Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe

by Lisa Randall

Our analogy isn’t...

What analogy is? Now I'm just poking at the legal bumpers.

For the analogy to be even better, only the space between the marked points would expand—not the points themselves.

Well, that's one question answered.

All types of matter that interacted sufficiently strongly with each other had a common temperature.

Gah. In addition to an explanation of wheat energy is, I think we need an explanation of what temperature is.

They behaved like hot air trapped inside a balloon, which becomes less dense and cooler as the balloon expands.

That is how gasses work, but not how balloons work. Balloons apply a pressure onto the gas, which would compact it as the air inside gets cooler.

Dark energy, on the other hand, doesn’t dilute at all.

The success of these predictions also tells us that, even today, the amount of ordinary matter cannot be much greater than what has been observed.

I am assuming that this is the ratio, and not the grand total amount. Otherwise, we have observed the entire universe, which would be both surprising and a little sad.

Even though I cannot tell you what happened at the very beginning of the Universe, I can say with reasonable certainty that at some time very early on in its evolution—perhaps as early as 10−36 seconds in—this sensational event called inflation took place.

Once inflation ended—also only a fraction of a second into the Universe’s evolution—it left behind a large, smooth, flat homogeneous Universe whose later evolution is predicted by the traditional Big Bang theory.

Is Dr. Randall using different time scales, or does she need an editor? If she is trying to say that the inflation lasted less than a second, there are clearer ways to say it. Also, But what if some of the dark matter experiences influential non-gravitational interactions too? Also, what does the "also" refer to?

This is puzzling because according to the original Big Bang scenario, the age of the Universe at the time when the cosmic radiation decoupled from charged matter was too low for light to have had enough time to travel even one percent of the way across the sky.

Does this mean that during the inflationary stage, the universe expanded so quickly that matter/energy/radiation would have appeared to travel faster than the speed of light to some neutral observer at the center of the universe?

Depending on when inflation began and how long it lasted, the original region a light ray could cross might have begun as 10−29 meters in size but expanded during inflation to be at least about a millimeter big—a little bigger than a piece of sand.

Is the author using some hitherto unknown meaning of "expanded"?

The Universe is uniform because the enormous expansion during inflation smoothed out the wrinkles in the spacetime fabric, much as stretching out your jacket sleeve eliminates the creases in its fabric.

I would have expected that a dramatically expanding universe would increase the size of any tiny idiosyncratic clusters. Much like when you fill your jacket sleeve with explosives, tiny imperfections in the fabric become massive gaping holes. I just don't understand. Much like how a jacket sleeve doesn't understand. Also, the jacket sleeve analogy doesn't help at all.

Quantum mechanics tells us that the exact time at which inflation ends is uncertain, which means that it ended at slightly different times in different regions of the sky. These tiny quantum effects were imprinted in the radiation as small deviations from perfect uniformity.

Ok, the universe was homogenous when the inflation began, and the tiny wobble of the inflation ended created the imperfections and differences from one region to another.

COBE discovered the quantum fluctuations that were generated when the Universe was roughly the size of a grain of sand, and which are ultimately the origin of you, me, galaxies, and all the structure in the Universe. These initial cosmological inhomogeneities were generated when inflation was ending. They started on minuscule-length scales but they were stretched by the expansion of the Universe to sizes where they could seed galaxies and all other measurable structure,

Am I very, very dim, or does Dr. Randall seem to be switching when these inhomogenieties came about from sentence to sentence? This part is really poorly written and unclear. I expect cosmology to be a bit hard to wrap my head around, but I don't think this is cosmology's fault.

This has been a difficult chapter. Dr. Randall just isn't being clear, to the point where I don't see the numbers adding up -- meters get expanded to millimeters, and seconds to milliseconds. I don't know if she is trying to tell us something very subtle that I am just missing (or something obvious that I am missing), or whether an earlier draft of this chapter was based on one set of numbers, and then it was partially updated as the science changed.

But dark matter was also important because electromagnetic radiation initially prevented ordinary matter from developing structure on scales smaller than about a hundred times the size of a galaxy. Only by hitchhiking with dark matter did galaxy-sized objects and the seeds of stars in our Universe have time to form.

Again, the numbers don't match. Things would not be able to be more than 100 times the size of our galaxy could not form, and therefore our galaxy could not form despite being the size of our galaxy and not 100 times larger. There is obviously a big simplification going on here, with some process being completely elided because this is aimed at non-physicists, but the inclusion of numbers make this missing process necessary to understanding the text. Is this some metatextual metaphor for the measurements that showed dark matter and dark energy were needed, or is this just poor communication? Either drop the numbers, or explain the simplification in a footnote. Or just don't simplify it this much.

Not only is there more of it, but because dark matter is essentially immune to the influence of light, electromagnetic radiation could not drive it apart the way it can with ordinary matter.

https://www.youtube.com/watch?v=yyZU4iNRdsM

Because it is immune to radiation, it could collapse even when ordinary matter could not, forming a substrate in which protons and electrons could be shepherded into collapsing regions.

Dr. Randall has repeated herself four times in this and the preceding paragraph. I believe it was Goëbels who said that if you repeat a lie enough times, it becomes truth.

Objects didn’t emit light because stars hadn’t yet formed, yet the microwave background radiation that earlier on had interacted with the ubiquitous electrically charged matter no longer lit up the sky. This time is invisible to conventional telescopes (See Figure 6.)

Do we see silhouettes of objects in the background radiation?

Further investigation shows that giant stars formed first—but they either quickly exploded into supernovae, releasing the first heavier elements into the Universe, or they collapsed into black holes.

I am still not sure how we can distinguish dark matter from black holes given the tools we have -- both bend light around them, and have gravitational effects, and both would contribute mass to a distant galaxy without increasing the brightness of the galaxy. Are there tell-tale signs of a black hole, which a blob of dark matter would not have? Almost certainly, but Dr. Randall hasn't mentioned any of this.

Only after metals—which is what astronomers call heavy elements—were present could the smaller stars (like our Sun) form in cooler, denser regions, and the structure we now observe be created.

Two questions: why do astronomers call all heavy atoms metals even when there are plenty of non-metal heavy atoms, like Carbon? Second, why do we need the heavier elements to create smaller stars?

But before those stars could form, galaxies had to emerge. Indeed, galaxies were the first complex structures to exist. Galaxies—each seemingly self-contained but, as we will soon see, all connected—were in many respects the building blocks of the Universe. Once formed, galaxies could merge into larger structures, like galaxy clusters.

I eagerly await the explanation for why this has to start with galaxies, than stars and galaxy clusters.

Underdense regions expand more rapidly than the Universe as a whole, whereas overdense regions expand more slowly.

I thought she had earlier said that the expansion was constant. Does the density of the space affect its expansion, or does all the space expand but the more dense areas retain the shape of their matter because the gravitational forces are much stronger than the expansionary forces? I have no idea if this "empty regions expand more" is one of the simplifications or not. Grr.

This yields a hierarchical bottom-up model where smaller structures form before larger ones, in which small galaxies formed first.

But, at this point, before the ordinary matter can form into smaller structures, there are massive structures being formed primarily of dark matter -- these filaments and nodules where they intersect.

The gravitational binding force of the Local Group prevents the Milky Way and Andromeda from receding from each other with the Hubble expansion. Their paths are actually converging and in about four billion years they will collide and merge.

Ok, this really sounds like the space is expanding, even where there is matter, but everything is being pulled together faster than the expansion. This is going to bother me and keep me up at night, isn't it?

Dark matter forms a diffuse spherical halo, whereas ordinary matter can collapse into a disk, such as the familiar disk of stars of the Milky Way plane.

Does "diffuse spherical halo" mean that our galaxy or galaxy cluster is sitting with a dark matter shell and that there isn't a whole lot of dark matter here? A halo does have a big hole in the center after all, at least in 20th and 21st century depictions. That shell would seem to be what the words suggest, but it would also seem to be absurd. Since there is 5x as much dark matter as light, wouldn't the structures inside this shell be torn apart by the gravitational effects of the shell?

Milky Way disk with its central bulge, black hole, and a surrounding dark matter halo.

Has Dr. Randall never seen a halo in her life? Either a real halo, or one on a painting of Jesus? That,s just a diffuse sphere of dark matter, not a halo.

Solar-sized star formation requires extremely cool temperatures—tens of kelvins. Overly high temperature gas never gets sufficiently concentrated to ignite nuclear burning.

Ok, PV=nRT applies here. That makes sense -- the higher the temperature, the greater the volume the ideal gas wants to take up,

In yet another amazing connection between basic fundamental processes and the nature of the Universe, without the heavy elements and molecular cooling that ordinary matter experiences, the gas that created the Sun would never have sufficiently cooled down.

This means that the original stars had an entirely different life cycle, or at least formation. They didn't have the heavy elements required to form.

Not only do stars evolve, but galaxies do as well.

Pop-science books are terrible for using colloquial definitions of words and scientific definitions willy-nilly. But "evolve" is one they should know to avoid. Also, my iPad's autocorrect thought that "willy-nilly" was best represented as "silly-hilly". I eagerly await the autocorrect errors that will enter our language -- "silly-hilly" is pretty good. Imagine a future where the etymology of a phrase is "iOS autocorrect of traditional phrase", and there are distinctions made between iOS autocorrect and Android autocorrect, and the various versions. Phrases get bastardized all the time by people until they form new entries into the lexicon. Why not have them mechanically bastardized with the determining human component being whether they bother to fix it?

For example, you might have thought Population I would be a good way to refer to the first stars in the Universe. But Pop I already referred to one later group of stars, and Pop II was used for another. So when the group of ephemeral earliest stars was hypothesized, it was called Pop III.

Was Pop 0 taken?

Moreover, the Sun sent out charged particles that swept away hydrogen and helium in the more local vicinity.

Why would charged particles affect Helium? Ok, now I am being picky. But still, why?

But in context it should be clear what I mean.

I had basically nothing to say about chapter 6. There just wasn't much in it.

I like to reflect on early-fourteenth-century Italians gazing upward

We all have our fetishes, but this seems oddly specific.

Planetary scientists call those elements with melting point below 100 degrees kelvin gases—independent of the actual phase the matter is in. Those with low melting point, but not so low as gases, are referred to—again by planetary scientists—as ices, though whether or not the material is actually an ice also depends on the actual temperature. That is why Jupiter and Saturn are called gas giants, while Uranus and Neptune are sometimes called ice giants. In both cases, the interior is actually a hot, dense fluid.

Planetary scientists are clearly assholes.

When in the 1950s the German scientist Ludwig Biermann (and independently another German, Paul Ahnert) made the extraordinary observation that the bright ion tail of a comet always points away from the Sun,

Did no one look at comets before then? I mean, really look at them?

Comet nuclei are very nonreflective, which makes them extraordinarily difficult to see, since the most common way to view nonburning objects (such as you and me) is through reflected light.

Dr. Randall is really trying to hard to personalize things right now. This chapter has a lot of this, and is something of a slog anyway. This doesn't help.

One further fascinating compositional feature of a coment is that it contains organic compounds, such as methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane, as well as long-chain hydrocarbons and amino acids, the precursors of life.

This would have been a great time to explain organic compounds don't mean we are being bombarded with dead animals and plants.

Meteorites on Earth have even been found to contain components of DNA and RNA that presumably came from either asteroids or comets.

much farther out is the hypothesized Oort cloud, which produces long-period comets and to which I’ll soon (figuratively) return.

Ok, give it a rest. We get it, you're personable.

An additional region that astrophysicists propose but that we will not focus on here lies between the scattered disk and the Oort cloud and is given the patronizing-sounding name detached objects.

While this entire chapter has been something of a painful slog, I feel better knowing that my kindred spirits lie just past the Oort Cloud.

New comets in the inner Solar System might enter a new orbit or they might appear only once before getting kicked out of the Solar System or colliding with a planet, as Shoemaker-Levy famously and gloriously did when it crashed into Jupiter not so long ago—in 1994.

Why only new comets? Could Halley's Comet not run smack into Jupiter some day? Is the assumption here that any comet with a stable orbit has been around enough times that it would have hit Jupiter if their periodicity was inclines that way?

Gerard Kuiper, for whom the Kuiper belt is named,

For some ungodly reason, I have learned this as the Kuniper Belt, adding an n. I was also raised to speak with a New Jersey accent, and as a child added an 'r' to wash to pronounce it 'warsh'. I can only assume that people in New Jersey refer to the "Kuniper Belt"

Kea—a beautiful viewing site with wonderfully clear skies (well worth visiting if you are on the Big Island).

Yes, yes, you're personable. Now stop interrupting.

there. Either KBOs formed elsewhere—closer to the Sun—or something dispersed most of the mass.

Or a lot of comets perished entering the solar system, either hitting a planet or falling into the sun.

Amusingly, the International Astronomical Union (IAU) requires that plutinos, like Pluto, have also to be named after underworld deities. We know of at least a thousand such objects, though, given the limited surveys to date, scientists suspect—as with the other categories I’ve discussed—the existence of far more.

So, Nixon is probably an underworld deity by now... Can we get something named after him?

A good fraction of these redder classical objects have very circular orbits.

Redder?

In this image of the Oort Cloud,mew see the same pattern as dark matter and the galaxy -- a diffuse sphere connecting to the plane of rotation. I bet there is something awesome to explain this. Or a farting camel: 🐪💨

Which is to say that even though no one will get to the Oort cloud anytime soon, small Solar System bodies—possibly from the Oort cloud—do occasionally descend to Earth.

Another chapter that basically taught me nothing.

Certainly most of the metals we mine here come from extraterrestrial impacts.

This seems so improbable, especially when combined with the claim that the water also comes entirely from meteors. The entire surface of the planet is pulverized meteors, covering what was there before? How does this square with tectonic plates? We can see those being formed in the middle of the Atlantic.

The dramatic phenomenon began with a high, dark cloud that emitted smoke, sparks, and slow-moving, red lightning that was followed by stones that rained down to the ground.

At this point I am far more interested in the red lightning. And we will never return to it.