The Big Bang Wasn’t All At One Point (Cosmos Commentary)

I finally got around to watching the first episode of Cosmos. I quite enjoyed it, although probably not as much as I would have were I still 9 (which is the age I was back when I used to watch Carl Sagan doing Cosmos… and that, indeed, is probably a nontrivial part of why I’m an astronomer today rather than a paleontologist). I think it’s awesome that once again we’ve got a very charismatic astronomer on TV sharing the wonders of the Universe with us. Alas, I doubt it will have anywhere near the cultural impact that the original Cosmos did, simply because there is so much more out there to pay attention to now. (Not only is there more out there to pay attention to, but over time American society has become more and more ADHD.) Back in the late 1970s, there was little more than three networks of TV to choose from; it was the rare household that had cable. Now, most people have many more options for TV, never mind the ability to download stuff off the Internet on demand. Even if it’s just as high-quality, just as cool, and just as engaging as the old Cosmos was, I fear that the new Cosmos will not be noticed by as large a fraction of the population, and will be more quickly forgotten as people move on to the next shiny thing.

As for the show itself: it all seemed pretty basic to me, but then again, I’m a PhD physicist and professional astronomer who does a fair amount of astronomy outreach, so I was not the primary target audience. I liked the homage to the old show– not just the explicit one at the end (which brought a tear to my eye), but the “we are starstuff” comment, and the Ship of the Imagination (which, as Tyson points out, allows you to travel much faster the speed of light, something I’m doing all the time when I teach astronomy classes).

I did have a couple of quibbles, though. My first was when he was flying through the Solar System’s asteroid belt. The asteroid belt was thick with rocks, creating a massive hazard. The real asteroid belt is not like that. There is less mass, total, of asteroids, than there is in any single planet, and they’re spread out over a huge area in the disk of the solar system. This is why we can fly spacecraft through the asteroid belt without worrying about weaving and dodging. There, asteroids just aren’t that thick.

What, is this Cosmos, or The Empire Strikes Back?

To be fair, when he was out in the Oort cloud, although yet again they were shown too thick (I know, for purposes of actually being able to see something), he did mention that the Oort cloud objects are typically as far apart as Earth is from Saturn. Still, the visual image will stick with people more than the words.

My primary quibble with the show, though, is the title of this post. One sentence of what he said promulgated one of the primary misconceptions about the nature of the Big Bang. “Our entire universe emerged form a point smaller than single atom.” GAH! No! Indeed, Tyson was (perhaps deliberately) cagey about the difference between our Universe and our Observable Universe. He did use the term “Observable Universe”, with a good description. (It’s as far away as we can see, a horizon defined by the speed of light and the 13.8-billion-year-old age of the Universe.) However, thereafter, he seemed to be conflating the Universe with the Observable Universe. While there are some good reasons why one might do this, the way in which he did it fed into a very common misconception about the Big Bang.

Even though our observable universe is finite, the whole universe is much bigger– indeed, perhaps (probably?) infinite.

Here’s the real story, given the Big Bang model as we best understand and use it in astronomy: the Big Bang didn’t happen all at one point. Rather, the Big Bang happened everywhere. The problem with describing it as happening at one point is that it gives you the misconception that we could identify a point in space away from which everything is rushing. This is not the description of our Universe that shows up in modern cosmological models. Every point in the Universe is equivalently the center. Any point in space you can identify: that is where the Big Bang happened. Everything is rushing away from everything else. It’s really not like an explosion, where there’s a center everything rushes away from. (I wrote about this years ago in my blog post “Big Bang”: A terrible name for a great theory.)

Strictly speaking, it is true that our observable universe was once upon a time compressed into a size smaller than the size of an atom. However, saying that by itself implies a misconception: that that compressed, less-than-an-atom size of extremely dense, extremely exotic matter is all there was. In fact, that’s not right. Our Observable Universe was that small… but just as today there is other Universe (filled with galaxies) outside the boundaries of our Observable Universe, at that early epoch there was more extremely exotic dense-matter Universe outside the atom-sized ball that would one day expand and become today’s Observable Universe. Indeed, if the Universe today is infinite, it was always infinite… even back at that early epoch we’re talking about.

The Observable Universe (or a 2d projection thereof) at a period a tiny fraction of a second later than what I’m talking about in the text.
The whole Universe (or a 2d projection thereof) at the same epoch.

This may seem like a minor quibble, but the notion of the Big Bang as an explosion, something everything is rushing away from, is a very tenacious misconception that leads to other misconceptions about our Universe amongst many people I run into. It’s a little difficult to wrap your head around the real model– indeed, people find talks about cosmology that try to describe the real situation (and also the cosmology section of my current ongoing astronomy class) very brain-hurty. But, to my point of view, that’s part of the fun!

There was one throwaway comment about the Big Bang that Tyson made in Cosmos that I really liked. Just before the comment about the atom-sized Universe that got me worked up to make this post, he said about this early Big Bang epoch that “It’s as far back as we can see in time… for now.” That “for now” is great, and spot on. If you read A Brief History of Time by Stephen Hawking, he’ll talk about how the Big Bang was the beginning of time, and how it’s not even really meaningful to ask what was “before” the Big Bang. While that’s true in a purely classical General Relativity description of the Big Bang, we know that such a description can’t be right… because our Universe also has Quantum Mechanics in it, and we have huge amounts of experimental evidence telling us that we need to take Quantum Mechanics seriously. The real story is that there is an extremely early epoch in the Universe (what I tend to think of as “the beginning” nowadays) about which we can make supportable statements based on our understanding of physics. However, we also know that we don’t understand physics well enough to really know what the Universe was like before that early epoch. So, it is meaningful to talk about a before, it’s just that that before is a “known unknown”.

For now.

15 thoughts on “The Big Bang Wasn’t All At One Point (Cosmos Commentary)”

  1. One quibble: I think it is a common misconception to say “Everything is rushing away from everything else” because it’s actually only the weakly bound things that are moving apart from each other, eg. the galaxies. Atoms, solar systems, galaxies and humans are not rushing apart since their electrostatic and gravitational forces hold them together. The “big rip” may change all that if the density of the universe decreases to the point where Dark Energy wins out in the end and then we’d see it all get ripped apart. At least that’s my understanding.

    1. Your understanding is not correct, in general. If the dark energy (Sean Carroll explained why this is a bad term—everything has energy and many things are dark) is the classical cosmological constant—and there is no observation which indicates otherwise (which doesn’t mean that we shouldn’t test the idea)—then there will be no big rip. Bound systems will remain bound and the universe will asymptotically approach exponential expansion.

      The big rip is a highly speculative idea, much more speculative than, say, inflation.

      1. He’s fine — he says the big rip ***may*** change this. The may is everything.

        With current (and, honestly, all future plausible) data, we can’t rule out dark energy of the nature that would eventually lead to a big rip, and I’m still holding out hope… even though ti’s most likely that dark energy really is just boring old w=0 vacuum energy.

        Re: the term Dark Energy, I often tend to capitalize it, to indicate that we’re talking about a specific thing, rather than “energy which is dark”. It’s much like the cosmological dark matter (which I often write as Dark Matter), which itself is also different from matter which is dark.

  2. Bryan — you are absolutely right. Sometimes I get in the cosmology frame of mind, at which point I think of galaxies as mathematical points, and “everything” meaning just galaxies (or, even, really, galaxy clusters) and forgetting about the substructure.

    Even the galaxies within galaxy clusters aren’t rushing apart from each other because the gravity of the cluster holds the galaxies bound to them.

    (Also, there’s the whole issue that I don’t really like the “galaxies flying apart” way of describing it, but prefer the “space itself expands” way of describing it, but that’s a whole nuther ball of wax.)

  3. ” He did use the term “Observable Universe”, with a good description. (It’s as far away as we can see, a horizon defined by the speed of light and the 13.8-billion-year-old age of the Universe.) “

    Yes, but this is also misleading. It is trivially 13.8 billion light years distance in light-travel-time distance, but it is at 43 or whatever billion light years proper distance, which is what most people think of as distance in this context.

  4. Hi, nice article–I noticed exactly the same things in that episode of Cosmos. I can only imagine how annoyed you would have been at the speculative stuff about black holes in the most recent episode, and the very misleading imagery. I came from the “Starts with a Bang” blog (Ethan used some of your diagrams). Could you help with this question: as I understand it there are three basic options for the shape of the universe beyond the observable patch. It’s either infinite, which means it came from an infinite patch pre-inflation (as you say above). If it’s finite (in which case it came from a finite patch pre-inflation, surrounded by what, we don’t know) then it’s either topologically flat, meaning there are edges to it beyond the observable patch, OR it’s wrapped in on itself in a higher dimension, like the hypersurface of a 3-sphere (or some other finite unbounded 3D hypersurface), and since there’s no detectable curvature with current methods then any closed hypersurface must be huge.

    Could you tell me if I’ve got this straight about the three basic options? And what lines of reasoning lead cosmologists to rate some more probable than others? You said “probably?” infinite; and Ethan suggests in his post “Comments of the Week #4” that assuming a flat, finite universe is not something cosmologists tend to do. Is there something particularly awkward about edges? I guess they would have to be receding at light speed or greater relative to all points interior to the universe, or else light (or a travelling observer) could reach the edge and then what?

    Thank you! I appreciate the outreach you and others perform!

    1. ” If it’s finite (in which case it came from a finite patch pre-inflation, surrounded by what, we don’t know) then it’s either topologically flat, meaning there are edges to it beyond the observable patch, OR it’s wrapped in on itself in a higher dimension, like the hypersurface of a 3-sphere (or some other finite unbounded 3D hypersurface), and since there’s no detectable curvature with current methods then any closed hypersurface must be huge.”

      No. Unless there is a non-classical topology, if it is finite it cannot be exactly flat.

      1. Could you explain (qualitatively) how this is proved? It’s not that I want finite-but-flat to be true, I just want to know what the principle is that allows us to reject it as absolutely as your statement implies. The “principles” of isotropy and homogeneity wouldn’t seem to be sufficient, to me–they describe what we’ve encountered so far in our patch (and they are philosophically and aesthetically pleasing) but if our patch was very far from any edges in a finite-but-flat volume then they would still serve us well.

          1. “which contradicts the assumption of global homogeneity and isotropy.”
            This assumption is being questioned though. A new Scientist article (28 june 2014) mentions David Wiltshire’s work on Dipole Anisotropy being explained by inhomogeneity.

            Disclosure: I’m just a layman interested in the subject.

        1. With regard to your last question, the answer is really that GR doesn’t allow a finite but flat universe.

          Also, you can’t just consider “our patch”; the observation of the CMB tells us what the universe was like back then.

          In other words, a finite but flat model doesn’t match the theory, and doesn’t match the data. 🙁

  5. Those three basic options are all possibilities, yes. There may be others. (I don’t know of others off of the top of my head, but don’t want to rule them out.)

    With regards to the “finite with slightly positive curvature” Universe, check out this post. There may be tighter constraints on the curvature of the Unvierse than the ones I used from a 2011 paper for that post, but the basic idea is the same.

    With regards to a topologically interesting Universe — something like the “Asteroids” Unvierse, as in the video game where if you went off of one side of the screen, you came back on the opposite, indicating a toroidal topology– that’s definitely possible, and would not cause any problems with the GR equations that describe the Universe as a whole However, if something like that happens, the repeat scale is larger than the size of our observable Unvierse. Some folks have looked for signatures of repeats in the CMB that would be there if we were in a topologically interesting universe with a repeat scale comparable to or smaller than the size of our Observable Unvierse, and haven’t found them.

    So, it’s *possible* the Universe is closed or has an interesting toplogy, but the scale of both of those is almost certainly way, way the heck huger than the size of our Obsevable Universe, and any consequences inside are thus far (and are likely to continue to be indefinitely) indistinguishable from an infinite flat Universe. So, we assume the latter, as that’s the simplest model, and that works for making predictions about what would ever be observed in our Universe.

    As for whether or not there’s an “edge”, that’s a different question. A flat Universe with an “edge” — either a boundary that you simply can’t go past (a sort of wall, or something), or just the end of galaxies, beyond which there is nothing– is not consistent with the standard model of cosmology we use– a standard model that has withstood huge numbers of observational tests. We we don’t absolutely know that our standard model is correct, there’s nothing better, nothing else even close to as good, so it’s what we go with. This model is based on General Relativity (which has withstood countless tests, and is almost certainly valid in the regime that’s relevant) plus just the two basic assumptions that on the largest scales, the Universe is homogeneous (same everywhere) and isotropic (same in all directons). Those two assumptions are inconsistent with any kind of boundary. Of course, we test those assumptions all the time, and they’ve held up. The strongest evidence for them is probably the uniformity of the CMB, but the fact that the standard cosmological model does so well for things like big-bang nucleosynthesis, the distribution of galaxy clusters and large-scale structure, and so forth is all evidence that these assumptions are likely good ones.

  6. Hey thanks, that old post you referenced was perfect. I can’t imagine how we could be in a finite volume with positive curvature *without* being embedded in a higher dimension, but I suspect this is the kind of thing that I’d need deeper mathematical assistance for than you’re able to convey in a few paragraphs on a blog.

    Would it be possible for a finite flat volume to have its boundaries receding at lightspeed relative to *every* point within it? This would solve the problem of a “wall”, as the wall could never be reached. But maybe it’s impossible for the boundaries not to carry some of the nearby space along with them as they recede–in which case they wouldn’t be receding at light speed from observers in that peripheral region?

  7. Wasn’t the early evidence responsible for the formation of the BBT precisely that – the observation that all observable galaxies are expanding away from a common origin, and the backward extrapolation of that motion the rationale for the formation of the theory?

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