Galaxies & Cosmology

Part 2

There are four chapters in Part 2 of Galaxies and Cosmology:

• Chapter Five - Introducing Cosmology - The science of the Universe.
  The remaining chapters of the book provide a detailed introduction to cosmology, and Chapter 5 provides an outline of the theoretical basis for relativistic cosmology. This chapter describes some of the most important concepts that are needed to understand how modern cosmology describes the behaviour and evolution of the Universe.
  
  The chapter starts by considering some of observed properties of the Universe, such as the fact that it is expanding. This sets the scene for an introduction to how cosmologists go about understanding the Universe by the process of constructing mathematical models (see Comments below).
  
  The chapter gives an account of how scientists, starting with Einstein, have developed a set of closely related cosmological models. These models encompass a range of possibilities - such as model universes that are not expanding, or universes that expand at an ever-increasing rate. The differences between models are determined by certain physical parameters - such as a parameter that describes the average density of matter. This chapter is concerned with illustrating the principle that the measurement of these parameters allows astronomers to determine which cosmological model best describes the Universe in which we live.
  
  The image at the top left of this page shows a temperature map of the cosmic microwave background radiation in the Universe, red being hotter, blue shades cooler, courtesy of NASA.
  

Comments:

The course team put the following on the web page for those of you less confident with maths:

"A full treatment of these concepts would be mathematically demanding, and the course team has attempted to present the key ideas in as straightforward a way as possible. However, we recognize that it is possible that some students might find the somewhat more mathematical nature of this chapter off-putting, so some notes of guidance are provided.

"If the mathematical aspects of Chapter 5 are a concern to you, here are some hints that might help:

Remember that the equations are designed to help support the argument given in the text. So, on first reading, you may want to simply ignore the equations, and read the text as if the equations weren't there. (You might be interested to know that even mathematical physicists do the same. In his essay in the book, It Must be Beautiful: Great Equations of Modern Science, Professor Roger Penrose reveals that this is how he sets about reading text that contains equations.)

On reading through the text again you may want to see whether trying to understand the equations helps you to better understand the argument. Once you have an idea about the context in which an equation is being used, you are in a good position to judge its significance and how it fits into the argument that is developed.

Finally, questions in the text will give you some practice in using those equations that you are expected to be familiar with.

• Chapter Six - Big Bang cosmology - The evolving universe.
  This chapter considers the evolution of those FRW models (Chapter 5) in which the scale factor was zero at some time in the past. The early stage of expansion of such models is referred to as the big bang, and the chapter starts by investigating why it is believed that the physical conditions in the big bang would have been characterized by very high temperatures and densities.
  
  The remainder of the chapter is a discussion of the history of the Universe, starting from the very earliest times that can be described by current physical theory. There is particular emphasis on aspects of the early Universe that can be tested by observation, such as the abundances of light elements and the cosmic microwave background. The account of cosmic history in this chapter ends with a discussion of the formation of structure in the Universe and the role that dark matter is believed to have played in this process.

Comments:

The learning outcomes are given as:


Explain why the dominant energy density in the early Universe was that of radiation, and why the radiation temperature was extremely high at early times.

Discuss the limits of current physical theory with reference to the Planck time and the experimental verification of physical laws.

Describe in qualitative terms the major events that have occurred in the Universe from the Planck time to the present day.

Discuss whether particular aspects of cosmological theory are well-understood.

Describe the observations that can be carried out that probe conditions in the early Universe.

• Chapter Seven - Observational Cosmology - Measuring the universe.
  Observational cosmology is the branch of science concerned with measuring the parameters that characterise the Universe. This chapter is mainly concerned with the determination of some of the parameter values, which helps us to determine which of the various Friedmann-Robertson-Walker models (FRW models, Chapter 5) we live in. The current 'best values' for these parameters are discussed, along with the methods used to arrive at those values.

• Chapter Eight - Questioning Cosmology - Outstanding problems about the universe.
  A short chapter which addresses some of the outstanding problems in Cosmology, and considers how the standard Big Bang model might be expanded to provide a more adequate account of reality. The problems considered are:

  What is the dark matter?

  What is the dark energy?

  Why is the Universe so uniform?

  Why does the Universe have a flat geometry?

  Where did the structure come from?

  Why is there more matter than antimatter?

  What happened at t=0?

  Why is the Universe the way it is?

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