Digging Up the Early Universe

By Sean Carroll | October 2, 2012 8:31 am

I wrote another column for Discover (the actual magazine), which is now available online. It’s about how far back in cosmological time we can push our knowledge on the basis of actual data, not mere theory.

Of course we literally look back in time every time we peer into a telescope, since it takes time for light to travel to us from distant objects. But there’s an earliest moment we can possibly see using light — the moment of recombination, about 380,000 years after the Big Bang, when electrons hooked up with protons and other nuclei to form atoms. Earlier than that, the electrons were floating around freely, bumping into photons, and generally making the universe opaque.

So we have to be a bit more clever. And we have been: using the fact that the early universe was a nuclear fusion reactor, and observing the surviving abundances of light elements to pin down what conditions were like at that time. This technique gets us within seconds of the Big Bang. But if things break just right — the dark matter turns out to be a weakly-interacting particle, whose properties we can study here on Earth — we might be able to push the data-informed era much earlier back than that.

Think about what that means: Sitting here on Earth, cosmologists extrapolated our understanding back 13.7 billion years, to a few seconds after the universe began. We used that understanding to make predictions about the current universe—and we were right. We may not know for sure whether it will rain tomorrow, but we do know exactly how protons and neutrons bounced around like Super Balls in the nuclear inferno of the Big Bang. This will surely go down as one of the most impressive accomplishments of the human intellect.

And yet cosmologists want to do better still. The goal is to discover relics that predate even Big Bang Nucleosynthesis. At the moment that’s not quite possible, but there is one promising candidate: dark matter, the dense but unseen stuff that holds galaxies together.

Roughly speaking, if we get lucky, we could learn about conditions in the universe about 1/10,000th of a second after the Big Bang. We’d like to go even much earlier than that, but let’s not forget to be impressed at how well we’ve already done.

  • Zathras

    You young’uns got it so easy…..back in my day we were nothing but electrons and protons bouncing around in an opaque, tiny baby universe…..;-)

  • philh

    Why no mention of the hoped for detection of primordial gravitational waves ? Its a long shot that Planck will find them via the B mode but you never know and there results are only a few months away now. If not some future mission either via the B mode or direct detection via some very far off mission such as the BBO.

    • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean Carroll

      Both gravitational waves and neutrinos could give us a window on even earlier times. But there’s a strict length limit to these columns, and I wanted to highlight the parallel between the nucleosynthesis case and the dark matter case.

  • David Lau

    Sean, how does the properties of dark matter allow us to be able to push the data informed era even more back in time?

    • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean Carroll

      By comparing our theoretical prediction (which depends on conditions in the early universe) to the observed abundance.

  • http://rohanmedia.co.uk Rohan

    When I’m a physicist discovering something I’m going to call it doesit matter.

  • Zathras

    @5 Rohan:
    Then would the complementary anti-matter version be called….doesnt matter?

  • martenvandijk

    I advise the physics community to hire forensic scientists to do the digging and the research on the ”remains”. They might discover a terrible secret.

  • Rahul

    My mistake, the question about fusion has been answered via a link in the main article. I should read first! However, I would be grateful for answers to my other questions or links to help my understanding.

  • Shantanu

    Sean, pardon my off-topic comment here , but I didn’t see any mention of Leonid Grishchuk passing away (about 3 weeks ago) on this blog, inspite of the fact that almost all CV authors are cosmologists.
    Anyhow just wanted to share this sad news(in case anyone was not aware of it.)

  • Marshall Eubanks

    If Dark Matter is really strange matter, i.e., if it is made up of quark nuggets or compact composite objects (CCOs), then there should be bits of the QCD phase transition at the time of baryogenesis floating around where we can detect them, which would mean we could study objects formed when the universe was roughly 5 microseconds old. (In the CCO idea the objects are massive enough that they don’t interact much because of their low numbers, not because of a very low cross section, so you don’t need WIMPs.)

    See http://arxiv.org/abs/astro-ph/0603064 for more about this fascinating idea. Zhitnitsky prefers a CCO mass of about 1000 tons; if all of the Dark Matter is made of this stuff at that mass then one such object should pass through the Earth roughly every 3 years, and could potentially be detected via seismology.

  • Uncle Al

    Given a universe filled with high density hot H + He plasma slowly cooling by adiabatic expansion, will it super-radiantly lase proximate in time to recombination? Deeply red-shifted, narrow bandwith sparkles would be telling.

  • Cosmonut

    Must say this kind of thing makes me feel quite disappointed with the Universe. :)

    I mean all those vast endless spaces and billions of galaxies, and it only takes a group of jumped up apes sitting in a tiny corner a few hundred years to figure it all out.
    C’mon there’s GOT to be more to it !!

  • http://empiricalperspectives.blogspot.com/ James Goetz

    “Of course we literally look back in time every time we peer into a telescope, since it takes time for light to travel to us from distant objects. ”

    Not only that, we look back in time every time we peer into a mirror….

  • martenvandijk

    In reaction to what Paul Brown has said about evolution and Big Bang, I like to mention that evolution is a hell of an idea, but as far as Big Bang is concerned, to hell with it!

  • http://www.cynthiaracette.com Cynthia

    Hubby and I just finished last DVD of your Learning Co. lessons on dark matter and dark energy. Since we graduated from college in late 60’s (chem. & phys.) things have changed a whole lot. Enjoyed the DVDs and learned a lot and got confused a lot. If you’re ever in W. NY for a talk at U of Buffalo, we’ll come see what’s changed since your DVDs were made. We’re also doing research on internet on newer discoveries and ordered your two books to read. Go for it, Sean!


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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] cosmicvariance.com .


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