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Questions Review for the AST309 Final Formulas: Flux (i.e., brightness) is equal to (Luminosity / 4d2) where d is distance from you. You measure the flux, luminosity is a standard candle, so you can solve for the distance to determine how far away an object is. Hubble's Law: The more distant a galaxy is, the faster it is moving away from us. The formula is V = H0D where V is velocity in km/sec, H0 is Hubble's Constant (approximately 71), and D is the distance in Mpc. 1. What are the two main reasons why gamma-ray bursts are so hard to study? Because GRBs are so short in duration, it was necessary to build faster-responding telescopes in order to determine their redshifts, and thus their distances. This has now been somewhat accomplished, and improvements are being made all the time. We now believe that they are not isotropic (although they are isotropic in distribution, they are not isotropic in their outputs), this means that we must be in a special orientation in order to see them. This in turn means that (a) we cannot be certain what their actual flux is, and (b) that we are only seeing about 1% of the GRBs that happen. Another problem is that because their energy output is so high, what we perceive of them is subject to Relativistic Beaming, or Doppler Boosting. Since the jet is coming almost straight towards our eyes and since it is traveling at relativistic speeds, the flux that we detect is exaggerated relative to what it actually is. This is a result of Einstein's theory of special relativity, since when objects travel very fast, they appear to increase their mass and therefore their energy. The relativistic effects cause GRBs to be perceived as about a factor of 100 more energetic than they really are. This also complicates the difficulty in determining their intrinsic brightness. 2. Why do we think that gamma-ray bursts are associated with star formation? Now that we have redshift information about many sources, we can look at their distribution in time (we already know that they are spatially isotropic). It appears that GRBs happened more often in the past. The other things that we know are more prevalent in the past are quasars and newly-formed galaxies. These imply that star-formation was more common, which possibly suggests that there might be a connection between starformation and these processes. However, we need more direct evidence rather than a temporal connection. We do this by looking at what type of galaxies these GRBs occur in and where in the galaxy they lie. It turns out that there are two additional bits that suggest GRBs are associated with star formation. The galaxy type is generally in young, active galaxies, such as disk galaxies that contain lots of gas and dust. The other bit of evidence is that the GRBs are NOT preferentially located in the galaxy centers, but are instead distributed throughout the galaxy with a disk-like distribution. This strongly suggests two things: that GRBs are probably not associated with supermassive black holes, and that they trace the young stellar population since it is that component that forms a disk structure. So the observational evidence, while sparse, does imply a connection to star formation. 3. Gamma-ray bursts release about the same energy as a supernova yet they are about 1000 times brighter. What is the reason for this difference? Relativistic Beaming, or Doppler Boosting. Since the jet is coming almost straight towards our eyes and since it is traveling at relativistic speeds, the flux that we detect is exaggerated relative to what it actually is. This is a result of Einstein's theory of special relativity, since when objects travel very fast, they appear to increase their mass and therefore their energy. The relativistic effects cause GRBs to be perceived as about a factor of 100 more energetic than they really are. This also complicates the difficulty in determining their intrinsic brightness. 4. Why do we think that most of the dark matter is non-baryonic? Because there doesn't seem to be enough baryonic matter to make up the necessary difference. First of all, we would need to see some 15x more matter than we do. Second, we should see that matter behave more like the rest of the universe, which we don't. (See article on Bullet Cluster. See also answer to Question 12.) 5. Explain the theory behind the existence of virtual particles and why they lead to black hole evaporation. The idea is that, as explained by Heisenberg's Uncertainty Principle, the universe is subject to random fluctuations that cause pairs of virtual particles to briefly come into existence. Most of these pairs find their mates and annihilate each other. However, Hawking suggested that energy fluctuations bring about the creation of matter-antimatter pairs of particles near the event horizon of a black hole. When one of the particles falls into the black hole, the other particle escapes; since the one that escapes has positive energy, the one falling in must have negative energy. This in turn implies that the black hole will lose energy, and therefore mass (by E=mc2) each time this happens. Over time, the black hole is diminished by this net loss of mass, and, in theory at least, slowly wastes away. However, while the effect might be detectable at the microscopic level, the rate of deterioration is so slow that for a Solar-mass black hole it would be more than offset by the much greater accretion of mass from macroscopic material such as stars and dust falling into and adding to the black hole. 6. Explain how a gravity wave is generated. These are ripples in spacetime due to massive structures that rotate (i.e., binary systems of neutron stars would do it). The distortion of local space-time takes energy away from the bodies generating it, so they begin to fall towards each other. Gravity waves are a prediction from Einstein equations. Taylor and Hulse won the Nobel Prize in 1993 for their discovery of a pulsar in a binary system. They showed that the orbit got smaller and smaller - and it fit perfectly with gravity waves. Neutron Star-Neutron Star or Neutron Star-Black Hole collisions are bound to happen since we know that both of these exist in the Universe. The most likely way to have these collide is to have them initially in a binary orbit that then "hardens" (they get closer together) until they eventually collide. The gravitational energy released will produce gamma-rays. This is one possible explanation for short duration GRBs. 7. Describe the theoretical reason for why we require dark matter to make up most of the mass in the Universe. 1. From knowing the theoretical density curves of various particles, and comparing them to what we actually find as remnants of those generated during Big Bang Nucleosynthesis, we can determine that the total density of baryons in the universe is about .02 of Critical Density. 2. From looking at the pattern of the Cosmic Microwave Background, as well as from the philosophical arguments, we have compelling reasons to believe that the universe does in fact match the "flat" model. This means the universe does match a Critical Density of 1. 3. By examining the supernova redshift data, we believe that the cosmological constant i.e., Dark Energy is about 0.7 of Critical Density. 4. Putting these together, we come up with: Cosmological Constant = Dark Energy: 0.70 Baryonic Density = Normal Matter: 0.02 Non-Baryonic Density = Dark Matter: 0.28 ----------------------------------------------------------Universe's Density = Critical Density = 1.00 8. Explain the two main predictions of the Big Bang, and describe whether the observations hold up to these predictions. There are two main predictions from it that have held true. These are the cosmic microwave background and the elemental abundance in the early Universe (or Big Bang Nucleosynthesis). Since the Big Bang starts with an explosion that happens throughout the Universe, then there must be residual light (or heat) from that explosion. And that light must be everywhere. This is the cosmic microwave background, which was verified in the 1960s. Second, since the Universe started out with such extreme temperatures, we can use our knowledge of physics at high energies to model it. This will then tell us the exact elemental abundances in the early Universe. These numbers have been well tested now and the predictions agree amazingly well with the observations. This is the Big Bang Nucleosynthesis modeling that has been used to show that baryonic matter makes up only some 2% of the universe. 9. What are some of the reasons why inflation was invented? Inflation is an important aspect of the Big Bang since it many explains problems: the structure problem, the smoothness problem, and the fact that we are at the critical density. (For more on these, see Questions 12, 16, 17, 22.) 10. Why is the SWIFT satellite so important? It has enabled us to determine redshifts for GRBs. This in turn enables us to learn more about them. 11. Name at least one theoretical way in which inflation is not required. Not discussed? Unless he's talking about the fact that no analogous inflation seems to have occurred when the other forces split off from the GUT. 12. What is the evidence for or against the detection of dark matter in our Solar System? In our Solar System, there is no need to invoke DM: all of the mass is accounted for using Newtonian dynamics. However, we have plenty of evidence that seems to indicate a need to invoke DM on larger scales: rotation curves of spiral galaxies, average speed (Sigma) of elliptical galaxies, mass estimates from observation of hot X-Ray gas in galactic clusters, gravitational lensing, and computer modeling of the universe. If DM consists of MACHOs, we should have seen stronger evidence of it by now, so it is now thought unlikely to be explained by them. We know macro-scale dark bodies (Jovian planets, brown dwarfs, white dwarfs, black holes, etc.) do exist, but they only make up a fraction of the missing matter. For instance, if DM was made up of Jovians, there would have to be a lot of them, and the planet-forming process would have to be much more efficient than we observe it to be. Similarly, if DM was made up of black holes, then they would have a significant effect on the galactic disks, making them much "puffier" than we see. Some have suggested that DM may simply be Neutrinos. For neutrinos, all experiments rely on trying to detect reactions from the neutrinos as they travel through a large body (the ocean in some cases). When the neutrino strikes something head-on, it creates a particle shower that is recorded by various detectors laid throughout the body. Each of the neutrino experiments so far can only detect a mass difference between two types of neutrinos, and not their absolute mass. They have found that the mass difference is very small. On probabilistic grounds, if the mass difference is small, then the actual mass is likely to be of that order. And at this mass, the neutrinos may contribute to some of the dark matter, but probably not all of it. Thus, it is possible that the answer to the dark matter is a combination of MACHOs and WIMPs, with each maybe contributing nearly equal amounts. It is never very attractive to have multiple answers to a problem, but that is what the data suggest at the moment. If DM consists of new types of particles, we may see them if/when the LHC becomes operational. Many scientists that we will discover these particles, and most of them are bet- ting on the Higgs Boson, aka Neutralino, Higgsino, and other names. (These all refer to the same particle.) If DM is none of the above, then we'll have to go back to the drawing board. 13. How were the baryons in the Universe made and why are there no anti-particles in our Universe? During the Particle Era, the Universe was just the right temperature for particles to be created and destroyed continuously. What happens is the photons have the right energy to come together and annihilate each other to form matter and antimatter. Then the matter and antimatter smash together and form gamma-rays. This continues until the Universe cools enough such that the particles do not have enough energy anymore to annihilate each other, and we are then stuck with whatever was left at that point. At the end of the particle era, there were slightly more protons than antiprotons. For every billion antiprotons there were a billion and one protons. Another way to look at it is one billion protons were annihilated with one billion antiprotons to make a billion photons, which would result in leaving one proton in the end. It is this slight excess that makes up most of the matter in the Universe. 14. Why does a GRB only last for a very short time? Smartass answer: Because by definition, a "burst" only lasts a very short time. Serious answer: Because they are so energetic, and because they are probably not very large in size. More here? 15. Why is the cosmic microwave background the furthest that we can see back in time using light? Because before that time the photons were absorbed almost immediately, so the universe was completely dark. 16. Explain how we use the power spectrum of the microwave background to tell us about structure in the Universe. By looking at the fluctuations of the CMB, we can see that there were fluctuations in the density of the universe at that time. Those fluctuations made possible the structure we see today (galaxies, etc.), and by looking at just how "clumpy" it was, we can make a guess as to which model of the universe best fits: open, flat, or closed (or flat + expanding, which is what we believe to be the case). 17. When we look at a picture of the microwave background, we normally see it as a bumpy, granulated image. If the universe is supposed to be smooth on large scales, then what is causing the bumpiness? Fluctuations in the time leading up to the generation of the CMB, i.e., in the coming into existence of matter itself in the periods before the CMB. Because these fluctuations are random in location, time, and energy, matter itself was being created in random "clumps." This clumpiness, which we see evidence of later on in the CMB, makes matter possible. If it were all perfectly even in distribution then every particle would have been cancelled out by its counterpart and we wouldn't be here nothing would. 18. What is one explanation for why inflation may have happened? The universe going through a `phase state change' as it cooled, like water turning to ice and expanding. 19. Why did Einstein invent the cosmological constant? Based on his GR equations, he discovered that the Universe could not be standing still since the mutual gravitational attraction would cause it to collapse in on itself. However, he firmly believed in a static Universe (unchanging). Thus, he proposed a repulsive force that counteracted gravity. He simply inserted a fudge factor into his equations, and this was called the cosmological constant. Einstein proposed this in 1915. In 1924, Hubble discovered that Andromeda was a galaxy significantly far away from us (this was a fundamental breakthrough), and subsequently measured HUBBLE'S LAW. This results instantly showed that the Universe was expanding. This proved to Einstein that we don't live in a static Universe and that there was no need at all to include his fudge factor. Einstein calls this his biggest blunder. Evidence shows that the universe is expanding 3x faster than can be explained by baryonic matter alone. This points up the need to invoke DM and DE. 20. The two main aspects that determine our universe are inflation and dark energy. What are the similarities between these two? Neither one is anything more than a conceptual explanation at this time there is no direct evidence to prove them. 21. How does the Heisenberg Uncertainty Principle lead to structure in the universe? First, in the universe popping into existence at all, the result of a huge energy fluctuation. Next, in the Planck Era, "random energy fluctuations were so large that we cannot explain the physics at these high energies. Energy and mass are equivalent and so energy fluctuations cause huge changes in space and time. These fluctuations arise naturally out of the Heisenberg Uncertainty Principle." Because these fluctuations are random in location, time, and energy, matter itself was being created in random "clumps." This clumpiness, which we see evidence of later on in the CMB, makes matter possible. If it were all perfectly even in distribution then every particle would have been cancelled out by its counterpart and we wouldn't be here nothing would. (At least until the next BB.) 22. Our measurements show today we are close to the critical density. Explain the theoretical reasons why this means that our universe should be exactly at the critical density. There are strong theoretical arguments for having the density equal to the critical density, but they are mainly philosophical. For example, during inflation the Universe expanded 1025 times in a matter of 10-33 seconds. Since measurements of the density today suggest something close to critical, then the only way for that to happen is to have the Universe be exactly critical or inflation would have resulted in a universe much further away from CD than we find it.

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