Is the Big Bang expansion really accelerating?

It sure looks that way! In fact, the discovery of this effect has been widely hailed as the Scientific Story of 1998 by a number of science journals.

For several decades now, astronomers have dutifully included the so-called cosmological constant in just about every research paper, as a valid alternate to the uniformly expanding Big Bang cosmological model. For more information on this, see my article in Sky and Telescope magazine. Of course, Big Bang cosmology includes versions of the universe where this constant is zero, or has a non-zero value. Astronomers who perform studies of the observational aspects of the universe have placed limits to the Hubble Constant, the value of 'Omega' and the so-called deceleration parameter 'q', but they are also obliged to include in their analysis cosmological models that have non-zero values of the Cosmological Constant as well.

What has changed in the last 5 years is that, from Hubble Space Telescope observations, astronomers studying such things as the number of gravitational lenses, and the dynamics of the clustering of galaxies over the last 5-10 billion years, have not ben able to say that the Cosmological Constant is zero. In fact, the very best they have ever been able to say is that, compared to the density of gravitating 'stuff' in the universe, this constant has a non-zero value that COULD be about as large as what visible and dark matter contribute to 'Omega'. That being the case, a non-zero value for this constant must mean that, at some level, the universe is not simply expanding and slowing down, but is expanding and slowing down more slowly than if purely gravitational influences were at work.

By 1997, the general picture was that the Hubble Constant was near 65 kilometers/sec per megaparsec to within 10 percent or so. This implied from the mathematics of Big Bang cosmology a specific 'critical density' of gravitating matter and energy in order for the universe to be a 'critical' universe balanced between collapse and eternal expansion. Now the question was whether all forms of identifiable gravitating stuff equalled this critical value or not. This inventoring of the matter-energy density of the visible universe has been going on for decades, and the current status of this is that only 1 percent of the critical density is in the form of luminous matter. However, in order to make distant clusters of galaxies stable and not fly apart, there must be some 'non-luminous' stuff out there too. In some clusters, there is so much of this 'dark matter' present that if it were present in the rest of the universe too it would amount to 40 percent or more of the critical density of the universe.

Now, astronomers have to compare there data, not just with models that have 'dark matter' but also the Cosmological Constant because we have no way of ruling out the Cosmological Constant beforehand. When these models are compared against the data, the most consistent cosmologies that come out are those that have very little luminous matter ( stars, galaxies etc), lots of dark matter in two forms ( hot and cold ), and a cosmological constant. Still, although these kinds of studies seemed to require a Cosmological Constant, there was never any actual detection of the required physical effects of such a constant. According to the Einstein-DeSitter model, whenever such a factor is present in the equations, it will result in a peculiar phenomenon...the rapid acceleration of the distances between two gravitating bodies. Gravity of course caused deceleration between bodies and can cause them to fall towards each other.

According to a review article about this in the December 18, 1998 issue of the journal Science, astronomers have just now found the first evidence for the expected acceleration effect. In 1998, two teams of astronomers who have been studying very distant supernova with the Hubble Space Telescope calculated the rate of expansion of the universe with these very distant supernova in galaxies located several billion light years from Earth. Although the numerous supernova detected in nearby galaxies did show the usual Hubble Expansion, when these teams independently studied the most distant galaxies covered by their supernova, they found them to be 10-15 percent dimmer than expected based on their redshifts and distances estimated from the Hubble Expansion. As they found more supernovae at these distances, this effect did not go away as it would if it had ben a statistical fluke.

What it means is that the distant supernova are farther away than they would be if the expansion has been a steady one over the last few billion years. This means that the universe has been expanding at an accelerating pace, not a steady one. This accelerated expansion is exactly what you would get if the Cosmological constant were non-zero, as other studies in the past had suggested, but not proved. According to their 'best fits' to the data they have, if the universe is at its critical density, then all forms of matter and energy ( luminous, dark, photons, neutrinos etc) amount to about 30 percent of the critical density, and the cosmological constant is about 70 percent of the total. Not only is the kind of matter we are made from a less than 1 percent 'impurity' with respect to the vast reservoirs of 'dark matter', but even dark matter itself doesn't completely determine the evolution and destiny of the universe.

Let me explain how the above logic works. Because Type 1A supernovae are produced by the detonation of white dwarfs, and because the maximum white dwarf mass is 1.4 times the Sun, these supernovae should all produce nearly the same peak luminosity at the maximum of the detonation. This provides the 'standard candle' to gauge distances. Now, the astronomers detect such a supernova in a distant galaxy, and measure its velocity spectroscopically. They then compare its apparent brightness against its luminosity to get its distance..say 2 billion light years. So far so good! Now there are three outcomes of this, but you have to keep in mind that what you are seeing is a snapshot of what the universe was like 2 billion years ago. the first outcome is that, when you divie the speed by the distance you get exacly the same value for the Hubble Constant as what we measure locally...65 km/sec/megaparsecs. This means that 2 billion years ago, the universe was expanding at the same rate it is now. the second possibility is that when you do the division you get a number that is bigger than 65 km/sec/mpc. This means that 2 billion years ago, the universe was expanding a bit faster than it is today, and so the universe is slowing down in its expansion as it gets OLDER. The third possibility is that you ge t a number that is smaller than 65 km/sec/mpc. This means that the universe was expanding slower 2 billion years ago than it is now, in other words the expansion is speeding up today. Another way of saying this is that objects sem to be farther away ( dimmer) than we would expect if the universe were expanding at the same rate then as it is now. It is this third case that SEEMS to be supported by the new data.

There is a fly in the ointment though. According to Big Bang cosmology, the value of the Cosmological Constant is...a matter how old the universe gets. The density of matter and energy, however, continues to decline with time. Why is it that we happen to live at a time in the history of this universe when these two quantities have about equal value? Astronomers and physicists do not like these kinds of 'anthropic' coincidences because it singles out the observer, and the time that he makes the observation in the history of the universe, as being special in some way. Since the Copernican Revolution, we have been very nervous about throwing in the towel on these kinds of issues.

There is another possibility too. Perhaps the environment around these distant supernova is dustier that the typical environment around nearby Type 1A supernovae. Astronomers will have to look at many more of these distant events to sort out the issue of reddening from dust absorption, and the cosmological constant effect.

In the next 2-5 years, there will be far more supernova detected, and new satellites flown by NASA to measure the Cosmological Constant and the other physical constants of the universe. If the 1998 findings are supported, we will have convincing evidence that we live in a very curious universe indeed!!

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All answers are provided by Dr. Sten Odenwald (Raytheon STX)