Big Bang
In 1922, Russian mathematician Alexander Friedmann derived equations from Einstein's field equations that suggested the universe might be expanding. This theoretical work stood in contrast to the static universe model favored by Albert Einstein at the time. Ten years later, Edwin Hubble published observations showing that galaxies were moving away from Earth at speeds proportional to their distance. This relationship became known as Hubble's law and provided the first empirical evidence for cosmic expansion. Independent of both Friedmann and Hubble, Belgian physicist Georges Lemaître proposed in 1931 that the universe emerged from a single point he called a primeval atom. By 1964, Arno Penzias and Robert Wilson discovered the cosmic microwave background radiation, which offered strong confirmation of the Big Bang theory. The discovery tipped the balance of scientific opinion decisively toward the Big Bang model over competing steady-state theories.
The earliest phase of the universe lasted only up to 10^-43 seconds into the expansion, known as the Planck epoch. During this period, all four fundamental forces existed as one unified force with temperatures around 10^32 degrees Celsius. At approximately 10^-37 seconds, cosmic inflation caused the universe to grow exponentially while temperatures dropped by a factor of 100,000. Microscopic quantum fluctuations frozen during inflation became the seeds for future large-scale structures. After about 10^-11 seconds, particle energies dropped to values achievable in modern particle accelerators. Quarks and gluons combined to form protons and neutrons within microseconds. A few minutes into the expansion, when temperatures reached about a billion kelvin, neutrons combined with protons to create deuterium and helium nuclei through Big Bang nucleosynthesis. Around 380,000 years after the beginning, electrons and nuclei formed neutral atoms, making the previously opaque universe transparent and releasing photons that now constitute the cosmic microwave background.
Four pillars support the Big Bang model: Hubble's law showing redshifted galaxies, the cosmic microwave background radiation, light element abundances from nucleosynthesis, and large-scale structure formation. In 1964, Penzias and Wilson detected an omnidirectional signal in the microwave band at approximately 2.725 Kelvin. NASA's COBE satellite launched in 1989 measured this spectrum as nearly perfect blackbody radiation with deviations less than one part in 10^4. The WMAP probe released its first results in early 2003, providing accurate cosmological parameters consistent with inflation theory. Measurements of hydrogen-4 to hydrogen ratios show about 0.25 by mass, while deuterium-to-hydrogen ratios reach roughly 10^-3. These predicted abundances match observations remarkably well despite systematic uncertainties in some cases. Galaxy distribution studies confirm structures formed within a billion years after the initial expansion, supporting Lambda-CDM models over steady-state alternatives.
Current measurements indicate ordinary matter makes up only 4.9% of total energy density according to Planck space probe data from February 2015. Dark matter accounts for 25.9% while dark energy comprises 69.1% of the universe's content. Observations from the 1970s and 1980s revealed insufficient visible matter to explain gravitational forces within galaxies. This discrepancy led scientists to propose that up to 90% of matter exists as invisible dark matter. Type Ia supernovae measurements show the universe has been accelerating since it was half its present age. Dark energy appears to homogeneously permeate all space with large negative pressure. The WMAP team reported in 2008 that their results align with a universe containing 73% dark energy, 23% dark matter, 4.6% regular matter, and less than 1% neutrinos. While dark energy density remains constant during expansion, matter density decreases as the universe grows larger.
Scientists cannot yet explain why the universe contains more matter than antimatter despite statistical equilibrium suggesting equal numbers initially. A process called baryogenesis must have violated conservation laws to create an excess of one part in 30 million quarks over antiquarks. Magnetic monopoles predicted by grand unified theories should exist but remain undetected after decades of searching. The horizon problem questions how distant regions achieved uniform temperature without causal contact before inflation. Flatness problems arise because any small departure from critical density would grow significantly over time, yet the universe remains remarkably flat today. Quantum gravity effects dominate conditions near the Planck scale around 10^32 Kelvin, but no accepted theory exists for this regime. Cosmologists continue investigating these mysteries while developing new observational techniques to probe the earliest moments of cosmic history.
English astronomer Fred Hoyle coined the term Big Bang during a March 1949 BBC Radio broadcast describing theories based on matter created in one big bang at a particular time. Hoyle explicitly denied that his phrase was meant pejoratively, though popular reports suggest he intended it as criticism of the explosive theory. Helge Kragh notes evidence claiming derogatory intent appears unconvincing and mentions indications supporting neutral usage. The term itself has been argued as misleading since explosions imply expansion into surrounding space whereas the Big Bang describes intrinsic expansion of contents. Santhosh Mathew points out that bang implies sound which is not important to the model. An international competition with 13,099 entries from 41 countries failed to produce a better alternative name despite judges including Hugh Downs and Carl Sagan. Timothy Ferris concluded that like it or not, humanity remains stuck with the original terminology.
Modern observations of accelerating expansion suggest more visible universe will pass beyond our event horizon and lose contact with us. If dark energy density continues constant while matter density decreases, galaxy clusters may eventually be torn apart by ever-increasing expansion in scenarios called Big Rip. Alternative models propose heat death where average temperature asymptotically approaches absolute zero after stars burn out. Some theories suggest protons might become unstable leaving only radiation and black holes before they evaporate via Hawking radiation. Pre-Big Bang cosmology explores possibilities including quantum fluctuations causing the initial state or emergent universe models featuring low-activity past-eternal eras. Brane cosmology proposes collisions between branes in string theory created the Big Bang while cyclic models suggest periodic collisions occur. Eternal inflation allows universal inflation to end locally here and there leading to bubble universes expanding from their own big bang events. These speculative proposals remain untested hypotheses requiring further development of quantum gravity treatments.
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Common questions
Who derived the equations suggesting the universe might be expanding in 1922?
Russian mathematician Alexander Friedmann derived equations from Einstein's field equations that suggested the universe might be expanding. This theoretical work stood in contrast to the static universe model favored by Albert Einstein at the time.
When did Edwin Hubble publish observations showing galaxies were moving away from Earth?
Ten years after 1922, which is 1932, Edwin Hubble published observations showing that galaxies were moving away from Earth at speeds proportional to their distance. This relationship became known as Hubble's law and provided the first empirical evidence for cosmic expansion.
What happened during the Planck epoch of the Big Bang theory?
The earliest phase of the universe lasted only up to 10^-43 seconds into the expansion, known as the Planck epoch. During this period, all four fundamental forces existed as one unified force with temperatures around 10^32 degrees Celsius.
How much dark energy comprises the universe according to Planck space probe data from February 2015?
Current measurements indicate ordinary matter makes up only 4.9% of total energy density according to Planck space probe data from February 2015. Dark matter accounts for 25.9% while dark energy comprises 69.1% of the universe's content.
Who coined the term Big Bang during a March 1949 BBC Radio broadcast?
English astronomer Fred Hoyle coined the term Big Bang during a March 1949 BBC Radio broadcast describing theories based on matter created in one big bang at a particular time. Hoyle explicitly denied that his phrase was meant pejoratively, though popular reports suggest he intended it as criticism of the explosive theory.