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Energy, Matter, andEnergy, Matter, and AntimatterAntimatterin the Universein the Universe

Eric LinderUniversity of California, BerkeleyLawrence Berkeley National Lab

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In the BeginningIn the Beginning……

It all starts with E=mc2.

Remember that the early universe was very hot(energetic) and dense.

With lots of energy, lots of particles (matter andradiation) can be created. When there are manyparticles per volume (dense), the particles caninteract and return to being energy (annihilate).

This makes all matter, radiation, and energy inthermal equilibrium - a very simple initial state,characterized by its energy or temperature,E=kT.

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Before the Beginning?Before the Beginning?

We don’t know the physics to describe the extraextra early universe, when the age is of order thePlanck time tPl=√Gh/c5 (~10-43 s).

But it doesn’t matter. These initial conditionswere wiped out by a combination of the thermalequilibrium (many interactions of energy ↔matter)and an episode of Cosmic Amnesia known asinflation.

Inflation vastly expanded each small volume of theuniverse (which had achieved thermal equilibrium)into a huge volume, such that any observer couldonly see conditions within the previously smallpatch (visible universe << full universe).

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InflationInflation

The only event as important as inflation is the endof inflation.

First, inflation makes the rest of the full universeout of contact. Then, at the end of inflation, all theenergy from the inflaton, the quantum field drivinginflation, gets converted into matter and radiation(reheating).

This one-two punch removes any “memory” of theearlier stages of the universe, hence cosmicamnesia.

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Probing InflationProbing Inflation

We know very little about inflation.

We don’t know the nature of the inflaton, whatmakes inflation stop, or when. It could havehappened anytime between 10-37 s and 10-12 s (1025 and 1012 eV).

But it does solve many puzzles (smoothness) andpredicts the origin of perturbations in matter andradiation from quantum fluctuations, with specialrandom (scale invariant) properties. Detected √!

Inflation also predicts fluctuations in spacetimeitself (gravitational waves), which experiments aregearing up to search for.

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Topological DefectsTopological Defects

Inflation is an example of a phase transition, wherea symmetry of nature is broken.

Abdus Salam, 1979 Nobel Physics laureate, gavethe example of napkins at a dinner party.

Phase transitions can leave behind discontinuitiescalled topological defects. These can be 0-D(pointlike) - magnetic monopoles, 1-D (linelike) -cosmic strings, 2-D (sheetlike) - domain walls.

We have never detected any on cosmic scales, butthey would point us toward deep physics.

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Radiation EraRadiation Era

After inflation, reheating repopulates the universewith particles. (Cosmologists often call any form of mass-energy with much more energy than mass by the nameradiation, and this is meant to include photons, neutrinos,and relativistic matter.)

Because the universe is still hot and dense, manyinteractions occur and thermal equilibrium holds.

What determines the interactions and whathappens when equilibirum breaks down?

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Particle InteractionsParticle Interactions

For one thing, remember E=mc2. Particles cannotbe created if there is not enough energy, so as theuniverse cools, very massive particles cease beingcreated. They may decay into lighter particles ormay stick around.

Particles can also be created from interactions ofother particles. To do this, the particles have torun into each other.

Although we call the universe dense, it’s actuallymore dilute than water by the time it’s 10 s old. Sothe odds of 3 particles being in the same place atthe same time are low (this is why primordialnucleosynthesis can’t create carbon or heavier elements).

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Particle InteractionsParticle Interactions

Particles have pair interactions, mano a mano. Theprobability of their running into each otherdepends on their cross section.

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Cross SectionCross Section

Cross section σ does not depend only on thegeometric size of the particle, but the strength of itsattraction (under one or more of the four forces).

The interaction rate also depends on the numberdensity n of particles (the more densely packed, theeasier to collide), and the velocity v of particles (thespeedier, the more chances for encounter).

The probability of encounter in a time t is theeffective volume covered, V = σ vt = σL, times thenumber density, i.e. the number of particles in thevolume.

vL=vt

σ

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FreezeoutFreezeoutAs the universe expands, the number density ofparticles become diluted. Also, the cross sectionoften depends on energy and can get smaller.

Eventually, it is very unlikely for particles tointeract within a time as long as the age of theuniverse, H-1, and reactions cease. This is calledfreezeout because the number of particles staysconstant thereafter.

Note that more weakly interacting particlesfreezeout sooner (smaller cross section), when theuniverse was denser, and so have a largerabundance today. Dark matter, contributing 20% ofthe energy density today, may well be made ofweakly interacting massive particles (WIMPs).

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AntimatterAntimatter

When energy is converted into particles, it mustconserve total energy and momentum, and alsomust obey certain quantum properties. E.g. alepton can’t become a baryon.

If the energy creates a particle and its antiparticle,then quantum numbers are automaticallyconserved. Photons are their own antiparticle, butmatter partners with antimatter, e.g. electron andpositron or proton and antiproton.

When a particle and antiparticle meet, they convertback into pure energy.

e-+e+→1 MeV person+person → 1000 Mton blast

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Where is the Antimatter?Where is the Antimatter?

So where is all the antimatter?

Or, why is there even any matter? When theuniverse cools from expansion it can no longercreate matter or antimatter, so they shouldannihilate each other, leaving only energy (the CMB).

Something must break down! We detect plenty ofmatter (look around) but the only antimatter is thatcreated anew in energetic processes. (Antimatterwas identified in the lab in 1932, the positron.)

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Matter Matter vsvs. Antimatter. Antimatter

Dirac (Nobel 1933) predicted antimatter in 1928. Herealized something was needed to balance matter inquantum theory, and treated antimatter as “holes”,what you have when you take away from nothing(the vacuum).

Sakharov (Nobel 1975) laid out three conditionsneeded for matter-antimatter asymmetry, such as wesee:

• Breakdown of thermal equilibrium (phase transition)

• Violation of matter-antimatter symmetry (CP)

• Violation of baryon number

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Where is the Antimatter?Where is the Antimatter?

How do we know the whole universe is matter?

Solar system - direct probes.

Larger scales - gamma rays. Annihilation radiationis very characteristic line (e.g. e+e-→1.02 MeV).

Problem of separation. Large scales have not beenin causal contact since inflation.

Particle scales - do see symmetry, except in specialCP violating systems (K mesons, Nobel 1980; B mesons).

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Matter-Antimatter AsymmetryMatter-Antimatter Asymmetry

Following Sakharov, we guess that some symmetrybreaking occurred at high energy, allowing a slightexcess in baryons over antibaryons to be created,about 30,000,001 per 30,000,000.

Once the universe cooled so matter-antimattercreation ceased and they only annihilated, this leftbehind the sole extra baryons (1 part in 3x107 ofwhat would be expected).

This is directly evident today in the photon-baryonratio. In thermal equilibrium, there should beequipartition - equal numbers of all types ofparticles. But today the photon-baryon ratio is2x109, not nearly 1 (extra species give 109 not 107).

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Photons and BaryonsPhotons and Baryons

The high photon-baryon ratio has importantconsequences for the universe. Since there are somany more photons than expected per baryon, thephotons have influence even when their energy isbelow the interaction energy (there are always a fewphotons more energetic than the average).

This keeps the universe ionized longer thanotherwise, preventing atoms from forming til later,and preventing perturbations in the baryon densityfrom growing (structure formation).

This moderating influence is like the ocean’s effecton the climate of land (heat capacity).

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Antimatter MysteriesAntimatter Mysteries

Today one of the great puzzles of physics isbaryogenesis, the origin of the matter-antimatterasymmetry. We measure an asymmetry in specialparticle physics systems, but don’t understandwhere the primordial 10-8 comes from.

We also don’t know if neutrinos are their ownantiparticle. Experiments may reveal this in thenext few years!

So we don’t understand dark energy, don’tunderstand dark matter, and don’t even understandbaryons! Exciting mysteries everywhere we look.