The Origin of the Universe
Light can be considered as a collection of electromagnetic waves of different frequencies. Different frequencies correspond to different colors. In the frequency spectrum, red and blue correspond to the low frequency and the high frequency end respectively.
From the point of view of a stationary observer, light emitted from a source that is moving away, will appear to have a lower frequency content. This is because its wavelength tends to increase. As the source moves away, the next wave would be emitted from a position further away than the previous one.
Under the same principle, when we observe a star that is moving away from us, its frequency spectrum would be shifted to the red end. This phenomenon is called red shift. On the other hand, the relationship between perceived frequencies and the relative motion of the light source is called the Doppler's effect.
In the 1920s, Hubble discovered that the spectra of distant galaxies appeared to be red-shifted, meaning the universe was expanding. This milestone discovery made people aware that the universe may not be static at all. Rather, it must have a beginning.
The Big Bang Theory
One possible theory explaining the Beginning is the gigantic explosion or the Big Bang Theory. According to the Big Bang Theory, the beginning happened approximately 15 billion years ago.
At the beginning, the universe was believed to be infinitely dense and hot. Particles moved so fast that they could move away from any attraction force. However, as the universe expands, the temperature begins to fall. Note that temperature is a measure of average kinetic energy of the matter present.
One second after the big bang, the temperature fell to about 10,000 million degree Kelvin. Approximately 100 seconds later, it dropped to 1,000 million degree Kelvin, which is the same temperature as that inside the hottest stars today. At this temperature, protons and neutrons would not possess sufficient energy to escape from the attraction due to the strong nuclear force. Pairs of a proton and a neutron would combine together to produce the nuclei of the atom deuterium (hydrogen with one neutron, one proton and no electrons). The deuterium(s) then combined with more protons and neutrons to form the nuclei of helium and a small number of heavier elements, such as lithium and beryllium. This way, one quarter of the protons and neutrons were consumed to form these elements whereas the remaining neutrons decayed into protons, the nuclei of ordinary hydrogen atoms. These phenomena explain the abundance of helium and hydrogen in the universe.
Within a few hours of the big bang, the formation of these elements stopped. During the next few million years, nothing in particular happened in the universe, except the temperature continued to fall. Eventually, once the temperature had dropped to several thousand degree Kelvin, electrons (which had been moving freely so far) and the nuclei no longer had enough energy to overcome the mutual electromagnetic attraction force. They would combine to form atoms.
While all these were happening, the expansion rate in regions with higher density were reduced due to their stronger gravitational forces. Eventually these regions stopped expanding and started to re-collapse. This would make them rotate gently. Nevertheless, as they became smaller, they span faster so that the angular momentum was conserved. In the end, they span fast enough to balance the gravitational attraction. This way, disk-like rotating galaxies were formed. On the other hand, regions that did not rotate formed oval-shaped elliptical galaxies. They stopped collapsing because their constituents individually span about themselves.
As time went on, the hydrogen and helium gas decomposed to smaller clouds, which then collapsed further due to their own gravity force. As these clouds contracted, the temperature rose until it was high enough to allow nuclear fusion to take place. The lighter atoms combined to form larger atoms, releasing a vast amount of energy in the process. Hydrogen was converted to helium and energy was released. A new star was formed. Towards the end of the star's life, heavier elements on its surface would be flung back to the galaxy and acted as the starting material for the next generation of stars. However, some of these materials collected themselves together to form the bodies of planets like the Earth.
Doubts about the Big Bang Theory
The Big Bang Theory revolutionized the way many people see the origin of the universe. However, it cannot answer some important questions. For instance,
First, why was the universe so hot at the beginning?
Second, why is the universe so uniform on a large scale? In fact, no matter where the observing location is, and no matter what direction one is observing, the universe will still look roughly the same.
Third, what was the cause of the local variations in density in different regions of the universe, making the births of stars and planets possible?
The Steady State Theory
Many people believed that the Big Bang Theory was philosophically imperfect. They did not like the idea that time has a beginning at the big bang. If expressed mathematically, the big bang itself is a singularity. It can be perceived as an infinitely curved space-time (space-time is the idea to express position in terms of a four dimensional space and time. It is proposed by Albert Einstein in his famous General Theory of Relativity). However, scientific theories break down when dealing with singularity. It is therefore meaningless to define time prior to the big bang.
Therefore, some people found another theory called the Steady State Theory. The idea was based on the fact that the universe looked roughly the same in a macroscopic scale. They believed that as the galaxies moved away from one another, new galaxies continued to form to fill in the gaps and that new matter was constantly created. Therefore, the universe would remain the same in the past, at present and in the future.
Doubts about the Steady State Theory
The Steady State Theory was not as widely accepted as the Big Bang Theory.
First, in 1965, two American physicists discovered the existence of microwave radiation that would by no means be emitted from the Earth or even from our galaxy. That radiation would have to be emitted from beyond the observable universe and it appeared to be identical in all directions. This phenomenon could only be explained using the Doppler's effect: Light from extremely distant parts of the universe (N.B. The microwave radiation was later found to be emitted from some extremely bright star systems called quasars) in the past took an extremely long time and distance to reach the earth. Taking the expansion of the universe into account, the light would have been so red-shifted that it became microwave. This meant the universe was much denser in the past and hence the "steady" theory was incorrect.
Second, in the late 1950s, scientists detected radio wave emitted from outside our galaxy. They showed that there were more weak sources than the strong ones. Note that weak sources correspond to sources farther away and vice versa. It also appeared that there were less common sources closer to the Earth. This meant there were more sources in the past. Clearly, once again, this contradicted with the Steady State Theory.
Third, as far as our understanding of the universe is concerned, there are no ways that matter can be created out of nothing. And even if it could, should one minor defect occurred during the creation process, it would upset the equilibrium and thus caused the universe to contract or to expand.
Nowadays, most people accept the Big Bang Theory as the standard explanation for the origin of the universe.
The Future of the Universe
To consider the future of the universe, we must consider dark matter. Dark matter, as its name suggests, is material in space that cannot be detected using any form of electromagnetic radiation. They do not shine or radiate. Their existence is merely "suspected" by their collective effect on their surroundings in the universe.
There are several possible beings that can make up dark matter.
Neutrinos
Although they appear to be "massless" individually, if there are enough of them around, their collective effects (mass) are enormous.
Brown Dwarfs
They are stars with too little mass to allow nuclear reaction to take place in their cores. Therefore, they cannot be detected.
Black Holes
They are points in the universe with infinite mass. Nothing, including lights can escape from its gravitational pull. Therefore they cannot be detected.
Dead Stars
Like cold white dwarfs and cold neutron stars, dead stars possess no energy at all. Their existence cannot be detected.
Most galaxies are found in clusters. If it is true that gravity is holding galaxies in clusters, from what we observe in the sky, there are simply less than 90% mass available for this level of gravity. Therefore, scientists believe that one possible explanation is the existence of dark matter.
So why are we talking about dark matters? As it is mentioned earlier, our universe is constantly expanding. No one knows what would happen at the end (if there is an end at all). However, in order to analyze the future of the universe, we have to find out the average mass density in this universe. This means dark matter is an important part.
Some Possibilities
Depending of the average mass density of the universe, there are three distinct possibilities.
First, if the average mass density is below a critical value of about three hydrogen atoms per cubic meter, the four-dimensional universe of space-time is open. It will expand forever, until every atom is so far away from one another that an energyless state is reached.
Second, if the average mass density is above the critical value, the universe is close. Gravity will eventually halt the expansion and the universe will collapse to a singularity.
Third, if the average mass is equal to the critical value, then the universe is flat, gravity will slow down the expansion but can never stop it. This phenomenon resembles a graph approaching an asymptote. The graph will forever approach the asymptote but can never reach.
Meanwhile, the average mass density has not yet been determined by current technology, the future of the universe is still uncertain.
The Life of Stars
A star begins its life when a large cloud of gas (mostly hydrogen atoms) falls in itself due to gravity. The cloud is then shrunk, raising the temperature. The denser the cloud becomes, the hotter it gets. Eventually it begins to glow.
Once formed, a star does not contract forever. At the extremely hot and dense center, nuclei of hydrogen atoms occasionally fuse to form a single nucleus of helium. However, the mass of a helium nucleus is less than total mass of the four constituent hydrogen nuclei. It turns out that the missing mass is released as energy. However, not all of this energy is radiated away. Therefore, the energy trapped inside the star heats up its interior and raises the internal pressure. This pressure is high enough to halt any further contraction, a state called hydrostatic equilibrium is reached.
Stars where hydrogen burning provides the main energy source are called main sequence stars. On a Hertzsprung-Russell diagram (which plots the absolute magnitude of luminosity against temperature), most stars are positioned along the main sequence across the diagram. It was once believed that the main sequence represented the evolution process of stars. In fact, it shows that stars have a range of masses. When a star settles to a stage of hydrogen burning, the more massive it is, the brighter and hotter it gets. For instance, in order to resist gravitational collapse, the sun has over 700 million tons of hydrogen gas fused to helium each second. Nevertheless, even at this enormous rate, our sun will not run out of hydrogen for another 5 billion years (our sun currently in the middle of its life). However, main sequence stars that is more massive than our sun will have a much shorter life. This is because more energy is needed to support its extra size. For example, a star of 20 solar masses is 10000 times brighter than our sun, but it would only have enough fuel for a few million years.
Eventually a star will run out of fuel. When most of the hydrogen in the core has turned into helium, energy production is slowed down and the star begins to contract again. This raises the temperature of the star, igniting some of the unburned hydrogen around the helium core. Meanwhile the outermost layers of the star expand enormously, resulting in a decrease in its surface temperature, making its color red, thus a red giant is formed.
Inside the red giant, hydrogen continues to burn around the core, which will contract even more. When the temperature reaches about 100 million degrees, helium atoms will fuse to form carbon atoms, releasing more energy. As this goes on, the core's temperature continues to rise, fusing carbon atoms to oxygen. On and on, heavier elements form in the core of the star, while lighter elements are still being fused in surrounding levels. The star develops into a structure rather like an onion.
However, not all stars will reach this stage. In order to have nuclear reactions to produce heavier elements, a star needs enough mass to compress its core more -- hence the existence of brown dwarf. Relatively few stars are that massive. Our sun will probably proceed as far as the carbon stage. Stars that are 20 times the mass of the sun produce heavier elements, but again, they will only go as far as the iron-56 stage. The energy released during the formation of the iron particles cannot sustain further contraction. Therefore, heavier elements cannot be formed. The stars will start running out of energy when they reach this stage.
After running out of energy, stars of the same size as our sun will start to shrink. They will reach a stage called electron degeneracy. The electrons surrounding the nuclei are packed so close that a white dwarf, which can support its own weight even without any energy supply, is formed. Such white dwarfs fade when they age because they glows only for reason of energy they had earlier in their lives. In a few million years a white dwarf simply turns dark in space.
On the other hand, dying stars more massive than the Chandrasekhar's Limit cannot shrink to form white dwarfs. It is important to note that it is not the star's initial but final mass that matters for this limit. Most stars during their lifetimes would throw away some of their mass from their outer layers into space, producing what is stellar winds. Nevertheless, stars that start off 8 times the mass of the sun can usually manage to end their life below the limit, forming a white dwarf.
For a star 15 times more massive than the sun, it will probably take several days to develop an iron core -- its last attempt to survive. This iron core is slightly smaller than the Earth, but it contains more mass than the sun does. However, comparing to the sun, the entire star is at least 50 times bigger in diameter.
Iron continues to form until the core exceeds the Chandrasekhar's limit, then the ultimate collapse starts. There are two factors that will speed up this process.
First, energy is taken by iron as it splits to form lighter elements. This lowers the pressure which is essential for the hydrostatic equilibrium. Second, electrons are pushed very close to the nuclei of the atoms. These electrons try to combine with protons to form neutrons, releasing numerous tiny, virtually "massless" particles called neutrino, which can penetrate all matters. They carry enormous amount of energy away instantaneously. This again lowers the pressure and speeds up the collapsing process.
Black Hole
For some massive stars, if they continue to shrink and hit a limit called the Schwarzchild's Radius, which is larger for more massive stars, they would simply collapse to a point of infinite density called a black hole. In a black hole, no radiation (including light) can escape from within the Schwarzchild's radius, because the gravity is so strong. This is why a black hole is "black". Besides, matter fallen into the radius cannot escape, this is why it is called a "hole".
Supernova
In most cases, however, the collapse will turn, instead, to a big explosion when a neutron star is formed. As the core collapsed to the density of an atomic nucleus, a situation similar to that of electron degeneracy results. This process proceeds so rapidly that the outer levels continue to move in. However, they are bounced back and thus move outwards again at 10% the speed of light. The outer levels, in effect, blow off what is ahead of them in the outermost layers, causing an explosion. This explosion is made even more powerful by the thermal neutrinos, emitted from the neutron-rich core because of the extreme temperature.
The density is extremely high, the neutrinos have to use up 1% of their energy to travel through the materials. Nevertheless, this energy is so enormous that the star heats up to the extreme limit and it explodes powerfully. Only the neutron core remains and this explosion is called a supernova.
Note that the materials blown off can contribute to the starting materials for the next generation of stars, and that the supernovae themselves can cause, as some theorists suggest, interstellar gas in dense regions to compress locally and form new stars.
As neutron stars come from the collapse of ordinary stars, they span rapidly - in order that angular momentum is conserved for a collapsed neutron star with less moment of inertia. Moreover, as it becomes a neutron star, the normal magnetic fields of the star from which it forms become very great in magnitude. Furthermore, it is likely that a neutron star, with its tremendous mass, is surrounded by a hot gas where delocalized electrons fly around at high speed. The combination of these three effects - moving magnetic field through charged particles - is a neutron star emitting radio waves like pulses. This is called a pulsar.
Finally, what is the ultimate fate of such neutron stars? Since a neutron star has no energy supply, its spinning motion, which also causes light and radio waves, would slow down year after year. In a thousand years, it will still rotate about 10 times a second. In the end, several million years after its birth, it goes dark, only an undetectable cold neutron star remains.
Nova
Novae are different from supernovae. Occasionally in the past, people declared they had discovered a new star. In fact it was only an existing variable star - a star that varies in luminosity - temporarily increasing its luminosity by hundreds to thousand times, so that it was visible. Novae occur in close binary star systems - a binary star system consists of two stars revolving about each other - in which one member is a white dwarf and the other one may be a star already reached the red giant stage.
For a binary star system, there is a surface of points on which the mutual gravitational attraction cancel exactly. However, the situation is that a red giant would constantly increase in size to the extent that some of its stellar material may extend through the surface and be attracted towards the white dwarf. These material, mostly hydrogen, continues to accumulate on the white dwarf's surface, raising its temperature until it reaches that of the degenerate interior of the white dwarf. At that time, hydrogen is converted to helium, liberating a vast amount of energy. This will blow off the materials in the outer layer on the white drawf's surface. After the explosion, the system will settle down, awaiting for another explosion as the accumulating process continues. This phenomenon is called a nova.
KP Pun
New Zealand
August 1995
Bibliography
George O Abell, Exploration of the Universe (fourth edition), CBS College Publishing, New York, 1982.
Stephen Hawking, A Brief History of Time, Bantom Books, UK, 1988.
Laurence A Marshall, The Supernova Story, Plenum Press, New York, 1988.
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Copyright 1995-2001 KP Pun. All rights reserved.
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