Chapter 27: The Early UniverseThe plan: 1. A brief survey of the entire history of thebig bang universe.2. A more detailed discussion of each phase, or“epoch”, from the Planck era through particle production, nucleosynthesis, recombination,and the growth of structure.3. Then back to the (near) beginning, and “inflation.” 4. Finally the demonstration from the WMAP CBR map that the universe is almost perfectlyflat. This ties together several separate strandsof the big bang model for history of the universe.It also shows that inflation probably did occur, and dark matter and dark energy exist, whether or not we know what they are made of.27.1 Back to the Big Bang27.2 The Evolution of the Universe More on Fundamental Forces27.3 The Formation of Nuclei and Atoms27.4 The Inflationary Universe27.5 The Formation of Structure in the Universe27.6 Cosmic Structure and the Microwave BackgroundUnits of Chapter 27The total energy of the universe consists of both radiation and matter.As the Universe cooled, itwent from being radiationdominated to being matterdominated.Dark energy becomes moreimportant as the Universeexpands.27.1 Back to the Big BangIn the very early Universe, one of the mostimportant processes was pair production:Virtual particles and photons were createdfrom the high-energy vacuum state, fromnothing. The temperature and densitywere trillions of times their current values.The upper diagrams show how twogamma rays can unite to make anelectron–positron pair, and vice versa.Electron-positron pairs are matter-antimatter pairs, and annihilate, producinggamma rays again. These two processescome into balance, or equilibrium.The lower picture is of such an eventoccurring at a high-energy particleaccelerator.27.1 Back to the Big BangWhen the temperature haddecreased to about 1billion K, the photons nolonger had enough energyfor pair production, andwere “frozen out.” Thismeans the remainingphotons (still gamma rays)were now permanent,although still frequentlyscattered by the electronsin the surrounding gas.We now see these photons, greatly redshifted,as the cosmic background radiation.27.1 Back to the Big BangIn the very earlyUniverse, the pairproduction andrecombinationprocesses were inequilibrium: Equalrates of productionand destruction, noparticle or photonpermanent.This table lists the main events in the different epochs of the Universe. You don’t have tomemorize the numbers, or even all the epochs (I’ll tell you which ones are important toremember), but you should be able to read from the top to the bottom and understand itis a summary of the primary components of the universe at each time.27.2 The Evolution of the UniverseCurrent understanding of the forcesbetween elementary particles is thatthey are accomplished by exchangeof a third particle.Different forces “freeze out” whenthe energy of the Universe becomestoo low for the exchanged particle tobe formed through pair production.27.2 The Evolution of the UniverseIf we extrapolate the Big Bang back to the beginning, it yields a singularity—infinitedensity and temperature.This result is probably artificial, reflecting our lack of understanding concerning matter attemperatures and densities so large that gravity had not yet “frozen out” from the otherforces. Any occurrence in physics or math of a “singularity,” where physical quantitiesappear to become infinite, is a sign of missing physics (or a mistake).This is called the era of “quantum gravity,” when quantum effects and gravitational effectswere important on the same size scales.This is the same reason we can’t say what happens when something falls into a black hole--another “singularity.”We can only (hope we) understand what happens back to 10− 43 seconds after the Big Bang.Therefore, we cannot predict anything about what happened before the Big Bang; indeed,the question may be meaningless. We don’t even understand whether “time” would havebeen relevant during these earliest of times, in which case talk of “before” and “afterwould not even make sense.However a lot happens after that time, and theories of big bang cosmology predict certaincrucial events that can be observationally tested.Time = 0 ? No singularity, just ignoranceThis first 10− 43 seconds after the Big Bang are called the Planck era.This is called the era of “quantum gravity,” when quantum effects andgravitational effects were important on the same size scale. Therewere no separate forces.At the end of that era, the gravitational force “freezes out” from allthe others, becomes separate. That is why today we can speak of“gravity” as if it acted independently from other forces.Planck era, GUT era, …The next era is the GUT (Grand Unified Theory) era. Here, the strongnuclear force, the weak nuclear force, and electromagnetism are allunified. At the end of it, there was not enough energy for thecorresponding production of heavy particles, and the forces (whichwere just one unified force before this time) became separate forces.The next era is called the quark era; during this era all the elementaryparticles were in equilibrium with radiation.About 10− 4 s after the Big Bang, the Universe had cooled enough thatphotons could no longer produce the heavier elementary particles; the onlyones still in equilibrium were electrons, positrons, muons, and neutrinos.This is called the lepton era.…quark era, lepton eraAbout 1 second after the Big Bang, the Universe became transparent toneutrinos. (Note: this is the equivalent of the era of recombination forphotons, to come much later; so there should be a “cosmic neutrinobackground” all around us, those same neutrinos from when theuniverse was 1 second old; no one has proposed a way to detect them.)After 100 seconds, photons became too low in energy forelectron–positron pair creation—this marks the end of the radiationera, and the end of particle creation.Epochs in cosmic historyThe evolution of the universe in a single graph--look at it slowly, especially as a reviewtool. Yellow and grey bands show how the density and temperature decreased as afunction of time (horizontal axis). Notice, on the bottom right, the transformation fromfundamental particles, to atoms, and finally to galaxies and stars.Meanwhile the mysterious “dark energy” has been increasing in importance.The next major era occurs when photons no longer are able to ionize atoms assoon as they form. This allows the
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