The evidence of Dark Matter

The evidence of Dark Matter

Sam Wang

http://www.symmetrymagazine.org/article/december-2013/four-things-you-might-not-know-about-dark-matter

        Dark matter has always been a mysterious matter to human, and I write this blog to provide you some information about the evidence of dark matter. first of all, what is dark matter? The dark matter is the name given to mass that emits no detectable radiation, like the light. Although dark matter has not been directly observed, its existence is inferred from its gravitational effects such as the motions of visible matter, gravitational lensing and so on. The reason why dark matter is so interesting is that we have the evidence that it exists, but we have no method to detect or observe it, this feature gives us a feeling of mysterious. Now I am going to provide some evidence of dark matter.

       Here is a video, we can get a quick idea of the evidence of dark matter.

https://www.youtube.com/watch?v=Af0_vWDfJwQ

1. What is the evidence for Dark Matter in Milk Way？

         In Milky Way, we calculate the gravitational mass from observing the motions of stars and gas clouds in the disk as they orbit the center. The rotation curve of a galaxy shows how the velocity of stars around the center varies as the distance from the center increases. We know that in the solar system, the orbital speeds of planets decrease as their distance to the Sun increase. This drop-off speed with distance occurs because virtually all the mass of the solar system is concentrated in the Sun. Based on this theory, if the mass of Milky Way in concentrated at the center, the relationship between orbital speeds and distance to the center should be described as line A in the figure. But the real situation is most spiral galaxies show flat rotation curves out as far as we can trace them, like line B, even where no more stars are visible. Orbital speeds in the Milky Way’s outer regions, therefore, imply that a large amount of dark matter lies beyond our galaxy’s visible regions.

https://www.e-education.psu.edu/astro801/content/l8_p8.html

2. What is the evidence for Dark Matter in other galaxies?

         We determined the amount of dark matter in a galaxy by comparing the galaxy’s mass to its luminosity. The process is that we use the galaxy’s luminosity to estimate the amount of mass that the galaxy contains in form of stars. In the other word, the total mass of visible things. Secondly, we can use the orbital velocities of stars and gas clouds we observed to calculated the galaxy’s total mass. Finally, we compare this value with the total mass of visible staffs. If this total mass is larger than the mass of visible stars, then we can infer that the excess mass must be dark matter. And in fact, we know that the orbital speeds in the outer region of other spiral galaxies indicate that they, too, harbor lots of dark matter.

3. What is the evidence for dark matter in clusters of galaxies?

*There are 3 ways to determine that dark matter occurs in the clusters of galaxies.

1. measuring the speeds of galaxies orbiting the center of the cluster. And then compare this mass to the cluster’s luminosity, and we can find that clusters of galaxies have much greater masses than their luminosities would suggest, and therefore we can infer that galaxy clusters contain the huge amount of dark matter.

2. studying the X-ray emission from hot gas between the cluster’s galaxies. The hot gas can tell us about dark matter because the temperature of the hot gas depends on the total mass of the cluster. And in fact, the temperature measurements of hot gas also tell us the amount of dark matter in the cluster.

3. observing how clusters bend light as graviton lenses. Graviton lensing occurs because masses distort the space around them. Massive objects can therefore act as gravitational lenses that bend light beams passing nearby. We can measure the masses of objects by observing how strongly they distort light paths.

http://imagine.gsfc.nasa.gov/news/22aug06.html

Finally, here is a video that may help you understand the evidence of dark matter.

https://www.youtube.com/watch?v=XCwWxrx1SIU

THANK YOU FOR READING THIS BLOG!

Sources

http://www.astro.cornell.edu/academics/courses/astro201/dm_evidence.htm

https://www.thunderbolts.info/wp/2012/02/29/essential-guide-to-the-eu-chapter-10/m33-rotation-curve/

https://www.e-education.psu.edu/astro801/content/l8_p8.html