Expansion
SCIENCE BACKGROUND - CLOSING IN ON UNDERSTANDING THE EXPANDING UNIVERSE
WHAT'S THE DIFFERENCE BETWEEN AN OPEN AND CLOSED UNIVERSE?
An open universe expands forever; a closed universe expands, but decelerates
until it eventually reverses direction and begins to contract; a "critical density"
universe is exactly midway between these scenarios and so will expand indefinitely,
always slowing down but never quite coming to a halt. If, for example, you throw
an object up in the air, it falls down due to gravity. But if the object moves
fast enough (say, by rocket) it can escape from the Earth. By analogy the Universe
itself may not have enough density to halt its own expansion.
WHAT'S THE RELATIONSHIP BETWEEN MASS DENSITY AND AGE OF THE UNIVERSE?
The rate of the Universe's expansion reflects how much gravity and hence, matter,
it has. Like going up a steep hill, the galaxies outward rush should have slowed
if the Universe has a lot of mass, and this implies a younger universe. If the
Universe has little mass, and so is barely decelerating, then galaxies would
have taken more time to reach their current positions, like rolling along a
flat floor.
The rate of the Universe's expansion should be slowed by the mutual gravitational
pull of all matter contained in the Universe.
WHY DO THEORISTS FAVOR A CRITICAL DENSITY UNIVERSE?
In formulating the simplest models of the expanding universe theorists favor
the notion that space contains the exact amount of matter that keeps the Universe
precisely balanced between expanding forever and collapsing under gravity. Assuming
such a "critical density" makes it easier to explain a number of observed properties
of the space, including the large-scale structure of galaxies.
DOES THE UNIVERSE CONTAIN ENOUGH MASS TO REACH CRITICAL DENSITY?
A fundamental problem is that telescopic observations show that the Universe
contains only 1/100 the luminous (i.e., stars and galaxies) mass that it needs
to reach critical density. Astrophysicists hold that dark matter must account
for the rest. Observational evidence showing that dark matter affects the rotation
rate of galaxies, and behavior of clusters of galaxies, boosts estimates of
the amount of matter in the Universe to 10% of the value needed to reach critical
density. To date the remaining 90% of the required mass to reach critical density
is missing and unaccounted for.
WHY HAS IT TAKEN MORE THAN 60 YEARS FOR ASTRONOMERS TO CALCULATE AN ACCURATE
VALUE FOR THE HUBBLE CONSTANT?
First, astronomers discovered that establishing an accurate distance scale to
faraway galaxies has been more difficult than anticipated. Second, while astronomers
can simply and accurately measure a galaxy's velocity, the measurement may not
represent the expansion velocity of the Universe at that distance. The reason
is that each galaxy possesses a gravitational force. Velocities are altered
when more massive galaxies, which have stronger gravitational forces, pull smaller
galaxies toward them.
WHY ARE THE TEAMS OPTIMISTIC THEY ARE CONVERGING ON A SINGLE VALUE FOR THE
HUBBLE CONSTANT?
The historically debated values of the expansion rate of the Universe have differed
by up to a factor of two, but the estimates of the two Hubble teams are now
within 25 percent. Hubble Space Telescope has taken this decades-old debate
out of gridlock and on toward a solution. That's because Hubble can see and
measure certain key celestial distance markers out to ten times farther from
Earth than ground-based telescopes.
HOW DO THE TEAMS MEASURE COSMIC DISTANCES?
Both teams base their results on studying a class of celestial milepost marker,
called Cepheid variable stars, whose pulsation rate is a direct indication of
their intrinsic brightness.
Freedman's team is systematically looking into a variety of methods for
measuring distances. They are using Cepheids in a large sample to tie into
five or six "secondary methods." One such secondary method relates the total
luminosity of a galaxy to the rate at which the galaxy is spinning, the Tully-Fisher
relation. Another secondary method makes use of a special class of exploding
star known as a type Ia supernova. These secondary distance indicators are
needed to look deeper into the Universe to get a more representative rate
for the expansion of space (the gravitational fields of nearby clusters may
yield an inaccurate value because the expansion rate may be affected by the
local motion of galaxies).
In contrast, the Sandage team took the "fast track" to focus on a single
secondary distance indicator, one of the same indicators also used by the
Key Project Team, the type Ia supernova. Sandage maintains that these stars
are "standard bombs" that all reach exactly the same intrinsic brightness.
They are visible 1,000 times farther away than Cepheids, allowing for an accurate
measurement of the Universe's overall expansion.
WHY IS OBSERVING THE FORNAX GALAXY CLUSTER IMPORTANT?
Earlier results derived from the Virgo cluster have been questioned because
that cluster is so large that possible inaccuracies in the distances of individual
galaxies from its center might affect some findings. The Fornax cluster is more
compact than the Virgo cluster, so there is much less range for uncertainty
in the distances of member galaxies from its center.
