Updates on Three Problems in Modern Astronomy
COBE and How We Know There Was A Big Bang, The Solar Neutrino Problem and How We Know the Stars Run on Nuclear Energy, Gamma Ray Bursters and How Sometimes We Don't Know Anything
Virginia L. Trimble
Professor Physics Department, University of California at Irvine and Astronomy Department, University of Maryland at College Park
About the Lecture
To judge from reports in the popular press, every scientific advance is either epoch-making or disastrous. In fact, science generally progresses rather like an amoeba, throwing out occasional pseudopods in directions that look promising, and then either pulling the rest of the body after, or retreating. This talk will focus on three issues that have made the science columns of newspapers in the last year or two, explaining how we came to our present understanding of cosmology (the large scale structure and evolution of the universe) and of the energy sources in stars and the progress being made on the very curious and puzzling phenomena of bursts of gamma rays coming from random directions in the sky, seemingly uncorrelated with anything else we know about.
About the Speaker
Ms. Trimble received her Ph.D. in Astrophysics from the California Institute of Technology in 1968. Her research has encompassed topics of astrophysics such as binary stars, open clusters, supernova remnants, and dark matter. Throughout her career, she has served on the boards of various professional societies, including the American Physical Society, the American Association for the Advancement of Science, as well as on the editorial boards of various scientific journals. She is currently a professor at the University of California at Irvine, a visiting professor at the University of Maryland, and editor or editorial board member of 6 journals, including “The Scientist”, and “Astrophysical Journal”.
The President Mr. Ohlmacher called the 2046th meeting to order at 8:15 p.m. on September 15, 1995. The Recording Secretary read the minutes of the 2044th meeting and they were approved with one correction. The President then read a portion of the minutes of the 439th meeting May 25, 1895.
The President introduced Virginia L. Trimble, professor of physics at the University of California, Irvine, and of astronomy at the University of Maryland, College Park, to discuss “COBE and How We Know There Was A Big Bang, The Solar Neutrino Problem and How We Know the Stars Run on Nuclear Energy, Gamma Ray Bursters and How Sometimes We Don't Know Anything (Updates on Three Problems in Modern Astronomy)”.
Ms. Trimble said she chose as case studies three stories on astronomical research that had been reported in newspaper science columns in the last few years and explain how we came to our present understanding of stars and cosmology. In looking at these phenomena we should follow the dictum of Sherlock Holmes, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” 
Taking it as a model star, consider the mass of the sun. If it were powered by chemical energy it could shine, though not so brightly, for only about 104 years and it seems to have been shining for somewhat longer than that. If it were powered by gravitational contraction only, it could shine for about 107 years. Because it is powered by fusion, it should be able to shine for about 1010 years.
How do we know that fusion powers the sun and stars? Cecilia Payne, the first female Ph.D., first female astronomer and first female professor at Harvard, showed that the sun and stars were mostly hydrogen. Helium had already been discovered spectroscopically on the sun in 1868 and not found on earth until 1898. Arthur Eddington in 1926 calculated that in order to support the mass of the sun its central temperature must be at least 109 degrees. In 1928 Condon, Gurney and Gamow taking into account quantum effects said that the central temperature would have to be only 107 degrees. F. W. Aston, who was awarded a Nobel prize in 1922, pointed out in 1929 that the nuclear reaction of 4 protons to form a helium nucleon should be an exothermic reaction. Subsequently, Hans Bethe proposed a series of nucleosynthesis reactions the net result of which would be the conversion of 4 protons to a helium nucleon plus positrons and, as we know today, electron neutrinos of various energies. It should be possible to measure the flux of neutrinos produced in the sun by various energy- sensitive detectors. The unit of flux has come to be called the solar neutrino unit (SNU). From the very first measurements in 1972 there has been a pronounced shortage, 1/3 to 2/3 of the expected number of SNU's. The deficit does not vary linearly with the energy of the neutrinos as would be expected if the higher energy processes were proceeding at a lower than expected rate. What is the problem? Do we not really understand what is going on in the center of the sun? Two explanations for this deficit are:
the neutrinos may have a small mass and oscillate among forms that cannot be seen by the detectors (the electron neutrino oscillates to become a muon or tauon neutrino), or
neutrinos may not have mass may have small magnetic moments and the magnetic field of the sun induces the oscillation.
How do we know there was a big bang? We presently measure the age of the universe since the big bang as 10-20×109 years by three independent methods, the age of the oldest stars, isotope ratios and rates of radioactive decay, and the observed expansion rate using remote galaxies. The big bang left two relics of a hot, dense, very smooth primordial state: the fraction of helium in the universe, and the 2.7 K isotropic microwave background radiation. We observe that although the background radiation is very smooth, there are galaxies and voids and the distribution of matter in the universe is very lumpy. The COBE observations have shown that the deviation in the background radiation is less than 10-5. This is itself overwhelming evidence for the big bang because nothing else could have produced such isotropic radiation. It is so good, in fact, that it leads us to question how the lumpiness of matter in the universe could have formed without ruffling the background radiation?
There have been two serendipitous results of the effort to detect “illegal” nuclear tests. One was that seismic sensing unequivocally showed tectonic plate boundaries and provided convincing geological evidence for the theory of plate tectonics. The second was that although the COBE, VELA and Cosmos satellite observatories for gamma rays never saw an “illegal” nuclear test, from the late 1960's through 1972 they did detect about 15 gamma ray bursts from space. The detectors in these satellites could detect an event the size of a flare on a nearby star or a supernova half way across the universe, but the spectrum of the gamma rays are not right for either type of event. The bursts have no associated visible or microwave radiation and appear to be both isotropic and random. We are presently unable to explain this phenomenon. In this case we seem to have the details right but the defects in our theories must be fundamental.
 A. C. Doyle, The Sign of the Four, chp. 6, 1890.
Ms. Trimble kindly answered questions from the audience. The President thanked the speaker on behalf of the Society, announced the date for the next meeting, restated the parking policy, and adjourned the 2046th meeting at 9:40 p.m.
Weather: partly cloudy
John S. Garavelli