The Origins of the Universe
galaxies formed billions of years ago, astronomers have been able
to paint an increasingly detailed picture of how the universe evolved.
According to the widely accepted Big Bang theory, our universe was
born in an explosive moment approximately fifteen billion years
ago. All of the universe's matter and energy-even the fabric of
space itself-was compressed into an infinitesimally small volume
and then began expanding at an incredible rate. Within minutes,
the universe had grown to the size of the solar system and cooled
enough so that equal numbers of protons, neutrons, and the simplest
atomic nuclei had formed.
After several hundred thousand years of
expansion and cooling, neutral atoms-atoms with equal numbers of
protons and electrons-were able to form and separate out as distinct
entities. Still later, immense gas clouds coalesced to form primitive
galaxies, and, from them, stars. Our own solar system formed relatively
recently-about five billion years ago-when the universe was two-thirds
its present size.
In April 2000,
an international team of cosmologists supported in part by
NSF released the first detailed images of the universe in
its infancy. The images reveal the structure that existed
in the universe when it was a tiny fraction of its current
age and one thousand times smaller and hotter than today. The project, dubbed BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) captured the images using an extremely sensitive telescope
suspended from a balloon that circumnavigated the Antarctic in late
1998. The BOOMERANG images were the first to bring into sharp focus
the faint glow of microwave radiation, called the cosmic microwave
background, that filled the embryonic universe soon after the Big
Bang. Analysis of the images already has shed light on the nature
of matter and energy, and indicates that space is "flat."
roots of the Big Bang theory reach back to 1929, the year Edwin
Hubble and his assistant Milton Humason discovered that the universe
is expanding. Between 1912 and 1928, astronomer Vesto Slipher used
a technique called photographic spectroscopy-the measurement of
light spread out into bands by using prisms or diffraction gratings-to
examine a number of diffuse, fuzzy patches. Eventually, Hubble used
these measurements, referred to as spectra, to show that the patches
were actually separate galaxies. Slipher, who did his work at Lowell
Observatory in Flagstaff, Arizona, found that in the vast majority
of his measurements the spectral lines appeared at longer, or redder,
wavelengths. From this he inferred that the galaxies exhibiting
such "red shifts" were moving away from Earth, a conclusion he based
on the Doppler effect. This effect, discovered by Austrian mathematician
and physicist Christian Doppler in 1842, arises from the relative
motion between a source and an observer. This relative motion affects
wavelengths and frequencies. Shifts in frequency are what make ambulance
sirens and train whistles sound higher-pitched as they approach
and lower-pitched as they move away.
these findings and eventually determined the distances to many of
Slipher's galaxies. What he found was amazing: The galaxies were
definitely moving away from Earth, but, the more distant the galaxy,
the faster it retreated. Furthermore, Hubble and Humason discovered
that the ratio of a galaxy's speed (as inferred from the amount
of red shift) to its distance seemed to be about the same for all
of the galaxies they observed. Because velocity appeared proportional
to distance, Hubble reasoned, all that remained was to calculate
that ratiothe ratio now referred to as the Hubble Constant.
what is the value of the Hubble Constant? After 70 years of increasingly
precise measurements of extragalactic velocities and distances,
astronomers are at last closing in on this elusive number.
Freedman is one of the scientists working to define the Hubble Constant.
As head of an international team at the Carnegie Observatories in
Pasadena, California, Freedman surveys the heavens using the Hubble
Space Telescope to measure distances to other galaxies. With grants
from NSF, she is building on the legacy of Henrietta Leavitt, who
discovered in the early 1900s that the absolute brightness of Cepheid
variable stars is related to the time it takes the stars to pulsate
(its period). Scientists can measure the period of a Cepheid in
a distant galaxy and measure its apparent brightness. Since they
know the period, they know what the absolute brightness should be.
The distance from Earth to the Cepheid variable star is inferred
from the difference between absolute and apparent brightness. Freedman
and her colleagues are using this method to determine distances
to other galaxies. With these Cepheid distances, Freedman's group
calibrates other distance-determination methods to reach even more
far-flung galaxies. This information, in turn, enables them to estimate
the Hubble Constant.
closing in on a definitive value for the Hubble Constant are doing
so in the midst of other exciting developments within astronomy.
In 1998, two independent teams of astronomers, both with NSF support,
concluded that the expansion of the universe is accelerating. Their
unexpected findings electrified the scientific community with the
suggestion that some unknown force was driving the universe to expand
at an ever increasing rate. Earlier evidence has supported the possibility
that the gravitational attraction among galaxies would eventually
slow the universe's growth. In its annual survey of the news, Science
magazine named the accelerating universe as the science discovery
of the year in 1998.
director of Mount Stromlo and Siding Spring Observatories in Canberra,
Australia, has studied another aspect of the expansion of the universe.
Scientists generally assume that everything in the universe is moving
uniformly away from everything else at a rate given by the Hubble
constant. Mould is interested in departures from this uniform Hubble
flow. These motions are known as peculiar velocities of galaxies.
Starting in 1992, Mould and his colleague John Huchra of the Harvard
Smithsonian Center for Astrophysics used an NSF grant to study peculiar
velocities of galaxies by creating a model of the universe and its
velocity that had, among other things, galaxy clusters. These galaxies
in clusters were accelerated by the gravitational field of all the
galaxies in the locality. All other things being equal, a high-density
universe produces large changes in velocity. This means that measurements
of peculiar velocities of galaxies can be used to map the distribution
of matter in the universe. Mould and Huchra's model has seeded major
efforts to collect measurements of the actual density of the universe
so as to map its mass distribution directly.
In the modular
universewhere stars are organized into galaxies, galaxies
into clusters, clusters into superclustersstudies of galaxies,
such as those conducted by Mould, give us clues to the organization
of larger structures. To appreciate Mould's contribution to our
understanding of these organizing principles, consider that a rich
galaxy cluster can contain thousands of galaxies, and each galaxy
can contain tens of billions to hundreds of billions of stars. Astronomers
now estimate that there are tens of billions of galaxies in the
observable universe. Large, diffuse groupings of galaxies emerging
from the empty grandeur of the universe show us how the universe
is put together-and perhaps even how it all came to be.
Only one of
those extragalactic islands of starsthe Andromeda Galaxyis
faintly visible to the naked eye from the northern hemisphere, while
two small satellite galaxies of the Milky Waythe Large and
Small Magellanic Cloudscan be seen from Earth's southern hemisphere.
Telescopes augmented with various technologies have enabled astronomersnotably
NSF grantee Gregory Bothun of the University of Oregonto discover
galaxies that, because of their extreme diffuseness, went undetected
until the 1980s. These "low-surface-brightness" galaxies effectively
are masked by the noise of the night sky, making their detection
a painstaking process. More than 1,000 of these very diffuse galaxies
have been discovered in the past decade, but this is only the beginning.
"Remarkably, these galaxies may be as numerous as all other galaxies
combined," says Bothun. "In other words, up to 50 percent of the
general galaxy population of the universe has been missed, and this
has important implications with respect to where matter is located
in the universe."