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Physics LibreTexts

A Cosmology Workbook

It has been quite a century for our understanding of the cosmos. As I write these words at the beginning of 2017 it was just over 100 years ago, in November 1915, that Albert Einstein finished development of his general theory of relativity, work that would provide the theoretical context for interpreting Edwin Hubble's distance-redshift law, published in 1929, as due to the expansion of the Universe. In the 40s, Gamow and collaborators speculated that the dense conditions that must have existed earlier in this expanding universe could provide another site, in addition to the cores of stars, for the fusion of light elements to heavier ones. In fact, to avoid over-production of the elements, this earlier, denser phase would have to be very hot. In the 1960s Bell Labs scientists accidentally stumbled upon the thermal radiation left over from that heat, that exists in the current epoch as a nearly uniform microwave glow. With that discovery, the idea that the universe used to be hot, dense, and expanding very rapidly became the dominant cosmological paradigm known as the "Big Bang."

The subsequent fifty years of the past century saw much progress as well. We now know that we do not know what constitutes 95% of the mass/energy in the universe. Only 5% of the mass/energy is composed of constituents in the particle physicist's standard model. Most of the rest is "dark energy" which smoothly fills the universe and dilutes only slowly, if at all, as the universe expands. The rest is "dark matter" that, like George Lucas's mystical Force, "pervades us and binds the galaxy together." Measurements of light element abundances, combined with modern, precision, version's of Gamow's big bang nucleosynthesis calculations, give us confidence we understand the expansion back to an epoch when the presently observable universe was \(10^{27}\) times smaller in volume than it is now. A speculative theory, known as cosmic inflation, has met with much empirical success, giving us some level of confidence we may understand something about events at yet higher densities and even earlier times. 

In this quarter-long course we will at least touch upon all the topics in the above two paragraphs. We will learn how to think about the expanding universe using concepts from Einstein's theory of general relativity. We will use Newtonian gravity to derive the dynamical equations that relate the expansion rate to the matter content of the universe. Connecting the expansion dynamics to observables such as luminosity distances and redshifts, we will see how astronomers use observations to probe these dynamics, and thereby the contents of the cosmos, including the mysterious dark energy.

We will introduce some basic results of kinetic theory to understand why big bang nucleosynthesis leads to atomic matter that is, by mass, about 25% Hydrogen, 75% Helium with only trace amounts of heavier elements. We'll use this kinetic theory, applied to atomic rather than nuclear reactions, to explore perhaps the most informative cosmological observable: the cosmic microwave background. Finally, we will study how an early epoch of inflationary expansion, driven by an exotic material with negative pressure, can explain some of the otherwise puzzling features of the observed universe. 

  • 1. Euclidean Geometry
    We want to emphasize that the labels themselves, x, y, z or x', y', z' have no physical meaning. All physical meaning associated with the coordinates comes from an equation that tells us how to calculate distances along paths.
  • 2. Galilean Relativity
    The idea is that the results of experiments done in any inertial frame should be the same; i.e., one should not be able to determine by some experimental means what frame is at "absolute rest" and which frames are moving -- all motion is relative. An inertial frame is one in which Newton's laws of motion are satisfied. These laws prescribe accelerations, not velocities, so a frame that is moving at constant relative velocity with respect to an inertial frame must itself be an inertial frame.
  • 3. Einstein Relativity
    While the principle of relativity holds, its specific implementation as Galilean relativity does not. As you know, because you have studied special relativity, this is indeed the correct solution to the puzzle of the Maxwell Equations lack of invariance under a Galilean transformation.
  • 4. Curvature
    Before continuing with spacetime, we temporarily retreat to the study of space alone. We do so to introduce the notion of "curvature'' in an attempt to loosen up your understanding of the nature of space, to have you better prepared to think about the expansion of space.
  • 5. Spacetime
    We can label spacetimes with coordinates, for example, we could label every point in a 1+1-dimensional space with a t value and an x value. These coordinates are just labels, with no physical meaning, until we also say something about the "invariant distance" between infinitesimally separated pairs of points.
  • 6. Redshifts
    We begin to work out observational consequences of living in an expanding spatially homogeneous and isotropic universe.
  • 7. Distances as Determined by Standard Candles
    Measuring the flux (energy/unit time/unit area) give us a way to figure out the distance to the object assuming we know its luminosity and that it is emitting isotropically. This is the so-called standard candle method of distance determination. Here we work out the theoretical relationship between flux, luminosity, curvature constant k, coordinate distance between observer and source, and the redshift of the source.
  • 8. The Distance-Redshift Relation
    For a given scale factor history, a(t) , one can work out a relationship between luminosity distance and redshift. This will be useful to us because it shows how we can infer  a(t) from measurements of luminosity distance and redshift, over a range of redshifts.
  • 9. Dynamics of the Expansion
    In the following set of chapters we will derive the dynamical equations that relate the matter content in a homogeneous and isotropic universe to the evolution of the scale factor over time.
  • 10. A Newtonian Homogeneous Expanding Universe
    Consider a homogeneous expanding universe in a Newtonian manner, where space is fixed, and the universe is filled with a fluid that is moving in such a manner as to keep the density spatially uniform. If homogeneity is to be preserved over time, then the motion must be such that the separation between any pair of fluid elements must scale up with time in the same way.
  • 11. The Friedmann Equation
    Sticking with our Newtonian expanding universe, we will now derive the Friedmann equation that relates how the scale factor changes in time to the mass/energy density. We will proceed by using the Newtonian concept of energy conservation. (You may be surprised to hear me call this a Newtonian concept, but the fact is that energy conservation does not fully survive the transition from Newton to Einstein).
  • 12. Particle Kinematics in an Expanding Universe: Newtonian Analysis
    We want to understand how an observer, at rest in their local rest frame, will observe the evolution of peculiar velocities of free particles. In a Newtonian analysis, the local rest frame will be an inertial frame (one in which Newton's laws of motion apply) only if acceleration is a constant.
  • 13. The Evolution of Mass-Energy Density and a First Glance at the Contents of the Cosmos
    We've seen that the rate of change of the scale factor depends on the mass density rho. In order to determine how the scale factor evolves with time, we thus need to know how the density evolves as the scale factor changes.
  • 14. Energy and Momentum Conservation
    The lack of energy conservation in an expanding universe is quite surprising to people with any training in physics and therefore merits some discussion, which we present here in this chapter. The student could skip this chapter and proceed to 15 without serious harm. If, subsequently, the lack of energy conservation becomes too troubling, know that this chapter is here for you.
  • 15. Pressure and Energy Density Evolution
    There is a sense in which energy is conserved in general relativity. We say it is locally conserved, which effectively means that in a sufficiently small region of spacetime, the change in energy is equal to the flux of energy across the boundary of the region, including that via any work being done on the region.
  • 16. Distance and Magnitude
    We have the invariant distance equation for a homogeneous and isotropic universe.  We now introduce no fewer than five kinds of spatial distances: Coordinate distance, Physical distance, Comoving distance, Luminosity distance, and Comoving angular diameter distance.
  • 18. The Early Universe
    To understand the "primordial soup" and its relics, we now turn our attention from a relativistic understanding of the curvature and expansion of space, to statistical mechanics. We begin with equilibrium statistical mechanics, before moving on to a discussion of departures from equilibrium. We will come to understand the production in the big bang of Helium, photons, other "hot" relics such as neutrinos, and "cold" relics such as the dark matter. We will also discuss the observations that test
  • 19. Equilibrium Statistical Mechanics
    Out of the early Universe we get the light elements, a lot of photons and, as it turns out, a bunch of neutrinos and other relics of our hot past as well. To understand the production of these particles we now turn to the subject of Equilibrium Statistical Mechanics.
  • 20. Equilibrium Particle Abundances
    At sufficiently high temperatures and densities, reactions that create and destroy particles can become sufficiently rapid that an equilibrium abundance is achieved. In this chapter we assume that such reaction rates are sufficiently high and work out the resulting abundances as a function of the key controlling parameter. We will thus see how equilibrium abundance changes as the universe expands and cools.
  • 21. Hot and Cold Relics of the Big Bang
    As the temperature and density drops, the reactions necessary to maintain chemical equilibrium can become too slow to continue to do so. This departure from equilibrium can occur while the particles are relativistic, in which case we have "hot relics," or when the particles are non-relativistic in which case we say we have "cold relics." The cosmic microwave and neutrino backgrounds are hot relics. The dark matter may be a cold relic.

Thumbnail: This is a modification of the Flammarion Woodcut is an enigmatic woodcut by an unknown artist. The woodcut depicts a man peering through the Earth's atmosphere as if it were a curtain to look at the inner workings of the universe. The original caption below the picture (not included here) translated to: "A medieval missionary tells that he has found the point where heaven and Earth meet...".

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