What fundamental particles do most Grand Unified and Inflationary theories predict existed before the Inflationary period? Basically, what do we expect the family of particles existing during the Grand Unification epoch to look like? It is my understanding that before electroweak symmetry breaking, particles had no mass. Some sources also say they had no electric charge. Would there still be 3 generations of leptons and quarks then? Are electrons and neutrinos even distinguishable? I know that photons and WZ bosons recombine into electroweak bosons, but before inflation do those recombine with gluons? Is that what XY bosons are? Where can I find a straightforward answer to what a non-symmetry broken standard model looks like, as a lot of what I see seems contradictory?

  • $\begingroup$ Before inflation? the inflaton field is during inflation, before, is quantized gravity which is not yet done, see hyperphysics.phy-astr.gsu.edu/hbase/Astro/timlin.html $\endgroup$
    – anna v
    Oct 5, 2021 at 14:53
  • $\begingroup$ A recent study from the University of Copenhagen has investigated the state of the universe in the shortest possible time (from 10-43 – 10-6) seconds after the big bang. They determined that the universe was made up solely of a quark-gluon plasma during this period. Associate Professor You Zhou stated “We have studied a substance called a quark-gluon plasma that was the only matter during the first microsecond of the Big Bang.”You Zhou, University of Copenhagen, as cited in Phys.org, May 21, 2021 “Study reveals new details on what happened in the first microsecond of Big Bang. $\endgroup$ Oct 5, 2021 at 15:13
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    $\begingroup$ @foolishmuse Note, the timeline in the link. Quark gluon plasma is after inflation. $\endgroup$
    – anna v
    Oct 5, 2021 at 15:28
  • $\begingroup$ @annav There is a period between inflation and quantized gravity, the grand unified epoch, and that is the epoch I would like more info about. That's why the question is tagged grand unification not quantum gravity. $\endgroup$ Oct 5, 2021 at 21:30
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    $\begingroup$ All of the studies of inflation, lengthy and serious as they are, remain a study of mathematical ideas in a physical regime for which there is almost no settled knowledge. Consequently credible answers should take the form either "not known" or "within model X, here is what is thought, but model X is itself far from well-established and has the following ad-hoc features or doubtful assumptions...". $\endgroup$ Oct 16, 2021 at 12:14

4 Answers 4


I think your question is about electroweak symmetry breaking and GUT symmetry breaking, and not inflation as such.

Here's a list of the fundamentally different particles in the Standard Model. There's one line for each family of particles that are related by Standard Model symmetries.

  • U(1) field (2 degrees of freedom, usually seen as 1 boson with 2 spin states)
  • SU(2) field (6 d.o.f., 3 bosons)
  • SU(3) field (16 d.o.f., 8 bosons)
  • 3 copies/"generations" of each of the following (okay, I lied about one line for each; I didn't want to copy and paste):
    • SU(2) and SU(3) charged Weyl fermions (12 d.o.f. = 2 spin states × 3 SU(3) colors × 2 SU(2) "colors")
    • SU(3)-charged, SU(2)-uncharged Weyl fermions (6 d.o.f.)
    • another SU(3)-charged, SU(2)-uncharged family with different U(1) charge (6 d.o.f.)
    • SU(2)-charged, SU(3)-uncharged Weyl fermions (4 d.o.f.)
    • U(1)-charged, SU(2)-uncharged, SU(3)-uncharged Weyl fermions (2 d.o.f.)
    • Weyl fermions with no charge at all (2 d.o.f.) if you're including those to explain neutrino mass
  • Higgs field (4 d.o.f.)

The fields after EWSB are the same; they are just "reinterpreted". As an analogy, if you have a wooden board that you initially covered with Cartesian coordinates, and the Texas sharpshooter of legend shoots a hole in the board, you may wish to switch to polar coordinates centered on the location of the hole, since they better respect the symmetry of the modified board. But the number of coordinates (2) is unchanged. If you had foreknowledge of where the hole would appear, then you could use the polar coordinates from the beginning. Presentations of the Standard Model tend to do that, which is why you'll sometimes hear that there are Higgs fields with different electric charges, even though those fields only make sense before EWSB and electric charge only makes sense after.

Post-EWSB, the up-type quarks are half of the SU(2)- and SU(3)-charged family of fermions coupled with one of the 6-d.o.f. families via the Higgs field, and the down-type quarks are the other half coupled with the other 6-d.o.f. family. The electrons and neutrinos work the same way if the uncharged fermions exist. If they don't exist then the neutrinos are just one half of the SU(2)-only family, not Higgs-coupled to anything.

The idea of grand unified theories is that something similar to EWSB happened at higher energy, leaving 12 massless bosons in three families (the U(1), SU(2) and SU(3) bosons), and some massive bosons like the W and Z, but much heavier, which are called X and sometimes Y.

One popular GUT is SU(5), which has a single SU(5) force with 24 gauge bosons / 48 d.o.f. (in general, SU(n) has $n^2-1$ gauge bosons). Instead of 5×3 charged fermion families, there are just 2×3 (while the uncharged fermions, if present, are still in their own 3 families).

Another popular GUT is SO(10), which has $10(10-1)/2=45$ bosons / 90 d.o.f., but has the advantage that all of the fermions in each generation, including the uncharged ones, are in a single family.

Are electrons and neutrinos even distinguishable?

The SU(2) charged halves of the electrons and neutrinos are aspects of the same thing pre-EWSB. The SU(2)-uncharged halves are different even pre-EWSB because they have different U(1) charges. The "halves" are not really associated with each other the way they are post-EWSB, although the Higgs coupling does exist. Pre SO(10)-symmetry breaking, they would all be aspects of the same thing, except that there are still 3 generations and they could be made of different mixes of generations. In other GUTs like SU(5), they may be different even before the GUT symmetry breaking.

It is my understanding that before electroweak symmetry breaking, particles had no mass.

Technically yes, but technically they have no mass even after EWSB.

The particles that "gain mass" from EWSB never behave like light propagating freely in a vacuum, which is what most people imagine when they imagine a massless particle. Post-EWSB, they're prevented from doing that by their coupling to the vacuum Higgs field. Pre-EWSB, they're preventing from doing it by couplings to the soup of other particles that are present.

Would there still be 3 generations of leptons and quarks then?

No one understands the reason for the 3 copies of the fermion fields. In most theories they are just added by fiat. There could be fermions that don't come in three copies; no one knows.

  • $\begingroup$ Thank you so much, this was exactly the type of answer I was looking for! Do you know where I can find more info on this topic in particular (instead of e.g. a full on dive into SU(5)/SO(10)). I would love more info on exactly which fields recombine into which! $\endgroup$ Oct 18, 2021 at 6:38
  • $\begingroup$ I sure would like to discuss about this topic further, do you have any way for me to contact you, or some other easily digestible resources on the subject? $\endgroup$ Oct 25, 2021 at 7:32
  • $\begingroup$ @AnthonyKhodanian Hi, I haven't forgotten this question, I just don't know any good references off the top of my head, and they're hard to find. I will try to find something, but no guarantees. I feel a bit guilty about getting the bounty despite the lack of references. $\endgroup$
    – benrg
    Oct 27, 2021 at 5:03
  • $\begingroup$ You deserved the Bounty. And don't worry about not having references on hand, if you would just like to give a few minutes I could just ask further clarifying questions? SE is bad about conversation so maybe another app like discord if you would prefer? $\endgroup$ Oct 27, 2021 at 10:33

The question, what particles existed during the Grand unification epoch ($t=10^{-43}\,\text{sec}$; $E=10^{19}\,\text{GeV}$; $T=10^{32}\,\text K$) and before Inflation ($t=10^{-36}\,\text{sec}$; $E=10^{14}\,\text{GeV}$; $T=10^{27}\,\text K$) is an open question.

Anything outside the range of energy that we can measure and observe is and remains speculative. Sorry.

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    $\begingroup$ Welcome to Physics! Hello! I have edited your answer using MathJax (LaTeX) math typesetting. For future posts, you can refer to MathJax basic tutorial and quick reference. Thanks! $\endgroup$
    – jng224
    Oct 16, 2021 at 12:03
  • $\begingroup$ Great Jonas, very helpful! Thank you for editing the superscripts in my answer using mathjax formatting. Kind regards, UN73 $\endgroup$
    – UN73
    Oct 16, 2021 at 12:42
  • $\begingroup$ Sorry, I should have been more clear. I am not looking for a definitive answer obviously, I am simply wondering what the current accepted theories predict and what would exist if Inflationary theory and GUT theories were true. $\endgroup$ Oct 16, 2021 at 15:25

In one sense, all types of fundamental particles (certainly everything in the Standard model, plus probably more besides, depending on which variety of GUT you prefer) existed in the grand unification epoch, since the energy density/temperature was so high that particle/anti-particle pairs of all types could be created very rapidly.

In another sense, no fundamental particles existed since the energy density/ temperature was so high that any particles that were created would be annihilated almost immediately.

So it’s either all or none at all - take your pick.

  • $\begingroup$ Okay but certain particles were indistinguishable, for example, the photon and WZ bosons became 4 identical massless bosons. I am wondering if the same happened to the quarks and leptons. Was there just a single indistinguishable quark and lepton, or were they still distinguishable? $\endgroup$ Oct 5, 2021 at 21:28
  • $\begingroup$ @AnthonyKhodanian what do you mean by indistinguishable? They aredistinguishable by their quantum numbers, that is the whole idea of having the group structure carried over in GUT theories. Even running masses as here arxiv.org/pdf/0804.0717.pdf . For quantization of gravity as it is still open it is an open question. $\endgroup$
    – anna v
    Oct 6, 2021 at 3:22
  • $\begingroup$ Several sources have mentioned that when a symmetry is unbroken, particles can become indistinguishable. For example, in the electroweak era the higgs and WZ bosons lose their mass, and together with the photon become 4 massless, chargeless, colorless particles with the same spin and therefore become practically identical and indistinguishable. This source says this also happens for GUT unification: "In the symmetric phase (all Higgs fields zero), electrons, neutrinos, and quarks would be indistinguishable." sciencedirect.com/topics/chemistry/electroweak-theory $\endgroup$ Oct 6, 2021 at 10:32

It's not possible to give a definitive answer to this question yet, in part because the detailed mechanism for inflation is not even known. Inflation could be caused by a scalar particle, usually called an inflaton, but this name is largely just a placeholder.

One can still ask what the particle content of various theories predicting inflation would be, but this is also still a very open-ended question. This video lecture by Nima Arkani-Hamed gives a great sense of why there are so many possible answers to your question, specifically from 9:25 to 22:00 or so. You can get a pretty compelling model for inflation in the Standard model just by adding a bunch of scalar fields to it. Inflation comes from such a toy model having lots of false vacua, which lead to varying values for the cosmological constant and hence varying amounts of inflation. A similar idea holds in string theory. Each string theory vacuum could have not only different values of the cosmological constant, but different numbers of fundamental particles, spatial dimensions, fundamental forces and values of physical constants. Inflationary eras would then corresponding to tunneling between these vastly different vacua.

I know this answer sort of dodges your more specific questions about unification of sets of particles, but in my understanding these questions are too specific to be relevant to the pre-inflationary universe. The particle content need not even organize into the same structure that we're used to in the Standard Model.


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