The Three Things Everyone Gets Wrong about the Big Bang

Ah, the Big Bang, our most science-y of creation myths. Everyone knows the story of how the universe and all its physical laws emerged from nothing in a massive explosion, growing from a singularity to the size of a breadbox until, over billions of years, it became the size it is today.

A hot dense state, if you know what I mean.

…actually, almost nothing in that paragraph is true. There are a lot of myths about the Big Bang, born from physicists giving sloppy explanations. Here are three things most people get wrong about the Big Bang:

1. A Massive Explosion:

When you picture the big bang, don’t you imagine that something went, well, bang?

In movies and TV shows, a time traveler visiting the big bang sees only an empty void. Suddenly, an explosion lights up the darkness, shooting out stars and galaxies until it has created the entire universe.

Astute readers might find this suspicious: if the entire universe was created by the big bang, then where does the “darkness” come from? What does the universe explode into?

The problem here is that, despite the name, the big bang was not actually an explosion.

In picturing the universe as an explosion, you’re imagining the universe as having finite size. But it’s quite likely that the universe is infinite. Even if it is finite, it’s finite like the surface of the Earth: as Columbus (and others) experienced, you can’t get to the “edge” of the Earth no matter how far you go: eventually, you’ll just end up where you started. If the universe is truly finite, the same is true of it.

Rather than an explosion in one place, the big bang was an explosion everywhere at once. Every point in space was “exploding” at the same time. Each point was moving farther apart from every other point, and the whole universe was, as the song goes, hot and dense.

So what do physicists mean when they say that the universe at some specific time was the size of a breadbox, or a grapefruit?

It’s just sloppy language. When these physicists say “the universe”, what they mean is just the part of the universe we can see today, the Hubble Volume. It is that (enormously vast) space that, once upon a time, was merely the size of a grapefruit. But it was still adjacent to infinitely many other grapefruits of space, each one also experiencing the big bang.

2. It began with a Singularity:

This one isn’t so much definitely wrong as probably wrong.

If the universe obeys Einstein’s Theory of General Relativity perfectly, then we can make an educated guess about how it began. By tracking back the expansion of the universe to its earliest stages, we can infer that the universe was once as small as it can get: a single, zero-dimensional point, or a singularity. The laws of general relativity work the same backwards and forwards in time, so just as we could see a star collapsing and know that it is destined to form a black hole, we can see the universe’s expansion and know that if we traced it back it must have come from a single point.

This is all well and good, but there’s a problem with how it begins: “If the universe obeys Einstein’s Theory of General Relativity perfectly”.

In this situation, general relativity predicts an infinitely small, infinitely dense point. As I’ve talked about before, in physics an infinite result is almost never correct. When we encounter infinity, almost always it means we’re ignoring something about the nature of the universe.

In this case, we’re ignoring Quantum Mechanics. Quantum Mechanics naturally makes physics somewhat “fuzzy”: the Uncertainty Principle means that a quantum state can never be exactly in one specific place.

Combining quantum mechanics and general relativity is famously tricky, and the difficulty boils down to getting rid of pesky infinite results. However, several approaches exist to solving this problem, the most prominent of them being String Theory.

If you ask someone to list string theory’s successes, one thing you’ll always hear mentioned is string theory’s ability to understand black holes. In general relativity, black holes are singularities: infinitely small, and infinitely dense. In string theory, black holes are made up of combinations of fundamental objects: strings and membranes, curled up tight, but crucially not infinitely small. String theory smooths out singularities and tamps down infinities, and the same story applies to the infinity of the big bang.

String theory isn’t alone in this, though. Less popular approaches to quantum gravity, like Loop Quantum Gravity, also tend to “fuzz” out singularities. Whichever approach you favor, it’s pretty clear at this point that the big bang didn’t really begin with a true singularity, just a very compressed universe.

3. It created the laws of physics:

Physicists will occasionally say that the big bang determined the laws of physics. Fans of Anthropic Reasoning in particular will talk about different big bangs in different places in a vast multi-verse, each producing different physical laws.

I’ve met several people who were very confused by this. If the big bang created the laws of physics, then what laws governed the big bang? Don’t you need physics to get a big bang in the first place?

The problem here is that “laws of physics” doesn’t have a precise definition. Physicists use it to mean different things.

In one (important) sense, each fundamental particle is its own law of physics. Each one represents something that is true across all of space and time, a fact about the universe that we can test and confirm.

However, these aren’t the most fundamental laws possible. In string theory, the particles that exist in our four dimensions (three space dimensions, and one of time) change depending on how six “extra” dimensions are curled up. Even in ordinary particle physics, the value of the Higgs field determines the mass of the particles in our universe, including things that might feel “fundamental” like the difference between electromagnetism and the weak nuclear force. If the Higgs field had a different value (as it may have early in the life of the universe), these laws of physics would have been different. These sorts of laws can be truly said to have been created by the big bang.

The real fundamental laws, though, don’t change. Relativity is here to stay, no matter what particles exist in the universe. So is quantum mechanics. The big bang didn’t create those laws, it was a natural consequence of them. Rather than springing physics into existence from nothing, the big bang came out of the most fundamental laws of physics, then proceeded to fix the more contingent ones.

In fact, the big bang might not have even been the beginning of time! As I mentioned earlier in this article, most approaches to quantum gravity make singularities “fuzzy”. One thing these “fuzzy” singularities can do is “bounce”, going from a collapsing universe to an expanding universe. In Cyclic Models of the universe, the big bang was just the latest in a cycle of collapses and expansions, extending back into the distant past. Other approaches, like Eternal Inflation, instead think of the big bang as just a local event: our part of the universe happened to be dense enough to form a big bang, while other regions were expanding even more rapidly.

So if you picture the big bang, don’t just imagine an explosion. Imagine the entire universe expanding at once, changing and settling and cooling until it became the universe as we know it today, starting from a world of tangled strings or possibly an entirely different previous universe.

Sounds a bit more interesting to visit in your TARDIS, no?

Advertisements

16 thoughts on “The Three Things Everyone Gets Wrong about the Big Bang

  1. Wyrd Smythe

    I’m confused by the apparent conflict between “infinitely many other grapefruits of space” and the idea that, although it wasn’t a singularity, it all began as something roughly Planck-sized.

    What kind of distances for the entire universe created by the BB are we talking for, say, 10^-50 seconds after the BB? Was all of it really close together at the very beginning or is that the wrong way to look at it?

    Like

    Reply
    1. 4gravitonsandagradstudent Post author

      It’s just the wrong way to look at it. When saying it all began as something roughly Planck-sized, that’s just the characteristic distance: the scale of the largest patterns in the universe, or the scale our Hubble volume was.

      If the universe is infinite now, it was likely infinite then as well, even 10^-50 seconds after the Big Bang.

      (By the way, one thing I didn’t mention in the article: the timing of the Big Bang is a bit ambiguous. Because there wasn’t one singularity, there isn’t one specific event that was “the Big Bang”. It was a process, and different people use the term to mean different parts of that process, so it’s probably not possible to say precisely what happened at a time like 10^-50 seconds because the Big Bang itself isn’t specified that precisely.)

      Like

      Reply
      1. Wyrd Smythe

        You mean infinite in the closed sense, like the surface of a sphere, right?

        Maybe this isn’t a valid analogy, but I’m starting with the idea of flatland creatures living on an expanding sphere. Each one would see the “universe” expanding away from them. They could extrapolate backwards to determine that everything they can see must have previously been where they are.

        So I’m imagining someone on the edge of my Hubble volume observing the universe. We’d both see the universe expanding from us. But wouldn’t we both see that, running backwards, we both converge on the same spot?

        If I extend that to third and fourth persons on the (respectively) second and third person’s boundary, each would project convergence on them. That logic would seem to extend for as many maximally separated observers as you care to name.

        So it seems like the entire universe must converge on one spot. Granted, not a singularity, but “one spot” for some definition of “one spot”?

        Like

        Reply
        1. 4gravitonsandagradstudent Post author

          Ah, I see where the problem is.

          At the moment, we don’t know whether the universe is truly infinite, or whether it’s finite in the sense you’re describing (like the surface of a sphere, curving over on itself).

          If it is infinite, then it would have been infinite around the time of the Big Bang as well.

          If it is finite, though (like the surface of a sphere), then indeed it would have all come from “one spot”: probably not a singularity, but everything would have been “close”. How close, though, depends on how big the universe is now, and that’s not something we know yet.

          Like

          Reply
          1. Wyrd Smythe

            So, if it is infinite and unbounded, there could be two observers who would “backwind” their observable universe to converge on them, but which would not converge on each other?

            What does that mean for the “chain of observers” thought experiment I mentioned?

            Like

            Reply
            1. 4gravitonsandagradstudent Post author

              The key thing here is that you dont end up converging on the same spot. You only converge on the same spot if there’s a singularity that everybody’s converging to. Without a singularity, two observers would just “backwind” to nearby spots, and a chain of enough nearby spots can still eventually get far away.

              Like

              Reply
  2. Collin Merenoff

    You say the laws of physics existed before the big bang? What could that even mean?

    A thousand years ago, people thought the entire layout of the world was ordained by God(s). Gradually we realized that various aspects of the world got the way they are by a chaotic process of self-development. Only one subject escaped such an enlightenment — the laws of physics.

    Every complexity that has ever been discovered in nature has been found to have evolved. The laws of physics are a very complicated system, but it’s still popular to make an exception and say God created them — even though the word “God” is meaningless. You make an exception and say they always existed — even though the word “always” is meaningless.

    There is evidence that the universe began with some kind of inflation. The simplest explanation is that inflation was what happened while the laws of physics were not yet complete.

    Like

    Reply
    1. 4gravitonsandagradstudent Post author

      The thing is, “the laws of physics” can mean different things, a point I try to make in my post.

      I agree that every complexity in nature has been found to have evolved (well, or at least emerged…crystals aren’t exactly Darwinian under most conceptions). But the sort of laws of physics that would predate the big bang are precisely those that are not complex. They’re rules like “probabilities should add up to one” or “things need to be close together to effect each-other”. More broadly, they’re the most general options: laws that simply say that anything is possible as long as it is consistent.

      Relativity and quantum mechanics may seem to be complex, but that’s only because they’re unfamiliar. When we think of complex things as having evolved/emerged, it’s because they have many contingent details: things which could have gone multiple ways, all ending up in the right configuration to achieve some goal. Quantum mechanics and relativity aren’t like that. You can’t propose lots of different quantum mechanicses, or lots of different relativities, that all make sense. There is one quantum mechanics, and one relativity. They’re much more like rules of logic than complexities of the natural world. Unless you want to argue that 1+1=2 evolved?

      Inflation doesn’t help you here. It only makes sense as a change in spacetime, and it only happens because of the general properties of quantum fields.

      Like

      Reply
  3. Pingback: The Balloon Analogy ... the Good, the Bad, and the Ugly - Comments | Page 2

  4. Mark Pounds

    I’ve often thought of the Universe being on the surface of a sort of multi-dimensional torus, with a magnetic north and south. This would mean the universe isn’t actually expanding. It’s actually contracting towards the other side..In essence, if you were looking left.. eventually youd see the right side of the universe speeding towards you..You’d be rushing towards the center of the torus.. which would eventually collect all matter, and restart the process. It’s the only logical explanation I can think of.

    Like

    Reply
    1. 4gravitonsandagradstudent Post author

      While a toroidal universe hasn’t been ruled out, I suspect it wouldn’t work the way you’re thinking. Remember, in general relativity the shape of the universe isn’t just a background, it’s determined by the behavior of matter inside it. I haven’t run the math on your idea (in particular because I’m somewhat confused by your phrasing…what part of a torus are you calling the center? I’m assuming you don’t literally mean “magnetic”, but if you do that’s another issue), but I don’t think the shape you’re thinking of would be stable, or do what you want it to do.

      Like

      Reply
  5. Dorene

    What did the “spot” or singularity or grapefruit sit in before expanding and existing?
    I picture the universe expanding as cracking an egg onto a plate and watching it expand. But where did the egg come from?

    Like

    Reply
    1. 4gravitonsandagradstudent Post author

      The thing it was “sitting in” was just the rest of the universe. The thing that was grapefruit sized was the part of the universe we can see today, but there’s lots of universe outside. The whole universe may well be infinite, and if so it was likely always infinite.

      Like

      Reply
  6. Hahaha

    Ok… there’s not gonna be a time that we occupy were we are gonna understand evolution. <
    We don’t understand enough about the human brain to fathom that there was no single beginning. The human race (needs) a start to it’s existence otherwise it just doesn’t make sense. I love listening to the chatter here like people think they know something just because the were taught it.

    Like

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s