Will we ever know what happened before the Big Bang?

(This piece originally appeared in the “Will We Ever?” column at BBC Future. I added some commentary at the end and changed the accompanying image, but otherwise left everything the same, including UK spelling.)

If you think theories about the universe are mind-bending, rest assured that many scientists feel the same way. But the question isn't a philosophical one: it has potentially real, testable aspects.  [Credit: ESA/Planck Collaboration/D. Ducros]
If you think theories about the universe are mind-bending, rest assured that many scientists feel the same way. But the question isn’t a philosophical one: it has potentially real, testable aspects. [Credit: ESA/Planck Collaboration/D. Ducros]
In many ways, it’s strange to us humans that the Universe should be the age it is. The Universe – by definition, everything that physically exists – should either be infinite in age, or somehow tied to the lifespan of the human species, as it does in many mythologies. However, thanks to studies on the rate the Universe is expanding, and applying this knowledge in reverse, we know its age. Roughly 13.8 billion years ago, all we can observe on Earth, in our solar system, other galaxies and everything in between expanded out rapidly from an initial point much smaller than an atom, which we call the Big Bang.

The Big Bang model is our best explanation for why the cosmos appears as it does. Nevertheless, it’s not able to answer some of the more challenging questions, including what – if anything – came before it? Despite how it might sound, this question isn’t a philosophical one: it has potentially real, testable aspects. But to understand any of the possible answers, we must first understand the question itself.

First of all, the language we use to describe what we know and don’t know can sometimes be muddy. For instance, the Universe may be defined as all that exists in a physical sense, but we can only observe part of that. Nobody sensible thinks the observable Universe is all there is, though. Galaxies in every direction seem similar to each other; there’s no evident special direction in space, meaning that the Universe doesn’t have an edge (or a centre). In other words, if we were to instantaneously relocate to a galaxy far, far away, we’d see a cosmos very similar to the one we observe from Earth, and it would have an effective radius of 46 billion light-years. We can’t see beyond that radius, wherever we’re located.

For many reasons, cosmologists think the early Universe underwent inflation: an incredibly rapid expansion right after the Big Bang. As the Universe expanded, it also cooled, so in the distant past, it was hotter, more dense, and opaque to all forms of light. When the cosmos became transparent, about 380,000 years after the Big Bang, it left behind a bath of photons, detectable today as the Cosmic Microwave Background (CMB). As space observatories like COBE, WMAP and more recently Planck have shown, the CMB is remarkably smooth, but not quite uniform. Within this were tiny ripples that were stretched to enormous sizes during inflation, and in turn these became the seeds for large-scale objects like galaxies and galactic clusters we see today.

While inflation comes in many possible versions, the gist is that random fluctuations in temperature and density produced by the Big Bang were smoothed out by the rapid expansion, much like a wrinkly uninflated balloon grows into a smooth object when filled with air. Inflation happened so quickly that, in many versions, the Universe has disconnected regions – parallel universes – that might even have different sets of physical laws. Inflation would have produced a lot of gravitational waves – fluctuations in the structure of space and time – which in turn should leave their mark on the CMB. Gravitational waves in the very early Universe churned up space-time, creating an environment that twisted light emission.

Pocket universes

However, none of this really tells us what, if anything, came before the Big Bang. In many models for inflation, as in some of the older Big Bang theories, this is the only Universe that exists – or at the very least, the only Universe we can observe.

A partial exception to this is a model known as eternal inflation. In this scheme, the observable Universe is part of a “pocket universe”, a bubble in a larger froth of inflation that is ongoing. In our particular bubble, inflation began and ended, but in other pocket universes – unconnected (“parallel”) and thereby unobservable to our pocket universe – inflation might have had different properties. Eternal inflation effectively emptied the regions outside of bubbles of all matter; these would have no stars, galaxies, or other familiar hallmarks.

If eternal inflation is correct, then the Big Bang is the origin of our pocket universe, but not the beginning of the whole Universe, which may have begun much earlier. The evidence for multiverses will be indirect at best, even with confirmation of inflation from Planck or other observations. In other words, eternal inflation could answer the question of what preceded the Big Bang, but still leave the question of ultimate origin out of reach.

Trillion-year cycle

Many cosmologists regard inflation as being the worst model we have, except for all the alternatives. Inflation’s generic properties are pretty nice, thanks to its usefulness in solving difficult problems in cosmology, but the specifics are slippery. What caused inflation? How did it begin, and when did it end? If eternal inflation is correct, how many pocket universes could there be with similar properties to our own? Was there a Bigger Bang that started the multiverse going? Finally, since we’re scientists and not philosophers, how can we tell all of these options apart: can we test them?

There is one possible alternative to inflation, which bypasses these questions and, along the way, resolves what came before the Big Bang. In Paul Steinhardt and Neil Turok’s cyclic universe model, the observable Universe resides in a higher-dimensional void. Coupled to our universe is a parallel shadow universe that we can’t directly observe, but is connected via gravity. The Big Bang was not the beginning, but a moment when the two “branes” (short for “membrane”) collided. The Universe in the cyclic model goes between periods when the branes are moving apart, accelerated expansion, and new Big Bangs when the branes re-collide. While each cycle would take about a trillion years to complete, the whole cosmos could be infinitely old, bypassing the philosophical problems with inflationary models.

The cyclic universe is not a popular model among working cosmologists, but at least it could be ruled out by experimental observations: if the gravitational-wave signature of inflation is found, then the cyclic model is dead. The cyclic model isn’t complete: it doesn’t explain how much dark energy there is in the Universe any more than standard cosmology does, for example. In other words, the cyclic model is not complete, so at present there’s no physical evidence to distinguish it from inflationary models.

If you think all these options are fairly mind-bending, rest assured that professional scientists feel the same way. Since the observable Universe is currently accelerating with no sign of re-collapse even in the far future, why should there be a cosmos with a beginning but no similar ending? If inflation or the Big Bang erases information of what (if anything) came before, are we stuck debating over the number of angels dancing Gangnam Style on the head of a pin? Even if eternal inflation or the cyclic model is correct, it pushes the question of ultimate origin into the realm of untestability.

In another decade or century, the questions and the methods we use to answer these questions will most likely have evolved. But for now, it’s unclear how we can possibly know what preceded the Big Bang.


I usually avoid the kinds of sexy big questions that often make cosmology books by Paul Davies or Stephen Hawking or Roger Penrose popular. The main reason for that is because those big questions may not be answerable, because they are beyond the reach of our telescopes or experiments. One such question—what, if anything, came before the Big Bang?—is cause for a great deal of speculation, and a good amount of nonsense. If memory serves, Pope John Paul II was the first pontiff to explicitly accept Big Bang cosmology, but he also forbade Catholic cosmologists from even pondering the question of whether anything came before.

However, BBC Future provided me a great opportunity to examine the meta-question: “Will we ever know what happened before the Big Bang?” That’s a question better suited to me: it’s not speculation, but pondering how can we know? Thanks again to Simon Frantz, my editor at BBC Future, who asked me to write the piece and helped turn it into something coherent, instead of Grumpy Matthew grumbling into his coffee.

10 responses to “Will we ever know what happened before the Big Bang?”

  1. I respect very much your final statement that ultimate cause must exist , an existence beyond testability….a supreme extra-cosmic fact.
    All laws of logic dictate this conclusion.

    1. Torbjörn Larsson, OM Avatar
      Torbjörn Larsson, OM

      Creationists shouldn’t comment on science, it is hilarious and makes deconverts from religion, see Dawkins’s Convert’s Corner.

      The article soundly _rejected_ an “ultimate cause” in that it was describing how only natural processes has been found to suffice, which are causal but not atomistic “causes”, and in how eternal processes could be responsible for cosmology so no time to place an “ultimate” event. But of course you are not interested in the science, only in what seemed to fit your anti-science preconceptions.

      Moreover, this is empirical findings so logic is a negligible part of its foundation. In fact, Gödel showed that first-order logic alone cannot “dictate” (result in) arithmetic, which is the basis for empirical quantification.

      Finally, if you want to find an existence beyond current testability, the emergence chemistry and especially its gas giant core high pressure regime out of first principles of quantum mechanics comes to mind. Yet gas giants exists and are known results of natural processes.

  2. Torbjörn Larsson, OM Avatar
    Torbjörn Larsson, OM

    So inflation is like democracy, it works but people are eternally dissatisfied.

    The cyclic model isn’t complete: it doesn’t explain how much dark energy there is in the Universe any more than standard cosmology does, for example. In other words, the cyclic model is not complete, so at present there’s no physical evidence to distinguish it from inflationary models.

    First, as I understand it standard cosmology does predict the density of dark energy. It is the amount that is needed to balance gravity potential with the matterenergy contributions, or inflation wouldn’t occur. Or at least that is what I got from Krauss 2009 video on cosmology.

    If so then the question becomes why the rest of the contributions are what they are, e.g. what predicts the standard particle model.

    Second, there is no distinguishing evidence except perhaps the WMAP and Planck observations that only inflation predicts!?

    Here is what one cosmologist say on the Planck polarization data to date:

    “Still, at the small angular scales, the polarisation data can be trusted and in this data Planck have one of their most impressive figures. The figure below shows how both the temperature multiplied by the polarisation (pixel by pixel on the sky) and how the polarisation itself varies with angular scale. The blue dots are the measured signal. Now, the red curve is not the best fit curve to this data. That is worth pausing and reflecting on. If it isn’t the best fit curve, then what is it?

    That curve is the unique prediction from analysing Planck’s temperature data. There are no free parameters in defining those red lines. Once the temperature data is analysed, we can make an unchangeable prediction for what the polarisation should look like. The fact that the red line goes straight through the blue data points is absolutely remarkable. However, if one believes in the big bang and standard cosmological model, this is all that could have happened.”

    The figure text is also worthwhile:

    “These curves reflect some of the best of humanity. These are the tiny fluctuations in the polarisation of a field of radiation, left over from a hydrogen plasma that permeated the entire universe, 14 billion years ago. The oscillations in the curves come from sound waves in this hydrogen plasma. The curve is our prediction for this data, with no free parameters to play with at all. Just reflect on that. I’m unable to describe how incredible this is. We don’t even know whether Shakespeare wrote Shakespeare’s plays, but we can predict exactly what the polarisation in the CMB should look like.”

    Maybe I am mistaken, but I have never heard that the cyclic model predicts these features but instead that it was pretty much excluded pre-WMAP 9 and Planck 4 year data releases.

    Also, I seem to remember Bousso claiming that cyclic models has inherent difficulties with singularity theorems. Admittedly, I would have to check that.

    The evidence for multiverses will be indirect at best, even with confirmation of inflation from Planck or other observations.

    Isn’t a claim of “indirect” evidence just a result of sloppy thinking? I haven’t seen a testable definition of it.

    Conversely, all our facts are achieved not by preexisting knowledge but by observation some distance and, from relativity by necessity, some time removed. It is all the same under the method of statistical hypothesis testing.

    Eg we should be able to decide between eternal inflation and cyclic models, say, given sufficient constraint.

    1. I’m not sure what you’re talking about with regard to Planck polarization – the Planck collaboration hasn’t released any polarization data yet. They’re going to release that early next year, and it may or may not have anything to say about inflation. The Planck power spectrum is a measurement of temperature fluctuations, and I wrote about that here: https://galileospendulum.org/2013/03/21/planck-results-our-weird-and-wonderful-universe/.

      On your other point, there are plenty of things we have indirect evidence for, and that’s fine. We don’t have direct observations of the nuclear fusion process at the heart of the Sun, but the indirect observations (including helioseismology and neutrino experiments) tell us our model for that process is correct. The problem about evidence for the multiverse isn’t that it’s indirect, but that even if we have that evidence, we can’t probe the properties of the parallel universes.

  3. Torbjörn Larsson, OM Avatar
    Torbjörn Larsson, OM

    Eg we _would_ be able to decide et cetera.

  4. OK , Dr. Larsson , would you please answer the question that no atheist was ever able to answer …..
    What is the physical natural mechanism by which laws . principles , rules , constants …..(.ie the whole realm of the formal ) , were generated ?
    please ; no bluffffing or change of subject or saying some irrelevant things.
    aa.sh with respect
    NB ; I really would be glad to read THE ANSWER.

    1. I am cutting off this discussion right now; you may continue it in another space, but not in my comments section. This article was about whether we can experimentally determine what preceded the Big Bang and what that might mean from a physics point of view. Discussions of philosophy and theology and atheism have many forums on the internet, but this post is not one of them.

      Please refer back to the commenting policy if you object to my heavy-handedness: https://galileospendulum.org/commenting-policy/.

  5. Dr. Francis ; But this is discrimination based of personal views , my question is 100% scientific …..OK let me read your scientific answer for it as it is very relevant to your topic , i am talking science and asking for non-biased scientific answer.

  6. […] the concordance model of cosmology, which also includes the basic ingredients of cosmic flatness, inflation, dark matter, and dark energy. However, “pretty well” isn’t always good enough, […]

  7. [removed for ignoring the commenting policy]

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