Dark matter and dark energy are two of the most profound mysteries in contemporary physics. Despite the evidence supporting them, we still cannot directly observe them or confirm they exist. To show the scale of the problem, just 5 percent of the observable universe consists of regular matter, the remaining 95 percent is thought to be this dark matter and dark energy. In this interview with Hongsheng Zhao, the IAI’s Max Rogers, enquires about Zhao’s theory that dark matter and dark energy are really one thing: Dark Fluid. This theory may have the potential to help us understand quantum gravity and the origin of mass.

Where does the story of dark matter and dark energy begin?

Up until the 1970s, particle physics experienced rapid advancement through the continuous discovery of new particles. Once this phase of discovery reached its limit, physicists constructed a model known as the standard model, which neatly incorporates all the particles that had been discovered up to that point—almost all of them. But there were still several missing particles, notably the mass-giving particle called the Higgs Boson, as well as some cosmological issues. This is where dark matter and dark energy enter the picture.

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Dark matter and dark energy were introduced to explain important observations that the standard model failed to account for

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Why are they called ‘Dark’ and why were they introduced?

Particle physicists postulated the existence of additional particles, coined dark particles (i.e., dark matter), for theoretical completeness. What distinguishes dark matter from normal matter is its lack of interaction with the electromagnetic force: dark particles neither absorb, reflect, nor emit light. Consequently, they are particles which astronomers cannot directly observe in light; we can gather indirect evidence, only through their gravity, e.g., their gravitational bending of background stellar light.

Dark matter and dark energy were introduced to explain important observations that the standard model failed to account for. The foremost among these are the centrifugal acceleration in rotating galaxies and the accelerated expansion of the universe. Galaxies rotate at a disproportionately fast rate compared to the amount of gravity from ordinary matter present, prompting the introduction of dark matter to balance this excessive rotational centrifugal force. In other words, there must be additional gravitational effects at play that are invisible to us, explaining the rotational patterns of galaxies. Additionally, cosmological observations reveal that the universe's expansion is accelerating over time. Dark energy was posited to address this phenomenon. Now, we are finding more evidence on even larger scales - larger than galaxies - supporting the need for new physics with effects like both dark matter and dark energy.

We often hear that approximately only 5 percent of the matter in the universe is ordinary matter, with the other 95 percent being composed of dark matter and dark energy. How do we know this relative proportion with such precision?

Picture galaxies springing out of the Big Bang into a marathon with a huge spread in speed, the gravitational attraction of dark matter would make the relative speed between galaxies reduce with time, until an epoch when their speed gap widens again as galaxies “feel” new energy.  The specific ratio of matter and energies is parametrised to explain why we are seeing such an epoch in galaxies now.

What distinguishes dark energy from dark matter, and why are they frequently grouped together?

Historically dark matter and dark energy were conceived without a connection between them. As previously mentioned, dark matter was formulated to account for the rotational behaviour of galaxies, while dark energy was proposed to explain the universe's accelerating expansion. We label them as ‘dark’ descriptively, as we cannot directly detect them non-gravitationally. It is only in recent times that researchers have begun to explore the potential relationship between dark energy and dark matter, prompted by various observed discrepancies that are hard to be thought of as unrelated coincidences.

What is your theory of Dark Fluid?

The concept of dark fluid posits that dark matter and dark energy stem from the same underlying fundamental field. This perspective offers a significant advantage: it enables transitions between the two entities, allowing for the exchange of energy and their existence in various forms. To illustrate, consider how a photon can be understood both as a particle and a wave. Depending on the context, it may be convenient to perceive light as a particle or as a wave or a field. Similarly, with dark fluid, I'm respecting the conservation rules of physics, but I'm adjusting the scale of description and how we approach our understanding of dark energy and dark matter.

Are you are using the quantum phenomena of wave-particle duality as an analogy to describe the dark energy / dark matter relation?

In a somewhat heuristic manner, yes! Are you familiar with something called a soliton? In a narrow channel of water, if you induce a shock, the shock front can propagate smoothly as a single particle-like entity for long distances. In this scenario, it is convenient to conceptualize the volume of fluid as an iceberg-like particle, a localized packet of energy. However, as the channel widens, the soliton will no longer have the necessary conditions to persist, and it will begin to disperse like a wave. Different environments alter our perception of the same underlying hydrodynamical physics.

The proposition of dark fluid suggests that dark matter and dark energy also exhibit this behaviour—they exhibit the energy flowing between localised and dispersed forms.

If you were a betting man, how likely do you think it is that this theory is correct and why?

My perspective is that outside galaxies, dark matter will exhibit particle-like behaviour, while toward the centre of galaxies, it will behave like a wave. To have lot of dark particles in centres of galaxies would gravitationally rob the rotational energy of the stellar structure near the centre, impeding their rotation. If dark matter can adopt wave-like characteristics in the galaxy’s dense regions, it would mitigate this honey-like friction, thereby allowing for the observed fast rotation of the central stellar structure, so-called a galaxy bar. Consequently, from this perspective as reviewed by my postdoc and I, I believe with 99 percent chance that my theory is the explanation. The transition between particles to waves must happen inside a galaxy.

What are your ideas on the nature of Dark Fluid?

Despite advancements made through large particle colliders, such as solving the Higgs mystery, the expected discovery of dark particles did not materialize.  However, there is a known particle that the standard model doesn’t explain: neutrinos. Nothing in the model could give an inertia mass to allow neutrinos to stay at rest. However, experiments later discovered that neutrinos can transition from a massless to a massive state, enhancing their gravitational interaction capabilities.

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This transition from massive to massless neutrinos forms the foundation of my dark fluid theory, offering a potential explanation for the rotation of galaxies

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I propose that neutrinos transitioning between mass and massless states play a crucial role in facilitating a form of dark fluid. When envisioning dark fluid, I visualize dark matter as neutrinos zooming around in bound pairs, a pair of neutrinos bound by a gravity-like force. However, under high-density high-gravity conditions, these pairs can be disrupted, causing them to lose their particle-like behaviour and transition into a wave-like state, exhibiting massless properties. This transition from massive to massless neutrinos forms the foundation of my dark fluid theory, offering a potential explanation for the rotation of galaxies. Although the underlying mechanism is not yet fully understood, my theory provides one possible explanation for the neutrino phenomena.

As an analogy consider ice: it consists of water molecules forced to vibrate with fixed neighbours. However, neighbours become unbound when ice is subjected to pressure changes, so ice melts into a fluid state, i.e., the fluidity of the substance varies based on the environment. Similarly, one could liken dark fluid to a chameleon, adapting its properties depending on environmental factors.

What do you consider the most significant philosophical themes?

If some iteration of the theory proves correct, we may conclude that there are fields within the universe previously unknown to us—similar to the Higgs field but potentially composite states of neutrinos. I believe that unravelling this microscopic realm will not only elucidate the origins of dark matter and dark energy but also provide insights into the origin of mass and gravity.

So, you believe that dark fluid could offer insights into quantizing gravity as well? Could it assist in bridging the gap between quantum mechanics and general relativity?

It depends on the level of ambition. Currently, no one knows how to attribute a quantum mechanical origin to gravity. Some propose maintaining Einstein's gravitational field intact while modifying forces between matter. Others suggest altering gravity while leaving matter unchanged. There's also an intermediary approach, as in this pyramidal illustration of the four known forces of Nature, the weak and strong forces on tiny scales, and the electromagnetic and gravity interactions on cosmic scales. At the base of the pyramid is the mysterious mass of the neutrino, a possible missing link that unifies all the known forces. A visual illustration below.