Unit 1 - Basic Concepts (Chapters 1 - 3)

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Earth Sciences
Earth Sciences 1086F/G

Introduction January-29-14 2:49 PM Additional Resources for Understanding • The Scientific Method • A Big Bang Theory discussion • Another page about the Big Bang Theory • The Nebular Hypothesis Temperature Scales - Celsius is used in more or less everydayenvironments - Kelvin tends to be more useful in very, very low temperature situations - Below is a figure that relates the three scales - Following is a figure developed by NASA http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=169 Chapter 1: Scientific Theory and the Big Bang January-29-14 2:57 PM The Scientific Method Hypothesis HYPOTHESIS:An educated guess based upon observation - Can be supported or rejected through experimentation or observation - Can be supported, cannot be proven to be true Theory THEORY: Summarizes a hypothesis (or group of hypotheses) that is supported by repeated testing and observation - A theory is considered valid as long as there is no firm evidence to dispute it - Theories attempt to explain the "why" of some action Law LAW: A law explains a body of observations - At the time it is made, no exceptions will have been found to that law - Scientific laws explain things, but they do not describe them - Laws explain the "how" of some action The Big Bang The Theory - An effort to explain exactly what happened at the very beginning of the universe - At the birth of the universe, time and space were created in gigantic expansion that emanated from a "singularity" SINGULARITY:An area in space-time where gravitational force is so high that all known laws of physics break down and do not apply - The gigantic explosion is difficult to explain as to have an explosion there has to already by space into which the explosion spreads → Think of it as an infinitesimally balloon, which in the tiniest fraction of time, suddenly expands and keeps on expanding → In this tiny instant, time and space had a finite beginning The Observations(The Evidence) - The Three Pillars of Proof: 1) Recession of stars/galaxies (as described by Hubble's Law) 2) The characteristics of cosmic microwave background radiation 3) The abundance of light elements Hubble's Law - Edwin Hubble: → Demonstrated that there were many galaxies in the universe — not just the one we are in (the Milky Way) → Proved that the universe is expanding → Showed us how to measure distances in space - Huge optical telescope installed into Earth's orbit is named after him, the Hubble Space Telescope Light'sRedshiftand Hubble'sLaw DOPPLEREFFECT: When an object coming toward you makes a sound, the sound waves are compressed by the motion of the noisy object and sounds differently to you than when the same sound waves are being carried off away from you - This is true for both sound and light waves - When a light source is: → Moving toward the observer, the light wavelength appears to shorten — to move into the blue spectrum or "blue-shifted" → Moving away from the observer, the light wavelength appears to lengthen — to move into the red spectrum or "red-shifted" - Hubble realized that the faster the light-emitting object was moving, the greater the shift - As the speed of light is fixed and cannot change, when Hubble observed apparent changes in speed of light (from a star), is meant that the stars had to be moving away from the Earth Unit 1 - Basic Concepts Page 3 moving away from the Earth - This applied to everything he could see, the whole universe had to be expanding, and with it the light waves moving though it - The more distant a galaxy is from us, the longer its light takes to arrive, thus the more "red-shifted" it appears when it finally arrives - Therefore, the amount of redshift can be used as a measure of a star or galaxy's distance from Earth - Hubble'sLaw: v: the speed expressed in kilometres per second d: the distance of the star/galaxy away from Earth in parsecs (1 parsec = 3.26 times the distance light travels in one year) H o the Hubble constant → The Hubble constant is the speed of the expansion of the universe, which Hubble assumed to be constant (this has turned out to be somewhat wrong) Cosmic Microwave Background Radiation - It is estimated that it was extremely hot in the first seconds of the universe and as it expanded, it cooled - The hot light photons, produced in the early period, have since lost energy and dropped from the visible light energy range into the microwave energy range — and that constitutes the cosmic microwave background (CMB) that we can still see today - Scientists figure that CMB can be seen from anywhere in the universe because it comes from all directions, and with nearly the same intensity - This CMB was first discovered as "noise" in a very sensitive microwave radio receiver - Above, the signal has been converted into temperature - The only explanation that makes any sense is that this CMB represents the very last remnants of the light/heat energy of the Big Bang's initial expansion - The general temperature of space should be 0 on the Kelvin scale (which is equal to -273° on the Celsius scale), while the actual average temperature is 2.726 K - The inhomogeneity amounts to temperature variation on the order of 1 degree in 100 000 degrees Abundance of Light Elements - The third pillar of proof has to do with the ratio of all the various atoms of the three lightest elements: → Hydrogen (75%) → Helium (25%) → Lithium (trace) - The observed abundance of all the different atoms of those elements can be explained only if they originated from one single ratio of the first subatomic particles of matter that can be formed from a super-hot environment - The only way to get that one critical ratio is through a unique event like a Big Bang Shape of the Universe - Knowing the shape of the universe will help answer the question of how the universe will end 1) A "closed" universe — positive curvature (a sphere) → Finite in size but without a boundary → Closed universes are also closed in time; they eventually stop expanding, and then contract in a "Big Crunch" → This model depends greatly upon there being sufficient matter in the universe that gravity can eventually pull things back together 2) An "open" universe — negative curvature (saddle-shaped) Unit 1 - Basic Concepts Page 4 2) An "open" universe — negative curvature (saddle-shaped) → Infinite and unbounded → Parallel lines eventually diverge → Open universes expand forever 3) A flat universe → Infinite in spatial extent and have no boundaries → Parallel lines are always parallel → Flat universes expand forever, but the expansion rate approaches zero - To all three models, a density parameter is critical → If space has a negative curvature, there is insufficient matter around to allow gravity to act and stop the expansion; the density parameter is less than 1 ( ) → If space has a positive curvature, there is more than enough matter around to allow gravity to pull everything back together ( ) → If space is exactly flat, we can say that there is exactly the "critical" value of matter around that will prevent the univer se from pulling back together or from expanding indefinitely to oblivion ( ) - So what is out there affecting gravity? There is conventional matter (stars, planets, asteroids, comets, etc.) which only acc ounts for less than 4% of the universe - The idea of dark matter has been hypothesized; it has never been seen because it gives off no (electromagnetic) energy, but we know it exists because we can detect its gravitational attraction to conventional matter - Scientists have recently detected that expansion is increasing, not decreasing - The force that seems to control the expansion of space is dark energy, which acts in opposition to gravity: it repels matter - When this force exactly counterbalances the kinetic energy of the Big Bang expansion, we are at the "critical" value of 1 for a density parameter (i.e. a flat universe) - As the expansion rate is apparently increasing, we seem to be accepting that the universe is almost perfectly flat — but has just the slightest negative curvature (*for the purpose of the course, "flat" will be accepted) Age of the Universe - There are several lines of investigation we can use to determine the age of things — even the universe Radioactivity - Certain elements have components that are radioactive — they breakdown (at fixed rates) to form other components and give off energy in the process - Scientists have found a star that could be radioactively dated at 13.2 billion years — therefore, the universe has to be older than that Hubble'sExpansionConstant - In any "rate" expression, there is a time factor - Therefore Hubble's equation can be used to determine the age of the most distant light sources we can find - White dwarf stars (remnants of stars that have consumed all their "fuel" and are sitting around cooling off) are prime candidates for dating because they have gone through the whole life cycle of a star - A cluster of white dwarfs were found and given dates between 12 and 13 billion years - Considering it would have taken something less than 1 billion years for the cosmos to cool sufficiently (from the Big Bang event) to form a star, the age of the universe had to fall between 13 and 14 billion years Cosmic MicrowaveBackgroundRadiation - Based on the best physics available, a sophisticated model of the universe (from the time represented by that CMB signal map back to the time when the Big Bang produced those first photons) has been produced - Assumingthe model is right, the universe is exactly 13.72 ± 0.12 billion years Unit 1 - Basic Concepts Page 5 Chapter 2: Time and Space January-29-14 5:02 PM Light Years LIGHT-YEAR: The distance that light travels in one year - Scientists use light-years to measure distances in space - Keep in mind that when you are viewing something in space, you are looking into the past Measuring Light Years - Different measurement techniques are needed for different distance ranges Up to 500 Light Years Distant TRIGONOMETRIC PARALLAX: When a nearby object is viewed against a more distant field, it appears to more slowly behind you as you pass it — this phenomenon relies on an object appearing to be at a different place relative to the background depending on your viewpoint - The same thing happens when we view a nearby star against a background of much more distant stars → For example, as Earth rotates around the Sun, the nearby star appears to "wobble" relative to the distant stars. Is we point a telescope at our nearby star in January, and then take another look in July, the telescope would have to be moved by some (tiny) angle. Once we have measured that angle, and knowing the distance that Earth has travelled in six months (2 astronomical units, or 300 million km), we can calculate the distance to the star 500 to 500 Million Light Years Distant 1) The first technique for stars in this range deals with the brightness of stars — astronomers use a chart called a Hertzprung-Russell Diagram relating the luminosity of stars (equivalent to brightness) to their temperature (related to color) → We draw a vertical line from that color-determined temperature to intersect our Main Sequence average best-fit line on the H-R diagram, and measure (using a horizontal line), on the brightness scale what the "true brightness" (called the "intrinsic" brightness) of that star must be → A star's brightness dims with distance, so the brightness you see from Earth (called the "apparent" brightness) is rather less than true → To get distance you use the following equation: → This method of determining distance from color is called main-sequence fitting, and it is good for distances up to about 150 000 light years away 2) The second technique for determining distance in this middle range makes use of "marker stars" which have a special property: they have a pulsing brightness that peaks with absolute regularity (its "period"), which is completely related to the star's brightness — these are called Cepheids → We find a Cepheid and carefully measure the time between one brightness peak and the next (we determine it's period), and this gives us the intrinsic brightness value (from a calibrated chart) → From there on, we just use the same procedure as for the first technique Unit 1 - Basic Concepts Page 6 → From there on, we just use the same procedure as for the first technique → Edwin Hubble was the first person to prove that the Milky Way Galaxy was not alone by using this Cepheid technique
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