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Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•What is the evidence for the cosmological constant?•What is the Cosmic Microwave Background and how is it produced?•What produces small ripples in the CMB?•How does geometry change with the curvature of the Universe?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2VS.Expansion from IgnitionGravity GravityGravity slows the expansion.Expansion from Big BangGravity GravityAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Flat, “critical” !=1“Empty” !=0 Universe: No gravity ! no decelerationExpansion vs Time, if gravity is the only force operating after the Big BangMassive !>1 Universe: Expansion slows down the expansion! “Decelerates” Typical Separation Between Galaxies !“Decelerating Universe”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4“Cosmological Redshift”The redshift of a photon tells you the factor by which the Universe has expanded since the photon was emitted ! Size of Universe NowSize of Universe Then= 1 + zSo, at z=1, the typical distance between galaxies was 1/2 the present value!Note: This is true no matter what the value of H0 or !!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5Flat, “critical” !=1Increasing redshiftTypical Separation Between Galaxies ! ! Size of Universe NowSize of Universe Then= 1 + zz=1z=2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6Flat, “critical” !=1Increasing distanceTypical Separation Between Galaxies !Longer travel time = photons emitted from farther away = appears fainterAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7Flat, “critical” !=1Typical Separation Between Galaxies !Longer travel time = photons emitted from farther away = appears fainterTypical Separation Between Galaxies !In a denser universe, objects will be fainter at a given redshift.!=0 !>1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8Distant white dwarf supernova are far fainter than any model predicted!• Fainter ! Further than expected!• Further ! Universe is bigger than expected.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWX• Fainter ! Further than expected!• Further ! Universe is bigger than expected.The expansion of the Universe must be accelerating!!! Must be some other force pushing space apart!!!Typical Separation Between Galaxies !!>1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9Typical Separation Between Galaxies !!=0Typical Separation Between Galaxies !!=???Distant objects are farther away than any decelerating model can predict!Typical Separation Between Galaxies !!>1Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Typical Separation Between Galaxies !!=0Typical Separation Between Galaxies !!=???Farther? Then universe has to be much bigger than expected. Expansion must have been accelerating at some point!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11More complicated histories possible if there’s more physics beyond gravity vs the Big BangDeceleration AND AccelerationAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12A period of acceleration fixes the age problem as wellOld enough to make globular clustersAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13This “pushing apart” was something that Einstein had predicted (but then rejected)."0: “Cosmological Constant” or “Lambda” or “Einstein’s Biggest Blunder”(Einstein “invented” "0 to fight gravity and thus avoid the uncomfortable conclusion that the Universe was expanding)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14The acceleration only starts after the Universe has expanded a lot.• Early Universe is small, but has the same mass ! DENSE!• "0 only becomes important when the Universe is fluffy & mostly empty!• Must be a property of empty space!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15One possibility for the “Dark Energy” due to "0:Virtual particles pop in and out of existence, even in empty spaceThese virtual particles can exert PRESSURE and push space apart!(But really, we have no idea!)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16We can find out more about the “Dark Energy” by observing the very Early (young) Universe• Smaller!• Denser!• Hotter!• Different!…at higher redshiftsRun “The Universe, the Movie” backwards for ~15 billion years.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17When the Universe was smaller, photons were more energetic!•Hotter!•Energetic photons would have ionized all matter!The Past (high z) !Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18At a redshift of z#1300…•Universe was very smooth!•All the matter in the Universe already existed, but it wasn’t yet very lumpy. •No galaxies or stars!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19At a redshift of z#1300…•Sea of very hot, ionized gas and energetic photons•Photons are continually scattering off of free electrons.H nucleiHe nucleiunbound “free”electronsAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20At a redshift of z#1300…•Photons bounce around, interacting with electrons and nuclei, just like in the center of a star!Early Universe is Opaque!These photons have a hot thermal spectrum!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Early universe contains a sea of photons with a hot (>3000K) thermal spectrumThese photons will redshift with the expansionAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22Redshifting makes a thermal spectrum appear coolerAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23As the Universe expands, the photons all redshift to lower energiesStarts hereBut at slightly lower redshifts the photons wind up hereAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24But, a cooler thermal spectrum can’t keep atoms ionizedH nucleiHe nucleiunbound “free”electronsNeed photons with <100nm to ionize HydrogenAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW25Universe is Ionized!Universe is Neutral! Photons are too low energyAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW26Universe is Ionized!Universe is Neutral! Photons are too low energyPhotons can scatter


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