What does the term "Dark Ages" mean to astronomers?

The years before the invention of the telescope could certainly be thought of as a dark age. The actual first telescope probably appeared in Holland a year or two before Galileo started making his famous astronomical discoveries in 1609. The telescope definitely brought light to astronomy.

These days, however, astronomers use the term "Dark Ages" to refer to a time much further back, when the Universe itself was dark. Based on current results, especially from the Wilkinson Microwave Anisotropy Probe (WMAP), our current thinking about the early Universe has this time line:

• 0 years Big Bang - the origin of our Universe
• 380,000 years Recombination - the time at which the Universe had
  cooled enough to allow protons and electrons to form
  atoms. This is the time from which the light seen with
  WMAP originated, now seen as the microwave background.
  In terms of redshift z, it represents z approximately 1100.
• 200,000,000 years At about this time (redshift approximately 20), the first
  stars formed. This is known as the time of reionization,
  when some atoms again separated into protons and
  electrons.
• 1,000,000,000 years The oldest galaxies and quasars that have been seen date from this time (redshift about 6)
• Between 380,000 years and 200,000,000 years from the time of the Big Bang, there were no sources of light in the Universe. The glow from the background radiation cooled as the Universe expanded, leaving a literal "Dark Ages" until the formation of the first stars.

WMAP finds evidence of the reionization, but not of the "first light" that ended the Dark Ages. The search for the original stars and galaxies is one of the principal goals of the James Webb Space Telescope (JWST), currently being built.

Here is a picture of the early Universe timeline, from the NASA/WMAP Science Team (http://map.gsfc.nasa.gov/m_ig/030624/030624_s.jpg).

The WMAP Web site is http://map.gsfc.nasa.gov/index.html

The JWST Web site is http://ngst.gsfc.nasa.gov/

I found an excellent summary of the evolution of the early Universe by Dr. David Alles, Western Washington University, at http://fire.biol.wwu.edu/trent/alles/Cosmic_Evolution.pdf
This is a PDF file, which requires Adobe Acrobat or Acrobat Reader.

This week's question is provided by Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.