The Greatest Story Ever Told — 06 — Goodbye antimatter, hello protons, neutrons, and electrons!

Posted by Ethan on March 8, 2010
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Electricity is actually made up of extremely tiny particles called electrons that you cannot see with the naked eye unless you have been drinking. -Dave Barry

Welcome back to our series on The Greatest Story Every Told, where we start from before the big bang and come forward in time to get the Universe we have today. (If you’re just joining us, go back for parts 1, 2, 3, 4 and 5.) Last time, we talked about how we made more matter than antimatter (and we clarified some questions). So what does our Universe look like at this point?

Well, the Universe is still full of hot radiation, flying around and slamming into everything, including nearly equal amounts of both matter and antimatter. Nearly equal, but not quite. For every ten billion antiquarks, there are something like ten billion and six quarks, and for every ten billion positrons, there are ten billion and six electrons. And while all of this is going on, the Universe is expanding and cooling as it continues to age. Everything moves away from everything else, just like dots on the surface of a balloon.

(For those of you wondering just how far back in time I’m talking about, the Universe is about one femtosecond, or 10-15 seconds, old at this point.)

Well, what happens as the Universe expands and cools? Things smack into each other far less often, for one, because there’s more space between them in general. And when they do smack into each other, because things are cooling in addition to expanding, they hit each other with less energy. Even though matter and antimatter are constantly running into one another and annihilating, what’s not happening anymore as the Universe ages is that you’re not making more matter and antimatter to replace what you’re destroying; it’s all becoming radiation. In other words, this happens.

And its counterpart, shown below, is no longer happening once the Universe gets cool enough.

So instead of a fiery inferno, where everything is continuously blasted apart mere instants after it’s created,

things are instead coming together a little more calmly. This means when two up quarks and one down quark get together, they can finally, when the Universe is about 10 microseconds old, form a stable proton. And when two down quarks and one up quark get together, they can form a stable neutron.

All told, the matter and antimatter has all annihilated away, and so our Universe is full of the radiation left over from that, and that tiny little excess of matter: protons, neutrons, and electrons. At this point, the Universe is still only one second old, and still hotter than the center of the Sun, but it looks an awful lot more like what you’re used to!

So come back for part 7, where we’ll see if we can’t save these unstable neutrons from disappearing!

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Comments

  1. #1 Rich
    March 8, 2010

    Ethan,

    I think one interesting thing you’re leaving out is the pressure of the universe…at least as a chemist, I’m interested in just how tightly packed together the matter that makes up the universe is at such early points in time. Is this something that has been calculated?

    ~Rich

  2. #2 Douglas Watts
    March 8, 2010

    So what you’re saying is that were it not for inflation, there would not have been enough “cooling” (through expansion of space) to stop the constant, new creation of matter by gamma ray photons smacking into each other, and the matter’s annihilation via particles and anti-particles hitting each other and creating gamma ray photons. Without inflation this 1:1 ferris wheel of matter into energy and energy back into matter would not have stopped. And inflation stopped it and let temps. cool enough to allow quarks to form stable nuclei.

    Is that even close?

  3. #3 Jon
    March 9, 2010

    I think your being a bit human-centric in only discussing the role of baryonic matter, which so were told, makes up only a small part of all the matter in the universe. What about dark matter? Should you not be explaining it’s role in the creation of the universe or do you have no confidence in CDM theory?

  4. #4 dg
    March 9, 2010

    @Jon

    Exactly what the dark matter is doing at this point, other than interacting gravitationally, is pretty strongly dependent upon what kind of particle(s) it’s made of. The interaction cross-section, decay rate, annihilation products (and any temperature dependence of these) are all things that are TBD.

    That said, we know from the success of CDM that it should be there (with some power spectrum, perhaps seeded by inflation), with the baryonic matter sitting on top of it, ready to create the CMB anisotropies in a few hundred thousand years. And it shouldn’t have much more of an influence than that (or we would have seen the effects already).

  5. #5 Bjoern
    March 9, 2010

    @Rich: At these early times, the universe is radiation-dominated. Hence the pressure is simply one third of the energy density, and the energy density of radiation goes with the scale factor a to the power of -4. Hence the pressure also goes with the scale factor a to the power of -4. And the scale factor a is equal to 1/(1+z) with the redshift z. Hence if you know the radiation pressure p_0 (or the energy density) today, you get the pressure at earlier times simply using p = p_0 (1 + z)^4. Both the radiation pressure p_0 today and the redshift z at the eras which Ethan discusses here are well-known, so this is quite easy to calculate (IIRC, there are some problems with treating the neutrinos, since they behave somewhat differently from photons, but that doesn’t change much in the results). I don’t have the relevant numbers available right now, but this should be easy to look up…

  6. #6 ScentOfViolets
    March 9, 2010

    @Bjoern

    Making reasonable assumptions, it’s easy to see that at some point in history, the universe must be radiation dominated. But for how long? What about the dark matter part? Hard to see how that contribution would be radiation dominated, or for that matter, how dark matter could ever fall into thermal equilibrium on a “reasonable” time scale.

    63999999

  7. #7 Bjoern
    March 9, 2010

    @ScentOfViolets: The energy density of matter (and that also holds for (cold) dark matter) goes with the scale factor to the power of -3, but the energy density of radiation goes with the scale factor to the power of -4. Using the known energy densities of matter and radiation today, it’s easy to calculate that at z values higher than about 3200, the universe was radiation-dominated.

    I’m not sure about dark matter being in thermal equilibrium, but I’d guess that using sensible assumptions like dark matter consisting of supersymmetric partner particles (e. g. the “neutralino”), it is probable that these particles where in thermal equilibrium before the breaking of supersymmetry. But the argument about the domination of radiation does not depend on thermal equilibrium anyway, as far as I can see.

  8. #8 ThirtyFiveUp
    March 9, 2010

    Following along with eyes so wide they may pop out. Thanks for linking to parts 1,2,3,4,5. Big help.

  9. #9 Patk
    March 10, 2010

    I remember my first encounter with this whole shebang – I’d been an astronomy buff my entire life, but when I walked in to my Astronomy class in college, I had no idea the Big Bang was so complicated, but it fascinated me because it wasn’t a neatly bound package of “Boom! Universe!”

  10. #10 ScentOfViolets
    March 10, 2010

    @Bjoern:

    But as I pointed out beforehand, you’re making some explicit assumptions about dark matter that I don’t think are warranted by what we know right now. Sure, if dark matter is some heavy cold particle set that only interacts via the weak force and gravitation this is true, power laws being what they are.

    But I haven’t heard anything about this if so – shoot, weren’t neutrino’s dismissed as a candidate for dark matter and then put back in . . . twice? – and afaik, we’re still looking at axions, WIMPs, et al as candidate particles.

  11. #11 Bjoern
    March 11, 2010

    @ScentOfViolets: As far as I can see, I’m only assuming that the dark matter is “cold” (i. e. made up of particles which are heavy enough that their rest energy is far greater than their kinetic energy – WIMPs would be an example for that). And the LCDM model, which is the standard model in cosmology today, uses (as the abbreviation says) essentially only cold dark matter. I don’t see where I used anything about the possible interactions of dark matter.

    I don’t know much about axions, but neutrinos have quite definitely been ruled out as a relevant contribution to dark matter.

  12. #12 ScentOfViolets
    March 11, 2010

    @Bjoern: See here for a start to how neutrinos could be a possible candidate. If you’ve got something within the last year or so that dismisses this possibility, I’d like to see it.

    Understand, I’m not trying to imply that you’re wrong – as I said, power laws are power laws. The point I am trying to make is that dark matter, afaict, is sufficiently poorly understood that we don’t know if it would behave like a fourth power or a third, or something in between, or perhaps something else altogether.

  13. #13 Bjoern
    March 11, 2010

    @ScentOfViolets: Your “here” link merely links back to this comment thread…?!?

    If you’ve got something within the last year or so that dismisses this possibility, I’d like to see it.

    Well, I have the WMAP 7 year results:
    http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/cosmology/wmap_7yr_cosmology.pdf

    They give the following relation between the density parameter for neutrinos and the neutrino masses (between equations 27 and 28): the sum of all neutrino masses, divided by 94 eV, is equal to the density parameter times h^2. Then, in section 4.7, they give an upper limit on the sum of all neutrinos masses of 0.58 eV (they also provide other upper limits, using additional data, but this is enough to show the point). Dividing by the 94 eV, one immediately sees that the neutrino density parameter times h^2 is at most about 0.006. Compare that to the density parameter of dark matter in total, which is about 0.11 (same paper, table 1 on page 3). Hence neutrinos make up at most about 5 to 6% of all dark matter.

  14. #14 ScentOfViolets
    March 11, 2010

    @Bjoern:

    Sorry about that, I’m home sick today with something I’ve caught from my daughter. This is the link. You can work outwards from there.

  15. #15 Bjoern
    March 11, 2010

    @ScentOfViolets: Hope you get well soon!

    With respect to your link: essentially, sterile neutrinos could be cold dark matter (and then everything I said above automatically also applies to them, I’d say), or they are warm dark matter. In the latter case, they behave something in between usual matter and radiation – and then you are right, then their contribution becomes harder to calculate. Nevertheless, I’d say one still could determine without much great work when the radiation-dominated era ended.

  16. #16 ThirtyFiveUp
    March 11, 2010

    Somewhat OT, but Daily Kos put up a What is Dark Matter post. Some of the 95 comments are good. Some are dumb.

    http://www.dailykos.com/story/2010/3/10/844892/-Dark-Matter-Mystery:-CERN-Collider-Aims-for-Record-Energy;-DK-Greenroots

  17. #17 Thomas Neil Neubert
    March 12, 2010

    The thing that I miss most in Ethan’s The Greatest Story Ever Told is the word “IF”. “IF” this hypothesis is true, “IF” such and such concept or interpretation is correct. But skepticism seems to have disappeared from the consensus world of cosmologists.

    Such certainty as is found in current cosmology has precedent. “By the same argument as the preceding it can be shown that the earth can neither move in any one of the aforesaid oblique directions, nor ever change at all from its place at the centre.” Claudius Ptolemy.

    So to find healthy skepticism, we must turn to leading physicists who are not cosmologists. For example, Robert B. Laughlin, Nobel Prize 1998, had this to say in 2005, “Exploding things, such as dynamite or the big bang, are unstable. Theories of explosion, including the first picoseconds of the big bang, thus cross Barriers of Relevance and are inherently unfalsifiable, notwithstanding widely cited supporting “evidence” such as isotropic abundances at the surfaces of stars and the cosmic microwave background anisotrophy. One might as well claim to infer the properties of atoms from the storm damage of a hurricane.”

  18. #18 Bjoern
    March 13, 2010

    @Thomas:

    But skepticism seems to have disappeared from the consensus world of cosmologists.

    There *is* still a bit of skepticism – but it has become so small that most people don’t bother mentioning it anymore. In the last decade, so much evidence has been amassed for the LCDM model that most cosmologists take it for granted – although they still know that it, in principle, could be overturned in the future. Hey, do you also expect people still saying “*if* General Relativity is true” or something like that?

    Comparing this to Ptolemy makes little sense – because all that Ptolemy attempted to do was “saving the appearances”. He didn’t really have a consistent theory explaining the solar system – he had a hodge-podge of mutually contradictory hypotheses.

    For example, Robert B. Laughlin, Nobel Prize 1998, had this to say…

    You do know what’s an “argument from authority”, and why it is in general not valid – don’t you? (oh, BTW, I don’t think his claim makes sense – starts right with the usual fault of comparing the big bang to an ordinary explosion…)

  19. #19 Enrique
    March 15, 2010

    whaht happens to the radiation after the objects are “running into each other?”

  20. #20 Thomas Neil Neubert
    March 18, 2010

    Bjoern, as for my “argument from authority”, you are missing my point. Yes I quoted Laughlin in part because he is an authority; but that is also in part why I listen to you and Ethan. But also you, Ethan and Laughlin make sense to me.

    You are correct that cosmologists’ “skepticism … has become so small that most people don’t bother mentioning it anymore.” To me that is the big problem with big bang cosmology.

    Back to Robert B. Laughlin, may I highly recommend his book A Different Universe for his sense of skepticism. For example, here is Robert B. Laughlin’s answer to the question, “Is Einstein relevant any more?”

    RBL answers, “Einstein’s ideas were certainly right, and one sees evidence for them every day, but the deeper sense of the question (is) not whether relativity was right as whether fundamental things mattered and whether there (are) any more of them left to discover. (I have) heard this concern voiced again and again in my travels around the world and have come to recognize it as technical hubris– like the suggestion in 1900 that the patent office should be abolished because everything had already been invented. Just look around you. Even this room is teeming with things we do not understand. Only people whose common sense has been impaired by too much education cannot see it. The idea that the struggle to understand the natural world has come to an end is not only wrong, it is ludicrously wrong. We are surrounded by mysterious physical miracles, and the continuing, unfinished task of science is to unravel them.”
    OK, that’s enough RBL quote to get the sense of him.

    Now back to my layman’s skepticism. The big bang theory is the best theoretical framework around; but it is an unfinished and unsustainable theory unless certain underlying extraordinary hypothesis are more fundamentally understood and proven to be correct(e.g. inflation, dark matter, baryogenesis, etc.) My personal view is that many of these extraordinary assumptions will be proven to be incorrect.

    Thus, the thing that I miss most in big bang cosmology is the word “IF” and the implication that all that is left is to fill in a few details.

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  28. #28 Sherrie
    November 11, 2010

    I love science but i wish it were possible to know what existed (and why) before the big bang that began the creation of the universe. Perhaps that’s where religion comes in.

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  30. #30 vigrx
    November 22, 2010

    Your introduction quote is really funny. lol. Kidding aside, it was said that antimatter can be a great source of energy. But the risk is too high using antimatter so I might as well be contented to our energy sources now.

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