7/16 Ellipticals: 1. Very little gas and dust an no star formation. 2. Composed of old stars. 3. Masses range from hundreds of thousands to 10's of trillions of solar masses. 4. Sizes range from 3000 ly to 600,000 ly. 5. Dwarf ellipticals are probably the most common galaxy in the universe. 6. Luminosities range from a million to 100 billion solar luminosities. Irregulars: 1. Contain gas and dust but not as dense as in spirals there is some star formation. 2. Masses range from a 100,000 to 100 million solar masses. 3. Sizes range from 3000 ly to 30,000 ly. 4. Luminosities from 10 million to a couple billion solar luminosities. 5. Probably dwarf ellipticals torn apart by large companions. Lives of Galaxies: Galaxies live in clusters some small such as our own local group containing about 3 dozen galaxies; some large with thousands of galaxies. Unlike stars that form in clusters, galaxies form first and then combine into clusters. Galaxies are large compared to the distances between them collide quite often. We see giant elliptical galaxies at the centers of all large clusters galaxies move to the center and merge. It is possible for a couple of elliptical galaxies to collide and become a spiral and for two spiral galaxies to collide and form an elliptical. Often, star formation is triggered by the collision of two galaxies. Active Galaxies: An active galaxy is one that emits a prodigious amount of energy from its nucleus. Driven by black hole. Three main types: Seyfert Galaxies Most often when taking pictures of a spiral galaxy, we over expose the nuclear bulge so that we can see the spiral arms. When doing so, a Seyfert galaxy looks just like a normal spiral galaxy. In low exposures, Seyfert galaxies have much brighter nuclei than
ordinary galaxies. About 2% of all spiral galaxies are Seyfert galaxies. Radio Galaxies generally elliptical galaxies that generate a lot of radio energy from their centers. Look at a radio galaxy from the side and see radio lobes. Matter spiraling into the black hole at higher latitudes gets spun out along the axis of ration and ejected from the galaxy it emits radio waves to form lobes. It s possible to be oriented so that we are looking down the lobe looks like a different type object. Quasars very far away and very bright may be young galaxies with much matter near their centers matter then is consumed by the black hole with a tremendous outflow of energy. Eventually, the matter in the center is consumed, and the quasar turns off. It then becomes an ordinary galaxy. All active galaxies are probably the same type of object at different stages of evolution and viewed from different perspectives. Chapter 12 Cosmology Cosmology is the study of the origin, evolution, and structure of the universe. We start with two assumptions: 1. Cosmological Principle: On a large enough scale (large compared to superclusters of galaxies), the universe is homogeneous (the same everywhere) and isotropic (the same in all directions). 2. The evolution and structure of the universe is governed by General Relativity (GR). Recall that GR tells us that matter and energy curve space. The universe contains matter and energy is it curved? There are three possible geometries: 1. Euclidian Geometry zero curvature geometry of a flat surface such as a tabletop. 2. Riemannian Geometry positive curvature geometry of the surface of a sphere. 3. Lobaschevskian Geometry negative curvature geometry of the surface of a saddle. A Euclidian universe is flat (and open). A Riemannian universe is closed. A Lobashevskian universe is open. There is a critical density 4 10 29 g/cm 3 works out to one H atom per cubic meter. If the density of the universe has the critical value, it is flat. If greater, it is closed. If less, open.
We define the density parameter is defined to be the ratio of the density of the universe to the critical density. If = 1, open and flat. If > 1, closed and positively curved. If < 1, open and negatively curved. How can we deduce if the universe is flat or curved either positively or negatively? One way is to count stars within an angle. If we see fewer stars than expected within a given angle, the universe is positively curved. If we see more, it is negatively curved. If we see the number expected, it is flat. The figure on the left is for a flat universe, the center, for a positively-curved universe, and the one on the right, for a negatively-curved universe. Such measurements and others indicate that the universe is flat! This means that should be 1. When we account for all the visible matter in the matter we can detect in the universe, we find = 0.05! Universe should not be flat. Yes but, we haven t included dark matter! We don t know what dark matter is, but we have some ideas. Two main ideas: MACHO and WIMP MACHO MAssive Compact Halo Object These would be objects such as neutron stars, black holes, white dwarfs, brown dwarfs, rogue planets floating around in the halos of galaxies. We can detect these objects using the gravitational lens effect when an object comes between us and a distant object, it will cause the distant object to brighten. We have found some MACHO s, but not enough to account for all the dark matter required to
explain the motions of galaxies. WIMP Weakly Interacting Massive Particle Two types of WIMP s hot dark matter and cold dark matter Hot Dark Matter Perfect Candidate neutrinos Two problems: 1. Even though there are more neutrinos in the universe than any other type of particle, their masses are too small to account for dark matter needed. 2. Computer models base on hot dark matter do not produce a universe like the one we see around us. Cold Dark Matter Computer models based on cold dark matter do produce a universe like the one we see around us. Problem no particle known to exist has the proper characteristics of cold dark matter. All known particles are detectable. There are hypothetical particles ones that might exist but ones we have yet to find that would fill the bill. 1. Axion The strong nuclear interaction conserves handedness or parity while the weak nuclear interaction does not. The axion is a hypothetical particle that, if it exists, would account for why one does and the other doesn t. 2. Magnetic Monopole We have never found an object with a north magnetic that doesn t also have a south magnetic pole. There is no theoretical reason why we can t have an isolated north or south magnetic pole. They would be very massive and it wouldn t take many to account for the dark matter needed. We have searched and are searching for them, but none have been found. 3. Lowest Mass Supersymmetric Partner What we want is symmetry when we exchange particles with one-half integer spin (call fermions) and particles with integer spin (bosons). Hypothesize that an electron (fermion) has as boson partner called selectron. The photon (boson) has a fermion partner called a photino. These new particles will interact only gravitationally with the rest of the universe. Supersymmetric partners would be much more massive. The lightest of these would be stable and account for dark matter. But, when we add dark matter into its value rises only to 0.25 still too small. New version of the flatness problem. Another problem isotropic nature of the universe.
When we measure the temperature of space (cosmic microwave background radiation) in one direction, we get the same value as in the opposite direction. Same temperature in two regions of space that could not possibly have interacted. Need to interact to be in equilibrium. Called the Horizon Problem. Inflation solves the problem to make sense of this, let s go over the history of the universe. 1. 13.7 billion years ago a cataclysmic event occurred called the Big Bang creation event of the universe when space, time, matter, and energy were created. After this event, the universe began to expand, creating more space as it did so. 2. During the first 10 43 s (the Planck time) we don t know what happened. Current physics is not sufficient to determine what happened. 3. During this interval, the universe was controlled by a single force that we might call the superforce. At the end of the interval, gravity separated from the superforce leaving behind the grand unified force thus begins the GUTs era. (Grand Unification Theory). 4. Until 10 35 s, the universe remains in the GUTs era. At the end of this period, the strong nuclear interaction separates from the GUT force leaving behind the electroweak force. 5. What happens here is a tremendous amount of energy given up as occurs when water changes to ice The universe undergoes a phase transition. 6. This outflow of energy causes the universe to rapidly increase its rate of expansion until time about 10 39 s. During this time the universe increases in size by 43 orders of magnitude by a factor of 10 43 from smaller than an atomic nucleus to larger than the solar system. 7. This is inflation. 8. The universe slows its expansion to the previous rate at the end of inflation. Solves the horizon problem since before inflation all points in the universe were close enough together to interact came to equilibrium before inflation began and maintained after inflation even though they could no longer interact. Let s us understand why the universe is flat. The universe in getting larger gets flatter just as expanding a basket ball to the size of the Earth makes it look flat. 9. At this point the universe is composed of electrons, quarks, neutrinos, photons and dark matter whatever that might be. 10. As the universe cools, it gets to the point where quarks can begin forming neutrons and protons and antiquarks can begin forming antineutrons and antiprotons. Somehow there was some asymmetry between the production of matter and antimatter for every billion antimatter particles created, a billion and one matter particles were created. The billion antimatter particles annihilated a billion matter particles creating photons and leaving behind that single particle of matter those matter particles form the matter in the universe today.