Astronomers succeeded in compiling a map of the distribution of dark matter in the universe, with Hyper Suprime-Cam observation data mounted on the Subaru Telescope!
The dark matter in question is nothing but the invisible matter that dominates the constituent material of the universe. Estimated, 85% of the compilers of the universe are dark matter that can only be detected through the gravitational influence of the material. Because of its abundance in the universe, the gravity of dark matter also dominates the formation of the universe on a large scale. Among these are galaxy clusters.
To be able to recognize dark matter in the universe is certainly not an easy matter. Gravity is the only trace to trace the existence of this mysterious material. For that, the astronomers traced it by observing the distribution of galaxies in the universe.
Galaxies or galaxy clusters are generally located in the middle of the halo whose contents are dark matter. The Milky Way is also known to have a halo that extends up to 300,000 light-years away. The existence of halo galaxies cannot be observed directly but again we can know its existence from the motion of stars and gas in the galaxy. Nonetheless, halo galaxies have an important role in the formation and evolution of galaxies.
Hunting Dark Material through Weak Gravity Lensing
In order to know the distribution of dark matter in the universe, astronomers use weak gravitational lensing techniques to find it. Thus, gravitational lensing does occur in the universe. On a small scale, we know there are planets found with microgravity lens techniques. For larger scales, massive structures, such as galaxies and galaxy clusters, can deflect light from objects behind it because of their very strong force of gravity.
Observations were made on galaxies and galaxy clusters. When the light of distant galaxies comes to Earth, they will pass through dark matter. Do not forget that this dark matter composes 85% of the universe and has mass. All objects that have mass will have gravity. The larger the mass, the greater the tensile force. And that means that the effect of levers is also stronger.
When light passes through the gravitational field of dark matter, a light will be deflected by the effects of a gravitational lens. However, the effect is much larger than it should be. It can even be more than 6 times stronger than the effect that normal matter should have on the galaxy. Light bend causes the distant galaxy image to be enlarged to become more extended and form an arc when the light passes near the foreground galaxy.
From here we can know that dark matter around the galaxy or cluster of galaxies that became the main suspect. The effects of this lenses are strong and some are weak. The effects of powerful gravitational lenses can be recognized by the eye as we see the resulting observational images. And usually, this only happens when massive galaxy clusters are between the observer's point of view and the light coming from distant galaxies. When the light from this distant galaxy passes by, it undergoes a deflection and shows the shape of the arc and some of the multiple images on the resulting image.
But, not all such cases. Gravitational lensing can be experienced by all galaxies though the effect is very small and is known as a weak gravitational lensing technique. The shape changes due to gravitational lensing can not be recognized by our eyes because they are so small. But, it can be measured and the information can show us how dark matter behaves in the universe.
To be able to recognize that small change, the cosmologists working in this field made some initial assumptions. Firstly, all galaxies are elliptical and have random orientation in the universe. If there is a gravitational lensing effect, the galaxy in the observed sky piece will appear parallel as its image is deflected and extended in the same direction.
Very small changes in galaxies are searched by using Hyper-Suprime Cam mounted on the Subaru telescope. Observations were made since March 2014 and the resulting dark matter distribution maps currently cover 11% of all maps to be made. Until now released, Hyper-Sprime Cam has completed 60% of the planned sky monitoring. Although only 11%, the resulting map is an important breakthrough because it covers a large sky area.
Tracing the Expansion of the Universe
Dark matter distribution maps created by these astronomers also have other goals. They want to track the accelerated expansion history of the universe. Initially, it was estimated that the expansion of the universe would slow down due to the interaction of matter in it. Apparently not so. In the late 1990s, astronomers managed to find out that the universe experienced an accelerated expansion since 8 billion years ago. The cause of the universe experiencing accelerated expansion is not yet known. And astronomers want to find out through the growth rate of the universe.
When the early universe, a matter is not uniformly distributed. This evidence can be seen in the observations of background radiation. There are temperature fluctuations that give an indication of the density fluctuations that continue to evolve as the evolution of the universe. These changes occur as a result of the interaction of gravity between matter in the universe on a large scale (the interaction of galaxies and galaxy clusters).
According to astronomers, we can know the history of the expansion of the universe from the rate of growth of large-scale structures in the universe. If the rate of expansion is large, then the material will be difficult to contract and the growth rate is suppressed.
But, the rate of growth of the universe on a large scale will not be known if only the observable matter is seen, in this case, stars and galaxies. There is dark matter that dominates the universe. That is why dark matter maps are created so that astronomers can reconstruct the distribution of dark matter in the foreground galaxies.
Reference :
- https://www.sciencedaily.com/releases/2015/07/150702112045.htm
- https://www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/astronomy-general/galaxies
- https://en.wikipedia.org/wiki/Void_(astronomy)
- http://www.esa.int/Our_Activities/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding
- http://www.spacetelescope.org/science/composition_of_universe/
- https://www.mpg.de/11170451/early-galaxies-dark-matter
- https://www.space.com/14176-dark-matter-biggest-map-unveiled.html
- https://theconversation.com/what-a-new-map-of-the-universe-tells-us-about-dark-matter-82074
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