[Reading Science] "Darkness, I'm curious about you"... Launched a 2 trillion won telescope

European Space Agency Launches Euclid Space Telescope
Exploring 95% of the Universe: Dark Matter and Dark Energy Investigation

In the 20th century, modern physics, armed with quantum science and the theory of relativity, seemed to have reached a completion stage by discovering the existence of atoms and tracing the evolution of the universe. However, it turned out that humanity had only uncovered less than 5% of the universe. The ordinary matter we know, that is, elements, constitute only a tiny fraction of the entire universe. The rest is filled with matter and energy that have only been confirmed to exist but are invisible and whose nature is unknown. This is why, over the past 50 years, 'dark matter' and 'dark energy' have emerged as major research topics in modern physics.

Measuring the Mass of Galaxies

Dark matter was discovered through scientists' challenge to measure the weight of galaxies. Scientists devised two methods to measure the mass of galaxies, which are too large to place on a scale. The first method is to estimate the number of stars by observing the brightness of the galaxy, called luminosity mass. Another method measures the speed of stars rotating within the galaxy to determine the strength of gravity and convert it into mass, known as dynamical mass. However, scientists made a surprising discovery here. When measuring the mass of the same galaxy using these two methods, the results were completely different. The value obtained from measuring the dynamical mass was 4 to 5 times larger than the luminosity mass. From this, scientists hypothesized the existence of some matter that does not emit light but fills the interior of galaxies.

In 1933, Swiss physicist Fritz Zwicky first proposed this idea. While observing the Coma Cluster (in the constellation Coma Berenices), located 320 million light-years from Earth, he discovered that galaxies on the outskirts of the cluster were rotating much faster than predicted. This was completely different from the hypothesis that galaxies closer to the center of the cluster, where mass is heavier, rotate faster, and those on the outskirts rotate more slowly. Zwicky named this invisible matter that interacts gravitationally but cannot be seen 'invisible matter.' The academic community's response at the time was cold.

However, in the early 1970s, American physicist Vera Rubin published research proving the existence of dark matter, making it a mainstream concept. Rubin measured the speeds of stars within our Milky Way galaxy, initially predicting that the center would have faster speeds and the outskirts slower. Surprisingly, she confirmed that the speeds were the same. She obtained the same results for about 200 other galaxies. This phenomenon could not be explained without the existence of 'dark matter.'

Additionally, phenomena such as the gravitational lensing effect, where galaxies with strong gravity bend light to magnify background celestial objects, the rapid rotation speeds of dwarf galaxies, the Bullet Cluster phenomenon occurring during galaxy cluster collisions, the large-scale structure of the universe, and cosmic microwave background radiation indirectly prove the existence of dark matter. These events would be impossible if galaxies were composed only of ordinary matter such as stars, gas, and cosmic dust observable with existing equipment, and the rest were empty space. Observations to date indicate that ordinary matter, i.e., atoms, make up less than about 5% of the universe, dark matter accounts for about 25%, which is 4 to 5 times more, and dark energy comprises about 70%.

(Source: NASA)

Dark Matter Created Stars and Galaxies

Dark matter played a decisive role in the formation of stars and galaxies in the early universe after the Big Bang. Theories explaining this include the large-scale structure of the universe and cosmic microwave background radiation theories. The universe began to cool rapidly 380,000 years after the Big Bang as it expanded. However, because it expanded so quickly, thermal energy was uniformly distributed throughout the universe regardless of location. This is known as the isotropy of the cosmic microwave background radiation. Observations of cosmic microwave background radiation by satellites such as WMAP and Planck confirmed that thermal energy was evenly distributed across the universe. However, there were clear differences between the hottest and coldest regions. This means that even in the early universe, some areas had more matter than others. Over a long time, these matter concentrations clumped and accumulated due to gravity, leading to the formation of stars and galaxies. In other words, the uneven distribution of matter in the early universe created stars and galaxies. But this explanation hit a wall. Ordinary matter, which accounts for only about 5% of the universe, cannot generate enough gravity for this process. It would require more time than the current age of the universe.

Therefore, astrophysicists explain that dark matter provided the gravity that clumped stars and galaxies together, making this possible. However, the exact nature of dark matter has yet to be captured. Physicists worldwide have been conducting research for decades using detectors placed underground and in Antarctica, but no results have been achieved yet. Only six candidate substances have been theoretically proposed: axions, sterile neutrinos, WIMPs, SIMPs, primordial black holes, and others. However, some properties have been somewhat identified. Dark matter does not interact with visible light or any electromagnetic force, or interacts so weakly that it cannot be seen. It has existed since the birth of the universe and is stable. Because it interacts gravitationally, it has mass and is presumed to be slower than light, heavy, and cold. In South Korea, the Institute for Basic Science (IBS) has installed the Yemi Lab with detectors 1,000 meters underground in Jeongseon, Gangwon Province, to observe dark matter. KAIST is also conducting research on axions.

The Force of Cosmic Expansion, Dark Energy

Dark energy, which is more abundant than dark matter, is an entirely different entity. While dark matter clumped ordinary matter to form stars and galaxies, dark energy is anti-gravitational energy that causes the universe to expand. In 1998, scholars such as Saul Perlmutter and Brian Schmidt discovered that the expansion of the universe is not proceeding at a steady rate but is accelerating. Using supernovae with consistent brightness, they measured the distances to very distant galaxies and calculated the rate of cosmic expansion, which was found to be increasing. This led to the hypothesis that an invisible energy exists in the vacuum of the entire universe that accelerates expansion, named 'dark energy.' However, there are still many unresolved issues, including criticisms that the cosmic distance measurements using supernova brightness, which formed the basis of the dark energy hypothesis, might be flawed.

Research to unravel the mysteries of dark matter and dark energy is active. A representative example is the Euclid space telescope launched by the European Space Agency (ESA) on the 2nd of this month. It is a state-of-the-art space telescope built at a cost of $1.5 billion (about 1.95 trillion KRW). Its core mission is to create the largest-ever 3D map of the universe by observing up to 2 billion galaxies distributed over more than one-third of the sky by 2029, using a large visible light imager (VIS) and near-infrared spectrometer and photometer (NISP).

Senior Researcher Ibo Mi of the Korea Astronomy and Space Science Institute explained, "What we can see accounts for only 2 to 3% of the entire universe, and the rest is composed of dark matter and dark energy. Using the high-resolution Euclid space telescope, we can observe distant galaxies and analyze how dark matter and dark energy in the universe have changed over time."

Euclid Space Telescope launched by the European Space Agency (ESA) (illustration). Photo by ESA website