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by: Antwon Cole


Antwon Cole
GPA 3.89


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Class Notes
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This 33 page Class Notes was uploaded by Antwon Cole on Thursday October 22, 2015. The Class Notes belongs to ASTRO 2 at University of California Santa Barbara taught by Staff in Fall. Since its upload, it has received 10 views. For similar materials see /class/227023/astro-2-university-of-california-santa-barbara in Astronomy at University of California Santa Barbara.




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Date Created: 10/22/15
tiny fraction 39A of a second l 137 billion years Lecture 16 June 2 2009 Previously on astro2 Jeans instability arises when a selfgravitating object is heavier than its pressure can support After in ation small quantum uctuation are ampli ed to macroscopic scales and we see them as anisotropy in the CMB Before decoupling they do not collapse because radiation is keeping the sound speed very high After decoupling chunks of the universe of about 100000 solar masses become Jeans unstable and form the rst objects in the universe Previously on astro2 Until z20 the universe has been neutral since recombination Neutral hydrogen is opaque to UV radiation and so light from the rst stars cannot propagate far These are called the cosmic dark ages However the rst stars and quasars carve bubbles of ionized gas around them When enough bubbles are formed and start to ll in the entire universe UV radiation can nally travel again this is called re10n1zat10n Finding out the epoch of reionization and its sources is one of the hot topics in cosmology at this time We may be close to an answer Between z7 and 10 maybe Previously on astro2 In the currently standard picture of galaxy formation galaxies form hierarchically from the initial quantum uctuations ampli ed by in ation Halos grow bottom up assembling small chunks into larger and larger halos Inside halos there are baryons Torques from nearby halos spin up the halos and the baryons Baryons cool by emitting radiation and collapse because of Jeans instability As the baryons collapse they need to preserve their spin and so they settle into a rotating disk Previously on astro2 Spheroids are formed by mergers of disks The details of galaxy formation are poorly understood As stars are born evolve and die they disperse heavy elements in the gas between stars via supernovae winds New stars are born from this gas starting the cycle over and over again At every cycle the gas is more abundant in heavy elements which them form planets dust etc Previously on astro2 What is Drake s equation How do we detect planets Is there life out there Is there intelligent life How do we communicate with them Today On Astr02 1 Alternatives to Big Bang 1 Tired light 2 Steady state universe 3 The phoenix universe 4 The cyclic ekpyrctic universe Tired light cosmologies What if we are totally wrong and cosmological redshifts are not due to expansion An alternative explanation is the socalled tired light cosmology In this model photons loose energy as they travel in space in a manner proportional to the distance traveled The universe is static ie steady state Experiments on Earth do not have enough precision to rule this altemative out Problems with tired light Time dilation of supernovae light curves Tolman test on the expansion of the universe Surface brightness in an expanding universe scales as lz394 One power for photon redshift One power for time delay Two powers for difference between luminosity distance and angular size distance In tired light there should be only one power CMB should not be a blackbody Tired light and CMB In the standard BB cosmology as the universe expands the CMB photons loose energy and they become less dense preserving the blackbody intensity In a tired light cosmology as the photons travel they loose energy but their number is preserved So the blackbody has the wrong intensity Cartoons from Ned Wright s UCLA website Intensity MJysr Tired light and CMB 500 400 300 200 100 l l l FIRAS datai4000 2725 K Blackbody 2998 K Blackbody Tired Light CMB 25 Main observational facts vs tired light The night sky is dark No Spectra of distant objects appear redshifted Ok Helium abundance is 25 very homogeneously No explanation Deuterium in distant gas clouds No deuterium is only burned in stars The Universe is filled with a blackbody radiation at 3K No cannot be a blackbody all the time This blackbody radiation is extremely isotropic No see above No object older than 15 Gyrs has ever been found No explanation Galaxies at high redshift look different than today No The cosmic star formation rate changes as a functlon of time No No detections of cosmic annhilations Ok Light curves of supernovae are observed to be stretched at highz No Surface brightness at highz fades as lz 4 No The universe is at in ation No explanation No magnetic monopoles have ever been observed in ation No The CW is isotropic over the entire sky in ation No Steady state expanding universe What if the universe is expanding eternal and t1me 1nvar1ant The main problem is that as the universe expands matter dens1ty decreases In the steady state model popular in the 50s matter was constantly being created everywhere at the exact required rate few atoms per MW every year After all if matter is created at the Big Bang why not create it all the time everywhere CMB is produced by scattered star light Problems with steady state Number counts of galaxies follow a well de ned law because distances and volume changes with z In SS the density of objects is smaller than in BB by a factor 1z3 because of continuum creat1on vs matter conservatlon Data agree with BB taking into acccunt evolution which is not SS CMB can be introduced ad hoc but its temperature doesn t scale right with z It should be constant because it s steady state while it s observed to increase how Main observational facts vs steady state cosmology The night sky is dark Ok Spectra of distant objects appear redshifted Ok Helium abundance is 25 very homogeneously No He abundance should scale with that of O Deuterium in distant gas clouds No deuterium is only burned in stars The Universe is filled with a blackbody radiation at 3K Perhaps but should not evolve with z This blackbody radiation is extremely isotropic No explanation No object older than 15 Gyrs has ever been found No explanation Galaxies at high redshift look different than today No The cosmic star formation rate changes as a function of time No No detections of cosmic annhilations Ok Light curves of supernovae are observed to be stretched at highz Ok Surface brightness at highz fades as lz 4 Ok The universe is at in ation No explanation No magnetic monopoles have ever been observed in ation No The CW is isotropic over the entire sky in ation No explanation Alternatives to Big Bang 1 Summary Many alternative scienti c theories to Big Bang have been formulated over the course of XX and XXI century As in all good scienti c theories the assumptions imply quantitative predictions that can be measured No single theory alternative to Big Bang to date has been able to stand the test provided observations Tired light and steady state for example are falsi ed by a number of facts e g the blackbody spectrum of the CMB The search is not over but it is more and more dif cult because of the amount of precise observational data collected in the past decades Alternatives to Big Bang 2 In the Big Bang theory we suspend our judgement as to what happens before Planck time What is Planck time Why do we suspend our judgement However some human minds cannot but wonder what happened before Planck time and whether there was really a singularity Is this a legitimate scienti c question Yes but we can only accept scienti c answers ie testable with experiments Early attempts The Oscillating Universe One theory that was once popular formulated in the 19303 is the so called oscillating universe In this theory a Big Bang is followed by a Big Crunch After the Big Crunch a Big Bang follows and so on Problems with the classic oscillating universe Nobody knows Why and how the universe should bounce through the singularity With the current value of cosmological parameters the universe will not recollapse and therefore there is no big crunch in our future quotl TANK w Noun 539 Marne mum Haze l STEP TWO Additional problems with the classic oscillating universe A fundamental physical properties of complex systems is entropy Entropy measures the amount of order of a physical system Statistically entropy can only increase eg when you put sugar in coffee Additional problems with the classic oscillating universe Defining entropy correctly is a very subtle and difficult problem It is not settled how to define entropy for the universe at all times However one of the qualitative arguments against oscillating universe is that entropy increases during the lifetime of the universe due to processes like stellar evolution or you mixing your coffee with sugar and if entropy carries over from one big bang to the next as increased energy in the CMB The ratio of total energy in radiation produced by stars as opposed to that in the CMB is a measure of how many previous cycles there have been If you do the math it turns out that there 7 MakP lagnusNormancom cannot have been more than 100 cycles before this one Problems with the classic oscillating universe model Fact there won t be a big crunch in our future No testable prediction has been made as to the consequences of a previous cycle before our Big Bang so this is not a scienti c theory Contemporary attempts The CyclicEkpyrotic Universe A recent idea inspired by string theory is that there are more dimensions than 4 but we are restricted to live into a 4D subspace called a membrane or brane Membranes move expand and collide When two membranes collapse energy is dumped from the brane to the space inside the brane causing a hot dense space like a Big Bang See URL httpWwwphysicsprincetonedusteinhnpr Features of the CyclicEkpyrotic Universe The cyclic ekpyrotic does not replace the main ideas of the Big Bang model The universe is expanding from a hot dense phase so that the CMB primordial nucleosynthesis etc are all ok The main difference is that the early stages are not described by in ation but this collision of branes The esthetic appeal is that it does not require a beginning of time and could be a comprehensive more fundamental theory More Features of the CyclicEkpyrotic Universe The cyclicekpyrotic scenario is very recent 2002 and it has not been fully calculated nor tested for internal consistency At this stage it is more of a conjecture that a full self consistent scienti c theory The alternative in ation is much more well developed and understood at a fundamental level Time is needed to understand whether this is areal competitor for in ation There are however testable predictions regarding The statistical properties of the initial density perturbations A speci c distribution of energies for fossil gravitational waves One day it may be falsi ed and that s what matters Alternatives to Big Bang 2 Summary Some of the alternatives to Big Bang seek a physical understanding of a universe Without a beginning and end of time The classic oscillating universe is not a scienti c theory because it does not make testable predictions Furthermore no big crunch is expected in the future The current proposal of a cyclicekpyrotic universe is build to be consistent with the observed properties that are the foundations of the Big Bang theory but replaces in ation with a collision of branes in a higher dimensional universe It is not clear if this is a fully self consistent scientific theory More time is needed to evaluate it Its proponents do claim however to make testable predictions and this is a key step tiny fraction 39A of a second l 137 billion years Lecture 17 June 4 2009 First of all Well done You guys have done very well in class About the nal Topics The nal will be on the entire course I know it is a lot of material so focus in particular on the following topics 0 Big BangIn ation basic features and observational tests evidence 0 Cosmography what is it How do you measure it 0 Galaxy properties and galaxy formation 0 Thermal history of the Universe 0 How do quantities scale with redshift time temperature densities photon energy Jeans mass cosmological constant Extraterrestrial life Modi ed Newtonian Dynamics AGN and Alternatives to Big Bang will NOT be part of the final About the nal Practicalities You have 3 hours for your nal There Will be 5 questions 3 qualitative 2 quantitative You are required to answer all 5 to get 100 The nal is worth 40 of the total grade The exam is open bookopen notes Bring a calculator We ll do our best to provide paper books for those with ebooks No laptops cell phone etc BRING BLUE BOOKS PUT NAME ON EXAMXIIXXI About the final How to get a good grade The rst 3 questions are qualitative and are worth 70 of the nal grade The last 2 questions are quantitative and are worth 30 of the nal grade Plan how to use your time read carefully the 5 questions before starting to write so that you do not write too much on one question and run out of t1me before you can address the other ones For the qualitative questions there is no need to write thousands of words Be complete and to the point For the quantitative questions pay attention to units ask yourself 1f the numbers that you are gettmg make sense


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