The amazing fact is that we can trace the Big Bang back to its earliest moments, at least as far back as 10-10 s, and possibly as far back as 10-43 s! This is an incredibly short time, and we can truthfully say that we can trace the evolution of the universe back to the first instant of creation. In so doing, we are probing not just the very earliest universe, but also the highest energy particle physics, so that particle physicists and astronomers are working on two aspects of the same puzzle. Show Assuming that the Big Bang actually happened, what would the early moments of the universe be like? The figure below, from the text, shows an overview of all of time and space, which you can refer back to as we discuss the different eras of the past. The eras of the universe, from the time of the Big Bang, are listed below. We will discuss each in turn. Today we see several lines of evidence that the Big Bang really happened. One of the earliest discoveries was made right here at Bell Labs in New Jersey by Penzias and Wilson, who received the Nobel Prize for their work. Using a radio telescope in 1963 to track down some unwanted noise in their receiving system, they found that when looking at the blank sky, no matter in what direction, they were receiving radiation with a temperature of 3 K. After discussions with some astronomers at Princeton University, they realized that they were seeing the 3000 K photons from the end of the era of nuclei, which has cooled since that time 500,000 years after the Big Bang, to a cool 3 K. This is known as the Cosmic Microwave Background. Since then, the COBE (Cosmic Background Explorer) satellite has measured this background radiation and found it to precisely fit a perfect blackbody spectrum at a temperature of 2.73 K. This perfect blackbody radiation coming from everywhere is strong evidence that we understand the universe at least back to 500,000 years before the Big Bang. To probe even further back, to only 1 millisecond after the Big Bang, we can look at the proportion of elements created in the Big Bang. In order to know how much helium the universe should have made, we need to know the precise temperature of the Big Bang. Luckily, we can get that directly from the cosmic microwave background temperature. Using this precisely known temperature, 2.73 K, we can deduce that the universe should have made 24% helium, exactly what we observe. We can also predict the ratios of other isotopes, which again agree to a remarkable degree. When we look at the universe today, we are immediately struck by the fact that all of the matter is clumped into galaxies, with almost no matter between the galaxies. We learned last time that there is also structure on vastly larger scales in the form of knots of superclusters and huge voids of empty space between them. We can now study this lumpiness of the universe to find out more details about the earliest moments of the Big Bang. The key is to look at the lumpiness, or anisotropy, of the Big Bang radiation. If the universe were too smooth, there would be few or no galaxies. Rather, the matter would be spread smoothly across the universe. However, if the universe were too lumpy early on, all the matter would be concentrated in small clumps, perhaps in the form of black holes of immense mass. There is now a spacecraft called MAP (Microwave Anisotropy Probe) that is studying the lumpiness of the cosmic microwave background, but many groundbased experiments and some spacecraft have also looked for fluctuations. It turns out that the fluctuations are very tiny -- for a long time they seemed too tiny. During the expansion of the universe, parts of the universe were no longer in contact and should have cooled separately. Yet we find that they are precisely the same temperature. Also, in the early Big Bang there should have been quantum fluctuations that, after expanding for 14 billion years, should still be on much smaller scales than galaxies. Why do we see such large-scale structure in the universe, yet relatively smooth structure on smaller scales? Both of these difficulties can be explained by the inflation theory. We already mentioned that inflation would have been driven by the decoupling of the strong nuclear force from the electroweak force. During this time, locally adjacent parts of the universe would have expanded far faster than the speed of light and ended up at opposite ends of the universe. Yet these far distant parts of the universe could have the same initial temperature. Also, the tiny quantum fluctuations would have grown in scale to larger than the solar system in a tiny fraction of a second, and hence the fluctuations would exist on the large scale we see today. Finally, another feature of inflation is to moderate the density of the universe to make space appear very close to flat. Imagine a balloon, whose surface is curved, then blow up the balloon to an immense size. As the size increases, the surface gets locally flatter and flatter. This is related to the density of the universe. Recall that we discussed last time the critical density of the universe, and said that the universe appears to be almost flat. Inflation can help that by taking an initially open or closed space and making it so large that it appears nearly flat. What we still have to understand is where is all the matter needed to make the universe flat. We said that there is far too little ordinary matter (maybe only 1 to 10% of the critical density), while there is also dark matter to help out, but matter and dark matter together only accounts for about 30% of that needed to flatten or close the universe. Current theories suggest that perhaps a new form of energy, dark energy, can make up the difference, but it has not been shown yet. Most scientists today would agree that the Big Bang is a successful theory, for which there are at least two bits of very clear observational evidence: 1) the cosmic microwave background radiation and 2) the relative amounts of hydrogen, helium, and other elements in the universe. However, from the above arguments you can see that several aspects of the Big Bang and related issues are not understood. From time to time you will see newspaper headlines that claim that the Big Bang is wrong, but those claims invariably turn out to be arguments over details, not fundamental disagreements in the theory as a whole. One observational fact stands out in favor of something like the Big Bang -- the sky is dark at night! If we imagine that the universe is infinite in all directions, then obviously there must be an infinite number of galaxies. If so, no matter which direction we look in the night sky, our line of sight must intersect a star in a galaxy somewhere. It is like being in a dense forest, in which no matter where you look you can only see tree-trunks, never the open sky. This argument is now called Olber's Paradox, after a German scientist in the 1800's. The universe is not like that -- the sky is dark at night -- so the observable universe cannot be infinite. The text casts this as showing that the universe must have a distinct beginning, since we can look back only to the cosmic horizon (14 billion years ago), but there is another possible horizon -- the one set by the expansion of the universe. If the universe is expanding such that the outer edges are moving away at greater than the speed of light, then we can only see a limited part of the universe up to a recession speed of the speed of light. So the universe may extend beyond this light-speed horizon. In fact, if the expansion were to slow down, as we expect due to gravity slowing the expansion, then the light-speed horizon would move outward with time, and new galaxies would appear inside our observable universe. However, recall that we mentioned last time that the universe may be increasing its rate of expansion through the action of some kind of anti-gravity producing dark energy. If that is the case, then galaxies that we can see now, at the edge of the universe, would speed up and go outside the speed-limit horizon. So the universe is very strange, and it seems to get stranger with each new discovery. But astronomers will continue to devise new observations to explore the universe, and new theories to explain it, and new particle experiments to verify the theories. But reflect for a moment on the following question from the first lecture: Time out to think Some people think that our tiny physical size in the vast universe makes When we look at how much we know (or think we know) about the universe -- things that seemed completely unknowable only a few decades ago -- we have to marvel at the power of our intelligence to understand the universe in which we live. Perhaps you will help humanity solve some of the mysteries. What are the 5 stages of Big Bang theory?Stages of the Big Bang Theory. Heavy Particle Era.. Light Particle Era.. Radiation Era.. Matter Era.. What happened after the Big Bang occurred?Origins. In the first moments after the Big Bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made.
What is the Big Bang timeline?Big Bang Timeline. What happened during the Big Bang?The universe began, scientists believe, with every speck of its energy jammed into a very tiny point. This extremely dense point exploded with unimaginable force, creating matter and propelling it outward to make the billions of galaxies of our vast universe. Astrophysicists dubbed this titanic explosion the Big Bang.
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