Test 1 Study Guide
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This 8 page Study Guide was uploaded by Alexandria Rollins on Sunday October 18, 2015. The Study Guide belongs to 123 at University of Oregon taught by Imamura J in Spring 2015. Since its upload, it has received 64 views. For similar materials see Galax & Expand Univers >3 in Astronomy at University of Oregon.
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Date Created: 10/18/15
o Cosmology Define cosmology How has cosmology as a science evolved from the time of the Greeks to current times Cosmology the study of the structure and evolution of the universe It has changed over time where before cosmology was based off of belief as well as empirical facts now cosmology is based solely on empirical data 0 What makes a model scientific Contrast science and philosophy A model is scientific when it can predict phenomena that have not been observed yet as it makes it falsifiable These models can reproduce the actions and behaviors of the universeThe scientific viewpoint is one where theories are based off of observation and these theories are tested by experiments With philosophy aspects of life are analyzed and synthetic structures explanations are created by logic Philosophical notions are only considered scientific if they lead to hypotheses and predictions amenable to experiments 0 Compare the terms and use of the terms Universe and universe When studying the universe in general you would use a lowercase u When you are talking about a specific universe or our Universe you use an uppercase u 0 List the basic observations which led to the Big Bang Theory Explain how each piece of data supports the Big Bang theory The Universe in past was much smaller and denser and has been expanding since that time The first observation is that the Universe is expanding this supports the big bang theory because in this theory the Universe started from nothing and was created in a massive explosion that ejected all the matter in the universe outward The galaxies appear to be streaming away from us which would be consistent with the continual spreading and expanding of matterThe second observation is that the Universe is bathed in a pervasive and uniform sea of radiation the Cosmic Microwave Background Radiation CMBRThe CMBR is thought to be the relic of the Big Bang In the past when the Universe was much smaller it was also much hotter As the Universe expanded it cooled down until today The third observation is that the chemical composition of the Universe is 90 hydrogen and 10 helium and tiny amounts of everything else Lastly spatial and temporal structure is observed in the Universe 0 Briefly describe the Big Bang theory Is the Big Bang theory on secure ground What why or why not The Big Bang Theory is a theory that deduces a cataclysmic birth of the universe from the observed expansion of the universe cosmic background radiation and abundance of the elements and the laws of physics In the context of scientific models the important point is that the Big Bang theory was proposed after Hubble and Slipher showed that the Universe was expanding Detailed investigations of the model then led to predictions and scenario described above Because the Universe was very hot at birth and had been expanding for billions of years it was predicted that it would have cooled as it expanded and today 1960s would have had a temperature on the order of 510 Kelvin This prediction was verified by the discovery of Penzias and Wilson giving strong support for the Big Bang theory 0 What is Hubble39s Law How was Hubble39s Law discovered Hubble s Law states that the larger the redshift the greater the distance to an object Slipher and Hubble acquired spectra of distant galaxies by passing their light through a dispersive device a device which separates a blend of light into its constituent colors a spectroscope Each pattern is unique to the element absorption lines of hydrogen helium carbon and other elements are shown Because distant galaxies are made of stars one expects that the light from a galaxy will appear as the combination of hundreds of billions of stars Slipher and Hubble acquired spectra of many galaxies and noticed something quite interesting They found that the light from distant galaxies was shifted to the red end of the spectrum the light was redshifted 0 Compare and contrast the Doppler shift and cosmological redshift The Doppler shift is the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond We use the shift to derive information about those stars and galaxies The cosmological redshift is a measure of the total quotstretchingquot that the universe has undergone between the time the light was emitted and the time it was received The difference between the two Doppler redshift and cosmological redshift is that in the case of a Doppler shift the only thing that matters is the relative velocity of the emitting object when the light is emitted compared to that of the receiving object when the light is received After the light is emitted it doesn39t matter what happens to the emitting object it won39t affect the wavelength of the light that is received In the case of the cosmological redshift however the emitting object is expanding along with the rest of the universe and if the rate of expansion changes between the time the light is emitted and the time it is received that will affect the received wavelength 0 What is the naive interpretation of Hubble39s Law How does the Principle of Mediocrity influence our interpretation of Hubble39s Law Describe the currently accepted notion for Hubble39s Law The Hubble law states that the more distant objects recede from the Milky Way galaxy at larger speeds than more nearby objects A natural explanation is that an explosion occurred and that we are at the center of the Universe Consequently the galaxies are flying away from us in all directions and that the ones with larger initial kicks are now farther off The current preferred explanation is that the Universe expands uniformly with no center In this scenario any observer in the Universe sees essentially the same thing and all observers find the same Hubble39s Law o For Homework 1 your determination of the distances to the given galaxies relied on the assumption that our Universe is flat among other assumptions How would your results change if the Universe is open saddleshaped If the Universe was open the parallel lines would be affected When looking at the redshift we would look at the line in between a closed and open Universe because we are flat If we were an open Universe the numbers would increase and we would correspond to the upper curve on a graph 0 Cosmic Microwave Background Radiation CMBR Describe the CMBR When was the CMBR producedarise Describe the properties of the CMBR Cosmic Microwave Background Radiation CMBR is is the thermal radiation left over from the time of recombination in Big Bang cosmology The study of this first began in 1965 by Penzias and Wilson The properties of the CMBR are 1 The radiation was of very low temperature T 2725 Kelvin or 455 Farenheit or 270 C and came from all directions in the Universe 2 The spectrum of the radiation was welldescribed by a blackbody Spectrum 3 The radiation was isotropic ie it had very close to the same temperature all across the sky temperature differences of lt 0004 on angular scales of 7 degrees excluding a wellknown 012 variation known as the dipole anisotropy and finer lower amplitude temperature variltions 0 Describe the horizon problem Describe why the CMBR39s properties lead to the horizon problem When I look in one direction on the sky and observe the CMBR see light that has traveled over the lifetime of the Universe to reach me When I look behind me in the opposite direction I see light that has also traveled over the lifetime of the Universe to reach me That is the part of the Universe in front of me and the part behind me are so far apart that they have never been in thermal contact and in fact in principle do not even know of the other39s existence The properties lead to the horizon problem because it was isotropic and homogeneous o The chemical composition of the normal matter in the Universe is roughly 90 hydrogen and 10 helium everything else heavier than helium is referred to as a metal Wand comprise only a small part of the Universe Describe the makeup of the Universe Be sure to consider both matter and dark energy The Universe is 72 dark energy 24 dark matter and about 4 of everything else such as planets and stars With this being said we don t know 96 of our Universe o Principle of Mediocrity Describe the principle of mediocrity The principle of mediocrity implies that we are the center of the Universe It is looked at by GeoCentric and HelioCentric theories Because of Hubble s Law we know we are not the center of the Universe 0 Give and describe examples of how the principle of mediocrity seemed to be violated at different times during history Be sure to mention how different experimental results that pertain to the principle of mediocrity were misinterpreted in the past Has it been violated yet 0 Cosmological Principle Describe the cosmological principle Cosmological principle is that we do not occupy a preferred place in the Universe and further that at a given time the Universe appears homogeneous and isotropic to all observers In other words no matter where you sit the Universe will look the same at a given time 0 Describe the meanings of homogeneity and isotropy Give simple examples that illustrate each concept Homogeneous means that in some average sense the Universe appears the same to all observers regardless of location An example of homogeneous would be a brick wall Isotropic means that the Universe appears the same in whatever direction one looks For example the Solar System is clearly inhomogeneous there is a star at the center of the system surrounded by planets The Solar System looks differently depending upon where you are and where you look 0 The Universe is clearly not bland and uniform in what sense is the cosmological principle valid There is structure in the Universe but the argument is that it is small compared to the overall size of the Universe and so in a sense it can be averaged over gt the Cosmologoical Principle is apparently valid 0 Perfect Cosmological Principle what is it Is it currently considered by astronomers when they model the Universe It is not what are some reasons why the Perfect Cosmological Principle is not in favor What is the Cosmological Constant It was first proposed by Einstein why Why did Einstein withdraw his suggestion Has this idea returned to favor today The Perfect Cosmological Principle is that it doesn t matter what time you look at the Universe it still looks the same We not only do not occupy a preferred place in the Universe we also do not occupy a preferred time The Universe appears homogeneous and isotropic and the same to all observers at all times in the Universe We don t use this idea that often because it does not account for evolution The cosmological constant is the value of the energy density of the vacuum of space and is usually denoted by the Greek capital letter lambda A Einstein proposed this to represent an unknown repulsive force which permeated the Universe Einstein made this assertion before Slipher and Hubble made the discovery that the Universe was expanding After Hubble made his announcement Einstein immediately recognized that because the Universe was not stationary it was expanding that there was no need to postulate a repulsive force Lambda was unnecessary Einstein immediately took back his suggestion considering it to be one of his greatest blunders Interestingly today it is suggested that there is some unknown force the Dark Energy which may in fact be the repulsive force which is parameterized by the Lambda Einstein added to his theory We do not yet know if this is correct however 0 Anthropic Principle What is the Anthropic Principle The Anthropic Principle states that if the Universe wasn t special then we wouldn t be here What we can expect to observe must be restricted by the conditions necessary for our presence of observers o What is the Big Bang Theory Briefly describe the Big Bang Theory Upon what observations is the Big Bang Theory based Is the Big Bang on a firm observational footing The Big Bang Theory is a theory that asserts only that the Universe in past was much smaller and denser and has been expanding since that time o What is a light year A light year is a unit of astronomical distance equivalent to the distance that light travels in one year which is 94607 x 1012 km nearly 6 trillion miles 0 Describe Olbers39s Paradox and the resolution of the paradox That is what are the assumptions which underlie Olbers39s Paradox and must they be modified to resolve the paradox How can the Big Bang resolve Olbers39 Paradox Olbers s Paradox is based around the question why is the night sky dark The assumptions are 1 the Universe is unchanging and infinite in size or at least large in the sense we define below 2 stars fill the Universe uniformly each star has the same luminosity L 4 the inverse square law holds ie the flux of energy from a star energy flow per second per unit area is given by f L I 4 pi D2 Here L is the intrinsic luminosity of the star and D is its distance from us Resolutions may include 1 obscuration by dust gt distant stars are blocked out and appear fainter Turns out this won39t work because dust if it absorbs energy will heat up and reradiate the energy This means that the Universe will still be filled with the same amount of radiation the dust acts simply as a gobetween so to speak 9 5 Expansion of the Universe 1 gt redshift of photons gt Wobserved is larger than Wemitted gt we absorb lower energy photons than are produced by the distant stars 6 Expansion of the Universe 2 gt Imagine that the star galaxy produces 1 photon every second If there is no relative motion between the star and us then we will also see photons energy fly by at the same rate gtsame luminosity However if the star is moving away from us then we will see the photons fly by a slower rate than 1 per second This means that the energy will arrive at the Earth at a slower rate gt a lower luminosity 7 The preceding effects conspire to make distant objects in an expanding universe have apparent brightnesses which fall off faster than the inverse square law This decreases the contributions from distant shells The expanding universe effects partially explain Olbers39s Paradox 8 One shell of stars covers a fraction 5 x 103916 x n x T of the sky So to make the night sky as bright as a star we would like to make the stars cover most of the observable sky This means that 5 x103916 x n x T x number of shells 1 9 To calculate the number of shells we note that there is roughly 1 star per cubic parsec in our galaxy gt average distance between stars in our Galaxy shell thickness T is 1 parsec explain why 10 gt number of shells 1 I 5 x103916 2 x1015 11 Because each shell is 1 parsec thick gt Universe needs to be at least 2 x 1015 parsecs in radius Recall that 1 parsec 33 light years and so the Universe must be at least 66 x 1015 light years in size in order to make the night sky as bright as the surface of the Sun 12 The current Universe is 137 billion years old and so has an observable size of 137 billion light years This is much less than needed to produce Olbers39s Paradox The fact that the Universe has a finite age is the principal explanation of Olbers39s Paradox 0 Describe the Friedman models for the Universe What assumption did Friedman make when finding his models for the Universe Was this assumption valid Friedman invented models for the spacetime structure of universes based on Einstein39s General Theory of Relativity GTR The assumption he made while finding his models for the Universe was that the cosmological principle was valid 0 What is meant by spacetime In terms of spacetime describe each of Friedman39s models for the Universe Spacetime is the spatial location and temporal location of the event considering we have four dimensions the position and time In terms of spacetime the Friedman models are flat Euclidean space critical universe positive curvature eg a sphere the closed universe and negative curvature eg a saddle open universe universes o In which type of Friedman universe are we thought to live Which universe is a closed universe and what does this mean Which universe is an open universe and what does this mean We are thought to live in a flat universe It is said to expand and will stop expanding after an infinite amount of time It is the dividing line between open and closed universes Closed universes are expanding but will reach a maximum size and then collapse An open universe expands and will expjand forever and is infinite in spatial extent 0 How can we distinguish open closed and flat universes Describe the tests and the results of the tests described in class Describe the behavior of parallel lines triangles in the different Friedman models Depending upon the geometry of the Universe the paths of parallel lines can cross or diverge A consequence of this is that if measures the angular sizes of distant galaxies they do necessarily have to decrease as 1ldistance as they would in a flat universe This effect has also been studied The results are suggestive but again not conclusive o Interpret and explain the Hubble Law in the context of an expanding Universe What is meant by cosmological redshift Describe cosmological redshift How does the cosmological redshift fit in with the Hubble Law What is meant by scale factor Rt The galaxies and other denizens of the Universe are simply carried along by the expansion of the spacetime In this sense they may be stationary in the expanding Universe the apparent recession of distant galaxies with respect to our home galaxy the Milky Way occurs because the distances of the galaxies from us grow with time as the Universe grows in size Each galaxy sees the other galaxies behave in the same manner No one galaxy sits at the center of the universe furthermore there is no center of the expansion This difficult conceptual point follows from the notion that the Universe itself is growing in size it is not expanding into anything and so has no edges and therefore no center All points in the Universe are simply moving apart with time Cosmological redshift is a measure of the total quotstretchingquot that the universe has undergone between the time the light was emitted and the time it was received Size Rt x size in the past where size in the past is usually the size of the universe at some point in the past 0 How does the scale factor behave for each type of Friedman universe Closed universes correspond to the bottom curve Open universes correspond to the top curve Flat universes fall right between closed and open universes o What is the Flatness Problem The flatness problem is a problem with the Big Bang model The question is why is the Universe density so nearly at the critical density In other words why is the Universe so flat The Universe is so wellbalanced between the positivelycurved closed Universe and the negativelycurved open Universe that astronomers have a hard time figuring out which model to choose If the curvature of the universe was just a few percent off from perfect flatness within a few seconds after the Big Bang the universe would have either recollapsed before fusion ever began or the universe would expanded so much that it would seem to be devoid of matter It appears that the densitycurvature was very finely tuned
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