Presbyopic Astronomy

Her phone call done, Cathleen returns to the Spitzer Memorial Symposium microphone with her face all happiness. “Good news! Jim, the grant came through. Your computer time and telescope access are funded. Woo-hoo!!”

<applause across the audience and Jim grins and blushes>

Cathleen still owns the mic. “So I need to finish up this overview of Spitzer highlights. Where was I?”

Maybe-an-Art-major tries to help. “The middle ground of our Universe.”

“Ah yes, thanks. So we’ve looked at close-by stars but Spitzer showed us a few more surprises lurking in the Milky Way. This, for instance — most of the image is colorized from the infra‑red, but if you look close you can see Chandra‘s X‑ray view, colorized purple to highlight young stars.”

The Cepheus-B molecular cloud
X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al

<hushed general “oooo” from the audience>

“Giant molecular clouds like this are scattered throughout the Milky Way, mostly in the galaxy’s spiral arms. As you see, this cloud’s not uniform, it has clumps and voids. By Earth standards the cloud is still a pretty good vacuum. The clumps are about 10-15 of our atmosphere’s density, but that’s still a million times more dense than our Solar System’s interplanetary space. The clumps appear to be where new stars are born. The photons and other particles from a newly-lit star drive the surrounding dust away. My arrow points to one star with a particularly nice example of that — see the C-shape around the star?”

The maybe-an-Art-major pipes up. “How about that one just a little below center?”

“Uh-huh. There’s so much activity in that dense region that the separate shockwaves collide to create hot spots that’ll generate even more stars in the future. The clouds are mostly held together by their own gravity. They last for tens of millions of years, so we think of them as huge roiling stellar nurseries.”

“Like my kid’s day care center but bigger.”

“Mm-mm, but let’s turn to the Milky Way’s center, home of that famous black hole with the mass of four million Suns and this remarkable structure, a double-helix of warm dust.”

False-color infra-red image of the Double-Helix Nebula
The double helix nebula.
Credit: NASA/JPL-Caltech/M. Morris (UCLA)

Vinnie blurts out, “That’s a jet from a black hole! One of Newt’s babies.”

Newt can’t resist breaking into Cathleen’s pitch. “Maybe it’s a jet, Vinnie. Yes, it’s above the central galactic plane and perpendicular to it, but the helix doesn’t quite point to the central black hole.”

“So take another picture that follows it down.”

“We’d love to, but we can’t. Yet. That image came from a long-wavelength instrument that only operated during Spitzer‘s initial 5-year cold period. Believe me, there are bunches of astronomers who can’t wait for the James Webb Space Telescope‘s far-IR instruments to get into position and start doing science. Meanwhile, we’ve got just the one image and a few earlier ones from an even less-capable spacecraft. This thing may be a lit-up part of a longer structure that twists down to the black hole or at least its accretion disk. We just don’t know.”

Cathleen takes control again. “The next image comes from outside our galaxy — far outside.”

Spitzer visualization of Galaxy MACS 1149-JD1
Credit: NASA/ESA/STScI/W. Zheng (JHU), and the CLASH team

The maybe-an-Art-major snorts, “Pointillism derivative!”

“No, it’s pixels from a starfield image with a very low signal-to-noise ratio. That red blotch in the center is one of the most distant objects ever observed, gracefully named MACS 1149-JD1. It’s a galaxy 13.2 billion lightyears away. That’s so far away that the expansion of the Universe has stretched the galaxy’s emitted photons by a factor of 10.2. Spectrum-wise, 1149-JD1’s ultra-violet light skipped right past the visible range and down into the near infra-red. Intensity-wise, that galaxy’s about 5200 times further away than the Andromeda galaxy. Assuming the two are about the same overall brightness, 1149-JD1 would be about 27 million times fainter than Andromeda.”

“How can we even see anything that dim?”

“We couldn’t, except for a fortunate coincidence. Right in line between us and 1149-JD1 there’s a massive galaxy cluster whose gravity acts like a lens to focus 1149-JD1’s light.”

The seminar’s final words, from maybe-an-Art-major — “A distant light, indeed.”

~~ Rich Olcott

Maybe even smaller?

There’s a sofa in my office. Sometimes it’s used to seat some clients for a consultation, sometimes I use it for a nap. This evening Anne and I are sitting on it, close together, after a meal of Eddie’s Pizza d’amore.

“I’ve been thinking, Sy. I don’t want to use my grow-shrink superpower very much.”

“Fine with me, I like the size you are. Why’d you decide that?”

“I remember Alice saying, ‘Three inches is such a wretched height to be.’ She was thinking about what her cat would do to her at that height. I’m thinking about what an amoeba might do to me if I were down to bacteria-size and I wouldn’t be able to see it coming because I’d be too small to see light. It would be even messier further down.”

“Well, mess is the point of quantum mechanics — all we get is the averages because it’s all chaos at the quantum level. Bohr would say we can’t even talk about what’s down there, but you’d be in the thick of it.”

She shudders delicately, leans in tighter. <long, very friendly pause> “Where’d that weird number come from, Sy?”

“What weird number?”

“Ten-to-the-minus-thirty-fifth. You mentioned it as a possible bottom to the size range.”

Now you’re asking?”

“I’ve got this new superpower, I need to think about stuff.  Besides, we’ve finished the pizza.”

<sigh> “This conversation reminds me of our elephant adventure.  Oh well.  Umm. It may have started on a cold, wet afternoon. You know, when your head’s just not up to real work so you grab a scratchpad and start doodling? I’ll bet Max Planck was in that state when he started fiddling with universal constants, like the speed of light and his own personal contribution ħ, the quantum of action.”

“He could change their values?”

“No, of course not. But he could combine them in different ways to see what came out. Being a proper physicist he’d make sure the units always came out right. I’m not sure which unit-system he worked in so I’ll just stick with SI units, OK?”

“Why should I argue?”

“No good reason to. So… c is a velocity so its units are meters per second. Planck’s constant ħ is energy times time, which you can write either as joule-seconds or kilogram-meter² per second. He couldn’t just add the numbers together because the units are different. However, he could divide the one by the other so the per-seconds canceled out. That gave him kilogram-meters, which wasn’t particularly interesting. The important step was the next one.”

“Don’t keep me in suspense.”

“He threw Newton’s gravitational constant G into the mix. Its units are meter³ per kilogram per second². ‘Ach, vut a mess,’ he thought, ‘but maybe now ve getting somevere. If I multiply ħ by G the kilograms cancel out und I get meter5 per second³. Now … Ah! Divide by c³ vich is equal to multiplying by second³/meter³ to cancel out all the seconds and ve are left mit chust meter² vich I can take the square root uff. Wunderbar, it is simply a length! How ’bout that?‘”

“Surely he didn’t think ‘how ’bout that?‘”

“Maybe the German equivalent. Anyway, doodling like that is one of the ways researchers get inspirations. This one was so good that (Għ/c³)=1.6×10-35 meter is now known as the Planck length. That’s where your ten-to-the-minus-thirty-fifth comes from.”

“That’s pretty small. But is it really the bottom?”

“Almost certainly not, for a couple of different reasons. First, although the Planck formula looks like a fundamental limit, it’s not. In the same report Planck re-juggled his constants to define the Planck mass (ħc/G)=2.2×10-8 kilograms or 22 micrograms. Grains of sand weight less than that. If Planck’s mass isn’t a limit, Planck’s length probably isn’t either. Before you ask, the other reason has to do with relativity and this is not the time for that.”

“Mmm … so if space is quantized, which is where we started, the little bits probably aren’t Planck-sized?”

“Who knows? But my guess is, no, probably much smaller.”

“So I wouldn’t accidentally go out altogether like a candle then. That’s comforting to know.”

My turn to shudder. <another long, friendly pause>.

~~Rich Olcott

Small, yes, but how small?

Another quiet summer afternoon in the office. As I’m finishing up some paperwork I hear a fizzing sound I’d not heard in a while. “Hello, Anne, welcome back. Where’ve you been?”

Her white satin looks a bit speckled somehow but her voice still sounds like molten silver. “I’m not sure, Sy. That’s what I’ve come to you about.”

“Tell me about it.”

“Well, after we figured out that I can sort of ‘push’ myself across time and probability variation I realized that the different ‘pushes’ felt like different directions, kind of. When I go backward and forward in time it feels a little like falling backward or forward. Not really, but that’s the best way I can describe it. Moving to a different probability is a little like going left or right. So I wondered, what about up and down?”

“And I gather you tried that.”

“Sure, why not? What good’s a superpower if you don’t know what you can do with it? When I ‘push’ just a little upward thIS HAPPENS.”

“Whoa, watch out for the ceiling fan! Shrink back down again before you break the furniture or something.”

“Oh, I won’t, I’ve learned to be careful when I resize. Good thing I was outside and all by myself the first time I tried it. Took some practice to control how how much my size changes by how light or heavy I ‘pushed’.”

“I think I can see where this is going.”

“Mm-hm, it’s good to know what the limits are, right? I’ve got a pretty good idea of what would happen if I got huge. What I want to know is, what’ll I be getting into if I try ‘pushing’ down as hard as I can?”

“Kinda depends on how far down you go. I’m assuming your retinas scale their sensitivity with your size. When you get bigger do green things look blue and yellow things look green and so forth?”

“Yeah, orange juice had this weird yellow color. Tasted OK, though.”

“Right. So when you get smaller the colors you perceive will shift the other way, to shorter wavelengths — at first, yellow things will look red, blue things will look yellow and you’ll see ultraviolet as blue. When you get a thousand times smaller than normal, most things will look black because there’s not much X-ray illumination unless you’re close to a badly-shielded Crookes tube.”

“Good thing this ‘push’ ability also gave me some kind of extra feel-sense that’s not sight. Sometimes when I try to ‘push’ it ‘feels’ blocked until I move around a little. After the ‘push’ I see a wall or something I would have jumped into.”

“That’s a relief. I was wondering how you’d navigate when you’re a million times smaller than normal, at the single-cell level, or a million times smaller than that when you’d be atom-sized.”

“Then what comes?”

“Mmm… one more factor of a thousand would get you down to about the size of an atomic nucleus, but below that things get real fuzzy. It’s hard to get experimental data in the sub-nuclear size range because any photon with a wavelength that short is essentially an extremely-high-energy gamma ray, better at blowing nuclei apart than measuring them. Theory says you’d encounter nuclei as roiling balls of protons and neutrons, but each of those is a trio of quarks which may or may not be composed of even smaller things.”

“Is that the end of small?”

“Maybe not. Some physicists think space is quantized at scales near 10—35 meter. If they’re wrong then there’s no end.”

“Quantized?”

Quantized means something is measured out in whole numbers. Electric charge is quantized, for instance, because you can have one electron, two electrons, and so on, but you can’t have 1½ electrons. Some physicists think it’s possible that space itself is quantized. The basic idea is to somehow label each point in space with its own set of whole numbers.  There’d be no vacant space between points, just like there’s no whole number between two adjacent whole numbers.”

“So how small can I get?”

“Darned if I know.”

~~ Rich Olcott

Thanks to Jerry Mirelli for his thoughts that inspired this post and the next.

Three Shades of Dark

The guy’s got class, I’ll give him that. Astronomer-in-training Jim and Physicist-in-training Newt met his challenges so Change-me Charlie amiably updates his sign.

But he’s not done. “If dark matter’s a thing, how’s it different from dark energy? Mass and energy are the same thing, right, so dark energy’s gotta be just another kind of dark matter. Maybe dark energy’s what happens when real matter that fell into a black hole gets squeezed so hard its energy turns inside out.”

Jim and Newt just look at each other. Even Cap’n Mike’s boggled. Someone has to start somewhere so I speak up. “You’re comparing apples, cabbages and fruitcake. Yeah, all three are food except maybe for fruitcake, but they’re grossly different. Same thing for black holes, dark matter and dark energy — we can’t see any of them directly but they’re grossly different.”

EHT's image of the black hole at the center of the Messier 87 galaxy
Black hole and accretion disk, image by the Event Horizon Telescope Collaboration

Vinnie’s been listening off to one side but black holes are one of his hobbies. “A black hole’s dark ’cause its singularity’s buried inside its event horizon. Whatever’s outside and somehow gets past the horizon is doomed to fall towards the singularity inside. The singularity itself might be burn-your-eyes bright but who knows, ’cause the photons’re trapped. The accretion disk is really the only lit-up thing showing in that new EHT picture. The black in the middle is the shadow of the horizon, not the hole.”

Jim picks up the tale. “Dark matter’s dark because it doesn’t care about electromagnetism and vice-versa. Light’s an electromagnetic wave — it starts when a charged particle wobbles and it finishes by wobbling another charged particle. Normal matter’s all charged particles — negative electrons and positive nuclei — so normal matter and light have a lot to say to each other. Dark matter, whatever it is, doesn’t have electrical charges so it doesn’t do light at all.”

“Couldn’t a black hole have dark matter in it?”

“From what little we know about dark matter or the inside of a black hole, I see no reason it couldn’t.”

“How about normal matter falls in and the squeezing cooks it, mashes the pluses and minuses together and that’s what makes dark matter?”

“Great idea with a few things wrong with it. The dark matter we’ve found mostly exists in enormous spherical shells surrounding normal-matter galaxies. Your compressed dark matter is in the wrong place. It can’t escape from the black hole’s gravity field, much less get all the way out to those shells. Even if it did escape, decompression would let it revert to normal matter. Besides, we know from element abundance data that there can’t ever have been enough normal matter in the Universe to account for all the dark matter.”

Newt’s been waiting for a chance to cut in. “Dark energy’s dark, too, but it works in the opposite direction from the other two. Gravity from normal matter, black holes or otherwise, pulls things together. So does gravity from dark matter which is how we even learned that it exists. Dark energy’s negative pressure pulls things apart.”

“Could dark energy pull apart a black hole or dark matter?”

Big Cap’n Mike barges in. “Depends on if dark matter’s particles. Particles are localized and if they’re small enough they do quantum stuff. If that’s what dark matter is, dark energy can move the particles apart. My theory is dark matter’s just ripples across large volumes of space so dark energy can change how dark matter’s spread around but it can’t break it into pieces.”

Vinnie stands up for his hobby. “Dark energy can move black holes around, heck it moves galaxies, but like Sy showed us with Old Reliable it’s way too weak to break up black holes. They’re here for the duration.”

Newt pops him one. “The duration of what?”

“Like, forever.”

“Sorry, Hawking showed that black holes evaporate. Really slowly and the big ones slower than the little ones and the temperature of the Universe has to cool down a bit more before that starts to get significant, but not even the black holes are forever.”

“How long we got?”

“Something like 10106 years.”

“That won’t be dark energy’s fault, though.”

~~ Rich Olcott

Dark Shadows

Change-me Charlie’s still badgering Astronomer-in-training Jim and Physicist-in-training Newt about “Dark Stuff,” though he’s switched his target from dark matter to dark energy. “OK, the expansion of the Universe is speeding up. How does dark energy do that?”

Jim steps up to bat. “At this point dark energy’s just a name. We frankly have no idea what the name represents, although it seems appropriate.”

“Why’s that?”

“Gravity pulls things together, right, and we have evidence that galaxies are flying away from each other. When you pick something up your muscles give it gravitational potential energy that becomes kinetic energy when you let go and it drops. In space, a galaxy moving away from its neighbors gains gravitational potential energy relative to them. If the Energy Conservation Law holds, that energy has to come from somewhere. ‘Dark energy’ is what we call the somewhere, but naming something and understanding it are two different things.”

Newt chips in. “Einstein came at it from a different direction. His General Relativity field equations contained two numbers for observation to fill in — G, Newton’s gravitational constant, and lambda (Λ), which we now call the Cosmological Constant. Lambda measures the energy density of empty space. The equations say the balance between lambda and gravity controls whether the Universe expands, contracts or stays static. Lambda‘s just a little bit positive so the universe is expanding.”

“Same conclusion, different name. Neither one says where the energy comes from.”

That’s my cue. “True, but Einstein’s work goes deeper. Newtonian physics maps the Universe onto a stable grid of straight lines. In General Relativity those lines are deformed and twisted under the influence of massive objects. Vinnie and I talked about how gravity’s a fictitious force arising from that deformation. Like John Wheeler said, ‘Mass tells space-time how to curve, and space-time tells mass how to move.’ Anyway, when you throw dark energy’s lambda into the mix, the grid lines themselves go into motion. Dark energy torques the spacetime fabric that pulls galaxies together.”

“So dark energy pulls things apart by spreading out the grid they’re built on? If that’s so how come I’m still in one piece?”

“Nothing personal, but you’re too small and dense to notice. So am I, so is the Earth.”

“Why should that make a difference?”

“Time for a thought experiment. Think of the Sun. The atoms inside its surface are trying to get out, right? What’s holding them in?”

“The Sun’s gravity.”

“Just like pressure on the skin of a balloon. In either case, as long as things are stable the pressure on an enclosing real or mathematical surface rises and falls with the amount of enclosed energy density and it doesn’t matter which we talk about. Energy density’s easier to think about. With me so far?”

“I guess.”

“Let’s run a few horseback numbers on Old Reliable here. Start with protons and neutrons trying to leave an atomic nucleus. Here’s the total binding energy of an iron-56 nucleus divided by its volume…”

“… so the nuclear particles would fly apart except for the inward pressure exerted by the nuclear forces. Now we’ll go up a level and consider electrons trying to leave a helium atom. They’re held in by the electromagnetic force…”

“Still a lot of inward pressure but less than nuclear by fifty-five powers of ten. Gravity next. That’s what keeps us from flying off into space. I’ll use Earth’s escape velocity to cheat-quantify it…”

“Ten billion times weaker than the electromagnetism that holds our atoms and molecules together. Dark energy’s mass density is estimated to be about 10-27 kilograms per cubic meter. I’ll use that and Einstein’s E=mc2to calculate its pull-us-apart pressure.”

“A quintillion times weaker still.”

“So what you’re saying is, dark energy tries to pull everything apart by stretching out that spacetime grid, but it’s too weak to actually do anything to stuff that’s held together by gravity, electromagnetism or the two nuclear forces.”

“Mostly. Nuclear forces are short-range so distance doesn’t matter. Gravity and electromagnetism get weaker with the square of the distance. Dark energy only gets competitive working on objects that are separated much further than even neighboring galaxies. You’re not gonna get pulled apart.”

~~ Rich Olcott

Dark Horizon

Charlie's table sign says "Dark Energy is bogus"

Change-me Charlie attacks his sign with a rag and a marker, rubbing out “Matter” and writing in “Energy.” Turns out his sign is a roll-up dry-erase display and he can update it on site. Cool. I guess with his rotating-topic strategy he needs that. “OK, maybe dark matter’s a thing, but dark energy ain’t. No evidence, someone just made that one up to get famous!”

And of course Physicist-in-training Newt comes back at him. “Lots of evidence. You know about the Universe expanding?”

“Prove it.” At least he’s consistent.

<sigh> “You know how no two snowflakes are exactly alike but they can come close? It applies to stars, too. Stars are fairly simple in a complicated way. If you tell me a star’s mass, age and how much iron it has, I can do a pretty good job of computing how bright it is, how hot it is, its past and future life history, all sort of things. As many stars as there are, we’re pretty much guaranteed that there’s a bunch of them with very similar fundamentals.”

“So?”

“So when a star undergoes a major change like becoming a white dwarf or a neutron star or switching from hydrogen fusion to burning something else, any other star that has the same fundamentals will behave pretty much the same way. They’d all flare with about the same luminosity, pulsate with about the same frequency —”

“Wait. Pulsate?”

“Yeah. You’ve seen campfires where one bit of flame coming out of a hotspot flares up and dies back and flares up and dies back and you get this pulsation —”

“Yeah. I figured that happens with a sappy log where the heat gasifies a little sap then the spot cools off when outside air gets pulled in then the cycle goes again.”

“That could be how it works, depending. Anyhow, a star in the verge of mode change can go through the same kind of process — burn one kind of atom in the core until heat expansion pushes fuel up out of the fusion zone; that cools things down until fuel floods back in and off we go again. The point is, that kind of behavior isn’t unique to a single star. We’ve known about variable stars for two centuries, but it wasn’t until 1908 that Henrietta Swan Leavitt told us how to determine a particular kind of variable star’s luminosity from its pulsation frequency.”

“Who cares?”

“Edwin Hubble cared. Brightness dies off with the distance squared. If you compare the star’s intrinsic luminosity with how bright the star appears here on Earth, it’s simple to calculate how far away the star is. Hubble did that for a couple dozen galaxies and showed they had to be far outside the Milky Way. He plotted red-shift velocity data against those distances and found that the farther away a galaxy is from us, the faster it’s flying away even further.”

“A couple dozen galaxies ain’t much.”

“That was for starters. Since the 1930s we’ve built a whole series of ‘standard candles,’ different kinds of objects whose luminosities we can convert to distances out to 400 million lightyears. They all agree that the Universe is expanding.”

“Well, you gotta expect that, everything going ballistic from the Big Bang.”

“They don’t go the steady speed you’re thinking. As we got better at making really long-distance measurements, we learned that the expansion is accelerating.”

“Wait. I remember my high-school physics. If there’s an acceleration, there’s gotta be a force pushing it. Especially if it’s fighting the force of gravity.”

“Well there you go. Energy is force times distance and you’ve just identified dark energy. But standard candles aren’t the only kind of evidence.”

“There’s more?”

“Sure — ‘standard sirens‘ and ‘standard rulers.’ The sirens are events that generate gravitational waves we pick up with LIGO facilities. The shape and amplitude of the LIGO signals tell us how far away the source was — and that information is completely immune to electromagnetic distortions.”

“And the rulers?”

“They’re objects, like spiral galaxies and intergalactic voids, that we have independent methods for connecting apparent size to distance.”

“And the candles and rulers and sirens all agree that acceleration and dark energy are real?”

“Yessir.”

~~ Rich Olcott

Dancing in The Dark

Change-me Charlie at his argument table

The impromptu seminar at Change-me Charlie’s “Change My Mind” table is still going strong, but it looks like Physicist-in-training Newt and Astronomer-in-training Jim have met his challenge. He’s switched from arguing that dark matter doesn’t exist to asking how it worked in the Bullet Cluster’s massive collision between two collections of galaxies with their clouds of plasma and dark matter. “OK, the individual galaxies are so spread out they slide past each other without slowing down but the plasma clouds obstruct each other by friction. Wouldn’t friction in the dark matter hold things back, too?”

Jim’s still standing in front of the table. “Now that’s an interesting question, so interesting that research groups have burned a bazillion computer cycles trying to answer it.”

“Interesting, yes, but that interesting?”

“For sure. What we know about dark matter is mostly what it doesn’t do. It doesn’t give off light, it doesn’t absorb light, it doesn’t seem to participate in the strong or weak nuclear forces or interact with normal matter by any means other than gravity, and no identifiable dark matter particles have been detected by bleeding-edge experiments like IceCube and the Large Hadron Collider. So people wonder, does dark matter even interact with itself? If we could answer that question one way or the other, that ought to tell us something about what dark matter is.”

“How’re we gonna do that?”

Newt’s still perched on Charlie’s oppo chair. “By using computers and every theory tool on the shelf to run what-if? simulations. From what we can tell, nearly everywhere in the Universe normal matter is embedded in a shell of dark matter. The Bullet Cluster and a few other objects out there appear to break that rule and give us a wonderful check on the theory work.”

The Bullet Cluster, 1E 0657-56 (NASA image)

“Like for instance.”

“Simple case. What would the collision would looked like if dark matter wasn’t involved? Some researchers built a simulation program and loaded it with a million pretend plasma particles in two cluster-sized regions moving towards each other from 13 million pretend lightyears apart. They also loaded in position and momentum data for the other stars and galaxies shown in the NASA image. The simulation tracked them all as pretend-time marched along stepwise. At each time-step the program applied known or assumed laws of physics to compute every object’s new pretend position and momentum since the prior step. Whenever two pretend-particles entered the same small region of pretend-space, the program calculated a pretend probability for their collision. The program’s output video marked each successful collision with a pink pixel so pinkness means proton-electron plasma. Here’s the video for this simulation.”

“Doesn’t look much like the NASA picture. The gas just spreads out, no arc or cone to the sides.”

“Sure not, which rules out virtually all models that don’t include dark matter. So now the team went to a more complicated model. They added a million dark matter particles that they positioned to match the observed excess gravity distribution. Those’re marked with blue pixels in the videos. Dark matter particles in the model were allowed to scatter each other, too, under control of a self-interaction parameter. The researchers ran the simulations with a whole range of parameter values, from no-friction zero up to about twice what other studies have estimated. Here’s the too-much case.”

“Things hold together better with all that additional gravity, but it’s not a good match either.”

“Right, and here’s the other end of the range — no friction between dark matter particles. Robertson, the video’s author/director, paused the simulation in the middle to insert NASA’s original image so we could compare.”

“Now we’re getting somewhere.”

“It’s not a perfect match. Here’s an image I created by subtracting a just-after-impact simulation frame from the NASA image, then amplifying the red. There’s too much left-over plasma at the outskirts, suggesting that maybe no-friction overstates the case and maybe dark matter particles interact, very slightly, beyond what a pure-gravity theory predicts.”

“Wait, if the particles don’t use gravity, electromagnetism or the nuclear forces on each other, maybe there’s a fifth force!”

“New Physics!”

A roar from Cap’n Mike — “Or they’re not particles!”

~~ Rich Olcott

Dark Passage

Change-me Charlie’s not giving up easily. “You said that NASA picture did three things, but you only told us two of them — that dark matter’s a thing and that it’s separate from normal matter. What’s the third thing? What exactly is in that picture? Does it tell us what dark matter is?”

The Bullet Cluster ( 1E 0657-56 )

Physicist-in-training Newt’s ready for him. “Not much of a clue about what dark matter is, but a good clue about how it behaves. As to what’s in the picture, we need some background information first.”

“Go ahead, it’s not dinner-time yet.”

“First, this isn’t two stars colliding. It’s not even two galaxies. It’s two clusters of galaxies, about 40 all together. The big one on the left probably has the mass of a couple quintillion Suns, the small one about 10% of that.”

“That’s a lot of stars.”

“Oh, most of it’s definitely not stars. Maybe only 1-2%. Those stars and the galaxies they form are embedded in ginormous clouds of proton-electron plasma that make up 5-20% of the mass. The rest is that dark matter you don’t like.”

“Quadrillions of stars are gonna make a super-super-nova when they collide!”

“Well, no. That doesn’t even happen when two galaxies collide. The average distance between neighboring stars in a galaxy is 200-300 times the diameter of a star so it’s unlikely that any two of them will come even close. Next level up, the average distance between galaxies in a cluster is about 60 galaxy diameters or more, depending. The galaxies will mostly just slide past each other. The real colliders are the spread-out stuff — the plasma clouds and of course the dark matter, whatever that is.”

Astronomer-in-training Jim cuts in. “Anyway, the collision has already happened. The light from this configuration took 3.7 billion years to reach us. The collision itself was longer ago than that because the bullet’s already passed through the big guy. From that scale-bar in the bottom corner I’d say the centers are about 2 parsecs apart. If I recall right, their relative velocity is about 3000 kilometers per second so…” <poking at his smartphone> “…the peak intersection was about 700 million years earlier than that. Call it 4.3 billion years ago.”

“So what’s with the cotton candy?”

Newt looks puzzled. “Cotton… oh, the pink pixels. They’re markers for where NASA’s Chandra telescope saw X-rays coming from.”

“What can make X-rays so far from star radiation that could set them going?”

“The electrons do it themselves. An electron emits radiation every time it collides with another charged particle and changes direction. When two plasma clouds interpenetrate you get twice as many particles per unit volume and four times the collision rate so the radiation intensity quadruples. There’s always some X-radiation in the plasma because the temperature in there is about 8400 K and particle collisions are really violent. The Chandra signal pink shows the excess over background.”

“The blue in the Jim’s picture is supposed to be what, extra gravity?”

“Basically, yeah. It’s not easy to see from the figure, but there are systematic distortions in the images of the background galaxies in the blue areas. Disks and ellipsoids appear to be bent, depending on where they sit relative to the clusters’ centers of mass. The researchers used Einstein’s equations and lots of computer time to work back from the distortions to the lensing mass distributions.”

“So what we’ve got is a mostly-not-from-stars gravity lump to the left, another one to the right, and a big cloud in the middle with high-density hot bits on its two sides. Something in the middle blew up and spread gas around mostly in the direction of those two clusters. What’s that tell us?”

“Sorry, that’s not what happened. If there’d been a central explosion the excess to the right would be arc-shaped, not a cone like you see. No, this really is the record of one galaxy cluster bursting through another one. Particle-particle friction within the plasma clouds held them back while the embedded galaxies and dark matter moved on.”

“OK, the galaxies aren’t close-set enough for them to slow each other down, but wouldn’t friction in the dark matter hold things back, too?”

“Now that’s an interesting question…”

~~ Rich Olcott

The Prints of Darkness

There’s a commotion in front of Al’s coffee shop. Perennial antiestablishmentarian Change-me Charlie’s set up his argument table there and this time the ‘establishment’ he’s taking on is Astrophysics. Charlie’s an accomplished chain-yanker and he’s working it hard. “There’s no evidence for dark matter, they’ve never found any of the stuff and there’s tons of no-dark-matter theories to explain the evidence.”

Big Cap’n Mike’s shouts from the back of the crowd. “What they’ve been looking for and haven’t found is particles. By my theory dark matter’s an aspect of gravity which ain’t particles so there’s no particles for them to find.”

Astronomer-in-training Jim spouts off right in Charlie’s face. “Dude, you can’t have it both ways. Either there’s no evidence to theorize about, or there’s evidence.”

Physicist-in-training Newt Barnes takes the oppo chair. “So what exactly are we talking about here?”

“That’s the thing, guy, no-one knows. It’s like that song, ‘Last night I saw upon the stair / A little man who wasn’t there. / He wasn’t there again today. / Oh how I wish he’d go away.‘ It’s just buzzwords about a bogosity. Nothin’ there.”

I gotta have my joke. “Oh, it’s past nothing, it’s a negative.”

“Come again?”

“The Universe is loaded with large rotating but stable structures — solar systems, stellar binaries, globular star clusters, galaxies, galaxy clusters, whatever. Newton’s Law of Gravity accounts nicely for the stability of the smallest ones. Their angular momentum would send them flying apart if it weren’t for the gravitational attraction between each component and the mass of the rest. Things as big as galaxies and galaxy clusters are another matter. You can calculate from its spin rate how much mass a galaxy must have in order to keep an outlying star from flying away. Subtract that from the observed mass of stars and gas. You get a negative number. Something like five times more negative than the mass you can account for.”

“Negative mass?”

“Uh-uh, missing positive mass to combine with the observed mass to account for the gravitational attraction holding the structure together. Zwicky and Rubin gave us the initial object-tracking evidence but many other astronomers have added to that particular stack since then. According to the equations, the unobserved mass seems to form a spherical shell surrounding a galaxy.”

“How about black holes and rogue planets?”

Newt’s thing is cosmology so he catches that one. “No dice. The current relative amounts of hydrogen, helium and photons say that the total amount of normal matter (including black holes) in the Universe is nowhere near enough to make up the difference.”

“So maybe Newton’s Law of Gravity doesn’t work when you get to big distances.”

“Biggest distance we’ve got is the edge of the observable Universe. Jim, show him that chart of the angular power distribution in the Planck satellite data for the Cosmological Microwave Background.” <Jim pulls out his smart-phone, pulls up an image.> “See the circled peak? If there were no dark matter that peak would be a valley.”

Charlie’s beginning to wilt a little. “Ahh, that’s all theory.”

The Bullet Cluster ( 1E 0657-56 )

<Jim pulls up another picture.> “Nope, we’ve got several kinds of direct evidence now. The most famous one is this image of the Bullet Cluster, actually two clusters caught in the act of colliding head-on. High-energy particle-particle collisions emit X-rays that NASA’s Chandra satellite picked up. That’s marked in pink. But on either side of the pink you have these blue-marked regions where images of further-away galaxies are stretched and twisted. We’ve known for a century how mass bends light so we can figure from the distortions how much lensing mass there is and where it is. This picture does three things — it confirms the existence of invisible mass by demonstrating its effect, and it shows that invisible mass and visible mass are separate phenomena. I’ve got no pictures but I just read a paper about two galaxies that don’t seem to be associated with dark matter at all. They rotate just as Newton would’ve expected from their visible mass alone. No surprise, they’re also a lot less dense without that five-fold greater mass squeezing them in.”

“You said three.”

“Gotcha hooked, huh?

~~ Rich Olcott

Fierce Roaring Beast

A darkish day calls for a fresh scone so I head for Al’s coffee shop. Cathleen’s there with some of her Astronomy students. Al’s at their table instead of his usual place behind the cash register. “So what’s going on with these FRBs?”

She plays it cool. “Which FRBs, Al? Fixed Rate Bonds? Failure Review Boards? Flexible Reed Baskets?”

Jim, next to her, joins in. “Feedback Reverb Buffers? Forged Razor Blades?
Fennel Root Beer?”

I give it a shot. “Freely Rolling Boulders? Flashing Rapiers and Broadswords? Fragile Reality Boundary?”

“C’mon, guys. Fast Radio Bursts. Somebody said they’re the hottest thing in Astronomy.”

Cathleen, ever the teacher, gives in. “Well, they’re right, Al. We’ve only known about them since 2007 and they’re among the most mystifying objects we’ve found out there. Apparently they’re scattered randomly in galaxies all over the sky. They release immense amounts of energy in incredibly short periods of time.”

“I’ll say.” Vinnie’s joins the conversation from the next table. “Sy and me, we been talking about using the speed of light to measure stuff. When I read that those radio blasts from somewhere last just a millisecond or so, I thought, ‘Whatever makes that blast happen, the signal to keep it going can’t travel above lightspeed. From one side to the other must be closer than light can travel in a millisecond. That’s only 186 miles. We got asteroids bigger than that!'”

“300 kilometers in metric.” Jim’s back in. “I’ve played with that idea, too. The 70 FRBs reported so far all lasted about a millisecond within a factor of 3 either way — maybe that’s telling us something. The fastest way to get lots of energy is a matter-antimatter annihilation that completely converts mass to energy by E=mc².  Antimatter’s awfully rare 13 billion years after the Big Bang, but suppose there’s still a half-kilogram pebble out there a couple galaxies away and it hits a hunk of normal matter. The annihilation destroys a full kilogram; the energy release is 1017 joules. If the event takes one millisecond that’s 1020 watts of power.”

“How’s that stand up against the power we receive in an FRB signal, Jim?”

“That’s the thing, Sy, we don’t have a good handle on distances. We know how much power our antennas picked up, but power reception drops as the square of the source distance and we don’t know how far away these things are. If your distance estimate is off by a factor of 10 your estimate of emitted power is wrong by a factor of 100.”

“Ballpark us.”

<sigh> “For a conservative estimate, say that next-nearest-neighbor galaxy is something like 1021 kilometers away. When the signal finally hits us those watts have been spread over a 1021-kilometer sphere. Its area is something like 1049 square meters so the signal’s power density would be around 10-29 watts per square meter. I know what you’re going to ask, Cathleen. Assuming the radio-telescope observations used a one-gigahertz bandwidth, the 0.3-to-30-Jansky signals they’ve recorded are about a million million times stronger than my pebble can account for. Further-away collisions would give even smaller signals.”

Looking around at her students, “Good self-checking, Jim, but for the sake of argument, guys, what other evidence do we have to rule out Jim’s hypothesis? Greg?”

“Mmm… spectra? A collision like Jim described ought to shine all across the spectrum, from radio on up through gamma rays. But we don’t seem to get any of that.”

“Terry, if the object’s very far away wouldn’t its shorter wavelengths be red-shifted by the Hubble Flow?”

“Sure, but the furthest-away one we’ve tagged so far is nearer than z=0.2. Wavelengths have been stretched by 20% or less. Blue light would shift down to green or yellow at most.”

“Fran?”

“We ought to get even bigger flashes from antimatter rocks and asteroids. But all the signals have about the same strength within a factor of 100.”

“I got an evidence.”

“Yes, Vinnie?”

“That collision wouldn’t’a had a chance to get started. First contact, blooie! the gases and radiation and stuff push the rest of the pieces apart and kill the yield. That’s one of the problems the A-bomb guys had to solve.”

Al’s been eaves-dropping, of course. “Hey, guys. Fresh Raisin Bread, on the house.”

~~ Rich Olcott

Friendly Resting Behemoths