The Dark Side

In the beginning, far, far away galaxies like our own Milky Way were formed and over billions of years they got bigger through the coming together of many smaller galaxies.  During the merger, small stars of the galaxy were pulled apart and scattered to form tiny relic galaxies while the big central globular cluster that survived the gravitational tug-of-war was able to gain more star material.  Astronomers were able to work out where dark matter is concentrated because it distorts light from distant stars.  The greater the distortion, the greater the concentration of dark matter.  Dwarf galaxies contain greater proportions of dark matter than large spirals and most of dark matter is in the outer parts of our galaxy.  A large region called a “halo” of dark matter encircles the Milky Way and represents the greatest concentration of the dark matter of the galaxy.  There is little to no normal matter in a hypothetical region that we call the halo.  Scientists theorize the existence of dark matter to explain observations that suggest there is far more mass in the universe than can be seen.  Dark matter is called “dark” because it does not appear to interact with the electromagnetic field, meaning that it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, it is difficult to detect.  Dark matter is often said to be the controlling force that organizes large structures such as galaxies and galactic clusters.  We know that dark matter exists because of the effect it has on objects that we can observe directly, but dark matter is still a mystery and it has been puzzling scientists for the last 90 years.

Dark matter was posited by Dutch astronomer Jan Oort in 1932 when he studied star motions in the Sun’s neighborhood.  Because the galaxy was not flying apart, he reasoned, enough matter must reside in the disk to keep the stars from moving away from the galaxy’s center.  By examining the Doppler shifts in the spectra of stars in the Milky Way Galaxy, Oort measured their velocities.  He found that the stars moved faster than expected.  Oort expected the stars to move only as fast as what would be expected from the gravitation force of the visible mass (stars, gas, dust) in the Galaxy.  In reality, the stars appeared to be moving faster than this – fast enough to escape the galaxy.  Since Oort knew that this couldn’t be the case, he hypothesized that there must be additional mass in the galaxy that wasn’t visible and would keep the stars bound to the galaxy.

In 1933, the Swiss astronomer Fritz Zwicky while working for Caltech used the Mount Wilson Observatory to measure the visible mass of a cluster of galaxies and he found that it was much too small to prevent the galaxies from escaping the gravitational pull of the cluster.  According to the measure of visible mass of the Coma Cluster of galaxies, they were moving too fast for the cluster to remain bound together.  Zwicky studied the light emitted by the more than 1,000 galaxies that are part of the Coma Cluster of galaxies.  Zwicky determined the mass of the Coma Cluster using two methods.  One method used the velocities of the galaxies, which he determined by measuring shifts in the light they emitted.  The second calculation method determined mass using the total brightness of the cluster.  In comparing the two resulting mass estimates, he found that the galaxy velocity measurement estimated that there were hundreds of times more mass in the Coma Cluster than the brightness estimate that he predicted.

Dark matter was inferred as an additional gravitational source that could explain the flattened rotation curves of spiral galaxies; spiral galaxies were found to rotate at a nearly constant rate, independently of its radius.  From Newton’s law and the distribution of visible matter, one would expect the rotational velocity of stars and gas inside a galaxy to decrease with distance, but in the 70’s astronomers found this flattened anomaly, and dark matter was born.  The American astronomer Vera Rubin studied the movement of stars around galactic centers and found something peculiar.  She uncovered the discrepancy between the predicted and observed angular motion of galaxies by studying galactic rotation curves and found that something was propelling the stars to move much faster than expected, given their mass, or pulling them fast enough to fling them out of the gravitational pull of the galaxy.   The vast spiral in the Andromeda Galaxy seemed to be rotating all wrong.  The stuff at the edges was moving just as fast as the stuff near the center, apparently violating Newton’s Laws of Motion, but this finally got people to pay attention to dark matter.

Dark matter is an invisible substance that makes up approximately a quarter of the universe, and its presence is indicated by its gravitational force.  However, scientists have yet to identify what this matter is, what it is made of, or where it comes from.  With the advancements in technology and the development of better instruments and detectors, scientists are starting to be able to explore new avenues in their research, yet the full answer still evades us.  One popular theory is that dark matter is composed of exotic particles, such as weakly interacting massive particles (WIMPs).  These particles have mass and interact with gravity, which is why they would have an effect on the surrounding matter and thus explain certain phenomena.  WIMPs neither absorb nor emit light and they don’t interact strongly with other particles, but when they encounter each other, they annihilate and make gamma rays.  WIMPs would be between 10 and 100 times heavier than a proton.  Thus far, scientists have yet to find any WIMPs and as such, even though it is a plausible answer for dark matter, it is one that has so far not been fully backed up by evidence.   It seems likely that dark-matter particles are much smaller than WIMPs.

The other leading contenders are axions, primordial black holes and neutrinos.  An axion is a hypothetical subatomic particle of low mass and energy that is postulated to exist because of certain properties of the strong force.  Even though the universe was very hot at the time, axions would have been very cold at birth and would stay cold forever, which means that they are absolutely cold dark matter.  In the 1970s, when physicists were developing the Standard Model, they noticed something odd about the strong nuclear force, which binds quarks together to form the protons and neutrons within the nuclei of atoms.  This led to the existence of a new particle, that was dubbed the axion.

Primordial black holes (PBHs) were first dreamed up decades ago.  Researchers proposed them as an explanation for dark matter, and some researchers think swarms of little black holes moving like clouds through space offer a cleaner explanation than what is termed to be dark matter.  Primordial black holes were created in the first instants after the Big Bang and tiny ones that are smaller than the head of a pin along with supermassive ones covering billions of miles could account for all of the dark matter in the universe.  Originally, some scientists conjectured that the missing mass in the universe was made up of small faint stars and black holes, though detailed observations have not turned up nearly enough such objects to account for dark matter’s influence.  Stephen Hawking developed this idea, and this theory was soon dismissed, but over time astrophysicists realized that there must be many more black holes lurking out there than they had originally thought.

Neutrinos are particles that do not emit light, just like dark matter.  Physicists have developed a new mathematical model that may shed light on the identity of dark matter, which showed that a non-interacting or sterile neutrino is probably a dark matter particle and contributes to the mass of dark matter.  However, neutrinos can make up only a fraction of the total amount of dark matter, because they are too light and, when they were created in the early universe, they were moving too fast.  Another leading hypothesis is that the universe is filled by a uniform sea of quantum zero point energy, which exerts a negative pressure, like a tension, causing spacetime to gravitationally repel itself.  This stuff is sometimes referred to as a cosmological constant, which was first introduced by Einstein, but later he referred to as his greatest blunder.

Dark matter’s gravity drives normal matter (gas and dust) to collect and build up into stars and galaxies.  Although astronomers cannot see dark matter, they can detect its influence by observing how the gravity of massive galaxy clusters, which contain dark matter, bends and distorts the light of more-distant galaxies located behind the cluster.  This phenomenon for dark matter to be able to bend light is called gravitational lensing.  The universe appears to have about five times more dark matter than regular matter and seems to be organized around an immense network of dark matter filaments that have grown over time.  If dark matter moved quickly, its properties would suppress the formation of structure on small scales, leading to different structures than what we can observe.

The universe isn’t a random jumble of objects, as it has a structure composed of galaxies and gas, which cosmologists call the cosmic web.  The cosmic web is composed of interconnecting filaments of clustered galaxies and gases stretched out across the universe and separated by giant voids.  These cosmic filaments are the universe’s largest known structures, and they contain most of the universe’s mass.  They look like tendrils, and they stretch out for hundreds of millions of light-years, spinning and twirling like giant corkscrews.  We don’t have a full theory of how every galaxy comes to rotate, or every filament comes to rotate.  A cosmic filament is able to twist galaxies and dark matter into a strand of the giant cosmic web of invisible gas which weaves throughout the universe, providing the scaffolding out of which galaxies were born.  Filaments pull matter into rotation and toward clusters at their ends.  Since the vast majority of matter in our universe is dark, much of the cosmic web is completely invisible to us.

For more than a decade the astrophysical community has been gathering observational evidence that seem to contradict the concept of dark matter in favor of new theories of gravity.  Although it’s since been superseded by Einstein’s theory of general relativity, Newton’s law of universal gravitation still holds pretty well as an explanation for the large-scale structure and movements of the universe.  But now, new observations have been made that don’t quite fit these currently accepted models.  A group of scientists knowing that stars can leave a cluster through two different doors, discovered that the leading tail in a cluster of stars seems to have far more stars than the trailing one, which is an apparent contradiction of Newton’s laws, as that predicts that both doors should be the same width.  Their hypothesis, known as Modified Newtonian Dynamics (MOND) could end up proving that dark matter doesn’t exist.  MOND states that gravity begins to behave differently from Newtonian physics when it becomes extremely weak as it occurs around the edges of galaxies.  They think that the ultrafast speeds at which stars and galaxies are seen to rotate around one another is a consequence of gravity behaving in surprising ways and it has nothing to do with dark matter.

For Friday Faithfuls this week I would like to know if you think that dark matter actually exists?  You don’t need to write anything technical here, as a poem or a story about dark matter would be perfectly acceptable.  If you prefer to answer questions, then describe how does dark matter affect the universe?  Does some type of missing matter need to exist in order to explain the errant motions that we see in the heavenly bodies?  What particles are responsible for dark matter, or does dark matter consist of more than one particle, or is it even a particle at all?  Is all dark matter made of the same “stuff,” or could there be multiple flavors of dark matter out there?  Do you think it is possible for dark antimatter particles to exist?  Has dark matter always existed in the Universe, or was it created at some later time?  Is it possible that the extra gravity attributed to dark matter is an effect of dark energy?  Will we ever come up with a better theory for gravity that doesn’t use dark matter?  Is dark matter going to remain stable, or will it all someday decay away?  Will any of our direct detection experiments ever find it, or is this a fruitless endeavor?

12 comments

  1. Reblogged this on A Unique Title For Me and commented:

    Mass is the measurement of the amount of matter in an object. All objects with mass will create gravity. Gravitation is the force of attraction that pulls any two bodies toward each other. We can’t see gravity, but we can see the effects that it has on objects and that allows us to detect its presence. An apple landed on Isaac Newton’s head and a brainstorm occurred that made him think about gravity and the forces of nature. Newton saw that the apple took a straight path when it fell and he knew that a cannon ball would follow a curved path when it was fired. Newton knew about Galileo’s study of projectile motion and how he was able to accurately analyze the flight path, or trajectory of a projectile. Isaac thought that if the cannon ball was able to travel faster that it would fall into the curved path around the Earth. Newton thought that the power of gravity (which brought an apple from a tree to the ground) was not limited to a certain distance from Earth, but that this power must extend much further than was usually thought. If this was the same force, then a connection would exist between the way objects fell to the ground on Earth and the motion of the Moon going around the Earth. Newton proposed that gravity was a universal force.
    Why do we search for things we can’t see? We can’t define or describe dark matter, but scientists tell us it is there. Should dark matter even matter to us? Is dark matter worth all the chatter? Would it be better if dark matter were put into the batter, so it could make us fatter or flatter? I would love to order a platter of dark matter and if I don’t eat it all, I will scatter or splatter it everywhere to prove that gravity exists. I would like to go on Twitter so I can twatter about the dark matter. That is a bunch of gibberish talk with nothing but nonsense orbiting in invisible haloes around everything. As Bonnie Dobson said in ‘Morning Dew’, “Well I guess it doesn’t matter anyway.” Look at me, I’m in tatters! I’m a shattered. All this chitter-chatter, shmatta, I’ve been battered, what does it matter? Pitter patter, mad hatter, it couldn’t get sadder than being sucked into the dark matter.

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