Arguably, one of the greatest sources of wonder is seen in the night sky. But we’ve set our sights on matters we cannot see. However, that hasn’t stopped us from understanding just what happens beyond the planet we call home.
by Akshitha Sahu
As kids, almost all of us must have looked up at the sky and wondered what exactly lied beyond our little bubble of air. Perhaps we even looked at those bright twinkles in the sky and thought they were bright enough to keep us busy forever. However, one of the greatest mysteries in our universe is detected well beyond what our eyes can perceive.
Neil deGrasse Tyson explains that the gravitational force, measured about 85% of the universe, comes from substances that don’t behave the way we know matter to. They don’t interact with light or any other form of energy. The question that comes to mind pertains to the means of measurement of this matter, especially since we cannot recognize them with the naked eye. Fritz Zwicky first discovered this strange phenomenon in 1937 by studying the movement of individual galaxies in the Coma cluster, which lies about 300 million light-years away from Earth. After using a few galaxies as tellers of the gravity field binding the entire Coma cluster, the average velocity uncovered was unexpectedly high. Using the knowledge that a larger gravitational field attracts objects at greater velocities, Zwicky inferred that the mass would be unusually high as well.
This is where the unusual starts to become a mystery. The huge mass we observe by studying the visible galaxies isn't enough to explain the speed that the galaxies are traveling at. From what we know, there must be more mass in these clusters that causes these galaxies to move at extreme velocities.
Dr. Becky Smethurst from Oxford University explains that the theory of dark matter relies on the fact that we’ve understood gravity, but a new study, published explains that Modified Newtonian Dynamics (MOND), an alternate theory of gravity, may be considered evidence against dark matter.
Now we see a trilemma. Does dark matter exist or not? What if our predictions fall off the mark and we need a new theory?
Dark matter doesn’t exist (a new modification of gravity)
A recent study presents evidence in favor of MOND over dark matter. To understand how MOND works, Dr. Smethurst explains that the basics of physics are the preface to this situation. Newton’s second law of motion explains the relationship between an object’s mass and the force required to accelerate the object. The “inertial mass,” or how much an object resists motion, is a very important idea. When we use Newton’s Law of Gravitation, the forces, the masses of the car and the earth, and the distance the car is from the center of the earth all work in a specific proportion. The mass in this case is called the “gravitational mass.” Newton’s laws assume that both the inertial mass and the gravitational mass are equivalent (which were proven by the Eötvös Experiments), which is now called the “equivalence principle.”
While these masses are equivalent in our lab experiments, with the preface of MOND, this concept does not hold true. The truth may be far more extravagant. The inertial mass is affected by every other object in the universe. This is called the “External Field Effect (EFE).” Putting this into the context of dark matter and whole galaxies, the EFE means that in areas where there are more galaxies found, the galaxies will pull on the outskirts of other galaxies, changing the speeds of the stars found on the outside. There is no need for the theories of dark matter, especially when we know that other galaxies will pull at the matter in different galaxies.
While Newton’s laws (Second law of motion and gravitation) work in our solar system, they
fail to meet the mark when it comes to scales as large as entire galaxies where the galaxy is accelerating very slowly. What we see is that the speed on the outsides of a galaxy is counterintuitively greater than it is in the seemingly more dense center. This discrepancy in what we’ve observed can be fixed with a simple change in Newton’s equations.
When we use MOND as the equations instead of Newton’s laws, the calculated speed of the stars in galaxies is the exact same as it is when observed. What the conclusion essentially comes down to is the reason why dark matter was theorized in the first place. When we use the unmodified equations, stars in the galaxy travel at unexplained speeds, hence the need to introduce the theory of dark matter. With these modified equations and the EFE, the need for dark matter vanishes, explains Dr. Smethurst.
Dark matter exists:
One of the greatest pieces of evidence we have for dark matter is seen in the way that
galaxies behave. They don’t behave in the way we expect them to. Dr. Smethurst explains that even though the brightest part of a galaxy is the center and we expect the greatest mass to be in the center, the truth is that there may be a greater force on the outskirts of the galaxy.
Another question that comes to mind after the previous statement is how this conclusion is derived. When we think about our solar system, there is a clear correlation between the orbital speed and the distance a planet is from the sun whereas the distance increases, the orbital speed decreases. According to Newton’s laws of gravitation, this makes sense because the rule of thumb is that the closer objects are, the more their gravitational fields will attract each other. But when we expect this to happen on the galactic scale with the assumption that the center of the galaxy has more mass, our measurements fail us. The trend is the opposite. As the distance from the center of a galaxy increases, the orbital speed also increases and flattens near the edge.
This leaves us with the assumption that there must be more matter on the outside of the galaxy. But we cannot see this, says Dr. Smethurst. Many may wonder if black holes are behind the chaos. NASA explains that the mass from black holes and other known matter cannot be the cause of this anomaly because there simply isn’t enough of this matter to make a difference. Others also wonder if what we see is simply antimatter, but NASA continues to rule out this possibility by explaining that the gamma rays that are released as antimatter and matter interact has not been detected.
As with all data, there is never enough, but there certainly is an amount that is not enough to subject a claim. Dr. Smethurst explains that while a sample of a few galaxies may prove dark matter wrong, the sample is nowhere near enough to debunk dark matter. We use dark matter to describe the matter we expect to see in the unexpected, and the term shall only become more refined as we continue our search for answers.
We need a new theory.
Perhaps the change starts with labeling the phenomena correctly. In an interview, Neil DeGrasse Tyson explains that the term, “dark matter,” may actually be a misnomer. He emphasizes the fact that this term is just the representation of a measurement. Dark matter labels what we observe as matter, especially when we don’t know if what we observe is matter in the first place. We simply see the gravitational forces at play in unexpected ways. This fact may alone be important enough to coin a new term for our observations.
With the shortcomings of dark matter, a vague description of what’s happening, and of MOND, which has a few kinks in the equations (e.g. this equation predicts that light and gravity don’t travel at the same speed, but a 2017 study in the Astronomical Journal showed that they do indeed travel at the same speed), it’s very possible that a new, more detailed answer is what we need.
While it may be hard to believe, it’s possible that Albert Einstein and Sir Issac Newton were half wrong in their predictions. While Newton’s laws of gravitation work on earth, Einstein’s can help us explain the world on a larger scale. But these scholars’ predictions don’t hold up when it comes to explaining dark matter at the cosmic scale. Neil DeGrasse goes on to explain that even with all of our wild predictions, we simply don’t know. Dark matter may only be a theory at best, and as we’ve seen in the history of astrophysics, there is always more than enough data to give us answers, and perhaps more questions. If there’s one thing that we can agree on, everyone from particle physicists to cosmologists, it may be that while theories come and go, our wonder will remain forever.