The Future of the Universe: Implosion or Continued Expansion?
Evidence for Accelerated Expansoin
Perhaps an equally compelling question to the Origin of the Universe is the
Future of the Universe. There are only two possible outcomes. Either gravity
will slow down the expansion of the Universe and bring everything together again
in a so-called Big Crunch, or the Universe will never cease in its expansion
and simply go on forever until the light of every star winks out and we are
left with darkness. A key to determining the fate of the Universe is an accurate
determination of the total mass and of the rate of expansion. We have a pretty
good handle on the dimensions, with a Universal radius of 12-15 billion light
It is reasonable to expect the expansion of the Universe to perhaps come to
a halt. Gravity is a force that exists between any two objects, and so long
as the Universe contains objects of mass, such as superclusters of galaxies,
gravity will exist between them. Many cosmologists theorize that the Universe
"has to come to an end." They predict that gravity will slow the expansion
rate of the Universe until it comes to a complete halt, and then slowly force
everything back into the smallest of volumes from which it all began so very
long ago. In order for gravity to exert this influence, astronomers must ascertain
the mass of the Universe, and here the problems lie.
The total amount of visible matter in the Universe is insufficient to hold
together many of the structures that we see. Our own Milky Way has an estimated
200 billion stars and 600 billion solar masses, but this observed material cannot
alone hold our galaxy together. Even though it takes 225 million years for the
Sun to orbit the galactic core, this is still a very fast speed, and the stars
themselves ought to be flung out into empty intergalactic space. Astronomers
theorize the presence of invisible mass known as "dark matter" that
we cannot find, but whose gravity we can detect. This dark matter is what must
be present in order to keep order in our Galaxy, and this dark matter may comprise
half of the mass of the Universe, and perhaps even more. A description of this
dark matter defies explanation, and perhaps in time will be better understood.
The search for the mass required to collapse the Universe continues, even
today. Several years ago, astronomers were reviewing old photographic plates
from the Palomar Survey when they discovered that the Universe consisted of
twice as many faint galaxies as had previously been believed to exist. Even
with the extra observable galactic masses and dark matter, the Universe is still
50% of the mass necessary for gravity to halt its expansion. At present, it
appears that the Universe might be expanding for a long, long, time ... if not
Then, in 1999, astronomers in two different observatories made a stunning
discovery. Using Type Ia supernovae as cosmic distance indicators, they found
the Universe is not only expanding, but us actually accelerating in that expansion.
Termed "The Scientific Discovery of the Year" by the editors of the
journal "Science," in the January, 2000 issue, this finding that the
Universe is accelerating against the force of gravity rocked the Physics world,
for such an activity should not be happening ... and yet there it is, observable
for anyone with the right equipment.
The implications of this finding are huge. If the Universe is accelerating,
then it will NEVER collapse, and there need be no fear of the Big Crunch. The
Universe is not someplace within an endless cycle of expansions and contractions,
but perhaps existing in the midst of a one-time expansion event and accelerating
toward its demise.
Magill's Science Annual, 2000
The Accelerating Universe
Data from supernovas and background radiation show that the rate of expansion
is increasing and the Universe is flat
During the last two years an assortment of data consistently suggests that
the Universe has a flat geometry and is expanding at an increasing rate. Data
was released in the spring 2000 from two balloon experiments, Boomerang and
Maxima, carrying microwave detectors examining the cosmic background radiation.
This radiation de-coupled from the matter in our Universe in the distant past,
thereby revealing the early structure of the Universe. This radiation shows
regions which are slightly warmer or compressed subtending an angular range
of about 0.9 degrees, the size predicted in a flat Universe which was 300,000
years old. A Universe curved like a sphere or a saddle would have larger or
smaller hot spots, respectively. A dynamically flat Universe requires a specific
total amount of mass plus energy density, each of which will influence the expansion
of the Universe.
Within a distance of about one billion light years of us the Universe is observed
to be uniformly expanding. The recessional velocity, v, of each object is proportional
to its distance, d, from us, d/v = 1/H = 12± 2 billion years (12±
2Gyr). This is the standard equation of distance divided by speed equals the
time to travel that distance. Assuming unchanging velocities into the past,
this time, the inverse of the Hubble Constant, gives us the date when all of
these objects were together, marking the Big Bang origin of our Universe. Of
course the expansion rate may not have been constant, thereby giving us an erroneous
date for the Big Bang. Mass has a gravitational attraction, which slows the
expansion. Recent data (see below) indicate the expansion is speeding up, thereby
requiring a repulsive force. In the 1920s when Einstein believed in a
static (unchanging) Universe he postulated such a force to counter balance the
gravitational attraction. This was called the cosmological constant, L . This
is usually interpreted as a vacuum energy density with a negative pressure pushing
the Universe outward.
Knowledge of an acceleration of the expansion of the Universe is important
for several reasons. It will give us a more accurate date for the Big Bang.
We will have a clearer idea of whether the Universe will expand forever, ending
in a cold heat death, or collapse into a hot crunch. Combined with other data,
it will give us a better understanding of the composition and energy/mass density
of our Universe. It will also provide data to test various cosmological models
of our Universe.
The mass density, estimated from measurements of the total mass in large clusters
of galaxies, is only 30 percent of what is needed to make a flat Universe. This
mass is estimated from the motions of these galaxies and most of it is dark
matter, invisible to our telescopes. For a flat Universe the insufficient mass
density must be complemented with a vacuum energy density, which also causes
an accelerating expansion of our Universe.
The recessional velocity is caused by the stretching of space between objects,
similar to a stretching rubber band. We refer to this stretching as an increase
of the distance scale of the Universe. The wavelengths, l , of electromagnetic
spectral lines from a distant object are compared to the same spectra on Earth.
Their shift, D l , to larger values (redshift) is a measure of the increased
distance scale of our Universe. For example, z = D l / l = 0.83 from an object
6 billion light years away means that the fundamental distance scale of our
Universe has increased 83 percent during the last 6 billion years that the radiation
was traveling to us. By comparing the expansion rate over the last 6 billion
years with the expansion rate over the last one billion years we can deduce
whether it is accelerating or staying constant. To know the expansion rate we
need to measure distance, which for the distant object is 6 billion light years.
Evidence for an accelerating expansion
The most common method for measuring astronomical distances is the use of
so called "standard candles", or objects whose luminosity (emitted
light intensity) are uniformly known. Since the brightness of a standard candle
decreases as the inverse of the distance squared from the observer, its brightness
gives us its distance from us. Cepheid variable stars are reliable standard
candles for measuring distances to nearby galaxies but are not luminous enough
to see in more distant galaxies. Supernova explosions are a 100,000 times more
luminous for a short period of time. Type Ia supernova have a fairly regular
luminosity, so that we can know it within 25% uncertainty, thereby giving us
their distance to about 12.5% uncertainty. There is also a systematic uncertainty
for all types of standard candles, which will not be important in this discussion.
Type Ia supernova are carbon-oxygen white dwarfs, with little hydrogen, which
gravitationally implodes after sufficient mass is accreted onto its surface.
It then burns into 56Ni, which decays producing much of its luminosity.
Two international collaborators: the Supernova Cosmology Project, led by Saul
Perlmutter of the USA, and the High-z Supernova search Team, led by Brian Schmidt
of Australia, have combined to analyze about 100 type Ia supernova with z ranging
from 0.2 up to 1. The supernova at z= 1 occurred when the Universe was 35% of
its present age and its light traveled about 8 billion light years to reach
us. The search for high z supernova involves a telescope, with a CCD imager,
taking 10-minute exposures of patches of the sky, each with about 5,000 galaxies.
A few weeks later another image is taken of the same patch in the sky. A subtraction
of the first image from the second can reveal the appearance of a new light
source in the sky. Once it is identified as a supernova, the powerful Keck telescope
takes a spectrum of it. Models in which the Universe is expanding at a constant
rate predict that almost all of the high-z supernova should be brighter than
observed. Specifically they are 25 percent dimmer than expected at z = 0.5.
This suggests they are 12.5 percent farther away than a z = 0.5 supernova should
be. With the electromagnetic radiation traveling a longer distance to us, the
expansion of the Universe has more time to redshift the supernova spectra. A
longer time for the shift, z, means the Universe was expanding more slowly in
the distant past, than in the recent past. The expansion rate is speeding up.
Implications of an Accelerating Universe
Other explanations for dimming the high z supernova seem to be unlikely. It
is possible for cosmic dust to absorb some of the radiation, but this would
normally absorb more strongly in the blue thereby reddening the spectra. No
such effect is seen. An accelerating expansion of the Universe means that there
must be a repulsive force pushing it apart. The fit to the high-z supernova
data is adjusted to give the amounts of mass and vacuum energy density in our
Universe. The fit gives densities which are consistent with a dynamically flat
Universe. The fit attributes thirty percent of this density to mass and seventy
percent to the vacuum energy. This agrees remarkably well with the other cosmological
Another welcomed result of this data is that it solves the age crisis. The
age of the oldest stars, found in globular clusters in our galaxy, are 11 to
14 Gyr old. This is perilously close to the age of 12± 2 Gyr, calculated
for our Universe without any vacuum energy. Also the age of our Galaxy is determined
to be 10 to 20 Gyr from the radiometric dating of isotopes produced in its stars.
An accelerating Universe with vacuum energy, which was expanding more slowly
in the past, adds 1.5 Gyr to the age required to reach its present distance
scale. With these new results the best estimate for the age of the Universe
is 13.5± 2 Gyr. This should give enough time for the first stars to form,
presumably within the first billion years of the Universe. Finally, if the vacuum
energy density remains constant, the acceleration of the expansion will increase
in the future, resulting in a sooner cold heat-death for our Universe. This
will still be far into the future.
William R. Wharton, Wheaton College
Back to Inflation, Big
Bang, Expansion, or just go to
the Syllabus for a time
to think about all of this.
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