2) EXPANDING UNIVERSE & THE BIG-BANG THEORY

Chapter :2 EXPANDING UNIVERSE & THE BIG-BANG THEORY

Todays modern scientists describe the universe in term of two basic partial theories; General Theory of Relativity that describes the force of gravity and the large scale structure of the universe, i.e. the structure on scales from only a few kms to as large as 1026 kilometers (the size of the observable universe); and the Quantum Mechanics that deals with a distance of a millionth of an inch. However, both the theories are known to be inconsistent with each other, it means both can not be correct.

In 1924, an American astronomer Edwin Hubble found out with his landmark observations and demonstrated that there are so many other galaxies existing except our one. He also found out that the galaxies are going away from each other. This confirmed the expansion of the universe what scientifically supported that at the beginning the universal objects or matters were closer together and condensed as a large primordial or primeval nucleus. This assumption established the Big-Bang Theory.

According to the Big-Bang theory, there was a time about 10-20 thousand million years ago, all the matters were extremely condensed in a very dense and hot primeval nucleus. And after that, a suddenly gigantic explosion (Big-Bang) occurred that threw out all the matters in the form of galaxies. Therefore, the galaxies are continuously moving away from each other, causing the expanding universe. In this way, the distances between galaxies are increasing on and the spatial gaps are also growing on.

However, Hubble discovered the distances of nine different galaxies only. But, now we know that our galaxy is only one of some hundred thousand millions that we can see using the modern telescopes. It is clear now that each galaxy containing itself some hundred thousand million stars. Our own galaxy is too vast about hundred thousand light years across and it is rotating slowly. The stars residing in its spiral arms orbit around its (galaxy) centre once in about several hundred million years. Our sun is just an ordinary yellow star with average size, besiding near the edge of one of the spiral arms.

Stars are so far way that they appear to us as fixed stars just like a pin point of light. So how can we confirm that they are being away from each other? Due to vast majority of stars, we only observe one characteristic feature of colour of their light-spectrum. Focusing the telescope on any individual galaxy or star, we can observe similar light -spectrum of different spectra, but the relative brightness of their colour is found always exactly as like the glowing red hot. In fact, the glowing red hot has a characteristic spectrum that depends only on thats temperature (thermal spectrum). In this way, we can suppose the approximate temperature of a star. Not only this, but we can acknowledge about the atmospheric and chemical or elemental characters of such star with this light-spectrum, because each condition absorbs a characteristic set of very specific colour.

In 1920 and afterward, the astronomical observations found the same characteristic sets of colour spectrum of stars of other galaxies in respect to the stars of our own galaxy, but they all were similarly sifting towards the red-ends. To understand the implications of this, we must follow the Doppler-Effects.

We know that the visible light consists of waves or fluctuations in the electromagnetic field. The frequency for number of waves (per second) of light is so high that ranging from 4-7 hundred million waves per second. Different frequencies of light that we see as different colours; the highest frequency appears at the blue-end of spectrum where the lowest to the red-end of spectrum. Now, suppose that a source of light at a constant distant from us, such as a star emitting light -waves at a constant frequency, certainly, we will receive the same frequency at which that is being emitting. Now, if that source is moving towards us, it will be closer every moment. It means, the time between two wave-crests reaching us is shorter, and therefore the number of waves (frequency), we receive each moment is higher than thats former state. Similarly if the source is moving away, the frequency of waves will be longer or lower. In this condition of light we will have their light-spectrum shifting towards red-end of spectrum. And those moving towards us will produce the blue-shifting spectrum. This relationship between frequency and speed is called the Doppler-Effect.

The Uniform Expansion of the Universe

Because of the astronomical observations of far away galaxies found only the red shifting spectrum of their light etc. waves, so that, it has obviously considered that all galaxies are moving away from us. This also established that the universe is expanding uniformly and as fast as the distance between them is growing. It means, as far as the distance is increasing, their speed of departure is also increasing proportionally with the time.

This discovery of expanding universe was one of the great revolutions at the 20th century. Before this, Newton and others believed in the static model of the universe. But, they did not think about the future prospectus as to why should it not contract under the fluency of gravity. Suppose that, if the universe is expanding fairly slowly, the force of gravity would cause it eventually to stop expanding, and then, to start contracting. And if, the expansion-rate is more than a certain critical rate, the gravity would never be strong to bind or stop it. In this way, the universe would continue its expanding forever. As far example, when one fires a cannon ball up perpendicular to the earth surface, and if it has a perfectly slow speed, the gravity would eventually hault the ball and it will start falling back. But if, the ball has a certain critical speed (about 7 miles per second), the gravity will not be able to pull that back. So that, such the ball will keep going way forever from the earth.4

However, Edwin Hubble found that galaxies of all directions are moving away from us and that there is a direct proportionality between the distance of the galaxies and their red shifts. Hubbles law states that the velocity of recession V of galaxy is directly proportional to its distance from the earth (R). Hence, he made an equation of V=HR (where H is the Hubbles parameter). And this is why; we can conclude that the universe is expanding.5

In addition, here it is quotable that the Russian physicist and mathematician Alexander Friedman had his two very simple assumptions about the universe that it looks identical in whichever direction we look; and that this would be true if we observe it from anywhere else. In this way he showed that we should not expect the universe to be static.

Freidmans assumption has been approximately proved by two American physicists Arno Penzias and Robert Wilson in 1965, when they were testing a very sensitive microwave-detector (microwaves are just like the light-waves but just with a low frequency of only 10 waves per second). They found that their detector was picking up more noise than it ought to be. This noise, like bird drippings, did not appear to be coming from any particular direction. Eventually, they found out and considered that it was coming from out side of the solar system and even beyond of the galaxy as the microwaves radiation. In fact, now we know that the radiation must have traveled to us across most of the observable universe and since it appears to be the same in different directions. It established the first assumption of Freidman, that the universe must be the same in every direction if only on a large scale. For this, Penzias and Wilson received the Noble Prize in 1978.

At roughly the same time of Penzias and Wilson, two American scientists Bob Dickey and Jim Peebles were also testing and working on microwaves with an important suggestion of Gorge Gamow (once a student of Freidman) that the early universe should have been very hot and dense, glowing white hot. So, Dicke and Peebles argued that we should still be able to see the glow of the early universe, because the light from very distant part of it would only just to be reaching us now. However, the expansion of the universe meant that this light should be so greatly red shifted that it would appear us now as the microwaves radiation.6

Now first apparently, all the evidences, that the universe looks the same from all directions, suggests that we may be in the centre place of the universe what was Freidmans second assumption too. In Freidman model, all the galaxies are moving way directly from each other. This situation is rather like a balloon with a number of spots painted on that which is being steadily blown up. As the balloon expands, the distance between two spots also increases accordingly. Thus there is no spot that can be detected as the centre. Similarly, in this model, the speed at which two galaxies are moving apart is really proportional to the distance between them. So it predicts that, the red shifts of a galaxy should be directly proportional to its distance from us as exactly Hubble found. Similar models were discovered in 1935 by the American physicists Howard Robertson and British mathematician Arthur Walker jointly.

Although, Freidman found only one, but in fact, there are different kinds of models that obey Freidmans two fundamental assumptions.

There are three models of Freidman concerned. In the first kind, that Freidman found, the universe is expanding sufficiently slowly that the gravitational attraction between different galaxies causes the expansion to slow down and eventually to stop and then to collapse (fig. 2:1).

This figure shows how the distance between two galaxies changes with the time. It starts at zero (big-bang) and increases to a maximum, and then it decreases or collapses to zero again.

In the second kind of solution, (see fig. 2:2), the universe is expanding rapidly that the gravitational attraction can never stop it though it does slow it down a bit only. This kind of model or figure shows that the separation between neighboring galaxies starts from zero and eventually the galaxies are moving apart at a steady speed. And finally, there is a third kind of solution, in which the universe is expanding only just fast enough to avoid re-collapse. In this case, the separation (fig: 2:3) of the universe starts also from zero and it increases forever. However, the speed at which the galaxies are moving apart gets smaller and smaller, but it never reaches to zero. It means, there is no any boundary of the universe to get an end. Thus in this model of expanding universe, the expansion holds just a critical speed that the space is flat and infinite.7

The modern physics* establishes the future of the universe that the expansion depends on how much matter the universe contains and on how fast it is expanding. This expansion can stop only if it contains sufficient mass for gravity to provide an adequate attractive force to slow down and reverse the expansion. The critical density (Pc) above which the expansion will be arrested is about 6x10-27 kgm-3, i.e. equivalent to about 3.5 hydrogen atoms per m3. Let P be the average density of the universe. According to this, there are three probabilities, such as in fig: 2:4.

(i) If PPc, the universe is closed and sooner or later the gravity will stop the expansion. Thus, the universe will start to contract. The progression of events will be the reverse of those that took place after the big-bang, with an ultimate big-crunch:

*Author R.Murugeshan, ed. 2001, Publ. S.Chand & Company, N.Delhia fiery death. In that case, perhaps the system will rebound in another big-bang. Thus, a continuing cycle of the big-bang, expansion, contraction and big-crunch is a possibility.

(iii) And if P=Pc, the expansion will continue at an ever decreasing rate but the universe will not contract. In this case, the universe is said to be flat because the geometry of space in such a universe would follow the figure: 2:5A.

If P< Pc, the space is negatively curved: a two dimensional analogy of a saddle (fig: 2:5B)

If P>Pc, the space is positively curved as the surface of a spherical ball has the positively curvature (fig: 2:5C).

In all cases, however, the space time is curved.

To find out the value of the critical density

First consider a spherical volume of the universe of radius R whose centre is the earth. Assume that the distribution of matters in the universe is uniform. Let P be the density of matters inside the volume. Then mass of the universe M=4/3 R3P.

If M=mass of the galaxy,

Therefore, the total energy of the galaxy is the sum of its kinetic and potential energies, i.e. E= 1/2 mv2 -m MG/R.

To escape the galaxy permanently from the universe, it must have a maximum kinetic energy 1/2mv, such that its total energy E=0.

E =KE+U=1/2 mv2- GmM/R=0

1/2 mv2 =GmM/R

(According to Hubbles law V =HR, where H is Hubbles parameter)

Therefore, 1/2mv2 =1/2m (HR)2 =GM/R/(4/3R3Pc)

Therefore, the critical density Pc=3H2/8G.

Thus, if the density is greater than the critical value Pc, the galaxy is bound in the universe and eventually will fall back. On the other hand, if P is less than Pc, the galaxy will move away and thus keep the universe expanding.

According to the astronomical observations and calculations, the present-mass density of the universe has been estimated to be ~5x10kgm-3 that is about ten times less than the critical value. This seems to indicate that the universe will go on expanding forever.8The Steady State Theory: Thomas Gold, Hermann Bond & Fred HoyelThey jointly suggested that as the galaxies move away from each other, new galaxies were forming continuously in the gaps between, from new matters that were being created continuously. The universe, therefore, would look roughly the same at all times as well as all points of the space. This was called the steady state theory. This theory required a modification of general relativity to allow the continual creation of matters, but the rate, that involved, was so slow (about one particle per cubic kilometer per year). Therefore, it was not to conflict with experiments. Although, it was simple and it made definite predictions that could be tested by observations. One of these predictions was that the number of galaxies or similar objects in any given volume of space should be the same wherever and whenever we look.

But a Cambridge group of astronomers, led by Martin Riley who had worked with Bondy, Gold and Hoyel on radar during the IInd world war, surveyed the sources of radio waves from outer space during 1950-60. They carried out the results that most of the radio sources must lay outside of our galaxy, indeed, many of them could be identified with other galaxies too. And also, that there were many more weak sources as being more distant ones. This could mean that we are at the centre of a great region in the universe in which the sources are fewer than elsewhere. On the other hand, it could also mean that the sources were more numerous in the past at that time that the radio waves left on their journey to us than they were now. Either explanation is contrary to the predictions of the steady state theory.

Moreover, the discovery of microwaves radiation by Penzias and Wilson, in 1965 also indicates that the universe must have been much denser in the past. Therefore the steady state theory had to be abandoned.9

Friedmans solution about the universe predicts that at the same time in the past (near about 10-20 thousand million years ago), the distance between galaxies must have been zero.

Another attempt to avoid the conclusion that there must have been a Big-Bang and therefore a beginning of time had made by two Russian scientists Evgenii Lifshitz and Isaac Khalantikev, in 1963. They suggested that if the Friedmans universe is moving apart, so, it is not surprising that at the same time in the past they were all at the same place. However, in fact the galaxies are not moving directly just away from each other: they also had some small sideways velocities. So, they need not have been all at exactly the same place, but may be very close only. Perhaps then, the current expanding universe resulted not from a Big-Bang singularity, but from an earlier contracting phase. As the universe had collapsed, the particles in that could not all collide but had flown past and then away from each other producing the present expansion of the universe. How then we can have the claim whether the real universe should have started out with a Big-Bang?

However, they withdrawn their claim in 1970.10

In 1965, a British mathematician and physicist Roger Penrose suggested, using the light-cones, that it behave in general relativity together with the fact that gravity is always attractive. He showed that a star collapsing under its own gravity is trapped in a region, whose surface eventually shrinks to zero size. And, since the surface shrinks to zero, so too must its volume and all the matters in that star will be compressed into a region of zero volume, so, the density of matter and curvature of the space-time would be infinite. In other words, one has a singularity contained into a region of space-time known as a black hole. In 1970, St. Stephen Hawking and Roger Penrose jointly found out the mathematical solutions and proved that there must have been a big-bang singularity provided only that the general relativity is correct and the universe contains as much matters as we observe.

But after some times, Hawking turned against his former assumption and re-established that there was in fact no any singularity but the universe or stars can be disappeared according to quantum effects. This assumption provides the probability of black hole singularity for the evolution of the universe.

What is a black hole? To understand this, we should follow the thermal history and the step evolution of the universe.

Please follow the next.........

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