Gamow,g - The Origin and Evolution of the Universe

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THE ORIGIN AND EVOLUTION OF THE UNIVERSE Author(s): G. GAMOW Source: American Scientist, Vol. 39, No. 3 (JULY 1951), pp. 392-406Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27826381Accessed: 01-04-2015 19:58 UTC

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AGE PHYSICAL CONDITION EVOLUTIONARY PROGRESS

ZERO POINT TO

5 MINUTES LATER

Temperatures of

many billion

degrees. Uniform very high densities.

Primordial y/em, in

the dense field

of radiant energy.

o - e/ectron

> QC ?

0-neutron Q

5 MINUTES

TO

HALF-HOUR

Temperatures of one billion

degrees and below.

Uniform material densities below 0.1 percent of

atmospheric air.

Neutrons and protons begin to stick

together forming

composite nuclei.

-deuferon

o

ABOUT 30

MILLION YEARS

Temperature of 300? abs. (room

temp.) Mean

density about

10-24g-'m3

Lukewarm gas breaks

up into primordial

gaseous galaxies.

ABOUT 3

BILLION YEARS

Temperature of intergalactic space, few

degrees abs. Same density

within galaxies.

Material within galaxies condenses

into stars. Planetary systems

formed. Elementary life begins.

FEW

HUNDRED

MILLION YEARS

Temperature maintained at

few degrees abs. by stars. Same density within

galaxies.

The developed human intellect attacks the

problems of the origin of the universe.

Pictorial History of the Universe

(See pages 397-405.)


AMERICAN SCIENTIST

SUMMER ISSUE JULY 1951

THE ORIGIN AND EVOLUTION OF THE UNIVERSE

By G. GAMOW The George Washington University

The Age of the Universe

THE problem of the origin of the world has been occupying human

mind ever since the dawn of history. All ancient religions, which were, essentially, the first attempts of awakening intellect to find its

place in the surrounding world, discussed the problem of creation at considerable length. Some of them even went so far as to give the exact date of the "creative act." Thus Archbishop Ussher, in the seventeenth century, concluded from the narratives of the Old Testament that the world was created in the year 4004 b.c. Much more elaborate calcula tions by the occult scientists of ancient India lead to the date which would make the world 1,972,949,052 years old as of today. Modern estimates, based on detailed studies of various evolutionary features of the universe, do not claim the precision of the ancient thinkers, but they all agree that the zero point of the history of the universe must be placed at a few billion years ago.

There are two different geological methods for estimating the date when the earth was formed: one of them leads to the age of the oceans, the other to the age of the continents. We can get a fair idea concerning the age of oceans by studying the salinity of ocean water. This water contains about 3 per cent of dissolved salts, which, if extracted and piled up on the land, would cover the area of the United States by a layer almost two miles thick. How did all this salt come into the ocean?

Strange as it may sound at first, salt is being brought into the oceans by the rivers, which wash it away from the rocks forming the crust of the earth. While water evaporates from the ocean surface, and falls again on the continents to repeat its eternal cycle, the dissolved salt stays in and gradually increases the salinity of ocean water.

Based on the Sigma Xi-Resa National Lectureship, 1950-1951, and on the author's

book, Creation of the Universe, soon to be published. All rights reserved.

393


394 American Scientist

Geologists estimate that every year the rivers bring into the ocean about 400,000,000 tons of salt. Since the present amount of ocean salt is 40,000,000,000,000,000 tons, the process must have lasted for at least 100,000,000 years. This figure must be increased by a factor of a few tens, since it is known that at the present epoch the erosion of con tinents is abnormally high, and that during most of the geological time (when there were much fewer mountain ranges than now) the erosion was

only a small fraction of its present value. Thus, the fact itself that the oceans are not saturated with salt proves that they could have existed only for a limited period of time, while the date of their formation may be set at a few billion years ago.

The age of the continents can be estimated by measuring the age of various rocks from which they are formed. It is known that many rocks contain small deposits of radioactive elements, uranium and thorium, which are slowly decaying into lead. Once the rock is solidified from the originally melted state, this radiogenic lead stays together with the original radioactive elements. Therefore, by measuring the uranium/ lead and thorium/lead ratios, we can get a rather exact figure for the age of a given rock, in the same way as one can find how long a furnace was

burning by comparing the amounts of remaining coal and accumulated ashes. Using this method, one finds different ages for the rocks of dif ferent geologic formations, but in not a single case does this age exceed the value of two billion years. We can consider this figure as the lower limit, and, possibly, as a good actual value for the age of the earth.

It may be noted that a few years ago a British geologist, A. Holmes, proposed a more intricate method based on the study of uranium-lead transformation prior to the solidification of the crust. By comparing the relative amounts of radiogenic leads in the rocks of different geological ages, and making certain assumptions concerning the processes of ore deposition, he arrived at the result that the formation of radiogenic lead must have started 3,350,000,000 years ago. This figure is supposed to represent, not so much the age of the earth itself, as the date at which the "freshly formed" radioactive atoms must have started to decay into lead.

Astronomers have essentially three different methods for judging the age of the stellar universe. The first is based on the study of stellar motion within our system of the Milky Way, and refers to the statistical distri bution of stellar velocities which is expected to approach a certain "limiting distribution" (the so-called equipartition of energy between all stars) when the stellar system has existed for a sufficiently long time. The observed velocity distribution is still some way off from that "limiting distribution," which, according to mathematical calculations, indicates that the system must have existed for only a few billion years. The second astronomical method is based on the study of stellar

energy sources. We know in fact that stars, and in particular our sun, derive their energy from the slow nuclear transformation of hydrogen into helium taking place in their hot central regions. Thus, the natural life span of a star is determined by the rate of its burning (that is, by its


The Origin and Evolution of the Universe 395

absolute brightness) and by the original amount of hydrogen it contains. Since the brightness of stars is known to increase as the cube of their mass, and the amount of nuclear fuel is simply proportional to the mass (hydrogen forms about half of the total mass in a normal star), the brighter stars will burn out faster than fainter ones, their life span being inversely proportional to the square of their mass. Our sun is a com paratively faint star; its total life span can be calculated to be about 50 billion years. If the sun is only a few billion years old, it may be com pared with a baby who is just learning to walk.

The stars, which are five times heavier than the sun, burn 25 times as fast and have a life span of only about two or three billion years. Ob servational astronomy reveals that the stars of just about that mass, seem to be on the verge of hydrogen exhaustion. The dwindling of their fuel supply is manifested in all kinds of "unquiet behavior" ranging from regular pulsations of their giant bodies (Cepheid variables) to terrific explosions (novae and supernovae) which tear these stars apart. It therefore seems reasonable to conclude that most of the stars forming the system of the Milky Way were originally formed two or three billion years ago, and that in the case of pulsating and exploding stars we ob serve the death agony of those members of the stellar community who are coming to the end of their natural life during the present epoch of the history of the universe.

The third astronomical method of estimating the age of the universe is based on the phenomenon of universal expansion discovered by the

Mount Wilson astronomer, E. Hubble, about a quarter of a century ago. We know that the stellar system of the Milky Way, containing our sun

along with several billions of other stars, is not a lonely island in the infinite expanses of the universe. Large telescopes reveal that the space outside our Galaxy is populated by myriads of similar stellar systems scattered more or less uniformly all the way to the limit of telescopic vision.

There are nearly one billion such galaxies within the range of the 200-inch telescope of the Palomar Mountain Observatory, and the author was told by Professor Harlow Shapley of Harvard that when ever he has a new graduate student he sends him up to the telescope with orders "to discover a new galaxy and to name it." The striking thing about these distant galaxies of stars is that the light emitted by them, while similar to the light coming from nearby galaxies, shows however the peculiar phenomenon of a shift of all spectral lines towards the red end of the spectrum. A simple physical explanation of this "red shift" lies in the assumption that the galaxies are receding from us at rather

high speeds. This so-called Doppler effect, consisting in the change in

frequency of waves emitted by an approaching or receding source, is a familiar phenomenon in the field of acoustics. Everyone has noticed how the aggressive high-pitched honk of an approaching, fast-driven car goes into a much lower departing tune as the car passes by.

In optics the same effect will make the light of an approaching source look bluer, and that of a receding source redder, than it actually is.



There is a story of how this Doppler effect in optics almost saved the famous American physicist, R. W. Wood, from paying the usual fine for crossing an intersection on a red light. The story goes that, being sum moned to the traffic court with the violation ticket, Professor Wood gave a brilliant (as usual) lecture to the judge on the subject of the Doppler effect, explaining how and why one can see a red light as green if one drives towards it. But, while the judge was highly impressed by that presentation, and was ready to waive the fine, one of Wood's students (recently flunked by him on an optics examination) happened to be in the courtroom and proposed that the judge ask the professor to estimate the velocity with which he must have been driving in order to see the red light as green. As a result, the fine was changed from that for crossing on a red light to that for exceeding the speed limit of the city of Balti

more.

Hubble's measurements of the red shift in distant galaxies indicate that they all move away from us with speeds proportional to their distances. It does not mean, however, that we actually are in the center of the universe with all its parts running away from us, and can, in fact, be interpreted simply as an optical illusion common to any observer located anywhere within a uniformly expanding system. If we imagine an inflated rubber balloon with black dots painted all over its surface in a polka-dot fashion (the galaxies scattered through the space of the universe), an observer sitting on any one of these dots will see all other dots receding from him when the balloon is gradually swelling to a larger and larger size. And the observed recession-velocity of more distant dots will be larger in proportion to their distances.

The observationally established expansion of the universe gives us a valuable clue to the history of the universe, indicating that all present features of the universe must have originated as the result of successive differentiation of a rapidly expanding primordial matter. The date of the "beginning/' that is, the epoch when the material forming the universe was in the original highly compressed homogeneous state, can be obtained by a simple division of the average distance between the neighboring galaxies by the measured velocity of their mutual recession. The result, 1.8 billion years, is of the same order of magnitude as all other approximate estimates of the age of the universe.

However, Hubble's exact figure of 1.8 billion years stands in sharp contradiction with Holmes' figure of 3.35 billion years (the discrepancy being far outside the claimed accuracy of both methods), and at the present stage it is difficult to say which of the two figures should be con sidered to be correct. It may be that the geological estimate is to be changed, since Holmes' calculations are based on certain specific assump tions concerning the process of ore formation. On the other hand, there are several ways in which the observed recession velocities of distant galaxies could be fitted into a longer time scale. One of these possibilities involves the introduction of the so-called cosmological term, which corresponds physically to the assumption of a repulsive force acting between individual galaxies and increasing with their distance. The



presence of such a repulsion would turn the expansion into an accelerated process, and thus move the zero point farther back in time. Another possibility was considered recently by Bondi, Gold, and Hoyle who suggest that, while the space of the universe is being gradually thinned out by the expansion, new matter is continuously being created between the receding galaxies at a rate which compensates for the effect of the recession. Thus, while older galaxies get farther and farther from each other, new galaxies are being formed in between to take their place, and the show is going on without any beginning or ending. Although such a hypothesis may be quite attractive from the philosophical point of view, it encounters serious observational as well as theoretical diffi culties, and should be taken at present with a good-sized grain of salt.

At the present writing it seems that the discrepancy between geologi cal and astronomical age-estimates can be removed by using better data for intergalactic distances. In fact, a recently published work by A. Behr indicates that, introducing various corrections into the older estimates of intergalactic distances, one may almost double the original Hubble figure for the duration of the expansion process.

The Formation of Atomic Species When we inquire about the early stages in the history of the universe*

we find that the most valuable archaeological document is presented by the relative abundance with which different atomic species are found in nature. In fact, there is every reason to believe that chemical elements were "cooked" very early in history when the density and temperature of the matter in the universe were both exceedingly high. If we imagine history running back in time, we inevitably come to that epoch of "big squeeze" with all the galaxies, stars, atoms, and atomic nuclei squeezed, so to speak, to a pulp.

During that early stage of evolution, matter must have been dis sociated into its elementary compounds: protons, neutrons, and elec trons. We call this primordial mixture ylem since in Webster's Dictionary this word is explained as: "O.F. Hem, fr. L. hylem, acc. of hyle. See Hyle. The first substance from which the elements were supposed to be formed. Cf. Hyle, 1. O?s." While the temperature of ylem was still very high (above one billion degrees centigrade) thermal motion of the particles

was too violent to permit their sticking together. This high temperature also prevented neutrons from decaying into protons and electrons, or, to state it more correctly, the production of fresh neutrons in the processes of proton-electron collisions at that time was compensating for their loss due to the decay process.

However, as soon as the density and temperature of matter dropped as the result of the progressing expansion, two processes must have started. The first process was the predominating neutron decay which was cutting sharply into the number of neutrons available for the nuclear reaction. The second was the aggregation of neutrons and protons into complex groups : the prototypes of the atomic nuclei of today. The result of the competition between these two processes must have determined



the relative numbers of various composite nuclei which exist in nature at the present time. If the expansion had been too fast or the original density of matter too low, very few nuclear collisions could have taken

place before all neutrons were destroyed (turned into protons) by the natural decay process. In this case, practically no complex nuclei would have been built, and the matter of the universe today would consist

predominantly of hydrogen. If, on the contrary, the original density had been too high, neutrons and protons would have had ample chance to unite into complex units, and most of the material of the universe would be present now in the form of heavier elements.

Apparently, the actual situation was somewhere between these two extremes, and we should be able to get rather exact information con

cerning the physical conditions which prevailed during the early stages of the expansion of the universe, by analyzing in detail the processes of nuclear formation which took place during that epoch. It must be re membered that, even though these processes occurred billions of years ago, we can discuss them on the basis of perfectly reliable nuclear in formation. In fact, the temperature of a billion degrees corresponds to thermal energies of the order of one million volts, and these are exactly the energies at which nuclear reactions are being studied in our labora tories using electrically accelerated nuclear beams.

The first attempt to calculate what must have happened to ylem dur ing the early stages of the expanding universe was made by the author and a former student, R. Alpher, several years ago.1 The problem pre sented by the building-up process of atomic nuclei is very similar to the classical problem of heat flowing along a bar heated at one end. In the latter problem the increase of temperature in any section of the bar is given by the difference between the heat inflow from the heated side and the heat outflow in the opposite direction. Similarly, the increase in the number of representatives of any given nuclear species, say the nuclei with atomic weight 100, is given by the difference between the rate of their production through neutron capture in 99-weight and their elimina tion as the result of moving into 101-weight through a subsequent neutron capture.

One can write simple differential equations containing the known neutron-capture cross sections, the solution of which will give us the ex

pected distribution of the original material between different atomic weights, for any given original density and any given time of cooking. Since, as was stated above, the expansion process must have started

spontaneous decay of neutrons, the entire "cooking period" could not

1 When the preliminary communication concerning these calculations was written, and signed by two names, Alpher and Gamow, we felt that something was missing. Thus, in accordance with the Greek alphabet, we have added the name of Bethe (in absentia), which resulted in the theory's being often referred to as the a?y theory. Dr. Bethe, who reviewed a copy of the article sent to Physical Review, did not object and, in fact, was quite helpful in further discussions. There was, however, a rumor that at the later date when the theory went temporarily on the rocks, Bethe was con

sidering changing his name to Zacharias. It may also be noticed, with chagrin, that Dr. R. Herman, who joined the team later, still refuses to change his name to Delter, which would be most appropriate.



have lasted much longer than the mean lifetime of a free neutron (the order of magnitude of half an hour). It may look silly to talk about the consequences of a process which took place a few billion years ago and lasted for only half an hour, but the ratio of half an hour to a few billion

Neutron capture cross section (in barns)

O 50 IOO I50 200 250

ATOMIC WEIGHT Fig. 1. Observed and calculated abundance curves.

years is about the same as the ratio of a few microseconds to several years, which represent respectively the reaction time within an explod ing atomic bomb and the period of time after which the radioactivity of fission products can still be noticed at the explosion site!

The results obtained by the integration of the building-up equations



for different original densities of y lem are shown graphically in Figure 1, along with the empirical abundance curve (shaded band) of different atomic species. We see that the best fit is obtained on the assumption that in the beginning of the building-up process (five minutes after the stage of maximum compression) the density of matter in the universe was in the vicinity of one microgram per cubic centimeter, that is, about one-tenth of 1 per cent of the density of atmospheric air. This is quite a

high density considering that today the mean density of the universe is about one atom per cubic meter or 10~30 g./cm3. Without going into the details of the theory, we may indicate that the characteristic shape of the empirical abundance curve, which shows a steep slope for the ele ments of the first half of the periodic system, and a horizontal run for all heavier elements, is a direct consequence of corresponding behavior of

Helium Tritium ^Tralphium

v Deuterium

Hydrogen

Neutrons

1 -1-1-1-1-1?

5 io 15 2 0 25 30 35 minutes from zero point

Fig. 2. Building of lightest elements. CAfter Fermi and Turkevich.)

neutron-capture cross sections which are shown in the inset diagram in the upper-right corner of Figure 1.

One should expect, in fact, that the abundance curve must level off when the capture cross sections become rather large, in the same way that the temperature distribution along a heated bar levels off at its far end if that part of the bar is made from a material possessing much higher heat conductivity than the part immediately adjacent to the source of heat.

The above-described calculations, involving the building-up process of all atomic species from the beginning to the end of the periodic system, are, by necessity, rather approximate. In fact, in order to be able to carry through the calculations in a finite period of time, we had to assume the "smoothed out" curve for capture cross sections, and to make several other simplifying assumptions. Another possible way of



obtaining the density of the matter in the universe during the "nuclear cooking" process would be to study exclusively the reactions between the few simplest elements, but to make this study in great detail using all the available data of nuclear physics. Calculations of this type have been worked out by the author for the process of deuterium formation in the reaction n+p-*d+y, and later by Fermi and Turkevich for all reactions involving neutrons, protons, deuterone, tritons, tralpha (He3), and alpha-(He4) particles. The results of the later authors, who used the same physical assumptions concerning the densities and temperatures as used in the previous calculations, are shown graphically in Figure 2. We notice that, whereas the decay of neutrons into protons begins almost immediately after the start of expansion, the aggregation process leading to the formation of complex nuclei starts only five minutes later when the temperature of ylem has dropped below one billion degrees. By the end of half an hour most of the original neutrons have decayed, whereas hydrogen and helium are forming about half and half of the entire mixture.

Deuterium is present in the amount of a few per cent, and will be completely exhausted in the process of building heavier elements. (As stated above, these further building-up processes were disregarded in the calculations.) Thus we see that, using this detailed method, we also arrive at a reasonably good result, since it is known that at present the matter in the universe consists of somewhat more than 50 per cent of

hydrogen, somewhat less than 50 per cent of helium, and about 1 per cent of all heavier elements. It is still too early to state that the above described theory of the origin of chemical elements provides a complete explanation of all observed facts. Indeed, there still remain some serious difficulties involving notably the problem of carrying the building-up process across the atomic weight five (because of the absence of any stable nuclei of that weight), and the explanation of the so-called "shielded isotopes" (that is, the isotopes which cannot be obtained directly through the beta-decay of the nuclei with the excess of neu

trons). It seems, however, that the theory gives a reasonably consistent picture of what must have happened during the very early stages of the evolutionary history of our universe.

The First Thirty Million Years, and the Beginning of the Differentiation Process

After atomic species were formed in the first half-hour or so of the

history of the universe, the expansion of newly formed matter continued in a rather monotonous way for quite a long time. The most characteristic feature of this entire period was the prevailing role of radiant energy as

compared with ordinary matter. It is well known that, according to Einstein's law, radiant energy possesses ponderable mass, the value of which can be obtained numerically by dividing the amount of energy, expressed in ergs, by the square of the velocity of light. Using this rule, we can easily find, for example, that the light which is filling a lecture room weighs only a few billionths of a microgram?about the weight



of one bacterium! This is, of course, a negligibly small figure as com

pared with the weight of the air in the same lecture room.

If, however, we make a similar calculation for radiant energy and ordinary matter during the early stages of the expansion of the universe, we shall arrive at a rather different result. Assuming that at the end of five minutes the temperature of the universe was about one billion de grees (as it follows from the previous discussion), we find that the mass

density of radiation (aTA/c2) was about 10 g./cm.3, thus being com

parable with the density of iron! Since, at the same time, the density of

ordinary (atomic) matter was only about one microgram per cubic centimeter, we conclude that at that epoch the situation was ruled ex

clusively by light (with very short wave length, of course) and that the material particles were helplessly thrown around like little chips of wood in the stormy ocean of radiation.

As the expansion of the universe proceeded, the situation was gradually changing in favor of matter. Indeed, whereas the total number of atoms was left unchanged, radiant energy was being spent doing the work of expansion. It can be calculated that the density of matter and radiation became equalized approximately at the age of about 30,000,000 years, when the temperature of the universe dropped to about 300? Kelvin (roughly room temperature), and its material density to the value of about 10~24 g./cm.3 (one H-atom per cm.3, or the present density within the galaxy).

At that point in history, matter took over the leading role in further developments, and its first deed was to break up the homogeneity of the hitherto continuous expansion process. The chief agent in this break ing-up process, which ultimately led to the present highly differentiated state of the material universe, was Newtonian gravitation between ma terial particles. In fact, as it was once shown by the British astronomer, Sir James Jeans, a gravitating gas filling uniformly an unlimited space is intrinsically unstable, and is bound to break up into separate "gas balls"

with completely empty space in between. The size of the condensations resulting from this so-called gravitational instability is determined by the condition that each "gas ball" should be sufficiently large so that the escape velocity from its surface is larger than the thermal velocity of gas particles. Using the initial temperature and density of the pri mordial gas, and remembering that the temperature in the expanding universe falls off in inverse proportion to the square root of its age, whereas the material density falls inversely to the three-half power of it, one can estimate the size and mass of "gas balls" which must have been formed during various epochs of the expansion of the universe. In doing this, one finds that the mass of the condensation always comes out the same no matter when they are formed, since the time-dependence cancels out in the mass formula.

Numerically, one obtains for the mass of the condensation the value of about 108 sun masses, which, though being on the low side, corresponds in order of magnitude to the average mass of galaxies. This is a very gratifying result indeed, since that mass is obtained exclusively by using



nuclear constants such as neutron-capture cross sections, etc. As was

mentioned above, no such condensations of gas could have taken place prior to the age of 30,000,000 years, that is, so long as radiant energy ruled the situation. As soon as matter took over, however, the break-up process must have taken place, so that the mean density of individual "gas balls" must equal the mean density of the universe at that time.

This conclusion also agrees with observational evidence, since the present mean density of matter inside the individual galaxies is the same (10-24 g./cm.2) as the above-quoted mean density of the universe at the moment of galactic separation. When, at the date mark of 30,000,000 years, the originally homo

geneous gas broke up into separate clouds (the progenies of today's galaxies), the space of the universe was quite dark since the original brilliance of the first days of creation was already dimmed out by ex

pansion, and the stars, which illuminate the universe today, were not yet formed. There was nothing at that time but giant clouds of luke warm gas which were being pulled away from each other by the pro gressing expansion of space. It goes without saying that the break-up of the expanding gas into separate clouds, or fragments, must have re sulted in a rather rapid rotation of these fragments around their axes distributed at random in all directions. We observe the same type of rotation in the fragments of an artillery shell exploded in mid-air. Here probably lies the explanation of the fact that most gnlaxies are found now in the state of rotation, manifested in their flattened elliptical shapes and in their spiral arms winding around their centrally con densed bodies.

Stars, Planets, Satellites

The next step in the evolution of the universe apparently was the formation of stars, which must have originated as the result of the secondary condensation, that is, the break-up of the original "galactic gas balls" into billions of smaller "stellar gas balls" by the same old process of gravitational instability. These smaller gas condensations contracted quite rapidly, and, as the result of compression, the material in their central regions was heated to the temperature of some 20,000,000 degrees, representing the threshold for nuclear reactions. The liberation of nuclear energy had started, and the universe became illuminated by billions and billions of stars.

Space does not permit us to consider here the detailed analysis of stellar evolution, and, in particular, the problem of the origin of plane tary systems. We shall mention only that, according to recent theories of the German physicist, C. von Weizs?cker, and the American as

tronomer, G. P. Kuiper, the formation of the planets took place in a way very similar to that proposed centuries ago by Kant and Laplace (the collision theory of Jeans, and Chamberlin and Moulton, being abandoned

by modern cosmogony). Since, as was mentioned above, the material

forming the original galactic gas balls was in a state of rapid turbulent

rotation, stellar condensations (or pre-stars) were rotating too. Thus,


404 American Sdentisi

whereas most of their material must have fallen towards the center, forming the main body of the star, some of it must have been left outside in the form of a strongly flattened or rotating disk. About 99 per cent of this disk was gaseous hydrogen and helium, whereas the remaining 1 per cent was formed by small dust particles of silicates, iron oxides, ice crystals, etc.

The dust particles of that swarm must have been always colliding with one another (forming the units of larger and larger mass "plane tesimals" of the Chamberlin and Moulton theory). Those chunks of the material which happened to grow larger than the others swept the space around them, capturing the smaller stones and dust particles, until they found themselves moving in practically empty space.

Mathematical analysis of this rather complicated process not only gives us a reasonably complete picture of planetary formation, but also leads to the understanding of various observed regularities such as the

Bode law which governs the distance of various planets from the sun. It goes without saying that this "rejuvenated Kant-Laplace theory" predicts the existence of planetary systems around practically any star within our own or any other galaxy. Recently this theoretical conclusion found confirmation in the actual discovery of planetary systems near two close stars. According to the theory, the entire process of the condensa tion of stars and the formation of planetary systems must have taken a few hundred million years.

There is one more important point to be mentioned in connection with the theory of planetary origin. As we have seen above, planets were formed by the accumulation of solid dust particles which were

floating in a hydrogen-helium, gaseous mixture. Such a process would produce rocky bodies similar to our earth, to Mars, and to two internal planets : Venus and Mercury. However, if the mass of a planet exceeded certain limits (a few earth masses), it would possess a sufficiently strong gravitational field to capture and hold quite large amounts of interstellar hydrogen and helium gases. Neither our earth nor the three other inner planets ever exceeded that limit, and so they have remained rocky bodies as we know them now. On the other hand, the original rocky bodies of outer planets, such as Jupiter and Saturn, managed to grow above that limit (because the original dust disk was thicker at these distances), and thus have acquired a lot of interstellar gaseous

material.

It was recently shown by H. Brown of Chicago that only about 2 per cent of the giant bodies of Jupiter and Saturn is made from the same

material as our earth. This material, forming the rocky cores of these planets, is covered by layers of frozen water, methane, and ammonia, which account for another 8 per cent. The rest of Jupiter and Saturn is nothing but highly compressed mixtures of gaseous hydrogen and helium. Thus, if Flash Gordon of the Sunday comic strips, or a serious rocket explorer of the future, were to land on the seemingly solid surface of these planets, he would sink deeper and deeper into the compressed gas, and would finally be crushed by the tremendously high pressures



lying deep in the body of these planets at the surfaces of their inner solid cores.

The formation of satellites took place in a way completely similar to planet formation, by the condensation of dust particles from the flat tened rotating envelopes which surround the proto-planets. The only possible exception is presented by our own moon, which stands out of the company of other satellites because of its exceptionally large rela tive mass. It is believed that the earth was originally born without any satellites (as were Venus and Mercury), and that it was later broken into two pieces (larger one, the earth; smaller one, the moon) by the tidal forces of the sun. In fact, a British astronomer, George Darwin, was able to show that in the distant past, the moon was much closer to the earth, and that several billion years ago, the earth and the moon

must have comprised one single body. We may note here again that the calculated date of the moon's birth fits well with other values quoted for the age of the universe.

The history of the universe as described above is shown schematically in the Frontispiece (page 392).

A Glance into the Future

Having learned that the universe, as we know it today, must have originated a few billion years ago from hot homogeneous ylem which was successively differentiated in the process of expansion of the uni verse, we may naturally ask: What lies ahead of us? Will the universe continue its present expansion beyond any limit, or will it stop and start collapsing back on us (or rather on our descendants) ? This question can be answered in a simple way by comparing the kinetic energy of galaxies flying away, with the potential energy of the Newtonian attraction be tween them. Using the available data, one can easily find that the kinetic energy of galactic recession is almost a hundred times as large as their mutual gravitational energy. Thus, the situation is similar to the case of a rocket fired from the surface of the earth with ten times the escape velocity (one hundred times the escape energy). The galaxies will fly apart forever without ever turning back. Within each galaxy the process of stellar evolution will be continuing, and the stars, which draw their death ticket by using up all their hydrogen fuel, will be ex

ploding and going into oblivion. Some 47 billion years from now, that fate will reach our own sun, and still later the other fainter stars. But all this is still so far away that it is hardly cause for anxiety.

Another question we could ask pertains to the forces which caused the initial expansion of the universe, and to the state of affairs which

must have existed prior to the maximum stage of contraction which was the starting point of all our discussion. Mathematically we may say that the observed expansion of the universe is nothing but the bouncing back which resulted from a collapse prior to the zero point of time a few billion years ago. Physically, however, there is no sense in speaking about that "prehistoric state" of the universe, since indeed during the

stage of maximum compression everything was squeezed into the pulp,



or rather into ylem, and no information could have been left from the earlier time if there ever was one.

This conclusion is in complete agreement with the statement made centuries ago by St. Augustine of Hippo who, in one of his writings, was trying to answer the question of what God was doing before He made heaven and earth. "He was making the hell," wrote St. Augustine, "for the persons who ask that kind of questions."

REFERENCES The A ge of the Universe

1. Holmes, Arthur. The age of the earth. Endeavour (London), July 1947. 2. Hubble, Edwin. The problem of the expanding universe. In: Science in

progress, third series. Yale University Press, 1942. 3. LeMa?tre, George. The primeval atom. D. Van Nostrand Co., 1950. 4. Hoyle, Fred. The nature of the universe. Harper & Brothers, 1951. 5. Behr, Alfred. Zur Entiernungsskola der Extragalactischen Nebel. Astr.

Nachrichten., 279, 97, 1951.

The Formation of Atomic Species

6. Alpher, Bethe, and Gamow. The origin of chemical elements. Phys. Rev., 78, 803,1948.

7. Alpher, Ralph, and Herman, Robeft. Theory of the origin and relative distribution of the elements. Rev. Mod. Phys., 22, 153, 1950. (This paper con tains complete literature on the subject.)

The First Thirty Million Years

8. Jeans, Sir James. Astronomy and cosmogony. Cambridge University Press, 1928.

9. Gamow, George. The evolution of the universe. Nature, 162, 680, 1948.

Stars, Planets, Satellites

10. Weizs?cker, Carl von. ?ber die Entstehung der Planetensystems. Zeit, f?r Astroph., 22, 319,1944.

11. Kuiper, Gerard. On the origin of the solar system. In: Astrophysics: A topical symposium. McGraw-Hill Book Co., 1951.

12. Brown, Harrison. The composition of our universe. Physics Today, April 1950.


Article Contentsp. [392]p. 393p. 394p. 395p. 396p. 397p. 398p. 399p. 400p. 401p. 402p. 403p. 404p. 405p. 406

Issue Table of ContentsAmerican Scientist, Vol. 39, No. 3 (JULY 1951) pp. 361-520Front MatterEDITORIAL MISCELLANY [pp. 366, 368, 370, 372, 374, 376, 378, 380, 382, 384, 386, 388, 390, 513-516, 518-519]THE ORIGIN AND EVOLUTION OF THE UNIVERSE [pp. 392-406]THE RELATIONS BETWEEN ASTRONOMY AND GEOPHYSICS [pp. 407-411]THE MEANINGS OF TIME AND SPACE IN PHILOSOPHIES OF SCIENCE [pp. 412-421]THE MEANING OF "ELEMENTARY PARTICLE" [pp. 422-431]ON THE PSYCHOLOGICAL ASPECTS OF AUTHORITARIAN AND DEMOCRATIC POLITICAL SYSTEMS [pp. 432-440, 451]ON THE BIOLOGICAL BASIS OF ADAPTEDNESS [pp. 441-451]FISHER AND FORD ON "THE SEWALL WRIGHT EFFECT" [pp. 452-458, 479]COMMUNICATIONSTHE CITIZEN AND THE HISTORY OF SCIENCE [pp. 459-461]SAMPLING THE UNIVERSE [pp. 462-465]LETTERS FROM A MEXICAN EXPEDITION [pp. 466-471]WILLIAM PROCTER, 18721951 [pp. 471-472]

MARGINALIA [pp. 473-479]THE SCIENTIST'S BOOKSHELF [pp. 480-484, 486, 488, 490, 492, 494, 496, 498, 500, 502, 504, 506, 508, 510-513]Back Matter

Gamow,g - The Origin and Evolution of the Universe

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