Particle Physics: Discovering and Describing Particle Interactions

Homework

• Reading Preparation
• Key Equations
• WebLecture
• Study Activity
• Chat Preparation Activities
• Chapter Quiz
• Lab Work

Reading Preparation

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 32: 1-6

Study Points

Study Points

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 33: All

• 33.1 Studies of objects visible in Earth-based and Earth-orbit telescopes show that stars are grouped into clusters and galaxies, and galaxies themselves into clusters and superclusters. We can use the rotation of a galaxy derived from the Doppler shift of its edges to determine its mass.
• 33.2 Stars form when clouds of dust and gas undergo gravitational collapse to a point where density and pressure trigger thermonuclear fusion, which balances collapse with an expansion force. The energy emitted by the star as light is its luminosity. The apparent brightness of a star depends on this luminosity and the distance to the star. The temperature of a star can be determined from Wien's law and the total energy flux of the star from the Stefan-Boltzmann relationship. Plotting stars by their surface temperature and intrinsic luminosity gives us a distribution that relates to stellar evolution, with hydrogen-fusing stars on the main sequence, and helium or carbon burning stars on giant "branches". Stellar evolution life cycles depend on stellar mass: the more massive a star, the fast it fuses the hydrogen at its core and reaches hydrogen burnout, at which point its core collapses and helium fusion to carbon begins. With low mass stars, helium fusion may never start (white dwarfs), while high mass stars continue fusing heavier and heavier elements, until the final core collapse causes a massive implosion or supernova. Depending on how much mass remains in the core, the remnant may become a neutron star or black hole.
• 33.3 Distance measurements to stars and galaxies rely on a chain of methods, beginning with parallax for nearer stars, then stellar types and intrinsic luminosities, and types of stars such as Cepheid variables, whose period and luminosity are related, and Type I supernovae, which have similar magnitudes.
• 33.4 Einstein's general theory of relativity explains that gravitational mass and inertial mass will be the same: the two are equivalent. Since we would observe light bending if we accelerate through a beam, we should also see light bending in a gravitational field. We can observe light from distant galaxies bending around intermediate massive objects, confirming Einsteins' prediction. Space is therefore curved by gravity; how much it is curved depends on how fast the universe is expanding. When sufficient mass is contained within a small enough volume, the escape velocity from the mass may exceed the speed of light, creating a black hole, where the curvature of space prevents the exit of light for any source within the Schwarzschild radius.
• 33.5 Evidence for the expansion of the universe comes from the apparent redshift of all distant objects: everything far enough away is moving away, not toward us. There are three redshifts:
  • Doppler shift: due to motions through space, toward (blueshift) or away from (redshift) the source.
  • Cosmological shift: due to expansion of space, always away (redshift).
  • Gravitational shift: due to potential energy loss as light leaves a massive object.
  The rate of expansion of the universe is given by the Hubble constant, H₀. Since different parts of the universe were once together in one place, conditions in widely separated parts of the universe are assumed ot be the same -- the universe is isotropic.
• 33.6 Running the expansion backwards at its current rate gives an origin-of-the-universe date of about 13 billion years ago. The event which caused the expansion was named the Big Bang. Models predicted the existed of cosmic microwave background radiation, which was detected (somewhat by accident) in 1964. Since then, examination of the CMB reveals anisotropy, variations in the emission temperature, which also fits the model of the Big Bang. Current cosmology theories identify different phases in the expansion by how energy was transmitted: initial high energy concentrations prevented the formation of stable matter and even the free transmission of photons. Subsequent phases as density decreased allowed transmission of photons and formation of atoms. When we look at distant objects, we actually are looking back in time. We can't look back into the first 300 000 years of the universe when photon transmission was blocked. We can only see those objects which were less than 13 billion light years away 13 billion years ago; light objects further away formed since then has not yet reached us.
• 33.7 The Standard Cosmological Model provides a basic sequence of phases and time periods which current observations support. Each phase is marked by the breaking of symmetry resulting in separation of forces (electroweak era splits into electromagnetic and weak forces) or the creation of new particles (hadrons, leptons). At around 300 000 years after the initial event, matter becomes less dens than energy, the universe becomes transparent, and stars and galaxies form. Expansion continued but something happened about 5-6 billion years ago to cause the rate to increase.
• 33.8 The expansion of the universe (and space itself) is countered by masses exerting gravitational attraction. The universe is nearly flat with gravitational attraction nearly equally offsetting cosmological expansion. Local masses attract each other enough to lead to formation of galaxies and galactic clusters.
• 33.9 To explain the structure and movement of galaxies, we need to assume more mass than we can observe through light-interacting matter. Astronomers posit a "dark matter" capable of gravitational attraction but not of photon interaction. Light-interacting "normal" matter is only about 5% of the total energy-matter substance of the universe; dark matter makes up another 25%. Astronomers suggest the remaining 70% is "dark energy" that may be repulsive to space, causing the observed acceleration of expansion.
• 33.10 The distribution of visible matter (galaxies) in space is non-random, rather more like the film of bubbles in a foam, with pockets of little to no matter inside.
• 33.11 One of the interesting conclusions of cosmology is the anthropic principle: that we see a universe which is a particular way because we are here, i.e., because we are the product of that universe. If the universe were different, we wouldn't be here to observe it.

Key Equations

Web Lecture

Read the following weblecture before chat: Cosmology

Study Activity

Chat Preparation Activities

• Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

Lab Work

• There is no lab for this unit.

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