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# 11: Particle Physics and Cosmology

At the very beginning of this text we discussed the wide range of scales that physics encompasses, from the very smallest particles to the largest scale possible—the universe itself. In this final chapter we examine some of the frontiers of research at these extreme scales. Particle physics deals with the most basic building blocks of matter and the forces that hold them together. Cosmology is the study of the stars, galaxies, and galactic structures that populate our universe, as well as their past history and future evolution.

• 11.1: Prelude to Particle Physics and Cosmology
The study of elementary particles requires enormous energies to produce isolated particles, involving some of the largest machines humans have ever built. But such high energies were present in the earliest stages of the universe and the universe we see around us today was shaped in part by the nature and interactions of the elementary particles created then. Bear in mind that particle physics and cosmology are both areas of intense current research, subject to much speculation.
• 11.2: Introduction to Particle Physics
The four fundamental forces of nature are, in order of strength: strong nuclear, electromagnetic, weak nuclear, and gravitational. Quarks interact via the strong force, but leptons do not. Both quark and leptons interact via the electromagnetic, weak, and gravitational forces. Elementary particles are classified into fermions and boson. Fermions have half-integral spin and obey the exclusion principle. Bosons have integral spin and do not obey this principle.
• 11.3: Particle Conservation Laws
Elementary particle interactions are governed by particle conservation laws, which can be used to determine what particle reactions and decays are possible (or forbidden). The baryon number conservation law and the three lepton number conversation law are valid for all physical processes. However, conservation of strangeness is valid only for strong nuclear interactions and electromagnetic interactions.
• 11.4: Quarks
Six known quarks exist: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). These particles are fermions with half-integral spin and fractional charge. Baryons consist of three quarks, and mesons consist of a quark-antiquark pair. Due to the strong force, quarks cannot exist in isolation. Evidence for quarks is found in scattering experiments.
• 11.5: Particle Accelerators and Detectors
Many types of particle accelerators have been developed to study particles and their interactions. These include linear accelerators, cyclotrons, synchrotrons, and colliding beams. Colliding beam machines are used to create massive particles that decay quickly to lighter particles. Multipurpose detectors are used to design all aspects of high-energy collisions. These include detectors to measure the momentum and energies of charge particles and photons.
• 11.6: The Standard Model
The Standard Model describes interactions between particles through the strong nuclear, electromagnetic, and weak nuclear forces. Particle interactions are represented by Feynman diagrams. A Feynman diagram represents interactions between particles on a space-time graph. Electromagnetic forces act over a long range, but strong and weak forces act over a short range. These forces are transmitted between particles by sending and receiving bosons.
• 11.7: The Big Bang
The universe is expanding like a balloon—every point is receding from every other point. Distant galaxies move away from us at a velocity proportional to its distance. This rate is measured to be approximately 70 km/s/Mpc. Thus, the farther galaxies are from us, the greater their speeds. These “recessional velocities” can be measure using the Doppler shift of light.  According to current cosmological models, the universe began with the Big Bang approximately 13.7 billion years ago.
• 11.8: Evolution of the Early Universe
The early universe was hot and dense. The universe is isotropic and expanding. Cosmic background radiation is evidence for the Big Bang. The vast portion of the mass and energy of the universe is not well understood.
• 11.A: Particle Physics and Cosmology (Answers)
• 11.S: Particle Physics and Cosmology (Summary)
• 11.E Particle Physics and Cosmology (Exercises)

Thumbnail: A proton is composed of two up quarks, one down quark, and the gluons that mediate the forces "binding" them together. The color assignment of individual quarks is arbitrary, but all three colors must be present. Image used with perimssion (CC BY-SA 2.5; Arpad Horvath).

## Contributors

• Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).