Close up of the EDGES antenna. The four panels are made from aluminum sheet metal and supported by PVC legs. The white bag under the antenna encloses analog amplifiers and calibration circuits. Credit: Judd Bowman/Arizona State Univ |
In the beginning, there was no
light.
After the Big Bang created the universe 13 billion
years ago, the universe remained enshrouded in darkness. Based on observations
of the radiation left over from the Big Bang, astronomers have theorized that
several hundred million years after this event, gravity caused hydrogen and
helium particles to condense into clouds. The energy from this activity
eventually ignited those clouds, setting in motion a chain of events that led
to the birth of the first stars. Although the transition between the so-called
cosmic dark ages and the birth of stars and galaxies may explain the origin and
evolution of many celestial objects, astronomers know very little about this
period.
Recently, two astronomers conducted an experiment to
try to learn more about this transitional period, which is known as the Epoch
of Reionization, or EOR. Because identifying any light from the earliest
galaxies is nearly impossible, Alan Rogers, a research affiliate at MIT’s
Haystack Observatory, and Judd Bowman, an assistant professor at Arizona State
Univ., instead focused their efforts on detecting radio waves emitted by
hydrogen that existed between the first galaxies. Some of these radio waves are
just reaching us today, and astronomers have theorized that certain
characteristics of the waves could hold clues about the EOR.
As the first stars started to form during the EOR,
their ultraviolet radiation (light) excited nearby hydrogen atoms, knocking off
their electrons and giving them a positive electrical charge. This process,
known as ionization, is important to cosmologists because it marks a pivotal
moment in the transition between the early universe, which contained only
hydrogen and helium gas, and today’s universe, which is filled with diverse
galaxies, planets and black holes. Figuring out exactly when—and for how long—this
ionization occurred is an important first step for confirming or modifying
current models of the evolution of the universe.
To understand more about this period, the researchers
focused their study on the frequency of radiation emitted by non-ionized, or
neutral, hydrogen. Specifically, they looked to see how the signal changed over
time, which would indicate how long it might have taken for the non-ionized hydrogen
to become ionized as a result of the birth of stars and galaxies. As the
researchers reported in a paper published in Nature,
it took at least 5 million years for the non-ionized hydrogen to become
ionized. It is a good bet, then, that the birth of the first stars and galaxies
took the same amount of time or more to develop into the stars and galaxies we
recognize today.
A new frontier
The finding isn’t surprising to Harvard astronomer Avi Loeb, who says that many
models predict that the EOR lasted for several hundred million years. Even so,
the study is significant because it provides the first observational results
about the EOR—a research area that Loeb calls the “major frontier” in astronomy
over the next decade. By showing that radio observations can probe ancient
radio waves, Rogers
and Bowman “have basically opened the window for using this simple technique”
in parallel with more sophisticated instruments, he says. To refine their
estimate, Loeb suggests that the researchers improve the calibration of their
antenna in order to remove any interference produced by the instrument itself.
Rogers and Bowman hope to deploy a system with improved
calibration later this month. They are also involved in developing a large
radio telescope that will attempt to make much more sophisticated measurements
from the EOR. Known as the Murchison Widefield Array, the telescope consists of
512 antenna “tiles” that will try to discover low-frequency radio phenomena
that may reveal details about how the galaxies formed and evolved.
Rogers and
Bowman’s study was supported by the National Science Foundation and NASA.
