How Did Structure Form in the Universe?
The Big Bang theory is widely considered to
be a successful theory of cosmology, but the theory is incomplete.
Astronomers observe considerable structure in the universe, from stars to
galaxies to clusters and superclusters of galaxies. The recently released
"Deep Field Image" taken by the Hubble Space Telescope, shown below, provides
a stunning view of such structure. How did these structures form?
Most cosmologists believe that the galaxies that we observe today grew
gravitationally out 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 MAP satellite will measure these small fluctuations in the
temperature of the cosmic microwave background radiation and in turn probe
the early stages of structure formation.
Hubble Deep Field Image:
[Text of 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
Most 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 overdensity 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 150 million
years ago), 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. The MAP satellite will measure these small fluctuations
in the temperature of the cosmic microwave background radiation. These
temperature fluctuations are minute: one part of the sky might have a
temperature of 2.7281 Kelvin (degrees above absolute zero), while another
part might have a temperature of 2.7280 Kelvin.
NASA's
Cosmic Background Explorer
(COBE)
satellite, has detected these tiny fluctuations on large angular scales.
MAP will re-measure the fluctuations with both additional
angular resolution and
sensitivity. The mission summary
page offers a quick introduction to how MAP achieves this sensitivity - more
details are available on the technical
information page.
What Made These Small Fluctuations?
While 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. MAP, with its superb
sensitivity, should be able to easily distinguish between the two theories,
or rule them out entirely.
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 the
home page at Fermilab or by reading an
introduction to the project.
-
The Grand Challenge Consortium
- This is a collaboration of scientists from Princeton University, MIT,
University of Illinois, Indiana University, U.C. Santa Cruz, University of
Pittsburgh and the NCSA and PSC supercomputer centers. You can learn about
the largest simulations of structure formation to date, and see computer
simulations of galaxy formation and mergers by visiting the GC3 home site.
-
The University of Washington supercomputing group
- This group maintains an excellent
science education page.
- 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 the
Hubble Deep Field
image. You can learn more about this image by clicking
here.
Back to the Introduction to Cosmology Page
Back to the MAP Home Page
Please help us make this web site more useful and enjoyable by telling us
what you would like to see at this site:
David N. Spergel / dns@astro.princeton.edu
Gary Hinshaw / hinshaw@stars.gsfc.nasa.gov
Charles L. Bennett / bennett@stars.gsfc.nasa.gov
Last updated: March 20, 1996