Dark Mysteries of Our Mysterious Universe

At first the Cosmologists assured us that the visible matter in the form of the elementary particles (also called baryonic matter) was only a small fraction of the total matter in the universe; it was estimated to be only 5%. Rest of the matter was invisible; we couldn’t see it or feel it. But it has to be there because it was deduced from Einstein’s celebrated theory of general relativity. This invisible matter is called ‘dark matter’. Paul Davies (2) wrote in 1992, “..there is strong evidence for a large amount of dark or invisible matter, possibly enough to make up the deficit. Nobody is sure what this missing matter is.”

But surprise of all surprises, the dark matter does not account for the whole 95% of the invisible matter; it is only about half of the critical density. It is now believed that the remaining unaccounted for invisible matter is in the form of ‘dark energy’. The phrase ‘dark energy’ was coined by the University of Chicago Cosmologist Michael Turner, in a paper which he wrote in collaboration with Saul Perlmutter. Davies did not mention anything about dark energy in his book ‘The Mind of God’, which was published in 1992 because the concept is very recent.

Dark Matter

Dark matter was explained by Alan Guth when he formulated his theory of inflation of the universe in early 1980’s. Postulation of the existence of dark energy is the result of the most recent observations discussed by Saul Perlmutter (4) which showed that the rate of expansion of the universe is in fact accelerating. This acceleration is caused by the force of ‘antigravity’. Newton’s gravity is an attractive force while antigravity is repulsive; it repels other objects. All these theoretical concepts emerge from Einstein’s theory of general relativity which still remains intact and unmodified. Theory of general relativity predicts the existence of antigravity which Einstein didn’t like. In order to suppress the effect of antigravity which made the universe to expand, Einstein had introduced a constant in his equation which he called cosmological constant. Although this insertion was arbitrary and made for the specific purpose of keeping the predicted universe steady and static (non-expanding and non-contracting) which appealed to the prevailing wisdom, it did not mar the authenticity of his equation. This cosmological constant was like a ‘constant of integration’ with which most of the lay-readers having only an elementary knowledge of calculus may be familiar.

Soon after Einstein’s fiddling with his equation, Hubble’s measurements showed that the universe was actually expanding. Einstein’s vision of a static universe was wrong and he believed that the introduction of cosmological constant was unwarranted. He called this as the biggest blunder of his life. But wait, science is full of mysteries and unexpected thrills. The cosmological constant remains an important piece of the scientific puzzle as we will see later on and is quite handy in constructing new theories.

Antigravity is a force of repulsion. Alan Guth argued that if antigravity existed at the time of big bang, actual rate of expansion of the universe would be much greater than the normal rate without accounting for antigravity. This led him to formulate his inflationary theory according to which, the tiny universe at the instant of big bang suddenly exploded into a huge expanse of spacetime due to antigravity. This inflation of the universe was described by Davies (3) in these words: “As the universe expanded and cooled, it seems probable that the repulsive force would overwhelm the effects of ordinary, attractive gravity and cause the universe to embark on a phase of violent runaway inflation. In the minutest fraction of a second a submicroscopic region of space would swell up exponentially to cosmic proportions, doubling its size every 10^-35 second or so.” Inflation theory also predicted a flat universe.

Davies (3) also described some facts which were heretofore strange but now were explained by the inflationary theory as follows: “..it explains why the universe is so uniform. Any initial irregularities would be drastically diluted by the huge inflation…Inflation would also explain the otherwise miraculous balance between the explosive vigor of the big bang and the gravitating power of the cosmic material…any excess or shortfall in the expansion rate is wiped out when the exponential inflation takes over.”

While the theory of inflation explained several mysteries, the new observations and measurements discovered several others. These observations led to the discovery of accelerating universe and dark energy. Writing on ‘Quintessence’ on PhysicsWeb, Robert Caldwell and Paul Steinhardt (1) wrote, “….in 1998, two independent groups – the Supernovae Cosmology Project and the High-Z Supernova Search – announced a spectacular result based on significantly more precise measurements of cosmic expansion. Their observations of the brightening and dimming of distant type 1a supernova revealed that the expansion of the universe is in fact accelerating…The measurements indicated that the distant supernovae are dimmer than they ought to be if the universe was expanding at a steady pace.”

Dark Energy

According to Tim Radford, science editor of The Guardian, “The dark energy story began in 1998 when astronomers reported that the most distant galaxies seemed to be receding far faster than calculations predicted,” (Guardian Unlimited, December 19, 2003). Quoting Sir Martin (Rees) of the Institute of Astronomy at Cambridge, UK, Radford also said, “The dark energy is spread uniformly through the universe latent in empty space. Its nature is a mystery. Whereas there is a real chance of learning what the dark matter is within the next five to ten years, I’d hold out less hope of understanding the dark energy unless or until there is a unified theory that takes us closer to the bedrock of space and time.” So we hark back to unified theory (Theory of Everything) which remains as elusive as ever.

However, there are various speculations as to the constitution and nature of the dark energy. According to Paul Preuss (5), “Various types of dark energy have been proposed including a cosmic field associated with inflation; a different, low energy field dubbed ‘quintessence’; and the cosmological constant, or vacuum energy of empty space. Unlike Einstein’s famous fudge factor, the cosmological constant in its present incarnation doesn’t delicately (and artificially) balance gravity in order to maintain a static universe; instead, it has ‘negative pressure’ that causes expansion to accelerate.”

Existence of dark energy in the form of cosmological constant was initially proposed rather diffidently. It was one way to account for the missing matter. Michael Turner (6) said, “Several of us made a suggestion to save inflation: Add a cosmological constant for the missing two-thirds of the critical density…Inflation was the inspiration for the very successful cold dark matter (CDM)…. CDM theory is based on a flat universe, dark matter made of slowly moving elementary particles, and density perturbations arising from quantum fluctuations.” The CDM theory with a cosmological constant predicts accelerated expansion of the universe and the frustrating fact was that the earlier supernova results did not show acceleration. However with the discovery of cosmic speed up in 1998, everything fell into place. According to Michael Turner (6), “The universe is flat, with one-third in matter and two-thirds in something like a cosmological constant. Overnight skeptical astronomers became believers in inflation.”

The dark energy hypothesis seems to have provided a suitable explanation for the accelerating universe and to have opened up a promising vista for further development. However, the astronomers and physicists are considering other possibilities also. One of such possibilities is quintessence as has already been mentioned. According to Caldwell and Steinhardt (1), “..the concept of quintessence was introduced … by ourselves and Rahul Dave, then at the University of Pennsylvania, in 1998. Quintessence is a dynamic, time-evolving and spatially-dependent form of energy with negative pressure sufficient to drive the accelerating expansion.. Whereas the cosmological constant is a very specific form of energy with negative pressure – quintessence encompasses a wide class of possibilities.”

Physics and Astronomy are presently going through a very exciting stage. They are on the trail of a supreme unified theory which will bind all the fundamental forces of nature into a single theory and astronomy is performing an extremely difficult and challenging task of explaining the missing matter in the universe and shedding light on the dynamics of our evolving mysterious universe.

References

1.Caldwell, R, and Steinhardt, P, “Quintessence,” PhysicsWeb, http://physicsweb.org/articles/world/13/11/8, November, 2000.
2.Davies, P. “The Mind of God,” Simons and Schuster, New York, 1992, p.51.
3.Davies, P, “God and the New Physics,” Simons and Schuster, New York, 1983, pp. 184-185.
4.Perlmutter, S, et al, “The Supernovae Cosmology Project,” Journal of Astrophysics, Vol. 517, 1998, quoted by Robert Caldwell and Paul Steinhardt in (1).
5.Preuss, P, “Dark Energy Fills the Cosmos,” science beat, http://lbl.gov/Science-Articles/Archive/dark-energy.html
6.Turner, M, “Dark Energy: Just What Theorists Ordered,” Physics Today, wysiwyg://157/http.physicstoday.org/vol-56/iss-4/p.10.htm