Theories of the Universe

Theories Of The Universe

Since the universe of galaxies is expanding in all directions, how did it all begin? According to one theory, proposed in 1927 by the Belgian mathematician Georges Lemaitre, all the material in the universe was once concentrated in a giant atom. The atom exploded some 10,000 million years ago, and the fragments, now galaxies, moved outward and away from each other forever. According to another theory the expansion will be followed by contraction to an extremely dense state. Another explosion will then bring about expansion, and so on ad infinitu, In yet another theory the universe is supposed to be in a steady-state, in the sense that as the galaxies move further away from each other new galaxies are constantly being formed to fill the gaps.

That each of these and other theories of the universe has its supporters indicates the great uncertainty of our knowledge. Science thrives on well-ordered speculation, but all the theorizing in the world is useless unless it is based on the facts of observation. In the next few years, many of our present theories will probably have to be revised or discarded, and a whole crop of new problems will arise. But this is as it should be if our knowledge, like the universe in which we live, is to remain in as a state of expansion.

Quasars

Astronomers have been studying star-like objects whose velocities of recession, judges from the red-shifts of the lines in their spectra, indicate that they out among the galaxies. These objects are called quasi-stellar radio sources or quasars. They are far too large and luminous to be stars. The lines in their spectra show that they are composed of intensely hot gases. They are intense source of radio emission. Some of them vary in their energy output and some are associated with one or more bright jets. Their precise nature remains a mystery and until this is solved we cannot be sure where they are comparatively near to us or extremely distant.

3C - 48

3C - 147

3C - 196

3C - 273

Four quasi-stellar radio sources photographed with the 200-inch Hale telescope

Expanding Universe

The Expanding Universe

In the universe of galaxies the situation is complicated because the galaxies recede from our Galaxy at speeds which increase with increasing distance. The rate of increase is estimated to be about 15 miles a second per million light-years, but is thought gradually to get larger at distances beyond about 1,000 million light-years. 3C-295, one of the most distant galaxies yet photographed, appears to have a velocity of recession of 76,000 miles a second, or about two-fifths the velocity of light. It is therefore unlikely that its distance will be more than 5,000 million light-years, although this estimate is little more than a guess.

If the value 76,000 miles a second seems absurdly large, we must realize that it refers to the speed of a galaxy, not to that of a relatively tiny object like a star. A galaxy comparable in size to our Galaxy, moving at that speed, would still take about 250,000 years to pass through a distance equal to its own diameter. Further, an observer on 3C-295 would regard himself as being stationary while our Galaxy receded from him at a velocity of 76,000 miles a second.

Cluster of Galaxies in Hercules. Source of the photo: NASA, photograph in public domain

In other words, no matter what particular galaxy we found ourselves in, we would regard that galaxy as being at the center of an expanding system of galaxies. Our bun-model now breaks down completely, for the bun has a raisin at or near its center. The universe of galaxies, on the other hand, has no absolute centre. There are as many centres as there are galaxies.

We must not think that astronomers measure the velocities of recession of the galaxies. All they can measure are the relative shifts towards the red of certain lines in the spectra of galaxies. They then interpret the shifts in terms of the wave theory of light, and thereby conclude that the shifts are Doppler effects due to velocities of recession of the sources observed, that is, of the galaxies themselves. This particular interpretation is undoubtedly the best one so far but its is by no means the only one possible.

Exploding Galaxies

Exploding Galaxies

One of the many exciting scientific discoveries of recent years was that certain peculiar-looking galaxies are intense sources of radio emission. Prominent in this respect is Cygnus A, the first discrete source to be discovered in the radio sky, and now identified with a faint optical object believed to be 500 million light-years away. Another strong radio source is Virgo A, identified with Messier 87. Photographs of the latter taken in short exposure-times reveal that a long, narrow jet extends from the galaxy’s central region. The jet probably consists of streams of high-energy electrified particles and presumably is responsible for the radio emission.

Similar jets are associated with other galaxies and indicate that these objects are in the throes of violent explosions. Perhaps the most spectacular case is the irregular galaxy Messier 82 in the Ursa Major, for there, if present interpretations are correct, we are witnessing the aftermath of an explosion of series of explosion which affected the entire nucleus. But since Messier 82 has an estimated distance of 10 million light-years, the events we now see took place 10 million years ago.

Messier 87, a short-exposure photograph showing the nuclear “jet”

Galaxies, like the stars in them, are moving objects. They have their own individual motions and, when considered as a whole or on the large scale, have motions away from us and from each other. The situation is somewhat similar to that of an expanding bun well-filled with raisins. As the bun expands the raisins move away from the center and also get further apart.. An insect on any one particular raisin would therefore get the impression that the other raisins were moving away from him. To make the model more apt, the individual raisins should be able to move about slightly, relative to the material of the bun, but they would still share in the overall expansion.

The Realm of the Galaxies

The Realm of the Galaxies

The Clouds of Magellan

Prominent among the stars and of southern skies are two large misty patches which look like detached fragments of the Milky Way. Portuguese navigators of the 15the century called them the “Clouds of the Cape”. They were first described with reasonable accuracy in 1521 by Antonio Pigafetta, chronicler of Magellan’s voyage round the world. When photographed with large telescopes the two clouds of Magellan, as they now are called, are seen to be magnificent objects. With their great population of stars and gas of shining gas, they are no less than galaxies, similar in several respects to our own Milky Way System.

The Clouds of Magellan lie eel outside our Galaxy at a distance of about 180,000 light-years. They extend far beyond their naked-eye limits, have diameters of the order of 40,000 light years and 25,000 light-years, and are companion galaxies to our own. The large Cloud contains objects similar to those found in our Galaxy and in particular a magnificent bright nebula associated with the star 30 Doradus. But its spiral arms are fragmentary and difficult to trace, The Small Cloud has no definite structure and is comparatively free from interstellar dust and bright nebulosity.

The Large Cloud of Magellan. Source of the photo: NASA

Cepheids

Both clouds contain large numbers of variable stars called Cepheids after their prototype Delta Cephei. In 1912, from a study of photographs of the Lesser Cloud, Miss Henrietta Leavitt of the Harvard Observatory discovered that these stars have an important property – the longer the period of their light variations, the brighter they appear. For practical purposes they can be assumed to be all equally distant from us. Hence the relationship is one between period of light change and luminosity. This provides astronomers with an important measurement tool for determining stellar distances. If a group of stars contain a Cepheid, the determination of the Cepheid’s period leads directly to its luminosity and this, compared with the apparent brightness, establishes its distance. The method has been applied with great success to globular star clusters, the Clouds of Magellan, and other neighbouring galaxies.

Messier 31

The only other galaxy visible to the unaided eye is Messier 31, the great Galaxy in Andromeda. It was first mentioned in 1612 by Simon Marius, a German astronomer, who compared its appearance in the telescope to that of a candle flame seen through a piece of horn. Astronomers in the 19th century were uncertain whether it was a mass of shining gas or a system of stars. But all doubts were removed in the early 1920th. Photographs taken with the 100-inch telescope of the Mount Wilson Observatory not only gave indications of separate star clouds, bright nebulae, and open star clusters, but also showed some of the brighter stars as individual objects. In 1923 Edwin Hubble, using the 100-inch telescope, detected the first of many Cepheids in this galaxy and was thus able to put the determination of its distance on a sound basis.

The light from Messier 31 takes about 2.2 million years to reach us – as interval of time far longer than the entire history of Man. If the sun was placed in Messier 31 it would be beyond detection even with the full photographic power of the 200-inch telescope on Palomar Mountain. The overall diameter is roughly 180,000 light-years, or almost double that of our Galaxy. If we represented our Galaxy by a disk one inch in diameter, Messier 31 would be another disk nearly two inches across separated from the first by a distance of 22 inches.

Messier 31, the Great Galaxy in Andromeda. Photograph: Adam Evans

Another Spiral Galaxy

Messier 31 is a spiral galaxy similar in content to the Galaxy. It even has its own system of about 200 globular clusters and also two small elliptical companion galaxies. It appears oval in shape because its plane is tilted by about 15 degrees to the line of sight and this tends to make its spiral arms difficult to trace. The latter are outlined by clouds of interstellar dust and contain numerous star clusters and patches of bright nebulosity. The entire object is rotating about its center, but, since one rotation takes many million years, the general appearance remains unchanged from one century to the next. Photographs of the central region taken with plates sensitized to red light show that the nucleus is an enormous assemblage of highly luminous reddish star and closely resembles a gigantic globular cluster.

Messier 33

Another nearby galaxy is Messier 33 in the constellation of Triangulum. This, too, has a spiral structure, exceptionally well shown since we happen to view it almost face on. Its distance is reckoned to be two million light-years, and its diameter about 50 thousand light-years.

Messier 31, the Milky Way Galaxy, the Clouds of Magellan, and Messier 33 are the largest members of at least 17 galaxies known as the Local cluster. All 17 objects are contained in a volume of space some three-million light-years across. Most of the other members are lightweight systems, several of them being faint and sparse, and resembling large galactic star cluster. On the other hand, Messier 31 and the Galaxy are themselves lightweight systems compared with giant elliptical galaxies like Messier 87 in Virgo. Likewise the local cluster is a small affair compared to many other clusters, some of which contain several hundred galaxies.

NGC 4565, a spiral galaxy in Coma Berenices, seen edge on. Photograph: Ken Crawford

Neutrino Count Level Debated

Neutrino Count Level Debated

For 15 years, Dr. Raymond Davis Jr.’s 100,000 gallon neutrino telescope has yielded low particle count levels, touching off speculation and concern about the reliability of the solar energy model. Four recent data runs at the neutrino “telescope” (in a mine beneath Lead, S.D.) have returned neutrino counts that approach the predicted levels. Optimistic observers greeted the news with statements that Davis’s equipment was at fault in the past and the mystery of the missed neutrinos is now solved.Davis disagrees with both conclusions. Refinements in the efficiency of detected argon nuclei (produced by the interaction of neutrinos and chlorine atoms) have increased the detection rate, but the change in efficiency has been corrected for. Also, the unprecedented high levels, David told Science News, are merely part of the expected statistical fluctuations. The predicted flux of solar neutrinos, he feels, is not yet warranted.

Davis hopes to build a neutrino detector containing 20 tons ($60 million worth) of gallium. His detector uses a relatively cheap dry cleaning fluid sensitive to high energy neutrinos. Gallium would probably increase detection rates since it is sensitive to the low energy neutrinos generated by the primary burning process held in the sun’s core.

Aliens. What is it? – I don’t know but just play dead and maybe it will go away.

A number of theories suggest there are fluctuations in the sun’s neutrino output, but Davis believes there is little likelihood that such a variation exists. There is a slight possibility of an 18 month period in the sun’s nuclear output, but this has not yet been established.

The neutrino detector is buried underground to shield it from virtually all particles except neutrinos.