GALAXY ABELL 1835 and DARK ENERGY
Neşever BALTACI1

1 Ümraniye Anadolu İ.H.ve İ.H.Lisesi-İstanbul, e-posta: nesever@yahoo.com

SUMMARY

Galaxy breaks distance record Astronomers have discovered a galaxy some 13.2 billion light years away – the most distant galaxy ever seen. Light from this galaxy, known as Abell 1835 IR1916, provides information about the universe when it was just 3% of its current age. However, this radiation has been red-shifted to longer wavelengths by the expansion of the universe, and astronomers can calculate the age of a galaxy by measuring the red-shift of its Lyman alpha line. Dark sides and golden ages Astronomers first started talking about a "golden age" of astrophysics and cosmology in the late 1990s. • Ironically, the outstanding questions in the golden age concern the dark side of the universe - what are the "dark matter" and the "dark energy" that cannot be seen but which make themselves known through their gravitational influence? • But dark matter and dark energy are just two puzzles, albeit two extremely difficult and important ones, in a galaxy of questions that still ...New evidence has confirmed that the expansion of the universe is accelerating under the influence of a gravitationally repulsive form of energy that makes up two-thirds of the cosmos.It is an irony of nature that the most abundant form of energy in the universe is also the most mysterious. Since the breakthrough discovery that the cosmic expansion is accelerating, a consistent picture has emerged indicating that two-thirds of the cosmos is made of "dark energy" - some sort of gravitationally repulsive material. But is the evidence strong enough to justify exotic new laws of nature? Or could there be a simpler, astrophysical explanation for the results? The dark-energy story begins in 1998, when two independent teams of astronomers were searching for distant supernovae, hoping to measure the rate at which the expansion of the universe was slowing down. They were in for a shock: the observations showed that the expansion was speeding up. In fact, the universe started to accelerate long ago, some time in the last 10 billion years. Like detectives, cosmologists around the world have built up a description of the culprit responsible for the acceleration: it accounts for two-thirds of the cosmic energy density; it is gravitationally repulsive; it does not appear to cluster in galaxies; it was last seen stretching space–time apart; and it goes by the assumed name of "dark energy". Many theorists already had a suspect in mind: the cosmological constant. It certainly fits the accelerating-expansion scenario. But is the case for dark energy airtight? The existence of gravitationally repulsive dark energy would have dramatic consequences for fundamental physics. The most conservative suggestions are that the universe is filled with a uniform sea of quantum zero-point energy, or a condensate of new particles that have a mass that is 10-39 times smaller than that of the electron. Some researchers have also suggested changes to Einstein's general theory of relativity, such as a new long-range force that moderates the strength of gravity. But there are shortcomings with even the leading conservative proposals. For instance, the zero-point energy density would have to be precisely tuned to a value that is an unbelievable factor of 10120 below the theoretical prediction. Until recently the supernova data were the only direct evidence for the cosmic acceleration, and the only compelling reason to accept dark energy. Precision measurements of the cosmic microwave background (CMB), including data from the Wilkinson Microwave Anisotropy Probe (WMAP), have recently provided circumstantial evidence for dark energy. The same is true of data from two extensive projects charting the large-scale distribution of galaxies - the Two-Degree Field (2DF) and Sloan Digital Sky Survey (SDSS)Now a second witness has testified. By combining data from WMAP, SDSS and other sources, four independent groups of researchers have reported evidence for a phenomenon known as the integrated Sachs-Wolfe effect. The case for the existence of dark energy has suddenly become a lot more convincing. One of the prime methods for measuring extragalactic distances is to use "standard candles" such as Cepheid variable stars. the total amount of matter in universe - including all the dark matter - accounts for just one-third of the total energy. This has been confirmed by surveys such as the 2DF and SDSS projects, which have mapped the positions and motions of millions of galaxies. But general relativity predicts that there is a precise connection between the expansion and the energy content of the universe. We therefore know that the collective energy density of all the photons, atoms, dark matter and everything else ought to add up to a certain critical value determined by the Hubble constant: ρcritical = 3H02/8π G, where G is the gravitational constant. The snag is that they do not. Mass, energy and the curvature of space-time are intimately related in relativity. One explanation is therefore that the gap between the critical density and the actual matter density is filled by the equivalent energy density of a large-scale warping of space that is discernable only on scales approaching c/H0 (about 4000 Mpc).In a universe where the full critical energy density comes from atoms and dark matter only, the weak gravitational potentials on very long length scales - which correspond to gentle waves in the matter density - evolve too slowly to leave a noticeable imprint on the CMB photons., gravitational collapse is slowed by the repulsive dark energy. Consequently, gravitational potentials grow shallower and photons gain energy as they pass by. Similarly, photons lose energy passing through underdense regions. Negative pressure;to examine this strange property of dark energy it is helpful to introduce a quantity w = pdark/ρdark, where pdark is the mean pressure and ρdark is the density of dark energy in the universe. The rate of change in the cosmic expansion is proportional to -(ρtotal + 3ptotal), where ρtotal is the density of all the matter and energy in the universe and ptotal is the corresponding pressure. To account for the accelerated expansion, however, this quantity must be positive. Since ρtotal is a positive quantity, and the mean pressure due to both ordinary and dark matter is negligible because it is cold or non-relativistic, we arrive at the requirement that 3w x ρdark + ρtotal < 0 for an accelerating expansion. Since ρdark ~ 2/3ρtotal, we find that w≥-1/2, so the pressure of the dark energy is not just a little negative but a lot negative!