From the Big Bang to the Biosphere ― Reading #3


Is Dark Matter Required for Life to Exist?

Ethan Siegel

Forbes Magazine | June 17, 2016



[1] Dark matter is the most mysterious, non-interacting substance in the Universe. Its gravitational effects are necessary to explain the rotation of galaxies, the motions of clusters, and the largest scale-structure in the entire Universe. But on smaller scales, it's too sparse and diffuse to impact the motion of the Solar System, the matter here on Earth, or the origin and evolution of humans in any meaningful way. Yet the gravity that dark matter provides is an absolute necessity for allowing our galaxy to hold onto the raw ingredients that made life like us and planets like Earth possible at all. Without dark matter, the Universe would likely have no signs of life at all.


[2] Stars make up 100% of the light we observe in the Universe, but only 2% of the mass. When we look at the motions of galaxies, clusters and more, we find that the amount of gravitational mass outweighs the stellar mass by a factor of fifty. You might think, however, that other types of normal matter could account for this difference. After all, we’ve discovered lots of other types of matter in the Universe besides stars, including: 

• stellar remnants like white dwarfs, neutron stars and black holes,

• asteroids, planets and other objects with masses too low (like brown dwarfs) to become stars,

• neutral gas both within galaxies and in the space between them,

• light-blocking dust and nebulous regions, ionized plasma, found mostly in the inter-galactic medium.


[3] All of these forms of normal matter— or matter originally made of the same things we are: protons, neutrons and electrons— do in fact contribute to what’s there, with gas and plasma in particular each contributing more than the sum total of all the stars in the Universe. But even adding all these components together only gets us up to about 15-to-17% of the total amount of matter we need to explain gravitation. For the rest of the motions that we see, we need a new form of matter that isn’t just different from protons, neutrons and electrons, but that doesn’t match up with any of the known particles in the Standard Model. We need some type of dark matter. 


[4] A minority group of scientists favor not adding some unseen source of mass, but to rather modify the laws of gravitation instead. These models all have difficulties, including the inability to reproduce the full suite of observations, including individual galaxies moving within clusters, the cosmic microwave background, galaxy cluster collisions (above) the grand cosmic web or the patterns observed in the large-scale structure of the Universe. But there’s an important piece of evidence that points to the existence of dark matter that you might not expect: our very existence.


[5] It might surprise you to learn that we don’t just need dark matter to explain astrophysical phenomena like galactic rotation, cluster motions and collisions, but to explain the origin of life itself! To understand why, all you need to remember is that the Universe began from a hot, dense state— the hot Big Bang—  where everything started off as a mostly uniform sea of individual, free, high-energy particles. As the Universe expands and cools, we can form protons, neutrons, and the lightest nuclei (hydrogen, deuterium, helium and a trace amount of lithium), but nothing else. It isn’t until tens or even hundreds of millions of years later that matter will collapse into dense enough regions to form stars and what will eventually become galaxies. 


[6] All of this will happen just fine, albeit differently in detail, whether there were plenty of dark matter or none at all. But in order to make the elements necessary for life in great abundance— elements like carbon, oxygen, nitrogen, phosphorous and sulphur— they need to be forged in the cores of the most massive stars in the Universe. They do us no good in there, though; in order to enable the creation of rocky planets, organic molecules and (eventually) life, they need to eject those heavier atoms back into the interstellar medium, where they can be recycled into future generations of stars. To do that, we need a supernova explosion. 


[7] But we’ve observed these explosions in great detail, and in particular, we know how quickly this material gets ejected from the stars in their death throes: on the order of a thousand kilometers per second. (The Cas A supernova remnant has ejecta leaving it between a whopping 5,000 and 14,500 km/s!) While this may not sound like that big a number, especially compared to the speed of light, remember that our own star orbits the Milky Way at only some 220 km/s. In fact, if the Sun were to move even three times as fast as that, we’d find ourselves— today— escaping well beyond our galaxy’s gravitational pull. A supernova remnant might see the fastest of its ejecta leave the luminous, star-based part of the galaxy, but combined with the intense gravitational pull of a diffuse, extended halo of dark matter, we’ll keep most of that mass inside our own galaxy. Over time, it will fall back towards the normal-matter-rich regions, form neutral, molecular clouds, and participate in subsequent generations of stars, planets, and more interesting, organic molecular combinations.  


[8] But without the additional gravitation of a massive dark matter halo surrounding a galaxy, the overwhelming amount of material ejected from a supernova would escape from galaxies forever. It would wind up floating freely in the intergalactic medium, never to become incorporated into future generations of star systems. In a Universe without dark matter, we’d still have stars and galaxies, but the only planets would be gas giant worlds, with no rocky ones, no liquid water, and insufficient ingredients for life as we know it. Without the copious amounts of heavy elements provided by generations of massive stars, molecule-based life like us would never have come to be.


[9] It’s only the presence of these massive dark matter halos, surrounding our galaxies, that allow the carbon-based life that took hold on Earth— or a planet like Earth, for that matter— to even be a possibility within our Universe. As we’ve come to understand what makes up our Universe and how it came to be the way it is, we’re left with one inescapable conclusion: dark matter is absolutely necessary for the origin of life. Without it, the chemistry that underlies it all— the heavy, complex elements, the ingredients necessary for biology in the first place, and the rocky planets that life takes hold on— could never have occurred at all.


The 'dark matter' of life on Earth

The Australian Academy of Science


[1] We share this planet with an astonishing number of living things. So far, taxonomists and biosystematists (scientists who classify living things) have been able to discover, describe and classify approximately 1.9 million living species on Earth, organizing all of life into an enormous, well-ordered library. Knowing which groups each species belongs to helps us better understand how life evolved, how living things interact with each other and their environments, and how we can better protect species on the brink of extinction.


[2] Living things are organized into groups (also called ‘taxa’) according to a hierarchy of different levels: domain, kingdom, phylum, class, order, family, genus and species. The criteria for classifying living things into these groups get narrower and more specific the further you go down the list. At the highest level, ‘domain’, living things can fall into one of three major lineages— eukaryotes, bacteria, and archaea— according to fundamental differences in how their cells and genetic material are organized. 


[3] But there are still a lot of empty shelves in this vast library of life: millions of species have yet to be discovered, described and classified. What’s more, many of these undiscovered species are microscopic organisms that are invisible to the naked eye, making discovery a much greater challenge compared to spotting a new kind of bird or butterfly, for example. 


[4] These microscopic species are the ‘dark matter’ of the biological world. For astrophysicists, the physical ‘dark matter’ of the universe is matter that we’re unable to detect because it doesn’t interact with light. However, they know that it exists based on how it interacts with gravity, which affects how stars and galaxies are distributed throughout the universe. 


[5] For life on Earth, biological ‘dark matter’ refers to the species that we haven’t yet been able to detect, but which we know must exist. We know there are millions, and some estimates say there are possibly trillions, of species yet to be found. So, how do we discover something that’s very hard to see? Take bacteria, for example. Microbiologists traditionally study bacteria in the lab by growing colonies on agar plates, which are small, contained environments which supply bacteria with nutrients and a surface on which to grow. This has been a key step in the discovery and documentation of bacterial species to date. However, some species cannot grow under laboratory conditions and are ‘invisible’ to such methods.


[6] And that’s not to mention all the other groups of microorganisms, including protists, nematodes and fungi. These groups also have millions of unknown species within their ranks, and most of them won’t grow into nice, observable colonies on a laboratory agar plate. In fact, many of them won’t grow on agar plates at all. To give you an idea of the scale of the challenge, just three per cent of all the estimated nematodes and protists that live in Australia and New Zealand have been discovered. 


[7] Scientists have made some progress, however. In the past decade, taxonomists have developed new methods of discovering and analyzing species based on just their DNA. DNA is extracted directly from soil or water, gut samples, rocks from deep within the Earth’s crust, the deep ocean or other environments, and sequenced (decoded) in small fragments. The fragments are then pieced together into larger gene or genome sequences using enormous amounts of computing power. These methods have helped scientists discover and describe a staggering number of new species that were previously undetectable and invisible. 


[8] Understanding how to classify and study these ‘dark matter’ species is important for many reasons. Microorganisms are abundant, incredibly diverse and essential for many ecological processes. Discovering and describing them might help us find better ways to improve farming productivity, enhance environmental sustainability, fight infectious diseases, or better understand some of the friendlier microorganisms that live inside us, undetected.


[9] Some organisms may even give us insights into how life first began. Earth was once a pretty hostile place before bacteria created the conditions for more complex life to thrive by oxygenating the atmosphere. Studying this biodiversity ‘dark matter’, much like the ‘dark matter’ of the cosmos, might hold the key to answering some of the biggest scientific questions of the century—and it could re-write our entire understanding of life on Earth.


Dark Matter Is in Our DNA

Adam Frank

Nautilus/Cosmos | February 2017


[1] “Family Physics” may be the best episode of Public Radio’s long running show, This American Life. Its premise was simple. Import key concepts from the realms of quantum mechanics and cosmology and use them to illuminate the everyday world of parents, kids, and their interactions. Introducing the show, however, host Ira Glass was quick to point out how much physicists detest this kind of enterprise. “They hate it when non-scientists … apply principles from physics to their petty little lives and petty little relationships.” Glass was equally quick to point out that he and his colleagues at the show just did not care. As he put it, “Once physicists name something the ‘mediocrity principle’ or the ‘uncertainty principle’ or the ‘grandfather paradox,’ well … they’re just asking for it.” Glass had a point. 


[2] The names we physicists bequeath our cosmic laws sometimes resonate with the more mundane, everyday struggles everyone has making sense of everyday life. There is a deep well of humor to be tapped in using “cosmic mediocrity” to sum up a bad day. But in some cases, the nomenclature we physicists choose reaches below just the everyday. In those cases it touches something much deeper in the collective basement of shared human culture. Sometimes the collision between physics and the cultural unconscious drops us into vast landscapes of the mythic. Nowhere is the cross-pollination more potent than the discovery of the Dark Universe. 


[3] While some point to galaxy cluster work by Fritz Zwicky in the 1930s, dark matter was truly “discovered” in the 1970s by Vera Rubin, who was studying the rotation of spiral galaxies. Rubin found that galaxies were spinning too fast for the matter we could see in them, yet they weren’t flying apart. Rubin’s work left astronomers with a choice: Either our laws of gravity were wrong, or there was something else out there pulling on the galaxy’s stars and speeding them up while keeping them together. Since physicists had already invested a few centuries on their theories of gravity, few of them signed up for the first conclusion. In that way the astrophysical community moved en mass toward the second idea: There was another kind of stuff out there we couldn’t see that was exerting a gravitational force on the stuff we could see. This “other” stuff could have been called “invisible matter.” It could have been called “unseen matter.” It could have been called lot things but, instead, the name astronomers settled on for their mystery was “dark matter.” The rest is a cultural history where, as Ira Glass says, we astronomers were just asking for it. 


[4] The Wikipedia page for “Dark Matter in Fiction” lists 47 entrees. That’s a lot. The bulk of this list is, however, science fiction stories. That’s where you would expect an idea as rich as most of the Universe existing in some other form to get picked up. What that list doesn’t include though is Dark Matter Productions, a theater company in New York that, in its own words, “takes pride in shining a light on under-served, and frequently difficult, subject matter.” The Wiki-list also doesn’t include Dark Matter, a London puppet theater performance about a character’s descent into dementia. It doesn’t include DARKMATTER, a transgender south-Asian performance art duo also based in New York City. There is also the off-Broadway play Dark Matters and the book Dark Matter about the role of invisibility in theatrical production. There is a chapbook of poems called Dark Matter by Christine Locke-Lim and another by Amy Nelson Smith and another coming out by Robin Morgan. There are numerous non-science fiction works of fiction called Dark Matter and a raft of art exhibits with the Dark Matter title. One could go on, but the point is clear: Dark matter strikes a nerve. The question then becomes, why?


[5] Obviously “Darkness” is an idea with a long and complicated history for human beings. Yes, on one level it simply means the absence of light. But unless you stick strictly to an electromagnetic definition for “the Light,” you’ll find that the phrase is also loaded with connotations reaching much farther and deeper than wiggling electromagnetic fields. The binary pairing of Darkness and Light is so fundamental to human culture that it has been enshrined in our most important kinds of story: the creation myths. Genesis from the Bible is the ur-example of a creation myth with its “darkness upon the face of the deep.” That unknowing darkness is all that exists until God speaks the Word and there is Light. And, of course, the Light is Good.


[6] In the Zoroastrian creation myth, for example, the world began with Ahura Mazda, known as the Wise Lord, who lived in the Endless Light. But there was also evil, in the form of Ahriman, living in the realms of Absolute Darkness. When the Wise Lord created the world of humans with all its light, Ahriman refused to give it praise. Instead, he began shaping an army of demons, witches, and monsters to wage war on the light.


[7] Now if this sounds like a backstory for every fantasy, horror, or even science fiction story you’ve ever encountered, that’s no accident. Mythologies like the creation myth of the Zoroastrians are not just old, untrue stories. Instead, as Joseph Campbell and others have shown us, myths are like warehouses of the human imagination. There is a reason why so many myths, from so many different cultures, across so many different eras, tell the same kind of story. It’s because these are stories we can’t help but tell. They are elemental, expressing something elemental about the way humans— all humans in all times— parse their basic experience of the world. 


[8] In his famous work The Cooked and The Raw, anthropologist and comparative mythologist Claude Levi-Strauss claimed that the structure of human consciousness locks us into seeing the world in binary pairs. From this perspective, the duality of Dark/Light has to be seen as more than just a statement about electromagnetic energy. Instead, it holds not only the tension between Good and Evil but also of Ignorance and Knowledge. As the 18th century Zen chant in praise of enlightenment puts it, “From Dark Path to Dark Path we’ve wandered in Darkness.” Don’t we all want to climb out of the Darkness and make our way into the Light? Isn’t that why the foundational historical period for modern democracy is called “the Enlightenment”? 


[9] Darkness and Light appear in the human imagination on the truest of mythic scales. They are archetypes expressing fundamental tensions every human being encounters. Given that deep root it is, perhaps, not a surprise that astronomers settled on dark matter as the name for their great astronomical mystery. After all, scientists must also reach into the same collective pool of associations when venturing into the undiscovered country of creativity. Now you could counter that “dark matter” just sounds cool, end of story. But the real question then becomes why does it sound cooler than, say, “invisible matter,” or “unseen matter.” I’d argue that because of its fundamental associations with good, evil, mystery, and enlightenment, calling something “dark” is bound to send small shivers down even the most ardent rationalist’s spine. 


[10] But words, meanings, and intentions can be a tricky triplet. When an artist names their book of poems Dark Matter, are they explicitly thinking about the binary pairing of Good/Evil or Knowledge/Ignorance. One of the hallmarks of artist expression is that, at its best, it can call on much more subtle incantations of experience. In this way one can imagine how the explicit meanings of dark matter in a scientific setting can find entirely new and creative renderings in paintings, poems, and theater. In a sense these possibilities illustrate the creative power of our mythic storehouses of meaning. We build representations of the world in science and in art that are always forward and backward looking. 


[11] So the deeper resonance between a scientific field’s choice of nomenclature and the public’s response seem baked-in when it comes to dark matter. Once you add “dark energy” to the mix, giving us the fact that 95 percent of everything in the Universe comes in “dark” form, the mythic associations only deepen. Should we be so surprised that poets, playwrights, and painters are drawing from this wellspring? Not at all. We humans are born from mystery and at our end return to it. Though science deals with the most specific and technical forms of unknowing at its frontiers, it is an unknowing none the less. We all want a path from Darkness to Light. We all are asking for it.