Cosmos Research CardsFrom Inflation to the First Stars

Real basis

• The universe is expanding: Hubble found that the farther a galaxy is, the faster it recedes. The universe isn't static; it is expanding.

  Source: Hubble, PNAS 1929, "A relation between distance and radial velocity among extra-galactic nebulae"

• Friedmann's universe: Friedmann derived from general relativity that the universe can expand or contract, seven years before the observational evidence.

  Source: Friedmann, Zeitschrift für Physik 1922, "Über die Krümmung des Raumes"

• The primeval atom: Lemaître independently proposed that the universe is expanding, and later imagined it began as a “primeval atom,” the forerunner of Big Bang theory.

  Source: Lemaître, Annales de la Société Scientifique de Bruxelles 1927, "Un Univers homogène de masse constante et de rayon croissant"

• Inflationary universe: Guth proposed that the early universe underwent exponential expansion, explaining in one stroke why it is so flat and why temperatures are almost the same everywhere.

  Source: Guth, Physical Review D 1981, "Inflationary universe: A possible solution to the horizon and flatness problems"

• New inflation: Linde fixed the problem of how inflation could end gracefully, making inflation theory truly workable.

  Source: Linde, Physics Letters B 1982, "A new inflationary universe scenario"

• Accelerating expansion: Using distant supernovae as “standard candles,” they found the expansion of the universe is accelerating. Dark energy entered the picture, and the work won the 2011 Nobel Prize.

  Source: Riess et al., The Astronomical Journal 1998, "Observational evidence from supernovae for an accelerating universe and a cosmological constant"

• 42 supernovae: Another team independently reached the same conclusion: some energy in the universe is driving the accelerating expansion.

  Source: Perlmutter et al., The Astrophysical Journal 1999, "Measurements of Omega and Lambda from 42 high-redshift supernovae"

• Recipe of the universe: The Planck satellite gave the most precise cosmological parameters yet: an age of about 13.8 billion years, with ordinary matter only about 5%.

  Source: Planck Collaboration, Astronomy & Astrophysics 2020, "Planck 2018 results. VI. Cosmological parameters"

• Where mass comes from: Higgs proposed a field filling space that gives particles mass, and predicted a new particle.

  Source: Higgs, Physical Review Letters 1964, "Broken symmetries and the masses of gauge bosons"

• Finding the Higgs particle: The Large Hadron Collider found a new particle at about 125 GeV, confirming a prediction made 48 years earlier.

  Source: ATLAS Collaboration, Physics Letters B 2012, "Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC"

• The quark model: Gell-Mann proposed that protons and neutrons are made of smaller “quarks,” bringing order to the particle zoo.

  Source: Gell-Mann, Physics Letters 1964, "A schematic model of baryons and mesons"

• Asymptotic freedom: The closer quarks get, the weaker their interaction; the farther apart, the tighter they pull. This explains why quarks are always locked together.

  Source: Gross & Wilczek, Physical Review Letters 1973, "Ultraviolet behavior of non-abelian gauge theories"

• Sakharov conditions: Sakharov listed three conditions needed for the universe to produce “more matter than antimatter.” They are still the starting point of research.

  Source: Sakharov, JETP Letters 1967, "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe"

• CP violation: The decay of the K meson showed that the laws of physics for matter and antimatter are not perfectly symmetric.

  Source: Christenson, Cronin, Fitch & Turlay, Physical Review Letters 1964, "Evidence for the 2π decay of the K2 0 meson"

• The positron: Anderson photographed a positively charged electron in cosmic rays, the first time humans saw antimatter.

  Source: Anderson, Physical Review 1933, "The positive electron"

• The Dirac equation: Dirac combined quantum mechanics and relativity to describe the electron, and a solution of his equation unexpectedly predicted antimatter.

  Source: Dirac, Proceedings of the Royal Society A 1928, "The quantum theory of the electron"

• Catching the neutrino: After waiting by a nuclear reactor for years, they finally caught this “ghost particle” that barely interacts with anything.

  Source: Cowan, Reines et al., Science 1956, "Detection of the free neutrino: a confirmation"

• Neutrino oscillation: The Super-Kamiokande detector found that neutrinos change between three types, which means they have mass.

  Source: Fukuda et al. (Super-Kamiokande), Physical Review Letters 1998, "Evidence for oscillation of atmospheric neutrinos"

• The perfect liquid: Heavy-ion collisions recreated the quark–gluon plasma of the first few microseconds of the universe, which behaves like a liquid with almost no viscosity.

  Source: STAR Collaboration, Nuclear Physics A 2005, "Experimental and theoretical challenges in the search for the quark–gluon plasma"

• The αβγ paper: Alpher and Gamow calculated how light elements formed in the Big Bang, and Bethe's name was added to make “αβγ.”

  Source: Alpher, Bethe & Gamow, Physical Review 1948, "The origin of chemical elements"

• Cosmic microwave background: Two Bell Labs researchers couldn't get rid of the noise in their antenna, and finally realized it was the afterglow of the Big Bang. They won the 1978 Nobel Prize.

  Source: Penzias & Wilson, The Astrophysical Journal 1965, "A measurement of excess antenna temperature at 4080 Mc/s"

• Explaining the afterglow: A Princeton team was preparing to look for this radiation, got scooped by the lab next door, and wrote down the theoretical explanation.

  Source: Dicke, Peebles, Roll & Wilkinson, The Astrophysical Journal 1965, "Cosmic black-body radiation"

• A perfect blackbody: The spectrum measured by COBE matched the theoretical blackbody curve almost perfectly, giving decisive support to Big Bang theory.

  Source: Mather et al., The Astrophysical Journal Letters 1990, "A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite"

• Ripples in the afterglow: The background radiation has temperature ripples of one part in 100,000. These tiny differences became the seeds of galaxies.

  Source: Smoot et al., The Astrophysical Journal Letters 1992, "Structure in the COBE differential microwave radiometer first-year maps"

• Baryon acoustic oscillations: Sound waves in the early universe left a characteristic scale of about 500 million light-years in the distribution of galaxies, which became a ruler for measuring the universe.

  Source: Eisenstein et al., The Astrophysical Journal 2005, "Detection of the baryon acoustic peak in the large-scale correlation function of SDSS luminous red galaxies"

• The 21 cm line: The first detection of 21 cm radio emission from neutral hydrogen let humans “see” gas in the Milky Way that is otherwise invisible.

  Source: Ewen & Purcell, Nature 1951, "Observation of a line in the galactic radio spectrum"

• Signal of the cosmic dawn: The EDGES experiment reported a 21 cm absorption signal left by the first stars, but the result is still disputed and awaits independent verification.

  Source: Bowman et al., Nature 2018, "An absorption profile centred at 78 megahertz in the sky-averaged spectrum"

• The atomic nucleus: Alpha particles occasionally bounced back off gold foil, and Rutherford concluded that an atom's mass is concentrated in a tiny nucleus.

  Source: Rutherford, Philosophical Magazine 1911, "The scattering of α and β particles by matter and the structure of the atom"

• What powers the stars: Bethe calculated that stars shine by nuclear fusion, solving the puzzle of how the Sun can burn for billions of years.

  Source: Bethe, Physical Review 1939, "Energy production in stars"

• Invisible mass: The outskirts of the Andromeda galaxy rotate too fast, so a large amount of unseen matter must supply the gravity. Dark matter was no longer just a guess.

  Source: Rubin & Ford, The Astrophysical Journal 1970, "Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions"

• First proposal of dark matter: Zwicky found that galaxies in the Coma Cluster moved too fast and proposed that “dark matter” exists. The idea was ignored for decades.

  Source: Zwicky, Helvetica Physica Acta 1933, "Die Rotverschiebung von extragalaktischen Nebeln"

• Bullet Cluster: After two galaxy clusters collided, the gas stayed in the middle while the centers of gravity ended up on either side. Dark matter had been “flung” out.

  Source: Clowe et al., The Astrophysical Journal Letters 2006, "A direct empirical proof of the existence of dark matter"

• Shadow of a black hole: Radio telescopes around the world formed an Earth-sized “virtual telescope” and photographed the shadow of the black hole at the center of M87.

  Source: Event Horizon Telescope Collaboration, The Astrophysical Journal Letters 2019, "First M87 Event Horizon Telescope Results. I. The shadow of the supermassive black hole"

• Gravitational waves: Two black holes merged 1.3 billion years ago, and ripples in spacetime were finally heard by humans, confirming Einstein's prediction from a century earlier.

  Source: Abbott et al. (LIGO & Virgo), Physical Review Letters 2016, "Observation of gravitational waves from a binary black hole merger"

• The Chandrasekhar limit: A white dwarf has a maximum mass, about 1.4 solar masses. Beyond it, the star is doomed to keep collapsing.

  Source: Chandrasekhar, The Astrophysical Journal 1931, "The maximum mass of ideal white dwarfs"

• Giant black hole in the early universe: When the universe was under 700 million years old, there was already a black hole of 800 million solar masses. How did it grow so fast? Still a mystery.

  Source: Bañados et al., Nature 2018, "An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5"

• The Galactic center: Tracking stars near the galactic center for over a decade proved a black hole of about 4 million solar masses hides there. Nobel Prize 2020.

  Source: Ghez et al., The Astrophysical Journal 2008, "Measuring distance and properties of the Milky Way's central supermassive black hole with stellar orbits"

• B²FH: This hundred-page paper explained, step by step, how stars forge the elements of the periodic table. We are all stardust, and this is where that idea starts.

  Source: Burbidge, Burbidge, Fowler & Hoyle, Reviews of Modern Physics 1957, "Synthesis of the elements in stars"

• Hoyle's prediction: For stars to make enough carbon, Hoyle predicted that carbon-12 must have a special energy level, which experiments then confirmed.

  Source: Hoyle, The Astrophysical Journal Supplement 1954, "On nuclear reactions occurring in very hot stars. I. The synthesis of elements from carbon to nickel"

• Kilonova: The first time the same cosmic event was both “heard” and “seen”: the merger of two neutron stars.

  Source: Abbott et al., Physical Review Letters 2017, "GW170817: Observation of gravitational waves from a binary neutron star inspiral"

• Where gold comes from: The material thrown out by neutron star mergers contains large amounts of heavy elements. The gold on your ring probably came from such a collision.

  Source: Kasen et al., Nature 2017, "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event"

• Pulsars: Graduate student Jocelyn Bell noticed a radio signal precise to the second. At first jokingly called “little green men,” it turned out to be a neutron star.

  Source: Hewish, Bell et al., Nature 1968, "Observation of a rapidly pulsating radio source"

• A galaxy at redshift 14: The Webb telescope confirmed bright galaxies that already existed about 300 million years after the universe began, much earlier than expected.

  Source: Carniani et al., Nature 2024, "Spectroscopic confirmation of two luminous galaxies at a redshift of 14"

• Hubble Deep Field: Hubble stared at a small patch of “empty” sky for ten days of exposure and captured about three thousand galaxies.

  Source: Williams et al., The Astronomical Journal 1996, "The Hubble Deep Field: Observations, data reduction, and galaxy photometry"

• Gaia star map: The Gaia satellite measured the positions and motions of over a billion stars, producing the most detailed 3D map of the Milky Way.

  Source: Gaia Collaboration, Astronomy & Astrophysics 2018, "Gaia Data Release 2. Summary of the contents and survey properties"

• Cold dark matter: If dark matter is “cold” and slow-moving, galaxies gather bottom-up step by step, which matches observations.

  Source: Blumenthal, Faber, Primack & Rees, Nature 1984, "Formation of galaxies and large-scale structure with cold dark matter"

• The shape of dark matter halos: Computer simulations show that cold dark matter halos, from dwarf galaxies to galaxy clusters, all follow the same density profile. The profile was later named after its three authors.

  Source: Navarro, Frenk & White, The Astrophysical Journal 1996, "The structure of cold dark matter halos"

• Recipe of the universe: The Planck satellite measured the tiny fluctuations in the cosmic microwave background: ordinary matter makes up about 5% of the Universe's total energy, dark matter about 27%, and the rest is dark energy.

  Source: Planck Collaboration, Astronomy & Astrophysics 2020, "Planck 2018 results. VI. Cosmological parameters"