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20 Cards in this Set
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Be able to describe observations that indicate there is a large amount of matter that can be detected by its gravitation effects on star and galaxy rotation but that does not emit light.
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One of the ways is through galaxy rotation curves we already discussed when talking about how we map the varying masses of the galaxy. Another way is by looking at clusters of galaxies themselves and seeing how fast the galaxies are moving within the cluster. The faster they are moving in a cluster, the more mass there has to be or else the galaxies would be able to escape! We indeed see galaxies moving fast enough that there must be a lot more matter within the clusters than is visible and luminous. Also, if we look at the gravitational lensing that occurs between to nearby galaxy clusters, the gravitational lensing can indicate to us that there is more mass than is luminous.
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What are MACHOs?
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These are some of the objects used to possibly explain what dark matter is. The acronym stands for MAssive Compact Halo Objects. These are things like white dwarves, black holes, and neutron stars, but in a very compact form so they wouldn't be shining. We do know however that not all dark matter can be made of MACHOs because we don't see as many lensing events as we do when these objects pass behind stars and other objects.
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What are WIMPs?
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This stands for Weakly Interacting Massive Particle. A neutrino is one example of what a WIMP might be like (e.g. something with mass that is very difficult to catch with any kind of chemical.) It's hard to detect them, but they provide mass.
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How did Hubble's Law lead to the development of the Big Bang Theory?
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If you have a uniformally expanding universe, you would see a Hubble's Law-type situation, where the farthest away things are moving away from us the fastest. So if everything's expanding, what led to the expansion? The Big Bang theory answers this question.
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How did the detection of the 3 K background radiation and the observed helium abundance support theBig Bang scenario?
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The Big Bang theory predicted that, if the universe has been expanding and cooling all along, we should be able to look far enough back in time to be able to see the entire universe when it was acting like a hot, dense gas. The last time this would have happened was when the universe was just before it was cool enough for atoms to form, about 3,000 K, so we can look for a black body distribution of photons with a peak wavelength of 3,000 K. Except we are way way redshifted from the time in the universe when this would have been occuring that the peak wavelength appears to us to be about 3 K.
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What else (other than three degree background radiation) does the Big Bang theory predict?
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The Big Bang theory also explains the helium to hydrogen ratio that you see whether it's in population 1 stars or population 2 stars. The heavy metal abundance in these two are really different, but the helium to hydrogen ratio is really very similar. The Big Bang theory can explain that by going through what happens when you have this expanding and cooling universe. When it was really hot, protons and neutrons would have been present in roughly equal numbers. Neutrons are a little more massive than protons, so to go from a proton to a neutron you have to add energy (which was there in the form of high temperatures), and then neutrons will decay back to protons. At high enough temps, both these reactions will be happening back and forth rapidly enough that you have equal numbers of protons and neutrons. Then the universe continues to cool down and you can no longer make neutrons out of protons. Meanwhile, the neutrons that are there gradually decay away. As it cools down you have more and more protons and fewer and fewer neutrons until it cools down enough for any helium nuclei to form and remain stable. Then the neutrons that remain combine with protons to make deterium, and deterium plus deterium makes helium. So you can figure out the hydrogen to helium ratio by knowing how many neutrons there still were relative to hoe many protons there would be (e.g. all the initial protons plus the neutrons that have decayed into protons. Use up the few remaining neutrons combining them with protons to make helium, and you get a hydrogen to helium ratio). The calculations in the Big Bang theory predict that just enough neutrons would have decayed to form protons between the time that you could no longer make neutrons and the time that you make helium that just the right amount of neutrons would have decayed over that time to give the 75-25 hydrogen to helium ratio that we see.
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What is an closed universe? (talk about the mass energy they contain without accounting for dark matter, which would cause the expansion of the universe to speed up.)
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If the density of the universe is greater than the amount of matter that allows gravity to eventully stop the expansion (see flat universe) you have a closed universe and the expansion will reverse. (we die in a big crunch)
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What is a flat universe?
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In a flat universe you have just the right amount of matter density such that, if the Big Bang started the expansion of the universe, gravity will gradually slow that expansion down so that, at infinite time, the expansion would stop altogether.
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What is an open universe?
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If the density of the universe is less than the amount of matter that allows gravity to eventually stop the universe's expansion (see flat universe) the expansion would continue forever and never stop.
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How do the terms Open, Flat, and Closed universe come about?
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From the geometry involved. if the universe is flat, the geometry of the universe will be Euclidian (parallel lines will continue to be parallel to infinite distance, the angles of triangles will always add up to 180, etc). If there is more mass energy there (e.g. closed universe) the geometry will be different. An analogy is taking a 2 dimensional sheet of paper and spreading it over a globe. (Parallel lines will converge here and angles of triangles sum up to more than 180.) If the universe is open, the geometry will be the opposite. Parallel lines will diverge and angles will add up to less than 180. Even if there is dark energy speeding up the expansion of the universe, the geometry of these models still holds true.
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We use white dwarf supernovae as standard candles to measure the distance to far away galacies. What do these measuremenets tell us? How does it help us solve the provlem that the universe seems younger than the age of globular clusters?
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White Dwarf supernovae are the wonderful standard candles that give us the distances to far away galaxies. They have told us that the actual distances to these galaxies these supernovae are in are bigger than what we would measure assuming Hubble's Law is uniform all the time. If you assume that the universe is steadily expanding, you would predict how far away a galaxy of a given redshift ought to be. It turns out our standard candle supernovae are farther away than their red shifts indicate. Thus, the universe's expansion has to be speeding up. We are forced to conclude therefore (hands are waving wildly) that some weird force is making it accellerate. So we know that if the universe is prettty flat (which it is) and has been decellerating the amount that a flat universe should decellerate, the universe should only be about 2/3 (9 billion yrs) as old as our initial guess from Hubble's Law (13 billion). However, if it is accellerating we get the age of the universe right but now we have to deal with dark energy, the force making it accellerate.
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What are the general "epochs" in our universe's history?
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First it started out slowing down. Then inflation kicked in and it accellerated for a while. Then it slowed down over time and now its accellerating again because the dark energy is kicking in. They've kind of averaged out to give us the age of the universe that we would assume with just plain old Hubble's Law.
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What observations did the WMAP satallite and other projects like Boomerang balloon make to show us the universe is flat?
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These measure patchiness (light and dark variations) and blotchiness in the Cosmic Microwave Background. How big these patches look to us depends on what the geometry of the universe is in complicated ways that we do not need to know for the test. They can make predictions for how big the patches should be and how those patches' sizes are distorted as they pass through the different types of geometry found in different models of the universe (e.g. open, flat, closed). A closed universe predicts bigger patches and an open universe predicts littler patches. The particular size of the patches we observe is the size predicted for the flat universe. So basically these projects corroborate what we already thought to be the shape of our universe.
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What do the combination of Type 1A supernovae measurements and WMAP flat-universe measurements tell us that the universe is roughly made up of?
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When you combine up the accelleration of the universe and the fact that it's flat (two things we derived from supernovae and WMAP) to properly explain the way the supernovae are moving away from us and the patchiness, you need to have this very specific balance between dark energy, exotic dark matter, and normal matter:
Dark Energy: 73% Exotic Dark Matter: 23% Normal (made of protons and neutrons and electrons): Matter: 4% |
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How can we tell that some of the matter providing density in the universe is not normal matter?
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We observe the deterium that was left over in the helium formation. Most of the protons and neutrons made it all the way to helium, but there were some proton-neutron pairs (e.g. deterium) that did not make it all the way to helium and if you had a low density of protons and neutrons in the early universe it would be more difficult for the two deterium nuclei to find each other and make helium. If you had a high density of protons and neutrons (normal matter), it would have been easy for the deteriums to find each other and you would have less left over deterium. So from looking at how much deterium is left over, they can make an estimate of how much normal matter there is. This is because the exotic matter (matter than isn't protons and neutrons) would not have effected the helium formation. So they can say how much there had to be to make the helium.
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What are grand unified theories?
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These attempt to blend all the four forces of nature into one. They've already managed to blend the weak nuclear force with electro magnetic theory and the grand unified theories that lead to inflation are ones that merge the strong nuclear force into the electro weak force and then, we hope, to merge gravity in as well. So these are theories by which we are trying to link these four forces together.
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How do Grand Unified Theories relate to an early inflationary epoch that has been siugested for our universe? How would inflation explain the uniformity of the cosmic microwave background on opposite sides of the sky?
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The early episode of inflation in the universe is believe to have come about when the strong force seperated out from the electro weak force. Somehow, gravity's brief role as a repulsive force explains this epoch of very early inflation in the universe. The inflationary universe model is pretty important in explaining things like helium abundance that the big bang just sort of glossed over. One of these things is that, when you loook one direction in the sky and see the same temperature for the black body radiation as we do when we look in the other direction. How did each side correspont their temperatures? They must have been in contact! These are pretty far away from each other and would not have been able to exchange information over the lifetime of the universe. Except: if that initial inflationary epoch really did happen they would have been able to be in contact with each other before the inflation started.
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How would an early inflationary epoch explain a lack of observable monopoles in the universe?
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The grand unified theories predict magnetice monopoles to exist. We haven't found one. How come? Because the inflation spread them all so far apart that it's not surprising that we haven't found one.
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How would an early inflationary epoch explain the flatness of the universe?
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There's no reason the universe should have been flat vs. open vs. closed. However, whatever curvature it might have started out with, if it inflated that rapidly, that curvature would be driven to make it look flat. So an inflationary universe would be expected to look flat.
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Remember the ant on the balloon.
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how do grand unified theories help us explain why we have all this matter and very little antimatter out there?
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The photon collisions in the early universe would generally produce equal amounts of matter and antimatter, and those matter-antimatter particles would anhialate and you should be left with nothing but energy, but you're left with matter and very little antimatter. Some Grand Unified Theories might help with that.
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