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April 25, 2014 — A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.
Virgo, the brightest star in Spica, is one of the 12 constellations of the Zodiac. Near its second brightest star, Zavijava (or Beta Virginis), lies a very important point in the sky, the autumnal equinox. This is the intersection of the celestial equator (the projection of the Earth’s equator onto the sky) and the ecliptic (the apparent path of the sun on the sky) and is the position of the sun on the first day of fall.
In mythology, Virgo has been associated with the Greek goddess of justice, Dike; the Syrian goddess of fertility, Atargatis; and Tyche, the goddess of fortune. Tyche normally holds a horn of plenty, and Spica, the brightest star, means “Virgo’s ear of grain.” Spica is the 15th brightest star in the sky.
This month, the Eta Aquarids meteor shower peaks around May 4-5 and is best viewed after midnight toward the constellation Aquarius in the southeast. This year, you may see up to 30 meteors per hour.
Jupiter is seen right after sunset in the constellation Gemini. Mars is very close to Spica in Virgo, and Saturn can be seen all night long in Libra, to the east of Virgo. Venus is the morning star this month.
May 2014 Interest: The Horizon Problem
(Best URL: http://en.wikipedia.org/wiki/Horizon_problem)
On distance scales much larger than superclusters of galaxies, which can span hundreds of millions to billions of light-years, the universe begins to smooth out. The average density and temperature of matter on these larger scales are uniform.
A causality problem is then apparent, because regions of the universe that are sufficiently far removed from each other have not had enough time to interact and make for the smoothness. This is because "locally," light travels at a finite speed, and all other effects, such as temperature equilibration processes, propagate more slowly.
Additionally, in earlier times, the universe's expansion was faster than it is at present. It is important to understand that the limitation of the finite speed of light does not apply to regions of the universe that are very far apart.
According to Einstein's general relativity, such very distant regions can be receding from each other faster than light speed. Thus, during all times, even when the universe was much more compact, mutually distant regions never had time to communicate with each other and equilibrate their densities and temperatures. This is the "horizon problem,” identified by cosmologists in the 1960s. It is so named because, beyond a region's causal horizon, there can have been no communication of effect.
In the 1980s, cosmologists began to envision "inflation scenarios" in which the very early universe (at ages of a small fraction of a second) could grow exponentially rapidly, after an even earlier epoch when all regions were in communication with each other and, therefore, equilibrated in density and temperature.
These scenarios are still in their infancy, essentially proto-theories; the actual physics are completely unknown. However, experiments on COBE, WMAP and Planck satellites have increasingly refined measurements of the cosmic microwave background (the radiation signature from the recombination epoch before which the universe was ionized), finding supporting evidence that something like inflation did happen during those earliest moments of the universe.
Some profound consequences follow if, in fact, there was an inflation epoch. One is that the whole of the universe would be enormously larger than we can ever observe beyond our horizon.
Several other large-scale cosmological problems are recognized, and solvable by inflation scenarios, one being the "flatness problem": Observations on large scales show that the universe is exceedingly flat. General relativity allows for curvature of space time, but the average curvature of the observable universe is very close to zero -- the universe is very "flat" -- to be discussed next time.
To view this month’s sky chart, click here.