Physical cosmology

• 1. Introduction

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  Physical cosmology asks the largest question physics can pose: where did the universe come from, and where is it going? It is the branch of physics that builds mathematical models of the universe from the laws of physics. A cosmological model describes the largest-scale structures and dynamics of the cosmos. From that description, scientists probe the universe's origin, structure, evolution, and ultimate fate. The field draws on particle physics, astrophysics, general relativity, quantum mechanics, and plasma physics all at once. Yet a model that agrees with many diverse observations still rests on two ingredients nobody can explain. Most of the universe is made of dark matter and dark energy, whose nature remains unknown. How did a discipline reach such precision while leaving its main contents in shadow? What ancient light still reaches us from the universe's first moments? And how do physicists weigh forms of energy that fill all of empty space?

• 2. Theory And Observation

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  In 1916, Albert Einstein published his theory of general relativity, describing gravity as a geometric property of space and time. Einstein believed in a static universe, but his original equations would not permit one. Because masses spread through the universe gravitationally attract, they drift toward each other over time. To hold the cosmos still, in 1917 he added a cosmological constant to his field equations to counteract gravity. His model describes a finite, unbounded space, like the surface of a sphere with no edges. That so-called Einstein model proved unstable to small perturbations, destined to expand or contract. In the early 1920s, Alexander Friedmann found the cosmological solutions of general relativity. His equations describe a universe that may expand or contract, with geometry open, flat, or closed. Meanwhile, observers were reading the sky for clues that theory alone could not supply.

• 3. Reading The Redshift

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  In the 1910s, Vesto Slipher, and later Carl Wilhelm Wirtz, read the red shift of spiral nebulae as a Doppler shift, a sign the nebulae were receding from Earth. Measuring distance to such objects is hard, requiring an assumed physical size or an assumed intrinsic luminosity used with the inverse-square law. Because of that difficulty, they did not realize the nebulae were galaxies beyond the Milky Way. In 1927, the Belgian Roman Catholic priest Georges Lemaitre independently derived the Friedmann-Lemaitre-Robertson-Walker equations. He proposed that the universe began with the explosion of a primeval atom, later called the Big Bang. In 1929, Edwin Hubble gave Lemaitre's idea an observational footing, measuring galaxy distances using the brightness of Cepheid variable stars. He found a relationship between a galaxy's redshift and its distance, now known as Hubble's law. Hubble's numerical factor was off by a factor of ten, because the types of Cepheid variables were not yet understood.

• 4. Two Rival Universes

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  Hubble's law, read through the cosmological principle, suggested an expanding universe, and two explanations competed to account for it. One was Lemaitre's Big Bang theory, advocated and developed by George Gamow. The other was Fred Hoyle's steady state model, in which new matter is created as galaxies move apart, leaving the universe roughly the same at any moment. For years, support for the two theories was evenly divided. The balance tipped when evidence began favoring a universe that evolved from a hot, dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model. After precise measurements by the Cosmic Background Explorer in the early 1990s, few cosmologists seriously proposed rival theories of the cosmos. A handful still advocate alternative cosmologies, but most agree the Big Bang theory best explains the observations.

• 5. The Cosmic Microwave Background

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  The cosmic microwave background is radiation left over from decoupling, after the epoch of recombination when neutral atoms first formed. At that point, radiation from the Big Bang stopped Thomson scattering off charged ions and traveled freely. First observed in 1965 by Arno Penzias and Robert Woodrow Wilson, it carries a perfect thermal black-body spectrum. Today it sits at a temperature of 2.7 kelvins and is isotropic to one part in 100,000. Cosmological perturbation theory lets scientists calculate the angular power spectrum of this radiation precisely. Satellite experiments such as COBE and WMAP, along with ground and balloon experiments like the Degree Angular Scale Interferometer, the Cosmic Background Imager, and Boomerang, have measured it. WMAP's results have even placed limits on the neutrino masses. On the 17th of March 2014, the BICEP2 Collaboration announced an apparent detection of B-mode polarization, read as evidence of primordial gravitational waves from the earliest phase of the Big Bang. The Planck collaboration later measured cosmic dust more accurately. On the 30th of January 2015, a joint BICEP2 and Planck analysis attributed the signal entirely to interstellar dust in the Milky Way.

• 6. The Energy Of The Cosmos

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  The lightest elements, mostly hydrogen and helium, were created during the Big Bang through nucleosynthesis. Stellar nucleosynthesis then fuses smaller nuclei into larger ones, ending at stable iron group elements such as iron and nickel, which hold the highest nuclear binding energies. Different forms of energy may dominate the cosmos: relativistic particles called radiation, or non-relativistic particles called matter. Relativistic particles have rest mass that is zero or negligible compared to their kinetic energy, so they move at or near the speed of light. As the universe expands, both matter and radiation dilute, but at different rates. Matter's density falls only as volume grows, while radiation also loses energy as photon wavelengths stretch. The very early universe was radiation dominated, and radiation controlled the deceleration of expansion. Once the average energy per photon fell to roughly 10 eV and lower, the universe became matter dominated. As expansion continues and matter dilutes further, the cosmological constant becomes dominant and the expansion accelerates. There is no clear way to define total energy in the universe under general relativity, leaving it controversial whether energy is conserved in an expanding cosmos.

• 7. The Unsolved Ingredients

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  About 23% of the mass of the universe consists of non-baryonic dark matter, while only 4% is visible, baryonic matter. Dark matter behaves like a cold, non-radiative fluid that forms haloes around galaxies, and its gravitational effects are well understood. It has never been detected in the laboratory, and its particle physics nature remains completely unknown. Candidates include a stable supersymmetric particle, a weakly interacting massive particle, an axion, and a massive compact halo object. Alternatives such as MOND modify gravity at small accelerations, and TeVeS is a version of MOND that can explain gravitational lensing. If the universe is flat, a further 73% of its energy density must be dark energy, which does not cluster in haloes like matter. The case for dark energy strengthened in 1999, when measurements showed the expansion of the universe had begun to accelerate. Quantum field theory predicts a cosmological constant 120 orders of magnitude larger than the value observed. A better grasp of dark energy may settle the universe's ultimate fate, whether a Big Rip, a Big Freeze, or some other scenario.

• 8. The Frontier Questions

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  The early, hot universe is well explained by the Big Bang from roughly 10 to the minus 33 seconds onward, but problems remain. There is no compelling reason in current particle physics for the universe to be flat, homogeneous, and isotropic. Grand unified theories predict magnetic monopoles that have never been found. A brief period of cosmic inflation resolves these, driving the universe to flatness and exponentially diluting the monopoles. A second puzzle is why the universe holds far more matter than antimatter, a process called baryogenesis. Andrei Sakharov derived three required conditions for baryogenesis in 1967, including a violation of CP-symmetry. Particle accelerators measure too small a CP-symmetry violation to account for the asymmetry. New tools press these frontiers further. In 2016, the LIGO Scientific Collaboration and Virgo Collaboration announced the first observation of gravitational waves, from a pair of merging black holes detected by Advanced LIGO. A second detection from coalescing black holes was announced on the 15th of June 2016. Observations suggest the universe began around 13.8 billion years ago, and according to the Lambda-CDM model it will continue expanding forever.