The Dark Matter Question?
- Scientists have long believed that some invisible “dark matter” particles must accompany the normal matter in the universe to explain how stars orbit in galaxies and how galaxies orbit in clusters.
- An alternative idea that "There is no extra matter and that our equations of gravity need updating" has received much less attention.
- Numerous experiments have failed to find evidence for dark matter particles, and the possibility remains that gravity should be modified.
- Some astrophysical evidence, such as recent observations of gravitation in galaxies, favors modified gravity theories over dark matter. It is time that physicists let go of their prejudices and reexamined this underdog idea.
What Exactly Is Gravity?
We all know that gravity attracts one mass to another, but how exactly does it do that? No one really knows, but also, nobody denies that gravity exists. We just do not understand how it works. Newton's equations accurately describe gravity events here on earth. Einstein's equations accurately describe celestial events extremely accurately. Newton's equations are a simplified version of Einstein's equations, so there is no conflict between them. Bottom line - Einstein describes outer space events very well while Newton takes care of normal events (as opposed to atomic events) here on earth.
But no one can describe exactly "how" gravity works. What exactly is the mechanism that makes a rock sink to the bottom of a pool? How do stars bend light rays coming around them? Is Einstein's explanation of a curved space-time correct? See the picture to the left of the famous Steven Hawkings weightless in a very special jet during a dive that leaves passengers weightless for a short period of time. Exactly how is he lifted into weightlessness?
A popular theory is that there are gravitons, tiny massless force particles, that move back and forth between all matter and that attract each other in a gravitational field. Gravitons are extremely weak on a local scale yet extremely strong on a celestial scale. While this theory has a high intellectual appeal, after hundreds years of observation and experimentation, there is no concrete evidence that gravitons exist. Most scientists have lost faith in the reality of gravitons, but no one has a better explanation.
When Einstein introduced his General Relativity Theory in 1915, many scientists were skeptical that the sun could bend light traveling around it because light photons had no mass. According to Newton, objects with mass attracted each other, but by 1915 most scientists believed light photons had zero mass. When in 1919 Einstein was proved correct, scientists were truly amazed. Einstein explained that there was a curve in space-time around large objects like the earth and sun. See the sketch to the left. But, when a rock sinks in water, is there such a thing as a curve in water-time? Is water just a denser form of space-time? Both Newton and Einstein describe extremely well "what" gravity does, but not "how" it works. Newton, until he died, was very disturbed that he could not explain "how"gravity worked.
Physics is currently divided into two different worlds. One is Einstein's world of large objects governed by his set of field equations and gravity. The other world is one of micro atomic phenomenon called Quantum Mechanics which excludes gravity. Both theories have extensive histories of precise experimentation that have successfully predicted outcomes to many decimal places. Although many people have tried to combine these two theories into one "grand" theory of everything, to date no one has been able to do so.
Some Quantum enthusiasts have gone so far as to suggest that gravity may not be a fundamental force at all. It may be the offspring of the three known fundamental forces acting in some unknown way with space and time. (This seems a bit far fetched to most scientific people.) On the other hand, string theorists have been able to incorporate gravity into their mathematical theory, but there is no evidence that String Theory as a whole is correct. However, many scientists believe that String Theory will eventually evolve into the "grand" theory that all scientists are looking for. Top
Is Gravity The Average Attraction Of Molecules?
Dr. Erik Verlinde is a respected Professor of Physics and String Theory at the University of Amsterdam and was previously a full Professor at Princeton. He published a paper in 2010 and has given lectures that challenge the historical view of gravity. Verlinde suggests that gravity is somewhat like temperature. What exactly is temperature? Temperature per se does not exist. Temperature is the statistical average energy of billions of molecules in a given volume. Similarly, Verlinde suggests that gravity is the "average attraction" of one set of zillions of molecules in a given region attracting another set of zillions of molecules. In other words gravity is not a large systems phenomenon but the combined contribution of many tiny individual molecules attracting one another per an unknown rule of Quantum Physics. Such a new Quantum rule would not obsolete either Newton or Einstein theories, their equations would simply be demoted to effective field equations. Verlinde's reasoning has caused many scientists to take a hard look at gravity on a molecular basis. Currently, Quantum Mechanics does not address gravity, but almost all scientists believe that one day it will. Verlinde has set the stage for a brand new theory of gravity. See a short gravity video by Erik Verlinde here.
In a paper which appeared in November 2016 on the ArXiv preprint server, Verlinde shows how his theory of gravity accurately predicts the velocities by which stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies. Also, a team led by astronomer Margot Brouwer at the Leiden Observatory, The Netherlands measured the distribution of gravity around more than 33,000 galaxies to put Verlinde's prediction to the test. She concluded that Verlinde's theory agrees well with the measured gravity distribution, but she emphasizes that dark matter could also explain the extra gravitational forces. However in the theory of dark matter, the amount of it is a "free parameter", which must be adjusted for each observation. But, Verlinde's theory provides a direct prediction, without any free parameters.
A Verlinde limitation is that the new theory is currently only applicable to isolated, spherical and static systems, while the universe is dynamic and complex. Many real observations cannot yet be explained by the new theory, so dark matter is still in the game. Top
Dark Matter Disbelievers
In spite of all the inferred evidence of dark matter, there are disbelievers in the astro-physics community. The current theory of the total universe makes sense only if astro-physicists either modify the theories of gravity or invoke additional gravitational forces due to unseen sources of mass which are referred to as dark matter.
See the Chandra x-ray image of galaxy cluster Abell 383 to the left. Two different teams of respected astro-physicists analyzed the cluster with competing conclusions. The purple halo cloud is believed by one team to be dark matter that is inferred from gravitational effects of the cluster.
The second team found evidence that the amount of dark matter is not peaked as dramatically toward the center as the standard cold dark matter model predicts. Their paper describes this as being the "most robust case yet" made for such a discrepancy with the theory.
Gravitational theory predicts that spiral galaxies, like our own Milky Way, must have massive dark matter halos that provide the galaxy's missing mass. But the Milky Way's own dark matter halo has yet to be detected or inferred, even indirectly.
Some such as Mordehai Milgrom, an astro-physicist at the Weizmann Institute of Science in Rehovot, Israel, think that the theory of gravity has to be modified. His MOND theory does not require dark matter. (See the MOND Theory below.) MOND explains a vast wealth of data on galactic rotation speeds using only their visible stars and gas. MOND does this with a mathematical prescription that strengthens the visible material's gravity, but only where the gravity gets very weak. Some scientists think that by now we should have found some independent evidence for dark matter and that hasn't happened. Top
Galaxy Rotation Curves
Several independent observations point to the fact that the visible mass in galaxies and galaxy clusters is insufficient to account for their dynamics when using Newton's laws. This discrepancy – known as the "missing mass problem" – was first identified for clusters by Swiss astronomer Fritz Zwicky in 1933.
The early studies were augmented and brought to the attention of the astronomical community in the 1960s and 1970s by the work of Vera Rubin at the Carnegie Institute in Washington, D.C. Rubin mapped in detail the rotation velocities of stars in a large sample of spirals.
While Newton's Laws predict that stellar rotation velocities should decrease with distance from the galactic centre, Rubin and collaborators found instead that they remain almost constant – the rotation curves are said to be "flat". This observation necessitates at least one of the following: a) There exists in galaxies large quantities of unseen matter which boosts the stars' velocities beyond what would be expected on the basis of the visible mass alone, or b) Newton's Laws do not apply to galaxies. The former leads to the dark matter hypothesis; the latter leads to MOND. Top
The MOND Theory
One possible theoretical explanation for the diverse rotation observations suggests that at large scales, the laws of gravity are different from Einstein’s theory of general relativity. As mentioned above, this idea, known as Modified Newtonian Dynamics (MOND), was proposed more than 30 years ago by Israeli physicist Mordehai (Moti) Milgrom of the Weizmann Institute, pictured at the left. Specifically, Milgrom proposed that for very small accelerations, the square of the acceleration, not the acceleration itself, is proportional to the gravitational force.
The basic premise of MOND is that while Newton's laws have been extensively tested in high-acceleration environments (in the Solar System and on Earth), they have not been verified for objects with extremely low acceleration, such as stars in the outer parts of galaxies. This led Milgrom to postulate a new effective gravitational force law (sometimes referred to as "Milgrom's law") that relates the true acceleration of an object to the acceleration that would be predicted for it on the basis of Newtonian mechanics.
Despite decades of trying, researchers have failed to capture a single piece of dark matter, even though it is supposed to make up roughly five-sixths of all matter in the universe. "However, until we fully understand dark matter we should keep an open mind", says Abraham Loeb, a theoretical astrophysicist and chair of astronomy at Harvard University, who did not take part in the research.
Please note that dark matter theory does do a "very good job" of explaining the variations in the cosmic microwave background (CMB) curve, the total mass of the Universe, and galaxy cluster halos. MOND "struggles" with all three of these items.
Therefore, one can conclude that while dark matter is no longer necessary to explain the rotational speeds of stars in a galaxy. At this time, dark matter still has its place in the cluster of galaxies, the mass of the Universe, and the CMB of the Big Bang. The mystery of dark matter continues. Top
New Evidence Questions Dark Matter
A team led by Stacy McGaugh (pictured to the left) from Case Western Reserve University in Cleveland has found a significant new relationship in spiral and irregular galaxies. The rotational speeds of stars inside galaxies tightly correlates with the gravitational speeds expected from visible mass only (no dark matter necessary).
The finding is consistent among 153 spiral and irregular galaxies, ranging from giant to dwarf, those with massive central bulges or none at all. It is also consistent among those galaxies composed of mostly stars or mostly gas.
In a September, 2016 paper in the journal Physical Review Letters, Stacy McGaugh, lead author and chair of the Department of Astronomy at Case Western, says that "the relationship we have found is tantamount to a new natural law".
The McGaugh data are exactly what MOND would have forecast. However, Stacy McGaugh and his team at this point are not pressing for any theoretical interpretation of their empirical data. They say that other models of dark matter also have the potential to explain their findings.
"We find a relation between what you see in normal matter in galaxies and what you get in their gravity," McGaugh said. "This is important because it is telling us something fundamental about how galaxies work."
Previous analyses of the orbital velocities of the stars in galaxies often depended on visible wavelengths of light. However, the stars that produce the most visible light are prone to fluctuations and may not provide the best picture of how matter is scattered throughout a galaxy. Instead, McGaugh and his colleagues analyzed near-infrared images collected by NASA's Spitzer Space Telescope over the past five years. Stay tuned and be patient.
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