time? In the early days of human civilization, in lieu of science and technology, different civilizations created their own answers to those questions.

At the beginning of human history, observational data was limited to a group’s local environment and what they could see of the sky with the naked eye. From these observations, cultures created mythological explanations for how Earth, the sky, and humans came to be. Looking back, these mythologies can seem entertaining but implausible. However, in early human history, science and mathematics were rudimentary and technology was limited.

As the role of science and mathematics grew in human culture, the study of the universe became increasingly scientific and cosmological. Over the last few centuries, scientists have relied more and more on an observation-based scientific method, documenting observable phenomena and coming up with testable hypotheses for those phenomena.

“Before there was any light there was only darkness, all was night. Before there was even darkness there was nothing.”

These sentences, taken from a myth from the Maori people in New Zealand, describe the very beginning of the universe.

The universe grew and produced Rangi (heaven) and Papa (Earth), a husband and wife pair intertwined in a close embrace. Rangi and Papa had many sons who lived in the darkness between their parents’ bodies and schemed to kill their parents. One son, Tawhiri (the god of wind and storm), advocated for separation instead and so Tane-mahuta (the god of the forest) pressed Rangi and Papa—sky and Earth—apart.

In the Christian creation story, there is a similar void until the creator, God, says, “Let there be light.” God continues to create the sky, waters, and Earth, followed by plants, celestial objects, animals, and eventually the first man and woman.

In Shinto, the indigenous religion in Japan, heaven and Earth separated out of chaos. Earth was initially formless until the brother and sister gods Izanagi and Izanami appeared and were tasked with creating structure. Together, the pair created the islands that formed the Japanese archipelago. The pair also gave birth to numerous other deities, including the god of fire, the sun goddess, the moon god, and the storm god.

Images of primordial chaos, darkness, and light permeate mythologies across the world, from the Maori mythologies to the biblical book of Genesis to Shinto and beyond. These creation myths (and many others) put forth lineages of gods and humans and other worldly creatures that give shape to Earth until it looks like the land we know today, filled with humans, plants, and animals.

These examples are just a few among a vast array of mythologies that have shaped cultures across the world.

In the academic definition of the word, a mythology is a powerful narrative that is considered to be of great importance, is recited at special occasions, and explains how and why the world has come to be as it is today. Mythologies contain meaning and truth for the groups of people that recite and maintain them, but they may not have meaning or truth for other groups of people.

Therein lies the difference between mythologies and science. Modern science seeks to find theories and laws that hold true, no matter who performs the experiment. Science seeks objective explanations for natural phenomena rather than subjective narratives.

The scientific method as we know it wouldn’t emerge as a dominant method until the Scientific Revolution. In the meantime, the ancient Greeks laid the foundation for much of Western science, philosophy, and cosmology.

The ancient Greek philosophers, scientists, and mathematicians were extremely influential in the Western world for millennia after their time.

In 387 BCE, Greek philosopher Plato founded the first institution of higher learning in the Western world. Plato and other Greek philosophers emphasized an intellectual approach to cosmology and other sciences that used evidence, reason, and debate to come to conclusions about the universe.

Yet Plato believed that knowledge was best gained from reasoning, not through observation. Plato divided the universe into two categories: the physical, visible world and the world of “Forms.” According to Plato, the Forms, though nonphysical, are the most accurate reality. He believed we can gain knowledge from them through deductive reasoning. The material world, Plato believed, is imperfect and therefore not a good source of knowledge.

Ancient Chronologies

Many ancient cultures observed the cyclical nature of things, from human life and death to the return of each season to the phases of the moon. Influenced by these rhythms, they created cyclical chronologies of the great cosmos.

The ancient Mayans, for example, as well as the Aztecs and Incas, believed in eternal cycles of life, death, and rebirth. They believed that the smaller cycles, such as the phases of the moon, were part of a larger cycle of the universe. The ancient Babylonians believed in a cosmic cycle made of 600 saros that were 3,600 years each, for a total of 2,160,000 years before the world recreated itself.

Similarly, in the writing Huang Chi Ching Shih by Shao K’ang-chieh in the Sung dynasty (960–1279 CE), Shao describes a cosmic timeline called the yuan that consists of twelve periods (hui) of 10,800 years each. Over the course of the cosmic cycle, the world improves and then declines. After a full yuan, 129,600 years in total, heaven and Earth end and are born again. The cycle repeats into eternity.

There was no way to determine if these large, repeating cycles were accurate, but they served at least one purpose: death was not so final if all was born again, and again, and again into eternity.

This artistic rendering, while more of a zodiac than a Ptolemaic model, is a beautifully crafted Earth-centered system.

Plato’s student Aristotle disagreed with this element of Plato’s approach and placed more emphasis on observation of the natural world. Aristotle created a model of the universe with Earth fixed at the center and the planets, sun, and stars moving around it in nested crystalline spheres.

This model was added to and refined by Ptolemy, an influential astronomer, mathematician, and geographer of Greek descent born in Egypt.

In 150 CE, Ptolemy published a new geocentric model of the universe that attempted to make Aristotle’s perfectly spherical model fit with the observed irregular motion of celestial bodies. Ptolemy added several mechanisms including an epicycle, which was a smaller orbit each planet makes in the course of orbiting Earth. Though incorrect, the Ptolemaic model of the universe predicted celestial motion relatively accurately and was used for over a millennium by European and Islamic astronomers.

In retrospect, the ancient Greeks were far from accurate in their cosmologies and models of the universe. However, their work was vastly different from ancient mythologies. The Greeks had their gods, but in their sciences they emphasized rational thought and mathematics.

The Greeks were limited by their desire to find perfection and mathematical harmony in the universe, but they moved the cosmological discussion from mythology to mathematics, nature, and rationalism. They believed that nature is knowable and understandable rather than under the control of supernatural beings.

After the contributions of the ancient Greeks, science in Europe moved forward relatively slowly, especially during the Middle Ages.

The Middle Ages in Europe began with the fall of the Roman Empire in the fifth century CE and lasted until somewhere between the thirteenth and fifteenth centuries, depending on the region of Europe.

The Middle Ages were characterized by a general decline in quality of life after the fall of the Roman civilization. The Roman Catholic Church was the major social and political force, and it dominated the intellectual sphere.

From the eighth to the fourteenth centuries, many of the advancements in astronomy and mathematics took place in the Islamic world, centered around an academy called the House of Wisdom in Baghdad. There, the motivation for better astronomy and the required geometry came from the desire to accurately observe Islamic holy days, which follow a lunar calendar.

In about the twelfth century, conditions began to change in Europe. Travel and communication became faster and safer. City-states emerged in Italy; national monarchies formed in France, England, and Spain; and secular (nonreligious) education became more common. Agricultural developments made it possible for lower classes to enjoy a balanced diet, and the population grew rapidly. As the Middle Ages ended, Europe entered a period known as the European Renaissance, or the “rebirth” of European civilization.

During this time period, the cultural decline of the Middle Ages reversed and art, scholarship, innovation, and commerce flourished. Leonardo da Vinci painted the Mona Lisa, European explorers set sail for far-off continents, and Johannes Gutenberg invented the printing press. The printing press had an enormous impact on education and scholarship as it led to the mass production of printed books and, consequently, the spread of information and ideas. From this flourishing of arts and sciences of the Renaissance grew a period called the Scientific Revolution.

In 1543, the Polish mathematician and astronomer Nicolaus Copernicus published De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres). The text was largely a mathematical work, but it also contained a heliocentric (sun-centered) model of the planetary system.

It should be noted that Copernicus was not the first to propose a heliocentric model of the universe. The ancient Greek Aristarchus had proposed a heliocentric model in the third century BCE, as had the Indian mathematician and astronomer Aryabhata in 490 CE. However, Copernicus’s work came right at the beginning of a massive shift in Western science and was an influential text for many subsequent scientists.

De revolutionibus outlined a simple theory that accounted for the movement of the planets and placed the planets in order from Mercury outward. Though his model was a significant improvement in terms of arrangement, Copernicus still believed in perfectly circular orbits and therefore calculations based on his model had many flaws. At first, few scientists paid much attention to the book’s heliocentric theory at all, for it simply did not fit into the Aristotelian way of thinking that still dominated Western science. Over time, however, as the physical sciences grew, De revolutionibus would find its place in the prevailing scientific view.

In the 1570s, the Danish astronomer Tycho Brahe observed two celestial phenomena that helped show that the universe was neither unchanging nor divided into an imperfect earthly realm and a perfect, impenetrable heavenly realm.

Brahe’s first observation was of a supernova in 1572 that shone brightly for about eighteen months. When a massive star reaches the end of its lifecycle, it dies in a brilliant supernova explosion. Though the explosion itself happens in about one hundred seconds, the peak brightness can last for several months.

At the time, astronomers did not know the true nature of supernovas, and Brahe mistook the phenomenon as the birth of a new star. Though Brahe was incorrect about the life cycle event he was observing, he correctly noted that the event took place among the stars.

Using the supernova’s parallax, Brahe was able to calculate where the supernova had occurred. Parallax is the difference in an object’s apparent position when viewed from two different lines of sight. (You can observe parallax yourself by closing one eye, looking at an object, and then looking at the same object with just the other eye closed.) The apparent difference is greater when objects are closer, which means that parallax can be used to determine the distance to comets, stars, and other celestial objects.

Copernicus dedicated De revolutionibus to Pope Paul III, but the church placed it on the forbidden books list decades later.

In the fall of 1577, Brahe made his second significant observation when he observed a comet that passed near Earth.

Comets are often described as dirty snowballs. More specifically, comets are small bodies of rock, ice (including ices of water, carbon dioxide, ammonia, and methane), and dust that orbit the sun. In their orbits, comets typically travel both very far from and very near to the sun. The center of a comet is surrounded by a large cloud of gas and dust called the coma, which is created as the ice at the center of the comet is heated and sublimated by the sun. Material streams out of the coma as the comet moves, creating a luminescent tail that can be up to tens of millions of kilometers long.

The earliest recorded supernova took place in 185 CE and was documented by Chinese astronomers.

Some comets orbit the sun on predictable schedules, such as Halley’s Comet, which passes by Earth about every seventy-five years.

In Brahe’s day, when comets came and went, scientists and philosophers thought that they must move beneath the moon in the imperfect and changing earthly realm. Aristotle himself had taught that comets, like lightning and meteors, occurred in Earth’s atmosphere. At the time, comets were also often believed to be bad omens that preceded major disasters such as plagues and floods.

Brahe’s calculations using parallax, however, showed that comets passed by far above Earth’s atmosphere.

“I conclude that it was in the sphere of Venus,” Brahe wrote in a 1578 manuscript. His measurements were not entirely accurate, but the implications were significant: if a comet was passing through the sky near Venus and then disappearing, not only were changes occurring in the celestial realm but also between Aristotle’s theoretical crystalline spheres.

Brahe had already suspected that Aristotle’s view of the universe was incorrect, and along with the supernova of 1572, his observations of the comet of 1577 suggested Brahe was right.

Though influential, not all of Brahe’s astronomical ideas were correct, including his model of the universe. Brahe proposed a hybrid of the Ptolemaic and Copernican systems in which the sun, moon, and stars orbited Earth while the known planets revolved around the sun. Mathematically, Brahe’s system was equal to Copernicus’s model of a perfectly spherical heliocentric system (which also had its flaws), but it was fundamentally inaccurate.

It was Brahe’s student Johannes Kepler who, after Brahe’s death, brought about the next significant advancements in determining the structure of the universe.

Kepler began his degree at the University of Tübingen in Germany in 1589 and studied under a mathematics professor, Michael Maestlin, who was an adherent to the Copernican system. Maestlin lent Kepler De revolutionibus, which Kepler sensed was correct. He decided that he would demonstrate it was so.

In 1600, Kepler moved to Prague to assist Brahe, who at this point was serving as the imperial mathematician for Holy Roman Emperor Rudolph II. Over decades of work and with instruments he had designed, Brahe had created an enormous data set of astronomical observations. Kepler’s task was to calculate planetary orbits from these observations.

When Brahe died in 1601, Kepler succeeded him as imperial mathematician to the emperor and inherited Brahe’s data. With the intellectual freedom granted by imperial court life and Brahe’s exceptionally accurate data, Kepler was able to determine that Mars moves in an elliptical orbit.

Kepler’s second law tells us that planets move faster when near the sun and slower when farther away.

Kepler published his findings on the orbit of Mars in 1609 in a book called Astronomia nova (New Astronomy). It marked the introduction of Kepler’s first law of planetary motion, which states that the planets move in elliptical orbits with the sun at one focus. (That is, the sun is located off from the center of the elliptical path.)

Also in 1609, the famous Italian astronomer Galileo Galilei built his first telescope. Over the next few years, Galileo made a multitude of discoveries, including the craggy surface of the moon, the four largest moons of Jupiter, and the phases of Venus.

The first observation showed that the moon’s surface was as imperfect as Earth’s surface. The discovery of Jupiter’s moons showed that Earth was not the only body to have a satellite. (Galileo at first thought that the bright objects he saw were “fixed stars,” but after observing them over time, he saw that they moved around, behind, and in front of Jupiter.) His observations of the phases of Venus showed that, with respect to Earth, Venus was sometimes on the far side of the sun and sometimes on the near side of the sun. In other words, Venus orbited the sun, not Earth.

The moon’s irregularities and the discovery of Jupiter’s moons were not part of the Aristotelian and Ptolemaic models of the universe, and Venus’s orbit was simply not possible in the Aristotelian and Ptolemaic models.

In 1610, almost seventy years after Copernicus published De revolutionibus, Galileo published these observations and conclusions in a treatise (or pamphlet) titled Sidereus Nuncius (Starry Messenger). The first page read, with great fanfare and enthusiastic capitalization:

HIGHLY ADMIRABLE sights, and presenting to the gaze of everyone, but especially PHILOSPHERS, and ASTRONOMERS, those things observed by GALILEO GALILEI, Patrician of Florence, Public Mathematician of the University of Padua, with the aid of a TELESCOPE …

Sidereus Nuncius referred to the motion of Mercury, Venus, and Mars around the sun and clearly promoted the Copernican heliocentric model. The Catholic Church had deemed that heliocentrism was heretical and ordered Galileo to avoid teaching or defending heliocentrism in the future.

In 1633, however, after Galileo printed an implicit defense of heliocentrism in Dialogue Concerning the Two Chief World Systems, he was charged with heresy and sentenced to house arrest for the remainder of his life. Though deemed a heretic, Galileo succeeded in popularizing the Copernican system.

The developments that occurred during the Scientific Revolution forever changed the way scientists, scholars, and common people understood astronomy and cosmology. The Aristotelian model had fallen. The universe was imperfect and changing, and Earth was just one of many planets orbiting the sun on elliptical paths.

The Scientific Revolution didn’t just bring new knowledge and discoveries—it also popularized the modern methodology for scientific research, which would play an important role in developing the prevailing modern cosmological theories.

Today, students in science classes across the world learn the scientific method as a standard element of conducting an experiment. The scientific method was not always a part of how intellectuals arrived at conclusions, however, as shown by the intellectual thought of the ancient Greek philosophers and church-influenced scholarship prior to the Scientific Revolution.

In general, the scientific method starts with the specific (an observation) and moves toward a more general principle (a theory that can explain observations). The tests must be replicable and, if subsequent tests show that a hypothesis does not hold true, the theory must be modified or thrown out entirely.

If analysis of test data shows the predictions are inaccurate, then the scientist must modify his or her hypothesis, make new predictions, and run more tests. When significant data shows that the predictions are accurate, the scientist can develop the hypothesis into a general theory.

During the Scientific Revolution, this focus on methodical observation, experiments, and inductive reasoning (finding general laws from particular instances) became the dominant scientific methodology. As scientists such as Galileo and Kepler focused on making observations over time and making conclusions based on large amounts of data, the pace of scientific discoveries accelerated and our knowledge of the universe began to expand rapidly.

Humans began making sense of the world around them through myths that told the story of creation. These myths were important to the cultures that created them, but they varied between societies. Today, we investigate the universe in a very different way.

Over the seventeenth, eighteenth, and nineteenth centuries, science and technology accelerated at breakneck pace, transforming, industrializing, and modernizing the world. There were developments in astronomy and physics, and scientists made huge advancements in their understanding of the scale of the universe and the movement of celestial bodies. These developments made way for a more scientific variation of the cosmological question: From a perspective embedded in the physical sciences and the scientific method, how did the universe come to be as it is today?

Before scientists could derive the big bang theory, the modern standard cosmological model, they needed specific advancements in science and technology. Our modern cosmology grew from a deeper understanding of optics, chemistry, and light, which we will explore in the next chapter.