"Summary: Using data from deep-space surveys taken by NASA's Hubble
Space Telescope and other observatories, astronomers have performed a
census of the number of galaxies in the universe. The team came to the
surprising conclusion that there are at least 10 times as many galaxies in the
observable universe than previously thought. The results have clear
implications for our understanding of galaxy formation, and also helps shed
light on an ancient astronomical paradox -- why is the sky dark at night?
This Hubble Space Telescope view reveals thousands of galaxies stretching
back into time across billions of light-years of space. The image covers a
portion of a large galaxy census called the Great Observatories Origins
Deep Survey (GOODS). Besides the myriad of galaxies visible in this image,
only 10 percent of the total number of galaxies in the universe are
observable for the current generation of telescopes, according to a new
analysis of the GOODS and other Hubble deep-field surveys. The study's
researchers concluded that at least 10 times more galaxies exist in the
observable universe than previously thought. The analysis places the
universe's estimated population at, minimally, 2 trillion galaxies. According
to the research, about 90 percent of galaxies in the observable universe are
too faint and too far away to be seen with present-day telescopes.
Credit: NASA, ESA, the GOODS Team, and M. Giavialisco (University of
Massachusetts, Amherst)
The universe suddenly looks a lot more crowded, thanks to a deep-sky
census assembled from surveys taken by NASA's Hubble Space Telescope
and other observatories.
Astronomers came to the surprising conclusion that there are at least 10
times more galaxies in the observable universe than previously thought.
This places the universe's estimated population at, minimally, 2 trillion
galaxies.
The results have clear implications for galaxy formation, and also helps shed
light on an ancient astronomical paradox -- why is the sky dark at night?
In analyzing the data, a team led by Christopher Conselice of the University
of Nottingham, U.K., found that 10 times as many galaxies were packed into
a given volume of space in the early universe than found today. Most of
these galaxies were relatively small and faint, with masses similar to those
of the satellite galaxies surrounding the Milky Way. As they merged to form
larger galaxies the population density of galaxies in space dwindled. This
means that galaxies are not evenly distributed throughout the universe's
history, the research team reports in a paper to be published in The
Astrophysical Journal.
"These results are powerful evidence that a significant galaxy evolution has
taken place throughout the universe's history, which dramatically reduced
the number of galaxies through mergers between them -- thus reducing
their total number. This gives us a verification of the so-called top-down
formation of structure in the universe," explained Conselice.
One of the most fundamental questions in astronomy is that of just how
many galaxies the universe contains. The landmark Hubble Deep Field,
taken in the mid-1990s, gave the first real insight into the universe's galaxy
population. Subsequent sensitive observations such as Hubble's Ultra Deep
Field revealed a myriad of faint galaxies. This led to an estimate that the
observable universe contained about 100 billion galaxies. The new research
shows that this estimate is at least 10 times too low.
Conselice and his team reached this conclusion using deep-space images
from Hubble and the already published data from other teams. They
painstakingly converted the images into 3-D, in order to make accurate
measurements of the number of galaxies at different epochs in the
universe's history. In addition, they used new mathematical models, which
allowed them to infer the existence of galaxies that the current generation of
telescopes cannot observe. This led to the surprising conclusion that in
order for the numbers of galaxies we now see and their masses to add up,
there must be a further 90 percent of galaxies in the observable universe
that are too faint and too far away to be seen with present-day telescopes.
These myriad small faint galaxies from the early universe merged over time
into the larger galaxies we can now observe.
"It boggles the mind that over 90 percent of the galaxies in the universe
have yet to be studied. Who knows what interesting properties we will find
when we discover these galaxies with future generations of telescopes? In
the near future, the James Webb Space Telescope will be able to study
these ultra-faint galaxies," said Conselice.
The decreasing number of galaxies as time progresses also contributes to
the solution for Olbers' paradox (first formulated in the early 1800s by
German astronomer Heinrich Wilhelm Olbers): Why is the sky dark at night if
the universe contains an infinity of stars? The team came to the conclusion
that indeed there actually is such an abundance of galaxies that, in
principle, every patch in the sky contains part of a galaxy. However, starlight
from the galaxies is invisible to the human eye and most modern telescopes
due to other known factors that reduce visible and ultraviolet light in the
universe. Those factors are the reddening of light due to the expansion of
space, the universe's dynamic nature, and the absorption of light by
intergalactic dust and gas. All combined, this keeps the night sky dark to
our vision.
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