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 years.

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 forever.

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.

Expanding 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 1920’s 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 data.

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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