The Big Bang: What We Know and How We Know It

When most people think of the Big Bang, they imagine a single moment: a whole universe emerging from nothing. That’s not really how it worked, though. The Big Bang refers not to one event, but to a whole scientific theory. Using Einstein’s equations and some simplifying assumptions, we physicists can lay out a timeline for the universe’s earliest history. Different parts of this timeline have different evidence: some are meticulously tested, others we even expect to be wrong! It’s worth talking through this timeline and discussing what we know about each piece, and how we know it.

We can see surprisingly far back in time. As we look out into the universe, we see each star as it was when the light we see left it: longer ago the further the star is from us. Looking back, we see changes in the types of stars and galaxies: stars formed without the metals that later stars produced, galaxies made of those early stars. We see the universe become denser and hotter, until eventually we reach the last thing we can see: the cosmic microwave background, a faint light that fills our view in every direction. This light represents a change in the universe, the emergence of the first atoms. Before this, there were ions: free nuclei and electrons, forming a hot plasma. That plasma constantly emitted and absorbed light. As the universe cooled, the ions merged into atoms, and light was free to travel. Because of this, we cannot see back beyond this point. Our model gives detailed predictions for this curtain of light: its temperature, and even the ways it varies in intensity from place to place, which in turn let us hone our model further.

In principle, we could “see” a bit further. Light isn’t the only thing that travels freely through the universe. Neutrinos are almost massless, and pass through almost everything. Like the cosmic microwave background, the universe should have a cosmic neutrino background. This would come from much earlier, from an era when the universe was so dense that neutrinos regularly interacted with other matter. We haven’t detected this neutrino background yet, but future experiments might. Gravitational waves meanwhile, can also pass through almost any obstacle. There should be gravitational wave backgrounds as well, from a variety of eras in the early universe. Once again these haven’t been detected yet, but more powerful gravitational wave telescopes may yet see them.

We have indirect evidence a bit further back than we can see things directly. In the heat of the early universe the first protons and neutrons were merged via nuclear fusion, becoming the first atomic nuclei: isotopes of hydrogen, helium, and lithium. Our model lets us predict the proportions of these, how much helium and lithium per hydrogen atom. We can then compare this to the oldest stars we see, and see that the proportions are right. In this way, we know something about the universe from before we can “see” it.

We get surprised when we look at the universe on large scales, and compare widely separated regions. We find those regions are surprisingly similar, more than we would expect from randomness and the physics we know. Physicists have proposed different explanations for this. The most popular, cosmic inflation, suggests that the universe expanded very rapidly, accelerating so that a small region of similar matter was blown up much larger than the ordinary Big Bang model would have, projecting those similarities across the sky. While many think this proposal fits the data best, we still aren’t sure it’s the right one: there are alternate proposals, and it’s even controversial whether we should be surprised by the large-scale similarity in the first place.

We understand, in principle, how matter can come from “nothing”. This is sometimes presented as the most mysterious part of the Big Bang, the idea that matter could spontaneously emerge from an “empty” universe. But to a physicist, this isn’t very mysterious. Matter isn’t actually conserved, mass is just energy you haven’t met yet. Deep down, the universe is just a bunch of rippling quantum fields, with different ones more or less active at different times. Space-time itself is just another field, the gravitational field. When people say that in the Big Bang matter emerged from nothing, all they mean is that energy moved from the gravitational field to fields like the electron and quark, giving rise to particles. As we wind the model back, we can pretty well understand how this could happen.

If we extrapolate, winding Einstein’s equations back all the way, we reach a singularity: the whole universe, according to those equations, would have emerged from a single point, a time when everything was zero distance from everything else. This assumes, though, that Einstein’s equations keep working all the way back that far. That’s probably wrong, though. Einstein’s equations don’t include the effect of quantum mechanics, which should be much more important when the universe is at its hottest and densest. We don’t have a complete theory of quantum gravity yet (at least, not one that can model this), so we can’t be certain how to correct these equations. But in general, quantum theories tend to “fuzz out” singularities, spreading out a single point over a wider area. So it’s likely that the universe didn’t actually come from just a single point, and our various incomplete theories of quantum gravity tend to back this up.

So, starting from what we can see, we extrapolate back to what we can’t. We’re quite confident in some parts of the Big Bang theory: the emergence of the first galaxies, the first stars, the first atoms, and the first elements. Back far enough and things get more mysterious, we have proposals but no definite answers. And if you try to wind back up to the beginning, you find we still don’t have the right kind of theory to answer the question. That’s a task for the future.

13 thoughts on “The Big Bang: What We Know and How We Know It

  1. ohwilleke

    It is worth observing how very little of the history of the universe is beyond definitive scientific backtracking.

    In the conventional chronology of the Big Bang universe, Big Bang Nucleosynthesis, which is largely confirmed experimentally, is expected to take place at 10 seconds to 1000 seconds after the Big Bang.

    The Planck epoch in which quantum gravity dominates, the GUT epoch during which the Standard Model forces are merged, the Electroweak and Inflationary epoch and Baryogenesis during which the electroweak force grows distinct from the strong force; inflation causes space-time to surge from smaller than an atom to 100 million light years; quarks come into existence, and perhaps leptons too, electroweak symmetry breaking, and the Quark Epoch, during which the electromagnetic force and weak force become distinct, the Higgs mechanism starts to function more or less as it does now, and there are quarks which have not hadronized in a quark-gluon plasma, all purportedly take place in the first 10^-6 seconds of the universe (about the mean lifetime of a muon).

    This is really the only portion of the chronology where our determinations become highly speculative, and even during the electroweak symmetry breaking and the Quark Epoch, the universe was at roughly the limits of the energies where the Standard Model is experimentally confirmed and so we can feel comfortable about relying on theory track back the present to the past that far.

    In between Big Bang Nucleosynthesis and the Quark Epoch we have the Hadron epoch during which quarks transform from a quark-gluon plasma to ordinary hadrons, and anti-hadrons are annihilated in collisions with ordinary matter hadrons, giving rise to the existing matter-antimatter asymmetry ratio in quarks, for about one second, which is also within the realm of Earth based high energy physics experiments.

    At the commencement of the next era, called the Lepton epoch, when the temperature of the universe is about 10 billion kelvins which translates into energy scales of 1 MeV, neutrinos “decouple” and cease to interact with ordinary matter. The Lepton epoch allegedly lasts nine seconds and during this era new lepton-antilepton pairs are created in abundance, but ultimately energy levels fall to a point where new pairs are not created and redundant lepton-antilepton pairs annihilate. This is also an era in which we know that the Standard Model works from experimental tests on Earth.

    So, we’ve got conservatively, 10 seconds in the history of the Universe that we haven’t directly observed, and less conservatively, more like 10^-6 seconds that we are really adrift in making confident statements about because we have no experimental evidence to extrapolate from. This is stunning impressive and that fact that we can’t get all of the way back to zero shouldn’t detract from how much we do know.

    The first 10^-6 seconds happen to be a very important and eventful fraction of a second, but simply putting that very early fraction of a second in a black box and inferring initial conditions at that point from what we do observe works very well for a great many purposes.

    Liked by 2 people

  2. hdhuffman

    All nonsense. If we knew so much, the “discovery” (heedless speculation) of “dark matter” and “dark energy” making up most of the universe would not have been a surprise.

    I left academic education behind at the very beginning of the present dogmatism that claims we know so much about so little, or maybe ten to twelve years after that (I left around 1979). In 1997, I made, on my own of course, the greatest discovery in history, that casually disproves the central theories of both the life and earth sciences. So I see the (huge) holes where consensus scientists see “settled science”.

    So, you have been warned. Take heed.

    Liked by 1 person

    1. Smitty Werbenjagermanjensen

      Making “the greatest discovery in history” is one thing, but making it casually is a real feat! I form a mental picture of you doing so with the easy nonchalance of, say, Archimedes getting into his bath. Or possibly of Newton and his apple tree, creating the gravity that now holds us all down! Kudos to you!


  3. Dimitris Papadimitriou

    The usual popularization , that the Big Bang singularity was a single point is a bit misleading , and in some cases ( as , for example , for an infinite FLRW universe ) is wrong.
    The same is true , in classical GR , for Schwarzschild black holes ( or white holes ):
    These are spacelike surfaces , and in the case of the initial BB singularity , the scale factor goes to zero , while the (Ricci ) curvature goes to infinity.
    Our universe’s beginning had also a special feature , that led to the approximately homogeneous and isotropic spacetime that we observe :
    That the gravitational entropy was very low at the beginning , a feature that inflation alone cannot explain (actually , it could made it even worse at the very beginning ,at least if something like the generalized second law holds , even approximately).


    1. 4gravitons Post author

      Maybe I’m missing something here, but is a surface with a zero scale factor not in fact a single point? Since the metric is proportional to the scale factor, and points in a metric space are identical iff the distance between them is zero.

      Popularization based on this does sometimes confuse people, because they have trouble imagining a point continuously expanding into an infinite space. And this isn’t helped by the occasional popularizer who says something like “the universe was the size of a breadbox” and forgets to say “visible universe”.


  4. Dimitris Papadimitriou

    A frequently asked question by many people , is how a possibly infinite universe ( as ours may be ),initiated by a single point , as you said.
    In that case , the universe was always infinite , even if the scale factor was zero!
    OK , more accurately , one could say that the ” zero moment of time” was mathematically undefined , but , anyway , the initial singularity was spacelike ,so ,in any case ,(even if the universe is finite ) ,the description of a spacelike singularity as a point is a bit misleading.
    More proper is , perhaps , the description of it as an initial moment of time.
    Something similar holds for black hole singularities , at least the most physically relevant ones , that are also spacelike.
    Unlike timelike singularities , as the idealized Kerr ( or charged BH) ones , these spacelike singularities , although different from the Big Bang one , are not pointlike.
    An interesting consequence is that the interior of a Schwarzschild BH has the “shape” of a 3d tube ( or spindle ) and its ( maximal) volume grows and grows as time goes by , because the geometry of spacetime inside the horizon is dynamical!
    Ok , l’m sure you already know all this stuff but l think that , maybe , professional physisists could have been more accurate in their descriptions of these subtle things , just to avoid answering the same questions again and again!

    Liked by 1 person

    1. 4gravitons Post author

      FWIW I haven’t worked much with singularities in GR directly, so I hadn’t put much thought into this. The impression I get from glancing around online is that this is even a bit more subtle, because when describing a singularity as spacelike you’re really making a statement about the neighborhood of the singularity, not the singularity itself (for which it doesn’t really make sense to speak about any geometric properties at all). Anyway, while I wouldn’t go so far as to call this a reason to avoid the “Big Bang singularity was a single point” popularization, it’s definitely a nice rabbit-hole to go down for audiences who are up for learning more.


  5. Dimitris Papadimitriou

    Maybe I have to emphasize that your posts , as far as I know them are ,in my opinion , very well written and have an intermediate level ( between pop sci and ” expert ” stuff ) that is suitable for most interested readers.
    I hope that my comments are not seem like nitpicking!

    Liked by 1 person

  6. Dimitris Papadimitriou

    As you said , the most interesting thing about spacelike ( or timelike or null ) singularities is not themselves ( they may not exist in reality after all ) , but what happens in their vicinity.
    In the case of the Big Bang , it seems to be conformally flat ( roughly speaking).
    The consequence is the homogeneous and isotropic universe that we observe.
    In the case of black holes is the internal structure and the dynamical nature of spacetime , with effects like the blueshift/ mass inflation instabilities in the vicinity of the inner horizon of rotating BHs , and the internal volume growth that l mentioned.
    By the way , most people who are concerned about the BH info loss problem , have not yet studied the backreaction effects of the Hawking radiation in the case of more realistic BHs that have these instabilities , or the consequences for BH complementarity , strong holography and the like.
    There is no consensus , also , about the explanation for the very low initial entropy of the universe.
    These are difficult problems , there is no need for hastiness.

    Liked by 1 person

  7. Pingback: The Big Bang: What We Know and How We Know It | Later On

  8. Kevin Zhou

    Sorry to be the bearer of bad news, but you’ve just had your article sent out on The Browser, an email newsletter for literary folk, with tens of thousands of subscribers! You might want to prepare for a deluge of traffic, as well as a deluge of comments declaring that this is obviously all wrong because of something Aristotle said.

    Liked by 1 person

  9. Pingback: What I learned last week (#133): digital nomad - Get On With It

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