Observations beginning in the 1990s support the idea that the universe is expanding at an accelerating rate. No longer considered just a hypothesis, this accelerating expansion is now an accepted fact. But a major leap of faith is still required for it to actually work.
Suppose the rate of expansion of the universe depended upon only matter’s inertia following the enigmatic split-second period of inflation. If this is true, the rate of expansion would not be increasing to the extent indicated by observations of Type 1a supernovae.
To explain this discrepancy, it has been necessary to hypothesize a dark energy permeating the entire universe. This is a weak force, but because there is so much of it, the end result is a powerful repulsive pressure that is applied to all matter, making it fly apart at an ever greater rate.
The energy is termed dark simply because we cannot see it or detect it in any way. Scientists only surmise that it is responsible for the observed accelerating rate of expansion of the universe.
Though dark energy has not been observed, evidence for its existence is convincing. For one thing, distance measurements vis-à-vis red shifts indicate the universe has continued to expand since its temporal midpoint. Moreover, additional energy is necessary to explain the flat nature of the universe as opposed to one having a spherical curvature. Additionally, dark energy is necessary to account for enormous wave patterns of mass density throughout the universe.
It is possible to calculate the amount of dark energy in the universe based on how it affects the rate of expansion. According to current thinking, about 68% of the universe is dark energy. Dark matter comprises 27%. The remainder, everything that we observe, is less that 5% of the total.
It has been proposed that dark energy is an intrinsic property of space. Since Albert Einstein vastly altered our understanding of reality at the start of the twentieth century, it has been known that space, rather than an empty abstraction, possesses properties such that it is an active rather than passive participant in the total picture.
It has been further theorized that dark energy is actually a cosmological constant, which is to say that it fills all space uniformly with no variation in density. Alternatively, dark energy is viewed as a scalar field, its density varying in time and space. In either case, its effect is that of a small but all-pervasive negative pressure that causes all matter to blow apart at an accelerating rate.
The existence of dark energy has been further (beyond supernovae observations) verified because it is a necessary component of any theory that would explain the amazingly flat distribution of the cosmic microwave background noise. For this distribution to be uniform, as verified by highly precise measurements, the density of mass together with energy must be a certain level. And dark energy must be present for this density level to be attained. For these reasons, it is quite certain that dark energy exists, and that it exists in the proportion that has been proposed.
Like dark energy, dark matter is hypothetical. The current estimate is that dark matter comprises about 85% of the total matter in the universe. This large amount is necessary to account for various observed phenomena including galaxy rotation curves, gravitational lensing of background objects, hot gas distribution in clusters of galaxies and minimal cosmic microwave background irregularities. Many researchers believe that dark matter is composed of an as yet undiscovered subatomic particle.
It is generally believed that dark matter consists of weakly interacting massive particles (WIMPs). These bodies would interact only by means of gravity and the weak force. It is held that most dark matter is non-baryonic, which means that it does not form atoms because it has no electric charge.
Researcher Vera Rubin, around 1970, was the first to accurately quantify dark matter. Using an improved spectrograph with enhanced sensitivity, she measured the angular velocities of spiral galaxies to unprecedented precision. She calculated that most galaxies should contain roughly six times their observed mass. Here again, dark matter had to be invoked to make up the difference.
While it is true that neutrinos, a form of dark matter, have been detected, most of this non-interactive material has eluded observers. But it is generally accepted that most galaxies have more dark matter than the visible variety.
However there are instances where dark matter is a minor ingredient if not completely absent. Globular clusters (spherical collections of stars that orbit a galactic core as a satellite), for example, do not appear to contain dark matter. Interstellar gas located at the peripheries of galaxies, in contrast, in addition to some elliptical galaxies, present evidence for a greater-than-typical fraction of dark matter, as much as 95%.
There are several varieties of dark matter. One set of categories terms them, somewhat whimsically, cold, warm and hot. Cold dark matter is the hypothetical type that fits most observations. Components have a small free-streaming length. (A free-streaming length is the distance dark matter moved due to random motions in the early universe before it slowed down because of the universe’s expansion.) Hot dark matter probably did not play a role in primordial galaxy and cluster galaxy formation. Warm dark matter hypothetically had particles the free-streaming size of a dwarf galaxy. Needless to say, no such particles have been observed. Hot dark matter is theorized to have a free-streaming length comparable to a proto-galaxy.
There have been possible laboratory dark matter sightings, but at this time not with absolute certainty. It seems probable that in the near future researchers will more accurately ascertain the nature of these elusive phenomena.
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