October 2002 Sky from Keeble Observatory
Last month, we discussed the observation that normal matter
does not contribute enough mass to the universe to account for the
observed gravitational effects on stars, galaxies, and clusters
of galaxies. Even if we add in the contribution of exotic matter
- theorized but not yet detected - there is not enough.
Gravity exerts an attractive force between objects which have mass.
The gravitational force on objects near the Earth's surface is experienced
indirectly as weight, pulling you towards the center of the planet.
(What you are actually aware of is the force of the floor pushing
up through the soles of your feet, or perhaps the chair pushing
on the seat of your pants.) Step into free fall, like a shuttle
in orbit, and you experience weightlessness. The floor no longer
pushes on you because you're falling at the same rate.
Except in science fiction, there is no antigravity - i.e. gravity
always pulls, never pushes. That suggests that the universal expansion,
which started at the Big Bang some 14 billion years ago, should
be slowing. The rate of expansion is measured by the Hubble Constant,
which is the proportionality constant between distance and recession
velocity, measured in kilometers per second per million parsecs.
One of the prime scientific objectives of the Hubble Space Telescope
was to precisely measure the Hubble constant and the so-called deceleration
parameter, which is the rate at which the expansion has slowed over
time, in order to determine the overall mass density of the universe.
How this should work would be to accurately determine both the distances
to a large number of galaxies and their recession velocity. Since
light travels at a constant speed in vacuum, the further into space
we look the farther back in time we are peering.
It was, therefore, a huge surprise when the expansion was found
to be accelerating! The Hubble constant isn't constant! Nearby objects
appear to be moving away faster than they would be if the rate of
expansion had remained fixed. Something's pushing, while gravity
should only be pulling.
In his General Theory of Relativity, Albert Einstein originally
included a long-range push, which he called the Cosmological Constant.
His intention was to account for the fact as understood in 1915
that the universe was static. It was a long-range repulsive force
which balanced the inward pull of gravity, but only over the distances
to far away galaxies. On the scale of the solar system or even one
galaxy, the effects of this constant were supposed to be insignificant.
The cosmological constant is back in a different form, now known
as dark energy. This dark energy provides the repulsive push to
accelerate the expansion. Current theory suggests that most of the
universe is made of dark energy, with normal and exotic matter less
than half the mix.
One of the consequences of this scenario of accelerating expansion
is that at some point in the far distant future, each galaxy will
essentially be an isolated island in space. The rest of the universe
will be receding so fast that light from other can never reach us.
Once the stars have consumed all the fuel available, the lights
will go out. Permanently.
Another scenario has been recently proposed, however. Andrei Linde
and his co-worker Renata Kallosh, both at Stanford University, have
taken anther look at the possible nature of dark energy. Their calculations
suggest that the long-range repulsion provided by dark energy could
reverse in the future and become an attractive force in addition
to gravity. The universal expansion would slow to a stop, and then
the whole thing would rapidly collapse back to the densities and
temperatures of the Big Bang. One version of their calculation suggests
that this might happen as soon as 10 billion years from now. It
might then rebound and start all over again!
Their models are still somewhat controversial, so the debate is
likely to continue for years. But, that sort of scientific free-for-all
is how we sort out and test the various theoretical models. Linde
approaches the debate with good humor, acknowledging the old joke
about cosmologists that, Astrophysicists are always in error, but
never in doubt.
Lunar phases for October: New Moon on the 6th, at 7:18 am; First
Quarter on the 13th, at 1:33 am; Full Moon at 3:20 am on the 21st,
Last Quarter at 12:28 am on the 29th.
Planet watchers will have to stay out late or come out early this
month. Venus is bright above and to the left of the Sun when it
sets. Venus will set earlier and earlier as the month progresses
and as it catches up to Earth in its orbit. Binoculars or a small
telescope will reveal a thinning crescent phase. Saturn rises about
4 hours after sunset early in the month, about 3 hours after sunset
at month's end. Pre-dawn observers will be able to see four bright
planets stretched across the sky. From east to southwest note in
order Mercury, Mars, Jupiter, and Saturn, starting the month spanning
about 90 degrees.
Looking directly overhead at about 8:00 pm at mid-month finds Cygnus
at zenith. The long neck of the Swan stretches toward the southwest,
from bright Deneb to the fainter binary Alberio. Check out this
binary in a small telescope to note the distinct color difference
between the two stars. The bright star a bit to the west from Deneb
is Vega in the constellation Lyra. Close to Vega your binoculars
will reveal a binary star, which resolves into four stars with a
small telescope. A clear, moonless night may allow you to find the
Ring Nebula, as well. To the south is Altair in the constellation
Aquila (the Eagle). This bright triangle of stars (Vega, Deneb,
Altair) marks the onset of fall and the approach to winter. The
Milky Way divides the sky from northeast to southwest. Below Vega
to the west you will notice that there are fewer stars - your line
of sight is now out of the plane of the Galaxy. Binoculars may permit
you to find a spectacular globular cluster in the lopsided square
of the constellation Hercules - this system of about a million stars
is orbiting the Milky Way. Toward the east from Deneb you are again
gazing out of the Galaxy. Just to the northeast, about halfway to
the horizon you can pick up the faint smudge of the Andromeda Galaxy.
Though part of the Local Group of galaxies which includes the Milky
Way, this large spiral is not orbiting our Galaxy - it is about
2 million light years distant, and on a trajectory which will bring
it into collision with the Milky Way in the distant future.
For your own monthly star chart, you can direct your web browser
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.