Crossing the phantom divide

At the risk of misleading those who stumble upon them, words by their depth and beauty can incite a naive wonder in one’s mind. The expression “crossing the phantom divide” reminds me of how much I feel poetry and philosophy are an intrinsic part of science. Human nature in its basic expression draws from the well of imagination and relies on the traditional pillars of learning.  It is challenging to write a post on another mystery involving abstract ideas based on theoretical analysis. In general terms, it seems natural to start by defining what energy is. Energy is “the substance from which all elementary particles, all atoms and therefore all things are made, and energy is that which moves. Energy may be called the fundamental cause for all change in the world” (Werner Heisenberg, physics and philosophy, p.61). As it is for matter, energy has a dark side that appears to be hidden from us.  The expression “dark energy” has been used for other subjects as a metaphor when we talk about the brain’s dark energy or the dark energy of the Unconscious to refer to an energy whose source we are unaware of. Even Leonard Susskind concedes that dark energy has more of a claim to being called mysterious than dark matter “but the mystery has much more to do with its absence than its presence… The mystery is not why dark energy exists but why so little of it exists” (The Cosmic Landscape: String Theory and the Illusion of Intelligent Design, Leonard Susskind).

Studies of dark matter and dark energy are extremely complementary from both a technical and scientific standpoint. The difficulty in disentangling the two components of the dark sector raises the possibility that both dark phenomena can be unified into a single negative mass fluid. A unifying model has been widely discussed, most commonly describing a (flat) universe filled with a fluid obeying the Chaplygin equation of state.  A possible scenario that might explain how dark energy and dark matter interact is that dark energy may decay into cold (or even warm) dark matter. Dark matter probes highlight the interconnectedness of dark matter and dark energy research. The Large Synoptic Survey Telescope (LSST), a major endeavor by the National Science Foundation and the Department of Energy, was originally envisioned as a Dark Matter Telescope. In recent years, studies carried out with LSST have been more focused on dark energy.

The universe is said to be overwhelmed by a cosmic fluid with negative pressure. A great number of theories have been advanced, discussed, modified and corrected over the years. Among them, dark energy has been defined as the unvarying cosmological constant (Λ) introduced by Einstein, whose origin is still an open debate. It may also be a massive cosmological vector field filling the Universe,  a scalar field that drives the inflation at early epochs and survives to date to become quintessence, that is the fifth component of the cosmos in addition to cold dark matter, baryons, photons, and neutrinos.

After it was discovered in the 1920s that the Universe is expanding, it was determined in 1998  that the expansion was accelerating. But that was put into question almost 20 years later by another team of scientists who claimed that there is only marginal evidence for cosmic acceleration and that the data are still quite consistent with a constant rate of expansion.  Nevertheless, the discovery of cosmic acceleration led to the establishment of the Cosmological Constant + Cold Dark Matter (ΛCDM) model as the standard cosmological paradigm. Ongoing and planned cosmic surveys, such as the Dark Energy Survey Project, are designed to test the aforementioned model and to shed light on the mechanism behind it.

Researchers are applying brane cosmology to explain how dark energy plays into the current accelerated universe. Dark energy may well be a non-linear kinetic energy of the scalar field named k-essence or a tachyonic scalar field. The word “tachyon” was coined by Gerald Feinberg in 1967 to describe a particle with imaginary mass. Then again, dark energy may be a vacuum energy of holographic origin which seems to be confirmed by current observational data. It may even be composed of two parts: the cosmological constant and the holographic dark energy.

After its scheduled startup in late 2019, the Dark Energy Spectroscopic Instrument (DESI) will begin a five-year observing run next year in the goal to measure the effect of dark energy on the expansion of the universe. It will scan one-third of the sky using an array of 5,000 swiveling robotscan to measure the velocities of 5,000 galaxies at a time. Another five-year project, this one called “Exploring Dark Energy through Cosmic Structures” (EDECS), ended in 2017. Its goal was to study the influence of Dark Energy clustering on the cosmic structure formation. To that end, the project developed new numerical algorithms to perform cosmological simulations.

As we are awaiting the Square Kilometer Array (SKA) to be operational early 2020s, the Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) based in South Africa will map nearly all of the southern sky in radio continuum and neutral hydrogen line emission and so doing, it will measure the expansion of the Universe from 12 to 7 billion years ago.

At the same time, a recent study has suggested that dark energy stars may be an alternative to black holes. The term “dark energy star” was coined by George Chapline when he proposed that gravitational collapse of objects with masses greater than a few solar masses should lead to the formation of a compact object called dark energy star with a much larger vacuum energy. The study may be viewed in the context of another paper which states that the important question concerns whether dark energy is a completely new physical entity or one which we already know, namely a gravitational energy within the vacuum of a closed gravitating system that has long been known to be a negative energy. It remains to be seen whether scientists currently analyzing the data gathered by the Event Horizon Telescope (EHT) will provide evidence that dark energy stars exist. EHT is a virtual Earth-sized telescope aimed to measure the size of the emission regions of two supermassive black holes, Sagittarius A* at the center of the Milky Way and M87 in the center of the Virgo A galaxy.

Finally, we know that dark energy is embedded in space, counteracting gravity. The way gravity and antigravity are interacting in my mind is somehow related to time. With the understanding that time’s arrow is perspectival,   I  picture the negative-energy particles of the dark sector traveling backwards from the future somehow meeting at the intersection of past and future those positive-energy particles traveling forward in time as if they both were traveling the same distance in their determination to meet. That is how I see a cosmic coincidence unfolding. Lastly, I imagine a phantom energy to be something that appears to have no physical reality and still is ultimately real. Raising the concept of a divide begs the question of what lies on the other side and what circumstances enable its crossing. The modified gravity approach is the focus of research and may be the key to unifying both components of the dark sector, a path to solving the coincidence problem.