Monday, July 2, 2012
Few things are as awesome as a crystal clear night sky far from city lights.
The Milky Way dominates in a band of countless stars from horizon to horizon. What are the large-scale structures in the universe, how large are they, and how did they form?
Humans are naturally drawn to patterns and structure. Ancient civilizations imagined seeing outlines of familiar things in what appeared as groupings of stars. They used star patterns to establish directional orientation at night.
Calendars were established based on figures in the Zodiac. Agricultural societies planted and harvested crops in synchrony with the appearance of iconic figures delineated by star groupings.
Modern astronomy demonstrates the three dimensional distributions of stars, revealing many recognized patterns to be illusions. However, it is no less romantic to contemplate the many more complex and fascinating structures astronomy brings into focus.
Imbedded in the much larger Milky Way galaxy is our solar system. The motions of the planets, asteroids, comets and other objects in the solar system are dominated by their gravitational attraction to the sun.
In recent years there have been discoveries of many star-planet systems in the Milky Way galaxy, each locked in gravitational embraces like ours.
For a perspective on relative sizes, imagine the outer boundary of the solar system to be Pluto's orbit. Scale that down to the size of a grain of dust. The same scaling applied to the Milky Way makes its diameter comparable to the length of two football fields.
The Milky Way is a fairly typical spiral arm galaxy having several hundred billion stars. At its center is a massive black hole surrounded at some distance by a very dense distribution of stars.
Beyond the central area, the distribution of stars resembles a toy pinwheel. The high density of the core gives way to lesser densities of stars as one moves outward on each arm.
The sun is located about two-thirds of the distance out from the center on the Orion-Cygnus arm.
It takes a little more than 100,000 years for light to travel across the entire Milky Way galaxy. The huge distances are more evident when one compares this with the 8 minutes it takes for sunlight to reach Earth.
The Andromeda galaxy is about 2.6 million light-years from the Milky Way. It has a little more than twice the number of stars as our galaxy, and will collide with it in about 4.5 billion years. It is also a spiral galaxy and, like the Milky Way, has several satellite galaxies, most of which are classified as dwarfs.
In total, there are about three dozen structures in this local group. The distance across this collection of structures is about 10 million light-years.
Our local group is little more than a small outlier among a collection of more than 100 galaxy groups and clusters called the Virgo Supercluster. The diameter of this supercluster spans 110 million light-years.
Even within this large collection there is structure. Something like 98 percent of galaxies in the Virgo Supercluster is concentrated in 11 groupings of galaxy clusters called clouds.
At the core of the Virgo Supercluster is the Virgo Cluster. It may have as many as 2,000 galaxies, some of which are spiral shaped, while others have elliptical, football-like shapes. Spiral galaxies in the Virgo Cluster are distributed in a long filament. That filament is about four times longer than it is wide.
If all these layers of structure don't cause your head to spin, there is more. Our supercluster is on the order of 100 million light-years in size. That is a couple orders of magnitude smaller than the observable universe. Galaxies have been observed in their formative stages at distances of over 13 billion light-years.
A study published in 1989 indentified something called the Great Wall. This is a sheet of galaxies having dimensions of about 500 million light-years long, 200 milion light-years wide and 15 million light-years thick.
This discovery was followed by others in the last decade. These were given suggestive names like Sloan's Great Wall and Newfound Blob. It had become increasingly evident the universe is made up of filaments of galaxies separated by large bubble-like voids. Superclusters are regions in the filaments where the density of galaxies is greatest.
When looking at scales greater than about 300 million light-years, structure diminishes. That is, there is more uniformity, less lumpiness, in the distribution of objects. Sloan's Great Wall is an exception, being on the order of 1.37 billion light-years across.
Mapping the distribution of observable objects is not easy. Volumes and distances are distorted by the curvature of space. A manifestation of this is the bending of light's path as it passes large masses, much like it is bent when passing through a glass lens.
Distances to objects are calculated from measurements of their red shifts, which are caused by motions away from us. Consequently, their own internal motions can introduce distortions in the apparent shapes of objects.
These effects have become increasingly manageable as our understanding has grown, but more puzzles remain to be solved.
This accounting only discusses the distribution of objects that emit light. Those stars and gases make up less than 5 percent of the mass-energy (remember Einstein's demonstration of the equivalence of mass and energy) of the universe. Of this, one-eighth is made up of stars, the rest gas.
Roughly 74 percent of the universe's mass-energy is dark energy and 22 percent dark matter. These are called dark because they emit no light and, therefore, cannot be directly observed.
However, the existence and distribution of dark matter and dark energy can be inferred from a number of sources. There are a number of hypotheses for what makes up these mysterious entities. Evidence in support of a winner is being sought from observations and research, but no one is claiming victory yet.
A full explanation must also account for the extreme uniformity observed in what is known as the cosmic background microwave distribution. This data indicates the universe was very smooth in the early stages of the Big Bang.
Cosmologist want to know what seeded the trend toward the lumpiness we observe on so many scales of size? They can explain much of it, but the picture is murkier for the very largest structures.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.