Dark Matter – Scienceandsf -A Blog Published by Robert A. Lawler

For Decades Astronomers and Physicists have been thinking that Dark Matter is made up some kind of exotic sub-atomic particle. Maybe they’re wrong, maybe Dark Matter is made up of ordinary matter but in objects that don’t shine.

Let me begin today but reminding everyone of the problem of Dark Matter. Over the last 70-80 years as astronomers studied the dynamic behavior of the galaxies they found that the gravity of the objects that they could see, i.e. the stars that shined, was not sufficient to account for the way galaxies moved. There had to be some form of missing mass, some kind of dark matter in galaxies in order to explain their dynamics.

The Rotation speeds of stars in the spiral arms of galaxies, top curve, do not fit the expected speeds based on the matter that we can see, bottom curve. Dark Matter is the generic term for whatever was causing the difference. (Credit: Wikipedia)

Back in the 1980s when I was an undergraduate the ideas about Dark Matter had basically coalesced into two types of matter. These two classes of matter were given the corny names of Machos, meaning Mass Concentrations, and WIMPS meaning Weakly Interacting Massive Particles. Mass Concentrations were thought to be composed of ordinary particles like protons, neutrons and electrons and could be anything from small black holes to dark stars, given the name brown dwarfs, or even smaller objects like planets.

Too small to be a star yet too big to be a planet the question is, just how many Brown Dwarfs are there in the spaces between the stars? (Credit: Space News)

Now astronomers didn’t like the idea of having to look between the stars for small objects that didn’t shine by their own light, so they didn’t like Machos. Let’s face it telescopes are the main tool of astronomers and telescopes gather light from objects that shine like stars.

Telescopes gather a large amount of light, more than our eyes do, as well as magnifying an image. But objects that don’t emit or reflect light can’t be seen in a telescope no matter how powerful it is! (Credit: Meade)

On the other hand physicists loved the idea of WIMPs because at the time they were coming up theories of ‘Supersymmetry’ that predicted the existence of a large number of massive particles some of which could be WIMPs. So starting in the 1990s Machos were largely ignored while everybody went looking for WIMPs either in outer space or at the big atom smashers like the Tevatron at Fermilab or the new Large Hadron Collider at CERN.

Physicists are searching for ‘Physics beyond the Standard Model’. Some of that physics could be Dark Matter. (Credit: Phys.org)

Problem is that after thirty years of searching no particles that could be WIMPs have been found. And now it seems that the wind has shifted and maybe it’s time to take another look at Machos.

The James Webb Space Telescope is designed to observe the Universe in the infrared portion of the EM spectrum. This will enable it to search for both Brown Dwarfs and Rogue Planets helping astronomers get a better idea of their numbers. (Credit: Space.com)

For one thing astronomers have new, bigger, better instruments that are more capable of looking for objects that don’t shine at visible wavelengths. Just a few months ago I published a post about how astronomers are beginning to discover large numbers of Brown Dwarf stars, objects too big to be called planets but too small to ignite nuclear fusion in their cores so they do not shine like a star. See my post of 22 September 2021.

Just a few of the observatory domes that make up the European Southern Observatory high in the mountains of Chile. (Credit: Physics World)

Now a new study from the European Southern Observatory in Chile and Bordeaux University has announced the discovery of as many as 170 rouge planets, that is planets that do not orbit any star but rather move through the Milky Way all on their own. The rogue planets were discovered in a star forming region of the galaxy relativity close to our solar system in the constellations of Scorpius and Ophiuchus.

As one of the zodiacal signs Scorpius is a well known constellation but nearby Ophiuchus is also a very interesting part of the sky. The stellar nursery where the Rogue Planets were discovered lays between these two constellations. (Credit: International Astronomical Union)

It was the fact that the rogue planets were very young, and therefore still warm that enabled the astronomers to find them in the infrared region of the electromagnetic spectrum. Even so the astronomers at Bordeaux had to shift through observations accumulated over 20 years and still aren’t certain how many rogue planets they’ve found, the best estimate being 70 to 170 Jupiter sized worlds.

How many Rogue Planets are out there? It’s difficult to say because, once they’ve cooled down from their formation they are nearly impossible to find! (Credit: The Verge)

Still if the star forming regions in space are also producing large numbers of solitary planets who knows how many older rogue planets there could be between the stars. Could there be as many rogue planets as there are stars? Or maybe even more? Finding out just how many rogue planets there really are could be a difficult task, remember once the planet cools down like our earth did after a few million years they’ll be almost impossible to find.

Actual image of a Rogue Planet that is so young that it is still warm enough to be visible. (Credit: Colorado College Sites)

And there’s one more candidate for a Macho because the possibility that there could be large numbers of small, ‘primordial’ black holes in the Universe is once again being seriously discussed. These would be black holes with a mass that of a planet that formed in the first seconds after the big bang and have just been floating around ever since then. Such black holes would also be very difficult to find, unless of course one of them came inside our solar system.

Primordial Black Holes could have been formed a millionth of a second after the Big Bang. How many are out there? Your guess is as good as mine. (Credit: Owlcation)

So maybe we don’t need physics beyond the standard model in order to explain Dark Matter. If there are a lot more Brown Dwarfs than we ever imagined, more Rogue Planets and more primordial Black Holes maybe Dark Matter is just protons, neutrons and electrons in objects that don’t shine by their own light.
Machos may not be as exciting as WIMPs, but reality is what it is and after thirty years of failing to find any exotic elementary particles maybe we need to give Machos a rethink!

The XENON experiment was designed to detect particles of dark Matter, has it detected Dark Energy instead?

Two of the greatest mysteries in all of science concern the nature of Dark Matter and Dark Energy which combined are estimated to make up as much as 95% of the matter and energy in the Universe. Dark Matter was first proposed in the 1920s to explain the rotation curves of the galaxies, which when all of the visible matter is taken into account, do not appear to obey Newton’s law of gravity. Dark Matter, particles that only interact with other particles by their gravitational attraction, is supposed to account for this discrepancy. The particles predicted by theories of Dark Matter have been given the generic title of WIMPs, which stands for Weakly Interacting Massive Particles. Trouble is that no particles of a type that could be called Dark Matter have ever been discovered.

Today scientists really only understand about 5% of everything that makes up the Universe. About 25% is Dark Matter and fully 70% id Dark Energy. (Credit: NDTV Gadgets 360)

There are literally dozens of different candidate particles that could be Dark Matter but at least that means we have some idea of what it’s like, we can design experiments to check our guesses. We have hardly any idea about what Dark Energy is like. (Credit: Physics – APS.org)

Dark Energy on the other hand, was proposed in 1998 to account for the shocking discovery that the Universe was not only expanding, it was accelerating its expansion. Something was pushing the galaxies apart and for want of a better name it was called Dark Energy, although many physicists would rather refer to it as Dark Pressure. We know almost nothing else about it.

We do know that since 70% of the Universe is Dark Energy it will determine the eventual fate of the Universe. (Credit: NASA)

Since Dark Matter is supposed to be made up of WIMPs, particles something like those we’ve been studying for decades experimental physicists have put a good deal more effort into trying to detect them than Dark Energy, which as I said we know almost nothing about. One of the current experiments designed to detect WIMPs is the XENON series of detectors operating at the Gran Sasso National Labouratory in Italy. The Gran Sasso facility is one of number of High Energy Physics labouratories that have been constructed deep underground, often in old mines, in order to help shield them from the cosmic rays that rain down from outer space.

The layout of the Gran Sasso National Labouratory in Italy, one of the world’s leading centers for High Energy Physics. (Credit: Nature)

To date there have been three XENON detectors with a fourth just beginning operation, all are designed to provide two distinct signals whenever a WIMP ‘bumps’ against an atom of liquid Xenon. The liquid Xenon is contained within a ‘time projection chamber’, see image below. The liquid Xenon is kept at its boiling point, 165 Kelvin or -108 degrees Celsius, and there is a layer of gaseous Xenon at the top of the chamber. At both the top and bottom of the chamber are arrays of photomultiplier tubes (PMTs). There is also a high voltage electric field across the chamber, positive at the top, negative at the bottom.

Setup of the Xenon Experiments. When an unknown particle strikes am atom of liquid xenon light is first detected at the bottom array of Photo Multiplier Tubes (PMTs). released electrons then float upwards in the liquid because of a large electric voltage until they reach the surface and ionize xenon gas that is detected by the upper array of PMTs. (Credit: NI Group at UC San Diego)

When, and if a WIMP strikes an atom of liquid Xenon it knocks off an electron causing a spark of ultraviolet light at a wavelength of 178 nm in a process known as scintillation. This signal is picked up by the lower PMTs and is referred to as S1.

The electron then begins to drift upward at a known speed, the deeper the collision with the WIMP took place the longer it takes for the electron to reach the top. Once the electron reaches the top of the liquid layer it pops out into the gaseous Xenon where it accelerates rapidly knocking into atoms and ionizing them. This ionization releases more photons of light that are picked up by the upper PMTs as signal S2. The X-Y location of the original collision is determined by the location of the PMTs that picked up S2. The Z coordinate, the depth in the liquid Xenon where the collision occurred is calculated from the time difference between S1 and S2.

The original Xenon10 experiment was just a small, proof of concept model. (Credit: UCLA Physics and Astronomy)

The first detector to be installed at Gran Sasso contained 15kg of liquid xenon as was known as XENON10. This experiment was operated between October of 2006 and February of 2007 as a proof of concept model and an analysis of the data provided by 58 days of operation found no excess signals that could have been caused by a WIMP collision.

The second experiment, called XENON100, contained 165 kg of liquid xenon and was operated between 2008 and 2014. Over several data collecting runs the detector found no excess signals beyond that which would be expected by random background events.

The latest experiment to complete its data collection run is known as the XENON1T and contains 3.2 tonnes of ultra-pure liquid xenon. This detector began operation in 2014 and gathered 278.8 days of data before being taken off line in order to make space for its successor. During its time in operation the XENON1T experiment recorded 285 events, 53 more events than were theoretically expected by random collisions with radioactive particles. These 53 signals are the first evidence for possible Dark Matter particles, specifically a type of theoretical particle known as an axion.

The Xenon1T experiment was much larger and was the first to give signals that could have been caused by unknown particles. (Credit: Science.Perdue.edu)

But a new theory suggests that the 53 events could be the sign of something even more exotic, particles of Dark Energy. In a paper written by astrophysicists at Cambridge University’s Kavli Institute for Cosmology and published in the journal Physical Review D the signals detected by XENON1T could have been caused by a hypothetical particle that screens the short range effects of Dark Energy and which the researchers have christened ‘Chameleons’. The idea is that; while Dark Energy is dominant over a distance scale of the entire Universe, at smaller scales, say within our galaxy, it is almost imperceptible, so some form of screening is necessary. The Chameleon particles would provide that screening.

Results from existing experiments already restrain the effective mass of any ‘Chameleon’ particles that could be Dark Energy. (Credit: Phys.org)

According to Doctor Sunny Vagnozzi, first author on the paper, “Our chameleon screening shuts down the production of Dark Energy particles in very dense objects…It also allows us to decouple what happens in the local, very dense Universe from what happens on the largest scales, where the density is extremely low.”

This whole idea of screening Dark Energy on the local scale while allowing it to have full effect at the cosmic scale is very new and so far without any observational data to back it up. So the question of just what are those 53 excess events detected by XENON1T, Dark Matter, Dark Energy or just some contamination inside the containment vessel will have to wait.

The Standard Model of Elementary Particles is nice and simple, it is also incomplete. Just what else there is in the Universe is the big question. (Credit: Quantum Diaries)

the XENONnT experiment is now completed and has begun taking data. (Credit: Pegasus WMS)

The XENONnT experiment, with 8 tonnes of Xenon gas, came on line just a year ago and is collecting data even now. Perhaps we’ll have the answer in a very few years.

As Searches for Dark Matter continue to come up empty Physicists are beginning to reconsider theories of Modified Gravity.

Physicists and Astronomers have had a big problem now for a very long time. Once astronomer Carl Hubble recognized that a large number of the fuzzy objects out in space called nebula were in fact entire galaxies astronomers and astrophysicists starting trying to work out the dynamics of how those galaxies behaved. Take a typical spiral galaxy like our own Milky Way, it has a central globe about 20,000 light years in diameter surrounded by a thin disk 200,000 light years in diameter but only about 5,000 light years thick, see image below. The density of stars is greatest in that central sphere and slowly but steadily decreases the further out you go along the disk.

The galaxy M33 (Triangulum) is a typical spiral like our Milky Way with a concentration of stars in the center and the density growing less the further you go out from the center. (Credit: Astronomy Picture of the Day – NASA)

Physicists immediately recognized that such a galaxy would only be stable if all of its stars orbited around the center and indeed our Sun is calculated to orbit around the Milky Way’s center once every 200 million years. If astronomers could estimate the mass distribution of the stars then the physicists could use Newton’s law of gravity to work out a velocity profile for the galaxy. Basically that would give them a formula for the velocity a star would have as a function of its distance from the center of its galaxy. That formula could then be checked by using the Doppler effect to measure the actual velocities of stars at various places in a galaxy.

Capculated (expected) versus measured radial velocity as a function of distance from center for the spiral galaxy M33. It’s easy to see that something is wrong somewhere! (Credit: Dark Energy, Dark Matter, Dark Gravity)

It didn’t work. Dozens of studies, dating back to 1933 have shown that the stars near the outer edge of a galaxy are moving too fast. Indeed the whole profile of velocity versus distance indicated that galaxies should have more than twice the mass that we can see and that mass should be spread out more evenly from the center.

That’s where the idea of ‘Dark Matter’ comes from, some form of matter that doesn’t radiate light but possesses a great deal of mass. There have been a lot of ideas about what dark matter could be, several of which have even been given ‘cutesy’ names. MACHOS, which stands for Mass Concentrations, could be anything from brown dwarfs, objects too small to become stars but larger than planets, to stellar mass black holes. Problem with either alternative is that you need huge numbers of them; remember we have to more than double the mass of the galaxies. Astronomers have found a few brown dwarfs and stellar mass black holes but nowhere near enough to solve the problem of the missing mass.

Every now and then a physicist will suggest that Dark Matter is an enormous number of undetected primordial black holes. There is no evidence to support that idea and circumstantial evidence against it. (Credit: The Tribune)

Then there are the WIMPS or Weakly Interacting Massive Particles. These are elementary particles like electrons or quarks except that they are electrically neutral so they don’t interact with light, and they are very massive. Particle physicists like WIMPS because they can connect them to the particles predicted by theories of ‘Super-Symmetry’. Problem is that despite decades of searching, and building powerful particle accelerators like the Large Hadron Collider (LHC) at CERN no evidence for any super-symmetric particles has ever been found.

One of the problems with Weakly Interacting Massive Particles (WIMPS) as Dark Matter is that there are so many possibilities that it’s hard to keep them all straight. (Credit: Physics (APS) American Physical Society)

A completely different approach was taken back in 1983 by Israeli physicist Mordehai Milgrom. Maybe he asked, Newton and Einstein were wrong about gravity, and then Milgrom proceeded to modify the gravitational field equations so that they would accurately predict the behavior of galaxies. Milgrom referred to his theory as MOdified Newtonian Dynamics or MOND the name by which it has since been known. If MOND or some similar altered theory of gravity is true then the failure to detect dark matter is easy to understand, there simply is no such thing as dark matter, gravity is different than what we thought.

Physicist Mordehai Milgrom has challenged both Newton and Einstein, not something for the faint of heart to try! (Credit: Wikipedia)

Now according to Newton and Einstein gravity obeys what is known as an inverse square law, see equation. 1. This means that the strength of gravity gets weaker the further two masses are by the square of the distance between them. Double the distance and gravity is one quarter as strong, triple the distance and gravity is one ninth as strong, four times the distance yields one sixteenth the strength and so on.

Newton’s law of Gravity is an Inverse Square Law and has been measured to be extremely accurate within our Solar System. (Credit: Wikipedia)

The changes Milgrom proposed to the inverse square law where very small. They had to be because Newton-Einstein works extremely well in our solar system and recently astronomers have even shown that stars orbiting around the supermassive black hole at the center of our galaxy follow Newton-Einstein very accurately, see my post of 6May2020. Crucially however, the small change proposed by Milgrom doesn’t grow weak as quickly with distance as inverse square. This means that at enormous distances, much larger than our solar system, tens to hundreds of light years, it is the modified term that starts to dominate over the Newtonian inverse square term.

In MOND a constant Ao is introduced that must be extremely tiny so that MOND only differs from Newton over huge distances. Physicists don’t like this kind of tweaking of equations just to make them fit the observed data without some rational behind it. (Credit: Slideserve)

All that makes it very difficult to test MOND or any similar small changes to Newton-Einstein. There is one difference however that just might be measured. One of the quirks of a pure inverse square law is that if you are sitting at the center of a mass distribution then you are being pulled by the gravity of those masses equally in every direction so that you literally feel no force! Think about it, if you are at the center of a planet there is a lot of matter all around you but you’re being pulled down just as much as up, back just as much as forward and to the left just as much as to the right. Being pulled equally in every direction you end up not being pulled in any direction, so you feel no gravity. This is known as the lack of effect from an external field.

Newton first performed the calculation that showed how there is no net gravitational force inside a uniform shell of matter. (Credit: Slideplayer)

In MOND however an external field can be felt and so the rotation curve of a galaxy at the center of a large cluster of galaxies would differ from the rotation curve of a similar galaxy that is far from any other large galaxy. To test this idea a group of astrophysicists from Sejong University in South Korea, Cardiff University and the University of Oxford in the UK along with Chase Western Reserve University and the University of Oregon in the USA has examined the rotation curves of 153 galaxies to see if there is any trace of such a difference. The study is entitled ‘Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies’ and has been published in the Astrophysics Journal.

What they discovered was that of rotational speeds of galaxies inside an external gravitational field were slowed when compared to the rotations in more isolated galaxies, something contrary to Newton-Einstein but exactly as predicted by MOND. Statistically the results so far give a 4σ confidence level, just below the golden 5σ confidence that physicists use to declare a ‘discovery’. With results so provocative you can be certain that the researchers will be working to both find more evidence as well as improve the data they already have.

Some of the data from the study in Astrophysical Journal in chart form. (Credit: Kyu-Hyun Chae et al)

If MOND does turn out to be correct it will not only eliminate the need for dark matter it will force a reevaluation of many other well established theories. Much of Cosmology and the Big Bang Theory are rooted in Einstein’s gravitational field equations but so far no one has ever been able to expand MOND to describe the Universe as a whole. So even while MOND has gained strong new evidence in its favour there’s still a long way to go before it becomes generally accepted by the majority of physicists. The problem of galactic rotation has been around a long time and it looks like it will continue to be so for a little while longer.

The LZ Experiment will be the most sensitive instrument ever to probe into the existence and nature of Dark Matter. And just why do we think there even is such a thing as Dark Matter?

(NOTE: Science and Science Fiction is having a few problems right now with its platform program WORDPRESS. One of these problems is that I can’t insert any images into the body of my posts!!!! Right now the IT support at my host IPAGE is working the problem and hopefully they’ll fix the problem soon.

In the meantime however I want to check and see if I can still publish my posts I’m going to try to publish this one without images. Once everything is back to normal I’ll edit the post to add images but until then at least you can still read about the latest in Science and Science Fiction!!!)

Modern physics right now has a couple of big problems that are called Dark Energy and Dark Matter. I say they are big problems because we estimate that they make up more than 95% of the energy in the Universe while we know nothing about them aside from a few educated guesses.

In this post I’ll be discussing Dark Matter and in particular the latest experiment to attempt to detect and learn something about the nature of Dark Matter, the LZ experiment. Before talking about LZ however let’s take a step back and consider some of the evidence that has led physicists and astronomers to even consider the possibility of Dark Matter.

About fifty years ago astronomers began to make careful estimates of the masses of the various galaxies that they were observing in their telescopes. They made these estimates using two different techniques. (In science if you can measure something in two completely different ways and get the same answer from both that’s usually considered strong conformation that you’re getting the right answer!)

The first technique was very straightforward, count, well estimate, the number of stars in the Andromeda galaxy let’s say, and since we know the mass of one star, our Sun, a simple multiplication gives you an estimate of the mass of Andromeda. The second technique employed Newton’s laws of gravitation and required measurements of the velocities at which stars of various distances from the center of their galaxy orbited around that center. Both techniques required an enormous amount of very careful, painstaking measurements and so at first no one was surprised that the initial results were not very close.

As time went on however and the measurements became more accurate the discrepancies didn’t go away. Worse, the results always differed in the same way, that is the Newton’s laws astronomers always had the galaxies weighting more than the astronomers who counted stars, and the ratio between the two groups was always a factor of about six times as much mass as could be seen. It was looking as if there was a lot of matter in galaxies that couldn’t be seen, a lot of Dark Matter!

As astronomers began to consider what this Dark Matter could be made of they first considered fairly normal objects like red dwarf and brown dwarf stars. Red dwarfs are stars that are so small and dim that they are all but invisible at stellar distances while brown dwarfs are even smaller, so small they can’t even start hydrogen fusion in their cores and so are not really stars. They’re bigger than a planet, but not big enough to be true stars. Collectively these objects were called MAssive Compact Halo Objects or MACHOS and large numbers of them, larger than the number of ordinary stars, could account for a sizeable amount of the required Dark Matter in the galaxies.

So astronomers began looking for these MACHOS in our local stellar neighborhood in order to get a measurement of how common they were. In particular the Hubble space telescope was employed in the search but the final results were disappointing. A few brown dwarfs and very small red dwarfs were found, but not nearly in the numbers needed to even double or triple the masses of the galaxies.

It was at this point that the physicists got interested, because some of their theories about sub-atomic particles were indicating that for every particle we knew about, there should be a massive ‘Supersymmetric’ partner that only interacts weakly with other particles. These particles were given the name ‘Weakly Interacting Massive Particles’ or WIMPS and particle physicists were actively searching for these WIMPS in their powerful particle accelerators and other experiments. The possibility that these WIMPS could be the Dark matter astronomers were looking for only increased the effort to detect them. So it is that we come back to the LZ experiment!

The LUX-ZEPLIN or LZ experiment represents a collaborative effort between two previous searches for Dark Matter, the America LUX (Large Underground Xenon) and British ZEPLIN (ZonED Proportional scintillation of LIquid Nobel gasses) experiments. The LZ experiment will be conducted at the Sanford Underground Laboratory 1.5 kilometers beneath the Earth’s surface in the Homestake mine in Lead South Dakota. The LZ experiment like its predecessors LUX and ZEPLIN has to be conducted deep underground in order to minimize the number of false detections caused by cosmic ray particles.

The LZ experiment’s main detector will consist of a vessel containing 7000 kg of liquid Xenon with a small layer of xenon gas at the top while both the top and bottom of the vessel are covered with photomultiplier tubes. Any interaction between a WIMP and a nucleus of a xenon atom will immediately release a few photons of light that will be detected by the photomultiplier tubes. At the same time an electron will be knocked away from the atom. This atom will travel upward due to a uniform electrical field surrounding the xenon container. After a few milliseconds the electron will reach the xenon gas where it will ionize some of the gas atoms releasing more photons that will also be detected by the photomultipliers.

It is this unique signature of a few photons followed a fraction of a second later by more photons that will indicate the detection of a WIMP. There is a problem however; an ordinary neutron passing through the xenon vessel could produce a very similar signal. In order to filter out the false signals from neutrons the entire xenon vessel will be placed inside an outer detector containing 17,000 kg of gadolinium-loaded liquid scintillator (GdLS), which is a neutron absorber, releasing more photons.

So if the outer GdLS detector signals a neutron absorption immediately after the inner xenon detector detects a possible WIMP, that WIMP candidate will have to be rejected as a false event. This shows you the enormous lengths that physicists must go to in order to make absolutely certain that they are detecting just the particle that they are searching for.

The LZ experiment is now under construction and is expected to begin operation in 2020. In a few years then we may hear about the first confirmed detector of a WIMP, we may for the first time observe and learn something about an actual piece of Dark Matter.

The SuperCDMS Experiment and the Search for Dark Matter.

It was back in my undergraduate days (early 1980s) that the topic of Dark Matter first began to be seriously considered by astro-physicists and cosmologists. The idea that there was some kind of matter in the Universe that was for some reason invisible to our telescopes was considered as a solution to two of the biggest problems in our study of the Universe.

The first problem concerned the stability of all of the rotating spiral galaxies we were studying. The idea that the stars in the outer reaches of a galaxy, like our own sun, would orbit around the center of the galaxy made perfect sense. After all, it was just Newton’s laws of gravity at work we thought. However, when we estimated the mass a galaxy, basically counting the numbers of stars, and measured the speed at which the stars were orbiting we found that there wasn’t enough mass, the galaxies should fly apart! There had to be some mass that we weren’t seeing, some invisible matter whose gravitational attraction was holding galaxies together. See image below.

At the same time other astronomers were studying the Cosmic Microwave Background (CMB), the leftover radiation from the original big bang, and used that data to calculate how the Universe should look today. Problem was that the calculations didn’t match the reality, not based on the amount of mass we could see. In order to make the calculations work the Universe had to have about four times as much invisible matter as the matter we could see.

O’k so the Universe had a lot of matter that didn’t emit light the way normal matter did in the stars, some sort of Dark Matter. The search was on to discover just what this Dark Matter was. The astro-physicists, with some help from the high-energy physicists, came up with a lot of ideas: Cold Dark Matter, Hot Dark Matter, MACHOS (Mass Concentrations) and WIMPs (Weakly Interacting Massive Particles).

That was almost forty years ago now, and we’re still waiting for direct experimental evidence of any kind of dark matter. Oh, we’ve made some progress, everybody pretty much agrees on WIMPs as Dark matter but that doesn’t mean everybody’s right. We need good hard evidence.

Hopefully soon we’ll get some from the Super Cryogenic Dark Matter Search now under construction by the Stanford Linear Accelerator Center (SLAC) at Stanford University and which will be set up over 2000 meters underground at SNOLAB at the Vale Inco Mine in Sudbury, Canada.

Setting up sophisticated, delicate physics experiments deep down in an old mine because all of the rock above the instruments helps insulate them from the interference of cosmic ray particles. And the SuperCDMS needs to eliminate all of the interference it can, it’s trying to measure the tiny amount of energy produced when a WIMP bounces against a normal atom. The image below shows the Sudbury neutrino telescope already in operation at Sudbury.

Neutrino Telescope in Sudbury Mine (Credit: Pinterest)

Physicists calculate that such collisions are very rare, you may have to wait many trillions of trillions of years for a particular atom to experience such a collision. Rather than waiting so long physicists will use trillion of trillions of atoms and then ‘listen’, that’s right listen for the sound of any collisions. Technically the intent is to detect the minute phonon signals of the collisions with germanium crystal detectors. The image below shows one of SuperCDMS’s detectors.

But in order to ‘hear’ the sound of a WIMP hitting an atom the physicists have to eliminate as much as possible the racket caused by all of the atoms hitting each other caused by thermal vibrations and the only way to do that is to reduce the detector’s temperature down to a small fraction of a degree above absolute zero, hence Super Cryogenic Dark Matter Search.

The experimental setup is shown in the image below. The cyrostat and detector section is modular in design allowing more detectors to be installed in the future. Around the detectors is a lining of lead (Pb) shielding with water shielding around that. The entire apparatus is then mounted on seismic isolators because even the slightest outside movement could be picked up as an erroneous signal. It’s often true that in today’s physics experiments, eliminating the unwanted signals can be a bigger job than detecting the minuscule signal you’re looking for!

SuperCDMS Experimental Layout (Credit: SuperCDMS)

SuperCDMS is scheduled to be up and running by the year 2020 but it will take four or five years of data collection before any results can be announced but I’ll let you in on my opinion. Now I’ll be very happy to be proven wrong but I’ve always been skeptical of WIMPs, we’ve been looking for them for forty years and have no evidence so far. Personally I was a MACHO supporter, basically the idea here was that for every star we can see there would be dozens of smaller brown dwarfs, objects just too small to start nuclear fusion and so don’t glow, and on top of that there would be literally thousands of planetary sized objects in interstellar space. I still think we need to consider MACHOS as a possible solution to the Dark Matter problem.