The Decay of the Neutron, The Big Bang and aDarka Matter in the Universe
Geoffrey L. Greene
National Institute of Standards and Technology
About the Lecture
One of the major constituents of ordinary matter, the neutron, is unstable when it is isolated from the atomic nucleus. On average, a “free” neutron has a lifetime of about 15 minutes. This time defines the time scale during which the light elements (hydrogen, helium and lithium ...) were formed during the “big bang creation” of the universe. The abundance ratios of these light elements has changed very little in the subsequent evolution of the universe.
As a result, astronomical observations can be compared with theoretical predictions to provide a test of the big bang theory. A remarkable conclusion of such comparisons is the possibility that a substantial, perhaps dominant, portion of the mass of the universe is not in the form of “ordinary” matter.
The reasoning which leads to the possibility of such “dark” matter will be discussed. The talk will also describe some of the experimental work on the neutron lifetime which is currently in progress at the National Institute of Standards Cold Neutron Research Facility in Gaithersburg, Maryland.
About the Speaker
Mr. Geoffrey Greene received his Ph.D from Harvard University in 1977. His research interests have focussed on tests of basic physical laws using very low energy neutrons. Mr. Greene was on the faculty of Yale University and came to the National Institute of Standards and Technology (NIST) in 1983 where he currently leads a group carrying out precision measurements at the NIST National Cold Neutron Research Facility in Gaithersburg, Maryland. He is the author of 80 scientific publications and edited the book The Investigation of Fundamental Interactions with Cold Neutrons.
The President, Ms. Enig, called the 2031st meeting to order at 8:16 p.m. on September 30, 1994. The Recording Secretary read the minutes of the 2030th meeting and they were approved. The President then read a portion of the minutes from the 422nd meeting, May 12, 1894.
The President introduced Mr. Geoffrey L. Greene, National Institute of Standards and Technology, who discussed “The Decay of the Neutron, the Big Bang and ‘Dark’ Matter in the Universe”.
In answering the question “what is the world made of” Mr. Greene said we must rely on a confluence of many disciplines. A simple abundance chart for the elements observable in the universe (by mass) shows that it is 75% hydrogen, 25% helium, <1% everything else. In considering the neutron and its decay, we must answer, why is there any ordinary matter in the universe at all. Why haven't all the neutrons decayed since, it has been somewhat more than 11 minutes since the Big Bang? Neutron decay is exothermic by 0.783 MeV. The reason why they have not all decayed is that a neutron combined with a proton, the deuterium isotope of hydrogen, is 1.490 MeV lighter than two protons. The process of forming a deuteron from two protons is exothermic; it happens in stars but not in a bottle or on palladium rods.
The question how much ordinary matter is there in the universe was determined in the first 3 minutes after the Big Bang. At about 1 sec of age only common “garden variety” particles were present. Of those that were nucleons about 25% were neutrons and 75% were protons. At about 300 sec (less than a neutron half-life) the temperature had dropped sufficiently for atomic nuclei to begin to freeze out and the ratio of neutrons to protons was sufficient to result in 24% of the mass being helium. As Steven Weinberg has said, nothing much of interest has happened since.
What will be the fate of the universe, a Big Chill or a Big Crunch? The answer depends on how much mass we find in the universe. The cooling rate of the early universe depended on the number of neutrinos and the density of particles. Presently the universe contains on average about 3 nucleons per 100 cubic meters. From the observed density nearby, there must be a lot of very empty space somewhere else. The expansion of the universe is thought to follow Hubble's law — the velocity of recession between two objects is approximately equal to their distance times Hubble's constant H, presently estimated to be 15 km per sec per 106 light-year. The critical density of the universe is given by 3*H2/8*p*G. If the density of the universe is greater than this, the universe will eventually collapse in a Big Crunch. If the density is equal to or greater than this, the universe will expand and cool forever in a Big Chill. Within the present limits of measurement the ratio of the density of the universe to the critical density is between 2.0 and 0.01; so, the question is presently undecidable.
It can be calculated that shortly after the big bang, the ratio must have been within 10-17 of being 1. Why should the ratio be so close to 1? Some arguments for it are (1) naturalness, which is compelling but not explanatory, (2) an early period, referred to as inflationary, when the expansion of the universe was much faster and which may be evidenced by the relative uniformity of the background microwave radiation, or (3) the fact that galaxies exist, because if the ratio were much different they wouldn't have had time to form.
How much mass can we see in the universe? From the observable mass in the universe the density ratio is only 0.1. Let us turn the issue around; assume we know where all the mass is and attempt to predict its movement. If there is missing mass, it should lead to discrepancies in the movement of what we can see. The rotation of most spiral galaxies appears to be like rigid disks, the angular velocity increases linearly with distance from the center. This can only be explained if there is an invisible galactic halo of dark matter.
How much mass is missing and what is it? Only 1% of the critical density can be accounted for by visible matter. Only 10% of the critical density can be accounted for by ordinary matter. About 20% of the critical density might be accounted for by other matter. What can account for this missing mass that is not gas, dust or snowballs? Two of the suggested possibilities are MAssive Condensed Halo Objects, or MACHOS, and Weakly Interacting Massive Particles, or WIMPS.
The President thanked the speaker on behalf of the Society. The incoming President for the membership chairman announced two new members. The President announced the speaker for the next meeting, made the parking announcement, and adjourned the 2031st meeting at 9:41 p.m.
John S. Garavelli