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Introduction to Astronomy The Solar System

by: Marcelo Yundt Jr.

Introduction to Astronomy The Solar System ASTR 101

Marketplace > University of North Carolina - Chapel Hill > Astronomy > ASTR 101 > Introduction to Astronomy The Solar System
Marcelo Yundt Jr.
GPA 3.87


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Class Notes
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This 3 page Class Notes was uploaded by Marcelo Yundt Jr. on Sunday October 25, 2015. The Class Notes belongs to ASTR 101 at University of North Carolina - Chapel Hill taught by Staff in Fall. Since its upload, it has received 11 views. For similar materials see /class/228836/astr-101-university-of-north-carolina-chapel-hill in Astronomy at University of North Carolina - Chapel Hill.


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Date Created: 10/25/15
SUMMARY FOR DECEMBER 1 CLASS Topics Covered l The Early Universe 2 Future of the Expanding Universe Summary for Each Topic 1 We examined the questions of the origin of matter in the universe the origin of helium and other light ele ments and the fate of the expanding universe Regarding the origin of matter the early universe would have been so hot that it would have been ooded with particle antiparticle pairs being continually created then anni hilating This happens as pairs of suf ciently energetic photons can be converted into a patrticle antiparticle pair and vice versa particle antiparticle pair can annni hilate to form two high energy photons Once the uni verse cooled su iciently that for example typical photon energies were less than the equivalent energy in the mass of protons and their anti protons we d expect all protons to annihilate with anti protons leaving us with nothing So the early universe at rst rich in particles would leave us with essentially nothing but a cooling bath 7 of photons To avoid this unsatisfactory outcome there must be some fundamental asymmetry between particles and anti particles such that for every billion anti protons there is an extra proton so that we are left with some thing after almost everything annihilates This matter antimatter asymmetry is something that particle physi cists are struggling to explain Regarding helium and the light elements at the earliest hottest times in the universe it would have been impos sible to form stable atiomic nuclei given the immense density of energetic photons that would destroy them as soon as they were created Basically the universe had to cool suf ciently that stable nuclei could be created The rst step in H to He fusion is the production of deuterium which is a particularly fragile nucleus The temperature at which deuterium lasts long enough to get fused into He is low enough that it would no longer be possible to fuse He into C N 0 etc In other words a rm prediction of the Big Bang theory is that a fair bit of helium would be formed in the hot early phase around 23 by mass and also a tiny amount of other light elements like Li Be and B Nothing heavier than that can be made in the Big Bang thus all the heavy elements must be synthesized in stars It is truly en couraging that when astronomers look at the spectra of the oldest stars in the Milky Way ie the rst ones to form out of the original material from the Big Bang the abundance of He is exactly the 23 predicted by the Big Bang calculations and absorption lines of the heavy ele ments are essentially non existent in the spectra of those stars 2 We posed the question of how the expansion of the universe will continue to evolve Due to the attractive gravitational in uence of all of the matter in the uni verse you d expect that as the universe expands the expansion rate would gradually decrease In fact if the density of matter in the universe were sui ciently high you d expect the expansion to eventually slow down to a stop then the universe would recollapse back to an in nitely hot dense state once again To everyone s great surprise data on the history of the expansion rate of the universe instead shows that at present the expansion rate of the universe is actually increasing ie the expansion is accelerating The only way to make that happen is to postulate the existence of a form of dark energy that creates a negative pressure which opposes gravity and is behind the observed acceleration While such an energy eld could perhaps be a product of a highly energetic vacuum state at extremely early times in the universe there is no conventional physics that can account for it at such currently low energies Understanding the origin of this dark energy will undoubtedly be a central issue in physics for the foreseeable future


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