College Physics for AP® Courses

# Section Summary

### 34.1Cosmology and Particle Physics

• Cosmology is the study of the character and evolution of the universe.
• The two most important features of the universe are the cosmological red shifts of its galaxies being proportional to distance and its cosmic microwave background (CMBR). Both support the notion that there was a gigantic explosion, known as the Big Bang that created the universe.
• Galaxies farther away than our local group have, on an average, a recessional velocity given by
$v=H0d,v=H0d, size 12{v=H rSub { size 8{c} } d} {}$

where $dd size 12{d} {}$ is the distance to the galaxy and $H0H0 size 12{H rSub { size 8{c} } } {}$ is the Hubble constant, taken to have the average value $H0=20km/s⋅Mly.H0=20km/s⋅Mly. size 12{H rSub { size 8{c} } ="20"`"km/s" cdot "Mly" "." } {}$

• Explanations of the large-scale characteristics of the universe are intimately tied to particle physics.
• The dominance of matter over antimatter and the smoothness of the CMBR are two characteristics that are tied to particle physics.
• The epochs of the universe are known back to very shortly after the Big Bang, based on known laws of physics.
• The earliest epochs are tied to the unification of forces, with the electroweak epoch being partially understood, the GUT epoch being speculative, and the TOE epoch being highly speculative since it involves an unknown single superforce.
• The transition from GUT to electroweak is called spontaneous symmetry breaking. It released energy that caused the inflationary scenario, which in turn explains the smoothness of the CMBR.

### 34.2General Relativity and Quantum Gravity

• Einstein's theory of general relativity includes accelerated frames and, thus, encompasses special relativity and gravity. Created by use of careful thought experiments, it has been repeatedly verified by real experiments.
• One direct result of this behavior of nature is the gravitational lensing of light by massive objects, such as galaxies, also seen in the microlensing of light by smaller bodies in our galaxy.
• Another prediction is the existence of black holes, objects for which the escape velocity is greater than the speed of light and from which nothing can escape.
• The event horizon is the distance from the object at which the escape velocity equals the speed of light $cc size 12{c} {}$. It is called the Schwarzschild radius $RSRS size 12{R rSub { size 8{S} } } {}$ and is given by
$RS=2GMc2,RS=2GMc2, size 12{R rSub { size 8{S} } = { {2 ital "GM"} over {c rSup { size 8{2} } } } ","} {}$

where $GG size 12{G} {}$ is the universal gravitational constant, and $MM size 12{M} {}$ is the mass of the body.

• Physics is unknown inside the event horizon, and the possibility of wormholes and time travel are being studied.
• Candidates for black holes may power the extremely energetic emissions of quasars, distant objects that seem to be early stages of galactic evolution.
• Neutron stars are stellar remnants, having the density of a nucleus, that hint that black holes could form from supernovas, too.
• Gravitational waves are wrinkles in space, predicted by general relativity but not yet observed, caused by changes in very massive objects.
• Quantum gravity is an incompletely developed theory that strives to include general relativity, quantum mechanics, and unification of forces (thus, a TOE).
• One unconfirmed connection between general relativity and quantum mechanics is the prediction of characteristic radiation from just outside black holes.

### 34.3Superstrings

• Superstring theory holds that fundamental particles are one-dimensional vibrations analogous to those on strings and is an attempt at a theory of quantum gravity.

### 34.4Dark Matter and Closure

• Dark matter is non-luminous matter detected in and around galaxies and galactic clusters.
• It may be 10 times the mass of the luminous matter in the universe, and its amount may determine whether the universe is open or closed (expands forever or eventually stops).
• The determining factor is the critical density of the universe and the cosmological constant, a theoretical construct intimately related to the expansion and closure of the universe.
• The critical density ρc is the density needed to just halt universal expansion. It is estimated to be approximately 10–26 kg/m3.
• An open universe is negatively curved, a closed universe is positively curved, whereas a universe with exactly the critical density is flat.
• Dark matter’s composition is a major mystery, but it may be due to the suspected mass of neutrinos or a completely unknown type of leptonic matter.
• If neutrinos have mass, they will change families, a process known as neutrino oscillations, for which there is growing evidence.

### 34.5Complexity and Chaos

• Complexity is an emerging field, rooted primarily in physics, that considers complex adaptive systems and their evolution, including self-organization.
• Complexity has applications in physics and many other disciplines, such as biological evolution.
• Chaos is a field that studies systems whose properties depend extremely sensitively on some variables and whose evolution is impossible to predict.
• Chaotic systems may be simple or complex.
• Studies of chaos have led to methods for understanding and predicting certain chaotic behaviors.

### 34.6High-Temperature Superconductors

• High-temperature superconductors are materials that become superconducting at temperatures well above a few kelvin.
• The critical temperature $TcTc size 12{T rSub { size 8{c} } } {}$ is the temperature below which a material is superconducting.
• Some high-temperature superconductors have verified $TcTc size 12{T rSub { size 8{c} } } {}$ s above 125 K, and there are reports of $TcTc size 12{T rSub { size 8{c} } } {}$ s as high as 250 K.

### 34.7Some Questions We Know to Ask

• On the largest scale, the questions which can be asked may be about dark matter, dark energy, black holes, quasars, and other aspects of the universe.
• On the intermediate scale, we can query about gravity, phase transitions, nonlinear phenomena, high- $TcTc size 12{T rSub { size 8{c} } } {}$ superconductors, and magnetic effects on materials.
• On the smallest scale, questions may be about quarks and leptons, fundamental forces, stability of protons, and existence of monopoles.