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ASTRON 1144 Final: OSU [ASTRON 1144] Stars & Galaxies Final Exam Study Guide

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Smita Mathur

Astro Final Exam Study Guide - 80 MC Questions Lecture 11/29/16 ● Olber’s Paradox ○ Why is the night sky dark? → The Universe is expanding… and We cannot see an infinite Universe. ● Interpretation of Hubble Law: ​Hubble’s Law demonstrates that the Universe is expanding in a systematic way​: Hubble Parameter→ Rate of expansion today. ● Nature of Expansion ○ General Expansion of Spacetime ■ All observers in different galaxies see the same expansion around them. ■ No center - all observers appear to be at the center. ○ What is the recession velocity? ■ NOT motions through space… ■ Expansion of spacetime: galaxies carried along. ● The Big Bang: If we run the clock back far enough, eventually the size of the Universe would be: zero size and therefore infinite density ○ Infinitely hot ○ This initial state must have existed at some finite time in the past. ○ We call this hot, dense initial state the “Big Bang” ■ ​In the beginning it must be very compact → The Big Bang Theory ■ • The Big Bang: the Universe is expanding from a hot, dense, initial state. ● As universe expands: recession velocities get larger; light waves get stretched and redder; “Cosmological redshift” of light ○ All galaxies (with very few exceptions) are receding from us. ○ Recession is quantified in terms of the “cosmological redshift” of the galaxy, z ■ Not a Doppler shift: measures expansion of spacetime, not motions through space. ○ Expansion of space also stretches light: wavelengths get stretched=longer=redder; bigger distances = bigger stretching ■ Result: The redshift of an object gets larger with increasing distance. ● Big Bang naturally explains the observed Cosmological redshifts. ● The Hot Big Bang ○ What we see now: the Universe is cold and low density; as it expands, it cools; matter (galaxies) gets further apart. ○ In the past: Universe was smaller, hotter, and denser ■ Is there any evidence of this early hot, dense phase in the past? ● Cosmic Background Radiation-•​ Prediction of the Big Bang: cosmic microwave background radiation at T ∼ 3 K ○ The Universe is filled with diffuse, “relic” blackbody radiation. ○ As the Universe expands further: Blackbody photons redshift; spectrum peak shifts to redder wavelengths, and hence cooler temperatures ■ Cosmological redshift: The stretching of wavelength of light as it travels through the expanding universe. ■ By today, the spectrum is redshifted down to T~3K ○ Discovery→ 1965: Penzias and Wilson (Bell Labs) ■ Mapping sky at microwave wavelengths ■ Found a faint microwave background noise. ■ First thought it was equipment problems; finally determined it was cosmic in origin ● Won the Nobel Prize in 1978 for discovering the Cosmic Background Radiation ● The Big Bang model makes very specific predictions: ○ The cosmic background radiation spectrum is a perfect blackbody ○ Characterized by a single temperature ○ 1965-1990: Experiments with balloons, rockets, & radio antennas show a rough blackbody spectrum; temperature ~2.7 K ● Cosmic Background Explorer Satellite (COBE); Launched in Nov. 1989 ○ Mapped the entire sky at Near-IR to microwave wavelengths ○ Searched for fluctuations in the background as evidence of early-large ● The COBE results confirm and greatly strengthen the Big Bang Model: The cosmic background radiation has (as predicted)... ○ Perfect bla
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