(Not So) Absurd Search For Dark Matter

Here is a transcript and some images from the excellent video: https://www.youtube.com/watch?v=6etTERFUlUI (in case you want to save electricity or simply prefer reading over watching).

“I am at a gold mine a couple hours outside of Melbourne,because, one kilometer underground, they’re putting in a detector to look for dark matter. Let’s go. It’s gonna take 30 minutes to go down a kilometer underground. Dark matter is thought to make up 85% of all the matter in existence.

It could form a shadow universe five times more massive than everything we can see.Over the past several decades, over 50 experiments have tried to make a direct detection of dark matter, but none of them has found anything, except one.

Under a mountain in the Italian Alps, there is a dark matter detector called DAMA/LIBRA.

It’s been collecting data for around 20 years and, every year, it sees the same peculiar results. The rate of detections increases to a peak in June and then decreases to a minimum in November. Some scientists think this could be the first direct evidence of dark matter. But why would dark matter create a periodic annual signal?

Well, this is our galaxy, or at least what it looks like with visible light. Astronomers suspect it is surrounded and permeated by a huge sphere of dark matter, invisible particles that are zipping around all in random directions. According to most theories, dark matter doesn’t interact with anything including itself, except through gravity. We think there should be five times as much dark matter as there is ordinary matter.

Now, our solar system is moving around the galaxy at 220 kilometers per second. That means we’re also moving through dark matter at this rate, except earth orbits the sun at 30 kilometers a second. So, for half the year, we’re moving with the sun, going faster through dark matter. And, the other half the year, we’re moving in the opposite direction,so going slower through dark matter. And, the idea is we encounter more dark matter when we’re moving through it fastest, which happens to be in June, and less of it when we’re moving slowest, which happens in November.

The actual geometry is a little more complicated. The solar system is tilted at 60 degrees relative to the plane of the galaxy, but the idea still works. So, the signal observed at DAMA/LIBRA may be due to this motion through dark matter or it might not be due to dark matter at all. It could just be something mundane, like the temperature, humidity, moisture in the soil, the snow on the mountain, or the number of tourists in Italy. All of these things fluctuate with a period of one year.

And, that is why they’re gonna build an almost identical experiment in the Southern hemisphere, down the bottom of this gold mine outside of Melbourne, because, there, the seasons are reversed, but our motion through dark matter is still the same. So, if we see the same signal, it’s pretty strong evidence for the existence of dark matter.

One of the big problems that DAMA/LIBRA has is that there are other very similar experiments that don’t see anything. (In May 2021, the ANAIS particle detector failed to replicate the results of the DAMA experiments after 3 years of data collection[19] and in November new results from COSINE-100 experiment after 1.7 years of data collection also failed to replicate the signal of DAMA.[20][21])

And, this has led to a lot of uncertainty about is the DAMA/LIBRA signal really dark matter?So, yeah, we don’t know, right? The favorite thing in science…

The Evidence for Dark Matter

But, why do we think dark matter exists in the first place?

1. In 1933, Swiss astronomer, Fritz Zwicky, was studying the coma cluster, a collection of more than a thousand galaxies. These galaxies are gravitationally bound together.

So, they all orbit around their collective center of mass. Zwicky measured the orbital speeds of these galaxies and found that somewhere moving way faster than he expected, it was as if there was a lot more matter in the cluster than he could see,

pulling everything inwards. So, he proposed the existence of invisible matter,

which he called dunkle materie, the origin of the term “dark matter”. No one really took this idea seriously, but 40 years later, dark matter turned up again.

2. Vera Ruben and Kent Ford observed the motion of stars in the Andromeda Galaxy.

And, they expected that the further out from the center you go, the slower the stars would be orbiting, but this is not what they found. The rotational velocity stays almost constant with increasing distance from the center. Without additional mass in the galaxy to pull those stars in, they should be flung off into space. The result was the same in other galaxies.

3. Using radio telescopes, Albert Bosma and others measured hydrogen gas even further out from a galaxy center, but the rotational velocity still stayed constant. One way to explain this is to posit the existence of matter we can’t see, dark matter, which holds all these galaxies together.

(Demonstration with hanging weights from a string at the center, in turn attached to an orbiting mass ) So, let’s say you have a star and this represents the mass of everything in the center of the galaxy that’s pulling the star in. The star can maintain a stable orbit if its centripetal force is equal to the gravitational attraction to all the mass in the rest of the galaxy. And, so you can see that at about a distance of one meter,

this is the speed of the orbit. Okay, but what happens if we add some dark matter?

So, this water bottle represents the matter we can’t see, now there is more mass pulling this star into the middle, which means, at the same orbit, it can now go much faster, and, in fact, it must go faster to maintain that orbit. And, this explains the observation. This is what we see.

By looking at the rotation speeds of stars, scientists estimate that about 85% of the mass of a galaxy is dark matter. But, there’s another way to explain these observations without invoking dark matter. And, that is to modify our theory of gravity. What’s the supporting evidence for thinking that the particle idea is totally misguided and we should actually be looking at a revised theory of gravity?

“We can either invoke something we can’t see or you just say, well, the universe is what we can see, and we need a way to explain what’s going on out there. And, the only way we can do that is by modifying the laws of physics. So, when you look at the outskirts of galaxies, they’ve got a lot of centripetal acceleration. Dark matter says that well, that centripetal acceleration is due to the gravitational effect of dark matter.

Whereas, the people who like MOND will say, no, that’s centripetal acceleration,

that’s just the fact that it’s now reached this floor and can’t get any lower. (Created in 1982 and first published in 1983 by Israeli physicist Mordehai Milgrom, the hypothesis’ original motivation was to explain why the velocities of stars in galaxies were observed to be larger than expected based on Newtonian mechanics. Milgrom noted that this discrepancy could be resolved if the gravitational force experienced by a star in the outer regions of a galaxy was proportional to the square of its centripetal acceleration (as opposed to the centripetal acceleration itself, as in Newton’s second law) or alternatively, if gravitational force came to vary inversely linearly with radius (as opposed to the inverse square of the radius, as in Newton’s law of gravity). In MOND, violation of Newton’s laws occurs at extremely small accelerations, characteristic of galaxies yet far below anything typically encountered in the Solar System or on Earth.)

So, they, they’re saying that there’s not additional force due to dark matter, but there’s a limit to how low the acceleration could go. I think the consensus is hugely in favor of it being a physical substance, in that it just seems reasonable that is that it could be other particles out there that we haven’t seen yet.

4. And, there’s more evidence. This is the Bullet Cluster, a site where two clusters of galaxies collided. Most of the ordinary mass of these clusters is in the interstellar gas.

And, when the collision occurred, the interstellar gas interacted, heated up, and slowed down. So, you’d expect that most of the mass of the bullet cluster would be in the middle where all of this gas is. But, if you use gravitational lensing, the way that gravity bends light, you can actually measure where most of the mass in this picture is. And, it isn’t in the middle. It’s actually on either side. So, the best way to explain this is that when the clusters collided, all that gas got stuck in the middle,but the dark matter passed right through, creating the most gravitational lensing

where we can see the least ordinary matter.

5. Even more evidence for dark matter comes from the oldest light in the universe.

380,000 years after the big bang, light could finally travel through the universe unimpeded. And, this is what we see as the cosmic microwave background or CMB. The red spots show where the early universe was a little hotter and the blue spots show where it was a little cooler, but these temperature differences were tiny. Just 0.01%, but they are there. And, you can turn this picture into a graph

by counting up how many blobs there are of different sizes.

So, there’s the most common size blob, which results in this peak, but there are also other common size blobs. And, so you get these other peaks of decreasing size. Now, the height of these peaks depends on how much dark matter there is. In a universe without dark matter, the graph looks like this, but as dark matter increases, the amplitudes of even numbered peaks decreases. To match the measurements of the CMB, we need about five times as much dark matter as ordinary matter. This figure also agrees with the amount of dark matter required to explain the motion of stars in galaxies and the motion of galaxies in clusters.

Looking For WIMPS

So, the dark matter hypothesis explains a lot of different observations with a simple theoretical framework, that there’s some type of particle out there that only interacts through gravity. But, what is this particle exactly? Well, since we don’t know, scientists have proposed a whole bunch of different things that it could be. And, now we have to try to go out and find them. The approach differs depending on what you’re trying to find. Dama Libra and the detector at the bottom of the gold mine are looking for WIMPs, weakly interacting massive particles. These particles are expected to weigh

about as much as a proton, but interact with ordinary matter extremely weakly.

At the heart of the detector, our seven seven-kilogram crystals of pure sodium iodide.

“So, that’s actually sodium iodide in there?”

Yeah.

“I didn’t expect it to be so clear.”

The idea is that very, very rarely a dark matter particle may hit a nucleus in the crystal and transfer its energy. This creates a flash of light called a scintillation, which is detected by photomultiplier tubes, very sensitive light detectors, which are positioned above and below each crystal. But, there’s a problem—even the purest sodium iodide crystal contains radioactive potassium. And, when a potassium atom decays, it emits an electron and a gamma ray. Now, the electron can cause a scintillation in the crystal just like the hypothesized dark matter particle. So, to eliminate these events, the sodium iodide crystals are submerged in a tank full of 12 tons of linear alkylbenzene.

This is a liquid scintillator that emits light when exposed to a gamma ray

and that light can then be detected by photo multiplier tubes in the tank. So, if there’s a simultaneous detection in the crystal and in the tank, it was most likely a potassium decay, not a dark matter event.

But there’s another problem, cosmic rays. Energetic particles from the sun and other galaxies hit the top of Earth’s atmosphere creating muons, essentially heavy electrons,

which stream toward the earth at close to the speed of light. Muons can also create flashes of light in the crystal. This is Muon detector, and it’s got these three paddles of plastic here separated by some pieces of steel. If we see a flash of light in all three,

basically the same time, then we know that Muon has passed through them.

So, if I hit reset, we can see, counting up the muons being seen. So, it’s at least a few a second. This is why all sensitive particle detectors are located deep underground…Here, we have the muon detector, now one kilometer underground,

and it’s been running for something like 15 minutes, and there have been no muon counts. Yeah, we have to leave this running for a long time, I think, even if we wanted to get a single hit. We expect the number of muons down here to be about a million less. And, we didn’t see a million at the top, so we’re probably not gonna see any down here. And, this is the whole point of putting a dark matter detector underground. You wanna get rid of all the background that would create noise in the detector. But, even this shielding is not enough. We all have muon detectors immediately above the tank. So, if a flash is seen in a crystal at the same time a muon is detected, it can be ruled out.

Being underground brings its own challenges. The walls of the mine contain trace amounts of radioactive elements like uranium and thorium, which decay into radon gas. The requirements here are fairly serious for dark matter experiments. We have to completely control the environment, in particular, the radon level. To counteract this, the walls of the cavern are coated with special paint to contain radioactive particles.

The crystals are immersed in a continuous stream of pure nitrogen gas. And, the entire detector is shielded by 120 tons of steel and plastic. “Wow, look at the size of that cavern. There is a lot riding on this experiment. It will validate or disprove one of the most contentious results in physics. So, if we see nothing, well, this is the death of DAMA/LIBRA but if we see something, well, we are all happy.”

“I actually like the idea that because 80% of the mass of the universe is dark matter or dark stuff, maybe there’s more than just one particle that dark matter is made of. It could be an entire dark standard model if you like. A dark version of everything that we can see or maybe something more complex, ’cause there’s so much more of it. Like I really hope it’s that.”

– Do you think that dark matter interacts with ordinary matter?

– If we want to find out what this stuff is, we better hope there’s some level of interaction that we can at least probe when it comes to doing experiments.

“If God gave me the great book of physics and there were two sections, section A and section B, one for the luminous matter and one for dark matter, and they didn’t talk to each other, I would say that was a very peculiar universe. But, in science, we have to live with the possibility that, at some level, we may never find the answer.

It may elude us, but at least we tried.”

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Pretend-Recycling Is Not the Answer. First Reduce. Then Repair and Reuse.

1. Despite all the Bins, We Are Still Wasteful

In most of the North American continent, we have pretend-recycling, given that about most of what we place in our green bins ends up as garbage. Over 90% of plastic isn’t actually recycled. U.S. exported 74,000 shipping containers of plastic waste to low-income countries, according to researcher Jan Dell. That’s a lot more fossil fuels burnt than if the recyclable material was handled locally.

Canada is not any better. From a 2022 CBC article:

An investigation by Radio-Canada’s Enquête shows that much of what is supposed to be paper actually contains tonnes of plastic bags, some of which litter the Indian landscape, and are often burned as a source of fuel.….The city’s(Montreal) two recycling centres — in Lachine and Saint-Michel — average between 20 and 26% contamination, according to numbers provided by the city.”

A cynic would point out that it’s not shocking that the program became adopted by the majority of cities, given that it does not threaten consumerism. Instead it gets people to focus away from reducing their consumption and does not encourage them to find a second use for packages and other materials that they have paid for.

2. Plastic is Not As Cheap As It Seems

Economics are blamed for low recycling rates when it’s pointed out that it’s cheaper to produce plastic from its raw materials than to collect existing plastics, sort them out, use solvents and remold them into new ones. But that’s because as a society, the price of “virgin” plastic does not factor in the environmental impact of producing excessive plastic, and it does not factor in the cost of recycling . Conveniently for producers and packagers of plastic, the responsibility to recycle and cleanup has become that of the government. Cities then handicap recycling plants by unwisely accepting unsorted recyclable materials out of fear that no one would bother otherwise.

3. Greenwashing

Grocery chains and dollar stores do their share of greenwashing by either not offering plastic bags to put groceries in or by charging customers for them, but the sad truth is that stores still put out s multitude of rolls of plastic weekly to carry away fruits and vegetables, which are totally unnecessary, and they get consumers to take home tons of other plastic in the form of jars, dispensers for detergents, pasta, meat and legume-packaging.

4. A Better Approach

Things can be reused without making it look like you are bordering on poverty, a kind of sin for a society obsessed with discarding old materials and buying more. Here is an example of how my garden looks relatively neat and tidy, even though it is full of reused materials.

This morning after finishing a container of detergent, I decided to convert it into a watering can. The procedure is very simple. Just remove the labels; add a little drawing and drill a few holes in the cap.

After starting this blog yesterday, coincidentally, I learned of a generous grant given to a faculty of a prominent North American university. With hundreds of millions of dollars to spend on demolishing the “old” structure and building a new one that would hold all of the undergraduate labs, some members of the faculty wanted to throw everything out. Luckily, the lab coordinator decided that this would be a terrible waste, and that it made more sense to give away the glassware and lab instruments to other labs within its own university and then to other institutions if there were still leftovers.

We live in a neighborhood where an incredible amount of good quality stuff is put out as garbage. Luckily, as others became aware of their bad habits, they started to patrol the neighborhood to collect the toys, skis, deck furniture and even broken appliances, which can either be repaired or be sold for their metal content.

Things don’t magically appear at the store. Material and energy resources go into making consumer goods, and as the Earth hosts an increasing number of buyers, if we acted rationally, waste would decrease. We would each buy less so that more people could have a little more. We would hang on to what we already have, repairing whatever is broken and finding new uses for purchased materials.