The density of matter is called the critical density. Critical density also marks a dividing line between eternal expansion and eventual collapse. If the universe were to collapse, gravity would be the only force that would affect the expansion rate. When you put all luminosities of the galaxies together, it makes up about 0.5% of critical density. Overall matter could exceed critical density if dark matter made up over 200 times as much mass as the luminous matter does. This is not so, by observations of galaxy clusters it is said that they hold about 50 times more in dark matter than that of luminous matter. If galaxy clusters had more amounts of dark matter this would cause deviations from Hubble's Law. There are no observed deviations so therefore scientist must assume that overall matter is less that critical density and this is why our universe continues to expand and accelerate its expansion. It seems that our universe will be destined to continue this expansion forever (Bennett, Donahue, Schneider & Voit, 2012, p.459).
Light travels at the speed of light, at the event horizon, which is the boundary line between the inside of a black hole, and the universe. At this boundary line the escape velocity is equals to the speed of light, meaning not even light can escape (Bennett, Donahue, Schneider & Voit, 2012, p.373). When masses distort the space around them gravitational lensing occurs. When light passes by a massive object it bends the beams, this act is called gravitational lenses. Depending on how strongly the massive object bends the light's path, scientist can take measurements of the masses. Clusters of galaxies can act as a gravitational lens. Scientist can measure a cluster's mass without replying on Newton's laws with gravitational lensing because the gravities light-bending effect tend to distort the images of the galaxies that are lying behind the cluster you are observing. Mass is what causes the curvature of space-time, and the stronger the gravity the greater of the space-time curvature. For example, our planet Earth is traveling as straight as it possibly can at the very moment, but because of the mass of the Sun, the gravity of the Sun is curving the path of the Earth with its mass (Bennett, Donahue, Schneider & Voit, 2012, p.451,372).
If matter density is exactly equal to the critical density our universe would be flat. But it is not, there is slightly more critical density than matter density and this gives us the space-time curvature. Cosmic microwave background is evidence left over from the big bang such as radiation that began to stream across the universe at the end of the era of nuclei; it is also still present today. Cosmic microwave background was first predicted by George Gamow and his colleagues in the 1940s they had the idea that is the big bang really occurred, radiation should still be present throughout the universe and it should be able to be detected by a microwave antenna. The background came from the heat of the universe, when the universe was young the temperature was about 3000 K, which is similar to a red giant star surface. Since that time when the universe was young it has expanded by a factor of about 1000, this has made the elements stretch out and has allowed the cosmic microwave background to cool to a few degrees above absolute zero (Bennett, Donahue, Schneider & Voit, 2012, p.481).
Some questions left unanswered by the big bang theory are as follows: Where does the structure come from?, Why is the large scale universe so uniform?, and Why is the density of the universe close to the critical density? Starting with the first question where does the structure come from, the models of the big bang theory all have assumed that gravity collected matter around regions of slightly enhanced density in the early universe, but to be able to explain the origin of the universe requires that the big bang somehow produced slight density enhancements, but how? Inflation could do this because inflation stretches tiny quantum ripples to enormous sizes, this could have become the density enhancements which later formed galaxies. When we look to the right in the universe there are not much differences if we were to compare them to the left, for the second question why is the large-scale universe so uniform, the big bang does not tell us why throughout our universe it looks pretty much the same no matter where you go (of course leaving Earth out of this). Inflation can also help explain this because even though these different galaxies have not been in contact since the time of inflation they still were in contact with each other prior to that time and did exchange radiation that would have equalized their temperatures and density. The third question why is the density of the universe close to the critical density can not be explained by the big bang theory alone and it needs the explanation of inflation. Inflation actually predicts that the geometry of the universe should appear to be flat. If the critical density was lower than the amount of density of mass then our universe would start to collapse. The only way the universe could be flat and expand forever is if the overall density of matter is a little less than that of the critical density (Bennett, Donahue, Schneider & Voit, 2012, p.486-488).
The scientific theory of inflation needs a special ingredient, some type of energy that was combined with gravity to make the universe expand a huge amount in only a brief second (Steinhardt, 2011). Dark Energy versus Dark Matter Dark matter is an unseen influence that provides gravity to the object and motion scientist observe. Dark matter is unseen for now; I believe we do not have the right equipments yet in our existence to actually see dark matter. Dark matter is something that was only proposed to exist. Over many decades their confidence in dark matter has become creditable because the scientific models that have assumed this substance exists have given predictions that were verified through observations. Through observations of orbital speeds and stars it has been discovered that most of our galaxy's mass lies beyond the Sun and tens of thousands light-years away from the galactic center. This mass as indicated by a more detailed study lies in the spherical halo, as we have learned our halo is not very luminous. If most of our galaxy's mass lies in the halo that is not very luminous then it must be dark matter, we are only able to detect it with rotation patterns and not by visible light. There is even evidence of dark matter in clusters of galaxies, through observations it was discovered that galaxy clusters have more dark matter than single galaxies do. By measuring cluster masses along with measuring the orbital speeds of the galaxies in the center, studying the x-ray emission from hot gases in between the clusters will give you a ratio between dark matter and luminous matter (Bennett, Donahue, Schneider & Voit, 2012, p.446-447).
Newton's laws of motion and gravity are the most trusted tools used in science to this day. Either dark matter really does exist or we need to redo what we understand about gravity and motions. If our understanding of gravity and motion is correct then dark matter really does exist and we are observing the effects of its gravity in different motions and orbits, Science has come so far, and I really believe that in our current scientific theory's about dark matter ad its existence. I also believe that if humans still exist 1,000 years fro now; they will have invented a satellite that can detect dark matter. I really doubt dark matter is a substance that is only made up of one chemical compound. Dark matter and what it is made up of will be discovered someday.
Another substance that has a dark side is dark energy. Dark energy is what is causing the expansion of the universe. Dark energy's existence is something completely different than dark matter. As we had said dark matter is a type of matter that exists, we just can not see it due to our lack in technology. A force (dark energy) is pushing the galaxies in out universe apart. Scientist can tell the universe is expanding by some unseen force by the observations of white dwarf supernovae. The white dwarf supernovae are observed in galaxies very far away, the distance is measured by there lookback time and their redshift tells scientist how much the universe has expanded since the supernovae explosion (Bennett, Donahue, Schneider & Voit, 2012, p.452-453). If a universe only contains ordinary matter the expansion rate would decrease over time not increase. Scientist are still not sure what is causing the expansion rate to accelerate, this is why it is called dark energy (Sapone, 2010). Lawrence Krauss is a theoretical physicist and cosmologist at Arizona State University, he and other theorist find it somewhat unsettling that our universe contains just enough dark energy and dark matter to balance out the objects and the existence of our universe (Pendick, 2009).
References :
Bennett, J., Donahue, M., Schneider, N. & Voit, M. (2012) The essential cosmic perspective. (6th ed.) San Francisco, CA: Pearson Addison-Wesley Steinhardt, P. J. (2011). The Inflation Debate. Scientific American, 304(4), 36-43.
SAPONE, D. (2010). DARK ENERGY IN PRACTICE. International Journal Of Modern Physics A: Particles & Fields; Gravitation; Cosmology; Nuclear Physics, 25(29), 5253-5331. Pendick, D. (2009). Is the Big Bang in trouble?. Astronomy, 37(4), 48-51.
No comments:
Post a Comment