Cosmos: A Journey Through Space, Space, Time, and Scientific Discovery

Principles of Scientific Inquiry and the Cosmic Voyage

The cosmos is always always. This journey traverses the scale from the infinitesimal to the infinite, covering the span from the dawn of time to the distant future. It encompasses the exploration of galaxies, suns, worlds, and the gravity waves of space-time. The story of the cosmos is essentially the saga of wandering bands of hunters and gatherers finding their way to the stars. This adventure is driven by imagination, yet imagination alone is insufficient because the reality of nature is far more wondrous than human invention. Success in this exploration depends on a simple, strict set of rules adhered to by generations of searchers:

• Test ideas by experiment and observation.

• Build on those ideas that pass the test.

• Reject the ones that fail.

• Follow the evidence wherever it leads and question everything.

Using a ship of the imagination that is free from the constraints of space and time, it is possible to explore any point in the universe. In this vessel, looking forward shows the expanse of space, while the past lies beneath. Earth looked significantly different $250,000,000$ years ago, and its appearance will continue to change, potentially appearing vastly different another $250,000,000$ years into the future.

Line One and Two of the Cosmic Address: Earth and the Solar System

The start of our journey begins at Earth, the only home known to humanity. However, the cosmic address extends far beyond this point.

• The Moon: The nearest neighbor, possessing no sky, no ocean, and no life, marked only by the scars of cosmic impacts.

• The Sun: The central star that powers the wind, waves, and all life on Earth's surface. Its gravity holds the entire solar system together.

• Mercury: The closest planet to the Sun.

• Venus: A world covered in clouds where a runaway greenhouse effect has created a hellish environment.

• Mars: A world with a landmass roughly equal to the land area of Earth.

• The Asteroid Belt: A collection of rocky debris circling the Sun between the orbits of Mars and Jupiter.

• Jupiter: More massive than all other planets combined, it functions like its own miniature solar system with four giant moons and dozens of smaller ones. It features the Great Red Spot, a hurricane three times the size of Earth that has raged for centuries.

• Saturn: Known for its rings consisting of countless orbiting and tumbling snowballs, each acting as a tiny moon.

• Uranus and Neptune: The outermost planets, discovered only after the invention of the telescope.

• The Kuiper Belt and Pluto: Beyond Neptune lies a swarm of tens of thousands of frozen worlds, including Pluto.

• Voyager 1: The spacecraft that has traveled farthest from home, carrying a message intended for a billion years in the future, documenting human identity and music.

• The Oort Cloud: A vast shell enclosing the solar system containing a trillion frozen comets left over from the formation of the solar system nearly $5,000,000,000$ years ago. Individual worlds within this cloud are as far from their neighbors as Earth is from Saturn.

Line Three to Five of the Cosmic Address: The Galactic and Supercluster Scale

• Exoplanets: Planets orbiting other stars are plentiful and actually outnumber the stars themselves. While most are hostile to life as we know it, our understanding is limited to the single example of Earth-based life.

• Rogue Planets: Billions of worlds adrift in the galaxy without a parent star. They were cast away during the chaotic birth of their native systems. While molten at the core and frozen at the surface, they may contain liquid water oceans in the intermediate zone.

• The Milky Way Galaxy: Seen in infrared, every dot is a star. Earth is located in a trailing outer arm, approximately $30,000$ light-years from the center. The galaxy spans $100,000$ light-years across.

• The Local Group: A collection consisting of the Milky Way, the Andromeda galaxy (the great spiral next door), and several smaller companion galaxies.

• The Virgo Supercluster: A massive structure containing thousands of galaxies, within which the Local Group is just a small component.

• The Observable Universe: A network of approximately $100,000,000,000$ galaxies. This is the current limit of our cosmic address. The "cosmological horizon" defines how far we can see; because the universe is $13.8$ billion years old, light from beyond this horizon has not had enough time to reach us.

• The Multiverse: A hypothesis suggesting our observable universe is merely one bubble in an infinite ocean of other universes.

Giordano Bruno and the Infinity of Space

Four centuries ago, the common belief was that the Earth was the stationary center of a small universe created for humanity. In $1599$, few questioned this geocentric view.

• Nicolaus Copernicus: A Polish astronomer and priest who proposed the radical idea that Earth was not the center but a planet revolving around the Sun. This was viewed as a scandalous affront to scripture by figures like Martin Luther.

• Giordano Bruno: A Dominican monk from Naples who sought to expand on Copernicus’s ideas. Banned books, specifically the works of Lucretius, inspired him to envision an infinite universe.

• The Arrow Thought Experiment: Proposed by Lucretius, this thought experiment suggests that if one stands at the edge of the universe and shoots an arrow, it either flies forever or hits a wall. If it hits a wall, that wall must be beyond the edge, meaning one can stand on the wall and shoot again. Thus, the universe has no boundary.

Bruno's vision included the idea that the Sun was merely another star and that stars were other suns with their own earths and life. He was excommunicated and expelled by the Roman Catholic Church, Calvinists, and Lutherans.

Questions & Discussion: Bruno at Oxford

Bruno: I have come to present a new vision of the cosmos. Copernicus was right to argue that our world is not the center of the universe. The earth goes around the sun. It's a planet just like the others. But Copernicus was only the dawn. I bring you the sunrise. The stars are other fiery suns made of the same substance as the earth, and they have their own watery depths with plants and animals no less noble than our own.

Oxford Authority: Are you mad or merely ignorant? Everyone knows there is only one world.

Bruno: What everyone knows is wrong. Our infinite god has created a boundless universe with an infinite number of worlds. I beg you, reject antiquity, tradition, faith, and authority. Let us begin anew by doubting everything we assume has been arithmetic into death. Your god is too small.

Following eight years of confinement and relentless interrogation by the Inquisition, Bruno refused to recant.

Inquisition Verdict: You are found guilty of questioning the holy trinity and the divinity of Jesus Christ, of believing that God's wrath is not eternal, that everyone will be saved, of asserting the existence of other worlds. All of the books you have written will be gathered up and burned in Saint Peter's Square.

Bruno: It may be that you are more afraid to deliver this judgment than I am to hear it.

Bruno was executed in $1600$. Ten years later, Galileo Galilei used a telescope to confirm that the Milky Way was composed of countless stars, proving Bruno's vision had been a lucky guess based on intuition rather than scientific evidence.

The Cosmic Calendar: From the Big Bang to the Formation of the Earth

To grasp the history of the universe ($13.8 \times 10^{9}$ years), the entire duration is compressed into a single calendar year.

• Scale: Each month represents approximately $1,000,000,000$ years. Each day represents nearly $40,000,000$ years.

• January 1: The Big Bang. The universe emerged from a point smaller than an atom. Evidence includes the ratio of helium and the cosmic microwave background (radio waves residue).

• January 10: The first stars burst into light.

• January 13: Stars coalesce into small galaxies.

• March 15: Formation of the Milky Way galaxy, approximately $11,000,000,000$ years ago.

• Supernovae and Nucleosynthesis: Elements like oxygen, carbon, calcium, and iron were cooked in the hearts of dying stars and recycled. Humans are essentially "star stuff."

• August 31: Birth of the Sun ($4.5 \times 10^{9}$ years ago).

• Earth Formation: Formed from a disk of gas and dust. A specific collision with debris led to the formation of the Moon. Early on, the Moon was $10$ times closer to Earth and appeared $100$ times brighter.

Evolution and Human Prehistory on the Cosmic Calendar

• September 21: Life begins on Earth ($3.5 \times 10^{9}$ years ago).

• November 9: Microbes grow in complexity, inventing activities like respiration, eating, and sex.

• December 17: The Cambrian explosion of life in the sea. Tiktaalik becomes one of the first animals to venture onto land.

• December 28: The first flower blooms. Ancient forests eventually transform into coal.

• December 30, 06:24 AM: An asteroid impact ends the $100,000,000$-year reign of the dinosaurs, allowing small mammals (human ancestors) to thrive. This highlights the contingency of existence.

• December 31, 09:45 PM: Human ancestors stand on two feet for the first time ($3.5 \times 10^{6}$ years ago).

• December 31, 11:00 PM - 11:59 PM: Humans spend $40,000$ generations as hunters and gatherers, mastering fire and tools.

• December 31, 11:59 PM: Cave painting begins ($30,000$ years ago). Astronomy is invented for survival (predicting seasons and migrations).

Recorded History and the Scientific Revolution

All of recorded history occupies the final $14$ seconds of the Cosmic Calendar.

• 14 seconds to midnight: Invention of writing ($6,000$ years ago) using clay tablets to record bushels of grain and thoughts.

• 7 seconds to midnight: Birth of Moses.

• 6 seconds to midnight: Birth of Buddha.

• 5 seconds to midnight: Birth of Jesus.

• 3 seconds to midnight: Birth of Muhammad.

• 2 seconds to midnight: The two halves of the Earth discover each other.

• 1 second to midnight: The birth of modern science.

In just $400$ years (one second on the calendar), science led humanity from Galileo’s first telescope observations to landing on the Moon.

The Legacy of Carl Sagan

Carl Sagan was a preeminent scientist and communicator. His scientific contributions include:

• Predicting methane lakes on Titan (Saturn's moon).

• Identifying greenhouse gases in the atmosphere of early Earth.

• Explaining seasonal changes on Mars as windblown dust.

• Pioneering the Search for Extraterrestrial Intelligence (SETI).

• Leading roles in major spacecraft missions for the first $40$ years of the space age.

In $1975$, Sagan invited a young student from the Bronx to Cornell University in Ithaca, New York. He spent a Saturday showing the student his labs and inscribed a book: "For Neil, a future astronomer." This interaction exemplified the nature of science as a cooperative enterprise where knowledge is a torch passed from teacher to student across generations.