EXAM_4.docx

20.1-20.3 100 billion galaxies are estimated in our observable universe Found by counting galaxies in a small piece of the sky, and multiplying by the number of such pieces it would take to fill the entire sky. Based on the number of galaxies visible in the Hubble Deep Field Three major categories of galaxies Spiral- prominent disks and spiral arms Tend to collect in groups, contain up to several dozen galaxies Elliptical- rounder and redder than spiral and contain less cool gas and dust More common in clusters that contain hundreds to thousands of galaxies bound together by gravity Irregular- neither disk-like nor rounded in appearance Lenticular Galaxies- galaxies with disks but no evident spiral arms The Milky Way & Andromeda galaxies- two largest galaxies in our local group. Standard Candle- a light source of known luminosity Cepheid Variable- a type of very luminous star that makes an excellent standard candle Pulsating variable stars for which we can infer the luminosity from the time between peaks of brightness, which makes them valuable as standard candles. Main-Sequence Fitting- method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram By comparing the apparent brightness of stars along the cluster's main sequence to the known luminosities of standard main sequence stars, we can calculate the distance (from the inverse square law for light) The period between a Cepheid variables peaks of brightness and luminosity are directly related. Hubble?s Law tells us the more distant a galaxy, the faster it?s moving away from us. From this we can infer we are living in an expanding universe White dwarf supernovae are great standard candles but they are rare events, so we have observed them in only a tiny fraction of all galaxies so we cant use them to measure distances to all galaxies. Stellar parallax is the most accurate technique of measuring the distance to a nearby star Hubble?s Law ONLY applies to distant galaxies When we use an analogy that represents the expanding universe with the surface of an expanding balloon, the surface represents our universe. The inside of the balloon does not represent anything. If a galaxy has a lookback time of 1 billion years it means that it?s light traveled through space 1 billion years to reach us. Lookback time is more meaningful than distance, because the galaxy's current distance is different from what it was at the time it emitted the light we now see (because the universe is expanding). Cosmological redshift is the result of the expansion of the universe It differs from what we usually think of as a Doppler shift because the galaxies are not really moving through the universe; rather, they only appear to be moving away from us because space itself is growing between the galaxies. Cosmological Horizon- the "boundary" of our observable universe is called It is a horizon in time, not in space. That is, there is no physical boundary at the cosmological horizon, but we cannot see beyond it because we'd be trying to look to a time before the universe was born The universe is estimated to be about 14 billion years old. You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only ¼ as bright as Supernova B. What can you say about their relative distances? Supernova A is twice as far away as Supernova B, Because light follows an inverse square law with distance, the dimmer one must be twice as far away, because 22 = 4. The fact that the universe is expanding means that space itself is growing within the observable universe Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy B. According to Hubble's law, what can you say about their approximate relative distances? Galaxy B is twice as far as galaxy A because galaxy b has a larger redshift The galaxies farthest away from us are in the earliest (youngest) stages of their life. Galaxies Spiral galaxies have younger stars than elliptical galaxies. Among the large galaxies in the universe outside of clusters, most are spiral Large elliptical galaxies are more common in clusters of galaxies than they are outside of clusters. The most basic difference between elliptical and spiral galaxies is that elliptical galaxies lack anything resembling the disk of a spiral galaxy Hubble's galaxy classification diagram relates galaxies according to their shapes, but not according to any evolutionary status the technique of main-sequence fitting to determine the distance to a star cluster requires that we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster If we discovered we were wrong about the distance from Earth to the Sun, and it is actually 10% greater than we'd thought. How would that affect our estimate of the distance to the Andromeda Galaxy? The distance to the Andromeda Galaxy is also 10% greater than we thought The Earth-Sun distance (1 AU) is the baseline for parallax, so all the distances we've measured by parallax would be 10% greater than we'd thought. Since parallax is used to help us calibrate all other standard candle techniques, all our other measurements also would have to increase by 10%. Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation? We can learn the distance to the galaxy Using the Cepheid's period to determine its luminosity from the period-luminosity relation. Then we can calculate its distance by comparing its luminosity and its apparent brightness in our sky, using the inverse square law for light. This tells us the Cepheid's approximate distance, which is also approximate distance to its host galaxy. In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. He was able to prove this by observing individual Cepheid variable stars in Andromeda and applying the period--luminosity relation. Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? 2000 km/s, Multiply 20 km/s/(million light-years) ( 100 million light-years = 2,000 km/s. Hubble?s law does not work well for galaxies in the Local Group because galaxies in the Local Group are gravitationally bound together. White dwarf supernovae are more useful than massive star supernovae for measuring cosmic distances because White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities. Cepheid?s follow a period-luminosity relation, supernovae don't. Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually, we would use a white dwarf supernova as a standard candle The best way to determine a galaxies redshit is to take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory. Even if the redshift is cosmological, we still measure it just like a Doppler shift. The faster the rate of expansion, the younger the age of the universe. A cosmological redshift stretches lights wavelength. The lookback time of the cosmological horizon is the age of the universe Whatever the age of the universe turns out to be, this is precisely how long ago light from the cosmological horizon left on its journey to our eyes. We can?t see past the cosmological horizon because beyond the cosmological horizon, we would be looking back to a time before the universe was born. Hubble's constant is about 22 km/s/million light-years, implying an age of about 14 billion years for the universe. If Hubble's constant were 11 km/s/million light-years, the age of the universe would be about 28 billion years Changing Hubble's constant should change the estimated age of the universe. Given that the universe is about 14 billion years old, the oldest galaxies we see at great distances are younger than the oldest galaxies we see nearby. 21.1-21.3 Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in infrared light. Some regions in the universe start out denser than others is an important starting assumption in models of galaxy formation. Irregular galaxies were much more common when the universe was 2 billion years old than they are today. Collisions between galaxies typically unfold over a period of hundreds of millions of years. Collisions between galaxies more likely than collisions between stars within a galaxy because relative to their sizes, galaxies are closer together than stars. On a scale where the Sun is the size of a grapefruit, the nearest stars are thousands of kilometers away. In contrast, on a scale where the Milky Way is the size of a grapefruit, the other galaxies in the Local Group all lie within just a few meters. Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies Features of central dominant galaxies found in clusters of galaxies thought to form by the merger of several smaller galaxies often have multiple galactic nuclei near their centers The distinguishing feature of a starburst galaxy is a rate of star formation that may be 100 or more times greater than that in the Milky Way Active Galactic Nuclei- The unusually bright centers found in some galaxies Starbursts- unusually high rates of star formation Quasar- an active galactic nucleus that is particularly bright lie in the centers of distant galaxies, indicating that they are very luminous active galactic nuclei Globular clusters orbit in the halo, far from the supermassive black hole in the center The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is roughly 1 billion solar masses According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of light emitted by hot gas in an accretion disk that swirls around the black hole As matter falls toward the black hole, as much as 40% of its mass-energy can be converted into thermal energy and radiation. According to the theory that active galactic nuclei are powered by supermassive black holes, the energy released as light comes from gravitational potential energy released by matter that is falling toward the black hole Intergalactic hydrogen clouds are easiest to study by looking at absorption lines in quasar spectra The clouds are made mostly of hydrogen and hence have hydrogen absorption lines when a quasar illuminates them from behind. Hubble Space Telescope observations have shown that when the mass of the central black hole is very large, then the mass of the bulge of the host galaxy is also very large The best evidence for the existence of supermassive black holes is very high orbital velocities in a very compact region We can study how galaxies evolve because the farther away we look, the further back in time we see. Galaxy Formation Some regions in the universe were slightly denser than others. The universe started out filled almost uniformly with hydrogen and helium. The universe is expanding. One possible explanation for a galaxy's type invokes the angular momentum of the protogalactic cloud from which it formed. Suppose a galaxy forms from a protogalactic cloud with a lot of angular momentum. Assuming its type has not changed due to other interactions, we'd expect this galaxy to be a spiral galaxy. Two ways in which the starting conditions in a protogalactic cloud might cause it to become an elliptical (rather than spiral) galaxy are if the cloud begins with either relatively little angular momentum or relatively high density Results of collisions or other interactions between galaxies Elliptical galaxies are more common in clusters of galaxies than outside clusters. Starbursts The presence of very large, central dominant galaxies in clusters of galaxies. If the Andromeda Galaxy collided with the Milky Way NOTHING would happen to Earth. Reasons that interactions can shape galaxies The presence of features such as "tails" extending out of galaxies, bridges between galaxies, and rings of stars around galaxies. Galaxies with distorted appearances are more common at great distances than nearby. Computer modeling of collisions between galaxies. Starburst Galaxy Characteristics The observed features that cause us to classify it as a "starburst" must be only temporary phenomena in the galaxy's history. Supernovae occur so frequently that their effects combine to drive a galactic wind that blows material into intergalactic space. Its rate of star formation is many times higher than the rate of star formation in the Milky Way. Galaxy collisions have been more common in the past than they are today because they were closer together in the past because the universe was smaller. As the universe expands, the average distance between galaxies increases, making collisions less likely (on average) as time passes. A quasar's spectrum is hugely redshifted, this tells us the distance to the quasar. Images and spectra show quasars to be embedded at the centers of distant galaxies. Most active galactic nuclei are found at large distances from us, with relatively few nearby. This implies that active galactic nuclei exist tend to become less active as they age. We observe a source of X rays that varies substantially in brightness over a period of a few days, we can conclude the X-ray source is no more than a few light-days in diameter. The period of variability sets a limit on the size of a source. Evidence that active galactic nuclei are powered by accretion disks around massive black holes The total luminosity of an active galactic nucleus can be as high as about 10 billion times that of the Sun. X-ray emission from active galactic nuclei can vary significantly in times as short as a few days. Spectra of active galactic nuclei show that clouds of gas are orbiting a central object at very high speed. Central black holes can be very efficient for converting the mass-energy of infalling matter to thermal energy in the accretion disk. Roughly what percentage of the mass-energy can be converted to other forms of energy as matter falls into a black hole? 10-40% The observed relationship between the masses of central black holes and the bulge masses of galaxies implies that galaxy formation and supermassive black hole formation must be related somehow. Quasar spectra often show many absorption lines that all appear to be due to the same electron transition (such as level 1 to level 2 in hydrogen) but that fall at different wavelengths in the spectrum. We are seeing absorption lines from clouds of gas that lie between the quasar, and us and therefore each cloud has a different redshift. 22.1-22.4 Dark Matter- matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light Emits no radiation that we have been able to detect These gravitational effects are easily observed, so either dark matter really exists or there is something wrong with our understanding of how gravity works on large scales. Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified. Dark Energy- It is a name given to whatever is causing the expansion of the universe to accelerate with time. Observations indicate that the expansion is accelerating, which means something must be causing this acceleration. We don't know what it is, but we call it dark energy. Dark energy probably exists, but we have little (if any) idea what it is. Dark energy accelerates the expansion Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be true, Dark energy does not exist and there is much more dark matter than we are aware of to date. The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg, this refers to the idea that dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way. Observations indicate that dark matter may represent more than 90% of the Milky Way Galaxy's overall mass, and that it extends far beyond the galaxy's visible disk and the visible stars of the halo. Rotation Curve- a graph showing how orbital velocity depends on distance from the center for a spiral galaxy We plot orbital velocity on the vertical axis and distance from the galactic center on the horizontal axis We determine the mass distribution of a spiral galaxy by constructing its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center. If spiral galaxies did not contain dark matter, their rotation curves would change by the orbital speeds falling off sharply with increasing distance from the galactic center. The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter The rotation curve tells us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center. When we say the rotation curve for a spiral galaxy is "flat", it means gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. We know less about dark matter in elliptical galaxies than in spiral galaxies. It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy because Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies. In spirals, we can measure the mass to distances well beyond the visible stars by detecting the 21 cm radiation from atomic hydrogen gas. Ellipticals have very little atomic hydrogen gas, so we can measure their masses only to the distances where we can see stars. We detect gas with X-ray telescopes which tells us that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies. In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter. Three main strategies used to measure the mass of a galaxy clusters (9) Observing how the cluster bends light from galaxies located behind it. Studying X-ray emission from hot gas inside the cluster. Measuring the speeds of galaxies orbiting the cluster's center. When a cluster of galaxies is acting as a gravitational lens it means it bends or distorts the light coming from galaxies located behind it. Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it. (gravitational lensing) Dark matter is the dominant form of mass in both clusters and in individual galaxies. The distinguishing characteristic of what we call ordinary or baryonic matter is that it consists of atoms or ions with nuclei made from protons and neutrons. Some dark matter may consist of what astronomers call MACHOs (massive compact halo objects). We have detected gravitational lensing of distant objects that appears to be caused by compact but unseen objects in the halo of our galaxy MACHOS found in the halo of a galaxy Brown dwarfs Dim, low-mass stars Planet-sized objects that do not orbit a star Weakly Interacting WIMPs or neutrinos mean they respond to the weak force but not to the electromagnetic force, which means they cannot emit light. Can neither emit nor absorb light. Are subatomic particles. Tend to orbit at large distances from the galactic center. Space is NOT expanding within clusters of galaxies because their gravity is strong enough to hold them together even while the universe as a whole expands. We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it. Large-scale Structure of the universe- overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe. Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members. Galaxies and clusters have grown around tiny density enhancements that were present in the early universe Critical Density- average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist Note the importance of the caveat "if dark energy did not exist." With dark energy causing the expansion to accelerate, then even the critical density would not by itself be enough to prevent the expansion from continuing indefinitely. Observations of white dwarf supernovae is the primary source of evidence that led astronomers to conclude that the expansion of the universe is accelerating The temperature between galaxies in clusters tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass. A supercluster is most likely to have formed in regions of space where the density of dark matter was slightly higher than average when the universe was very young The actual density of matter, even with dark matter included, is less than about a third of the critical density. Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant today (such as 24 km/s/Mly), the age of the universe is oldest if the expansion rate has been increasing with time (an accelerating universe). An increasing rate of expansion would mean the expansion rate was slower in the past. In that case, it took longer for the universe to reach its current size than it would have at today's rate, so the universe is older than we would predict from today's rate of expansion alone. Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter that decays into photons or other forms of energy about 50 billion years from now. This would cause the galaxies in clusters would begin to fly apart. 23.1-23.4 Based on our current understanding of physics, we can understand the conditions that prevailed in the early universe as far back in time as about one ten-billionth of a second after the Big Bang During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles. This is why we live in a universe of almost entirely MATTER not ANTIMATTER The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. If we discovered a galaxy with a helium abundance of only 10% by mass it would call our theory into question. To determine conditions early in the universe, we work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size When a particle of matter meets its corresponding antiparticle of antimatter, The combined mass of the two particles is completely transformed into energy (photons). Planck time- before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred. The Planck time marks the end of the Planck era, which occurred when the universe was 10-43 seconds old. Current understanding says that all four forces should have been merged together before this time, but we lack a theory that explain how such a unification of the four forces works. Current theories cannot explain what happened during Planck time because we do not yet have a theory that links quantum mechanics and general relativity When we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang, it means these two forces first became distinct at this time. The four fundamental forces that operate in the universe today are Strong force Weak force Electromagnetic force Gravity A "GUT" (grand unified theory) refers to theories that unify the strong force with the electromagnetic and weak forces Gravity is not expected to unify with the other forces until much higher energies than those at which the GUT force operates. According to the Big Bang Theory, 2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces operated the universe during the GUT era. Inflation- a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence Summary of events in the early universe according to the Big Bang theory The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe. Cosmic Microwave Background With the exception of very small variations, it appears essentially the same in all directions in which we look into space. Thought to be radiation that began its journey to our telescopes when the universe was about 380,000 years old. Its spectrum corresponds to a temperature of just under 3 degrees above absolute zero. (just under 3 degrees Kelvin) Has a perfect thermal radiation spectrum Its temperature is the same everywhere, except for small variations at the level of 1 part in 100,000. Two major lines of evidence that alternative models have not successfully explained support the Big Bang theory. The theory predicts the existence of and the specific characteristics of the observed cosmic microwave background The theory correctly predicts the observed overall chemical composition of the universe. Measuring the amount of deuterium in the universe allows us to set a limit on the density of ordinary (baryonic) matter the universe Studies of the early universe add further support to the idea that dark matter really exists and is made of non-ordinary (nonbaryonic) matter, such as WIMPs. If we assume an episode of inflation, the Big Bang could explain the fact that the temperature of the cosmic microwave background is almost the same everywhere. The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. The prediction was that the universe should be geometrically "flat" (in the four dimensions of spacetime). Olbers's paradox- means ?Why is the sky dark at night?? Its resolution suggests that the universe is finite in age 3K- temperature of the universe (as a whole, today) We cannot scientifically test that prior to the Planck time, our universe sprouted from another universe. Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies the electromagnetic and weak forces into the electroweak force The era of nucleosynthesis- when the universe was about 5 minutes old- was when the basic chemical composition of the universe had been determined. In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. This is because before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely. Until about 380,000 years into the history of the universe, hydrogen atoms were ionized and photons would scatter off electrons before they could travel very far; the universe was opaque. If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for how galaxies came to exist In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life) This process didn?t produce carbon or other heavier elements because by the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur. How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed? Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure. WMAP satellite Cosmic Microwave Observations The universe is geometrically "flat" (in the four dimensions of spacetime). The matter density (both luminous and dark matter combined) in the universe is only about one-fourth of the critical density. Dark energy, whatever it is, represents the majority of the energy content of the universe. Based on the results from the WMAP satellite, the overall composition of the universe is 4% ordinary (baryonic) matter, 23% nonbaryonic dark matter, 73% dark energy Features of the early universe Dense Hot Filled with intense radiation