Astronomers observe considerable structure in
the universe, from stars to galaxies to clusters and superclusters of galaxies.
The famous "Deep Field Image" taken by the Hubble Space Telescope, shown
below, provides a stunning view of such structure. How did these structures
form? The Big Bang
theory is widely considered to be a successful theory of cosmology, but the
theory is incomplete. It does not account for the needed fluctuations to
produce the structure we see. Most cosmologists believe that the galaxies
that we observe today grew from the gravitational pull of small fluctuations
in the nearly-uniform density of the early universe. These fluctuations leave
an imprint in the cosmic microwave background radiation in the form of temperature fluctuations
from point to point across the sky. The WMAP satellite measures these small
fluctuations in the temperature of the cosmic microwave background radiation
and in turn probe the early stages of structure formation.
Text Link to the HST press release describing this image
In its simplest form, the Big Bang theory assumes that
matter and radiation are uniformly distributed throughout the universe and
that general relativity is universally valid. While this can account for
the existence of the cosmic microwave background radiation and explain the origin of the light elements,
it does not explain the existence of galaxies and large-scale structure.
The solution of the structure problem must be built into the framework of
the Big Bang theory.
Gravitational Formation of Structure
cosmologists believe that the galaxies that we observe today grew gravitationally
out of small fluctuations in the density of the universe through the following
sequence of events:
- When the universe was one thousandth its present size
(roughly 500,000 years after the Big Bang), the density of matter in the
region of space that now contains the Milky Way, our home galaxy, was perhaps
0.5% higher than in adjacent regions. Because its density was higher, this
region of space expanded more slowly than surrounding regions.
- As a result of this slower expansion, its relative
over-density grew. When the universe was one hundredth its present size (roughly
15 million years after the Big Bang), our region of space was probably 5%
denser than the surrounding regions.
- This gradual growth continued as the universe
expanded. When the universe was one fifth its present size (roughly 1.2 billion
years after the Big Bang), our region of space was probably twice as dense
as neighboring regions. Cosmologists speculate that the inner portions of
our Galaxy (and similar galaxies) were assembled at this time. The stars
in the outer regions of our Galaxy were probably assembled in the more recent
past. Some cosmologists suspect that some of the objects recently detected
by the Hubble Space Telescope may be galaxies in formation.
HST Images of Galaxies in Formation?
Observing These Small Fluctuations
Tiny variations in the density of matter in the early universe leave an imprint in the cosmic microwave background radiation in the form of temperature fluctuations
from point to point across the sky. These temperature fluctuations are minute:
one part of the sky might have a temperature of 2.7251 Kelvin (degrees above
absolute zero), while another part might have a temperature of 2.7249 Kelvin.
NASA's Cosmic Background Explorer (COBE) satellite, has detected these tiny fluctuations on large angular scales. WMAP re-measures the fluctuations with both higher angular resolution and sensitivity. The mission summary page offers a quick introduction to how WMAP achieves this sensitivity - more details are available on the technical information page.
What Made These Small Fluctuations?
gravity can enhance the tiny fluctuations seen in the early universe, it
can not produce these fluctuations. Cosmologists speculate about the new
physics needed to produce the primordial fluctuations that formed galaxies.
Two popular ideas are:
These different theories make very different predictions about the properties of the cosmic microwave background fluctuations.
For example, the inflationary theory predicts that the largest temperature
fluctuations should have an angular scale of one degree, while the defect
models predict a smaller characteristic scale. WMAP, with its superb sensitivity,
indicates that the inflationary model is more likely.
Learn More About Structure Formation at These Sites:
The Sloan Digital Sky Survey (SDSS)
This group plans to map the positions of over 100 million galaxies and
determine the distances to over a million galaxies and quasars. The effort
will produce the largest (known) survey to date of cosmic structure in the
universe. You can learn more about the details of the SDSS by visiting their
home page at Fermilab.
The Virgo Consortium
The Virgo Consortium is an international grouping of scientists carrying
out super computer simulations of the formation of galaxies, galaxy clusters,
large-scale structure, and of the evolution of the intergalactic medium.
Although most of the consortium members are British, there are important
nodes in Canada, the United States, and Germany.
The University of Washington N-Body Shop
This group creates software simulations for studying large-scale structure
formation and planet formation, and host an interesting image gallery.
The Hubble Space Telescope
HST has been able to observe distant galaxies and study the formation
and evolution of galaxies. The lead figure on this page is a Hubble Deep Field image. You can learn more about this image by clicking here.
Last updated: Wednesday, 03-09-2005