My personal vision of understanding gravity through the initial assumption that the earth along with the rest of the universe expand, three-dimensionally is getting stronger with the recent discovery of the possible existence of dark matter and dark energy. But first of all, we need to know, at least, the very basic ideas about them. Dark matter makes up about 26 percent of our Universe. Its presence was already been suspected since 1933, when astronomers observed and calculated that there were numerous unexplained gravitational effects floating in the 'unseen' void. It convinced them to believe that there must be more 'stuff' existing, in the Universe that cannot be seen by the telescopes. There is also this high possibility that it is the one responsible in making the galaxies spin faster than what should be expected. Its gravitational field also, seems to deviates the lights of the objects behind it, strengthening the evidence that dark matter, does exist. Dark energy, on the other hand, makes up approximately 70 percent of our Universe. It is also suspected to be in connection with the 'unseen' vacuum in space. It is all over the Universe, homogeneously distributed in the empty space. Distinctive about dark energy is that, it is not diminishing with time - it constantly spreading out, three-dimensionally as the Universe gets older. It also been suspected to be unaffected by gravitational attraction and speculated to be having repulsive force that is now becoming responsible in the accelerating rate of expansion of the Universe, in general. Together, they comprise 94 percent of our Universe, leaving only about 4 percent to all the combined mass and energy in the Universe that we can directly detect and observe. These recent developments in the study of the Universe (via Cosmology and Astrophysics), show that it is not much of the 'visible' things such as the planets, the stars and galaxies that define our Universe but rather, the 'unseen' things in the void around them that truly define it. But what are really these dark matter and dark energy? What are the secrets behind them?
In his theory of General Relativity, Einstein initially thought that the Universe was static. He also assumed that any celestial system, such as our Solar System had the tendency to contract, due to the very strong gravitational pull of its central body. So, he introduced a 'cosmological constant' (with an anti-gravitational force), to counter this contraction and create a mechanism that maintains the stability of the whole system (such as, it keeps the planets in our solar system to stay in their corresponding orbits while circling around the sun). But a certain Alexander Friedmann challenged his view. Using the general relativity's equations, Friedmann derived a mathematical solution where he showed an expanding pattern of the Universe (popularly known as the 'metric' or distance function of General Relativity). He then concluded that we must not expect the universe to be static. It took years, not until Edwin Hubble made a logical analysis of the mode of movements of the galaxies that convinced the proponents of a static universe to gradually accept the notion that the universe is expanding. Then, in 1998, the supernova reports confirmed the accuracy of Friedmann's prediction. But more than the confirmation, the results also show that the Universe is expanding in an inflationary rate, indefinitely, meaning, the Universe will expand forever. These only lead to more confusions rather than create concrete answers.
Is It Space that Expands?
One of the confusing issues in dealing with this notion that galaxies are moving apart from one another is the question of an 'expanding space'. There are people who are insisting that space is not a material thing (or substance), such as a dough or a rubber and it is not capable of expanding. It is the 'metric' or distance function of galaxies that expands (to be specific, the measures of the gaps of space between receding galaxies), which is nothing more than a 'mathematical value'. But there was also this hypothesis (or theory), postulating that during the Big Bang scenario, 'space' along with time, matter and energy were all been simultaneously created. In that case, it is valid argument to say that space is also expanding as the universe matured (since both time and space are directly proportional).
Different Behaviors of Space?
There are people who are reconsidering Einstein idea of cosmological constant as the right answer to the problem of what pushing galaxies apart. Does this means that there are portions in space where gravity is positive and other portions having negative gravity? Our most fundamental understanding of an atom is that it consists of negative electrons, positive protons and non-polarized or neutral neutron? Isn't possible that one day we might discover a neutral gravity? Or are we just complicating our understanding of the Universe?
Are Dark Matter and Dark Energy Related?
Another issue that facing today’s scientists is about the question of the relations of dark matter and dark energy. Are they connected or two separate entities? String theorists believe that dark matter is another type of 'supersymmetric particles' that has no relation to dark energy. Other scientists, on the other hand are trying to connect the two as due to a kind of 'scalar' field called "k-essence" (short for, kinetic energy driven quintessence), in which dark matter and dark energy are inversely related. The explanation is that, as the universe expands, the dark matter's 'piece' of the k-essence decreases, falling below the density of the dark energy's 'piece' of k-essence (believed to be constant), leading to behave more as dark energy. A more complicated hypothesis, try to connect this issue to M-theory. The idea is to confine all forces in the universe in a four-dimensional 'brane' and let gravity escape into a fifth dimensional plane (or brane), perpendicular to the four-dimensional brane. The effects of the dark matters are then the gravitational influence of the other branes on ours. Again, are we just complicating things?
Secrets Reveals
The truth lay behind this Copernican philosophical belief -"God created our world in simple ways". Dark matter and dark energy do exist and they are indeed, connected to one another. Like the fragile lens of the Hubble Telescope, which are solids but transparent and colorless...like the pure water which is a liquid but also transparent and colorless too, dark matter is indeed, more refine than any kind of gas in the universe, much perfectly transparent and extremely colorless. We do not see it directly but we can glance, it. Like the swaying of trees caused by the blowing winds, we can view (or detect), its effects. Dark energy is pushing the galaxies apart because these galaxies are immersed in an ocean of dark matter that constantly producing from its source - a simple way of understanding, the secrets behind them.
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