Cosmic Clump Coincidence?

Now this is just weird: the Zodiac, an apparent ring of distinct extra bright stars centered on us, is not an illusion – it really exists out there.  Called Gould’s belt, it is ~1000pc (parsec) in diameter, we are within about 100pc of its center, and it formed together 30-60 million years ago:


Some think it formed when a clump of dark matter, massing about ten million suns, passed through a molecular cloud, an event they say should happen every ~300 million years.  (Clumps pass elsewhere more often – Smith’s cloud, massing a hundred million suns, is passing right now. There should be about a thousand  such clumps near our galaxy.)

Since our galaxy’s diameter is about 40,000pc , the chance that the most recent clump would hit that close to us was roughly one in (40,000/200)2, or one in 40,000.  Since about a tenth of galaxy area has molecular clouds, the chance the most recent clump to hit a cloud would hit that close to us is one in 4000.  Coincidence?  Consider this plot from Jerison:


This dark matter clump hit us just about when the last big burst of max brain size growth began on Earth, with primates ~50 million years ago.  Yes, it is hard to see how a rare dark matter clump passing near could induce a primate brain growth spurt, so it is probably coincidence.  But big apparent coincidences should at least make us pause and ponder.

More details:

In this rational age, we have come to recognise constellations as chance alignments of groups of stars. … Yet it seems that an unseen hand may after all have been responsible for placing these stars in the sky.  Hints are emerging that around 30 million years ago, a giant clump of dark matter struck our part of the Milky Way, creating a rippling disc of star formation that eventually produced Orion’s belt, the bright ruby jewel of Antares in Scorpius, and many more of the sky’s most notable stars. …

In the middle of the 19th century, … John Herschel noticed that we are surrounded by a ring of bright stars. But it was Boston-born Benjamin Gould who brought this to wider attention in 1874. Gould’s belt, as it is now known, supplies bright stars for many famous constellations including Orion, Scorpius and Crux, the Southern Cross, which appears on the official flags of five countries and several territories. Perseus and Canis Major in the north, along with Vela and Centaurus in the south, also contain stars in Gould’s belt.

It is a sizeable structure, some 3000 light years across, and can be traced as a bright band of stars tilted at about 20 degrees to the Milky Way. Within it are several thousand high-mass stars as well as up to a million low-mass ones. Most importantly, these stars appear to have formed separately from the rest of the stars in the galaxy … “Gould’s belt is not part of the spiral structure … It must have been triggered by some local, violent event.” …

Every galaxy is thought to be surrounded by a giant sphere of dark matter more than 10 times the width of the visible portion of the galaxy, and containing at least 10 times the mass of the visible stars. The dark matter “halo” is clumpy rather than smooth, because it contains residues of smaller halos that the galaxy has swallowed. … In the halo surrounding the Milky Way – which is a pretty typical sort of galaxy – there could be a million billion (1015) such clumps of dark matter. … Dark matter clumps do not normally emit radiation, and so continue to circulate in the same orbits into which they were born …

Kenji Bekki … cranked up a computer model and began simulating what happened as clump after clump of dark matter smashed into one giant molecular cloud after another. Sure enough, he found that it was child’s play to generate something resembling a Gould’s belt. … as a clump of dark matter with a mass of about 10 million suns passes through a giant molecular cloud, it pulls gas in towards the centre of the collisions and triggers a wave of star formation. …

Bekki estimates that a collision with a chunk of dark matter happens only rarely – about once every 300 million years somewhere in the Milky Way – and that the resulting Gould’s belt will typically fade away in a few tens of millions of years. That makes us very lucky indeed to be sitting almost at the bull’s eye of the collision.

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  • Sanjay

    Surely it just adds to the weirdness that the belt lines up pretty well with the ecliptic, which is of course how the ancients used it.

  • We should also be happy we are not too close to those open clusters. Young hot stars often go bang.

    I wonder if one could make an anthropic argument? Given that nearby supernovae might preclude the emergence of intelligent life with a few million years, we should expect to find ourselves far away from them – like in the centre of Goulds belt.

    Might of course be other factors influencing location in addition, like galactic habitable zones and stellar density (we are very close to the plane of the disk).

  • josh

    Is this parody?

    • My thoughts exactly. It’s a pretty astrophysical coincidence, but dark matter causing brain growth? The polite term for this is “wild-ass speculation”. I shall refrain from mentioning the impolite term.

    • Eric Johnson

      Its possible celestial events can influence evolution. For instance, wiping out a lot of species or just a lot of individual animals can probably break up ecologies and cause a punctuation of equilibrium.

      However, I’m guessing we dont know of a mass extinction from this point in time. And theres probably only a rather narrow range of amounts of high-energy radiation that would kill lots of vertebrate individuals without causing a mass extinction.

      The putative binary partner of Sol, Nemesis, was once a hypothesis for causing mass extinctions that appear to be rather periodic. However, I think most workers do not believe that Nemesis exists, and never have.

  • Aron

    The ~50M years ago when brain expansion accelerated was prior to the most likely time of the dark matter passing (30-50M). Therefore, the monolith likely swung by after we had already begun using animal bones as bludgeons. Likely, it was interested in seeing whether our intelligence was anything the universe should have to fret about.

  • Sigivald

    Not so fast.

    Note that we’re nowhere near the “center” of it, and the constellations in the Gould Belt aren’t the ones in the zodiac anyway.

    This is the sort of thing I’d expect, loaded with “seemingly” and “possibly” and the like, from the intro to some really bad History Channel filler.

    • Huh? your source also says a diameter of 1000 pc and we are 100pc from the center. Just what I said.

  • Dan in EuroLand

    The causation is backwards. Brain growth caused the collisions!!


  • spriteless

    Humans see patterns. If it were a different pattern, we would have seen it and then maybe marveled at the coincidence of that, instead. We are going to create some pattern out of what we no matter what. What are the odds that I’d get my exact DNA from my parents Dr. Manhattan?

  • Your blog post reminds me of why I’m a big fan of intellectuals attaining 2+ disparate Ph.D.’s (or at least masters). I think you have a (unfortunately too) unique vantage to discuss topics like these with a masters in physics and a Ph.D. in the social sciences.

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  • haig

    This post needs ‘Also Sprach Zarathustra’ as background music to do it justice.

  • Granite26

    It seems to be testable. Have SETI spend extra time looking at similar bits of the sky…

  • J

    Can you translate this into some Powerball numbers?