begining of the universe

@kaniam (582)
December 19, 2006 11:58am CST
The vast majority of scientists accept the Big Bang theory for the origin of the universe. According to this model the universe is about 13,700 million years old. At the beginning of the universe, all the material that we can observe with our telescopes today (known as the observable universe) was confined to a very small space. Every point and everything that would become the space, matter and energy in the present-day Universe was contained within this entity – in our familiar three dimensions nothing exists outside it. If we go as far back as the Big Bang (the starting point for our current theory of the history of the universe) this material was squeezed tightly together and was extremely hot – leading to the full name of the theory as the 'Hot Big Bang'. After the Hot Big Bang, the universe expanded at an enormous rate, eventually becoming the cosmos that we inhabit today. In the contemporary universe astronomers can see that virtually all galaxies (large collections of stars like our own Milky Way) are moving away from each other. This implies that the whole universe is expanding and that the volume of space is getting larger. Scientists are still not exactly sure how the universe will end, although there are several different ideas. Science has advanced to the point where we can infer something about the entire universe. This has been a great challenge considering how unimaginably vast the universe is. The countless stars you see in the darkest sky constitute merely 3000 neighbors out of about 300,000,000,000 stars in our galaxy, and as many as 100,000,000,000 galaxies exist in the universe. Humans have always wondered: Has the universe always existed like we see it now, or did it somehow start all of a sudden? In the beginning of this past century, we found out in amazement that the entire universe is expanding. This led physicists to deduce that the universe started out in the finite past with a minuscule size. Realizing that the universe had a beginning, and awed by its vastness and its creations, people have asked: How did the universe begin? After all, we are here to be amazed by it because the universe eventually created lives like us. Now, after decades of observing and thinking, we have come to answer confidently the question of the origin of our universe... with what is known as the "big bang". What is the Big Bang?According to the big bang theory, the universe began by expanding from an infinitesimal volume with extremely high density and temperature. The universe was initially significantly smaller than even a pore on your skin. With the big bang, the fabric of space itself began expanding like the surface of an inflating balloon – matter simply rode along the stretching space like dust on the balloon's surface. The big bang is not like an explosion of matter in otherwise empty space; rather, space itself began with the big bang and carried matter with it as it expanded. Physicists think that even time began with the big bang. Today, just about every scientist believes in the big bang model. The evidence is overwhelming enough that in 1951, the Catholic Church officially pronounced the big bang model to be in accordance with the Bible. Until the early 1900s, most people had assumed that the universe was fixed in size. New possibilities opened up in 1915, when Einstein formulated his famous general relativity theory that describes the nature of space, time, and gravity. This theory allows for expansion or contraction of the fabric of space. In 1917, astronomer Willem de Sitter applied this theory to the entire universe and boldly went on to show that the universe could be expanding. Aleksandr Friedmann, a mathematician, reached the same conclusion in a more general way in 1922, as did Georges Lemaître, a cosmologist and a Jesuit, in 1927. This step was revolutionary since the accepted view at the time was that the universe was static in size. Tracing back this expanding universe, Lemaître imagined all matter initially contained in a tiny universe and then exploding. These thoughts introduced amazing new possibilities for the universe, but were independent of observation at that time. Why Do We Think the Big Bang Happened?Three main observational results over the past century led astronomers to become certain that the universe began with the big bang. First, they found out that the universe is expanding—meaning that the separations between galaxies are becoming larger and larger. This led them to deduce that everything used to be extremely close together before some kind of explosion. Second, the big bang perfectly explains the abundance of helium and other nuclei like deuterium (an isotope of hydrogen) in the universe. A hot, dense, and expanding environment at the beginning could produce these nuclei in the abundance we observe today. Third, astronomers could actually observe the cosmic background radiation—the afterglow of the explosion—from every direction in the universe. This last evidence so conclusively confirmed the theory of the universe's beginning that Stephen Hawking said, "It is the discovery of the century, if not of all time." * Expansion of Universe Around the same time that people began to come up with the idea of an expanding universe, astronomer Vesto Slipher noticed that there are more galaxies going away from us than approaching us. Astronomers know that a galaxy is approaching or receding by looking at the spectrum of its light. If the spectrum is shifted toward shorter wavelength (blueshift), then the galaxy must be approaching, just like the sound of an approaching racing car has a higher pitch (shorter sound wavelength). If the spectrum is shifted toward longer wavelength (redshift), then the galaxy must be receding, just like the sound of a racing car that has passed us has a lower pitch (longer sound wavelength). The degree of the shift depends on the speed of approach or recession. So in other words, Slipher observed more galaxies whose spectrum was redshifted than those whose spectrum was blueshifted. In 1929, Edwin Hubble discovered that farther galaxies are going away from us at higher speeds, proportional to their distance. In other words, the spectra of more distant galaxies had higher redshifts. From distant galaxies, light takes millions or even billions of years to reach us. This means we are seeing an image from millions or billions of years ago. In redshift, the spectrum is shifted from shorter wavelength to longer wavelength as the light travels from the galaxy to us. This increase in wavelength is due to expansion of the very fabric of space itself over the years that the light was traveling. If the wavelength had doubled, space must have expanded by a factor of two. Thus, Hubble's discovery was that this expansion factor was roughly proportional to the distance light traveled, or equivalently, to how far back in time you looked. This means that the universe was smaller and smaller earlier and earlier. The universe has been expanding.Tracing back this expanding universe, we see that the separations between galaxies become smaller while the density becomes higher. This continues until all matter is compacted into a completely shrunk volume of the universe with an incredible density—the moment of the big bang. We can estimate how long ago this was by dividing the distance to a galaxy by its recessional velocity. This way we estimate how long ago the distance between that galaxy and ours was essentially zero. Calculation shows that the big bang occurred as long as 10-15 billion years ago, which is about three times the age of the Earth. As a way of checking this age estimate, we can examine the oldest things we find in the universe to verify that they are 10-15 billion years old, but definitely not older. From radioactive dating of uranium isotopes, we know that the oldest isotopes were created (through nuclear reactions in supernovae) about 10 billion years ago. From our current model of star evolution, we know that the oldest stars in our Galaxy are about 12 billion years old. These ages are consistent with the age estimated from the observed expansion of the universe. This agreement suggests that the universe really began a finite time ago, providing an encouraging reason to believe in the big bang model of the universe. * Helium and Deuterium Abundance in Universe The notion that the expanding universe was extremely hot in the beginning provides a reasonable explanation for why helium and deuterium seem to have existed even before star formation. Both these species are created by nuclear fusion. Fusion of a proton and a neutron produces deuterium (also known as heavy hydrogen), while fusion of two deuterium nuclei produces helium. These reactions can occur only at very high temperatures, such as in the interiors of stars. In 1946, George Gamow, once a student of Friedmann, suggested that nuclear fusion must have taken place when the universe was so hot in the beginning. This process, called the "big bang nucleosynthesis", would have created helium and deuterium (plus trace amounts of elements like lithium and beryllium) out of an initial sea of energetic protons and neutrons. In the early 1960s, spectroscopic studies of local stars showed that the abundance of helium was about 20-30% by mass, the rest being mostly hydrogen. Stars and hydrogen bombs are the only things we know of that make helium in the present universe. They both combine hydrogen nuclei (protons) into helium nuclei through nuclear fusion, releasing great amounts of energy. Astronomers calculate that the night sky should be much brighter if all the helium we now observe had come from stars burning (or bombs exploding). Some, if not most, of the helium must have existed before star formation. Based on theories of the big bang nucleosynthesis, physicists in the mid-1960s calculated that roughly 1/4 of mass was converted into helium in the beginning, while the res
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