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
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
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
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
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.