Lightweek (vol.1 Issue 1 - Sept.1-6, 2009)

September 1st – 6th, 2009

Vol. I

Issue 1

IT IS ALL SO DARK OUT THERE We’ve been uncovering some of the most unexpected secrets of the universe, lately. No one has been able to provide a satisfactory explanation for these, hence making them retain the dark truth behind them and behind all interconnected phenomena; and for that simple reason, we’ve been calling (like you might have guessed) dark. To name a few, we have dark matter, dark energy, and dark flow – all of which we will briefly discuss this week. In astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted ©The Online Physics Club

radiation, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter and dark energy could account for the vast majority of the mass in the observable universe. Dark matter is postulated to partially account for evidence of "missing mass" in the universe, including the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in http://theonlinephysicsclub.clubdiscussion.net

galaxies galaxies.

and

clusters

of

Dark matter is believed to play a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is frequently called the "dark matter component," even though there is a small amount of baryonic dark matter (i.e. dark matter composed of baryons, i.e. protons and neutrons). Dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, ©The Online Physics Club

dark energy currently accounts for 74% of the total massenergy of the universe. Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow. High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the http://theonlinephysicsclub.clubdiscussion.net

cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today. Dark flow is a name given to a net motion of galaxy clusters with respect to the cosmic microwave background radiation which was found in a 2008 study. According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year WMAP data using the kinematic Sunyaev-Zel'dovich effect (or simply SZ effect), the authors of the study found evidence of a common motion of at least 600 km/s toward a 20-degree patch of sky between the constellations of Centaurus and Vela. The authors suggest that the motion may be a remnant of the influence of no-longervisible regions of the universe ©The Online Physics Club

prior to inflation. Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the Cosmic Microwave Background); this corresponds to the particle horizon at a distance of about 46 billion (4.6×1010) light years. Since the matter causing the net motion in this proposal is outside this range, it would in a certain sense be outside our visible universe; however, it would still be in our past light cone. The results appear in the October 20, 2008, issue of Astrophysical Journal Letters, (which is available online). The authors state that they plan to extend their analysis to additional clusters and the recently released WMAP fiveyear data. Astrophysicist Ned Wright posted an online response to the study arguing that its methods are flawed. The authors of the dark flow study released a statement in return, refuting three of Wright's five http://theonlinephysicsclub.clubdiscussion.net

arguments and identifying the remaining two as a typo and a technicality that do not affect the measurements and their interpretation.

All-in-all, the best (and, in fact, the only) thing we could do – apart from some ingenious research – is wait till some light is shed on this dark issues!

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