The universe is the poor man's particle accelerator

By Sean Carroll | December 19, 2005 11:41 am

David SchrammOne thing I wanted to add to Mark’s post about the New Views conference. The conference as a whole was dedicated to the memory of David Schramm, whose 60th birthday would have been this year; he died while piloting his own airplane in 1997. Schramm was an enormously influential figure in contemporary cosmology, one of the prime movers in bringing together particle physics and astrophysics in the study of the early universe. In particular, he was a pioneer in the use of Big-Bang Nucleosynthesis as a way to understand both particle physics and cosmology.

Between a few seconds and a few minutes after the Big Bang, the universe was a nuclear reactor, converting nucleons (neutrons and protons) into nuclei of helium, lithium, and deuterium. At very high temperatures the nucleons can’t bind together without being knocked apart; at low temperatures they would like to be bound into their lowest-energy state, which would be iron nuclei. But the universe is rapidly expanding, so we get a competition: as the temperature declines and it’s possible to form nuclei, the density is also falling, making reactions less frequent. We end up with several light nuclei, but don’t have enough time to make anything heavier.

The relic abundances of these nuclei depend on everything about physics when the universe was one minute old: particle physics parameters that govern the reaction rates, the number of species that governs the energy density, and the laws of general relativity that govern the expansion of the universe. (For example, if the universe were expanding a little bit faster, the reactions would happen a little bit earlier, implying that fewer neutrons would have decayed, allowing for the production of more helium.) Miraculously, the observed abundances fit precisely onto the predictions that come from extrapolating what we know about physics here and now all the way back to a minute after the Big Bang. The helium abundance provided the first empirical evidence that there were only three families of matter particles, long before Earth-based particle accelerators verified the result. And BBN assures us that Einstein’s general relativity works without modification in the very early universe; in particular, we know that Newton’s constant of gravitation had the same value then as it does now to within about twenty percent.

Personally, I find the success of BBN to be one of the most impressive feats in all of modern science. Here we are, 7,000,000,000,000,000 minutes after the Big Bang, making quantitative statements about what was going on 1 minute after the Big Bang — and it’s a perfect fit. I’ll never cease to be amazed that we know exactly what the universe was doing when it was one minute old.

  • Science

    Sean, what is interesting is public ignorance of the evidence for the big bang. You find a large dismissive reaction to the big bang as being speculative, when the evidence is conclusive that:

    (1) nuclear fusion is the only way known that could have created the light elements in the abundances observed in the universe, and

    (2) the redshifted cosmic background radiation from 300,000 years after the big bang is the most perfect Planck spectrum ever observed.

    You always find people trying to dismiss it all as speculative, despite the overwhelming evidence above. “Tired light” or “scattered light” speculations, violating conservation of energy somehow, with no tested predictions and no mechanism, are put up as a “counter-argument” to explain redshift. People don’t seem to grasp that this is just like putting up a flat earth theory to counter the well-tested facts.

    When you claim +/- 10% agreement on G could you provide reference please? (The one big failure of general relativity for the big bang is that the gravitational effect is out by a factor of 10, implying unobserved dark matter.)

  • Sean

    Best I could find after 30 seconds of searching was this paper, which puts a 20% limit on the variation of G. So I edited the post, just to be accurate.

  • Science

    Thanks! Fusion rate would increase (due to compression) if G rises, but would be reduced if the Coulomb repulsion between protons also rises: the two effects offset one another. So G will appear constant if you it is really varying and you ignore a similar variation with time of Coulomb’s law. The strong nuclear force can’t cause fusion beyond a very limited range, so the longer range forces control the fusion rate.

  • Count Iblis

    Michael Duff had some comments on that paper:

    Comments: In the light of a recent paper by Copi, A. Davis and Krauss which claims to place constraints on the time variation of Newton’s constant, $Delta G/G$, we reiterate that dimensionless ratios such as $Delta G/G$, $Delta e/e$ and $Delta c/c$ are every bit as unit-dependent as their dimensional counterparts $Delta G$, $Delta e$ and $Delta c$. An obvious example is provided by units in which time is measured in years and distance in light-years. Here $c=1$ and $Delta c/c$=0, whatever the theory. We also respond to remarks by Magueijo and T. Davis

  • Eugene

    Just a note on those constraints by Copi They didn’t use He or Li abundances constraints; they just use deuterium abundances and WMAP baryon-photon ratio. I also don’t like their direct use of the latter : since if G is changing, then one should not trust the WMAP results as the CMB peaks will change too. Anyway, their constraints are pretty weak, the 20% values quoted is 68% confidence. At 95%, it’s about 40%.

    Standard dG/G values with the full load of abundances, are around 10% give or take a few. I think He gives the best constraint, but somebody please correct me if I am wrong since I’ll like to know the real answer… gives the most recent He abundances constraint.

  • Aaron

    Whoa! I’d never heard of BBN before… it sounds really cool!

  • Plato
  • Helge

    Hey Sean :-)
    Something I have always wondered about is: You write: “Between a few seconds and minutes after Big Bang.” How do I have to understand this? Thinking about everything I know about general relativity it makes no sense to speak about “that event took place at that time.” But all physicists and cosmologists do it when speaking about the Big Bang.
    And now some random bitching: Why is your book so expensive? Can’t you bring it out in paperback, so it would be a little less?

  • Sean

    Hi Helge– A perfectly good question. When we speak informally of something happening a certain time after the Big Bang, we’re being sloppy in three different ways: first because the Bang itself is a singularity that isn’t really part of spacetime, second because all sorts of unexpected things might be happening in the early universe that make things different from our current expectations, and third because time is not universally defined in relativity. But all that is okay. What we really mean is “the time that would be experienced by an observer co-moving with the cosmological fluid between this moment and a moment arbitrarily close to the singularity, in a standard radiation-dominated Friedmann-Robertson-Walker cosmology.” But that’s not nearly as snappy as “the time since the Big Bang,” so we’re going to keep on saying that in the hope that all the caveats are understood.

    As far as the book is concerned, I’m the wrong guy to bitch to, you should talk to the publisher. But I estimate the amount of intellectual fulfillment contained in my book to be worth at least $200, so it’s still a bargain!

  • Elliot


    Is the Big Bang “really” a singularity? Or is that what the best available (GR based) theories tell us. Don’t people generally believe that singularities are eventually going to be extinguished by a quantum theory of gravity. (both in black holes and the big bang) or is there something special about the Big Bang which truly make it singular.



  • Sean

    We use the term “Big Bang” loosely, to refer either to the singularity predicted by classical GR or to whatever will replace it in the eventual theory. Most of us believe that the singularity will be replaced by something, but we’ll still call it the Big Bang.

  • Plato
  • Science

    On quantum gravity: the error is not necessarily in quantum mechanics, but in the incompleteness of GR.

    My point is that if the difference between electromagnetic force and gravity is the square root of the number of charges in the universe (and there is only one proof of this available), this ratio is nearly fixed to the present-day figure within 1 second of the big bang by the creation of quarks.

    If gravity was x times weaker during the fusion of nucleons into light elements in the first few seconds to minutes, then Coulomb’s law would also be x times weaker. So the gravitational compression would be less, but so would the Coulomb barrier which hinders fusion. The two effects cancel each other out. Therefore you have no basis whatsoever to claim G only varies by 20%.

    What you should say is that fusion calculations validate that the ratio of gravity to electromagnetism was the same to within 20% of today’s ratio.

  • Plato

    Hope you all have a Merry Christmas


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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