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Size 2.1Mb Date Jan 17, 2006 |
Figure 3. Birkhoff's Theorem and the Expansion of the Universe. A number of galaxies are shown, together with their velocities relative to a given galaxy G, indicated here by the lengths and directions of the attached arrows. (In accordance with the Hubble law, these velocities are taken to be proportional to the distance from G.) Birkhoff's theorem states that in order to calculate the motion of a galaxy A relative to G, it is only necessary to take into account the mass contained within the sphere around G that passes through A, shown here by the dashed line. If A is not too far from G, the gravitational field of the matter within the sphere will be moderate, and the motion of A can be calculated by the rules of Newtonian mechanics....
Figure 5. Red Shift vs. Distance. The red shift is shown here as a function of distance, for four possible cosmological theories. (To be precise, the 'distance' here is 'luminosity distance'— the distance inferred for an object of known intrinsic or absolute luminosity from observations of its apparent luminosity.) The curves labelled 'density twice critical', 'density critical', and 'density zero' are calculated in the Friedmann model, using Einstein's field equations for a matter-dominated universe, without a cosmological constant; they correspond respectively to a universe that is closed, just barely open, or open. (See figure 4.) The curve marked 'steady state' will apply to any theory in which the appearance of the universe does not change with time. Current observations are not in good agreement with the 'steady-state' curve, but they do not definitely decide among the other possibilities, because in non-steady-state theories galactic evolution makes determination of distance very problematical. All curves are drawn with the Hubble constant taken as 15 kilometres per second per million light years (corresponding to a characteristic expansion time of 20,000 million years), but the curves can be used for any other value of the Hubble constant by simply rescaling all distances....
ENERGY PER UNIT VOLUME PER UNIT WAVELENGTH RANGE: 3°K (electron volts per cubic centimetre per centimetre)...
This example also helps us to understand the shifting meaning of what we call 'conserved' quantities...
(The free neutron is actually unstable, with an average life of 15.3 minutes, but nuclear forces make the neutron absolutely stable in the atomic nuclei of ordinary matter.) Also, as far as we know, there is no appreciable amount of antimatter in the universe...
The cosmic rays are overwhelmingly matter rather than antimatter -in fact, no one has yet observed an antiproton or an antinucleus in the cosmic rays...
The First Three Minutes
We are now prepared to follow the course of cosmic evolution through its first three minutes...
In order eventually to predict the abundances of the chemical elements formed in the early universe, we will also need to know the relative proportion of protons and neutrons...
This account of the early universe has one consequence that can be immediately tested against observation: the material left over from the first three minutes, out of which the stars must originally have formed, consisted of 22-28 per cent helium, with almost all the rest hydrogen...
Our knowledge of the cosmic deuterium abundance was put on a much firmer basis by ultra-violet observations in 1973 from the artificial earth satellite Copernicus...
It is argued that the 20 parts per million of deuterium found by Copernicus could not have been produced by any conventional astrophysical mechanism without also producing unacceptably large amounts of the other rare light elements: lithium, berylium and boron...
Of course, at the same time that information was flowing badly from experimenters to theorists, it was also flowing badly from theorists to experimenters. Penzias and Wilson had never heard of the Alpher-Herman prediction when they set out in 1964 to check their antenna. Third, and I think most importantly, the 'big bang' theory did not lead to a search for the 3° K microwave background because it was extraordinarily difficult for physicists to take seriously any theory of the early universe. (I speak here in part from recollections of my own attitude before 1965.) Every one of the difficulties mentioned above could have been overcome with a little effort. However, the first three minutes are so remote from us in time, the conditions of temperature and density are so unfamiliar, that we feel uncomfortable in applying our ordinary theories of statistical mechanics and nuclear physics. This is often the way it is in physics - our mistake is not that we take our theories too seriously, but that we do not take them seriously enough. It is always hard to realize that these numbers and equations we play with at our desks have something to do with the real world. Even worse, there often seems to be a general agreement that certain phenomena are just not fit subjects for respectable theoretical and experimental effort. Gamow, Alpher and Herman deserve tremendous credit above all for being willing to take the early universe seriously, for working out what known physical laws have to say about the first three minutes. Yet even they did not take the final step, to convince the radio astronomers that they ought to look for a microwave radiation background. The most important thing accomplished by the ultimate discovery of the 3° K radiation background in 1965 was to force us all to take seriously the idea that there was an early universe. I have dwelt on this missed opportunity because this seems to me to be the most illuminating sort of history of science. It is understandable that so much of the historiography of science deals with its successes, with serendipitous discoveries,...
The Great Galaxy M31 in Andromeda. This is the nearest large galaxy to our own. The two bright spots to the upper right and below the centre are smaller galaxies, NGC 205 and 221, held in orbit by the gravitational field of M31. Other bright spots in the picture are foreground objects, stars within our own galaxy that happen to lie between the earth and M31. This picture was taken with the 48-inch telescope at Palomar. (Hale Observatories photograph)...
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