# I Understanding the Early Chronology of the Universe

1. Jul 1, 2017

### Mr Davis 97

I am trying to understand how the beginning of the universe unfolded. I've tried to read up on it, but some specific questions never seem to be addressed. What is the stuff that is in the beginning of the universe? I commonly read that it's just "energy," but what is energy in this context? Also, isn't each fundamental force associated with a boson? Does this mean that in the beginning there was fundamentally one type of boson, and after the fundamental forces split, there came to be four bosons? Also, where do quarks and leptons come from? If the beginning of the universe was just energy, how do two distinct forms of matter, quarks and leptons, come into being?

2. Jul 1, 2017

### Staff: Mentor

We don't know fully how this happened. We have pretty reliable knowledge back to the hot, dense, rapidly expanding state that is the proper meaning of the term "Big Bang", but prior to that our knowledge is still tentative. Our best current model is that that "Big Bang" state resulted from the ending of a previous inflation era, but we don't know for sure that that's the case, or, if it is the case, what caused the inflation era to start or what came before it. All we have are various speculative models.

Not quite. This question can't really be answered without talking about the models we have for all of the fundamental particles, bosons and fermions.

Our current model is the Standard Model of particle physics, which, roughly speaking, looks like this (at low energy, i.e., what we see today--in the early universe it's a bit different, as we'll see below):

Bosons

- Photon: electromagnetic interaction

- W+, W-, Z bosons: weak interaction

- 8 gluons: strong interaction

- Higgs boson: no interaction, this is what is "left over" after all of the fermions and the weak bosons get their masses (see below)

Fermions

There are 3 generations of each kind of fermion; each generation has (roughly) a pair of fermions of two types, quark and lepton, as follows:

- quarks: down/up, strange/charm, bottom/top

- leptons: electron/e neutrion, muon/mu neutrino, tauon/tau neutrino

Roughly speaking, the quarks "listen" to all three interactions, the electron/muon/tauon "listen" to the weak and electromagnetic interactions, and the neutrinos "listen" to the weak interactions only.

However, as I said above, this is all at low energy, i.e., in the universe as it is today. At high energies, i.e., in the early universe, there is a period before "electroweak symmetry breaking" where things look somewhat different:

Bosons

W1, W2, W3, B: electroweak interaction

8 gluons: strong interaction

H+, H-, H0, H0*: Higgs "interaction"

Fermions

These are the same as above, but now they are all massless (where above they all had nonzero rest mass). Roughly speaking, as the energy goes down (which we can think of in the early universe as the temperature decreasing due to expansion), a phase transition happens in which the electroweak interaction splits into the electromagnetic and weak interactions. Roughly speaking, this involves three of the four Higgs bosons being "eaten" by three of the electroweak bosons, giving the latter mass. (Fermion masses also arise from this process, but more indirectly, and AFAIK we don't fully understand this part of it; we don't need to go into that here.)

You will notice that the strong interaction is still separate in the above. It is natural to speculate that, at still higher energies (higher temperatures, so earlier times in the early universe), a further unification happens between the electroweak and the strong interaction, so that we have 12 bosons of a single "grand unified" interaction. However, there are multiple possible models that give this result, and all of them entail that there are additional bosons that give rise to other interaction processes that we have not observed (for example, proton decay). These models also model the fermions we know, quarks and leptons, as particular states of underlying "unified" fermions; but again, AFAIK all of the models also predict additional fermions that we have not observed. So we're not entirely sure about this level of unification at this point. (We also don't really understand how the Higgs fits into the picture at this level.)

You will also notice that gravity is not included in the above at all either. AFAIK nobody has proposed a "particle physics" type model that unifies gravity with the other interactions. The general belief seems to be that the only way to get this level of "unification", if it's possible, is to come up with a full theory of quantum gravity, which we have not yet done.

Finally, even the "grand unification" of the electroweak and strong interactions would not take us back before the Big Bang into the inflation era. There are various inflation theories, and all of them don't change any of the above; they just add on various mechanisms to drive inflation. During the inflation era, all of the Standard Model fields were in their vacuum states--i.e., there were zero particles of all of the Standard Model particle types (bosons and fermions). All of the energy in the universe was contained in the energy associated with whatever mechanism was driving inflation. What happened at the end of inflation was that all that energy was transferred to the Standard Model fields, meaning that a huge density of all of the Standard Model bosons and fermions was created at very high temperature, rapidly expanding--i.e., this is what created the "Big Bang" state.

It wasn't. See above.

See above.

3. Jul 8, 2017

### phinds

I recommend Weinberg's "The First Three Minutes"

4. Jul 8, 2017

### rootone

I would like to know what it was that inflated as well.
It could not have been any state of matter which we now recognize.

5. Jul 9, 2017

### timmdeeg

This point interests me for a long time. Discussing this era there appears the term "matter field" in some articles, but without further characterization or definition (I have no reference at hand in the moment, unfortunately). Does "matter field" mean what you describe saying "All of the energy in the universe was contained in the energy associated with whatever mechanism was driving inflation"? Could one say that said energy has the potential to create particles (perhaps in the first instant of time unknown particles, which then decay into known particles) under certain conditions, e.g. comparable to a phase transition? Whereby looking at the transition gas-liquid both phases contain the same particles which makes a decisive difference.

Matter antimatter particles can be created (if I remember correctly) but here I think the involved particles are known in this case. And if I see it correctly it is the time reverse process of annihilation. So this is understood. Whereas in contrast matter antimatter particles created finally during "reheating" stem from "energy associated with whatever mechanism was driving inflation". Does this leave the possibility open that this energy is due to unknown particles or it necessarily potential energy?

6. Jul 9, 2017

### Staff: Mentor

No. The term "matter field" means the Standard Model fields, which had no energy in them until the end of inflation.

7. Jul 10, 2017

### George Jones

Staff Emeritus
I am going to try to elaborate on Peter's answer a bit.

In the simplest models, inflation is driven by a single scalar inflaton field $\phi \left(t \right)$ that (assuming spatial homogeneity and isotropy of Friedmann-Lemaitre-Robertson-Walker universes) depends only on cosmological time. Initially, the "kinetic energy" $\dot{\phi}^2/2$ is much smaller than the potential energy term $V\left( \phi \right)$ (the form of $V\left( \phi \right)$ depends on the particular model). According (2.3.27) and (2.3.28) of Daniel Baumann's (Cambridge) excellent cosmology lecture notes
http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf
this means that density are almost related by $\rho = -p$, i.e., that during inflation, the universe is dominated by a large cosmological (almost) constant. Eventually, the inflaton field "rolls down" to near the minimum of the potential energy. For many models, the kinetic energy term $V\left( \phi \right)$ then dominates, and the universe acts briefly like a matter-dominated FLRW universe; see the discussion around (2.3.49) in Baumann. In order to get Standard Model stuff out, the Lagrangian contains interaction terms between the inflaton field and the Standard Model fields.These cause the universe to transition to a radiation-dominated Standard Model universe.

Last edited: Jul 10, 2017
8. Jul 12, 2017

### timmdeeg

This is the interesting point, thanks for the link!

Page 40
Inflaton decay.
To avoid that the universe ends up empty, the inflaton has to couple to Standard Model fields. The energy stored in the inflaton field will then be transferred into ordinary particles. If the decay is slow (which is the case if the inflaton can only decay into fermions) the inflaton energy density follows the equation ...

I'm afraid without knowing the math its hardly possible to support some intuitive understanding what that means, especially "the inflaton has to couple to Standard Model fields". If it is, I'm grateful.

9. Jul 12, 2017

### George Jones

Staff Emeritus
It is the coupling between the inflaton field and the Standard Model fields that allows the transition from a universe whose content is just the inflaton field to a universe whose contents are the Standard Model fields.

10. Jul 13, 2017