Chương 5: SPACE, DARKNESS, MATTER, AND STARS
Sent into space to pursue the dark and mysterious structures of the universe, Euclid's mission is both difficult and important. It is estimated that 95% of the universe consists of dark matter and energy. Researchers have a hard time finding evidence for these dark structures because their presence causes minute and subtle changes in the appearance and motion of objects we can observe. For this reason, the information that can be obtained from the data regarding the celestial bodies that Euclid would observe in the depths of the universe is quite valuable and important.
The Euclid space mission was organized and prepared by the European Space Agency (ESA). 15 countries and 2,000 scientists contributed to the preparation of this space telescope, its cameras and other equipment. Euclid is a space observatory that includes a 1.2 m diameter telescope and two scientific instruments that will record in the visible and infrared regions of the electromagnetic spectrum. This observatory, which has both imaging and spectral capabilities, is planned to continue its observations for at least six years at the Sun-Earth Lagrange-2 point, 1.5 million km away, in the neighborhood of the Gaia and James Webb satellite telescopes. Euclid will be able to take images at least four times sharper than those taken by advanced telescopes on Earth.
The first images show that the Euclid Space Telescope (EUT) and its cameras are performing extremely well, and that astronomers can use it to study the distribution of matter in the universe and its changes on the largest scales. Sharp images covering a large area of sky (larger than the area covered by the full moon) have the potential to reveal traces of darkness and hidden things. Euclid will provide us with information about the physics of stars and galaxies, as well as dark matter and energy research. Routine scientific observations of Euclid will begin in early 2024, following final fine-tuning.
Over the next six years, the EUT will make observations to reveal the shapes, distances and motions of billions of galaxies out to about 10 billion light-years away, in order to reveal dark influences or traces in the observed universe, thus generating data to create the largest three-dimensional cosmic map ever made. As the mission continues, Euclid's data will be released once a year and made available to scientists. What is special about the EUT is its ability to photograph a very wide area in a single image very sharply, in the visible and infrared energy regions. The first images from Euclid clearly demonstrate these important features.
The Dark Side of the Universe:
Dark matter holds galaxies together, causing them to spin faster than expected based on the amount of visible matter they contain. Additionally, scientists find the existence of this unknown or unobservable matter among the galaxies possible because the bending of light from the galaxies behind it (gravitational lensing) can be detected through observations. Dark matter, also called missing mass, interacts only with light and normal matter through gravity. Euclid will make observations to understand how dark matter is distributed. It will measure how the structure of matter in the universe is changing and the effects of gravitational lensing.
Astronomical measurements in the 1990s showed that the rate of expansion of the universe was increasing. This was not an expected situation because it was impossible to explain it with the physical knowledge of that time. The universe has been expanding continuously since the Big Bang, but it was assumed that this rate of expansion would slow down over time due to the gravitational pull of all matter in the universe. Based on the name dark matter, the source of this acceleration in expansion was called "dark energy".
According to current research results obtained using data from ESA's Planck mission, dark energy contributes 9668 percent to the matter-energy budget of the universe. Dark energy is added to theoretical models as a "cosmological constant" to explain the current expansion rate of the universe. However, dark energy may not be constant and may change over time. Perhaps dark energy is a new fundamental force that unifies the four forces we currently know of: electromagnetic, weak and strong interactions, and gravity. Today, understanding the nature of dark energy remains one of the most difficult research topics in cosmology and physics. Euclid will map more than a third of the sky using the distribution of galaxies over the past 10 billion years of cosmological history. Looking back in time could provide important clues to understanding how dark energy is accelerating the expansion of the universe. Euclid, which is expected to measure the acceleration of the universe much more precisely, may show whether the dark energy used as the cosmological constant is constant or not. In addition, the theory of general relativity will be tested for the first time on such a large scale and over such a long time period, thanks to Euclid's observations.
Euclid will allow cosmologists to investigate these two competing dark mysteries: dark matter and energy.
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Dark Matter
Various data regarding the large-scale structure of the universe cannot be explained by physical theories currently accepted as correct. One of the hypotheses proposed as a solution to this problem is that the universe is filled with a type of dark matter that cannot be seen because it does not interact with light (via the electromagnetic force).
One of the prominent ideas regarding dark matter is that dark matter consists of massive elementary particles (WIMPs) that have not yet been discovered and interact via the weak force.
Dark matter is a type of matter proposed to explain observations in cosmology and astronomy. Dark matter particles cannot be observed directly because they do not interact with light, but their existence can be understood through the effects they cause in their environment. It is thought that approximately 84% of the total amount of matter in the universe is dark matter. The nature of the particles that make up dark matter is still a matter of debate today. Many research groups are working to identify dark matter particles, either directly or indirectly.
There is a lot of observational data indicating the existence of dark matter. Firstly, in order to explain the change in the rotation speeds of celestial bodies around the centres of galaxies depending on the distance to the centre of the galaxy, the amount of matter interacting with light alone is not sufficient. The reason for this situation, called the missing mass problem, is thought to be dark matter particles that cannot be directly observed because they do not interact with light.
Another observational phenomenon indicating the existence of dark matter is related to the bending of light in space. The theory of general relativity states that mass curves space. The fact that light rays are affected by the curvature of space causes some celestial bodies to appear larger than they are.
This phenomenon, called gravitational lensing because it is similar to lenses making objects appear larger than they are, allows the amount of mass contained in a system to be calculated by examining only its geometry. Observations of galaxy clusters also indicate the existence of dark matter. For example, it is calculated that the amount of dark matter in the Abell 2009 galaxy cluster is more than 1014 times the mass of the Sun.
The debates and research on the nature of dark matter are still ongoing. Some of the discrepancy between observations and theoretical calculations based on the existence of ordinary matter alone may be due to ordinary matter, which is very difficult to observe because it emits very little light. However, there is an upper limit to the amount of ordinary matter that could have been produced by the Big Bang, and this amount is not enough to explain the observations. Although there are theories that attempt to explain the data by modifying Newton's and Einstein's laws of gravity, it can be said that the dark matter hypothesis is widely accepted among physicists. Among the dark matter particles that are suggested to exist, there are those that have only gravitational attraction with other particles. Among the dark matter particles that are suggested to exist, there are particles and axions that interact with other particles only through gravity and the weak force (one of the four fundamental forces).
A great deal of research is being done to observe dark matter particles and determine their characteristics. These studies can be divided into two categories: direct observations and indirect observations.
In direct observation studies, which are usually conducted in laboratories built underground, attempts are made to determine the scattering of dark matter particles from atoms in detectors. In indirect observations, the products that may form during the decay or destruction of dark matter particles are investigated.
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Dark Stars
The source of energy emitted by ordinary stars is fusion reactions. During nuclear reactions that occur in the cores of stars like the Sun, protons fuse to form alpha particles (helium atom nuclei), releasing a large amount of energy.
In an article published in Physical Review Letters in 2008, Douglas Spolyar, Katherine Freese and Paolo Gondolo claimed that among the first stars to appear in the universe, there may have been stars powered by WIMP-type dark matter particles that annihilate each other and turn into energy.
The researchers also proposed a mechanism in their article regarding how these stars formed. In short, the formation process of dark stars proceeds as follows: In the first galaxies that were forming, there were also high-density dark matter clumps as well as hydrogen and helium. The hydrogen and helium gases cooled and compressed into a small volume, pulling the dark matter towards them. As the density increased, the dark matter particles began to annihilate each other and turn into energy more frequently. The resulting high temperatures prevented the gas cloud from becoming dense enough for fusion reactions to begin. But the gas cloud continued to grow by capturing hydrogen, helium and dark matter from its surroundings. This created dark stars that are much less dense than ordinary stars but much brighter, powered by the annihilation of dark matter particles and their luminosity. The researchers estimate that dark stars could have masses millions of times that of the Sun, and luminosities around ten billion times that of the Sun.
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Quadrantid Meteor Shower
Quadrantids are one of the meteor showers observed in the northern hemisphere in early January. The source of the shower is known to be Asteroid 2003 EH1, which has completed its orbit around the Sun in approximately 5.5 years and has a diameter of 3 km. This meteor shower is expected to reach its maximum activity on the night of January 3-4. Unlike most meteor showers, the intense period of the Quadrantids lasts only a few hours. Before the day ends, the point of origin of the shower will have passed 60 degrees above the northeastern horizon. Although meteor showers are usually named after the constellation in which they spread, the point of origin of the Quadrantids is in the Bootes Constellation. While the number of meteors that can be seen per hour for the Quadrantid shower is around 100, it should be noted that these numbers can vary greatly depending on the location of the observation due to light pollution caused by both moonlight and artificial light sources. In the early hours of January 4, the light of a first-quarter Moon will make it difficult to observe anything other than bright meteors.
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