Universe

• 1. Introduction

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  The universe is all of space and time and their contents. That deceptively simple phrase hides a staggering reality: roughly 93 billion light-years of observable space, an estimated 2 trillion galaxies, and approximately 10 to the power of 24 stars. Yet 95 percent of everything that exists remains invisible to us. What is the dark matter that outweighs everything we can see? What is the dark energy accelerating the expansion of space itself? How did the universe go from a state of pure, unimaginable heat to the vast, structured cosmos we observe? These are the questions cosmology has been wrestling with since Albert Einstein published his general theory of relativity in 1915, and they are far from settled.

• 2. The First Moments

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  At time zero, all of existence was compressed into a state of infinite density. The Planck epoch lasted from that zero point to roughly 10 to the minus 43 seconds, a period so brief it defies any intuitive sense of time. During those first instants, gravity is thought to have been as strong as the other three fundamental forces, rather than the weakest of them as it is today. Physics has no confirmed model of what happened in that window.

  Within the first 10 to the minus 32 seconds, a brief and intense period of cosmic inflation is proposed to have occurred. This inflation would explain why the universe looks so geometrically flat: space expanded so rapidly that any initial curvature was smoothed away. Shortly after inflation, the four fundamental forces separated from one another as the universe continued to cool.

  Big Bang nucleosynthesis is the name for the process that followed, and it lasted just 17 minutes. It ended approximately 20 minutes after the Big Bang. During that narrow window, about 25 percent of all protons and every neutron in the universe were fused into helium, with small amounts of deuterium and traces of lithium also formed. The remaining 75 percent of protons stayed as hydrogen. No heavier elements were built; carbon, oxygen, and iron would have to wait for the first stars.

  After nucleosynthesis, the universe entered the photon epoch: a hot, opaque plasma of electrons, nuclei, and photons. About 377,000 years later, temperatures dropped enough for electrons and nuclei to combine into the first neutral atoms. Because neutral atoms are transparent to light in ways that plasma is not, the universe suddenly became see-through. The photons released at that moment still travel through space today as the cosmic microwave background, a faint thermal glow at roughly 2.72548 kelvins.

• 3. The Invisible Majority

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  Ordinary matter, the atoms, stars, and gas clouds that make up everything a person can see or touch, accounts for only 4.9 percent of the total mass-energy of the universe. Stars and visible gas inside galaxies represent less than 10 percent of even that modest slice. The great majority of what the universe contains is something else entirely.

  Dark matter accounts for 26.8 percent of the cosmic total. It is invisible to every part of the electromagnetic spectrum; it neither emits nor absorbs light at any significant level. Its existence is inferred entirely from its gravitational effects on visible matter, on radiation, and on the large-scale structure of galaxies and galaxy clusters. Other than neutrinos, a form of hot dark matter, dark matter has never been directly detected. It remains one of the greatest unsolved problems in modern astrophysics.

  Dark energy takes up the remaining 68.3 percent. Its density is far lower than that of ordinary matter within galaxies, at roughly 7 times 10 to the minus 30 grams per cubic centimeter. But unlike matter, dark energy is uniform across all of space, and that uniformity gives it overwhelming dominance at cosmic scales. Two proposed forms are the cosmological constant, representing a fixed energy density in empty space, and dynamic scalar fields such as quintessence, whose energy density can vary over time and space.

  In 1998, two independent research groups measured what is called the deceleration parameter and found it was negative, approximately minus 0.55. Before that measurement, most physicists expected the expansion of the universe to be slowing down under the pull of gravity. Instead, the universe's expansion has been accelerating for the past 5-6 billion years. Dark energy is the leading explanation, but its underlying nature remains unknown.

• 4. Structure on Every Scale

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  Galaxies are not scattered randomly through space. At the largest scales, they distribute homogeneously in all directions across distances greater than about 300 million light-years. Below that threshold, matter clumps in a hierarchy: atoms into stars, stars into galaxies, galaxies into clusters and superclusters, superclusters into immense filaments separated by enormous voids. The result resembles a foam of cosmic proportions.

  Typical galaxies range from dwarfs hosting as few as 10 million stars up to giants containing a trillion. The Milky Way sits inside the Local Group of galaxies, which in turn belongs to the Laniakea Supercluster. Laniakea spans over 500 million light-years, while the Local Group itself spans over 10 million light-years. The largest known void in the observable universe stretches 1.8 billion light-years across.

  For comparison, the Milky Way is roughly 87,400 light-years in diameter. The nearest large neighbor galaxy, Andromeda, lies roughly 2.5 million light-years away. The edge of the observable universe is about 46 billion light-years from Earth, giving the observable universe a diameter of approximately 93 billion light-years. That figure is larger than the age of the universe times the speed of light because space itself has expanded while the light traveled.

  A 2011 estimate calculated that if the cosmological principle holds, the total universe must be more than 250 times larger than the observable Hubble sphere. Whether the total universe is finite or infinite is unknown. Some disputed estimates place its diameter, if finite, as high as several megaparsecs.

• 5. From Geocentric to Galactic

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  The earliest written astronomical records from ancient Egypt and Mesopotamia date to roughly 3000-1200 BCE. Babylonian astronomers of the 7th century BCE pictured the world as a flat disk ringed by ocean. The first coherent geometric model of the heavens came from Eudoxus of Cnidos, a student of Plato, who used 27 nested celestial spheres to account for planetary motions. Aristotle expanded that count to 55.

  Aristarchus of Samos proposed a heliocentric model in antiquity, placing the Sun at the center. His original text did not survive, but Archimedes described it in The Sand Reckoner. The only other ancient astronomer known by name to support this view was Seleucus of Seleucia, who lived a century after Aristarchus. According to Strabo, Seleucus was also the first to argue that tides result from the Moon's attraction, and that their height depends on the Moon's position relative to the Sun.

  The geocentric model dominated Western thought for roughly two millennia. Nicholas of Cusa proposed Earth's rotation in his 1440 book On Learned Ignorance, about a century before Copernicus. Tusi (1201-1274) and Ali Qushji (1403-1474) provided empirical evidence for Earth's rotation using comets. Copernicus revived the heliocentric view, and the model was later accepted by Isaac Newton and Christiaan Huygens.

  The true scale of the galaxy remained unknown until the 20th century. In 1919 the Hooker Telescope was completed, and the prevailing assumption was that the Milky Way was the entire universe. Edwin Hubble used that telescope to identify Cepheid variable stars in spiral nebulae. By 1922-1923 he had proven conclusively that Andromeda and Triangulum were independent galaxies far outside our own. From that work, Hubble formulated the Hubble constant, which allowed the first calculation of the age and size of the observable universe, starting at 2 billion years and 280 million light-years and growing more precise with subsequent data.

• 6. Myth, Philosophy, and the Cosmos

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  Long before telescopes, cultures across the world developed creation narratives. The Finnish epic Kalevala, the Chinese story of Pangu, and the Indian Brahmanda Purana all describe a world hatched from a cosmic egg. The Babylonian epic Enuma Elish tells of the universe crafted from the corpse of the slain god Tiamat. Norse mythology uses the body of the giant Ymir in a parallel way. The Maori story of Rangi and Papa attributes creation to the union of a male and a female deity.

  The pre-Socratic philosophers of Greece attempted a different kind of explanation. Thales proposed that water was the single primordial material underlying all things. His student Anaximander proposed the limitless apeiron instead. Anaximenes argued for air. Empedocles proposed four elements: earth, water, air, and fire. Democritus, following the earlier work of Leucippus, argued that the universe consists of indivisible atoms moving through a void.

  In India, the philosopher Kanada, founder of the Vaisheshika school, developed his own atomism and proposed that light and heat were varieties of the same substance. In the 5th century AD, the Buddhist philosopher Dignaga proposed atoms to be point-sized and made of energy, not substance. The notion of a temporally finite universe was taken up by John Philoponus, a Christian philosopher whose arguments were later used by the Muslim philosopher Al-Kindi, the Jewish philosopher Saadia Gaon, and the Muslim theologian Al-Ghazali.

  The modern era of cosmology opened in 1917 when Einstein applied his 1915 general theory of relativity to the structure of the universe as a whole. John Archibald Wheeler later summarized the core insight in a phrase that became proverbial among physicists: 'Spacetime tells matter how to move; matter tells spacetime how to curve.' That bidirectional relationship is still the foundation on which all quantitative cosmology rests today.