Big Bang

• 1. Introduction

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  The Big Bang places the beginning of the universe at an estimated 13.787 billion years ago. That single number, drawn from detailed measurements of how fast the universe is expanding, marks the moment cosmologists call the initial singularity. The Big Bang is a physical theory describing how the universe expanded from an initial state of high density and temperature. It does not describe a noise, and it does not describe an explosion into surrounding space. It describes the intrinsic expansion of everything the universe contains.

  Fred Hoyle gave the idea its name in a March 1949 broadcast on the BBC Third Programme. He favored a rival picture and called this one, in passing, the big bang. The label did not catch on until the 1970s. By then the question was no longer whether the universe had a beginning, but how to read the fossil evidence it left behind.

  That evidence raises stubborn puzzles. Why does the universe look the same in every direction. Why is it made almost entirely of matter and almost no antimatter. What accounts for the unseen 95% of the cosmos that emits no light. The answers live in faint radiation, in the ratios of the lightest elements, and in a theory whose earliest moments still lie beyond the reach of known physics.

• 2. Four Pillars

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  The redshifts of galaxies form the most direct line of evidence for the Big Bang. Light from distant galaxies and quasars is shifted toward longer wavelengths, and these redshifts are spread evenly across the sky in all directions. When recessional velocity is plotted against distance, a straight line appears. This is Hubble's law, and it implies the universe is expanding uniformly everywhere.

  The cosmic microwave background is a second pillar. It is an omnidirectional signal in the microwave band, today measured at approximately 2.725 K. Through the 1970s it was found to match the spectrum of a blackbody in all directions, a glow stretched and cooled by the expansion of space itself. The radiation comes from the surface of last scattering, the moment neutral hydrogen became stable and the universe turned transparent.

  The abundance of light elements is the third. Big Bang models predict how much helium-4, helium-3, deuterium, and lithium-7 should exist relative to ordinary hydrogen. The ratios come out near 0.25 by mass for helium-4, around one part in a thousand for deuterium, near one in ten thousand for helium-3, and roughly one in a billion for lithium-7. The match is excellent for deuterium and close for helium, though lithium-7 is off by a factor of two, an anomaly called the cosmological lithium problem.

  Galaxy formation and the distribution of large structures complete the set. Distant galaxies, seen as they were long ago, look markedly different from nearby ones. The first quasars and galaxies appear to have formed within a billion years after the Big Bang, and larger clusters and superclusters built up afterward. These changing populations are strong arguments against a universe that stays the same forever.

• 3. From Heat To Atoms

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  The Planck epoch is the universe's first chapter, lasting up to 10 to the minus 43 seconds. In this stage the four fundamental forces, electromagnetic, strong nuclear, weak nuclear, and gravitational, were unified as one. The characteristic scale was the Planck length, and the temperature reached roughly 10 to the 32 degrees. Here even the concept of a particle breaks down, and a full account waits on a theory of quantum gravity that does not yet exist.

  Around 10 to the minus 37 seconds, a phase transition triggered cosmic inflation. The universe grew exponentially, unconstrained by the speed of light, and temperatures dropped by a factor of 100,000. All the mass-energy of the currently visible galaxies began in a sphere with a radius near 4 times 10 to the minus 29 meters and grew to a radius near 0.9 meters by inflation's end. Tiny quantum fluctuations, frozen in during this burst, became the seeds of all later structure.

  Baryogenesis tilted the balance between matter and antimatter. An unknown reaction violated the conservation of baryon number, leaving a slight excess of quarks and leptons over their antiparticles, of order one part in 30 million. When the great annihilations followed, that sliver of surplus is what survived as the matter we see.

  At about 10 to the minus 6 seconds, quarks and gluons combined into protons and neutrons. A few minutes in, when the temperature was about a billion kelvin and the density rivaled Earth's atmosphere, neutrons and protons fused into deuterium and helium nuclei through Big Bang nucleosynthesis. At about 380,000 years, electrons and nuclei joined into neutral atoms in an event called recombination, and the photons released then make up the cosmic microwave background.

• 4. The Dark Majority

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  Luminous matter, the stars and planets and everything that shines, makes up less than 5% of the universe's density. The rest is hidden. Dark matter accounts for 27% and dark energy for the remaining 68%. This accounting comes from combining astronomical observations with the laws of thermodynamics and particle physics across the lifespan of the cosmos.

  Dark matter announced itself through gravity it should not have if only visible matter existed. During the 1970s and 1980s, observations showed there was not enough visible matter to explain the gravitational forces within and between galaxies. The idea emerged that up to 90% of the matter in the universe is dark, emitting no light and not interacting with normal baryonic matter. It is inferred from the cosmic microwave background, galaxy rotation, cluster velocities, gravitational lensing, and X-ray measurements of clusters, yet no dark matter particle has been seen in a laboratory.

  Dark energy revealed itself in supernovae. Measurements of the redshift and magnitude of Type Ia supernovae show the expansion has been accelerating since the universe was about half its present age. To drive this, models require energy with large negative pressure that permeates all of space. Early on, with everything closer together, gravity won and braked the expansion. Only after billions of years, as matter thinned out relative to dark energy, did the expansion begin to speed up.

  In its simplest form, dark energy is modeled by a cosmological constant in Einstein's field equations. Its energy density stays nearly constant as the universe expands, while the density of matter falls. The gap between the measured energy density of dark energy and the value naively predicted from Planck units has earned the name the most embarrassing problem in physics.

• 5. Building The Theory

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  Vesto Slipher measured the first Doppler shift of a spiral nebula in 1912, and soon found that nearly all such nebulae were receding from Earth. He did not grasp the cosmological meaning, and at the time it was hotly contested whether these objects even lay outside the Milky Way. The pieces were on the table before anyone saw the picture.

  Alexander Friedmann derived his equations from Einstein's field equations in 1922, showing the universe might be expanding, against the static model Einstein then favored. In 1924, Edwin Hubble used the 100-inch Hooker telescope at Mount Wilson Observatory to measure great distances and prove the spiral nebulae were other galaxies. By 1929, working with Milton Humason, he found the correlation between distance and recessional velocity now called Hubble's law.

  Georges Lemaitre, a Belgian physicist and Roman Catholic priest, independently derived Friedmann's equations in 1927 and tied the recession of the nebulae to an expanding universe. In 1931 he pushed further, proposing that running the expansion backward led to a single point he called the primeval atom, where space and time came into being. He wrote that if the world began with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning.

  That beginning unsettled many. In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that a beginning in time smuggled religion into physics. Arthur Eddington, siding with Aristotle, found a beginning in time repugnant and held that matter is eternal. The discovery and confirmation of the cosmic microwave background in 1964 settled the contest in the Big Bang's favor.

• 6. Listening To The Glow

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  Arno Penzias and Robert Wilson stumbled onto the cosmic background radiation in 1964, picking up a signal that came from every direction. Their find confirmed predictions made around 1950 by Ralph Alpher, Robert Herman, and George Gamow. For this they shared the 1978 Nobel Prize in Physics.

  NASA launched the Cosmic Background Explorer in 1989, and it delivered two breakthroughs. In 1990, high-precision spectrum measurements showed the radiation to be an almost perfect blackbody, with no deviation at a level of one part in 10,000, and a residual temperature near 2.726 K. In 1992, the satellite found tiny temperature fluctuations across the sky, at a level of about one part in 100,000. John C. Mather and George Smoot shared the 2006 Nobel Prize in Physics for this work.

  BOOMERanG and other experiments measured the shape of those fluctuations in 2000 and 2001, finding the universe to be spatially almost flat. The first results from the Wilkinson Microwave Anisotropy Probe came in early 2003, giving the most accurate cosmological parameters then available. WMAP placed Hubble's constant at 70.4 kilometers per second per megaparsec and supported a Lambda-CDM model with cold dark matter.

  A crack remains in this precise picture. The Hubble tension is an unexplained gap between two ways of measuring the expansion rate. Methods based on the cosmic microwave background give a lower value than methods built on the cosmic distance ladder, and no one yet knows why.

• 7. Unfinished Business

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  Baryon asymmetry is among the deepest open questions. It is not understood why the universe holds more matter than antimatter. The young, hot universe is assumed to have held equal numbers of baryons and antibaryons, with an imbalance of only one part in 10 billion. For baryogenesis to explain the surplus, the Sakharov conditions must hold, requiring violated baryon number, broken C and CP symmetry, and a departure from thermodynamic equilibrium. All occur in the Standard Model, but not strongly enough.

  The horizon problem asks how distant regions came to share the same temperature. If radiation or matter had always dominated, the particle horizon at last scattering would span only about 2 degrees on the sky, leaving no way for wider regions to match. Inflation answers this. A rapid early expansion drove regions now on opposite sides of the observable universe into causal contact before inflation began.

  The flatness problem, also called the oldness problem, turns on a knife's edge. Any small departure from the critical density grows over time, yet the universe today remains very close to flat. Even at the age of a few minutes, the density had to sit within one part in 10 to the 14 of its critical value. Inflation also resolves the magnetic monopole objection, raised in the late 1970s, by sweeping point defects out of the observable universe.

  The Planck epoch still resists description, and there are no easily testable models for conditions before roughly 10 to the minus 15 seconds. Roger Penrose, Stephen Hawking, and George F. R. Ellis showed in 1968 and 1970 that singularities were an inevitable starting condition of these models. Whether time itself existed before the Big Bang is a question physics cannot yet answer, and future gravitational-wave observatories may one day detect relics from less than a second after the beginning.