Tag: Galactic Rotation

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.