Big Picture Part Four: Complexity

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Four, Complexity:

  • 28. The Universe in a Cup of Coffee
  • 29. Light and Life
  • 30. Funneling Energy
  • 31. Spontaneous Organization
  • 32. The Origin and Purpose of Life
  • 33. Evolution’s Bootstraps
  • 34. Searching Through the Landscape
  • 35. Emergent Purpose
  • 36. Are We the Point?

One of the most annoying arguments a scientist can hear is that “evolution (or the origin of life) violates the Second Law of Thermodynamics.” The idea is basically that the Second Law says things become more disorganized over time, but the appearance of life represents increased organization, so what do you have to say about that, Dr. Smarty-Pants?

This is a very bad argument, since the Second Law only says that entropy increases in closed systems, not open ones. (Otherwise refrigerators would be impossible, since the entropy of a can of Diet Coke goes down when you cool it.) The Earth’s biosphere is obviously an open system — we get low-entropy photons from the Sun, and radiate high-entropy photons back to the universe — so there is manifestly no contradiction between the Second Law and the appearance of complex structures.

As right and true as that response is, it doesn’t quite address the question of why complex structures actually do come into being. Sure, they can come into being without violating the Second Law, but that doesn’t quite explain why they actually do. In Complexity, the fourth part of The Big Picture, I talk about why it’s very natural for such a thing to happen. This covers the evolution of complexity in general, as well as specific questions about the origin of life and Darwinian natural selection. When it comes to abiogenesis, there’s a lot we don’t know, but good reason to be optimistic about near-term progress.

In 2000, Gretchen Früh-Green, on a ship in the mid-Atlantic Ocean as part of an expedition led by marine geologist Deborah Kelley, stumbled across a collection of ghostly white towers in the video feed from a robotic camera near the ocean floor deep below. Fortunately they had with them a submersible vessel named Alvin, and Kelley set out to explore the structure up close. Further investigation showed that it was just the kind of alkaline vent formation that Russell had anticipated. Two thousand miles east of South Carolina, not far from the Mid-Atlantic Ridge, the Lost City hydrothermal vent field is at least 30,000 years old, and may be just the first known example of a very common type of geological formation. There’s a lot we don’t know about the ocean floor.

Lost City

The chemistry in vents like those at Lost City is rich, and driven by the sort of gradients that could reasonably prefigure life’s metabolic pathways. Reactions familiar from laboratory experiments have been able to produce a number of amino acids, sugars, and other compounds that are needed to ultimately assemble RNA. In the minds of the metabolism-first contingent, the power source provided by disequilibria must come first; the chemistry leading to life will eventually piggyback upon it.

Albert Szent-Györgyi, a Hungarian physiologist who won the Nobel Prize in 1937 for the discovery of Vitamin C, once offered the opinion that “Life is nothing but an electron looking for a place to rest.” That’s a good summary of the metabolism-first view. There is free energy locked up in certain chemical configurations, and life is one way it can be released. One compelling aspect of the picture is that it’s not simply working backwards from “we know there’s life, how did it start?” Instead, its suggesting that life is the solution to a problem: “we have some free energy, how do we liberate it?”

Planetary scientists have speculated that hydrothermal vents similar to Lost City, might be abundant on Jupiter’s moon Europa or Saturn’s moon Enceladus. Future exploration of the Solar System might be able to put this picture to a different kind of test.

A tricky part of this discussion is figuring out when it’s okay to say that a certain naturally-evolved organism or characteristic has a “purpose.” Evolution itself has no purpose, but according to poetic naturalism it’s perfectly okay to ascribe purposes to specific things or processes, as long as that kind of description actually provides a useful way of talking about the higher-level emergent behavior.

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Big Picture Part Three: Essence

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Three, Essence:

  • 19. How Much We Know
  • 20. The Quantum Realm
  • 21. Interpreting Quantum Mechanics
  • 22. The Core Theory
  • 23. The Stuff of Which We Are Made
  • 24. The Effective Theory of the Everyday World
  • 25. Why Does the Universe Exist?
  • 26. Body and Soul
  • 27. Death Is the End

In Part Three we get our hands dirty diving into some of the central features of how our world actually works: quantum mechanics, field theory, and the Core Theory describing the actual particles and forces that make up the visible universe. The discussion of the basics of quantum mechanics itself is quite brief, and I mention the Many-Worlds formulation only to emphasize that there’s nothing about QM that implies we need to be idealist, anti-realist, or non-determinist. (Those options are open, of course — but they’re not forced on us by what we know about quantum mechanics.)

More directly relevant to this discussion are the ideas of effective field theory and crossing symmetry that let us conclude the laws of physics underlying everyday life are completely known. (I used to say “…completely understood,” but too many people chose to quibble about whether we “really understand” them rather than grasping the point, so I’ve switched to “known.”) (No, I don’t think it will really help either.) In early drafts I went on a bit too long about all the quarks and gluons and so forth, since personally I think that stuff is endlessly fascinating. But it dragged down the pace a bit, so now I have an Appendix in which I give the full Core Theory equation and explain — tersely but accurately! — every single term that appears in it.

In the body of the text I concentrate more on explaining what the claim actually says and why it has a chance of being true. For example, why it doesn’t matter for everyday purposes that we don’t yet understand quantum gravity.

Physicists divide our theoretical understanding of these particles and forces into two grand theories: the Standard Model of Particle Physics, which includes everything we’ve been talking about except for gravity, and general relativity, Einstein’s theory of gravity as the curvature of spacetime. We lack a full “quantum theory of gravity” — a model that is based on the principles of quantum mechanics, and matches onto general relativity when things become classical-looking. Superstring theory is one very promising candidate for such a model, but right now we just don’t know how to talk about situations where gravity is very strong, like near the Big Bang or inside a black hole, in quantum-mechanical terms. Figuring out how to do so is one of the greatest challenges currently occupying the minds of theoretical physicists around the world.

But we don’t live inside a black hole, and the Big Bang was quite a few years ago. We live in a world where gravity is relatively weak. And as long as the force is weak, quantum field theory has no trouble whatsoever describing how gravity works. That’s why we’re confident in the existence of gravitons; they are an inescapable consequence of the basic features of general relativity and quantum field theory, even if we lack a complete theory of quantum gravity. The domain of applicability of our present understanding of quantum gravity includes everything we experience in our everyday lives.

There is, therefore, no reason to keep the Standard Model and general relativity completely separate from each other. As far as the physics of the stuff you see in front of you right now is concerned, it is all very well described by one big quantum field theory. Nobel Laureate Frank Wilczek has dubbed it the Core Theory. It’s the quantum field theory of the quarks, electrons, neutrinos, all the families of fermions, electromagnetism, gravity, the nuclear forces, and the Higgs. In the Appendix we lay it out in a bit more detail. The Core Theory is not the most elegant concoction that has ever been dreamed up in the mind of a physicist, but it’s been spectacularly successful at accounting for every experiment ever performed in a laboratory here on Earth. (At least as of mid-2015 — we should always be ready for the next surprise.)

Princess Elisabeth of BohemiaOne of my favorite chapters in the book is 26, Body and Soul, where I relate the story of Princess Elisabeth of Bohemia and René Descartes. And how, you may ask, does quantum field theory relate to an epistolary conversation carried out in the seventeenth century? Descartes, of course, was famously a champion of mind/body dualism. Elisabeth challenged him on this, asking how something (the immaterial soul) that had no location or extent in space could possibly influence something (the physical body) that manifestly did. The updated version of Elisabeth’s challenge is to ask, “How could an immaterial soul possibly affect the evolution of the particles and fields in the Core Theory? How should that gloriously precise and well-tested equation be modified?”

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Big Picture Part Two: Understanding

One of a series of quick posts on the six sections of my book The Big PictureCosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part Two, Understanding:

  • 9. Learning About the World
  • 10. Updating Our Knowledge
  • 11. Is It Okay to Doubt Everything?
  • 12. Reality Emerges
  • 13. What Exists, and What Is Illusion?
  • 14. Planets of Belief
  • 15. Accepting Uncertainty
  • 16. What Can We Know About the Universe Without Looking at It?
  • 17. Who Am I?
  • 18. Abducting God

If, as a naturalist, you want to seriously engage with people who might not agree with you already, you’re going to have to talk a bit about epistemology — how we know what we know. It’s common for non-naturalists to level the accusation (perhaps sincerely felt) that naturalists simply assume their conclusion, and use methodologies that are not sensitive to the possibility of something outside the natural world.

I don’t agree, so in The Big Picture I spend a decent amount of time talking about how we actually go about the task of understanding the world. In particular, science doesn’t presume naturalism, it concludes that it’s the best explanation for the world we experience. Provisionally, of course — science never “proves” anything in the logical sense, so we should always be open to changing our minds in the face of new evidence. I talk a good deal (maybe too much) about Bayesian reasoning and how to update our beliefs.

As an extremely simple — but usefully illustrative — example of this philosophy in action, I concocted a straw-man example of theory choice:

Simplicity is sometimes easy to gauge, sometimes it is less so. Consider three competing theories. One says that the motion of planets and moons in the Solar System is governed, at least to a pretty good approximation, by Isaac Newton’s theories of gravity and motion. Another says that Newtonian physics doesn’t apply at all, and that instead every celestial body has an angel assigned to it, and these angels guide the planets and moons in their motions through space, along paths that just coincidentally match those that Newton would have predicted.

Most of us would probably think that the first theory is simpler than the second — you get the same predictions out, without needing to invoke vaguely-defined angelic entities. But the third theory is that Newtonian gravity is responsible for the motions of everything in the Solar System except for the Moon, which is guided by an angel, and that angel simply chooses to follow the trajectory that would have been predicted by Newton. It is fairly uncontroversial to say that, whatever your opinion about the first two theories, the third theory is certainly less simple than either of them. It involves all of the machinery of both, without any discernible difference in empirical predictions. We are therefore justified in assigning it a very low prior credence. (This example seems frivolous, but analogous moves become common when we start talking about the progress of biological evolution or the nature of consciousness.)

Some people don’t like the Bayesian emphasis on priors, because they seem subjective rather than objective. And that’s right — they are. It can’t be helped; we have to start somewhere. On the other hand, ideally the likelihoods of making certain observations can be objectively determined. If you have a certain theory about the world, and that theory is precise and well-defined, you can say with confidence what the chances are of observing various bits of data under the assumption that your theory is correct. In realistic circumstances, we are often stuck trying to evaluate theories that aren’t so rigorously defined in the first place. (“Consciousness transcends the physical” is a legitimate proposition, but it’s not sufficiently precise to make quantitative predictions.) Nevertheless, it’s our job to try to make our propositions as well-defined as possible, to the point where we can use them to objectively establish the likelihoods of different observations.

Everyone’s entitled to their own priors, but not to their own likelihoods.

Sadly, as much as we might aspire to be, we humans are not always completely rational. One concept I talk about is that of a “planet of belief” — rather than grounding our beliefs on an unshakeable foundation, we assemble a collection of beliefs that hold together under a mutual epistemological attraction. That’s not a mistake, it’s the best we can do. The secret is not to grow so attached to our planets that we equip them with impregnable defense systems, so that they can never be altered no matter what new things we learn.

Planets of Belief

Here we see some representative components of three plausible planets of belief. Can you figure out what kind of person each might belong to?

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Big Picture Part One: Cosmos

One of a series of quick posts on the six sections of my book The Big Picture — Cosmos, Understanding, Essence, Complexity, Thinking, Caring.

Chapters in Part One, Cosmos:

  • 1. The Fundamental Nature of Reality
  • 2. Poetic Naturalism
  • 3. The World Moves By Itself
  • 4. What Determines What Will Happen Next?
  • 5. Reasons Why
  • 6. Our Universe
  • 7. Time’s Arrow
  • 8. Memories and Causes

It wasn’t easy to settle on how to organize all the material in The Big Picture. Ultimately I decided to start with the universe — a bit of cosmology, yes, but also some basic features of how the universe physically operates. Features that played a crucial role in the transition from an ancient view of the world, which still lingers on in our informal “manifest image” of how things work, to our modern scientific view.

Ibn-SinaOne important step in that transition was the seemingly-innocuous realization that momentum is conserved. In Aristotle’s physics, to keep something moving you had to keep pushing it. That’s a very sensible thing to believe, since it’s absolutely true in our everyday experience. It took many centuries of thinking by very smart people (including Persian polymath Ibn Sina, right) to realize that things tend to move by themselves, and are only slowed down by external forces such as friction. That’s important for physics, of course, but there is a deeper implication for our picture of what kinds of things we need to invoke to explain the universe. As I put it in the book:

Aristotle’s argument for an unmoved mover rests on his idea that motions require causes. Once we know about conservation of momentum, that idea loses its steam. We can quibble over the details — I have no doubt Aristotle would have been able to come up with an ingenious way of accounting for objects on frictionless surfaces moving at constant velocity. What matters is that the new physics of Galileo and his friends implied an entirely new ontology, a deep shift in how we thought about the nature of reality. “Causes” didn’t have the central role that they once did. The universe doesn’t need a push; it can just keep going.

It’s hard to over-emphasize the importance of this shift. Of course, even today, we talk about causes and effects all the time. But if you open the contemporary equivalent of Aristotle’s Physics — a textbook on quantum field theory, for example — words like that are nowhere to be found. We sometimes talk about causes, but they’re no longer part of our best fundamental ontology.

What we’re seeing is a manifestation of the layered nature of our descriptions of reality. At the deepest level we currently know about, the basic notions are things like “spacetime,” “quantum fields,” “equations of motions,” and “interactions.” No causes, whether material, formal, efficient, or final. But there are levels on top of that, where the vocabulary changes. Indeed, it’s possible to recover pieces of Aristotle’s physics quantitatively, as limits of Newtonian mechanics in an appropriate regime, where dissipation and friction are central. (Coffee cups do come to a stop, after all.) In the same way, it’s possible to understand why it’s so useful to refer to causes and effects in our everyday experience, even if they’re not present in the underlying equations. There are many different useful stories we have to tell about reality to get along in the world.

The idea that “cause and effect” isn’t fundamental to the workings of the universe hasn’t spread as widely as it should have, despite the efforts of smart people such as Bertrand Russell. In this first section of the book I sketch how we moved from a picture of the universe animated by causes and reasons to one that obeys patterns, without the need for anything to cause it or sustain it. Of course the idea of causality is still crucial to our everyday lives, so I talk a bit about how cause-and-effect relations are emergent phenomena in a macroscopic world with a pronounced arrow of time.

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Entropic Time

A temporary break from book-related blogging to bring you this delightful video from A Capella Science, in which Tim Blais sings about entropy while apparently violating one of my favorite laws of physics. I don’t even want to think about how much work this was to put together.

Tim was gracious enough to tip his hat to a lecture of mine as partial inspiration for the video. And now that I think about it, entropy and the arrow of time play crucial roles in The Big Picture. So this is a book-related blog post after all! Had you fooled.

Posted in Entertainment, Music, Time | 6 Comments

The Big Picture: What It’s All About

Many years ago I had the pleasure of attending a public lecture on cosmology by Martin Rees, one of the leading theoretical astrophysicists of our time and a wonderful speaker. For the most part his choice of material was unimpeachably conventional, but somewhere in the middle of explaining the evolution of the universe he suddenly started talking about the possibility of life on other planets. You could sense the few scientists in the room squirming uncomfortably — this wasn’t cosmology at all! But the actual public, to whom the talk was addressed, loved it. They didn’t care about fussy academic boundary enforcement; they thought these questions were interesting, and were curious about how it all fit together.

Since then, following Martin’s example, I have occasionally slipped into my own talks some discussion of how the particular bit of science I was discussing fit into a larger context. What are the implications of quantum mechanics for free will, or entropy for aging and death, or the multiverse for morality? To (what should be) nobody’s surprise, those are often what people want to follow up on in questions after the talk. Professional scientists will feel an urge to correct them, arguing that those aren’t the questions they should be asking about. But I think it’s okay. Science isn’t just about solving this or that puzzle; it’s about understanding how the world works. The whole world, from the vastness of the cosmos to the particularity of an individual human life. It’s worth thinking about how all the different ways we have to talk about the world manage to fit together.

That idea is one of the motivating considerations behind my new book, The Big Picture: On the Origins of Life, Meaning, and the Universe Itself, which is being released next week (Tuesday May 10 — I may be visiting your area). It’s a big book, covering a lot of things, so over the next few days I’m going to put up some posts containing small excerpts that give a flavor what what’s inside. You can get a general feeling from glancing at the table of contents. I also can’t resist pointing you to check out the amazing blurbs that so many generous people were thoughtful enough to contribute — Elizabeth Kolbert, Neil Shubin, Deborah Blum, Alan Lightman, Sabine Hossenfelder, Michael Gazzaniga, Carlo Rovelli, and Neil deGrasse Tyson.

This book is a culmination of things I’ve been thinking about for a long time. I’ve loved physics from a young age, but I’ve also been interested in all sorts of “big” questions, from philosophy to evolution and neuroscience. And what these separate fields have in common is that they all aim to capture certain aspects of the same underlying universe. Therefore, while they are indisputably separate fields of endeavor — you don’t need to understand particle physics to be a world-class biologist — they must nevertheless be compatible with each other — if your theory of biology relies on forces that are not part of the Standard Model, it’s probably a non-starter. That’s more of a constraint than you might imagine. For example, it implies that there is no such thing as life after death. Your memories and other pieces of mental information are encoded in the arrangement of atoms in your brain, and there’s no way for that information to escape your body when you die.

More generally, ontology matters. What we believe about the fundamental nature of reality affects how we look at the world and how we choose to live our lives. We should work to get it right.

The viewpoint I advocate in TBP is poetic naturalism. “Naturalism” being the idea that there is only one world, the natural world, that follows the laws of nature and can be investigated using the methods of science. “Poetic” emphasizes the fact that there are many ways of talking about that world, and that different stories we tell can be simultaneously valid in their own domains of applicability. It is therefore distinguished from a hardcore, eliminativist naturalism that says the only things that really exist are the fundamental particles and forces, and also from various varieties of augmented naturalism that, unsatisfied with the physical world by itself, add extra categories such as mental properties or objective moral values into the mix.

Along the way, we meet a lot of fun ideas — conservation of momentum and information, emergent purpose and causality, Bayesian inference, skepticism, planets of belief, effective field theory, the Core Theory, the origin of the universe, the relationship between entropy and complexity, free energy and the purpose of life, metabolism-first and replication-first theories of abiogenesis, the fine-tuning argument, consciousness and philosophical zombies, panpsychism, Humean constructivism, and the basic finitude of our lives.

Not everyone agrees with my point of view on these matters, of course. (It’s possible that literally nobody agrees with me about every single stance I take.) That’s good! Bring it on, I say. Maybe I will learn something and change my mind. There are plenty of things I talk about in the book on which respectable good-faith disagreement is quite possible — finer points of epistemology and metaphysics, the connections between different manifestations of the arrow of time, how to confront radically skeptical scenarios, the robustness of the Core Theory, approaches to the mind-body problem, interpretations of quantum mechanics, the best way to think about complexity and evolution, the role of purposes and causes in our ontology, free will, moral realism vs, anti-realism, and so on.

The point of the book is not to stride confidently into multiple ongoing debates and proclaim that I have it All Figured Out. Quite the opposite: while the subtitle correctly implies that I talk about the origins of life, meaning, and the universe itself, the truth is that I don’t know how life began, what the meaning of it all is, or why the universe exists. What I try to advocate is a particular framework in which these kinds of questions can be addressed. Not everyone will agree even with that framework, but it is very explicitly just a starting point for thinking about some of these grand issues, not the final answers to them.

There will inevitably be complaints that I’m writing about things — biology, neuroscience, philosophy — on which I am not an academic expert. Very true! I’m pretty sure nobody in the world is an expert on absolutely everything I talk about here. But I’m just as sure that different kinds of experts need to occasionally wander outside of their intellectual comfort zones to discuss how all of these pieces fit together. My primary hope in TBP is not to put forward some dramatically original view of the universe, but to work toward a synthesis of a wide variety of ideas that have been developed by smart people of the course of centuries. It is at best a small step, but if it helps spark ongoing conversation, I’ll consider the book a great success.

So from people who don’t read the book very carefully, I’ll no doubt get it from both sides: “Knowing physics doesn’t make you an expert on the meaning of life, how dare he presume?” and “I read the whole book and he didn’t tell me what the meaning of life is, what a cheat!” So be it.

One thing that I meant to include in the Acknowledgements section of the book, but unfortunately it slipped my mind at the last minute: I should have mentioned how much my ideas about many of these topics have been shaped and sharpened by interacting with commenters on this very blog. Longtime readers will recognize many of the themes, even if the book presents them in a different way. I’ve definitely learned a lot from questions and arguments in the comment sections here (even if I’m usually too busy to participate very much myself). So — thank you, and I hope you enjoy the book!

 

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Youthful Brilliance

A couple of weeks ago I visited the Texas A&M Physics and Engineering Festival. It was a busy trip — I gave a physics colloquium and a philosophy colloquium as well as a public talk — but the highlight for me was an hourlong chat with the Davidson Young Scholars, who had traveled from across the country to attend the festival.

The Davidson Young Scholars program is an innovative effort to help nurture kids who are at the very top of intellectual achievement in their age group. Every age and ability group poses special challenges to educators, and deserves attention and curricula that are adjusted for their individual needs. That includes the most high-achieving ones, who easily become bored and distracted when plopped down in an average classroom. Many of them end up being home-schooled, simply because school systems aren’t equipped to handle them. So the DYS program offers special services, including most importantly a chance to meet other students like themselves, and occasionally go out into the world and get the kind of stimulation that is otherwise hard to find.

carroll-davidson-scholars

These kids were awesome. I chatted just very briefly, telling them a little about what I do and what it means to be a theoretical physicist, and then we had a free-flowing discussion. At some point I mentioned “wormholes” and it was all over. These folks love wormholes and time travel, and many of them had theories of their own, which they were eager to come to the board and explain to all of us. It was a rollicking, stimulating, delightful experience.

You can see from the board that I ended up talking about Einstein’s equation. Not that I was going to go through all of the mathematical details or provide a careful derivation, but I figured that was something they wouldn’t typically be exposed to by either their schoolwork or popular science, and it would be fun to give them a glimpse of what lies ahead if they study physics. Everyone’s life is improved by a bit of exposure to Einstein’s equation.

The kids are all right. If we old people don’t ruin them, the world will be in good hands.

Posted in Academia, Humanity | 14 Comments

Being: Human

cover6-150Anticipation is growing — in my own mind, if nowhere else — for the release of The Big Picture, which will be out on May 10. I’ve finally been able to hold the physical book in my hand, which is always a highlight of the book-writing process. And yes, there will be an audio book, which should come out the same time. I spent several days in March recording it, which taught me a valuable lesson: write shorter books.

There will also be something of a short book tour, hitting NYC, DC, Boston, Seattle, and the Bay Area, likely with a few other venues to be added later this summer. For details see my calendar page.

In many ways, the book is a celebration of naturalism in general, and poetic naturalism in particular. So to get you in the mood, here is a lovely short video from the Mothlight Creative, which celebrates naturalism in a more visual and visceral way. “I want to shiver with awe and wonder at the universe.”

Being: Human from Mothlight Creative on Vimeo.

Posted in Big Picture, Humanity | 19 Comments

Science Career Stories

The Story Collider is a wonderful institution with a simple mission: getting scientists to share stories with a broad audience. Literal, old-fashioned storytelling: standing up in front of a group of people and spinning a tale, typically with a scientific slant but always about real human life. It was founded in 2010 by Ben Lillie and Brian Wecht; I got to know Ben way back when he was a postdoc at Argonne and the University of Chicago, before he switched from academia to the less well-trodden paths of communication and the wrangling of non-profit organizations.

By now the Story Collider has accumulated quite a large number of great tales from scientists young and old, and I encourage you to catch a live show or crawl through their archives. I was able to participate in one about a year ago, where I shared the stage with a number of fascinating scientific storytellers. One of them was one of my mentors and favorite physicists, Alan Guth. Of course he has an advantage at this game in comparison to most other scientists, as he gets to tell the story of how he came up with one of the most influential ideas in modern cosmology: the inflationary universe.

It’s a great story, both for the science and for the personal aspect: Alan was near the end of his third postdoc at the time, and his academic prospects were far from clear. You just need that one brilliant idea to pop up at the right time.

But everyone’s path is different. Here, from a different event, is my young Caltech colleague Chiara Mingarelli, who explains how she ended up studying gravitational waves at the center of the universe.

Finally, it is my blog, so here is the story I told. I basically talked about myself, but I used my (occasionally humorous) interactions with Stephen Hawking as a hook. Never be afraid to hitch a ride on the coattails of someone immensely more successful, I always say.

Posted in Academia, Personal | 8 Comments

Did LIGO Detect Dark Matter?

It has often been said, including by me, that one of the most intriguing aspects of dark matter is that provides us with the best current evidence for physics beyond the Core Theory (general relativity plus the Standard Model of particle physics). The basis of that claim is that we have good evidence from at least two fronts — Big Bang nucleosynthesis, and perturbations in the cosmic microwave background — that the total density of matter in the universe is much greater than the density of “ordinary” matter like we find in the Standard Model.

There is one important loophole to this idea. The Core Theory includes not only the Standard Model, but also gravity. Gravitons themselves can’t be the dark matter — they’re massless particles, moving at the speed of light, while we know from its effects on galaxies that dark matter is “cold” (moving slowly compared to light). But there are massive, slowly-moving objects that are made of “pure gravity,” namely black holes. Could black holes be the dark matter?

It depends. The constraints from nucleosynthesis, for example, imply that the dark matter was not made of ordinary particles by the time the universe was a minute old. So you can’t have a universe with just regular matter and then form black-hole-dark-matter in the conventional ways (like collapsing stars) at late times. What you can do is imagine that the black holes were there from almost the start — that they’re primordial. Having primordial black holes isn’t the most natural thing in the world, but there are ways to make it happen, such as having very strong density perturbations at relatively small length scales (as opposed to the very weak density perturbations we see at universe-sized scales).

Recently, of course, black holes were in the news, when LIGO detected gravitational waves from the inspiral of  two black holes of approximately 30 solar masses each. This raises an interesting question, at least if you’re clever enough to put the pieces together: could the dark matter be made of primordial black holes of around 30 solar masses, and could two of them have come together to produce the LIGO signal? (So the question is not, “Are the black holes made of dark matter?”, it’s “Is the dark matter made of black holes?”)

LIGO black hole (artist's conception)

This idea has just been examined in a new paper by Bird et al.:

Did LIGO detect dark matter?

Simeon Bird, Ilias Cholis, Julian B. Muñoz, Yacine Ali-Haïmoud, Marc Kamionkowski, Ely D. Kovetz, Alvise Raccanelli, Adam G. Riess

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M≲Mbh≲100M where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they can radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will then rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2−53 Gpc−3 yr−1 rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.

Given this intriguing idea, there are a couple of things you can do. First, of course, you’d like to check that it’s not ruled out by some other data. This turns out to be a very interesting question, as there are good limits on what masses are allowed for primordial-black-hole dark matter, from things like gravitational microlensing and the fact that sufficiently massive objects would disrupt the orbits of wide binary stars. The authors claim (and quote papers to the effect) that 30 solar masses fits snugly inside the range of values that are not ruled out by the data.

The other thing you’d like to do is figure out how many mergers like the one LIGO saw should be expected under such a scenario. Remember, LIGO seemed to get lucky by seeing such a big beautiful event right out of the gate — the thought was that most detectable signals would be from relatively puny neutron-star/neutron-star mergers, not ones from such gloriously massive black holes.

The expected rate of such mergers, under the assumption that the dark matter is made of such big black holes, isn’t easy to estimate, but the authors do their best and come up with a figure of about 5 mergers per cubic gigaparsec per year. You can then ask what the rate should be if LIGO didn’t actually get lucky, but simply observed something that is happening all the time; the answer, remarkably, is between about 2 and 50 per cubic gigaparsec per year. The numbers kind of make sense!

The scenario would be quite remarkable and significant, if it turns out to be right. Good news: we’ve found that dark matter! Bad news: hopes would dim considerably for finding new particles at energies accessible to particle accelerators. The Core Theory would turn out to be even more triumphant than we had believed.

Happily, there are ways to test the idea. If events like the ones LIGO saw came from dark-matter black holes, there would be no reason for them to be closely associated with stars. They would be distributed through space like dark matter is rather than like ordinary matter is, and we wouldn’t expect to see many visible electromagnetic counterpart events (as we might if the black holes were surrounded by gas and dust).

We shall see. It’s a popular truism, especially among gravitational-wave enthusiasts, that every time we look at the universe in a new kind of way we end up seeing something we hadn’t anticipated. If the LIGO black holes are the dark matter of the universe, that would be an understatement indeed.

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