The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/94_15.txt

Text File | 1996-01-12 | 7KB | 152 lines

CONTACT:  Ray Villard, STScI         FOR RELEASE:  Monday, March 21, 1994
          [phone]

          Karie Meyers, NOAO       PRESS RELEASE NO.: STScI-PR94-15
          [phone]

          Dr. Marc Postman
          [phone]   (3/13 - 4/5)
          [phone]   (4/6  -    )

          Dr. Tod R. Lauer
          [phone]   (3/13 - 3/22, 3/26 - 4/5)
          [phone]   (3/23 - 3/25, 4/6 -     )



         GALAXY DRIFT CHALLENGES IDEAS ABOUT UNIVERSE'S EVOLUTION

Two astronomers have discovered that our own Milky Way galaxy and
most of its neighboring galaxies, contained within a huge volume
of the universe, one billion light-years in diameter, are
drifting with respect to the more distant universe.  This
startling result may imply that the universe is "lumpier" on a
much larger scale than can be readily explained by any current
theory.  "The new observations thus strongly challenge our
understanding of how the universe evolved," says Dr. Tod Lauer of
the National Optical Astronomy Observatories (NOAO).

This surprising conclusion comes from the deepest survey of
galaxy distances to date, conducted by Dr. Tod R. Lauer in
Tucson, Arizona, and Dr. Marc Postman of the Space Telescope
Science Institute (STScI) in Baltimore, Maryland.

The two astronomers used NOAO telescopes at Kitt Peak National
Observatory, near Tucson, Arizona, and at Cerro Tololo
Inter-American Observatory, near La Serena, Chile to study galaxy
motions over the entire sky out to distances of over 500 million
light years.  They explored a volume of space about thirty times
larger than had been surveyed previously.  The results of this
survey will be published in the April 20 issue of The
Astrophysical Journal.

The expansion of the universe causes all the galaxies in the
volume surveyed to be moving away from us.  Galaxies at the edge
of the volume are receding from us at 5% of the speed of light.
The large flow that Postman and Lauer discovered comes from
looking at the galaxy motions "left over" once the expansion of
the universe had been accounted for.  The flow means that the
nearby universe, as well as expanding, appears to be drifting
with respect to the more distant universe.

Astronomers generally assume that the diffuse glow of microwave
radiation left over from the Big Bang provides the backdrop or
rest frame of the universe.  In the mid 70's astronomers found
that temperature of this radiation is slightly hotter towards the
direction of the constellation of Leo.

This effect has been interpreted to mean that the Milky Way is
drifting with respect to the rest of the universe at about 380
miles per second in this direction.  It has also been assumed
that most of this motion is due to the gravitational attraction
of more distant galaxies; however, these galaxies have never been
positively identified.

In the mid-80's a group of seven astronomers surveyed the motions
of galaxies out to about one-third of the distance studied by
Lauer and Postman, finding the galaxies to be flowing as a group
with respect to the more distant universe.  This team postulated
that this flow was due to the gravitational pull of a large
concentration of galaxies dubbed "The Great Attractor."  The
Great Attractor is located deep inside the volume surveyed by
Postman and Lauer, however, and would not be massive enough to
cause their much larger sample of galaxies to drift.

In fact, the new result implies that the Milky Way and its
neighbors are affected by much larger concentrations of galaxies
at much larger distances than can be easily explained by popular
theories of how the universe is organized.

Lauer and Postman started their project in 1989 to measure the
drift of the Milky Way with respect to 119 clusters of galaxies
located all over the sky at distances as far as 500 million
light-years.  If the motion of the Milky Way was caused by
galaxies closer to us than the distant clusters, as was then
presumed to be the case, then its motion with respect to the
clusters should have been essentially identical to that with
respect to the microwave background radiation.

Because the galaxy clusters are at a variety of distances from
us, galaxies in the more distant clusters appear dimmer than the
ones more nearby.  However, once the various distances are
accounted for, the brightest galaxy in each cluster is always
found to give off roughly the same amount of light.  Astronomers
refer to such objects as "standard candles."  The distances to
the clusters are estimated from how fast they are moving away
from us as the universe expands.

If the Milky Way Galaxy is drifting, however, its motion makes
measurement of the expansion speeds dependent on which direction
we are looking.  If the drift is not corrected for, then the
cluster galaxies will appear to vary slightly in brightness in a
smooth pattern across the sky.  Postman and Lauer used images of
the cluster galaxies to detect this pattern and determine the
motion of our own galaxy.

The motion of the Milky Way that Postman and Lauer measured from
the distant clusters is in a completely different direction from
that inferred from the microwave background.  The most likely
solution to this dilemma is that the clusters themselves are
moving with an average velocity of 425 miles per second towards
the constellation of Virgo.  Because of the enormous size of the
volume containing the clusters, however, this implies the
existence of even more distant and massive concentrations of
matter.

Most theories explaining the structure of the universe predict
that the universe should be nearly uniform on the scale of the
Lauer and Postman cluster sample.  The motion of the Milky Way
and its neighbors would then be due to concentrations of mass
relatively close by.

If instead, the portions of the universe as big as a billion
light-years in diameter are still drifting with respect to the
larger universe, then the universe has structure or "lumps" of
matter on much larger scales than predicted by most theories.
The detection of galaxy flows across large volumes of space
should improve our understanding of how the universe came to be
organized the way we see it today.

A more provocative but probably less likely interpretation of the
Postman and Lauer result is that the large volume of clusters
really is at rest, with the temperature variation of the
microwave background around the sky being a relic of the
conditions of the Big Bang, rather then being caused by the
motion of our galaxy.  In this case, the microwave temperature
variation would tell about the properties of the very early
universe rather than about large scale motions of galaxies.


                                 ********
The Space Telescope Science Institute is operated by AURA for
NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD.  The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).

The National Optical Astronomy Observatories are operated by the
Association of Universities for Research in Astronomy, Inc.
(AURA) under cooperative agreement with the National Science
Foundation.