Dark Matter
Introduction
Dark matter is a theorized form of matter that is believed to measure for about eighty per cent of the matter in the universe, and accounts for about twenty-five per cent of its total energy density of the known universe. The name “dark matter” refers to the fact that it does not seem to interact with the electromagnetic radiation such as light and is invisible (or ‘dark’) to the entire electromagnetic spectrum, so it does not absorb or emit any electromagnetic radiation, making it extremely difficult to detect using usual astronomical equipment. Most of the dark matter is thought to not be a baryonic particle in nature, possibly being composed of some undiscovered subatomic particle. Dark matter has not been directly observed, but its presence is shown in many different observations, for this reason, most experts believe dark matter is to be omnipresent in the universe and to have had a strong influence on its structure and evolution.
Why does scientist believe in the presence in “Dark matter”? Scientist study dark matter by looking at how it effects visible matter and energy and believe that they account for the unexplainable motion of stars within galaxies. In 1997, a Hubble Space Telescope image showed light from a faraway galaxy cluster being bent around another cluster. Based on how the light was bent, scientist estimated the cluster to have a mass 250 times greater than the visible matter from cluster. This unusual force of gravity made the light bend in an unusual way, in which was not explainable as the mass of the visible matter could not weigh that much to generate that much gravity, hence why scientist believe that there must be some sort of matter present in which cannot be seen visibly, in which they call this theorised matter “dark matter” By measuring the angle of bending, astronomers can calculate the mass of the gravitational lens (the greater the bend, the more massive the lens). Using this method, astronomers have confirmed that galactic clusters indeed have high masses exceeding those measured by luminous matter and as a result, have provided additional evidence of dark matter.
Scientist also believe in the presence of “dark matter” through the mass distribution of Galaxies. The mass density of a spiral galaxy decreases as one goes from the middle of the galaxy to the outskirts of it. (only for the mass which is visible) And according to Kepler’s Second Law, the rotational velocity of an object as you go further from the centre should be lower than the rotational velocity in the middle of that object. This was predicted by scientist as when looking at a distant galaxy. However, they were wrong.
B represents the actual results measured of the galaxy rotational velocity and A represents what scientist thought would happen. The line A on the graph shows how the rotational velocity is less when you are more distant from the galaxy centre, but the actual results shown on line B shows how the rotational velocity of the galaxy from the centre to the outskirts are constant. If Kepler’s laws are correct, then the obvious way to resolve this unusual motion is to conclude that the mass distribution in spiral galaxies is not like that of the Solar System and that there is a lot matter in the outskirts of the galaxy that we can’t see.
What is Cold, Warm and Hot “Dark Matter” Astronomers have used NASA Chandra X-ray observatory to determine to the distribution of dark matter in huge cluster of galaxies. The astronomers observed a cluster of galaxies called Abell 2029, which is composed of thousands of galaxies in a giant cloud of gas. Most of the hot gas is mostly held together by the gravity from dark matter, so the distribution of hot gas is determined by how much dark matter there is and where. By accurately measuring the number of X-rays emitted from the hot gas and what part they are coming from, we can determine the distribution/where most of the dark matter is. From the astronomer’s result most of the dark matter was in the centre of a galaxy cluster. From outside of their data set, it indicates that eighty per cent of the universe consist of a form of Dark Matter called “Cold Dark Matter”. Cold and Hot Dark Matter can be distinguished not from their temperature, but from their speed at which dark matter particles were moving when galaxies began to form millions of years ago. Some models show how Dark matter would have been behaving at the time, where Cold Dark matter would have been moving slowly by the time of galaxy formation and hot dark matter would be moving quickly.
The image on the left is an image captured by the CHANDRA X-ray telescope(which is a galaxy cluster called Abell 2029), which shows how all the hot gas is located in the middle, as this is where the majority of the x ray is emitted from. The image on the right is captured by a DSS Optical, which captures an image of a galaxy cluster gamma radiation (Abell 2029). The cluster does not behave as scientist would expect it to, as if it was all visible matter then the right image would not have bright spots all over the place and would be all in the centre just like the left image. This show how the dark matter theory, which suggest that dark (invisible to direct observation) matter can interact with normal visible matter, exist.
Structure formation
Structure formation is the formation of galaxies, galaxy cluster and larger structures. The universe is now known from observations of the cosmic microwave radiation (which is an electromagnetic radiation as a remnant from an early stage of the universe in Big Bang cosmology), began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. Initially to the “Structure Formation”, the Friedmann solution describes a homogeneous universe. (Defined as “the same in all locations”) Later, small anisotropies (The property of being directionally dependant) gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, as it was the most dominant element of the universe at the very early times so the density perturbations (disturbance of density) of the ordinary matter should be washed out, unable to condense into structures of galaxies, galaxies clusters and larger structure. If they were only ordinary matter in the universe, there would not be enough time for density perturbations to grow into galaxies and clusters currently seen. Dark matter solves the solution to this problem, because it is unreactive to radiation. So, it’s density perturbations grow first and form galaxies and galaxy clusters.
What are WIMPS?
Dark Matter likely takes the form of a yet undiscovered subatomic particle, in which they are called Weak Interacting Massive Particles (WIMP). WIMPs represent one hypothesized class of particles. They neither absorb or emit radiation and don’t interact strongly with particles, but when they encounter each other, they annihilate and make by-products such as neutrinos and gamma rays, Fermi Nasa gamma ray telescope can detect most of this gamma ray dark matter annihilation from dwarf spheroidals and use this telescope in a way to research on the distribution of Dark matter by detecting the amount of gamma ray made from Dark matter WIMPs annihilating each other. The WIMP is classified as cold dark matter, because it is massive and slow.
How does it affect the growth of black holes and effect life on Earth
Every galaxy is surrounded by Dark matter that weighs as much as a trillion suns and extends for thousands of light-years (a unit of astronomical distance equivalent to the distance that light travels in one year). Every galaxy has a black hole in the centre of it and the heavier and larger the galaxy is, the bigger its black hole. Astronomers studied elliptical galaxies to understand the connection between a galaxy and its black hole. Dark matter somehow influences black hole growth and the formation of galaxies. Astronomers figured this out by studying more than three-thousand elliptical galaxies. They used star motions to weigh the galaxies’ central black holes. They used X-rays to measure the amount of hot gas present. Knowing the amount of hot gas surrounding the galaxy helped weigh the dark matter, because the more dark matter a galaxy has, the more hot gas it can hold onto. They found a distinct relationship between the mass of the dark matter and the black hole mass. This relationship can most likely be connected to how elliptical galaxies grow. An elliptical galaxy is formed when smaller galaxies combine, their stars and dark matter mingling and mixing together. Because Dark Matter weighs more than everything else, it moulds the newly formed elliptical galaxy (WHY) and guides the growth of the central black hole. The merger creates a gravitational blue print (WHAT?) that the galaxy, stars and black hole will follow in order to build themselves
It seems that Dark Matter could also build up within Earth’s core and eventually they would annihilate each other, generating a considerable amount of heat. The heat accumulated in the Earth’s core could trigger volcanic eruptions, mountain building, magnetic field reversals (what does this mean?), and changes in sea level.