Visions of the Universe
Visions of the Universe ISP 205
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This 15 page Class Notes was uploaded by Ms. Mckenzie Labadie on Saturday September 19, 2015. The Class Notes belongs to ISP 205 at Michigan State University taught by Stephen Zepf in Fall. Since its upload, it has received 39 views. For similar materials see /class/207722/isp-205-michigan-state-university in Integrative Studies Physical at Michigan State University.
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Date Created: 09/19/15
ISP 205 Final Exam Expansion of the Universe I How do we know the universe is expanding o Hubble39s diagram showed that the redness of a galaxy39s light and thus the speed with which the galaxy moved away from Earth increased with its distance from Earth and that the increase graphed into a straight line 0 The farther away a galaxy is the faster it moves away from us I How fast is it expanding 0 With enough matter the expansion will slow or even become a contraction 0 Dark energy drives the universe towards increasing rates of expansion 0 The current rate of expansion is usually expressed as the Hubble Constant in units of kilometers per second per Megaparsec or just per second I How do we measure this red shifts and distances 0 Redshifts I The higher the redshift the farther the galaxy I Redshifts means that it is traveling away from us Expanding I Observing spectrum that it emits Doppler39s shift 0 Distance I Standard candle method Observing Cepheid variable stars Determining intrinsic luminosity Comparing the apparent to the true luminosity I For really far galaxies use supernovae I Parallax cannot be used because the shift is to small for anything farther away than our milky way I What is Hubble39s Law 0 VH0D o The principle that the distance of external galaxies from the earth is proportional to theirs Balloon Equal distances I How do we estimate the age of the universe from the expansion rate 0 1 By looking for the oldest stars 0 2 By measuring the rate of expansion of the universe and extrapolating back to the Big Bang 0 We use Hubble39s constant to measure the farthest away galaxy which indicates when the big bang took place relatively I Was it always expanding this fast 0 The three possible types of expanding universes are called open at and closed universes o If the universe were open it would expand forever 0 If the universe were at it would also expand forever but the expansion rate would slow to zero after an infinite amount of time o If the universe were closed it would eventually stop expanding and recollapse on itself possibly leading to another big bang o In all three cases the expansion slows and the force that causes the slowing is gravity I How can we tell 0 The standard way of determining the properties of darkmatter is to look for its in uence on luminous matter 0 A similar analysis can be done of dark energy In particular the dark energy strongly affects the expansion of the universe 0 0 Remember the universal expansion of the universe causes galaxies to move away from each other 0 However galaxies have mass and hence the mutual gravitational attraction of the galaxies will tend to make the expansion slow down 0 Now consider the effect of adding dark energy to the universe The energy or mass of the dark energy will add more gravitational attraction to the universe and hence will tend to amok the expansion slow down even more 0 HOWEVER if the dark energy has pressure as well the pressure could act to speed up the expansion I Will it expand forever 0 Based on the observation of supernovae and the acceleration of the expansion yes I If the universe were expanding faster than we think it is would the age of the universe be younger or older 0 Younger Fate ofthe Universe I Observable consequences of the different possibilities for our universe 0 Recollapsing I If the density is larger than the critical density than the expansion will halt and reverse I Will end in a Big Crunch Will look like it began I Closed universe 0 Critical I If the matter density is equal to the critical density I The universe would expand progressively slowing down I Flat universe 0 Coasting I Matter is smaller than the critical density I The universe will keep expanding forever with little change in its rate of expansion I Open universe 0 Accelerating I If dark energy exists a repulsive force that cause the expansion to accelerate then the expansion rate will grow with time I Will become cold and dark more quickly I Can be open at or closed I In our case it is at I How adding up the matter in the universe can help us determine this 0 The critical density is the average matter density the universe would need for the strength of gravity to halt the expansion 0 By seeing how much matter including dark matter we have we can determine if it falls below or above the critical matter I What that result is o The universe is at andinfinitely expanding Becoming colder and darker as it does so I How looking at galaxies and other objects like supernovae far in the past can help us determine this 0 Using standard candles we can determine the age of the planets and estimate the distance of the galaxies O The early Universe I Physical properties of the universe at early times 0 The universe is becoming less dense as it expands so it must have been hotter and denser in the past I In ation period I When the galaxy expanded in size dramatically after the universe cooled down I Helium formation I Temperatures were so hot after big bang that elementary particles could form still cooled but hot enough to convert hydrogen into helium I Cosmic Microwave background formation I Photons bounced around among the free electrons in the universe until about 380000 years I Then captured by atoms The photons began to move freely through the universe I Galaxy formation I Galaxies began to form by the time the universe was 1 billion years old How the cosmic microwave background photons originate from the early universe 0 Photons frequently collided with free electrons during the era of nuclei and thus could travel freely only after electrons became bound into atoms 0 Dense fog to clear air When the universe changed from being opaque o Photons kept bouncing off free electrons to being transparent because electrons joined up with the photons How the current temperature of these photons tells us about the conditions in the early universe 0 It has a perfect thermal radiation spectrum So the heat cooled at a consistent rate How small peaks and valleys in these photons from the early universe tell us about how structure forms in the universe 0 The patterns of slightly higher and lower microwave emissions and temperatures come from regions of slightly higher or lower densities 0 These grow into galaxies When the universe was even hotter and denser how photons and neutrons came together at even earlier times about 3 minutes since start of Big Bang to make helium but not enough time to make heavier elements 0 The universe cooled off quickly and a ration of 25 helium and 75 hydrogen What the horizon and atness problems are and how a very early super in ationary period in the universe solves them 0 Horizon I Light travels faster before and how can they be so similar when they are farther apart 0 Flatness I How is the universe at with density being close to the critical density How the darkness of the night sky is related to finite age of the universe Olbers paradox Lecture 1 Tuesday January 11 2011 100 PM Astronomy is the physical science of the cosmos 7 planets stars galaxies the universe 0 Earth 0 Solar System a The milky Way Galaxy a The Local Group a The local Super cluster a The universe The solar system 0 Sun planets moons comets asteroids dust gas Our galaxy the milky way 0 Originally all gas 0 Now billions and billions of stars similar to our sun 0 Stars are borne evolve then die 0 Material processed through stars 0 This source of all chemical elements 0 Except hydrogenH heliumHe lithiumLi 39 Made in big bang The Hubble deep eld 0 Tiny area of sky 0 Some of the faintest objects ever measured 0 Made with the Hubble space telescope 0 Only a few stars nearly all objects are very distant galaxies Implications of Constant Speed of Light cosmic messenger o C 3x1005kms Period It is the law 186000 miles per second 0 Light travels at nite speed Universe is enormous so that we can see things that are billions of light years ago 0 Because the speed of light is constant when we see a very distant galaxies we see them as they were billions of years ago This allows us to study the universe the way it was when it was young aIMPORTANT concept we will return to later 0 We humans have only been around for a tiny fraction of the age of the universe but we can study and learn about nearly all of it Two items to note 0 Scienti c notation 7 range of distance scales in astronomy is astronomical It is tiring to write all those 0 s So we use scienti c notation o The exponent on the 10 tells how many times to multiply by 10 or divide if it is negative So 1003 1000 and 10 A3 001 0 Metric units 7 astronomy is an international subjects Everyone needs to agree on units So we use metric o 1 kilometerkm 6 miles Distances 0 Earth diameter 13000 km 13 x 1004km 0 EarthMoon 380000 km 38 x 1005 km 0 Earth 7 sun 150000000 km 15 x 1008 km 0 Sun to nearest Star 38 x10013 km 4 light years 0 Sun to central of our Galaxy 25000 lyrs 0 To nearest other large galaxy 22 x 1006 lyrs or 21 x 10018 km 0 Light years lyr 0 Distance light travels in one year 0 310 x1005 kms x 315 x1007syear 94 x10012 km Exam 1 Page 1 Lectu re 2 Thursday January 13 2011 100 PM o The eorTh spins This is The origin of doy ond nighT Seosons o The orbiT of The eorTh oround The sun 0 Why we see differenT sTorTs oT nighT oT differenT Times of The yeor o BuT There ore some sTors we con see oll yeor ond oThers we never see from The norThern hemisphere o Coupled wiTh The 23 TilT of The eorTh wiTh respecT To The plon of iTs orbiT why There ore seosons This TilT couses uneven heoTing by sunlighT ThroughouT The yeor The disTonce from sun To eorTh doesn T chonge The TilT does n winTer The norThern hemisphere is TilTed owoy from The sun ond in summer The norThern hemisphere is TiTle Towords The sun 0 If 0 ploneT which spins in exochy The some plon os iT orbiTs iTs sun how will The seosons be effecTed There will be no seosons o NoTilTno seoson The moon 0 The moon roToTes oround The EorTh wiTh o 27 doy period Tokes 295 To geT some phose bocllt becouse of The eorTh s roToTion obouT The sun 0 This couses us To see phoses o FirsT quorTer moon 7 doys 0 Full moon 14 doys 0 Third quorTer moon 22 doys o The moon roToTes obouT iTs oxis in exochy The some Time os iT orbiTs The eorTh o The some side of The moon olwoys is Towords The eorTh The oTher side is noT depends on The phose buT we never see iT direcTIy o Eclipses o Lunor EorTh beTween sun ond moon cosTs shodow on moon 0 Solor moon beTween sun ond eorTh cosTs shodow on eorTh o OrbiT of The moon oround The eorTh is TiTled by 5 from The plone of The orbiT of The eorTh oround The sun 0 o solor eclipse occurs during 0 new moon The PloneTs Exam 1 Page 2 ds simply ds possible Scientific models mollte testdble predictions dbout ndturdl phenomeno If they don t dgree the model needs to be revised or obondoned A theory in science is 0 simple yet powerful model whose predictions hove been borne out by repedted ond voried testing Science worllts forwhdt it dims to do Effectiveness from systemdtic testing ond observotion ond ndturdl expldnotions Exam 1 Page 3 Lecture 3 Tuesday January 18 2011 100 PM The Copernican Model 0 Ptolemaic model known to be complex and inaccurate after 1500 yrs Copernican model offered some simpli cation but was not a hu e improvement in accuracy because Copernicus used circular orbits they are actually elliptical as shown by Kepler and explained by Newton Heliocentric model shown to be correct by helio 7 sun 0 Galileo s observations 0 Keplers improved calculations Galileo 7 early 1600s 0 First to use telescope to look at night time sky 0 Galileo s telescopes inch in diameter x 2430 long 0 Discovered o Milky way myriads of stars Phase of Venus 7 con rmed heliocentric model Sunspots Craters Maria on Moon Rings of Saturn 4 moons orbiting Jupiter I image through modern telescope showing 2 of Galileo s satellites and their shadows Johannes Kepler 15711630 0 Tycho Brahe1546 1601 0 Was Kepler s thesis advisor 0 Great observer I 20 years data on planetary motions o Kepler analyzed data after Tycho died 0 Concentrated on orbit of Mars 0 Discovered 3 laws that together describe the motions of all the planets 0 Each planet moves around orbit in ellipse with sun at one focus 0 The straight line joining the planet and the sun sweeps out equal areas of space in equal amounts of time o For solar system and Ptime it takes to go around the sun once in yrs and a in AUdistance from the earth to the sun periodA2 radiusA3with P in years and r in AU 00000 0 Laws 0 Planets move on ellipses Key to improvement in accuracy of sun centered system 0 Planets move faster when they re closer to the sun 0 Period radius Also governs all orbiting systems Newton s theories on gravity and motion explain this and more Isaac Newton 1643 7 1727 0 One of the great geniuses of the millennium o Invented calculus o Mechanics 7how things move Exam 1 Page 4 o Keplers laws can be derived from Newton s laws 0 But Newton s laws are a general descriptions of a far wider range of phenomena 0 Universally valid I Except when Einstein s law of relativity takes over 0 Newton s laws of motion 0 Every body continues doing what it is already doing 7 being in a state of rest or moving uniformly in a straight line7unless it is compelled to change by an outside force I Conservation of momentum The change of motion of a body is proportional to the force acting on it and is made in the direction in which that force is acting I Force mass x acceleration To every action there is an equal and opposite reaction or the mutual actions of two bodies upon each other are always equal and act in opposite directions I A spaceship needs no fuel to keep moving in space I A baseball accelerates as the pitcher applies a force by moving his arm I A rocket is propelled upward by a force equal and opposite to the force with which gas is expelled out its back 0 Gravity 7 Newton s huge leap forward Newton needed to understand what force caused the acceleration to make the planets move in their orbits and also that caused applies to fall on his head I FGM1M2RA2 Force of gravity gravitational constant x massl x mass2 distance between objects squared I Mutually attractive force pulls objects towards each other Universal law of gravitation 7 force of gravity will pull them together 0 Next big step 0 Mass in f ma same as inertial mass gravitational mass I Mass and gravity are inseparable I Example dropping balls of different weight 0 Both fall at the same time 0 We have the force that causes things to move Gravity For each planet mass of sun x mass of planets o Gravitational force is always towards the center but the object at that instant is moving perpendicular to the center so the gravitational acceleration changes that direction so that this traces out an ellipse 0 Final Note Kepler s Laws can be fully accounted for by Newton s Laws 0 Full 3rd law is RA3 M1 M2 x PAZ o The fact that both masses both sun and earth mass for example are in the equation isn t easily noticeable in the solar system because the sun s mass is so much bigger than the earths but this is measurable and shows up in other cases like extrasolar planets o Tides Earth experiences gravitational attraction to sun and moon The amount of gravity depends on the distance to other object The closer you are the stronger the force 0 O O O 0 Exam 1 Page 5 Lecture 4 nuqu January 20 2011 1 no PM 0 Gravity is important because it determines how objects move in the universe from the orbits of planets in the solar system to the motions of stars moving in galaxies to the gravitationally pull ofgalaxies on each other 0 Light is the other major physical phenomenon we need to understand to learn about the universe because it is what we see Light is how we get information about the universe Light The Cosmic Messenger 0 Light as awave O n H i L distance between successive crests ght wave moves at speed of light c u 4 v f rate at which crests pass a stationary observer s us how many times any point on the wave bobs up and down each second 0 Frequency velocity distance 0 ch cyclessecond 0 Light as aparticle 0 Light can also be treated as 11 mm packets of energy 0 The energy carried by each photon depends on its frequency color I E h hc L h is called 39 I Eh x frequency h x c wavelength 0 Bluer light carries more energy per photon o The electromagnetic Spectrum 0 Light is given different names according to its wavelength 4 o In visible region Different L different color 39 Blue smaller I Redlarger o EphutunhvhCh 0 So longer wavelengths photons have lower energies Radio waves are lower energy than infrared which are lower than visible which are lower than ultraviolet which are lower than gamma rays ultraviolet light has a smaller wavelength than infrared light Which is true about this statement swer r Ultraviolet photons have more energy than infrared photons o The ill spectrum range ofpossible wavelengths is seen in Nature 0 The eanh s atmosphere prevenm some of them from reaching us in space I Only visible and radio waves pass freely through the Earth s atmosphere 0 Review of light 0 Critical asper r photon 7 part wave part particle has wavelength and energy 0 Eh x o lrZ law 0 Radiation from glowing objects 0 er objects emit more photons and more of these are higher energy photons 0 Lines as well as continuous spectrum 0 Inverse square law lrZ aw o A light bulb looks fainter at greater distances 0 Outgoing light wave spreads out over more and more surface area 0 If same amount of light emitted in al directions rface area increase as r I Light per unit surface area decrease as lrZ law 0 LuminosityL 39 trinsic brightness of light source 0 For 11 r I at Mummy 0 FluxF I Apparent brightness of object as it appears from distance r I L41t r2 0 How do light and matter interact 0 Emission I The sun emits li t 39 Light bulb emits white light 0 Absorption I Red chair 7 absorbs all colors except red Re ects scatters red I Snow absorbs some light which aids melting Scatters most light so it looks bright 0 Transmission 0 Re ection or scattering o Terminology 39 Transparent transmits light I O ue locks absorbs light 7Why is the rose red er 7 because the red rose re ects red light 0 How does light tell us the temperatures of planets and stars ermal Radiation I Most objects emit thermal radiation including stars planets etc thermal radiation spectrum depends on only one property is temperature 0 Two properties of thermal radiation objects emit more light at all frequencies per unit area 39 Honer objects emit photons with a higher average energy 0 Exam 1 Page 5 o INTENSITY IS PER AREA iLARGER OBJECTS CAN EMIT MORE TOTAL LIGHT EVEN IF THEY ARE COOLER Why don t we glow in the dark A iPeople only emit light that is invisible to our eye Exam 1 Page 7 Lecture 5 Tuasdav January 25 2011 102 PM I Thermal Radiation a Heat up a frying an i It glows more brightly has it gets hotter It changes color as it gets hotter H Spectrosco a 91 instrument that separates light by is wavelengths i Uses the principles of diffraction and re action quot I a tool used to spread the light 1 measures brightness of light at each point pixel in vertical direction b What are the three basic types ofspectra i Emission Line Spectran thin or lowdensity cloud of gas emiw light only at speci c wavelengths that depend on its composition and temperature producing a spectrum with bright emission lines Continuous spectrum e spectrum of a common incandescent light bulb spans all visible wavelengths without interruption iii Absorption line spectrum 1 A cloud of gas between us and a light bulb can absorb light of speci c wavelengths leaving dark absorption lines in the spe ctrum c Different chemical elements have different spectra d Lines come from the physics of the atoms e Lines at different wavelengths energies for different elemenw because the atoms of different elements are set up differently f Lines are important examples include determining what kinds of elements are present and using shi s in lines to determine velocities of obj ecw Doppler shi g How does light tell us what things are made of 39 ctrons in atoms have distinct energy levels Each element ion molecule has a unique set of energy levels HI Doppler Shi ange of wavelength X from moving source b Measuring Radical Velocity The Doppler effect i lfwave39s source is moving observer measures different frequency wavelength True for water waves sound waves and light waves 1 One difference is emitter can go faster than speed of sound but not light iii Shi in wavelength is AXX observed 9 mt v For vvelocity of emitter c velocity of wave AM i 7 v c v This Do ler shi only measures velocity along line of sight c we see a set of spectral lines at 500 nanometers and 400 nanometers in the lab We observe a star and see this set of spectral lines at 510 nanometers and 408 nanometrs respectively What can we conclude 39 Answer The star is moving away from us Exam 1 Page 8 Lecture 6 Thursday January 27 2011 1253 PM Test has 44 questionspoints There39s a diagram we have to ll in the rest is multiple choice For the test you can sit in any seat as long as your in the right row 1 Review from last class a What are the three basic types of spectra i Emission Line Spectrum 1 This spectrum shows a smooth continuous rainbow of light ii Continuous Spectrum 1 We see bright emission lines at speci c wavelengthscolor but no other light iii Absorption line spectrum 1 We see dark absorption lines where the cloud has absorbed lights of speci c wavelengthscolors 2 The graph shows a dip in intensity at the wavelength of each absorption line b The Doppler Effect for visible light i V 9 c ii Towards us negative velocity gives smaller wavelength higher frequency iii Away from us gives larger wavelength lower frequency iv The Doppler shift is important because we use it to measure motions throughout the universe 1 On small scales it was how planets around other stars were found 2 On large scales it was how the expansion of the universe was discovered 11 Telescopes a Galileo s telescopes N lquot in diameter X 2430quot long b Modern Telescope i The twin Keck lOm400quot telescopes on Mauna Kea in Hawaii ii The European very large telescope four 8 meter telescopes in Chile iii The SOAR telescope in Chile in which MSU is a partner c What does a telescope do i lVIagni es images see details ii Gathers light over large surface area see fainter objects 1 The mirror area determines the lightgathering power the area goes as the mirror diameter2 iii Technological advances have allowed larger telescopesall mirrors these days up to 10 meters 30 yards d SOAR MSU s New 4m telescope i Superb image quality Superb site in Chile gt highly competitive for opticalinfrared observations ii Located in Cerro Pachon Chile iii An intemational Partnership 1 MSU 2 University of North Carolina 3 National Optical Astronomy Observatories 4 Brazil e Telescopes in Space i Atmosphere blocks light at many wavelengths ii Atmosphere turbulence smears out images iii Hubble Space Telescope l 24m diameter mirror 2 Ultravioletopticalinfrared 3 Abovemost of Earth39s atmosphere a Unblurred view Exam 1 Page 9 ISP Final Study Guide Expansion of the Universe 1 S P Equot 9 N 99 How do we know the universe is expanding a Galaxies are redshifted moving away from us How fast is it expanding a H little 0 23kmsMLyr How do we measure this redshifts and distances a Take spectra of other galaxies b Identify absorption and emission lines from various atoms c Compare wavelengths of these lines to those on Earth the lines come in patterns provides an important check on the process d Calculate motions of other objects What is Hubble s Law a Direct linear relationship with velocity and distance b The velocity with which galaxies are moving is proportional to their distance away from us How do we estimate the age of the universe from the expansion rate a By measuring the rate of expansion Was it always expanding this fast a No it was slowed down by gravity may be sped up by dark energy Will it expand forever a Maybe very close What are the ways we try to determine this If the universe was expanding faster than we think it is would the age of the universe be younger or older What the expansion rate was like in the past or will be in the future for different answers about matter and dark energy in the universe a It expanded faster in the past Connection between the mass and fate of the universe and its shape Ex how more or less mass changes the fate of the universe and its geometric shape a mass density of dark matter is about 25 of that required to stop the expansion of the universe Fate of the universe l l Pm Determining the history and predicting the future of the universe Observable consequences of the different possibilities for our universe How adding up the matter in the universe can help us determine this and what the result is How looking at galaxies and other objects like supernovae far in the past can help us determine this and what those data suggest for the history of the universe The Early Universe H 0 Physical properties of the universe at early times H l Understanding how red shift is a time machine when used to look at very distant objects in the early universe a Light travels at finite speeds 9 General idea of how the universe was different physically in the past a Extremely hot and dense 5 How the quotcosmic background photons originate from the early universe about 380000 years when the universe changed from being opaque because photons kept bouncing off free electrons to being transparent because electrons joined up with the protons N 53 How the current temperature of these photons tells us about the conditions in the early universe N H How the small peaks and valleys in these photons from early universe tell us about how structure forms in the universe N N How protons and neutrons came together at even earlier times 3 minutes since the start of the Big Bang to make Helium but not enough to make heavier elements N W What the horizon and flatness problems are and how a very early superinflationary period in the universe solves them N 4 How the darkness of the night sky is related to finite age of the universe Olber s paradox N U39I What the anthropic principle is a the conditions in the universe have to be such to allow life to exist since we know we exist HOM EWORKS 11 1 How do we determine distances to galaxies a By observing Cepheid variable stars in those galaxies By observing their period we can determine their intrinsic luminosity We can then determine the distance by comparing the apparent brightness to the true luminosity Cepheid s are bright stars so we can observe them in nearby galaxies with the Hubble For really distant galaxies Cepheid s are too faint to see To determine distances to these galaxies we observe supernovae when they happen to have one Supernovae can also be quotstandard candlesquot We can only use parallax for stars near the sun in our own Milky Way because the shift is so small for anything further away 2 If you observe a galaxy with a measured redshift of 015 and a second one with a redshift of 030 which is further and by how much a The galaxy with a redshift of 030 is twice as far away as a galaxy with a redshift of 015 3 Why is every galaxy except the nearby Andromeda galaxy redshifted away from us a The universe is expanding so the light from the other galaxies is stretched by this expansion This stretching makes the wavelengths longer redder Andromeda is not redshifted but in fact blue shifted because it is close enough to us that it and our Milky Way are being pulled toward each other by their mutual gravitational attraction 4 If the Hubble Constant is 23 kmsMlyr and there is no matter in the universe how old is the universe a If there is no matter there is no mass to slow down the expansion over time through gravity That means it has always been expanding at the same rate The Hubble Constant tells us how fast that is It says that if a galaxy is 1 Mlyr away it is moving 23kms away from us How long does it take to go 1 Mlyr at 23 kms 1 Mlyr is about 10quot1 9 km at 23 ms it takes about 44 x 10quot16 seconds or 14 billions years 5 If a galaxy with a distance from us of 14 billion light years is moving away from us at 1 light years per year approximately how old is the universe a At a speed of 01 light years per year it will take something 14 billion years to go 14 billion light years If you are moving 01 mph how long will it take to go 14 miles 14 hours 6 What force works to slow down the expansion of the universe a Gravity 7 If the universe is really accelerating will future astronomers measure a Hubble constant that is larger smaller or the same as the one we measure now a If the universe is accelerating its expansion rate will be larger in the future so future astronomers will measure a Hubble constant that is larger than the one we measure now HOMEWORK 12 1 Arrange chronologically by earliest to most recent a Inflationary period helium formation cosmic microwave background formation and galaxy formation 2 What are some of the critical pieces of evidence for the Big Bang a Redshift of other galaxies and also Hubble s Law everything is expanding now so it must have been more compact and hotter in the past b We have detected the leftover radiation from the Big Bang c This theory correctly predicts the abundance of helium and other light elements 3 Why are there no stars composed of 90 Hydrogen and 10 Helium a Because nucleosynthesis in the universe when it was young made the universe 75 Hydrogen and 25 Helium So stars don t form with any less Helium than that 4 If the universe has more dark matter than we think there is today how would that change the age we estimate from the expansion rate a This would mean that there is more mass in the universe that has been slowing its expansion So its expansion has been slowed down more which means it was expanding faster in the past and got to where it is now faster ie a younger universe
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