Bouncing Back

In Cosmology by Brian Koberlein20 Comments

The Universe began with a big bang. Not an explosion from a single point, but rather an early dense state. Of course an obvious question this raises is what came before the big bang? While it’s possible that the answer is “nothing,” that hasn’t stopped some theorists from postulating an earlier cause for the Universe. One of these ideas is known as the big bounce. 

The basic idea of the big bounce is that the Universe goes through a series of expansions and contractions. Right now we live in an expanding Universe, but at some point, the model argues, the Universe will start to contract. Eventually it will contract to a dense fireball again, and this will trigger a new big bang. This solves the “what came before” problem of the big bang by postulating an infinite series of big bangs, but it’s not without problems. For one, as we currently understand dark energy the Universe will likely continue to expand forever. For another, if the Universe did re-collapse into a dense state, we have no idea how it would trigger a new big bang.

A new work in Physical Review Letters proposes a solution to this second problem. The key to the idea is to introduce quantum theory into the mix. In a purely classical model, a shrinking universe will eventually collapse into a singularity. It’s long been thought that quantum theory could provide a solution to this problem, but the devil is in the details. To prevent the formation of a singularity, the work introduces a symmetry known as conformal invariance. As long as the Universe has this symmetry during its dense period, it could enter the dense period at the end of one “universe” and re-expand to form a new “universe.” The authors call this a perfect bounce.

So with the right conditions it’s possible that our Universe could simply be the period between bounces.

Paper: Steffen Gielen et al. Perfect Quantum Cosmological Bounce. Phys. Rev. Lett. 117, 021301 (2016). DOI: 10.1103/PhysRevLett.117.021301


  1. If that dense state was actually extruded from a zero-dimensional entity or membrane, then couldn’t that exist absent of time?

    1. Can’t edit. If it did, then that entity would probably “still” exist and might be verifiable.

      1. As a mathematical entity, yes. However, I rather doubt that the relevant physics has been worked out, in a fully self-consistent way.

  2. What I like about this theory – or at least its version originally proposed by Paul Steinhardt – is that it is falsifiable. Namely, it predicts that there could be no gravitational waves leftover after the Big Bang, which is the case in the theory of inflation.

  3. (Forgive my bad English) The human mind cannot handle neither the idea that all the matter-energy and space-time was always there (big bounce) nor the idea that the Universe appeared from nothing (just big bang) It’s a matter of Faith (so far) that we explain any of these ideas with God creating the Universe, or God being the Universe, but even if God is the explanation, the same Big Question remains: how.

    1. “The human mind cannot handle neither the idea that all the matter-energy and space-time was always there (big bounce) nor the idea that the Universe appeared from nothing (just big bang)”.

      Well, those ideas were developed using mathematics, and applying them to the universe as a whole. The human mind can, with appropriate training, easily handle these ideas; for example, the relevant mathematics is well-understood.

      In any case, these ideas concerning cosmology are far easier to grasp than many aspects of quantum mechanics (QM), yet thousands of people have no real difficulty in understanding QM sufficiently to apply it; for example in designing things like MRI equipment, the repeaters in the optical fiber cables that span oceans, and lasers.

  4. Yes, there’s a problem to be addressed at the quantum level, and maybe this gets us there, and maybe it doesn’t. There is also a problem on the other end of the scale. Think of it like this.

    If the universe contains all matter and all energy, how can it expand at an accelerating rate if it is a closed system?

    1. “If the universe contains all matter and all energy, how can it expand at an accelerating rate if it is a closed system?”

      Very easily; the relevant mathematical models are self-consistent. Though how to grasp this intuitively, or at a physics-as-just-words level, may be more difficult.

  5. Thank you Jane for answering. As far as I could read and understood, scientists have theorized about how the Big Band could have appeared from nothing, but there is always a primordial “something”, i.e. quantic fluctuations or whatever. Well, but how did this “whatever primordial” appear from The Nothing? I mean, let’s try to imagine the really Nothing: nothing, nothing, nothing. No quantic fluctuations, no any mathemathical construct used to initiate anything: Nothing. How any primordial seed appear? This is the Last Question, in my opinion,

    1. Let me ask that in a different way. Where does nothing come from? Is nothing a natural state in the universe? We’ve never found anything in space that could be described as nothing, not even black holes. I’m not sure there’s anything in the laws of physics that would require the universe to have been nothing at some point.

      1. I guess that since the “beggining” of the universe, there isn’t Nothing anymore, but the Nothing could have been existed before, I don’t know (I’m just a chemical engineer, jajaja) But even if it is as you propose, that perhaps never was nothing, whatever was before the Big Bang must have had an origin, at least is what we humans tend to think, that everything had an origin.

        1. I think there’s some confusion over “before”. If it refers to time, and if time itself has an origin at the Big Bang (that’s two “ifs” ;-)), then asking about what happens before is a bit like asking where is north when you’re standing at the North Pole. Or what happens when the temperature of something is reduced “below 0K” (actually, negative temperatures do have some meaning, in certain, very carefully defined, situations). Etc.

          If, on the other hand, “before” refers to something other than time, …

    2. @Bernardo: the idea that the universe could have arisen “from nothing” (at its origin) is common in popsci articles. In the relevant papers on this, I suspect that the authors (scientists) did not propose anything like this. Rather, at least two distinct possibilities: a) that the two most powerful theories in physics – QM and GR – are so mutually incompatible that it’s impossible to say anything sensible about this physical regime; b) that everything, including time, began “then” (i.e. no primordial quantic fluctuations, or anything else).

      In his article, Brian was careful to write this “While it’s possible that the answer is “nothing,” that hasn’t stopped some theorists from postulating an earlier cause for the Universe”, with the N-word in quotes.

      As we have, as yet, no testable ideas re the physics which applies in the extremely dense, hot state that the observable universe was likely in, all that time ago, the best (scientific) answer is surely “we don’t know”?

  6. I have always considered this a fantastic theory, the big bang theory itself in my opinion makes scientists seem like a bunch of dum dums. “A very long time ago there was just a big bang… no one knows what causes big bang it just… happened?”

    1. The “it just happened” is not part of the scientific explanation, surely.

      There are lots of “fundamental questions” whose answers are, today, some variation of “we don’t know” or “no one knows”. One example: the nine Yukawa couplings for the quarks and leptons (equivalent to specifying the rest mass of these elementary particles; thank you Wikipedia) … why do they have the values they are observed to have? No one knows.

      And it’s quite easy to re-cast such questions in a form very similar to Drew Hoffmann’s quote: “A very long time ago the nine Yukawa couplings for the quarks and leptons got the values we observe them to have today … no one knows what caused this to happen.”

      So why does a fundamental question in cosmology make scientists seem like a bunch of dum dums, but similar questions about particle physics do not?

    2. In the immortal words of Dara Ó Briain: Science doesn’t know everything, and science knows it doesn’t know everything, otherwise it’d stop.

  7. I like it because it demonstrates that there is nothing new under the sun. Astronomers Burbidge proposed this expanding/contacting theory decades ago.

  8. This is a really outstanding, this is science.
    The universe is hanging out in the darkness. After the big bang formed the universe, two main forces which you mentioned earlier chose the big stars to control the universe. Time after time, level after level, civilization after civilization raises a question. How many civilizations are in the universe? We are the first or are we the last? You have to know the shape of the universe to answer these questions. I will show you what it looks like but you have to find a way but not here. No one is going to find data for that. Everything has timing. No one sees what you see, no one hears what you hear, and know one knows what you know. This is not a dream, its more than that.

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