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Dark Matter – Dark Energy! Galactic Scale
The chapters to come are going to be very different from the preceding ones. Here we will deal not with science that is mostly known but with science that is mostly unknown. Bear with us for a moment while we pile up a little imagery. Imagine a field of knowledge as being like an apartment building that is continually under construction. At the top of the building there is a swarm of workers constructing new stories, while in the completed parts of the building, the lower floors, people have already moved in and are living according to the ways the building has been built to work. They enjoy the amenities of the building, and they like or at least tolerate the style of decoration and the built-in appliances. Occasionally a worker comes down and remodels or fixes something in a previously built part of the structure, ripping out a wall, replacing an oven, repainting, and so on. Very rarely the building needs to be reworked on some fundamental level, rewiring, new plumbing, etcetera. When that happens, the crew drops back down for serious rehabbing and the apartment dwellers complain to the management, which shrugs and says, “That old Newtonian wiring couldn’t handle the faster communication and heavier traffi c. Einstein and Co. are putting in new cables from the ground up. Don’t worry: you won’t notice the difference until you need it.” In the buildings of science, most people live in the lower stories, using the results of science (technology and methods of problems solving) but not being at all concerned with how the building is being built on the top levels. People who need more of the science for the things they do in their lives (such as engineers) occupy higher constructed fl oors, but they do not need to know what is going on in the act of construction, nor must they worry about the infrastructure of the building itself, strange and interesting though that is. Anyone who has ever dealt with a building contractor will know that there’s a lot of weird stuff hidden in the walls and floors of one’s home. On occasion news trickles down from the top that some new fl oor is open for occupancy, and sometimes the noise of construction reaches the lower fl oors when an argument breaks out among the workers. This separation between those who live in the building (the users of the fi eld) and those who are building the building (the creators/discoverers of the fi eld) is one of the things we have been trying to overcome in this book. We want to break down the barrier of mystery between living in the works of science and building the science itself. We do not expect or want everyone to become a worker in the fi elds of science, but we think things will be better if everyone understands more about what those weird noises are and why big dark holes have been cut in their walls. All of which is by way of prefacing the point of this step: Welcome to the construction zone. Hard hats are required. In the following chapters, we will look at elements of cosmology that at the time of this writing are the subject of ongoing research and discussion. Since we are dealing with the presently unknown, it may turn out that some of the ideas that we put in these chapters are wrong. This is not the case for most of the material of the previous chapters. We can be confi dent of what we wrote about the Sun, most of what we wrote about stellar evolution, and even much of what we wrote about black holes. In these chapters, much of what we say about cosmology, and about observations of dark matter and dark energy, is well established. But when we come to the nature of dark matter and darkenergy, we enter the realm of the unknown, and confidence is not an asset in that realm. Exploring the really unknown is where the methods of science fully show their value. When some new phenomenon is at fi rst detected, the question “What is that thing?” begs to be answered. Theories will be created all over the place. Anybody can come up with something in the theoretical universe. Indeed, when a new thing is detected, it can be guaranteed that someone will posit the theory “It’s nothing” and someone else will say some variation of “It is proof of my pet ideas.” Theory alone, as we have said, is not enough. There needs to be detection to test the theory. Theory acts as a guide to creating the apparatus of detection, because if a thing is whatever is theorized, then there should be consequences of that and hopefully some of those consequences will be detectable. We are going to lay out the paths presently being followed to create new science out of ignorance. The ignorance came about because something was found that could not at the time be well explained. Since then effort has been made to mine understanding from ignorance. We hope by showing these scientific works in progress to further illuminate the process by which the work of science is done.

To be fair, we could do this with any topic presently under new study. The ones we’ve picked, like the topics in the earlier chapters, have a certain dramatic quality: balls of fusing gas, galaxy -eating black holes, things like that. Dark matter and darkenergy may have that level of drama—we don’t know yet—but we do know that in trying to understand them, we have to reach to the beginning and the ends of the universe, so the search has its own dramatic quality. In short, in pursuit of these topics, we hope to show some of the most interesting questions presently being asked about the composition of our universe. We will reach back to the beginning of time, hint about its end, conjoin the largest objects in existence with the smallest, and show how one blunder might hide two different truths. Enough showmanship. On to the science. Source: [http://www.rugo.co/dark-matter-dark-energy-galactic-scale/ Dark Matter – Dark Energy! Galactic Scale]

Gravitational Waves
Despite its tantalizing analysis of the characteristics of black holes, the Hawking effect is for now firmly theoretical, so we must ask again what we can, at present, detect about black holes. The observations of the effects of accretion disks around black holes seem to tell us little beyond the fact that a black hole is a compact object with a strong gravitational fi eld. Is there anything else about black holes that we can detect and that will yield any more information about them than the simple fact of their strong gravity? There is something, and it has to do with a phenomenon known as gravitational radiation. Gravitational radiation is analogous to electromagnetic radiation, something we are more familiar with. A radio station makes radio waves by driving an electric current rapidly up and down in its antenna. This makes electromagnetic waves that propagate outward from their source. When a radio wave comes to the antenna of a radio receiver, it makes electric currents move up and down in that antenna, and these currents are detected by the receiver. This, by the way, is a case where a very abstract aspect of the theoretical universe (Maxwell’s equations of electromagnetism) leads to a relatively simple way to make objects: radio transmitters and receivers, which in turn lead to radical changes in human life (the entire interconnected web of communications we rely upon today). Just as electric charges moving back and forth create electromagnetic radiation, so masses moving back and forth create a phenomenon called gravitational radiation. And just as electromagnetic radiation can be modeled as light waves, so gravitational radiation can be modeled as a kind of wave called a gravitational wave. These gravitational waves propagate outward from their source, and when they encounter other masses, these other masses are moved by the gravitational wave (this is exactly analogous to the radio transmitter and receiver). The main difference between these two kinds of radiation is that gravity is much weaker than electromagnetism, and this makes gravitational radiation much more diffi cult to detect than electromagnetic radiation. That gravity is much weaker than electromagnetism is perhaps best illustrated by the example of a magnet lifting an iron nail. Here the magnet, a little object easily held in the hand, is pulling upward on the nail with its magnetic force, while at the same time the entire Earth is pulling down on the nail with its gravitational force, and yet the tiny magnet wins! Though gravitational radiation is weak, it is not completely theoretical. It has been detected indirectly in a system called the binary pulsar. This system, first detected by Russell Hulse and Joseph Taylor in 1974, is a binary star system where both stars are neutron stars and one is a pulsar. For our purposes, an important property of this system is that the regularity of the pulsar’s pulses make it an extraordinarily accurate clock, as good as (or perhaps better than) our best atomic clocks. But remember that the Doppler effect changes our observations of the pulse period depending on the speed of the pulsar. We can thus use the underlying regularity of the pulse as a means of measuring the speed. Because of the high accuracy of measurements of the pulse rate, we get very accurate measurements of the speed of the pulsar over time, and from that very accurate measurements of the orbits of the neutron stars, the masses of the neutron stars, and the orbital period (the time it takes the neutron stars to go in orbit around each other). Notice that just one very accuI rate measurement of a single phenomenon can be put through a number of calculations to produce accurate measurements of other related phenomena. The binary pulsar is a very good system, since it tells us how to detect and calculate its own characteristics. If only more things were like that. Of course, as with so many of these interesting star systems, you wouldn’t want to live there. And you’re better off touring them from a distance through telescopes. Since the neutron stars are moving masses, general relativity predicts that the binary pulsar emits gravitational radiation. Furthermore, given the numbers for the neutron star masses and the details of the orbits, general relativity makes a precise prediction for how much energy the binary pulsar will lose in a given period of time, radiated away in the gravitational waves. This loss of energy should affect the orbits of the neutron stars. At fi rst one might expect that a loss of energy would lead to the stars slowing down and therefore to the orbital period becoming longer. However, for a gravitationally bound system, a loss of energy means that the system is more tightly bound; in other words, that the neutron stars are closer together. Just as in the solar system, a smaller orbital size means a smaller orbital period (planets closer to the Sun orbit faster: they have shorter years than ones farther out). So, as the neutron stars lose energy, they speed up. For our purposes, what is important is that general relativity makes a precise prediction for the (very small) change in the orbital period due to loss of energy in gravitational radiation, and that a pulsar is such an accurate clock that this small change can be (and has been) measured. The measured orbital period change agrees with the prediction, thus yielding an indirect detection of gravitational radiation. In other words, we can tell there is gravitational radiation because the system acts as if there is. If there were some other cause for the energy loss, then that other cause would have to produce exactly the same effects as the predicted gravitational radiation. That is certainly possible, but most unlikely. Source: Gravitational Waves