In the last post (part 1), we discussed the phenomenon of the accelerating universe, namely that the universe is expanding and all galaxies are moving away from all other galaxies. Based on the paradigm of “cause and effect,” mainstream science argued a mysterious new force was causing the expansion. The force was named dark energy.
We also noted, that the accelerating universe was characterized by two unusual features:
1. The more distant a galaxy, the faster it is accelerating away from us.
2. There is no expansion of space occurring within a galaxy.
We ended the last post with questions: Why was there no expansion of space within a galaxy? Was the space between stars and other celestial bodies within our galaxy somehow different than the space between galaxies? In this post we will address those questions. Let’s start at the beginning.
In 1933, Fritz Zwicky (California Institute of Technology) made a crucial observation. He discovered the orbital velocities of galaxies were not following Newton’s law of gravitation (every mass in the universe attracts every other mass with a force inversely proportional to the square of the difference between them). They were orbiting too fast for the visible mass to be held together by gravity. If the galaxies followed Newton’s law of gravity, the outermost stars would be thrown into space. He reasoned there had to be more mass than the eye could see, essentially an unknown and invisible form of mass that was allowing gravity to hold the galaxies together. Zwicky’s calculations revealed that there had to be 400 times more mass in the galaxy clusters than what was visible. This is the mysterious “missing-mass problem.” It is normal to think that this discovery would turn the scientific world on its ear. However, as profound as the discovery turned out to be, progress in understanding the missing mass lags until the 1970s.
In 1975, Vera Rubin and fellow staff member Kent Ford, astronomers at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, presented findings that re-energized Zwicky’s earlier claim of missing matter. At a meeting of the American Astronomical Society, they announced the finding that most stars in spiral galaxies orbit at roughly the same speed. They made this discovery using a new, sensitive spectrograph (a device that separates an incoming wave into a frequency spectrum). The new spectrograph accurately measured the velocity curve of spiral galaxies. Like Zwicky, they found the spiral velocity of the galaxies was too fast to hold all the stars in place. Using Newton’s law of gravity, the galaxies should be flying apart, but they were not. Presented with this new evidence, the scientific community finally took notice. Their first reaction was to call into question the findings, essentially casting doubt on what Rubin and Ford reported. This is a common and appropriate reaction, until the amount of evidence (typically independent verification) becomes convincing.
In 1980, Rubin and her colleagues published their findings (V. Rubin, N. Thonnard, W. K. Ford, Jr, (1980). “Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc).” Astrophysical Journal 238: 471.). It implied that either Newton’s laws do not apply, or that more than 50% of the mass of galaxies is invisible. Although skepticism abounded, eventually other astronomers confirmed their findings. The experimental evidence had become convincing. “Dark matter,” the invisible mass, dominates most galaxies. Even in the face of conflicting theories that attempt to explain the phenomena observed by Zwicky and Rubin, most scientists believe dark matter is real. None of the conflicting theories (which typically attempted to modify how gravity behaved on the cosmic scale) was able to explain all the observed evidence, especially gravitational lensing (the way gravity bends light).
Currently, the scientific community believes that dark matter is real and abundant, making up as much as 90% of the mass of the universe. However, the nature of dark matter itself is still a mystery. Just what is this mysterious substance that appears to glue a galaxy together?
The most popular theory of dark matter is that it is a slow-moving particle. It travels up to a tenth of the speed of light. It neither emits nor scatters light. In other words, it is invisible. However, its effects are detectable, as I will explain below. Scientists call the mass associated with dark matter a “WIMP” (Weakly Interacting Massive Particle).
For years, scientists have been working to find the WIMP particle to confirm dark matter’s existence. All efforts have been either unsuccessful or inconclusive. The Department of Energy Fermi National Accelerator Laboratory Cryogenic Dark Matter Search (CDMS) experiment is ongoing, in an abandoned iron mine about a half mile below the surface, in Soudan, Minnesota. The Fermilab is a half mile under the earth’s surface to filter cosmic rays so the instruments are able to detect elementary particles without the background noise of cosmic rays. In 2009, they reported detecting two events that have characteristics consistent with the particles that physicists believe make up dark matter. They may have detected the WIMP particle. However, they are not making that claim at the time of this writing. The Fermilab stopped short of claiming they had detected dark matter because of the strict criteria that they have self-imposed, specifically there must be less than one chance in a thousand that the event detected was due to a background particle. The two events, although consistent with the detection of dark matter, do not pass that test. Where does that leave us? To date, we are without conclusive evidence that the WIMP exists.
Does the WIMP particle exist? Consider the facts.
1) The Standard Model of particle physics does not predict a WIMP particle. The Standard Model, refined to its current formulation in the mid-1970s, is one of science’s greatest theories. It successfully predicted bottom and top quarks prior to their experimental confirmation in 1977 and 1995, respectively. It predicted the tau neutrino prior to its experimental confirmation in 2000, and the Higgs boson prior to its experimental confirmation in 2012. Modern science holds the Standard Model in such high regard that a number of scientists believe it is a candidate for the theory of everything. Therefore, it is not a little “hiccup” when the Standard Model does not predict the existence of a particle. It is significant, and it might mean that the particle does not exist. However, to be totally fair, the Standard Model has other issues. For example, it doesn’t explain gravity. Because of these issues, numerous variations of the Standard Model have been proposed, but none have gained wide acceptance.
2) All experiments to detect the WIMP particle have to date been unsuccessful, including considerable effort by Stanford University, University of Minnesota, and Fermilab.
That is all the evidence we have. Where does this leave us? The evidence is telling us the WIMP particle might not exist. We have spent about ten years, and unknown millions of dollars, which so far leads to a dead end. This appears to beg a new approach.
To kick off the new approach, consider the hypothesis that dark matter is a new form of energy. We know from Einstein’s mass-energy equivalence equation (E = mc2), that mass always implies energy, and energy always implies mass. For example, photons are massless energy particles. Yet, gravitational fields influence them, even though they have no mass. That is because they have energy, and energy, in effect, acts as a virtual mass.
If dark matter is energy, where is it and what is it? Consider these properties of dark-matter energy:
- It is not in the visible spectrum, or we would see it.
- It does not strongly interact with other forms of energy or matter.
- It does exhibit gravitational effects, but does not absorb or emit electromagnetic radiation.
Based on these properties, we should consider M-theory (the unification of string theories discussed in previous posts). Several prominent physicists, including one of the founders of string theory, Michio Kaku, suggest there may be a solution to M-theory that quantitatively describes dark matter and cosmic inflation. If M-theory can yield a superstring solution, it would go a long way to solving the dark-matter mystery. I know this is like the familiar cartoon of a scientist solving an equation where the caption reads, “then a miracle happens.” However, it is not quite that grim. What I am suggesting is a new line of research and theoretical enquiry. I think the theoretical understanding of dark matter lies in M-theory. The empirical understanding lies in missing-matter experiments.
What is a missing-matter experiment? Scientists are performing missing-matter experiments as you read this post. They involve high-energy particle collisions. By accelerating particles close to the speed of light, and causing particle collisions at those speeds, they account for all the energy and mass pre- and post-collision. If any energy or mass is missing post-collision, the assumption would be it is in one of non-spatial dimensions predicted by M-theory.
Why would this work? M-theory has the potential to give us a theoretical model of dark matter, which we do not have now. Postulating we are dealing with new unknown form of energy would explain why we have not found the WIMP particle. Postulating that the energy resides in the non-spatial dimensions of M-theory would explain why we cannot see or detect it. Real-world phenomena take place in the typical three spatial dimensions and one temporal dimension. If dark matter is in a different dimension, it cannot interact with “real”-world phenomena, except to exhibit gravity. Why is dark matter able to exhibit gravity? That is still a mystery, as is gravity itself. We have not been able to find the “graviton,” the mysterious particle of gravity that numerous particle physicists believe exists. Yet, we know gravity is real. It is theoretically possible that dark matter (perhaps a new form of energy) and gravity (another form of energy) are both in a different dimension. This framework provides an experimental path to verify some of the aspects of M-theory and the existence of dark matter (via high-energy particle collisions).
Although dark matter is a mystery, we know from scientific observation it is real. Without dark matter our galaxy would fly apart. In fact, dark matter makes up most of the mass of a galaxy, over 90%. In a sense, you can think of a galaxy similar to the way we think of an atom. An atom can act like a single particle, an entity unto itself. However, we know the atom is composed of subatomic particles, like electrons, protons and neutrons. We also know that some of those particles are composed of other subatomic particles, which I will not go into detail here. The point is a galaxy may act on a cosmic scale as though it is particle, similar to an atom, with subatomic particles we call stars, planets and other celestial bodies. I know this is mind boggling, but it fits the observable evidence. It provides insight into the difference regarding space between galaxies and the space within a galaxy. It is consistent with our observations of the accelerated expansion of the universe.
Let us summaries our understanding from the first two post. First, the universe is expanding and the expansion is accelerating. Second, there is no expansion of space within a galaxy. Third, science believes that the accelerating expansion of the universe is caused by a mysterious new force, dark energy. Fourth, it appears galaxies are glued together via another mysterious entity, dark matter. Lastly, dark matter only exists within a galaxy and not between galaxies.
If we are willing to accept that a galaxy on a cosmic level acts essentially like a particle, as discuss above, we are still left with a mystery. What is really causing the space between these “particles” (i.e., galaxies) to expand. In other words, we are back to the question: What is dark energy? In the next post we will discuss a new theory, first proposed in my book Unraveling the Universe’s Mysteries, that seeks to explain the fundamental nature of dark energy.
