The Expanding Universe

In the 1920s, Edwin Hubble made the surprising discovery that, on average, the galaxies are all moving away from us. The farther away a galaxy is, the faster it is moving away. Hubble found a simple rule for this recession, a galaxy twice as far away is receding twice as fast.
At first you might think that we are at the exact center of the universe if the galaxies are all moving directly away from us. But that is not the case. Hubble’s discovery indicates that the universe is expanding uniformly. You can see how a uniform expansion works by blowing up a balloon part way, and drawing a number of uniformly spaced dots on the balloon. Then pick any dot as your own dot, and watch it as you continue to blow the balloon up. You will see that the neighboring dots all move away from your dot, and you will also observe Hubble’s rule that dots twice as far away move away twice as fast.
Hubble’s discovery provided the first indication that there is a limit to how far away we can see things. At distances of about fourteen billion light years, the recessional speed approaches the speed of light. Recent photographs taken by the Hubble telescope show galaxies receding at speeds in excess of 95% the speed of light, galaxies close to the edge of what we call the visible universe.
The implications of Hubble’s rule are more dramatic if you imagine that you take a moving picture of the expanding universe and then run the movie backward in time. The rule that galaxies twice as far away are receding twice as fast become the rule that galaxies twice as far away are approaching you twice as fast. A more distant galaxy, one at twice the distance but heading toward you at twice the speed, will get to you at the same time as a closer galaxy. In fact, all the galaxies will reach you at the same instant of time. Now run the movie forward from that instant of time, and you see all the galaxies flying apart from what looks like a single explosion. From Hubble’s law you can figure that the explosion should have occurred about fourteen billion years ago.

Did such an explosion really happen, or are we simply misreading the data? Is there some other way of interpreting the expansion without invoking such a cataclysmic beginning? Various astronomers thought there was. In their continuous creation theory they developed a model of the universe that was both unchanging and expanding at the same time. That sounds like an impossible trick because as the universe expands and the galaxies move apart, the density of matter has to decrease. To keep the universe from changing, the model assumed that matter was being created throughout space at just the right rate to keep the average density of matter constant.
With this theory one is faced with the question of which is harder to accept—the picture of the universe starting in an explosion which was derisively called the Big Bang, or the idea that matter is continuously being created everywhere? To provide an explicit test of the continuous creation model, it was proposed that all matter was created in the form of hydrogen atoms, and that all the elements we see around us today, the carbon, oxygen, iron, uranium, etc., were made as a result of nuclear reactions inside of stars.
To test this hypothesis, physicists studied in the laboratory those nuclear reactions which should be relevant to the synthesis of the elements. The results were quite successful. They predicted the correct or nearly correct abundance of all the elements but one. The holdout was helium. There appeared to be more helium in the universe than they could explain.

By 1960, it was recognized that, to explain the abundance of the elements as a result of nuclear reactions inside of stars, you have to start with a mixture of hydrogen and helium. Where did the helium come from? Could it have been created in a Big Bang? As early as 1948, the Russian physicist George Gamov studied the consequences of the Big Bang model of the universe. He found that if the conditions in the early universe were just right, there should be light left over from the explosion, light that would now be a faint glow at radio wave frequencies. Gamov talked about this prediction with several experimental physicists and was told that the glow would be undetectable. Gamov’s prediction was more or less ignored until 1964 when the glow was accidently detected as noise in a radio telescope. Satellites have now been used to study this glow in detail, and the results leave little doubt about the explosive nature of the birth of the universe.

What was the universe like at the beginning? In an attempt to find out, physicists have applied the laws of physics, as we have learned them here on earth, to the collapsing universe seen in the time reversed motion picture of the galaxies. One of the main features that emerges as we go back in time and the universe gets smaller and smaller, is that it also becomes hotter and hotter. The obvious question in constructing a model the universe is how small and how hot do we allow it to get? Do we stop our model, stop our calculations, when the universe is down to the size of a galaxy? a star? a grapefruit? or a proton? Does it make any sense to apply the laws of physics to something as hot and dense as the universe condensed into something smaller than, say, the size of a grapefruit? Surprisingly, it may. One of the frontiers of physics research is to test the application of the laws of physics to this model of the hot early universe.

We will start our disruption of the early universe at a time when the universe was about a billionth of a second old and the temperature was three hundred thousand billion (3× 10¹⁴ ) degrees. While this sounds like a preposterously short time and unbelievably high temperature, it is not the shortest time or highest temperature that has been quite carefully considered. For our overview, we are arbitrarily choosing that time because of the series of pictures we can paint which show the universe evolving. These pictures all involve the behavior of matter as it has been studied in the laboratory. To go back earlier relies on theories that we are still formulating and trying to test.
To recognize what we see in this evolving picture of the universe, we first need a reasonably good picture of what the matter around us is like. With an understanding of the building blocks of matter, we can watch the pieces fit together as the universe evolves. Our discussion of these building blocks will begin with atoms which appear only late in the universe, and work down to smaller particles which play a role at earlier times. To understand what is happening, we also need a picture of how matter interacts via the basic forces in nature.
When you look through a microscope and change the magnification, what you see and how you interpret it, changes, even though you are looking at the same sample. To get a preliminary idea of what matter is made from and how it behaves, we will select a particular sample and magnify it in stages. At each stage we will provide a brief discussion to help interpret what we see. As we increase the magnification, the interpretation of what we see changes to fit and to explain the new picture. Surprisingly, when we get down to the smallest scales of distance using the greatest magnification, we see the entire universe at its infancy. We have reached the point where studying matter on the very smallest scale requires an understanding of the very largest, and vice versa.

مواضيع ذات صلة

Four-vector algebra

Relativity and the philosophers

Four-vectors

Simultaneity

The Lorentz transformation

The principle of relativity

Gravity and relativity

الجاذبية.. ما بعد آينشتاين، للباحثين فرانك دانينوس، هيلين لومور

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