By Mikayla Kauinana & Archit Tiwari
Modern physics has transformed the world and the lives of the people in it. Its applications can range from microwave ovens, smoke detectors, and GPS to nuclear power, synthetic elements, and quantum mechanics. The speed of such advancements and our knowledge of classical physics has led some to suggest that we might be reaching ‘the end of physics itself.’ But have we really learnt everything there is to learn?
One of the biggest unsolved problems are that of dark matter and dark energy. Despite being theorised to permeate all parts of existence, their natures and processes have almost completely eluded physicists. Rampant theorising regarding its nature in the media has clouded the mystery even further in the eyes of the public. It may seem like humanity has hit a dead end in its quest to understand dark matter and dark energy. However, this is not the case. We have come far in expanding our knowledge regarding the nature of dark matter and dark energy. Modern research has also allowed us to strongly support some theories and rule out others. So what do we know about dark matter and dark energy, and what have recent studies revealed to us?
The first scientist to suggest the existence of dark matter was Dutch astronomer Jacobus Kapteyn in 1922.1 Then in 1933, Swiss astrophysicist, Fritz Zwicky, who studied galaxy clusters, determined the approximate total mass of the Coma Cluster. Zwicky showed that the average galaxy mass differed from the mass expected. By estimating that the cluster had about 400 times more mass than was visible, he obtained evidence of an unseen mass, and therefore unseen matter, that doesn’t interact with light he called Dunkle Materie (dark matter). Zwicky inferred this was providing the mass and the gravitational attraction to hold the Coma Cluster together.2 In the 1960s and 1970s, Vera Rubin and Kent Ford worked to measure the velocity curve of spiral galaxies, which showed that stars at the edge of the galaxy moved faster than expected.3 The results showed that most galaxies must contain about six times as much dark as visible mass. Soon, dark matter was recognized as a major unsolved problem in astronomy.
Cold Dark Matter
In the most widely accepted model of dark matter, the Universe is dominated gravitationally by cold dark matter (CDM), with only a small portion being composed of stars, planets and living organisms. In the cold dark matter theory, small objects collapse under their self-gravity first to merge together to form larger and more massive objects. Dwarf galaxies are a great example since they were created in the early Universe and are now used to form larger structures.4 This model is in general agreement with observations of the Universe’s cosmological largescale structure. Most cosmologists favor the cold dark matter theory as a description of how the Universe went from nothing to its present day large-scale structure, including galaxies and their clusters. Cold dark matter is believed to be composed of some sort of particles with unobservable qualities. These particles have been hypothesized to be weakly-interacting massive particles (WIMPS), massive compact halo objects (MACHOs), or sterile neutrinos.5
WIMPS with Gamma Rays
The candidate regarded as most promising for the composition of dark matter is called WIMPs. The idea of WIMPs originated from the notion of a new elementary particle, which interacts through gravity and weak nuclear force. WIMPs are thought to have been produced in the early universe and are a probable component of cold dark matter.6 Today, experimental efforts to confirm WIMP theory include detection of the division products of WIMPs, including gamma rays and cosmic rays in nearby galaxies and clusters. Experiments also include measuring the collision of WIMPs with nuclei in a laboratory and to directly produce WIMPs in colliders. WIMPs annihilate each other, creating a cascade of particles and radiation that includes gamma rays.7 However, the existence of WIMPs in nature still remains hypothetical.
An alternate theory of dark matter, produced by astrophysicist Kim Griest, is called Massive Compact Halo Objects (MACHOs), which is any kind of astronomical body that might explain the presence of dark matter in galaxy halos. A MACHO is a body composed of matter that emits little or no radiation and drifts through space without a planetary system, such as black holes or dwarf planets. The search for MACHOs consists of using the objects between a distant light source and an observer as a lens which bends the light from the source. as the light travels towards the observer (gravitational lensing). This gravitational lensing is used to detect the effects of MACHOs on background galaxies.8 The EROS2 collaboration conducted observations with the Hubble Space Telescope’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to analyze MACHOs that are found not to contribute significantly to the composition of dark matter. This suggests that the mystery of dark matter will not be solved by MACHOs and are not likely to account for the large amounts of dark matter present in the Universe.
Sterile Neutrinos: Ghost Particles?
A third theory called sterile neutrinos are hypothetical particles that interact only through gravity. Some extensions of the standard model of the universe predict that the known neutrinos—electron, muon, and tau—can oscillate in and out of sterile neutrinos, which might not interact with any other ordinary matter.9 Physicists working in the Mini Booster Neutrino Experiment have found strong evidence for the existence of the sterile neutrino through nuclear or radioactive sources. Other groups have looked and found nothing, such as the IceCube collaboration, which operates a detector at the South Pole, and the now completed Main Injector Neutrino
Oscillation Search (MINOS) at Fermilab.9- 11 Currently there are numerous other dedicated experiments to settle the existence of sterile neutrinos as a component of dark matter.
While dark matter is responsible for the formation of large-scale structures, scientists have observed a mysterious process which seems to be ripping them apart. Termed ‘dark energy,’ this mysterious force or property is responsible for the acceleration of the Universes’ expansion. Similar to dark matter, its name is a bit of a misnomer since it does not interact with electromagnetic radiation and is hence more ‘invisible’ than ‘dark.’ Explanations of its nature are highly theoretical and involve potentially controversial concepts.
It first appeared in Albert Einstein’s field equations of General Relativity in the form of the Cosmological constant. He proposed it as modification to his equations so they would allow for a static universe. However, in 1929, Edwin Hubble, an American astronomer, discovered that galaxies outside the 10 Megaparsec (33 million light years, or 380 quintillion km) range were speeding away, causing emitted light to move to the redder side of the visible light spectrum. This movement was attributed to the expansion of space itself. Observations of supernovae redshifts in the 1990s further showed that the expansion was in fact accelerating, proving Einstein’s assumption of a static universe wrong. Since then, scientists have long postulated about the cause of the expansion, and various theories and models have been developed to explain its existence.
As a Property of Space: Vacuum Energy
The most commonly embraced theory of dark energy views it as an intrinsic property of space itself. Combined with the theory of Cold dark matter, it forms the Lambda-CDM (ΛCDM) model of our Universe, the most widely accepted one by cosmologists and astrophysicists. The idea stems from the notion that for objects in space to be accelerating, empty space must exhibit some sort of ‘negative’ energy that pushes itself apart, called Vacuum Energy. It is expressed as the Cosmological Constant (Λ), resurrecting Einstein’s ‘greatest blunder’ by assigning it a value that does not contradict the results of General Relativity and allows for a dynamic universe. Using the rate of expansion and E=mc2, dark energy can be said to constitute 68% of the Universe’s total energy density. However, quantum field theories explaining Vacuum Energy predict a Λ about 100 magnitudes higher than it currently is and requires an equally large gravitational factor to cancel out.12 While research continues to be conducted for alternate theories, the Lambda-CDM model yet remains the economical solution for both dark energy and dark matter.
Quintessence: The Potential Fifth Force
Quintessence refers to the idea that dark energy is a yet undiscovered field—a dynamic scalar field that acts in opposition to gravity. A field such as this is constant throughout space, explaining the fixed value of acceleration previously observed. However, its strength evolves over time. This contrasts from the Vacuum Energy model as, unlike the latter, it does not assume a property intrinsic to space but simply the existence of another field. A study conducted in Hungary in 2019 potentially detected the existence of a hypothetical particle called X17, using collisions between a beam of protons and Helium-3 atoms. The energies of the particles emanating from the collisions showed a discrepancy with expected results suggesting the involvement of a yet undiscovered force or creation of a particle inconsistent with the standard particle model of physics.13 Another paper published in 2019 by Indian scientists characterized the equations necessary to equate the existence of such a force.14 However, this model of a fifth force has not yet been been used to completely describe the mechanism of dark energy. Most models of Quintessence predict the runaway expansion of the Universe, resulting in the ‘Big Rip’ causing space to rip itself apart. However, there is no possible way to ascertain that scenario, and the scientific community at large does not accept the Big Rip model.
A Simple Sign Error: Negative Mass
One of the most fundamental axioms in physics is that mass can only hold positive values, which is why the notion of negative masses is an extremely controversial theory of dark energy. Instead of Vacuum Energy, it postulates that mass with negative values can have elusive and weird properties, such as repelling objects instead of attracting them. Both Einstein’s theory of General Relativity and the Standard Model of physics allows space for negative masses. However, attempts to reconcile them into a unified theory have so far been unsuccessful. Efforts to observe such a negative mass have been futile as well in lab-based experiments as well as observations from the Voyager probes. In 2018, a paper published in the journal Astronomy & Astrophysics proposed a ‘Dark Fluid’ with negative mass to explain both dark matter and dark energy. The Fluid at smaller scales would behave as dark matter and as dark energy at larger, galactic scales. Simulations of the negative mass model also form halos as dark matter has been theorised to do.15
Generic objects of dark energy, or GEODEs, is one of the more convoluted theories that has been proposed to explain dark energy. GEODEs are objects thought to be scattered in space that mimic black holes, except they have extraordinary cores filled with dark energy. Such objects would be impossible to differentiate from normal black holes and both are thought to form from the collapses of large stars.16 If small amounts of GEODEs were formed in the early Universe, they would be enough to explain the expansion of space we see today. There is not yet serious consideration for them though, as some researchers expect inconsistencies between GEODE collisions and black hole collisions and hope to further constrain or rule out the GEODE theory.17
Are Dark Matter and Energy Even Real?
While scientists have explored various theories to explain the nature of dark matter and dark energy, none have been able to concretely marry those theories with current models of physics and reality. This inevitably brings up doubts in the scientific community: Are dark matter and dark energy even real? Do we completely understand current phenomena in physics such as gravity and quantum mechanics? Are these theories simply there to prevent physics as we know it from collapsing? Scientists have actually examined such possibilities, namely that our current understanding of gravity may not be correct.
Modified Newtonian Dynamics, or MOND, is a proposed model that tweaks Newtonian physics and General Relativity in a way that makes them account for the effects of dark matter and dark energy. This theory is just one of a collection of many called ‘Modified Gravity’ that redefines the way we consider major concepts in physics. These theories gained popular support when they were able to explain holes in observations that particle cold dark matter theories were not. For example, MOND was able to predict star velocity curves more accurately than any dark matter theory present, and was able to account for ‘low-surface brightness galaxies,’ galaxies that have a smaller stellar population yet high amounts of gravitational lensing (bending of light passing near the galaxy).18 So this must be the fix? Dark matter, and by extension, dark energy is just a farce. We just need to re-evaluate our previous assumptions right? Not so fast. MOND and ‘Modified Gravity’ as a whole has serious flaws. For one, it is unable to account for the web-like structure of the Universe, whereas cold dark matter is actually one of the only models that can. Also, if gravity worked differently than we think it does, the distribution of heavy elements in the universe would be considerably higher.19 These major discrepancies have led the scientific community to mostly reject the concept of alternate gravity. The message is clear: dark matter is definitely out there.
What Does All This Mean For The Future?
Modern physics is definitely not yet complete, and we might not see the conclusion of this undertaking in our lifetimes, These are just a few theories that have been proposed to decode the grand riddles of dark energy and matter. Like most ongoing fields of research, and contrary to clickbait headlines, we aren’t even close to unfolding this mystery. It’s in the nature of science itself that all theories and knowledge must be continuously challenged and tested. After all, science is like the mythical Hydra: if we answer one question, ten more take its place. Even if explanations like Modified Gravity further complicate things, it still illustrates the importance of the scientific method in respectfully uncovering facts from the fog of uncertainty that surrounds every modern day mystery.
Each discovery has the potential to revolutionize the totality of science as they have in the past. Research is going on stronger than ever before. We know more about the structure and composition of the Universe, black holes, and gravity than we could have ever hoped to know. Rest assured, the research isn’t going anywhere and we can all watch with bated breaths as we uncover the secrets of the deep Universe.
1. Kapteyn, and J. C. “First Attempt at a Theory of the Arrangement and Motion of the Sidereal System.” NASA/ADS, ui.adsabs.harvard.edu/abs/1922ApJ….55..302K/abstract.
2. Zwicky. “On the Masses of Nebulae and of Clusters of Nebulae.” NASA/ADS, ui.adsabs.harvard.edu/abs/1937ApJ….86..217Z/abstract.
3. Rubin, et al. “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions.” NASA/ADS, ui.adsabs.harvard.edu/abs/1970ApJ…159..379R/abstract.
4. “The C star population of DDO 190” Astronomy & Astrophysics, 6 Oct. 2005, https://www.aanda.org/articles/aa/pdf/2006/08/aa2829-05.pdf
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6. Tate, Jean. “What Are WIMPs?” Universe Today, 19 May 2017, www.universetoday.com/41878/wimps/.
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8. Gravitational Lensing, w.astro.berkeley.edu/~jcohn/lens.html.
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10. “Sterile Neutrinos Are a No-Show in MINOS Experiment.” Physics World, 11 Mar. 2019, physicsworld.com/a/sterile-neutrinos-are-a-no-show-in-minos-experiment/.
11. Ice cube sterile neutrino: Blennow, M., Fernandez-Martinez, E., Gehrlein, J. et al. IceCube bounds on sterile neutrinos above 10 eV. Eur. Phys. J. C 78, 807 (2018). https://doi.org/10.1140/epjc/s10052-018-6282-2
12. Carroll, Sean M. “The Cosmological Constant.” Living Reviews in Relativity 4.1 (2001): n. pag. Crossref. Web.
13. Krasznahorkay, Attila & Csatlos, Margit & Csige, Lorant & Gulyas, J. & Koszta, M. & Szihalmi, B. & Timar, Janos & Firak, D. & Nagy, A. & Sas, Nandor & Krasznahorkay, A.. (2019). New evidence supporting the existence of the hypothetical X17 particle.
14. Sidharth, Burra & Das, Abhishek. (2019). Zitterbewegung field and the fifth force.
15. Farnes, J. S. “A Unifying Theory of dark energy and dark matter: Negative Masses and Matter Creation Within a Modified ΛCDM Framework.” Astronomy & Astrophysics 620 (2018): A92. Crossref. Web.
16. Croker, Kevin & Nishimura, Kurtis & Farrah, Duncan. (2019). The GEODE mass function and its astrophysical implication.
19. “Only dark matter (And Not Modified Gravity) Can Explain The Universe”. Ethan Siegel on March 6, 2018. Forbes