By Jeanne Bishop, Westlake High School, Westlake, OH
In this activity students will model the time after the Big Bang when the first nuclei of hydrogen and helium were created. The students will move and display cards that show the elements that are formed. The teacher should emphasize the need to be quiet and follow directions for this activity. Use a large area--an outside location, a large classroom with seats moved back, or a gym.
Each student receives a set of 5 cards made from index cards or stiff paper (The cards or paper may be color coded for easy identification)
1. To begin, students arrange themselves in a tight central group representing the matter that emerges in the first second, a soup of elementary particles. Students in the model represent parts of forces that will explode apart to become protons (p's or normal H nuclei) and neutrons (n's). Give one student the two temperature cards. Students have sets of cards with names or formulas: p or n, 2H, 3H, 3He and 4He.
Students hide all cards, since there are no elements before the Big Bang. If indoors, turn out the lights, representing the absence of electromagnetic energy before the Big Bang
2. When the lights are turned on, the teacher or a student calls "Big Bang." With this cue, the student inside the tight group with the temperature sign 10 BILLION DEGRES K holds it up. Most students hold up PROTON card and a few hold up NEUTRON. Students start moving out from the dense center.
3. Some students should encounter (that is, safely bump) other students. For a while, the "particles" should not stick together.
4. After allowing time for the students to move and expand out a short ways (which may be as short as 15 seconds in real time), the teacher gives the cue call of "100 seconds." The student with the temperature signs pulls away the 10 BILLION DEGREES K and sticks up 1 BILLION DEGREES K. Students hold the signs and continue to move outward. When two PROTONS meet, the two students decide if they want to bounce away or stick by holding hands (or locking arms) to form a deuterium nucleus. Then these students hide their PROTON cards and one displays a DEUTERIUM (2H) card. Couples representing deuterium continue bending and moving outward to encounter other PROTONS. Couples representing deuterium cannot stick to other single or couples of students until the next time cue is given (at step 6). The reaction is: p + p -> D + positron + neutrino.A PROTON and NEUTRON may also decide to stick together to form DEUTERIUM. This reaction is p + n -> D
5. If a DEUTERIUM bumps into a neutron and they join, TRITIUM (3H) is formed. The three students should hold hands (or lock arms) and exchange their cards for a 3H. The reaction is: D + n -> 3H
6. After seeing that the students have started forming Deuterium and some 3H, the teacher gives the cue call of "200 seconds." The DEUTERIUM couples do not have to join to other single or couples of students when they bump. However, now students can join if they wish. For a DEUTERIUM sticking to a free PROTON, the three students should all hold hands, hide the DEUTERIUM and NEUTRON cards, and show a HELIUM (3He) card. The reaction is: D + p -> 3He
7. Following formation of 3He, if an 3He joins another 3He, only four of the six students should hold hands, hide the 3He cards, and show a HELIUM 4 (4He) card. Two of the students holding PROTON cards should leave the HELIUM 4 group and hold up their original PROTON cards. (Note: Students may want to stick more often that what correctly models the early universe.) The reaction is 3He + 3He -> 4He + p + pOther possible reactions to produce 4He are:
8. The teacher directs one of the remaining HELIUM 3 (3He) join with an HELIUM 4 (4He). The teacher gives them the BERILLIUM 7 (7Be) card and a ping-pong ball representing an electron (e-). First this group should hold up the BERILLIUM 7(7Be) card and the ping-pong ball (e-). They then touch the BERILLIUM 7 (7Be) to the electron, forming LITHIUM-7 (7Li). They should then hide the BERILLIUM 7 (7Be) card and the ping-pong ball, and hold p the LITHIUM-7 (7Li).This reaction is 3He + 4He -> 7Be, 7Be + e- -> 7Li.
9. The teacher gives the cue call of "End". Students freeze in position with signs up so that all can see what has occurred. Ask students if what the model shows is the correct ratio of nuclei from element formation in the Big Bang (90 percent total H and 10 percent total He with almost no free neutrons). Probably the outcome will be wrong. Ask students what can be done to make the H to He ratio correct. Students probably will need to split some nuclei. Discuss who should split and then do it. Student with the temperature card pulls it down, indicating a further cooling of the universe.
Follow-up questions (while still in model positions or later)
1. What is wrong with "freezing" the motion at
the end of the activity?
(The nuclei continue to move.)
2. How are the numbers of particles and atoms we have
formed different from what really occurred?
(Actually there were trillions times more hydogen and helium formed right after the Big Bang. The small number os students in this model woudl be a tiny part of the total. In addition, most electrons, positrons and neutrinos were not included.)
3. Why can't the groups keep combining like they did in
this activity to form all the elements heavier than hydrogen, helium, and
(The universe is not hot and dense enough fro further joining of nuclei (fusion) in this way.)
4. Since the atoms are moving, what caused the atoms to
clump together to form stars, quasars, and galaxies?
Inflation fluctuations were frozen into space-time. That means they were converted into slightly denser and slightly less dense regions. The force of gravity locally collected large groups of atoms to clump them together.)