[Science Paper] Research about Dark Matter.docx
[서론]
고등학교 물리시간때 선생님이 잠시 소개해주셨던 암흑물질에 대한 설명을 듣고난 후, 흥미가 생겨서 간단하게 조사를 통해 암흑물질의 정체가 무엇인지, 인류는 현재 무슨 연구를 하고 있는지 알아보았습니다.
[Introduction]
After hearing the explanation of dark matter that my teacher introduced for a while during high school physics class, I became interested and briefly investigated what dark matter was, and what kind of research humanity is currently doing.
“Don't assume that what we currently think is out there is the full story. Go after the dark matter, in whatever field you choose to explore.” Would you believe, if the universe you can see is only 4.6% of the universe? What composes the other 95.4% are called the dark matter and the dark energy, which are not visible to us.
What is Dark Matter?
In the 1930s, Fritz Zwicky, a Swiss astronomist, observed the star’s orbits at the outer areas of the Galaxy, and realizes that the speed of the stars was extraordinarily fast, exceeding the theoretical speed. The problem was that, if we follow Newton and Einstein’s law of gravitation, the rotation speed at the outer space should also be decreasing, which was contradictory to the theory. In this speed, the stars should have escaped the Galaxy a long time ago, however it was still tied to the gravity of the Galaxy. This meant that, the gravity of the Galaxy was strong enough to maintain the star’s extraordinary speed, which lead to a hypothesis that “something very heavy, with an enormous gravity was pulling the stars in”, which eventually became the first ideas of the “dark matter”.
Then what is dark matter, and how does it compose 26.8% of the entire universe, while the ordinary matters only take up 4.6% of the universe? The logic behind it is easy to understand if we use Kepler’s equation: M=rv2G, G=6.67×10-11 . If we calculate the mass of the universe using the equation with the observed distance and velocity of the stars, we reach a conclusion that we need quite a lot of mass, which means that “something” does exist in the assumption that the Kepler’s equation is correct. Scientists call this issue the “Missing Mass Problem”, and various answers from different perspectives have been made till now, without anyone knowing the actual answer to it.
Does dark matter exist?
Scientists can only assume its existence, and the reason behind it lies in its characteristics. Dark matter can’t be seen – or scientifically speaking – the dark matter doesn’t interact with light, meaning it can’t be detected by any types of light rays, from radio, microwaves, infrared to visible light, UV ray, X ray, and gamma rays. As we know, matter uses the four fundamental interactions of gravitational interaction, electromagnetic interaction, weak interaction and strong interaction to interact, but the dark matter is only interactable through the gravitational interaction.
These characteristics of the dark matter that it can’t be detected brings up a reasonable refutation – Does dark matter exist? [1]Isabelle Grenier and her colleagues came out with this idea, that the dark matter doesn’t exist, and it is only undetected cold gas. When we measure gases, we detect the radio waves emitted by the warmer gas. But the story becomes different if we talk about cold gases, which emits no detectable signal, researchers must “measure radio waves emitted by carbon monoxide and assume that the two molecules are found together in a certain ratio”. By using the Energetic Gamma Ray Experiment Telescope (EGRET) satellite, the team observed a dusty region in the galaxy, which “generate more gamma rays than expected, indicating that they contain vast quantities of cold gas that the radio studies had missed”. This observation concluded that the amount of ordinary mass is double the amount of what we have been observing, accounting for the extra gravity provided instead of a new concept called the dark matter. Now some scientists even came up with a theory stating that dark matter doesn’t exist as a matter, but just that the gravity in the far-outer space just has a totally different gravitational law from our universe. We assume that the [2]Cosmological Principle is true, that the universe is homogeneous and isotropic, but what if it is not? The theory based on this idea is called the [3]Modified Newtionian Dynamics (MOND), however it still doesn’t perfectly prove the existence of dark matter.
The observations made in the collision of galaxy clusters shows a different result for the distribution chart of gravity and gas, which can’t be explained by the MOND theory which states that how gravity is applied changes by the scale. Recent researches made by the astronomists showed that after analyzing the [4]“mass profiles for 36 Dwarf Disk Spirals and 72 LSB galaxies… Our analysis shows that the McGaugh et al. relation is a limiting case of a new universal relation that can be very well framed in the standard "DM halo in the Newtonian Gravity" paradigm”, meaning that the movements of the galaxies can’t be explained only by using the MOND theory, but the estimation of dark matter is necessary to explain the observations perfectly.
Now, there is another theory that modified the MOND theory, named the Newton Bubble Theory, which states that there are virtual bubbles around the matter with high density that applies the original gravitational law of Newton, and the areas outside the bubble has a different gravitational law. This explains the difference in the distributions of the gas and the gravity in the collisions of clusters – as the gas in the galaxy cluster doesn’t have a density high enough to produce the Newton Bubble, it will have a different movement with the galaxy cluster.
Finding dark matter
There has been recent research to find dark matter from across the world. Since it is still being debated over its existence, when a new possible theory or a track of evidence is found, the impact is huge – and scientists are still working on to find the clue of their existence.
Some scientists estimate that the dark matters are composed of Weak Interacting Massive Particles (WIMP), which are also only estimated to exist, just like the dark matter. WIMP does not interact with any of the ordinary matter nor light, however it does rarely interact with each other leaving a special evidence. As the signals produced are very weak and thus has a possibility of being covered by the signals of the Cosmic Ray, the experiments to detect the signals are made underground. The DAMA detector of the Laboratori Nazionali del Gran Sasso (LNGS), the largest underground research center in the world located at Italy, [5]“have recorded a seasonal variation in data…the seasonal change arises because the Earth is moving through a halo of dark-matter particles that surrounds the Milky Way, resulting in a repeating pattern.”
Conclusion
It has been around 80 years after the first question of the ideas of the invisible matter made by Fritz Zwicky and 50 years from the observations by Vera Rubin, but we still do not know what dark matter is, or does dark matter exist. Few astronomists even uses the term “Dark Gravity” instead of dark matter to clear any possible confusion or assumption that the dark matter exists. It might take days, months, years, decades and even centuries to find out what accounts for everything happening out there, but we believe that eventually, we will reveal what it is at last. We might find a signal from the WIMP detector someday, or maybe some scientist will find a massive amount of undetected ordinary matter, or maybe someone will come up with another theory that is so perfect that accounts for every situation. We are in a fight without knowing who we are fighting nor how it is going to end, ignorant of when it will end. “The discrepancy between what was expected and what has been observed has grown over the years, and we're straining harder and harder to fill the gap,” Jeremiah P. Ostriker.
[1] Cho, Adrian. "Dark Matter Need Not Apply." Science | AAAS, 31 Oct. 2014, www.sciencemag.org/news/2005/02/dark-matter-need-not-apply.
[2] "Cosmology." Astronomy Notes, www.astronomynotes.com/cosmolgy/s3.htm.
[3] "The MOND Paradigm of Modified DynamicsScholarpedia." www.scholarpedia.org/article/The_MOND_paradigm_of_modified_dynamics.
[4] Paolo, C. Di, Salucci, P. Fontaine, J. P. "The Radial Acceleration Relation (RAR): Crucial Cases of Dwarf Disks and Low-surface-brightness Galaxies." IOPscience, Mar. 11, iopscience.iop.org/article/10.3847/1538-4357/aaffd6/meta#artAbst.
[5] Castelvacchi, Davide. "Mystery of dark-matter signal deepens with replication attempts." nature - International Journal of Science, 19 Mar. 2019, www.nature.com/articles/d41586-019-00865-9.
Works Cited
Castelvacchi, Davide. "Mystery of dark-matter signal deepens with replication attempts." nature - International Journal of Science, 19 Mar. 2019, www.nature.com/articles/d41586-019-00865-9.
Cho, Adrian. "Dark Matter Need Not Apply." Science | AAAS, 31 Oct. 2014, www.sciencemag.org/news/2005/02/dark-matter-need-not-apply.
"Comparison of Rotating Disc Galaxies in the Distant Universe and the Present Day." YouTube, 15 Mar. 2017, www.youtube.com/watch?v=gjBcCYl27iM.
"Cosmology." Astronomy Notes, www.astronomynotes.com/cosmolgy/s3.htm.
Fontaine, J. P. "The Radial Acceleration Relation (RAR): Crucial Cases of Dwarf Disks and Low-surface-brightness Galaxies." IOPscience, Mar. 11, iopscience.iop.org/article/10.3847/1538-4357/aaffd6/meta#artAbst.
"The MOND Paradigm of Modified DynamicsScholarpedia." www.scholarpedia.org/article/The_MOND_paradigm_of_modified_dynamics.