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MM #2: Cosmology

July 9, 2024

This is the second in the Metastatic Modernity video series of about 17 installments (see launch announcement), putting the meta-crisis in perspective as a cancerous episode afflicting humanity and the greater community of life on Earth. This episode provides a cosmological perspective on our insignificance.

As will be the custom for the series, I provide a stand-alone companion piece in written form (not a transcript) so that the key ideas may be absorbed by a different channel. I record unscripted videos in one take—usually keeping the first attempt—which has the advantage of being fresh and natural, but I inevitably leave out all the “right” things I would say if given more time. Writing allows more careful reflection and optimization of how I say things. I’m not as collected in real-time.

The write-up that follows is arranged according to “chapters” in the video, navigable via links in the YouTube description field.

Introduction

This just names the series and myself, acknowledging my professional connection to the topic at hand.

The Big Bang

The Big Bang happened 13.8 billion years ago, which created our space and time. We are not geared to think about a beginning of time and non-existence of space, but that’s what the Big Bang is about. The universe did not “explode” into a pre-existing space, the way our brains are hard-wired to picture any explosion. Space itself is the very thing that was (and is) exploding.

Think of inflating a balloon. Flecks of glitter are sprinkled on the balloon to represent galaxies. As the balloon inflates (space expands), the glitter flecks (galaxies) move apart, though not by moving within or through the rubber (space). Or a loaf of raisin bread expanding in the oven has raisins (galaxies) moving apart as the bread (space) expands between the raisins.

These analogies fail to capture the full essence because our 3-D mental model assumes (in practice requires) a space into which the balloon expands or the loaf enlarges. The real universe has no analog of the oven or the room into which the balloon inflates. The dough/rubber is the entire space—or what we identify as space, anyway—and was once minuscule. Again, our analogies break down since the balloon or bread were never arbitrarily tiny.

Early on, when the universe was tiny, it was also very hot: too hot for atoms or even nuclei to exist—essentially boiled into a plasma soup of quarks, gluons, and electrons. As the universe rapidly expanded and cooled (spreading the same energy over a larger volume), nuclei condensed out of the soup (called Big-Bang Nucleosynthesis, or BBN). This happened within the first 20 minutes after the Big Bang—we can’t accuse it of laziness! About 400,000 years later, it was finally cool enough for electrons to stay put around nuclei, forming neutral atoms and allowing the universe to finally become transparent to the propagation of light (called the epoch of recombination).

Galaxies

Gravity brought friendly atoms together to make dust, planets, stars, and galaxies. We all are personally familiar with the first three of these entities. Galaxies are gravitationally-bound collections of stars (and gas and dust and mostly dark matter, in fact). Our galaxy, the Milky Way, contains about 100 billion stars.

Portion of the Hubble extreme deep field

I show an image from the Hubble Space Telescope called the eXtreme Deep Field (XDF) that is littered with galaxies to the edge of our vision. Only one star appears, along the bottom edge toward the right (given away by the diffraction spikes that are apparent from a point source but not from fuzzy things).

Our Sun

Our sun and solar system are roughly 4.5 billion years old—about a third the age of the universe. As mentioned above, it is just one of approximately 100 billion in our galaxy, and it’s not a particularly special or bright one.

When you walk under a dark sky and see lots of stars, how many are intrinsically dimmer than our sun? Not many. Only 15 of the 2,000 most-visible naked-eye stars are dimmer than the sun.

Diversion: Dim Stars

As an unimportant tangent (feel free to skip!), here are the specifics of those 15 stars:

HIP. ID MV MV DIST (L.Y.) BAYER DESIG.
5336 5.78 5.17 24.6 μ-Cas
8102 5.68 3.49 11.9 τ-Cet
15457 5.03 4.84 29.9 κ-Cet
15510 5.35 4.26 19.8 e-Eri
16537 6.18 3.72 10.5 ε-Eri
19849 5.91 4.43 16.5 ο2-Eri
29271 5.04 5.08 33.1 α-Men
57443 5.06 4.89 30.1
64924 5.09 4.74 27.8
71681 5.70 1.35 4.39 α-Cen
72659 5.41 4.54 21.9 ξ-Boo
84405 5.44 4.33 19.5 36-Oph
88601 5.50 4.03 16.6 p-Oph
96100 5.86 4.67 18.8 σ-Dra
108870 6.89 4.69 11.8 ε-Ind

Columns are Hipparcos ID, absolute (visual) magnitude, apparent magnitude, distance in light years, and Bayer designation (for easier finding). Our sun has an absolute magnitude of 4.83. The other 2,000 stars down to apparent magnitude 5.17 (the cutoff here) have absolute magnitudes brighter (lower number than) 4.83. Note that only one bright star appears: Alpha Centauri. The third-brightest star in the sky (behind Sirius and Canopus), Alpha Centauri is actually a double star unresolved by naked eye. The angular separation between the two stars ranges from 2 to 22 arcseconds depending on orbital phase, so can be resolved in a telescope. The one that’s dimmer than the sun is the dimmer of the two, sensibly. I’ve never seen it myself (southern hemisphere). Besides Alpha Centauri, only two of the 15 intrinsically dim stars are brighter than fourth magnitude, and at sub-equatorial declinations are visible from anywhere south of the Arctic Circle.

I picked 2,000 stars rather than the 6,000 often cited (based on the aspirational limit of magnitude 6.5) because: have you ever tried to see a star dimmer than about 5.2 magnitude? It’s very hard, requiring a dark sky and young pupils that can still open wide. Anyway, don’t take my word for it: try for yourself on a real sky!

Most of this detail was skipped in the video, but the main point is: our sun is no super-star: just one of 100 billion and at the tail end of prominence. Our sun would not be on the list above for anyone beyond about 40 light years: very close compared to the 100,000 light years that our galaxy spans.

Scale of the Solar System

Adding insult to injury, we now compare our insignificant sun to a mere chickpea! A chickpea is about a centimeter across, as demonstrated in the video using the enviable MATH RULER. Earth would be about 100 times smaller, or about the diameter of a human hair—which I attach to a piece of tape for demonstration. Holding these two a meter apart—harder than it sounds in a non-mirrored display—captures the scale of the Earth–Sun system. Earth is a mere dust speck that would be hard to even see/notice.

Jupiter is a one-millimeter sand grain that would be 5 meters away. The solar system would require something like a football field (whichever way you wish to define “football”) to accommodate. It’s mostly very extremely empty. The sun itself accounts for 99.85% of the mass in the solar system. Add Jupiter and we’re up to 99.95%. Saturn, Uranus and Neptune account for 80% of the remainder, leaving 0.01% for all the dust specks including Earth. Think about a football field with one very bright (and too hot to touch) chickpea, one grain of sand, three other barely-visible grains, and the rest (including Earth) basically too small to notice. Everything in between in this lush oasis is empty, empty, empty.

Interstellar Emptiness

The next star from our sun (another chickpea) would be 300 kilometers away. More typically in our region off the Milky Way, it’s about 1,000 km between stars as big/bright as the sun, and 500 km between the puniest barely-stars. So our nearest neighbor is atypically close (4.4 light years but would normally be 15 before getting something as big/bright).

In any case, interstellar space makes the football-field solar system look like a hoarder-house, crammed with a few specks as it is. Spend a moment trying to imagine traveling hundreds of kilometers between two chickpeas, with nothing to run across in between! On this scale, our galaxy would be about a thousand times larger than Earth—itself hard to grasp even in this dramatically-shrunken (100-billion-fold) scale.

Galaxy Collision

This image from the Gemini North telescope in Hawai‘i reveals a pair of interacting spiral galaxies — NGC 4568 (bottom) and NGC 4567 (top) — as they begin to clash and merge. The galaxies will eventually form a single elliptical galaxy in around 500 million years.

So empty is interstellar space that when two galaxies, each containing 100 billion stars, collide with each other—as shown in the image of NGC 4567 and 4568—chances are that not a single star strikes another. That’s how empty galaxies are, and how tiny stars are compared to their separations. And galaxies are dense concentrations—swarms—of stars on the scale of the universe. I will note that in a galaxy collision (something I worked on for my PhD project), clouds of gas and dust do slam into one another, so that the overall encounter is not frictionless.

Copernicus to the Fifth

Part of the reason we go through all this is to give a sense of scale but also of insignificance. The Copernican revolution was very important in shaping our views and shaking the foundations of how important we thought we were. We still continue that journey to this day. Tying some of the previous steps together with some new ones, I can formulate five layers to Copernican-like revolutions of insignificance.

Copernican to the First Power

This is the original Copernican Revolution: Earth is not at the center of creation. Earth goes around the sun. Moreover, Earth is a tiny speck compared to the sun: 0.0003% of its mass. In a scale model of the solar system occupying a football field, Earth would be an inconspicuous dust mote.

Copernican Squared

But don’t go thinking the sun is the proper center of it all. The sun is one of about 100 billion stars in the suburbs of the Milky Way galaxy, orbiting the center every 225 million years. It’s not particularly bright or important.

Copernican Cubed

Well, then, is the Milky Way the center? After all, we see galaxies receding from us in all directions. No. Neither is the Milky Way nor any galaxy at the center. The universe, in fact, no more has a center than the surface of a sphere has a center. The Milky Way is one of about 100 billion galaxies in the visible universe (a number that keeps showing up in this post). A few concepts here warrant extra attention.

The Visible Universe

What do I mean “the visible universe?” Well, light has only been able to travel 13.8 billion light years in the 13.8 billion year lifetime of our universe. Looking into the distance is looking back in time. But a being sitting on what we judge to be the edge would see us at its edge, and opposite our direction could see normal universe that is situated beyond our visible horizon—another 13.8 billion light years farther.

It’s like being in the ocean (e.g., snorkeling or scuba diving). You can see maybe 10, 20 or 30 meters depending on water clarity. But if you were to swim to a rock barely discernible on the edge of your vision, you have no doubt that you’d see more stuff beyond it, and the rock you currently float beside would itself become hard to make out. Likewise, on the surface of the earth you can only see so far on its curved surface. That horizon is not the literal edge of the world, but simply the edge of your direct vision. Traveling to that “edge” would reveal more wonders and your current location becomes the edge from the new position.

The universe works similarly. There’s more to it than meets the eye. How much bigger is the real thing than what our time horizon allows us to see? Measurements of the Cosmic Microwave Background reveal that the geometry of our universe is “flat” to about 1%, implying that if it is curved back on itself at all—one possibility—the scale of this curvature is at least 100 times the horizon scale, and possibly very much bigger. Think of it as a patch on a sphere (like the Earth). The larger the sphere, the flatter the patch appears to be. If the universe is at least 100 times as large as the horizon in every direction, then it is at least a million times the volume of our visible patch (linear dimension, cubed).

Flying Away From Us?

One other bit of clean-up. The expansion of the universe indeed appears to be away from us in every direction, which might seem to support the notion that we are at its center. But this is how any galaxy would perceive the state of affairs when the entire space is expanding in every direction. Think of any glitter fleck on the inflating balloon, and imagine how it would describe the motion of other glitter flecks. All are moving away from it. No matter which one you start with, the story will be the same. And the farther the glitter speck in question, the faster it appears to recede (the Hubble Law).

Copernican to the Fourth Power

The clarification of the visible universe above did the heavy lifting for this stage of the Copernican journey. Our visible patch of the universe isn’t even a special region within the much larger space. At least a million times the familiar volume lies outside the part we can see, due to finite light speed and a finite age of the universe. Each sub-region of the whole has the same experience: causally cut off from most of the space in our complete universe.

Copernican to the Fifth Power

The final step is a recognition that our universe may not be the only one. In fact, it would be rather odd for this to be true. Quantum mechanics, inflation cosmology, and string theory are supportive of multiverse notions. According to the string theory Landscape idea, most instances likely don’t share our exact physics and may not even be able to form stable atoms—let alone stars, planets, and life. But some get “lucky,” and by pure selection effect we’d have to find ourselves in one of those (see post on the Anthropic Principle).

It’s Not About Us!

The point of going through all this is that it’s not about us. The universe isn’t here for us. We’re an insignificant component of life on an insignificant speck orbiting an insignificant star in an insignificant galaxy within an insignificant sector of the universe which itself might be an insignificant member of a large collection of disconnected universes.

Wrap-up & Supplements

That, I think, is an important perspective and forms one component of what I wanted to share in the overall series. If this expression of our insignificance is in any way unsettling to you, then I strongly recommend pondering why it would be. Doing so may reveal a critical disconnect in myth vs. reality.

The next episode will look at early life on Earth and the degree to which we still utterly depend on its solutions to the question of how to live.

Tom Murphy

Tom Murphy is a professor of physics at the University of California, San Diego. An amateur astronomer in high school, physics major at Georgia Tech, and PhD student in physics at Caltech, Murphy has spent decades reveling in the study of astrophysics. He currently leads a project to test General Relativity by bouncing laser pulses off of the reflectors left on the Moon by the Apollo astronauts, achieving one-millimeter range precision. Murphy’s keen interest in energy topics began with his teaching a course on energy and the environment for non-science majors at UCSD. Motivated by the unprecedented challenges we face, he has applied his instrumentation skills to exploring alternative energy and associated measurement schemes. Following his natural instincts to educate, Murphy is eager to get people thinking about the quantitatively convincing case that our pursuit of an ever-bigger scale of life faces gigantic challenges and carries significant risks. Note from Tom: To learn more about my personal perspective and whether you should dismiss some of my views as alarmist, read my Chicken Little page.