Quasars are the most distant distinct objects
that astronomers have been able to directly detect. Because of their intrinsic
brightness, the most distant quasars are seen at a time when the universe
was one tenth its present age, roughly a billion years after the Big Bang.
However, astronomers believe that some objects must have formed earlier than
quasars, because the ambient gas in the universe is observed to be ionized
at a relatively early time, presumably due to ionizing radiation from a population
of early objects. Since ionized gas can interact with cosmic microwave background photons, WMAP observations help to elucidate the nature of the ionized gas and the objects that caused the ionization.
light travels at a finite speed, distant objects are seen as they existed
in the past. We see the Sun not as it is now, but how it was eight minutes
ago. (The Sun is eight light minutes away from the Earth). We see the nearby
stars as they were several years ago. We see Andromeda, the nearest spiral
galaxy as it was roughly two million years ago. Thus, the most distant objects
that we see are the oldest objects that we can directly detect.
are the most distant distinct objects that astronomers have been able to
detect. In a region smaller than our solar system, a quasar emits more light
than our entire Milky Way
galaxy. Quasars are believed to be supermassive black holes, whose masses
exceed that of a million Suns, and whose pull is swallowing gas and stars
from their host galaxies. They shine brightly by converting the gravitational
energy of the infalling material into light. The most distant quasars are
seen at a time when the universe was one tenth its present age, roughly a
billion years after the Big Bang.
HST Image of a Quasar Interacting With a Galaxy:
Text Link to HST press release describing this picture
light from a quasar illuminates all of the material along its path to us,
quasars serve as distant flashlights revealing the properties of the early
universe. By observing quasars, astronomers have learned that almost all
of the hydrogen gas in the early universe was ionized into protons and free
electrons within a billion years after the Big Bang. They have also inferred
that the known quasars were not energetic enough nor common enough to ionize
all of the gas in the visible universe.
What Ionized the Gas in the Early Universe?
are not certain what objects ionized the gas in the early universe nor do
they know when this ionization occurred. Some speculate that an early generation
of massive stars
ionized the gas. Others speculate that most galaxies contain supermassive
black holes and that the formation of these supermassive black holes illuminated
the early universe.
When Was the Gas Ionized?
of quasars enable astronomers to infer that the gas was ionized within the
first billion years of the universe, we need to observe something more distant
than quasars to learn when the gas was first ionized: the cosmic microwave background radiation.
Since the cosmic microwave background photons were emitted roughly 380,000
years after the Big Bang, much earlier than the photons from quasars, their
properties tell us about the subsequent evolutionary history of the universe.
Microwave photons move freely through neutral gas, but they scatter off of
ionized gas. This scattering reduces the amplitude of fluctuations in the temperature of the cosmic microwave background and produces new "polarized" microwave background fluctuations.
Scattered light is often polarized. On a bright day, we see not only sunlight
directly from the Sun, but also light that scatters off of dust in the air.
This scattered light, or "glare", is polarized and can thus be filtered out
by a good pair of polarized sunglasses. Similarly, scattered cosmic microwave
background photons are polarized by scattering off of free electrons in the
early universe. WMAP
is designed to detect polarized photons. In principal, their properties reveal
the number of free electrons in the early universe and the ionization history
of the universe. This enables astronomers to infer that the first objects
in the universe capable of ionizing the gas formed at about 200 million years
after the Big Bang. We hope that the time history of the ionization will
help determine the nature of these first objects.
Last updated: Tuesday, 03-01-2005