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Lecture 17 : the Early Universe I Lecture 17 : The Early Universe I ªCosmic radiation and matter densities ªThe hot big bang ªFundamental particles and forces ªStages of evolution in the early Universe This week: read Chapter 12 in textbook 11/4/19 1 Cosmic evolution ª From Hubble’s observations, we know the Universe is expanding ª This can be understood theoretically in terms of solutions of GR equations ª Earlier in time, all the matter must have been squeezed more tightly together ª If crushed together at high enough density, the galaxies, stars, etc could not exist as we see them now -- everything must have been different! ª What was the Universe like long, long ago? ª What were the original contents? ª What were the early conditions like? ª What physical processes occurred under those conditions? ª How did changes over time result in the contents and structure we see today? 11/4/19 2 Quiz ªWhat is the modification in the modified Friedmann equation? A. Dividing by the critical density to get Ω B. Including the curvature k C. Adding the Cosmological Constant Λ D. Adding the Big Bang 11/4/19 3 Quiz ªWhat is the modification in the modified Friedmann equation? A. Dividing by the critical density to get Ω B. Including the curvature k C. Adding the Cosmological Constant Λ D. Adding the Big Bang 11/4/19 4 Quiz ªWhen is Dark Energy important? A. When the Universe is small B. When the Universe is big C. When the Universe is young D. When the Universe is old 11/4/19 5 Quiz ªWhen is Dark Energy important? A. When the Universe is small B. When the Universe is big C. When the Universe is young D. When the Universe is old 11/4/19 6 Cosmic radiation ª Since the 1960’s, it has been known that in addition to starlight, space is filled with a nearly-uniform, very faint radiation field ª Pervasive throughout Universe ª Range of wavelengths, with peak in microwave range ª Present density is 411 photons/cubic centimeter ª Known as cosmic background radiation, or CBR (or CMB, for Cosmic Microwave Background, since there are other backgrounds) ª We’ll discuss CBR discovery & properties in detail later on 11/4/19 7 Planck data 11/4/19 8 Cosmic radiation ª At earlier times, this radiation field must have been more intense ª Energy density of background radiation increases as cosmic scale factor R(t) decreases at earlier time t: ª Multiply current number of photons/volume by 3 3 (R0/R(t)) =(1+z) ª Multiply current energy per photon by (R0/R(t))=(1+z) ªWhy? Because photon energy is inversely proportional to wavelength, and wavelength at earlier time = present wavelength ´ (R(t)/R0) ª Overall, radiation energy density must vary as (1+z)4 11/4/19 9 Matter and radiation densities ª Already know matter density compared varies inversely with volume ª Thus: 3 3 ª rmatterµ(R0/R(t)) =(1+z) 4 4 ª rradiationµ (R0/R(t)) =(1+z) ª At early times, energy density of CBR must have exceeded energy density of matter! ª When radiation field is strong, matter is heated up ª Therefore earlier and earlier in the Universe, it must have been hotter and hotter ª This suggests that origin of the Universe was a hot Big Bang! 11/4/19 10 Hot big bang ª In fact, hot early Universe was previously (1948) predicted by George Gamow, with Alpher and Herman ª They suggested that the universe started off in an extremely hot state of matter and radiation mixed together ª High temperatures and densities would provide conditions where atomic nuclei could form! ª As the Universe expands, the radiation energy would be spread over in increasing volume of space, and cool ª They predicted “relic radiation” with temperature about 5K ª Work not fully recognized until revived by Jim Peebles in 1960s ª Original observation of CBR that confirmed hot big bang theory was serendipitous! 11/4/19 11 A brief look at the stages of the Universe’s life… ª We will discuss this diagram in detail in future classes… ª Crude overview: ª t=0: The Big Bang ª For first 400,000 yrs, an expanding “soup” of tightly coupled radiation and matter ª Earliest epochs were “extreme” physics ª Then more “normal” physics: protons & neutrons form ª Then came nucleosynthesis ª After 400,000 yrs, atoms form (“recombination”) and radiation and matter “decouple” ª Following decoupling, matter and radiation evolve independently ª Galaxies, stars, planets, etc can then form and evolve 11/4/19 12 SOME TERMINOLOGY ª Our terminology… ª Very Early Universe: from BB to t=10-35 s ª Early Universe: from t=10-35 s to t=3 mins ª The study of the early universe: ª No direct observations to constrain theories… ª .. but, the basic physics governing the universe is well understood and tested in laboratories on Earth (particle accelerators). ª The study of the very early universe: ª Still no observations to constrain theories… ª … and the basic physics gets less and less certain as one considers times closer and closer to the big bang. 11/4/19 13 THE TEMPERATURE OF THE UNIVERSE ª The universe started off very hot and cooled as it expanded. ª In fact, the radiation temperature is inversely proportional to the scale factor 1 T µ R ª The evolving temperature is crucial in determining what goes on when in the early (and very early) universe 11/4/19 14 ª At a given temperature, each particle or photon has the same average energy: 3 E = k T 2 B ª kB is called “Boltzmann’s constant” (has the value of -23 kB=1.38´10 J/K) ª In early Universe, the average energy per particle or photon increases ª In early Universe, temperature was high enough that electrons had energies too high to remain bound in atoms ª In very early Universe, energies were too high for protons and neutrons to remain bound in nuclei ª In addition, photon energies were high enough that matter particle pairs could be created 11/4/19 15 Particle production ª Suppose two very early Universe photons collide ª If they have sufficient combined energy, a particle/anti-particle pair can be formed. ª So, we define Threshold Temperature: the temperature above which particle and anti-particle pairs can be created. 2mc2 Tthres = 3kB ª Different particles with different masses have different threshold temperatures ª Protons : T»1013K ª Electrons : T »5´109K 11/4/19 16 Particle production ª Suppose two very early Universe photons collide ª If they have sufficient combined energy, a particle/anti-particle pair can be formed. ª So, we define Threshold Temperature: the temperature above which particle and anti-particle pairs can be created. 2mc2 Tthres = 3kB ª Different particles with different masses have different threshold temperatures ª Protons : T»1013K ª Electrons : T »5´109K 11/4/19 17 ªAbove the threshold temperature… ªContinual creation/destruction of particles and anti-particles (equilibrium) ªBelow threshold temperature… ªCan no longer create pairs ªParticles and anti-particles annihilate ªSmall residual of particles (matter) left over 11/4/19 18 Quiz ªWhen will the most massive particles be created? A. When the universe is very young B. When Dark Energy becomes important C. When the universe is very old D. Whenever 11/4/19 19 Quiz ªWhen will the most massive particles be created? A. When the universe is very young B. When Dark Energy becomes important C. When the universe is very old D. Whenever 11/4/19 20 Types of particles ª Particles created/annihilated in the very early Universe were more than the ordinary types of particles abundant today (protons, neutrons, electrons, photons, neutrinos) ª Other more massive and unusual particles could be created/annihilated ª These types of particles are observed today only as products of collisions in high-energy accelerators ª Two types of particles: fermions and bosons (see Ch. 4) ª Primary duty of fermions is to make up matter ª Primary duty of bosons is to mediate forces ª Fermions include: ª Those particles based on quarks, called hadrons ª Baryons are made of 3 quarks (e.g. proton, neutron) ª Mesons are made of 2 quarks (e.g. pion) ª Those particles that are not, called leptons ª Electrons, muons, tauons ª Neutrinos ª The hadrons are generally more massive than leptons 11/4/19 21 Forces ª There are four fundamental forces in the Universe ª Each has an associated particle (a boson) that mediates the force by constant “exchanges” ª Electromagnetic force (mediated by photons) ª Electric & Magnetic fields are familiar in everyday life! ª Strong nuclear force (mediated by gluons) ª Holds the nuclei of atoms together ª Binds quarks together into hadrons ª Does not affect leptons ª Weak nuclear force (mediated by W and Z particles) ª Responsible for neutron decay ª Gravitational force (mediated by gravitons) ª Gravitons have never been detected… still theoretical 11/4/19 22 Fundamental interactions 11/4/19 23 From http://universe-review.ca Stages of the early Universe ª In the high-temperature very, very early universe, these forces were all unified (in the same way that electricity and magnetism are unified today). ª As universe cooled down, they started to “decouple” from each other. 11/4/19 Graphics: University of Oregon Astronomy Dept 24 Theories and unification of phenomena 11/4/19 From http://universe-review.ca 25 Quiz ªThe forces ordered in strength are A.
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