When you have eliminated the impossible, whatever remains, howsoever improbable, must be the truth. This was the maxim of Sherlock Holmes, Arthur Conan Doyle’s famous fictional detective. I wonder how Holmes would have reacted to some of the findings of modern astronomy.
On December 12, 1970, a satellite carrying an X-ray telescope was launched by the National Aeronautics and Space Administration (Nasa) from Kenya. To honour that country’s independence, the satellite was named Uhuru, meaning “freedom” in Swahili.
Although space astronomy was in a primitive state those days, Uhuru came up with the remarkable discovery of sources of X-rays in binary stars. Binary star system consists of two stars in which the member stars go round each other. The binary stars emit X-rays in the following situation.
Imagine the two stars in a binary system moving close to each other with one star compact and the other blown up as a giant star. Then the gravitational attraction of the compact star on its companion will extract matter from the surface of the giant star and pull it towards the compact star. As this matter falls towards the compact star it develops high temperature through friction and begins to shine, not because of optical radiation but due to X-radiation.
This scenario conjectured by theorists received observational support from Uhuru. However, there were a few anomalous cases and the most well known was Cygnus X-1. The name indicates the brightest X-ray source in the Cygnus constellation. Using Newtonian gravitation and dynamics, one can estimate the masses of compact star and the giant star as respectively around 15 and 20-40 solar masses. While there is no problem in believing in a giant star as massive, astronomers were at a loss to identify a compact companion as massive as estimated from the Cygnus X-1 data.
For the only known compact stars were the white dwarf and the neutron star whose masses, as per rigorous theory were not expected to exceed 1.4 and 2 solar masses respectively. So what was this strange beast of a star? There were no direct observations to tell the astronomers what this compact star looked like.
This was where the astronomers followed Holmes’ dictum. No known species of stars fitted the description of the compact star. Besides, it was invisible. Theorists had an object that fitted the description but it was too esoteric to contemplate. It was the black hole. A black hole is massive but compact, so much so that its surface gravity pulls back even light trying to escape from its surface. Imagine our planet Earth as a black hole if compressed from all sides to a radius no more than eight millimeters. As nothing else seemed to describe the situation, astronomers opted for the black hole interpretation.
Naturally, such “last resort” type of conclusion requires patience. The phrase “When you have eliminated the impossible” demands that all alternatives are worked out, tested and rejected. Unfortunately, similar patience was not shown by astronomers in many other cases as seen in the case of dark matter.
To appreciate the dark matter example one needs to look at the motion of planets in our solar system. Mercury, the planet nearest to the Sun, moves around nine times as fast as the most remote planet, Neptune. The reason is found in Newton’s laws of motion and gravitation. However, when we apply the very same laws to galaxies, we encounter problems. A number of galaxies including our own Milky Way have clouds of atomic hydrogen going round much the same way as the planets orbiting the Sun. Thus, following the Mercury-Neptune example, astronomers expected that clouds orbiting farther away would be seen moving slower. To their surprise, this did not happen. The clouds farther away continued to move with undiminished speeds.
If one feels strongly (as most physicists and astronomers do) that the Newtonian laws continue to apply here, then the only alternative left is to assume that there is unseen or dark matter surrounding the galaxy which extends far beyond its visible part. The alternative scenario involves modifying the existing Newtonian framework. Some scientists feel that this may well be needed. Indeed, a theory called Modified Newtonian Dynamics (Mond), proposed by Israeli physicist Mordehai Milgrom, has been developed to this end.
However, taking the Newtonian view as correct, astronomers have conjectured the nature of dark matter. Earlier preferred options like black holes, planetary masses, objects that were not massive enough to shine as stars do, etc., have today given way to more esoteric possibilities. The reason cannot be considered entirely scientific. It seems at least partly motivated by the desire to save the Big Bang cosmology, the theory that starts with the premise that the universe began with a mighty explosion with ejecta of high (literally infinite) density and temperature.
A finding quoted in support of this theory is the observed abundance of heavy hydrogen gas also called deuterium. A deuterium nucleus is made of neutron and proton, which combined together in the high temperature furnace that the universe was a few seconds after the Big Bang. Indeed the deuterium density observed today should be very much as calculated in the Big Bang theory. Indeed it was, until the early 1980s when dark matter evidence gathered strength. The Big Bang calculations on the other hand tell us that if all the dark matter were made of neutrons and protons, the resulting abundance of deuterium would be negligibly small.
As per normal scientific practice, even if the predictions of a theory are not supported by well-tested observations, the theory should be abandoned. However, in this case, in an attempt to save the Big Bang theory it is argued that the dark matter is not made of neutrons and protons, but it is made of a new, as-yet-unknown species of particles. Not only that, the quantity of unknown matter density is around six times the density of ordinary matter that is observed by astronomers through their telescopes and measured by physicists in their labs.
This brings us to a second dictum of Sherlock Holmes: “It is a capital mistake to theorise before one has data”. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
The writer, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy
and Astrophysics, Pune University Campus