“Es ist immer angenehm, über strenge Lösungen einfacher Form zu verfügen.” (It is always pleasant to have exact solutions in simple form at your disposal.) -Karl Schwarzschild

The Universe is a vast and complex place, full of a diversity of structure from the smallest scales to the largest. And yet, by many accounts, it's a wonder that it came to be this way at all.

If things were just a little bit different at the very beginning, the Universe could have recollapsed in on itself in a mere fraction-of-a-second after the Big Bang. That very clearly *didn't* happen, but why not? And, if it *did* happen, would we have formed a black hole?

These excellent questions, along with some amazing science history, are on tap for this week's Ask Ethan!

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Sorry, I don't think this or your longer article answered the question. You seem to say that because there was an initial expansion it had to go into one of the three scenarios. But surely material within a black hole cannot escape regardless of whether it is expanding or not. Extrapolating back to the posited singularity, there must have been a time when all the mass in the universe was dense and compact enough to form a black hole (is that right?), but then the expansion of all that mass could not have escaped and formed our universe ... This is the nub of the question and I do not see how you have answered it. The fact of expansion does not enable it to escape. Thanks, Jay

"Finally, there’s the “Goldilocks” case, or the case where the Universe is right on the bubble between recollapsing (which it would do if it had just one more proton) and expanding into oblivion (which it would do if it had one fewer proton), and instead just asymptotes to a state where the expansion rate drops to zero, but never quite turns around to recollapse."

I suspect there are undiscovered laws of physics making

our current "on the bubble" expansion rate more than an extremely lucky coincidence.

I'm not saying it's aliens, but ALIENS.

I am an armchair astrophysicist and I have been following developments in our measurements of the universe and wonder why no one else has come up with this idea: If the universal expansion is 'speeding up' instead of 'slowing down' and there is this 'dark energy' and 'dark matter' we have to invent to explain away things, why don't we make the matter simpler and instead of working on all our ideas based around a BIG BANG, that this big bang came from a single point seems awkward and ridiculous, what if we think of the previous universe as a bathtub and the water in it is spacetime? After we get that far, we can then imagine that someone pulling the drain plug would cause it to empty into another bathtub beneath it. If the big bang were not a 'instant event' but rather a continuous one that is constantly pouring more matter and energy into the universe but that we can't see it because we're on the outer edge of the expanding pool of space time, it would explain away the question to this much easier than the lengthy explanation given. Of course, my thought-idea isn't scientific, yet.

Tell me if I'm wrong, but isn't the fact that we're here to observe the Goldilocks case just because we're the Goldilocks case? In other words all the other cases do not favor the emergence of 'intelligence' whatever form it may take.

I have been working on this subject many years ago. It seems to me that the universe started as a black hole, and still is. One reason is that the ratio of its mass to its size is precisely the black hole condition, fulfilled. Also an initial black hole, that we know as Planck´s quantum black hole, very small, with very low mass, can be scaled to the present universe by a huge factor of 10^61. Then, if you start with a black hole, you always are in a black hole:an expanding one as observed.

Why doesn't this blog turn into a black hole? Because everything blogged here is always the same I read somewhere else. ALWAYS THE SAME, COME UP WITH SOMETHING NEW MAN

@J Kennedy #1: I think you'll need to dig into the actual mathematical details of cosmology in order to answer your own question. As Ethan explained, the Schwarschild metric (the solution to GR which describes a black hole) is _not_ the metric which describes cosmology. The latter is the Robertson-Walker (or technically the FLRW) metric, and that metric neither starts as, nor ends as, a black hole in the Schwarschild sense.

To summarize (and answer the question) space itself was expanding so fast that the energy and proto-matter could not be accelerated toward each other fast enough to collapse. Think of Zeno's Paradox, where if every step you take is one half the length of the previous one you'll never get to your destination, instead invert it a bit; imagine every step you took your destination got twice as far away. We know gravity travels at the speed of light, so as the space between all matter expanded the attractive force reduced respectively. Ethan's final words talking about the density of the early Universe are the important ones. The amount of matter per unit volume /of space/ was certainly immense, but space was expanding faster than the matter could clump up to collapse into a singularity again. Luckily for us, the rate of expansion of space wasn't SO high that none of the matter clumped up - only some of it.

At first, it seemed the article did not answer the question at all. On re-reading, I suspect I just don't know enough about expanding spacetime to understand what Ethan was trying to say. Here is my popular-science-reader's view of the problem:

1. The early universe occupied a volume whose radius was less than its Schwarzschild radius. By definition, the escape velocity at the Schwarzschild radius is the speed of light.

2. No matter or energy can exceed the speed of light. Therefore, static or not, the initial universe should have proceeded directly to a Big Crunch.

Presumably, expanding spacetime changes the picture. But I have no idea how or why.

Maths Alert - the human body contains about 10^28 electrons, so while close to the 10^24 number a difference of a factor of 10000 is significant.

If I remember my cosmology lectures correctly (and this was a dozen years ago, so I probably don't!) I think I heard that if you were to calculate the Schwarzschild radius of all the mass in the visible universe, it would be the size of the visible universe. At the time I thought this was quite remarkable.

Is this an equivalent way of stating that we're nearly in the Goldilocks zone of a flat universe?

If so, could you interpret this as saying that in fact, we are living inside a black hole? We certainly cannot escape this universe!

@ Robert

nor sure how that can be, since mass of the Universe is an unknown. Yes, we know our visible radius, but that's all... Just like we don't know the size of the universe. we only know the size of our observable horizon, dictated by c.

As for black holes and living in one.. I really don't like when people bring that up, because it has nothing to do with physics and a lot to do with fiction. Yes, our visible universe has a horizon, but not due to gravity. And the fact that you can measure gravity and the fact it's so incredibly small that you can't even test far end implications of relativity on earth but instead have look for orbiting black holes and quasars out there is nothingness is proof enough. How do you explain gravity if you're supposedly living in an area of infinite gravity and curvature??

So forget about living in a black hole and instead focus on universe or black holes.. they are bizzare enough on their own without the need to put them together.

sorry.. typos..

*have to look

*in nothingness

Ethan gave a stock answer, but it's wrong. Because a gravitational field alters the motion of light and matter through space, but it doesn't suck space in. So the universe didn't collapse when it was small and dense, and it isn't going to. Ever.

@10: My interpretation of what Ethan is saying is this: the Schwartzchild radius calculation only holds for static starting conditions. Our starting conditions were not static, therefore the Schwartzchild model does not give an accurate prediction of how our universe was expected to evolve over time.

In short: it's a spherical cow calculation, but our cow (universe) is not spherical.

Because laws of physics are an outcome of the Big Bang. During the ginormous-bubble-of-stuff-state, laws of physics as we know them did not apply.

"What’s remarkable is that the amount of fine-tuning that needed to occur so that the Universe’s expansion rate and matter-and-energy density matched so well so that we didn’t either recollapse immediately or fail to form even the basic building-blocks of matter is something like one part in 10^24, which is kind of like taking two human beings, counting the number of electrons in them, and finding that they’re identical to within one electron. In fact, if we went back to a time when the Universe was just one nanosecond old (since the Big Bang), we can quantify how finely-tuned the density and the expansion rate needed to be. A pretty unlikely story, if you ask me!"

This is fun to play around with. If we introduce inflation, it pulls back the finetuning to the field.

If BICEP2 is correct, so Linde's chaotic inflation is in, the flatness finetuning solution is caused by finetuning of the field potential curvature. (At least if I understand Steinhardt's criticism during the World Science Fair panel discussion. [youtube])

I have no idea why a simple V(phi) = m*phi^2 potential has its curvature (dV/dphi)/(2*phi) = m "efficient mass" finetuned to 10^-24. But some naive layman ideas occur:

1. A dynamic process puts the mass clustered around the flat outcome. The Higgs field finetuning (close to quasistability of the vacuum) ought to go into this scenario too, so maybe that is preferred.

2. There is a correlation between the (near) zero energy density of the initial spacetime vacuum and the (near) zero energy density of the final spacetim cosmic constant vacuum.

In Linde's chaotic inflation the initial condition is a quantum fluctuation of less than 10^-3 g matterenergy. I don't know if that means the initial spacetime needed to be near zero energy density, but as I remember it the usual claim is that inflation can't start otherwise.

That pulls back the finetuning one step further... Doesn't really help re finetuning, but could happen.

@Panos Tsibo: You are confusing things, I think.

During the Hot Big Bang, the current Standard Model particles "froze out". It is true that they obey particular [sic!] laws, and that they may be different in other universes [though Ethan is on record here for pointing out that is unlikely], but if we "reheat" a part of the universe again we would recapture the initial laws. Those laws would include the symmetries of spacetime itself for one (relativity) as well as quantum field theory (relativity and quantum mechanics) for another. We know those laws.

Those laws are part of our universe today, and where part of the universe during the Hot Big Bang era, and before that during the Cold Inflation era. It is true that we don't know the laws of the inflation field that well, but inflationary cosmology and WMAP/Planck (and now perhaps BICEP2!) has captured some of its behavior. And that doesn't deviate from the expected FLRW metric.

[Quite frankly I don't understand how it could. The FLRW is supposed to be _the_ cosmological solution for expected well behaved copernican principle (isotropic and homogeneous) universes, as far as this layman understand.]

Im no scientist, so I do have some questions/theories about black holes. First off what if we're alot smaller than we already think? Looking at a supernova thats a big bang of sorts isnt it? What if nature is showing on a smaller scale of whats happening to us? Which brings me to ask what do you call the space outside our universe that were expanding into? Maybe were looking at the big bang wrong what if there was a black hole and we are the result of what comes out the other side. It keeps sucking mass through so we keep expanding and growing. One last semi unrelated question. Like I said earlier about nature showing us whats happening with the black hole could that also be a demonstration of what life is? When we grow old and die we create a supernova of sorts our mass goes back into the environment. What if our souls implode on a smaller scale in another dimention hence the "light at the end of the tunnel" effect. Having a death experience ive witnessed this effect and have been thinking about it ever since. Reading this alot of you will probably think im an idiot or whatever but if someone does have genuine answers or is willing to help me understand these things feel free to shoot me an email as id like to discuss it more if im worth the time. corey.tildesley@hotmail.com

@corey #20: No, a supernova is _not_ "a big bang of sorts". A supernova is a simple explosion of pre-existing matter in a pre-existing spacetime, due to nuclear chemistry (fusion). The cosmological expansion is a monotonic varying in the size ("scale factor" is the term you should look up) of spacetime itself.