General Astronomy/Current Unsolved Mysteries
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Dark Matter and Dark Energy
Dark Matter is invisible, but has been postulated from its apparent influence on normal matter. It is one explanation for the lack of gravity needed to hold our universe together. The current estimations of the total mass in the universe wouldn't support the observations and given the current estimates of mass; our star systems would simply fall apart. The dark matter theory states that there is matter that doesn't emit radiation and therefore we cannot observe, as all of our methods rely on detecting electromagnetic waves of some sort.
This is an observation that has thrown everyone off guard. There is a certain type of supernova called type Ia. The nice thing about type Ia supernova is that they all have the same absolute brightness. (We will discuss the mystery of why they have the same absolute brightness, and how they are used to measure distances, in a later chapter, General Astronomy/The Death of High Mass Stars). This makes them very useful for measuring distances. The thing that has everyone confused is that it seems that the rate of expansion of the universe is increasing. This suggests that there is some sort of dark energy in space. As space expands, this energy appears and causes space to expand even more. As of now, one of the major research efforts is to gather data on fainter supernova in order to have more data points to define the expansion curve. The goal is to gain a more exact idea of how the universe is expanding so that we have some idea of what is causing the acceleration.
This is still confusing people. One of the problems is that we have some pretty strong arguments as to what dark matter isn't. It's probably not regular matter made of protons and neutrons. The reason why we believe that is that if it were ordinary matter, then when you calculate you get too little deuterium in the universe. Another problem is that the "clumpiness" of the galaxies and the cosmic background radiation just doesn't look right. If you go through the properties of dark matter, it turns out that it doesn't correspond to any known particle.
Some other theories explaining dark matter deal directly with our way of thinking about gravity. There is speculation that there is another large-scale force that is keeping our universe together. Another possible explanation is to think of space as a gas-and-space solid. If you place two objects apart from each other then pressurize the area, the two objects will be forced towards each other. This reverses our current ideas of gravity from an object having a pull on other object, to an object being pushed from all directions. (An object alone has no movement, but two objects create an uneven pressure pushing the objects together.)
An estimated 23% of the matter in the universe is dark matter. Ordinary matter only makes up 4% of the universe. The remaining 73% is an even more mysterious, repulsive "dark vacuum energy".
The most popular theory right now is that the repulsive force is actually a property of space itself: it is caused by waves of energy, created by particles and anti-particles popping into existence and then annihilating each other with no net effect. Early in the universe's life, when there wasn't much space, the effect was small compared to gravity. But as the galaxies moved apart, the effect became greater. 
History and Ideas of Composition
The term dark matter can be applied to solar objects that have a large mass to luminosity ratio. Dark matter cannot be seen or measured however it plays an essential part in our universe. First theorized in 1933 by Swiss astrophysicist Fritz Zwicky who attempted to find an explanation for the absence of matter between objects in space. He observed that there was more mass in a cluster of galaxies than there should be from the visible objects. So there was something unseen adding to the mass of the cluster. Then in 1950 Vera Rubin discovered that contrary to Newton’s law that objects orbit around a center point move slower the farther away they are from the center. She found that in fact an objects speed remained the same regardless of the proximity to the center point. So in order for the speed to remain the same there had to be some extra matter, which was present on the outer rings of an objects orbit. Building off Zwicky’s work she concluded that this extra mass was dark matter. It was also discovered that dark matter explained the flat shape of rotational curves of the planets. The term dark matter represents matter that we perceive to be there because of its effect on the objects around it. While the exact make up of dark matter is still unknown scientist have theorized a list of possible components that dark matter could be. Possibly candidates or ingredients of dark matter are:
Ionized gas Emits thermal free-free radiation which cannot be observed. Dust Emits radiation and is made up of elements heavier that helium. Main Sequence Stars Could be an ingredient but could not be sole property of dark matter because a great amount of them would be visible. Black Holes are highly unlikely because they would disrupt the binary separations of dark matter. However not much is known about the explosions that produce black holes, so it is still an option. White Dwarfs When forming white dwarfs produce many intermediate-mass elements (He, N, Ne, C, O) or halo gas, which is not visible. Neutrinos Unlikely but they do have enough mass to be a candidate. WIMPS or Cold Dark Matter Weak interacting particles but move at nonrelativistic speeds. A more in depth flow chart depicting how the above suggestions are connected is picture below taken from Modern Cosmological Observations And Problems (140). http://ned.ipac.caltech.edu/level5/Bothun2/Figures/dm1.gif
The cosmic background radiation was formed when protons and electrons combined to form atoms. The trouble is that we know that the matter between galaxies today is ionized (i.e. it's separate protons and electrons) with clumps of hydrogen atoms. We know this because when we look at all but the most distant galaxies, we don't see the spectra lines of hydrogen. So at some point the hydrogen in the universe reionized. The notion was that starlight caused the hydrogen in the universe to reionize, but the latest observations seem to indicate that this reionization occurred before the first stars were there.
The idea is that galaxies started from tiny fluctuations in density that formed after the big bang. By assuming that the universe consists mainly of cold dark matter, you can almost get the clumpliness that you see with the current galaxies. But there are still puzzles. There is an annoying lack of tiny galaxies, and the rotation curve that cold dark matter predicts, isn't quite the one that we see.
Before the Big Bang
Now to get really speculative, there have been some papers written recently that try to figure out what happened before the Big Bang. One of the strange ideas is that the universe is merely one plane in a multidimensional space, and that what happened was that two membranes in a multidimensional space collided causing a massive expansion in three of the dimensions. This is all really speculative, but the weird thing is that it isn't totally disconnected from observation. The idea is that you can use this model to predict the initial expansion of the universe, and this might have some effects on the ripples that you see in the cosmic microwave background. The big problem is that the matter that began expanding had to have always existed, yet, because of the predictable nature of the elements, it had to have had a definite, external force to set it in motion that could decide when to start the "chain reaction". Something cannot just be in a stable form, or even an unstable form, forever and finally explode, it has to go in a cycle. In other words, consider the following. Out of nothing, a theretofore nonexistent dense mass spontaneously emerged, which erupted in an enormously powerful fireball by its own theretofore nonexistent energy to spontaneously and immediately create from this chaos the defined fundamental forces of physics and the subatomic fundamental particles, which eventually organized themselves into a variety of atomic species, then into molecules, and then into a diverse assortment of inorganic matter that gravitationally assembled itself into this highly structured and precisely ordered universe. We all know that this is ridiculous, but it is equally ridiculous to say "a theretofore stable mass spontaneously became unstable".
With all of these puzzles, its not clear what is going to happen next. There is a lot of data coming in, and it may be that with new data, it will be possible to make our models of the universe work with minor tweaks here and there, and we can go on in the mode of what Kuhn calls "normal science." It's also possible that one day there will be some observation which is like Galileo seeing the phases of Venus. Some observation that makes absolutely no sense in the current paradigm of things, and this will force people to fundamentally change how we view the universe.
1) Find an old astronomy textbook, and compare it with a very recent one. What mysteries in the old astronomy textbook are now believed to be resolved, and what facts and statements in the old astronomy textbook are now believed to be incorrect?
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