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Chapter 11.pdf - Part IV: Galaxies and the Universe Chapter...

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Part IV: Galaxies and theUniverse
Chapter 11CosmologyThe Big Bang and theFate of the Universe
11.1The Universe on the Largest Scales
This galaxy map shows the largest structure knownin the Universe, theSloan Great Wall. No structurelarger than 300 Mpc is seen.
Therefore, the Universe ishomogenous(any 300-Mpc-square block appears much like any other) on scalesgreater than about 300 Mpc.The Universe also appears to beisotropicthe same inall directions.Thecosmological principleincludes the assumptions ofisotropyandhomogeneity.
11.2The Expanding Universe
OlbersParadox:If the Universe ishomogeneous,isotropic, infinite,and unchanging, theentire sky should beas bright as thesurface of the Sun.
So, why is it dark at night?If the Universe is homogeneous and isotropicIt mustnot be infinite and/or unchanging.We have already found that galaxies are moving awayfrom us faster the farther away they are:recessional velocityH0distance
Hubble’s Law suggests that the galaxies aresomehow moving away from us.By Cosmological (Copernican) principle, this istrue for observers at any place in the UniverseHow can it be possible?Answer: The Universe is expanding!!RaisinBread.swfRaisin-Cake Model
cause_of_hubble_law.swfAlthough we see every galaxy moving away from us, itdoes not mean that we are at the center of theuniverse.Space itself is expanding: every galaxy sees every othergalaxy moving away from it.
Big BangHubble’s lawv=H0dsays that the further agalaxy is, the faster it is moving away from us.It implies that the Universe is expanding.Expansion implies a beginning.So the Universe has a finite age!
(The above assumes thatH0isindependent of time, which is nottrue, but it gives an approximatevalue of the age of the Universe)estimate_age_of_universe.swf00669dd110 pc3.26 10300,000km/ s 1yt14 10 yvH dH70km/ s70km/ sAge of the UniverseSince time = distance/velocity, a galaxy at adistancedfrom us must have been very closeat a previous time about
The Universe is expanding, and has a beginningDue to the finite speed of light, even if the Universe isitself infinite, the observable part is finiteThis solves Olbers’ paradox
So, where was the Big Bang?It was everywhere!No matter where in the Universewe are, we will measure thesame relation betweenrecessional velocity and distance,with the same Hubble constant.These concepts are hard tocomprehend, and not at allintuitive. A full descriptionrequires the very high-levelmathematics of general relativity.Balloon_model_galaxies.swf
11.3The Fate of the Cosmos
It can be shown by general relativity that, ifassuming thatthe only relevant force is gravitythe Universe is isotropic and homogeneousthen which way the Universe goes depends on itsdensityrand the spatial curvature.

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Term
Fall
Professor
NoProfessor
Tags
Big Bang, Physical cosmology, Cosmic microwave background radiation

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