At the Carnegie Institution of Washington, astronomers have been measuring the speed of distant galaxies for years. They do that by measuring the amount of red shifts of bright stars at various distances from the galactic center. Their conclusion, based upon many measurements, was a tremendous surprise. In spiral galaxies, the stars move in circular orbits, with velocities that increase with increasing distances from the center. At the edges of spiral disks, velocities of 300 km/sec (about 185 mi/sec) have been measured at distances as great as 150,000 light-years. This increase in velocity with increase in distance is unlike planetary velocities in the solar system, where the orbital velocities of planets decrease with increasing distance from the sun. This difference tells astronomers that the mass of a galaxy is not as centrally concentrated as is the mass in the solar system. A significant portion of galaxy mass is located at large distances from the center of the galaxy, but this mass has so little luminosity. Although scientists have seen its gravitational effects, no one has identified this "dark matter." But apparently there is a phantom universe out there, consisting of 90 to even 99 percent of the mass of the cosmos, and we have little knowledge of it!
Besides the ultimate question of how really the universe started, there are many other questions still hanging in the minds of cosmologists:
♦ What is the age of the universe?
♦ Where and what is the missing 90 - 99 % of the mass of the universe?
Since Hubble’s heyday in the 1920’s, astronomers have known that the universe is expanding. Scientists use the Hubble constant to deduce the age of the universe. As mentioned before, Hubble constant is the ratio of the speed of recession of the galaxies and their distances. There are these two loopholes though, what is the right distance and what is the right speed? It is extremely hard to measure how far away galaxies are. If they came in standard brightness, like 100-watt light bulbs, the astronomers could just figure that a dimmer galaxy was more distant than a bright one. Unfortunately, they don’t. Edwin Hubble himself did not realize this and triggered an earlier "age crisis" in the 1940’s, when he announced that the universe is 2 billion years old. Geologists already knew that the Earth was over 4 billion years old!
To make this puzzle more complex, Hubble Constant may not be constant after all. It is hard to imagine that distances, speeds, and distribution of the galaxies do not affect Hubble constant. Hubble constant is a simple ratio of speed and distance. But what if Hubble constant is called Hubble variable, and the latter is in the form of much more complicated formula.
Moreover, astronomy’s most reliable light bulb is a type of star called the Cepheid variable, whose inherent brightness can be easily calculated. But Cepheid variable can be spotted only in the neighboring few galaxies. But nearby galaxies are virtually useless in filling the other half of the equation - the expansion rate. The reason is that: in a universe that is expanding everywhere, neighboring galaxies are flying apart at a much slower speed than distant galaxies. Nearby galaxies are also subject to their neighbor’s gravity. The Andromeda galaxy, for example, is being pulled closer to our Milky Way, despite the overall cosmic expansion.
Therefore, since accurate distances can be measured only nearby, while useful speed of galaxies are found only deep in space, astronomers do the best they can to bridge the gap. They use the close galaxies to estimate the distances to the far away galaxies. But the method is inexact, which is why astronomers have not been able to agree on what the age actually is. The current estimate of the age of the universe ranges from 8 to 25 billion years, which indicates that "something" is very wrong!
Over the past few years, astronomers have uncovered the existence of the Great Wall, a huge conglomeration of galaxies stretching across 500 million light years of space; the Great Attractor, a mysterious concentration of mass pulling much of the local universe in the direction of the constellation Hydra and Centaurus; the Great Void, where few galaxies can be found; and galaxies caught in the agony of formation a mere billion years after the Big Bang, when they should not exist.
The existence of the Great Wall, the Great Attractor, the Great Void, superclusters, and clusters of galaxies indicate that the universe is full of extremely massive matters with heavy gravity pulling those structures together. These mysterious matters account for 90 to 99 % of the mass of the universe! Lately scientists have revived an old idea called the cosmological constant. This is a kind of powerful "antigravity" force that forces the galaxies to fly apart even as ordinary gravity tries to draw them together. It was first conceived by Einstein himself, who then rejected it as "the greatest blunder of my life." Einstein in 1915 thought that he needed it in his general relativity to balance the influence of the gravity. He was sure that the universe had to be static that he modified his theory to make this possible by introducing this cosmological constant. The relativity equations showed that without a cosmological constant, the universe would have to be either contracting or expanding. If he had stuck to his guns, he might have won another Noble prize. Recent research work suggests that the cosmological constant may be responsible for 65% of the expansion of the universe!
The universe makes a lot of sense if one can assume that just after it was born, all of the space went into overdrive, exploding outward for the briefest fraction of a second. This inflation theory explains, among other things, such mysteries as why the universe looks pretty much the same in all directions, and how a smooth distribution of matter evolved into today’s lumpy distribution, with clusters of galaxies surrounded by empty space. The inflation theory does not just explain cosmic phenomena; it makes predictions. It suggests that the blackness of space is only seemingly empty. In fact, it probably is abundant with vast amount of matter and energy that cannot be directly detected because they do not shine.
Dark matter is more than merely theoretical. The first hint that the cosmos contains more than what meets the eye came back in the 1930’s, when astronomer Fritz Zwicky pointed his telescope at the Coma cluster of galaxies and realized that it should not exist. Individual galaxies in the cluster were orbiting each other so fast that they should long since have flown out into deep space - unless gravity from unseen matter was keeping them together. No one took Zwicky too seriously; the idea was crazy, first of all, and besides, the measurements of orbital speed were difficult to make and prone to errors. In the 1970’s astronomers discovered that some galaxies are rotating too fast on their own axis indicating an extra gravity from unseen matter.
Not until a decade ago was the dark matter finally accepted as a huge problem rather than a nagging anomaly. Observation after observation showed that galaxies moved as if they were embedded in cloud of an invisible matter containing 10 times as much mass as was accounted for by visible gas and stars. Clusters of galaxies behaved as if there was 30 times as much dark matter as visible matter exerting its gravitational pull. To satisfy inflation theory, the ratio would have to be even greater: 100 times as much dark matter as visible.
The challenge of identifying and understanding the dark matter that forms 90 to 99 % of the mass of the universe has become one of the most irresistible and frustrating quests in science. For about a decade, the search of the missing universe has proceeded on two fronts:
♦Attempts to directly observe the missing matter.
♦Attempts to identify it using computer models. This is based upon the assumption that dark matter is made of a given particle or substance. Then the behavior of the universe is simulated to see if the result will look like the present universe.
The missing universe could be composed of any, some or none of the following dark matters:
Neutrinos: These are ghostly subatomic particle that have no electrical charge and interact only weakly with ordinary matter. They are also known as hot dark matter because they fly through the space at nearly the speed of light. They are known to exist in great numbers that they may account for some 20 % of the dark matter.
Wolfgang Pauli first suggested neutrinos in 1930 as a factor to permit an understanding of the energy distribution of electrons. They have been detected in 1953 in a high-power nuclear reactor. The sun emits plenty of neutrinos from the nuclear furnace at its core and, at night, these particles from the sun come up from below, the Earth being almost transparent to them. In 1987, light from an exploding star in the galaxy of the Large Magellanic Cloud reached Earth after traveling for 170,000 years. Enormous numbers of neutrinos were generated in this explosion and a sensitive neutrino detector in Japan picked up about 10 of them.
Neutrinos, left over from the Big Bang, are the most abundant particles of physics. In the time it takes to read this sentence, billions and billions of them pass through the body of every human being on Earth, leaving no trace! They pass through ordinary matter as though it was not there at all. Unless a neutrino scores a direct hit on an atomic nucleus, it leaves no hint of its passage. And such hits are so unlikely that the average neutrino can easily penetrate a slab of thick lead a trillion miles thick without impacting a single atom.
Recently, physicists from Canada and Japan have found the most convincing evidence yet that neutrinos have a tiny mass after all. The neutrino's mass cannot be much, around one billionth of a proton's. This finding means that scientists will have to adjust their theories of the universe.
WIMPs: (Weakly Interacting Massive Particles). These are also known as cold dark matter because they are slow moving. However, they are purely hypothetical particles derived from speculative theories. They perform somewhat better in computer models, but cold dark matter cannot account for the newly discovered features of the cosmos as Great Wall, Great Void, and Great attractor.
MACHOs: (Massive Compact Halo Objects). They are assumed to be large planets similar to the size of Jupiter or very dim stars made of ordinary matter. This is the simplest theory, but so many would be required that it seems unlikely that all the dark matter could be made of them.
Recently, scientists found a planet orbiting a star known as 47 Ursae Majoris, 200 trillion miles from Earth in the Big Dipper. This planet is twice the size of Jupiter (the size of Jupiter is 1300 times that of Earth). A second planet, circling the star 70 Virginis, in the constellation Virgo, has six times the mass of Jupiter. These planets, like Jupiter, probably consist of gases such as hydrogen, ammonia and methane.
Picking out a planet by a telescope against the glare of a star is like trying to spot a 100-watt bulb next to the sun. Astronomers find it much easier to look for the subtle influence of a planet on its parent star, such as the effect of gravity of planets on the motion of an orbiting star.
Black holes: These are objects with gravitational pulls so intense that light cannot escape from them. They are strongly predicted by the general theory of relativity, but their presence in such abundance should have been detected already.
Astronomers had evidence that some galaxies were strong emitters of X-rays, the source of which was not known. Donald Lynden-Bell at Cambridge University suggested that super dense bodies could provide the answer. Such a body would attract matter, accelerating it to a huge speed as it fell in. The falling matter would move at an immense speed, emitting X-rays in huge quantities. The extreme density of such a body would create an intense gravitational field. This would mean that space-time around the body would be so strongly curved as to cause the interior to be closed off from the outside universe. Nothing could escape from them. This is why such objects are now called black holes.
Figure 2.18 - Space-time diagrams of the sun, white dwarf, Neutron star
and black hole