Why Do We Exist?
Humans arrived at the cosmic stage relatively late, in the last 0.01 percent of cosmic history. We are not at the center of stage, since we reside on planet Earth which orbits the Sun and the Sun orbits the center of the Milky-Way galaxy, which is one out of a trillion galaxies in the observable Universe. Given that we arrived late and we are not featured centerstage, the cosmic play is not about us. Yet, we can still ask how we got here.
The cosmic play started in the Big Bang, 13.8 billion years ago. We can only trace our roots out to the distance travelled by light since then. This sets the finite size of the cosmic stage which is visible to us, and we do not know what lies beyond its horizon.
On that stage, the initial conditions were remarkably synchronized to a part in 100,000. During the first 50,000 years of cosmic history, the mass budget was dominated by radiation. Subsequently, as the radiation cooled matter dominated, and small inhomogeneities in the matter distribution started growing gravitationally. The cosmic expansion of regions that were slightly denser than average was slowed down by self-gravity, leading ultimately for their turnaround and collapse into bound objects. The Milky-Way galaxy was one of these bound regions.
The first generation of galaxies formed 50 million years after the Big Bang. Inside of them, gas cooled and condensed to make the first stars out of the primordial hydrogen and helium left over from the hot Big Bang. These stars were much more massive than the Sun, as cooling into smaller fragments was inefficient without heavy elements. The first heavy elements were synthesized inside these early fusion reactors. Within a few million years, the first massive stars exploded as supernovae and enriched their surroundings with the elements that enable life-as-we-know-it, such as the oxygen in water molecules or the carbon in organic molecules.
Subsequent generations of stars enriched the interstellar medium further and eventually led to the formation of metal-rich stars like the Sun in the last third of cosmic history.
The Sun formed 4.6 billion years ago. Inside the debris disk that was left from its formation process — dust particles settled to the disk midplane where they coagulated to make bigger and bigger rocks. These so-called planetesimals eventually grew in size to make rocky planets. One of these planets — the Earth — resided in the so-called habitable zone where liquid water could flow on its surface owing to atmospheric pressure.
An early collision of Earth with a Mars-sized impactor birthed the Moon which stabilized the spin of Earth. As the Moon receded, its growing orbit reduced Earth’s spin and the duration of a day grew from 4 hours to 24 hours (as reported here).
The soup of chemicals on Earth enabled the emergence of primitive life in the form of the Last Universal Common Ancestor (LUCA) 4.2 billion years ago. Complex life, in the form of early eukaryotes, appeared on Earth around 2.1 to 2.9 billion years ago (as reported here), during the Earth’s midlife, roughly when Mars lost its atmosphere and surface liquid water to become a desert.
Humans emerged only in the last few million years. This cosmic history should teach us humility. We are minor actors on the cosmic stage and we exist because of everything that preceded us. Terrestrial life is fragile and could disappear once the delicate conditions enabling it will disappear as they did on Mars.
Nobody would mourn the loss of humanity unless we make sure that our descendants travel to the stars where the history books are being written by the most accomplished cosmic survivors.
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In this essay, I featured four amazing watercolors from a series created by the celebrated artist, Greg Wyatt. These watercolors incorporate inspiring statements by Marcus Tullius Cicero, John Milton and Cervantes. This is the fourth in a sequence of essays, where Greg and I collaborate on the interface between art and science. The first essay in this series, titled “Music of the Cosmic Spheres,” appeared here; the second essay, titled: “Cosmic Waterfalls in Spacetime Cliffs,” appeared here; and the third titled “Missing Elements in the Cosmic Jigsaw Puzzle,” appeared here.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
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