During the last decades, astronomers have been faced with a bittersweet reality: the more scientific progress we make, the more mysterious the universe appears. This is because with the development of technology, we are able to obtain data with unprecedented accuracy, which, far from validating our advances in astronomy, often contradict the theories upon which our knowledge is built.
Although discarding decades of theoretical work can be daunting, many curious minds are beginning to rethink the way we conceive of the universe and its complexity. In fact, our current cosmic model is being called into question, with several new theories now taking the stage.
ΛCDM: Our current model of the cosmos
The Lambda Cold Dark Matter model — ΛCDM model for short — is the parametrization of the structure of the cosmos that scientists have employed to provide an account of the otherwise mutually inconsistent properties of the universe.
According to this model, the cosmos is made of three major components: Dark energy (Lambda “Λ”), Cold Dark Matter “CDM,” and ordinary matter.
Dark energy is an unknown form of energy that affects the universe on the largest scales and accounts for 68 percent of its composition. Cold Dark Matter, on the other hand, makes up about 27 percent of the universe, and refers to cosmic elements that cannot be seen directly and that move slowly and interact weakly with other objects. Ordinary matter, in contrast, refers to everything we can see or detect with telescopes.
This cosmic model has helped astronomers explain the large-scale distribution of galaxies, as well as the abundant presence of hydrogen, helium and lithium — the lightest chemical elements. It has also provided a reasonable account of the existence and structure of the cosmic background radiation that fills all space, and the constant expansion of the universe.
However, this last cosmic property — the expansion of the universe — has become one of the theory’s Achilles heels. Although the ΛCDM model succeeded in explaining the continuous motion of galaxies away from Earth — attributing it to the workings of the mysterious dark energy (Λ) — systematic estimates of the expansion rate have revealed inconsistencies in the current cosmic model that could render it obsolete.
Two puzzling cosmic properties
Techniques for estimating the rate of expansion of the universe — known as the Hubble constant (Ho) — have improved greatly in recent years, and although measurements have become more accurate, astronomers have found that the difference in values is growing larger and larger. This discrepancy has been called the Hubble Tension.
Similar discrepancies have been observed for another cosmic property known as the sigma-8 (σ8) value — the degree to which matter clumps together throughout the universe. In this case, the measurements predicted by the ΛCDM model and the empirical data differ with a statistical significance of about 2σ to 3σ. In fact, research has found that our universe may be thinner than we suspect. Moreover, different observational techniques often yield different values.
Discovering that the ΛCDM model no longer satisfies the cosmological principle, which states that the universe looks the same in all directions — isotropy, and in all locations — homogeneity; astronomers wonder whether the problem lies in our basic understanding of the cosmos.
Alleviating both tensions
The Hubble and sigma-8 discrepancies have become long-standing mysteries to scientists, who have noted that these two parameters are so intrinsically correlated that once one is resolved, the other diverges even more.
“You fix one problem and another problem pops up,” Arsalan Adil, a PhD student in theoretical physics at the University of California, told Vice. “There’s many many proposals out there that fix this [Hubble] discrepancy, but then they make the sigma-8 discrepancy much worse.”
Trying to solve the enigma, Adil and his colleagues have proposed a new theory called “Quintessence” which promises to alleviate both tensions simultaneously. According to this theory, the vast majority of the universe is composed of an undiscovered substance called quintessence, which, in simple terms, is a form of dark energy with striking physical properties and novel behavior.
Quintessence is repulsive to gravitational forces. This means that instead of causing the expansion of the universe to slow down due to the attractive force of gravity — as other forms of energy such as matter and radiation would — quintessence causes the expansion of the universe to accelerate.
This hypothetical type of dark energy differs from the ΛCDM model in that it is dynamical, i.e., it changes with time, which resolves the Hubble discrepancy among other tensions. And although this theory does not always agree with observational data, it has shown that our current cosmic model can be outperformed by another standard cosmological model.
Rethinking the universe
Faced with the inconsistencies of our current model, scientists have begun to think of ΛCDM as just another good approximation to the structure of the cosmos and a starting point on a long journey to understand the elusive nature of the universe.
With Hubble and Sigma-8 being just the tip of the iceberg, there are other discrepancies that remain to be explained on a large scale, such as the lower-than-expected amounts of lithium in the universe, the existence of supervoids and the possibility of the universe having a closed shape.
In a field where questions abound more than answers, our seemingly frustrating uncertainty may prove beneficial to our cosmological investigations, where our ways of studying and understanding the universe may be the obstacles themselves.