## Key points

• Fermions (particles that obey Fermi statistics) come in generations with each generation heavier than the preceding one.

• electron neutrinos interact with protons and neutrons; in the early universe this set of reactions keeps the protons and neutrons in balance:

## anti-neutrino+proton<---->neutron+anti-electron neutrino + neutron <------> proton + electron

• Hadrons are "heavy" particles made of quarks. Leptons are not made of quarks (and are light). A hadron made of 3 quarks is called a baryon; one made of 2 quarks is a meson.

• Forces between quarks are mediated by gluons in the sense that has the separation between quarks increases, so does the binding force.

• A proton is made of UUD; if a U quark changes to a D quark one has a neutron (. So the decay of the free neutron really means that one of the quarks has changed from D to U.)

Note: In the very early Universe, it is possible that its so small that the separation between the quarks vanishes if quarks are arbitrarily close together the gluon force vanishes and the quarks are "free". During this free quark period, one can not have protons or neutrons. This period is called the Quark-gluon plasma. The origin of the matter - anti-matter assymmetry may be related to the end of this period. Its unclear. Extra Material: Now let's do some funny things with numbers:

Let's assume that the Universe, for no apparent reason, knows about these three constants at arbitrarily early times:

• Gn = Newton's Constant Gravity/Mass
• h-bar = Planck's constant which we can maybe relate to some kind of length
• c = constant speed of light necessary to define time in a consistent manner.
Now look what we can do:

1. (Gh/c3) has units of length; in real units this length is approximately 10-36 meters pretty damn small! This is called the Planck Scale

2. (hc/G)1/2 has units of mass; in real units this mass is approximately 2 x 10-8 kg. If we multiple this mass by c2 we get an Energy scale which is about 1019 Gev. You might think of this as the energy per unit volume at the "beginning of the universe". This is called the Planck Mass.

3. since we have length scale, and a speed limit, we then define a time which is the planck scale divided by c. or (Gh/c5) or 10-43 seconds. One can think of this planck time, as the time when G,h-bar and c now exist. Back now to the neutrinoes.

When the universe is about 1 second old, or at an energy scale of 1 Mev, the neutrinos no longer interact with matter. The density of the Universe has dropped below in the interaction scale of the neutrinos and now the neutrinos "free-stream" away from the matter.

After this decoupling, the electrons and positrons annihilate (to one part in one billion) to reheat the universe and give us the photon background. However, the neutrinos are not around to participate in this reheating and so the neutrino background is colder than the photon background. We can actually calculate this exactly:

1. The entropy per volume of the universe, S is given by this

S goes as NTT3

where NT is the number of species of particles in thermal equilbrium at temperature, T.

2. The total entropy is constant; so SR3 is constant so NT(TR)3 is constant.

3. The total number of species of a particle is the product of its total number of spin states, whether or not it has an anti-particle, and whether it obeys the pauli exclusion principle. So

• a photon has 2 spin states, no anti particle, and obeys bose statistics so its species number is 2x1x1

• an electron is 2x2x7/8 = 7/2.

4. Before the electron position annihilation NT is therefore 7/2 + 2 = 11/2

5. After the electron position annihilation NT is 2.

6. Since NT(TR)3 is constant we have

11/2(TR)3 (before annihilation) = 2(TR)3 (after annhilation).

7. Before annihilation everything was in thermal equilibrium so the Tneutrino = Tphoton. Therefore (TneutrinoR)after = (TneutrinoR)before = (TR)before.

So we now have (TR)after/(TR)before = TR(after)/(TneutrinoR)after = T/Tneutrino = (11/4)1/3 = 1.4

In real numbers this means the neutrino background is 1.4 times colder than the photon background (or 2K right now).

8. Furthermore, the energy density scales as T4 so the energy density in the neutrino background compared to the photon background is:

(7/4)(4/11)4/3 = 0.45

This means that if neutrinos have a tiny mass, then their energy density (in the form of rest mass) is large. If they have no mass, then they are just like photons and their energy density is a consequence only of the neutrinos being at finite temperature.

Structure Formation:

In a completely generic way, structure formation involves the amplification of density enhancements through the actions of self gravity.

In a static case, once sufficient mass is available to overcome the internal energy in some volume, that mass will collapse and the size of the density enhancement will increase. This condition is known as the Jeans criteria - we will return to it later.

In a normal static situation, the internal energy or pressure of a gas is simply related to its temperature. Hence, cool clouds of gas can gravitational collapse while hot clouds would dissipate.

However, in the early universe there is an additional source of "pressure" and that is radiation pressure. Since the radiation and matter are coupled, then the radiation is trying to smooth out the distribution of matter (e.g. wash out any density enhancements).

At a general level, its unclear if purely baryonic density fluctuations could survive the effects of radiation pressure (also called radiation drag). To mitigate the effects of radiation pressure, it is desireable to have a new form of gravitating matter, one that can gravitationally clump but not interact with radiation. This matter, often called dark matter, could therefore provide the seed density fluctuations around which structure will later grow.

When discussion the gravitational collapse of object it is useful to refer to a timescale, known as the dynamical timescale. This is the characteristic timescale of a system after which the virial theorem holds. Its universal in all kinds of systems and I will derive this timescale under three different scenarios:

1) A pressure wave travelling through a hydrostatic fluid. Here we use the sound velocity as the characteristic velocity.

2) Gravitational free fall (or motion in 1D under constant acceleration).

3) Cluster crossing time. Here we equate the dynamical mass of the cluster with the mass obtained by considering it a spherical region of constant density.

These three cases are shown below in equation form.  We can now also do a quick estimate, based on observed large scale structure, what we might expect the for the amplitude of any anisotropy in the microwave background.

Structure will amplify in the matter dominated universe as z3/2. Since the redshift at decoupling was z ~ 1100, the amplification factor is 40,000. Hence, perturbations as small as 1/40,000 could have been amplified to produce the factor of 2 overdensities that we observe in large scale structure today. Hence, the expected temperature anisotropy at the surface of last scattering is 1/40000 = 2.5 x 10-5 which is consistent with the WMAP observations.

To illustrate the profound effects of radiation pressure on the suppression of the growth of structure in the radiation dominated era (e.g. the first 300,000 years), we can do a Jeans Mass analysis.

The Jeans criterion is that the gravitational potential energy of a could of gas must overcome its internal energy (IE) in order to for collapse to occur. The IE for a fluid is its pressure times its volume. This criteria can be stated in terms of density and pressure as shown below: In the radiation dominated era the pressure is significant and is 1/3c2r (see Lecture 14 notes). After the radiation era, the pressure will drop by about a factor of 109 (the effective photon to baryon ratio).

At recombination the density of the universe is ~ 10-21 g/cc.

The Jeans mass prior to recombination is 5 x 1018 solar masses and we don't observe structures this large.

After recombination the jeans mass lowers to 2 x 105 solar masses, which is the mass of a globular cluster.