Big Bang Cosmology
Topic/Concept: Big Bang Cosmology
Prerequisite knowledge required: Working model of the structure of the universe: the Milky Way is one of innumerable galaxies, most of which are at great distances away: Light and the electromagnetic spectrum: concept of Doppler shift in light (redshift vs. blueshift), finite speed of light means that we see astronomical objects as they were not as they are. Gravitation: all objects attract each other no matter what.
Resources required:
- Dark balloons and white-out
- Computer projector and screen
- Xeroxed handouts
Learning Goals: Students will gain an appreciation for both the power and limitations of a scientific theory. Students will differentiate between scientific and non-scientific theories. Students will apply scientific reasoning to construct their own theories based on available evidence. Studying the specific example of the Big Bang theory, students will begin to approach the current understanding of the origin and evolution of the Universe. Students will come to believe that acceptance or rejection of a scientific theory is not a matter of belief.
Learning Objectives: By means of an involved in-class small group activity followed by lecture/discussion and homework assignment, we will expose the Big Bang theory in its historical context and retrace the initial steps used by astronomers and physicists to construct this picture. Content covered will include:
- Cosmological redshift as evidence for expansion of space. Measurement of redshift and brief treatment of cosmological distance ladder. Hubble flow.
- Problems with gravitation. Newton invokes God to explain why Universe doesn’t collapse in upon itself.
- Comparison of the Steady-State “Old” Universe model to the Big Bang “Young” Universe cosmology. Olbers’ Paradox. Conflict among the scientists—Fred Hoyle coins the derogatory term “Big Bang.”
- Evolving universe—How galaxies observed to be at greater distances are also younger.
- The Cosmic Microwave Background—prediction, discovery, measurement, and implications.
In our treatment of the Big Bang as a scientific theory, we will draw some parallels between the history of its development and that of another major scientific theory—biological evolution by means of natural selection.
Detailed description of activity: It is 1929. Astronomer Edwin Hubble, using the world’s largest telescope, has just made a shocking discovery. Measuring the light from distant galaxies, he has observed that not only is nearly every galaxy, redshifted, but that the amount of redshift appears to be directly proportional to the distance of the galaxy from Earth…
Phase 1
Students will be divided into small groups of 5 or 6. Armed with handouts that contain evidence available in 1929, specifically Hubble’s observations of galaxy redshifts, the Newtonian and Einstein perceptions of gravity (including Einstein’s “fudge factor”), and Olbers’ Paradox, students will be asked to address the following questions:
- What may we infer about the origin of the Universe?
- How old is the Universe? Did it have an origin at a fixed point in time, or has it been around forever?
- Can we say anything about the possible fate(s) of the Universe?
It will be important to emphasize that we are interested only in scientific answers to these questions; namely, those that follow logically from the available evidence and can be used to make testable predictions. A viable theory will be both self-consistent and offer approaches to answering the above questions.
Phase 1a
Assuming that a majority of the groups develop one or more theories involving an expanding Universe, partway through Phase 1 they will be thrown the following teaser question: —Is there any way to explain the fact that all galaxies are moving away from us in every direction without placing the Earth at the center of the Universe?”
Phase 2
After 15 minutes or so, depending on the progress of the small group discussions, the class will reconvene to share conclusions. We will summarize a few of the most popular student theories to questions on the blackboard. Then a second handout will be distributed, outlining two important new pieces of evidence that were unavailable in Hubble’s time: 1) The prediction by Gamow, Alpher and Herman of an afterglow to the Big Bang and the subsequent discovery of the CMB by Penzias and Wilson and 2) Observations that indicate different types of galaxies at different distances. Students will then return to their small groups and attempt to refine their theories in light of this new evidence.
Phase 3
Finally, an in-class lecture/discussion of the currently accepted Big Bang Cosmology and its emergence over the Steady State model will anchor this activity. This discussion may take place during the next class period, since Phases 1 and 2 are likely to consume a full 50 minutes. The lecture may include examples of calculating redshifts from spectra and applying Hubble’s Law to calculate the age of the Universe.
Assessment: The students will likely find this activity to be very challenging. For this reason, it is important that the instructor constantly make rounds of the small groups, offering guidance and clarification but being careful not to push the students in too specific a direction. The instructor must be open to the many alternative interpretations of cosmology that the students may put forth. Success in this exercise is not necessarily coming up with a replica of the Big Bang theory in one short class period. It is assumed that students have not been living in a vacuum with respect to Cosmology, most will have heard of the theory before. Instead, evidence of scientific thinking observed as the students participate in the activity is the most important objective. Later on, accurate understanding of the Big Bang Cosmology as a scientific theory will be assessed by means of an essay, while some quantitative topics will be assessed by means of a problem set.
Assignment
- READING: “In the Beginning,” by Simon Singh. Playboy, May 2005, page 66 (Xeroxed handout)
- SHORT ESSAY: Evaluate the merits of the Big Bang as a scientific theory. What are its strengths and weaknesses? What outstanding questions does the theory fail to address? What additional evidence might serve to strengthen or weaken this theory?
- PROBLEM SET: Quantitative problems in the Big Bang Cosmology. Calculating redshift from spectra. Calculating velocity from redshift. Using Hubble’s Law to calculate the age of the Universe.
Associated file:Origin of the Universe.pdf