Dark Matter

Either dark matter exists or we do not understand how gravity operates across galaxy-sized distances. We have many reasons to have confidence in our understanding of gravity, so the majority of astronomers believe that dark matter is real.

Measurements of the mass and luminosity of galaxies and galaxy clusters indicate that they contain far more mass in dark matter than in stars.

Despite the fact that dark matter is by far the most abundant form of mass in the Universe, we still have little idea what it is.

MACHOS = Massive Compact Halo Objects Trillions of faint red stars, brown dwarfs and Jupiter-sized objects left over from the formation of the Milky Way still roam our galaxies halo, providing much of its mass. Brown dwarfs are “failed stars” that did not have enough mass to sustain nuclear fusion (and thus be on the Main Sequence). WIMPS = Weakly Interacting Massive Particles Unusual particles that have no electrical charge and thus cannot emit any kind of electromagnetic energy. Regions of density can grow into galaxies because the extra gravity in these regions draws matter together even while the rest of the universe expands. Detailed calculations show that, to explain that galaxies formed within a few billion years of the Big Bang, the density enhancements at the end of the “era of nuclei” must have been extremely significant. Perhaps WIMPS are particles not yet discovered??

Superclusters, walls and voids much larger than clusters of galaxies extend many millions of light years across the Universe. Each of these structures probably began as a very slight enhancement in the density of dark matter early in time, and these enormous structures are still in the process of forming.

Gravity can bend light, allowing huge clusters of galaxies to act as telescopes, and distorting images of background galaxies into elongated strands. Almost all of the bright objects in this Hubble Space Telescope image are galaxies in the cluster known as Abell The cluster is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. As a result, multiple images of these background galaxies are distorted into long faint arcs -- a simple lensing effect analogous to viewing distant street lamps through a glass of wine. The cluster of galaxies Abell 2218 is itself about three billion light-yearsaway in the northern constellation of the Dragon (Draco). The power of this massive cluster telescope has allowed astronomers to detect a galaxy at the distant redshift of Hubble Space Telescopecluster gravity bends and focuses the lightgalaxies that liemultiple imageslensing effectwinecluster of galaxieslight-yearsDracoallowed astronomersredshift

For many years it was thought that all the mass of the universe was present in a form that we can detect due to its emission of electromagnet radiation. Since the 1930’s evidence has mounted that this assumption was incorrect. This change of thought was based on the observations that galaxies in clusters had orbits that could not be accounted for by the amount of luminous matter. Studies of the rotation of galaxies also indicated the presence of more matter than that observed. This lead to what is called the dark matter (DM) problem. Today we know that only 4% of matter in the universe is in a form that emits electromagnet radiation (Carrol & Ostlie 2007, 1232). Many studies have been initiated to find exactly what this matter is and how it is distributed.

DEFINITION: Critical density – the precise density marking the dividing line between between external expansion and eventual collapse. If the matter density of U. > CD, then the collective gravity of all of it’s matter will eventually halt the U. expansion and reverse it! The galaxies will come crashing back together, and the entire U. will end in a fiery “Big Crunch!” If the matter density of U. = CD, then the collective gravity of all of it’s matter is exactly the amount needed to balance the expansion. In this case, the U. will never collapse but will expand more and more slowly as time progresses. If the matter density of U. < CD, then the collective gravity of all matter cannot halt the expansion. The U. will keep expanding forever with little change in its rate of expansion. This is often referred to as an “Open Universe.” THE FOUR SCENARIOS FOR THE FATE OF THE UNIVERSE

Because observations of distant supernovae suggest that a repulsive force opposes gravity on very large scales, astronomers are now seriously considering a fourth possibility: If repulsive forces cause the expansion of the U. to accelerate with time, then we live in an accelerating universe In this case, galaxies will recede from one another increasingly faster and it will become cold and dark more quickly than a coasting universe.

Final word on Black Holes

In a black hole the force of gravity ( attraction ) is so strong, that all matters are attracted. The space curvature is so strong that even light cannot escape any longer. Thus, the diameter of a sun for e.g. 1.4 million kilometers are shortened to 6 kilometers and the matter are pressed together.attraction