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Midterm

Midterm Notes Astronomy 2022a.docx

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Department
Astronomy
Course
Astronomy 2022A/B
Professor
Chris Racknor
Semester
Fall

Description
Midterm Notes Astronomy 2022a Lecture 0 Creation Stories  Genesis- God created Heavens and Earth in seven days and nights. Earth was waste and void  Greco-Roman- beginning only Chaos, out of void Erebus…  Nordic-Germanic- In the beginning the was only Ginnungagap, a great void  Hopi- Creator Taiowa, no time, no shape and he created finite Sotuknang  Chinese- Pangu in egg until he finnaly broke it What is Science?  Concerted human effort to understand (or better) o History of natural world and how the natural world works. o With observable physical evidence as the basis of understanding o Done through observation, or experimentation under controlled conditions  The Scientific Method (Karl Popper “Science is a history of corrected mistakes”) o Observation o Induction o General principle o Deduction o Prediction o Experiments o Revision Science and the Universe  Supreme being cannot be examined, thus science is powerless to answer question o However doesn’t mean one doesn’t exist  Philosophy helps fill that void Lecture 1 the Universe: Early Thoughts Rough numbers  The symbol ~ means roughly equal to o Ex. 24+32 ~10^2 What is the Universe or Cosmos?  All space, matter and energy that is “accessible” to us o Cosmology being the study of the universe  Cosmogeny/genesis: birth and start  Cosmogony: infancy  Eschatology: senility or last days  Cosmothanatos: death end Flat Earth  Was a common assumption across cultures o Such as in Egyptian cosmology which was similar to Mesopatmaian o Nut (Sky), Shu (Air), Geb (Earth), Nun (Water), Naunet (Abyss)  All in a flat line  However the Greek Aristotle was most famous to advocate a spherical earth o Eratosthenes (240BC) measured Earth’s diameter within 2%  Shadows in columns in Egypt disappeared, meaning sun was dirtectly overhead  Thus because this didn’t happen in Alexandria, deducted that the earth was spherical  Deducted the sun was far away and its rays would shine parallel on the Earth  Thus the shadows at Alexandria would be titled in respect to Syene, Egypt  Was not enough to prove earth was a sphere, because of the assumption that the sun was far away, Earth could still be flat if sun was close Round Earth  Hull-down effect o A ship a few km off-shore will have its lower part to appear below the horizon  Was well known to ancient sailors  Most Greeks believed Aristotle o But he incorrectly believed that the Earth was the centre of the Universe  Because if the Earth moved, there would be a howling wind  (all stars seem to spin on asingle sphere which we some from the insde?)  It is because planets(wanderer) were points of light in the sky that moved relative to the stars  Thus Aristotle’s universe required one sphere per planet, to explain what was being seen Planets  The naked eye can see 5 planets: Mercury, Venus, Mars, Jupiter and Saturn o The constellation the sun rises or sets in on your birthday determines your astrological sign o The moon’s motions against the stars is fastest and easiest to see  Moves its own width every hour  Over days and weeks the planets can be seen relative to the stars o Their motion is confined to a band of twelve constellations, the Zodiac Ptolemy (85-165 AD)  Claudius Ptolemaus, believed Aristotle geocentric model, but improved it  Accounted for the fact that planets sometimes appear to backwards o Retrograde motion  Some people tried to explain it by sudden stops and reverses but didn’t seem likely  Suggested that planets move in small circles (epicycles) while it orbited the earth  Was widely believed for 1500 years o Was clunky and problematic  Only gave good predictions for a few years or decades  Thus predictions had many errors by circa 1400 AD o King Alfonso X “God should of recommended something simpler” Copernican Theory  Polish monk, proposed a heliocentric model o Pros  Was simpler, and explained why the inner planets never travel far from the Sun  Reproduced much better the observed changes in brightness of planets  Natural reason for the seasons  No epicycles for retrograde motion o Cons  Could not predict the positions of planets any better (unless you added epicycles”  Did not explain why no wind was head  New the earth would have to go at least 100,000 per hour  Was no longer philosophically in the center  Ugly things were on the ground the center, where as great things were in the air o But now the earth was circling and in the realm of good  Because we should sit on the bottom of universe The fall of the Ptolemaic system  Tycho Brahe saw supernova in 1572 o Undermined the idea that the stars were perfect ad unchanging in the Ptolemaic systems  Galileo made a spyglass in 1609 o Found 4 moons circling Jupiter  Again undermining the idea that things orbited because they avoided the Earth, why would moons be orbiting the pure Jupiter o Ended up in hot water because of his advocacy for heliocentric models o The church was strongly opposed to this because  First university was founded to teach church law  The church kept much of Greek teaching and early scientists were largely church men  Vatican observatories date back to 1582 o However Copernicus (heliocentric Godfather) was a monk and his book of theory was dedicated to Pope Paul III  So why did Galileo really get in trouble?  Galileo had already met Pope Urban VIII, and was till permitted hypothetical discussion of the heliocentric theory, as long as it was stated to be true o Even had papers with the crest of the Pope  Was also personally given permsiion by the new pope to discuss the heliocentric theory  But Galileo published Dialogues Concerning Two Chied World Systems o Conversation between Salviati, Sagredo and Simplicio  Simplicio is made out to be a fool to defend geocentric  Using Barberini (Pope Urban) argument o Thus was placed under house arrest and recant his views Fall of Ptolemaic System  Johannes Kepler o From Germany and improved on Copernicus’s theory by introducing slightly eccentric ellipses instead of perfect circles o Reproduced observation of the planets with accuracy o Was a blow to geocentric because required ellipses to be around the sun  Isaac Newton introduced the concept of all bodies exerting gravity proportional to their mass… o Predicted elliptical orbits that matched well with Kepler o His laws of motion explained how Earth can move with no wind. o Thus heliocentric wins. What of the Stars  Are stars bodies of ordinary matter to? o Seemed unlikely because telescope revealed features on planets but just brightness for stars o Since earth moved around sun, stars should undergo a motion called parallax Parallax  A surveryor’s method of measuring distance o Because stars appear slightly shifted different positions of Earths orbit we can determine their distance o GET THE SLIDE shows better o The farther away the star (d=distance in parsecs) the smaller the parallax angle p  Was only way of measuring for early scientists o As early as 1672, Cassini for distance of Mars o However stars were hard to determine, thus meaning that they w  Parallax is measured in arcseconds o 1 arcsecond of movement= one parsec away  1 PARSEC equals 3.26 lighyears  1 year of light travel = 9 x 10^12 km  A circle as 360 degrees  1 degree=60 minutes of arc thus an arcminute  1 arcminute= 60 seconds of arc thus an arcsecond  1 arcsecond= 1/1296000 of a circle, thus really small  d=1/p Stellar Parallax  1838 by Fredrich Bessel revealing star 61 Cygni, putting it 3.15 parsecs or 10.3 lightyears o Was a parallax of 0.0000872 degrees o Was later revealed to one of our closest stars Distribution of stars  The initial assumption was stars were uniformly spread o Later became clear that the Milky way was actually many stars, that were somewhat clustered together  However telescopes reveals more and more progressively fainter stars o That there does seem to be an edge to the group of stars that surround us  We call that group the Milky Way  Greek mythology, Hera’s milk hence Milky Way o “galaxias” which is derived from the world milk: gala (galaktos)  Galaxy is now used to describe a massive collection of stars Milky Way Structure  Hard to determine because: we are in it, distance measurements are not easy to make, our view is often obscured by gas and dustclouds  Too figure out where we were in the MW Herschel counted star in all directions o Found that MW is flat, and at some point (Kapetyn) found that the numbers of stars is roughly the same at in both directions o Look at diagrams pg.59 Counting Global Clusters  Harlow Shapley studied Global Clusters and found that the sun was not the center of the galaxy o Since GC’s are massive and fairly far away they should orbit the center of our galaxy  But found that GC’s were not centered around our Sun Is our Milky Galaxy alone?  Debate centered on faint cloud-like objects called nebulae o Distant star clusters vs small nearby objects o Came in a variety of shapes and colours o Diagram at pg.63 o The moon and Andromeda scale the same when they are looked at in the sky The nature of the Spiral Nebulae  Couldn’t see individual stars so looks like fuzzy blobs with spiral shape o Thus couldn’t measure distance o Were they close or far?  Shapley-Curtis debate o Shapley argued that nebulae were nearby small objects while Curtis believed they were distant star clusters o The debate was solved by new large telescope  Hubble observed the nebula M31(Andromeda) and realized they were are individual stars  However it was later discovered that some nebulae were nearby gas clouds Expansion of Universe Page Lecture 2: the expansion of the Universe What is theory?  ….write later What is the Universe or Cosmos?  Space-time: a vast space which seems to have existed for a long time  Also includes the contents of this space o And to date everything in this space is made of either matter or energy  E=mc^2 describes how matter can turn into energy and vice-versa  A vast space-time with matter-energy o Matter refers to material objects, solids, liquids and gases…composed of atoms o Energy includes a vast array of forms, light, heat, sound, motion and others Einstein and Space-Time  Special Relativity in 1905 o Time and space do not stand apart  General Relativity in 1916 o Objects in space influence space itself Curved Space  GR says the space responds to the presence of mass by curving, the more massive the object the more curved space is o This curvature is what causes gravity  Ex. The moon simply rolls around the bowl of curved space created by the planets mass  Curved space does have some visible consequences o Abandons the idea of flat static space ad gravity being magical  Look at pg.13 14 Conservation of mass-energy  E=mc^2  Through experiments it appears that the total amount of matter-energy in the Universe is constant o None gets lost or created  Hydrogen fusion o Hydrogen one proton and helium 2 protons and 2 neutrons  The sun takes 4 hydrogen atoms and makes a helium atom through complex fusion o Makes more energy because helium has a smaller nucleus than 4 hydroen, thus that mass has been converted into energy  This energy is made in gamma rays and becomes sunlight Matter and Space  “space tells matter how to move. Matter tells space how to curve” Alan Guth  Einstein made a mathematical equation showing their relationship and thus summarizes properties of the whole universe, that can technically calculate the origin, current state and fate o Get Equation on pg.19 o However the stress-energy tensor describes every bit of matter and energy in the universe and an impossible task  We could make a good approximation for the tensor o For example, if we assume the universe is roughly uniform with matter and energy, we can determine if universe is expanding or shrinking  Thus changeable and changing  If the universe was infinite, unchanging and the same everywhere, then the stars would cover the night sky. Universe Change  We can’t tell by watching the distant galaxies cause it would take too long  Before Hubble looked at Nebulae o Slipher had already measured the spectra of some of them and found they exhibit  A Doppler shift  And a fairly systematically as shift toward the longer wavelengths or a redshift? o Hubble(used to be a lawyer) then measured the distances and redshifts for galaxies and found  That the more distant the galaxy, the greater is redshift and the faster it moves away from us Lecture 3: Black Holes Motion  An effect that fights against gravity (could be the initial explosion of the universe, or just ordinary motion) Heavy-weight fight  Gravity get’s stronger the more mass there is in an object, and the more closely it’s packed o The moon has about the same density as the earth, but less mass Escape Velocity  On earth, escape velocity is 11.3km/s, the minimum speed to escape Earth’s gravity o Would increase of Earth was more massive and had more gravity Gravity Well  It takes the same amount of energy to launch something on an escape trajectory away from Earth as it would to launch it, 6000 km upward under constant 9.81m/s earth’s gravity, thus Earth’s well is 6000 km deep  Pg. 7 Einstein strikes again…  Special Relativity states that no matter or energy in the Universe can go faster than the speed of light o Thus “motion” has a inherent speed limit, while gravity has no similar limit o Thus for very heavy objects, gravity will always be able to overcome the fastest motion and all other phenomena  Black hole: escape velocity is higher than the speed of light, therefore nothing can escape the blackhole o Since nothing can go faster than c o Black since no light can escape it, making the compact object invisible o Hole because it is like a hole in the universe, if you enter it you leave the region of the Universe that is observable and never return  Misconception o Not an interstellar vaccum o Does not “suck in” matter  Is just a much more compact form of the same amount of mass  If Sun turned into a black hole, the force of gravity on earth would be exactly the same, thus no change in orbit  An object would have to be 10km close to the black hole center before they began to spiral in Event Horizon  The boundary between the “inside” of a black hole and universe outside o The point at which the escape velocity=the speed of light  Outside of the horizon, light can get away (no sucking in) o But nothing that passes within this boundary can ever escape, thus we can have no hope of knowledge past it Schwarzschild radius (k)  The radius of the event horizon o For a black hole the mass of the Sun, this radius is about 3 km o Thus if you crushed the Sun’s mass into a 3 km radius sphere it become a black hole  More massive black holes have larger radii; o 10 M sunhas radius of 30 km  My Schwarschild radius at 95 kg pg. 17 o 1.41x10 m5 At the Event Horizon  If you could get close to the event horizon 1. Gravity would increase greatly in even a short span of space  Different Gravity at your head then your feet (Tidal forces)  Object descending into would be stretched vertically and squeezed horizontally o May occur before or after passing of EH, depending on mass of BH 2. Gravity is so great, it would stretch light  All wavelengths of emissions from near outside the EH are stretched (redshifted)  Thus blue light emitted near the EH would appear redder to distant observers 3. Gravity will slow down time  Slower time closer you are to black hole  At the event horizon, time will LOOK like it stops to people observing  Time dilation- pg. 23  Clocks closer to the BH seem to run more slowly to an outside observers  Space-wise, nothing really special is located at the event horizon (no major events) Black-Hole Properties  When a stellar core shrinks smaller than the k radius, it becomes a black hole: the core disappears within it’s own event horizon  3 measurable properties: mass, charge and angular momentum  Black hole is in 3 dimensions, not 2-D Limits of knowledge  All we can say is if nothing stops the crush of gravity, then all matter must be crushed in an infininetly tiny and dense point in the center called the singularity o This is the limit of our present-day knowledge: GR and Quatum physics offer different predictions about the nature of singularity What happens if my friend falls into a blackhole?  Would appear different to the falling a friend and the outside observer  Outside observer o Light from falling friend becomes more redshifted o Appears to fall more slowly, and any motions they make will appear slower closer to the EH o Would never see the friend cross the EH  Friend o Will reach and cross the EH in finite time without any ill effects  But as one falls, tidal forces will increase. For smaller BH’s, the astronaut will be stretched to spaghetti before reaching the EH, where as bigger BH’s the astronaut may pass the EH before those forces become appreciable  Thus you could fall into one, and not realize it for awhile o Nothing special happens while crossing, but after you will have a short time before your are stretched and crushed upon reaching the singularity  Stars would look distorted pg. 26 Cosmic censorship principle  Singularity: a point at which space-time diverges o Infinite forces are acting while laws of physics break down o Turns out this is not a problem as long as a singularity is shielded from view from the outside world by an EH  Hypothesis: every singularity is surrounded by an EH, thus no naked singularities Orbiting a blackhole  In orbit at 2 Schwarszchild radii, the images of various nearby stars are tremendously distorted (by the gravity of the blackhole) o This is due to the curvature of space produced by this very massive object No-hair theorem (John Wheeler)  Properties of a blackhole 1. Has mass 2. May have electric charge 3. May have spin (angular momentum)  Simple thus blackholes have no hair? o A blackhole made of one thing is indistinguishable from one made of anything else o Information tossed into a BH is lost (information is closely related to the concempt of entropy, so this is a puzzle for scientist Entropy o Essentially disorder nd o 2 law of thermodynamics says the total amount of entropy can never decrease, only increase o Any individual region of the Universe might see a decrease, but as a whole keep increasing entropy o Ex. If a house is left on its own fill to fall to pieces it’s entropy grows o If you repair it, you can decrease the amount of entropy, however the increased entropy in the waste, materials, garbage, energy used. Thus always exceeds decrease Black Holes and entropy o If black hole really has “no hair”, then it doesn’t have any entropy o Thus you can drop something into the black hole and decrease the amount of entropy in the Universe  Violating the 2 law of thermodynamics o Jacob Bekenstein proposed correctly that black holes do have entropy, and bigger ones have more o Black holes only get bigger and never smaller, thus their entropy always increases  But where does the singularity keep all the disorder? o Entropy and temperature  Linked when temperature refers to intrinsic motions of atoms and molecules, which is a big component of their intrinsic disorder (difference between a snowflake and waterdrop) Hawking and Blackhole entropy  Opposed to idea of blackholes having entropy o Because anything having entropy must have temperature and must emit atleast some heat radiation, when blackholes don’t emit anything  But when calculating he found that blackholes, can in fact “emit” radiation and particles Space and virtual particles  Empty space may occasionally “sprout” virtual particle-antiparticle pairs which spontaneously appear and quickly disappear o Always appear of pairs of matter and anti-matter, which annihilate each other on contact, causing them to disappear Hawkings Radiation  Hawking realized that near the EH, one particle could fall into the BH while the other escapes o Thus a BH could spontaneously “emit” particles and radiation  Energy of escaped particle actually comes from mass energy of BH, so mass of BH must decrease o Thus black holes will eventually evaporate, because of these emissions  Black hole radiation very similar to heat radiation of normal matter o Temperature of BH extremely low, near absolute zero, thus evaporate very slowly  0 Kelvin= -273.15 C, lowest possible temperature  Smaller BH’s are hotter and evaporate more quickly 65 o Evaporation time of BH of Sun’s mass about 10 years 2 (3?) main types of BH  Stellar mass black holes- created by the collapse of a massive star at the end of it’s life o 30-100 M solar 30  1 M SOLAR mass of our Sun= 2 x 10 kg  Supermassive black holes (SMBH)- found in the centers of the galaxies, quasars o A few times 10 10 M solar  Extreme blackholes- extremely small BHs, could elementary particles (ex. Electrons, quarks) Black Hole Recipe  Get a big Iron Core (naked white dwarf) o Can’t fuse for energy  Gravity of the core will make it collapse o No mixing required  Gravity is too strong for electron degeneracy o Neutron Star  Keeps collapsing, gravity is too strong ^^^ o Black Hole Stellar Black Holes  Material from a companion star may get pulled into a BH, but will typically spiral in, resulting in o Accretion disk  That is very hot and bright (observable) Super Massive Black Holes  1 million+ the mass of our Sun  They also form accretion disks, as can their effect on nearby stars falling into them o Called Active Galactic Nuclei (AGN)  Bright (hence active) and appear at the centre of galaxies o These accretion disks often form, extremely bright objects called quasars, among the brightest objects in the Universe o They also form extremely powerful jets that shoot out their poles, releasing material from the disk into space before it plummets into the BH pg. 44 Exotica  White holes- a phenomenon analogous to a BH from which only light can escape o However there is no obvious way to make or power one  Wormholes- conduits between two points in spacetime o Curved space into a short tunnel that links two widely separated regions o Are allowed in Einstein’s theory, but syas they are unstable and require tricky engineering Lecture 4: Origin of Matter Are they basic ingredients that make up all matter?  Yes  People once thought Earth, wind, fire and water were basic elements that made up everythin o Wood (earth, fire, and air) o Greek Empedocles was first associated with these idea (450-500BC) Would you eventually get pieces so small they were no longer gold?  Yes, smallest piece of elements is the atom o If the material in question isn’t an element, the smallest piece is a molecule, which can be divided into atoms  Greek “a-tomos”- indivisible o Leucippus of Miletus (420 BC), more commonly associated with his student Democritus  Not a scientific fact until 20 century o We now know atoms can be “split” The atom consists of:  Atomic nucleus- protons and neutrons, almost all the mass, both have same mass (Neutrons have slightly higher mass than proton)  Cloud of electrons surrounding it, almost all the space (1/1800 of proton) o But 100, 000 times bigger than the nucleus o Electron arrangement governs how atoms will interact with each other Atoms and molecules  Atoms normally have one electron for every proton o However quantum mechanics creates an effect where atoms have additional tendency to prefer specific number of electrons around them if they can o As a result atoms will sometimes share electrons, by sharing bonds and this result in compounds or molecules. Creating a variety of substances  Atoms don’t pass through each other because, electrons of opposite charge repel each other making objects seem solid o However if to push on them hard enough to could overcome this, but needs tremendous force (BH) Atomic Density  A teaspoon full of ordinary atoms weights several grams, but a teaspoon with material as dence as the matter in the nucleus would weight 2 billion tonnes Different Kinds of Atoms  The kind of element/atom depends on the atomic number o Number of protons in the atom  Two atoms with the same # of protons are the same element o A different number of neutrons would result in different isotopes and a different atomic weight  Isotopes are usually named with their atomic weight ex Carbon-12 VS Carbon-14 What’s matter made of?  Electrons are irreducible, but protons and neutrons are each made of 3 smaller particles called “quarks” o Protons  2 up quarks, 1 down quark o Neutrons  1 up quark, 2 down quakrs o An up quark has a charge of +2/3e, down quark -1/3e  This results in the neutral and positive charges o We don’t yet know if quarks are made of anything  Nuclei don’t fall apart because these quarks have an attractive force called the “strong nuclear force” o Three charges/colours red, green and blue  These different colours attract each other and bind adjacent prtons and neutrons into the atomic nuclei Four fundamental forces of nature 1. Gravity o Weakest force, but longest range 2. Electromagnetism o Strong, long range force 3. Strong Nuclear Force o Strongest, but only short range 4. Weak nuclear force o Involved in some kinds of radioactivity, but very short range Electromagnetism  Lights, chemistry and biology all use electromagnetic interactions between electrons o We are all electromagnetic beings Light and matter interact through 1. Reflection a. Specular reflection- smooth surface, like a mirror b. Diffuse reflection- rough, thus scattering reflection i. Most objects of everyday life reflect this way (like a movie screen) ii. This is how we see some Solar objects, like planets and moons but not the Sun 2. Transmission a. Through a medium, light may be bent in the process, creating a refraction (distorted image) i. Lenses, water 3. Absorption a. Opaque- some light is reflected b. Translucent- some light is transmitted 4. Emission  Solid or liquid objects will interact with a combination of reflection, transmission and absorption o Gas objects will interact primarily through absorption and emission What is light?  Pg. 35, fucked up  Waves- a pattern of motion that can carry energy without carrying matter along with it o Properties include: wavelength, frequency, speed, energy, height of waves, etc  Light:Electromagnetic o A light wave is a vibration of electric and magnetic fields  Interacts with charged particles through these fields o In normal matter, light interacts with the outer electron first  It is the properties of the atoms of any object that determine how light interacts with it Light photon quality  1. All photons travel at the speed of light c- 300 000 km/s  2. Any 2 photons of same frequency, or wavelength, or energy, or colour are indistinguishable o One colour, has the same of all of these things Back to matter, Atomic transitions  Electrons orbits are restricted to very specific radii by quantum mechanics, different for each element  Atoms in a gas, photons and atoms can only interact by absorption, transmission or emission o Absorption, an electron is kicked into a higher orbit when it absorbs a photon of light of a specific energy  These specific energies correspond to the energy differences between electron orbits o All other photons that don’t read the specific energy, are unabsorbed and transmitted unaltered o As a result a gas will let most light through, but will absorb some specific light and leave absorption lines  These are the source of spectral lines, used by Hubble to determine velocity of galaxies  An atom in an excited state, can return to its ground state by emitting photons o These emitted photons must have energies that correspond exactly to some transition between electron orbits, which results in red-shift? o The new photons get emitted in random direction pg. 44-45 Matter and energy  2 primary materials in the Universe: matter (atoms) and energy (photons)  Conservation of matter-energy: Einstein’s E=MC2 saying that total amount of matter-energy doesn’t change Big Bang Theory  Georges Lemaitre, Belgium father of the big bang, even though he didn’t develop much o Universe began as a primeval atom L’atom primitif  All matter must have been at the same single point  Using Einstein’s General Relativity equations result in a Universe that is infinitely dense and infinitely hot in a finite amount of time o This point, dubbed the Big Bang  Entire Universe started from an incredibly tiny, hot, dense collection of matter and radiation that evolved following tested laws of physics o Does not explain why the Universe came into existence, but how it evolved after that George Gamov (1904-1968)  Concluded the early Universe must have been very dense and hot o Because the Universe is expanding, and matter-energy is conserved  Important predictions: o Should be radiation (light) still present in the Universe that dates back to this early time o During the early phases, atomic nuclei would have been changed by the intense conditions. Proportions of diff-27nt atoms we see today, should reflect the conditions that existed in the Big Bang  Single point- 10 seconds, about the size of an orange After the Big Bang  Since then the Universe has cooled and expanded, its contents undergone different stages as their temperature dropped, some has transformed energy to matter and vice-versa What is temperature?  Temperature is measured by how fast the particles are moving History of the Universe  Pg.63  Early history o 1.At the earliest times, temperatures only allowed subatomic particles to survive (quarks and electrons), and gamma rays (high energy photons), changing from one form to the other o 2. Collisions are too violent for protons and neutrons to exist until after 1 second  Then these form but still too hot for atomic nuclei to form o 3. 3-30 minutes, nucleosynthesis, atomic nuclei form  Hot enough to overcome proton repulsion, cool enough the impact doesn’t break the atom  Still too hot for electrons to stay in orbit o Most matter formed during this phase, particles stick together 1 by 1, so more light elements and fewer heavier ones  Almost no elements heavier than helium are produced because the nuclei would break apart before more baryons can be added o Gamov predicts we end up with 75% of total mass in hydrogen-1, 24% in helium-4, then heavy hydrogen and lithium-7  When we look at the universe this almost exactly what we see since Stars and gas clouds are mostly composed of hydrogen and helium Stellar nucleosynthesis  The small fraction of heavy elements (anything heavier than He-4) about 1%, can be explained by processes at work in stars long after the Big Bang o The fusion processes in stars are sufficient to produce all the heavy elements we see today o Only released in the death throes of the most massive stars, explode and supernovae and spread their elements Recombination (380 000 years and 3000K)  Protons and electrons recombine to form atoms => universe becomes mostly transparent o Redshift z=1000?  Before this point, matter and radiation were in equilibrium o That radiation would thus be thermal radiation, with a blackbody spectrum at the temperature of the Universe  This is because ions and electrons can absorb or scatter photons of sorts of energies and redistribute energies  Now that electrons and nuclei combine, atoms only absorb photons of specific energies o Meaning radiation can freely stream through the Universe resulting in  Transparent Universe  Matter and radiation are now decoupled o Radiation still has some blackbody spectrum, but it redshifts as the universe expands  Until this point radiation pushed matter around, preventy gravity from making it clump Cosmic Microwave Background Radiation  Most photons that were around the time when atoms formed should still be flying around the Universe and detectable at a cooled long-wavelength microwave region  When atoms first came into being, the universe was about 3000 K, thus light emitted was visible o Photosphere (surface) of the Sun is 5800 K  The universe is now 1000 times larger, meaning this radiation is also 1000x stretched o Visible->microwave lengths  Wavelength: ~micron->~0.1 cm  Discovered in 1960’s, radiation had redshifted (cooled) to a temperature of 2.73 K as the Universe expanded  The CBR is very distant and comes from a redshift z=1100, about 300 000 years after the bgi bang o About 1% of the noise you see when you tune your TV to a non-existent channel is cosmic microwave background  1990’s Cosmic Background Explorer was sent out to test these predictions o And the observed spectrum of cosmic microwave background exactly matches the theoretical model General acceptance of the Big Bang Model  Until mid 60’s very controversial, but after little doubt on validity  4 pillars o Hubbles Law o CMBR o The origin of elements o Structure formation in the universe  Pg. 84-85 Lecture 5- Riddle of Dark Matter  Ordinary matter is composed of protons, neutrons and electrons and is called baryonic matter o Electron isn’t a baryon but only makes up 0.05% Big Bang Nucleosynthesis  Takes what we know of particle physics to predict the total mass of ordinary matter  The abundance of heavy hydrogen (deuterium), helium-3 and lithium can be measured from absorption lines in the spectra of distant stars o The composition of stars should give us a good idea of the overall composition of the Universe, since the Gas that stars are composed of are all from the Big Bang  Based on these absorption lines, BBN predicts the density of baryonic matter should be 4% of critical density  b0.04 o However, when adding up all matter we can, we only come to 1% of critical density  Thus 3% is too dark to see  Difference probably means that 75% of baryons do not shine  Nebulae of gas and dust  Stellar remnants and failed stars Dark Matter  Refers to any matter too faint to see through telescopes, whether that be baryonic or something exotic Detection  One way to detect, see if absorbed or blocked out light from background objects o Dark nebulae- clouds of small smoke-like particles called dust o However the survey of baryonic matter already includes such Detection through Gravity  Spiral galaxies, like are Milky Way are pancakes of stars orbiting the galaxy’s center nucleus  Edge of the spirals allow us to observe the velocity at which the stars orbit the galaxy o Rotation curve- plot of rotational velocity VS the distance from the center of the galaxy o Rotating toward us should be blueshifted, where way is red shifted  1. Weighing the Galaxies o Based on rotation curves and gravity, we can estimate the mass of galaxy, including all matter rd o Newton’s version of Kepler 3 law tells us how much mass is required within an orbit to result in a given velocity  Ex. In the solar system all mass in concentrated on the Sun, orbital speed of planets decrease the farther we go out o However in the Milky Way we do not see this, we find that there is 5-10x more mass then expected after adding all the visible mass in a glaxy  Galaxies are spinning too fast, unless there are 5-10x more mass in them  So must be more mass farther out in the MW then we can see o Milky Way Mass  Total visible- 200 billion solar masses  Total deuced from rotation curve- 1 trillion solar masses  Only 20% we can see o Using Doppler shifts we can do that same for other spiral galaxies, and they also show flat curves o Thus DM exists, but also most DM is distributed spherically, ex the halo of galaxies  Typically 10x more DM than there are stars o Different techniques for elliptical galaxies also show the 10x more o BBN has much more matter than predicted, but still does not include non-baryonic matter  2. Looking for DM in galaxy clusters o Cluster- a group of galaxies that orbit each other o By measuring their velocities with respect to each other, we can determine how much mass is holding them together o Fritz Zwicky (1898-1974), did this and concluded 5-10x more matter then expected but no one believed him  3. Hot Gas in Galaxy Clusters o Tend to contain hot gas which is visible in X-rays  This gas contains a lot of mass, up to 5-10x more mass in this gas than in all the stars in the cluster’s galaxies o Are identifiable by blue o Gravity compresses the intracluster gas and the gas is heated up  So from the temperature we can estimate the mass of an entire cluster o This led to the conclusion of up to 50x as much mass in DM as there are stars  4. Gravitational lensing o The bending of light and distortion of image, because the mass of very large objects (like galaxies and clusters) can deflect light passing nearby it  Space near it is curved o Means some galaxies, are just different images of the same thing o Einstein’s GR reveals mass can bend lights rays, and the more mass the more bending causing ghost images All DM Searches  Produce the same result of about  = 0.3 (all types of matter) m o 30x more than counting visible things, and 8x more than BBN predicts o If BBN is correct, most of this matter is not ordinary, and this difference is called the DM Problem The Dark Matter Problem  Pg. 45 Nature of Dark Matter  If DM IS ordinary matter it’s mass would come from baryons o The density of baryons in the BigBang leaves a unique imprint in the abundances of deuterium, certain isotopes of helium and lithium, which we can measure accurately by looking o Measurements of the abundance of the four different atoms most sensitive to baryon density during the initial stages of the Big Bang, are all consistent with the value o0 0.04   Evidence that BBN is correct, thus most astronomers conclude that the Universe is full of some unknown invisible matter  A clear prediction that eventually could be tested observationally o Leverrier, proposed Neptune, and that was dark matter at the time  Interestingly there is no evidence of DM, in our solar system MOND (Modified Newtonian Dynamics)  Most test of gravity have been within our Solar System, Over these distances theories of gravity seem to work very well  However a minority of astronomers believe that gravity may act different in different situations  MOND proposes that Newton/Einstein gravity holds for strong gravity but is different for weak gravity (about 10 of the Earth) o But proposing a different gravity at large distances doesn’t work  But galaxies and galaxy clusters orbit very slowly and are actually very weakly held together by gravity MACHOS (Massive Compact Halo Objects)  Brown dwarfs (stars not massive enough to shine), dim white dwarfs (relics of stars like the Sun), massive black holes  Are basically unseen baryonic matter, but if DM are MACHOS, BBN must be wrong  We can see MACHOS through gravitational microlensing o If a MACHO passes in front of a more distant star, under special conditions, it can focus the light from that background star and cause it to seem brighter  Doesn’t make sense because of eclipse complaint, but turns out the dimming is outweighted by the microlensing o Takes about 10 million years for a MACHO to fly in front of a star, thus we monitor 10 million stars for one year  Then we can compute it’s path and mass  There is only a small number of MACHO’s detection made, imply they exist but too uncommon to account for dark matter  Atleast between the Large Magellanic Cloud and the Milky Way WIMPS (Weakly Interacting Massive Particles)  More exotic, doesn’t consist of baryons or electrons but maybe: o Neutrinos (known to exist) o Axions, neutralinos, etc (hypothetical)  Ex. Bullet Cluster pg. 63 o From lensing: most of the mass in the blue regions, ex distribution is unaltered by collsion o But the gas distribution clearly is different  So must unseen mass must be in a fundamentally different form than gas particles  Candidate 1: massive neutrinos o Neutrinos (v) are produced in some radioactive decays, in the fusion process of the Sun, and in other exotic objects like supernovae o They have very low mass, would take about 5 million neutrinos to weigh as much as a single electron  May travel faster than light, but not know o The elusive neutrino  The sun produces over two hundred trillion trillion trillion neutrinos every second, a supernovae can unleash 1000x more than our Sun  About 65-billion neutrinos stream through ever square cm on the Earth every second, yet we’re oblivious  Because they don’t interact much with atoms, only a 1/trillion chance od o Catching neutrinos  We still expect about 7 billion neutrino interactions throughout Earth every second  This means every cubic meter only sees an interaction every 3700 years  These interactions can sometimes change the nature of atoms  Most reactions produce a miniscule burst of blue light (Cerenkov light) o Sudbury Neutrino Observatory  Has 9600 photon detectors, surrounding 12 meter diamer plastic vessel with ultra pure heavy water  Is located 2000 m, down INCO’s Creighton mine to protect cosmic rays and other radioactivity o Are Neutrinos the missing dark matter?  ~100 neutrinos per cm squared  Have mass but less than 0.3 eV 
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