The Foundation of the Dark Universe

Four hundred years ago, Newton articulated the laws of gravity. While this was a straightforward task—many had laid the groundwork for understanding gravity before him—his true brilliance lay in extending the application of gravity beyond Earth to the realms of the Moon and Sun.

Newton recognized that the Moon remained in orbit around Earth due to the same gravitational force that causes apples to fall from trees. He also posited that this force governs the motion of planets around the Sun. Thus, the cycles of the seasons, lunar phases, and the falling of fruit were all derived from a single fundamental principle of nature.

This led to the formulation of the universal law of gravitation, aptly named for its applicability to both celestial and terrestrial bodies. This revolutionary concept challenged prior views regarding Earth's position in the cosmos and laid the groundwork for modern scientific thought.

For centuries, Newton's universal law prevailed. Although some anomalies arose in Mercury's orbit, Einstein's refinements to gravitational theory in 1915 addressed those discrepancies. Gravity, even after modifications, remained a steadfast natural law applicable throughout the cosmos.

However, in the 1930s, astronomers encountered a new dilemma. They began identifying clusters of galaxies seemingly held together by gravitational forces. Yet when calculating the visible mass within these clusters, they found it insufficient to prevent disintegration.

Shortly thereafter, it was discovered that the Andromeda Galaxy was rotating more swiftly than Newton’s Law would predict based on its visible mass. This presented astronomers with a stark choice: either the law of gravity was not as universal as previously believed, or there existed a significant amount of unseen mass.

Given the reverence for universal laws of physics, the latter option became the more acceptable hypothesis. The challenge was estimating how much additional mass was required—approximately five times more than what was visible. While some of this may consist of dark objects like black holes and dim planets, tallying such an immense quantity remains complex.

Over the decades, physicists have proposed numerous theories about the nature of this "dark" matter, ranging from microscopic black holes to swarms of neutrinos. Yet, concrete evidence supporting these ideas has yet to materialize, leaving the true nature of dark matter—and whether it exists at all—uncertain.

The situation worsened in the 1990s when astronomers recognized that the universe, birthed from the Big Bang, was not merely expanding but doing so at an accelerating pace. Traditional theories suggested that this expansion would slow over time, perhaps even reversing and leading to a collapse akin to a backward Big Bang.

However, studies of supernovae dispelled that notion. The universe was indeed expanding, but at an increasing rate—something the known laws of physics could not explain. To address this discrepancy, physicists introduced the concept of dark energy, positing that it was the force driving this accelerated expansion.

The mechanisms behind dark energy remain elusive. In fact, it is significantly more mysterious than dark matter. As dark energy is presumed to be the dominant influence in the universe—outstripping even gravity—it highlights a critical gap in contemporary physics.

Today’s Understanding of the Cosmos

Cosmologists estimate that only five percent of the universe, when assessed by mass-energy, consists of the visible matter we can observe and interact with. The remaining ninety-five percent is dark, composed of dark matter and dark energy. This revelation poses a significant challenge to modern physics, which struggles to explain so much of existence.

Since the identification of dark matter in the 1930s, astronomers have traced its effects throughout the universe. Evidence of its presence can be seen in galaxy clusters, such as the one imaged by Euclid in Perseus. Dark matter is also invoked to explain gravitational lensing, where light bends around massive objects. It has even been credited with the formation of galaxies, stars, planets, and ultimately, life itself.

The most compelling evidence comes from the Cosmic Microwave Background (CMB)—radiation remnants from the Big Bang. Analysis of the CMB indicates that the early universe was nearly uniform, suggesting that matter was once evenly distributed across space.

Researchers believe that gravity led to the clumping of this matter into stars and galaxies over time. Yet, simulations based on the initial conditions suggested by the CMB revealed a problem: gravity and matter alone could not recreate the universe as we observe it. Only by incorporating dark matter and, later, dark energy, could researchers achieve a realistic model.

One particularly successful model, known as Lambda-CDM, is considered strong evidence for the existence of both dark matter and energy. It posits that a specific mix of these components was created during the Big Bang, along with ordinary matter, and that their interactions followed the laws of gravity.

Simulations utilizing the Lambda-CDM framework yield model universes that closely resemble our own, producing the correct proportions of atomic elements and accurately distributing galaxies and clusters across space. Many physicists believe this serves as compelling evidence that the universe is indeed governed by its dark components.

If this is accurate, the universe's fate appears bleak. An accelerating expansion could isolate galaxies, ultimately leaving them in vast voids of darkness. Following this, galaxies may disintegrate, followed by stars and planets, and eventually even atoms and black holes. The universe would be left devoid of form or life.

The Mission of the Euclid Space Telescope

As dark matter and dark energy elude direct observation, Euclid will seek out subtle indicators of their existence. This will involve scanning billions of galaxies, mapping them, and observing the distortions caused by these unseen forces.

Euclid's instruments are designed to capture images of galaxies in two types of light: visible light, which records galaxies in a manner perceptible to the human eye, and infrared light, which lies just beyond the visible spectrum.

Using its visible light camera, Euclid will document the shapes of over a billion galaxies. The presence of both dark and visible matter distorts light from more distant objects, allowing scientists to map the distribution of matter across the celestial sphere.

Due to the universe's expansion, nearly all galaxies are moving away from us, with those farthest away receding the fastest. This results in a slight stretching—and consequently, reddening—of the light waves emitted from them. The infrared camera will measure this "redshift" phenomenon.

By analyzing redshift data, scientists can infer two key aspects. Broadly, by assessing the redshift of millions of galaxies, they can deduce how the universe's expansion has evolved over time, illuminating the role of dark energy. More specifically, they can determine the distance of individual galaxies.

By merging the data from both cameras, Euclid will position galaxies within the night sky, creating the most extensive cosmic map to date. This will enable researchers to understand how galaxies cluster in the sky, a crucial metric for revealing the distribution of dark matter.

Ultimately, Euclid aims to enhance our understanding of the locations of dark matter in the cosmos and its influence on galaxy movement. Researchers will compare Euclid's findings with simulation results, validating—or challenging—models like Lambda-CDM. This will refine our comprehension of how dark matter and dark energy have shaped the universe.

In summary, Euclid represents a crucial endeavor to illuminate the two most significant enigmas in modern cosmology, aiming to unveil and chart the universe's hidden dark side. Should it succeed, it will produce a map of unparalleled detail and magnitude. Conversely, if dark matter proves to be illusory, it will prompt a reevaluation of the fundamental laws of gravity.

This story originally appeared in The Quantum Cat, a newsletter dedicated to space and science. Subscribe for free today!