The Dark Universe
About 95 percent of the Universe is dark. Whatever we see (humans, animals, plants, Sun, stars, galaxies) is only 5 percent of the Universe. All of these are made of atoms and atoms are made of only three kinds of particles electrons, protons and neutrons. We now know much about these but that is only 5 percent of everything.
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According to astrophysicists and cosmologists, 70 percent of the Universe is Dark Energy and 25 percent is Dark Matter. Research is going on about this
70 + 25 = 95 percent Dark Universe, but no definitive conclusions have been reached.
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We shall first discuss the 25 percent Dark matter before we go to the stranger Dark Energy.
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Dark Matter
Dark Matter was discovered by astronomers many years ago. Both visible matter and Dark Matter obey Newton's Law of Gravitation. Using this, they discovered that Dark Matter existed throughout the Universe.
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There are many galaxies and galaxy clusters in the Universe. Since the galaxies in a cluster attract each other, all the galaxies are moving around the centre of the cluster. This motion follows Newton's law of gravitation and hence from this motion the mass of the whole cluster can be calculated. This mass can also be calculated by adding all the masses of the galaxies. The mass calculated from the velocities was many times larger than the mass obtained by adding the masses of the galaxies. So it was concluded that invisible or Dark Matter existed so that the two calculations would agree.
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Next let us consider the motion of stars in the galaxy. All the stars are moving around the centre of the galaxy because of the gravitational attraction. That is why
many galaxies appear to be rotating. From the velocities of the stars and Newton's law the total mass of the galaxy was calculated. By adding the masses of all the stars also the total mass of the galaxy could be obtained. Again the mass calculated from the stellar velocities was many times larger than the mass obtained by adding the stellar masses. So again the existence of Dark Matter was inferred.
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Lord Kelvin was the first to guess the existence of Dark Matter. He inferred this in 1884 from his studies of stellar velocities. In 1906 Henri Poincare described
Kelvin's results in his essay and named it as Dark Matter.
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Fritz Zwicky and Vera Rubin were the astronomers who found out the precise details about Dark Matter. In 1938 Zwicky found the Dark Matter from the velocities of galaxies in galactic clusters. Rubin found it from the stellar velocities in galaxies.
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After that, the existence of Dark Matter was confirmed from many astronomical and cosmological researches. We will mention those briefly. They are gravitational lensing, Cosmic Microwave Background Radiation, cosmological evolution and structure formation in the Universe.
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In the sixteenth and seventeenth centuries the existence of atmosphere around the Earth was discovered. Everybody knows the importance of the atmosphere consisting Oxygen and Nitrogen. Dark Matter is more important than atmosphere since the latter exists only for a height of 30 or 40 kilometers while Dark Matter exists all over the Universe.
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Although astronomers have discovered the existence of Dark Matter through gravitational force, its nature can be discovered only by physicists.
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Such research in physics has been going on for more than 20 years. The basis of this research is the assumption that Dark Matter is made of particles not yet discovered. Although these particles do not interact much with the known particles such as proton, electron etc., if they interact weakly, many kinds of experiments are possible. We shall describe this research briefly.
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1. Direct Interaction
Since Dark Matter Particles (DMP) are everywhere, they may sometime collide with nuclei of ordinary matter. These nuclei will move and ionize the surrounding matter and hence can be detected by particle detectors. Many such experiments are being conducted for the past 20 years, but so far definitive
positive results have not been obtained.
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2. Indirect Detection
A DMP may collide with another DMP and particles such as proton, electron and photon may be produced. Such collisions will occur only in those locations where DMP's are abundant. Since it is believed that Dark Matter abounds at the galactic centres and the cores of the Sun and other stars, experimenters are searching for energetic particles coming from these directions. But these searches also have not succeeded so far.
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3. Production by particle accelerators
When two protons collide with high energy, many particles are produced. In 2012, Higgs boson was discovered in such collisions at the Large Hadron Collider. So DMP also may be produced in such collisions. With this expectation, many experiments are being conducted at the collider, but without any success so far.
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Since experiments conducted for many years all over the world have not succeeded, the question arises:
Does Dark Matter really exist?
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As already mentioned, sofar, Dark Matter has been inferred only from its gravitational interaction. All the Dark Matter experiments assume that dark Matter interacts weakly with ordinary matter. If this assumption is wrong, above experiments will never catch Dark Matter. In other words if Dark Matter has only gravitational interaction and does not have electromagnetic, weak, strong or any other interaction with ordinary matter, we will never know the true nature of Dark Matter. This is one way. There are three other ways.
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The second way is that for some reason the density of Dark Matter in our region is very small, in which case the direct detection experiment done on the Earth cannot catch it. But the other two experiments can detect it.
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The third possibility is Dark Matter may not be made of particles. Dark Matter may be the primordial black holes. Just as all matter was produced in the Big Bang, primordial black holes also might have been produced and that may be the Dark Matter.
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The fourth possibility is to change Newton's law of gravitation (and Einstein's gravitational field equations) a little. Then there will be no need for Dark Matter since Newton's law was used in getting Dark Matter. If this possibility turns out to be true, it will be a big revolution in Physics and Astronomy!
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Among the four possibilities above, which is correct? Or is there any other way? Continuing research only can answer this. So at present we cannot say anything definite about Dark Matter.
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Dark Energy
Energy is Mass, Mass is Energy. Einstein's equation declares this. In spite of this, we talk about Dark Matter and Dark Energy as two separate things. Why? Reason is: Dark Matter has mass and other properties like visible matter, but Dark Energy is not matter at all. It is pure energy!
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To understand Dark Energy, mathematics and Einstein's General Theory of Relativity are necessary. So it is difficult to explain it here.
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Nevertheless, some facts have to be mentioned. From Einstein's gravitational equations, it is possible to calculate the evolution of the Universe. How is the Universe expanding from the time of its birth in the Big Bang? This can be explained from Einstein equations. That is what cosmologists also found.
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But, in 1999, a major discovery was made, namely the speed of the expansion of the Universe is increasing. This discovery was made by studies on a large number of supernovae. So one had to include Dark Energy in Einstein's equations. That is required to explain the accelerating Universe. This Dark Energy can also be called the Energy of the Vacuum.
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Another consequence of Einstein's General Relativity is the presence of Dark Pressure along with Dark Energy. This Dark Pressure is not like the pressure due to gases with which we are familiar. It is a negative pressure. From the discovery of the acceleration of the Universe, Dark Energy and Dark Pressure have been inferred.
If there were no Dark Energy, there would have been only one constant, namely Newton's constant of Gravitation G in Einstein's equation. Presence of Dark Energy necessitates the addition of another constant,(denoted by the Greek letter Lambda), called Cosmological Constant in Einstein's equation.
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This Cosmological Constant has an interesting history. After Einstein discovered his gravitational equations, he used them to study the evolution of the Universe. In that period of time, Universe was thought to be in a steady state. But Einstein's equations contradicted the steady state. So Einstein included the cosmological constant Lambda in his equations to stabilise the Universe. He did not like this, since he had been very happy that his original equations did not have any new
constant apart from G, but still they led to so many new consequences. The addition of Lambda destroyed that feeling.
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But after Einstein's addition of Lambda, two major developments took place in Cosmology. In 1924 the Russian mathematician Friedman found an exact
solution of the original Einstein equations and this solution described an expanding Universe. Second, in 1929, astronomer Edwin Hubble discovered
that all the galaxies that he studied through the big 100 inch telescope in California appeared to be running away from us. If the Universe were expanding, this would happen. Thus Friedman through theory and Hubble through observation discovered the expansion of the Universe.
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Immediately Einstein removed the Lambda that he added in his equations against his will. He called the addition of Lambda as his greatest blunder. For, if he had not done that, he could have predicted the expansion of the Universe from his original equations.
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During that period of time, cosmology had not advanced much. After that, cosmological research progressed during the 80 years and reached the stage when the acceleration of the Universe could be discovered. So Einstein's Lambda was brought back. As we mentioned already, it refers to the vacuum energy.
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Nevertheless, we cannot be sure that this is the real reason for the acceleration. Instead of Lambda, an all-pervasive field may be the cause. This field may be the vacuum energy. There may be other possibilities. Research is going on. Future only can give the answer.
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We understand only 5 percent of the Universe. The major part of 95 percent is dark. Although active research is being conducted all over the world to find the true nature of this 95 percent, the truth is still not known. So we see how strange our Universe is. It is full of unsolved mysteries.