New light on dark matter
(appeared in Mar 2018)

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A galaxy found to contain no dark matter has put cosmologists in a fix, says S.Ananthanarayanan.

The current view of the universe is that the bulk of its mass is a form of matter that we have not been able to detect. As opposed to matter that we see, this invisible kind is called dark matter. The current understanding is that there is many times more dark matter than ordinary matter and dark matter must always be there.

An apparent contradiction, however, has been reported, in the journal, Nature, by a group of cosmologists. Pieter van Dokkum, Shany Danieli, Yotam Cohen, Allison Merritt, Aaron J. Romanowsky, Roberto Abraham, Jean Brodie, Charlie Conroy, Deborah Lokhorst, Lamiya Mowla, Ewan O’Sullivan and Jielai Zhang, from the universities at Yale, San Jose and Santa Cruz in California, Toronto, Max Planck Institute in Heidelberg and the Harvard-Smithsonian Centre, Cambridge, report that measurements on a galaxy some 650 million light years away show that it contains almost no dark matter. This is in contrast to galaxies the size of the Milky Way, where dark matter is 30 times the mass of ordinary matter. The ratio is much higher in larger as well as in smaller galaxies, a whole 400 times higher in dwarf galaxies.

The idea that there must be unseen matter in the universe is based on features observed in the rotation of galaxies and was formally suggested a century ago. When objects are in orbit, the gravitational attraction towards the centre reduces as the objects move further away. For objects that are in stable orbits, the centrifugal, or centre-fleeing tendency because of their, is balanced by the force of gravity that draws them to the centre. Objects that are further away, hence, move slower, as it is a weaker gravitational force that their motion needs to balance.

An example is our own Solar System. The inner planets, Mercury, Venus, the Earth and Mars are at distances from 57 to 227 million kms from the sun and the time they take to go once around the sun ranges from 57 to 887 days. The average speed of motion ranges from 47 km/sec, for Mercury, to 24 km/sec in the case of Mars. The outer planets, Jupiter, Saturn, Uranus and Neptune are at 780 million to 4.5 billion kms from the sun and they go around in 11.9 to 165 years. The range of speeds is from 13 km/s, for Jupiter, to 4.3 km/sec for Neptune. We can see that the planets at greater distances from the centre move more slowly.

Surprisingly, this is not what we see in the case of large celestial formations like galaxies. In the case of rotating galaxies, it is found that the portions further away from the centre move at the same or at increasing speeds compared to parts that are closer to the centre. The mass of the galaxy, which can be taken to be at its centre, towards which the extremities are drawn by gravity, can be estimated based on parts of the galaxy that are detectable, visually, by radio telescopes and so on. Given this mass of a rotating galaxy, the force of gravity at the outer reaches can be worked out and hence how fast objects need to be moving. Observation, however, shows that the peripheral objects in galaxies are moving at higher speeds than worked out. In the normal course, these fast moving objects cannot stay in orbit and the galaxy should fly apart. One way of explaining why this does not happen is to propose that the galaxy actually has more mass than we can see.

As early as 1884, Lord Kelvin had suggested that the mass of the Milky Way, as determined from the speeds at which stars in the galaxy moved, appeared to be greater than the total mass of the visible stars. His was perhaps the first proposal that there was an unseen component that made up the mass of the galaxy. Subsequent studies, with superior equipment and instruments that became available, placed the concept of dark matter on a firm footing. Apart from the rotation speed of galaxies, other evidence was the bending of light rays, or gravitational lensing, which brings into view objects that should be hidden behind intervening galaxies. The extent of bending of light could not be explained by the mass of the visible part of the galaxy, and this became a tool to estimate the mass of dark matter that the galaxy contained.

The contradiction

The discovery reported by the group writing in Nature is of a galaxy, NGC1052–DF2, which appears to be different from previously studied galaxies and clusters of galaxies. The galaxy was observed using an automated array of multi-lens cameras, the Dragonfly telephoto array, which has been optimized for observing the surveying extremely low brightness parts of the universe. The case of this galaxy stood out, the group explains, because the Dragonfly image was one of a dim object with some substructure, but a star data base, the Sloan Digital Sky Survey, shows the galaxy as a collection of point-like sources of light.

The group hence collected more data of the spectral structure of the light that came from the galaxy. This data enables estimation of the speeds of motion of the objects from which the light emerges and this in turn, enables an estimate of the mass of matter exerting gravitational forces. The mass of the galaxy, based on the visible matter, they say, is about 200 million times the mass of the sun, or some 250 times less than the Milky Way. Typically, the mass, as estimated from the rotation speed of the galaxy, is many times the visible mass, on account of dark matter. But in this case, the data of movement speeds of 10 luminous objects in the diffuse galaxy revealed a mass that was practically the same as the visible mass – or a zero component of dark matter

The finding places a large question mark before the concept of dark matter. While dark matter itself has not been detected so far, its supposition is central to explain the structure of the universe. That dark matter is associated with the existence of ordinary matter, again, is the position that helps understand the distribution of a faint, but pervasive background of microwave radiation, a remnant of the ‘big bang’, 13 billion years ago. The present finding, however, is an instance of a large region in space where there is no dark matter. Other findings, as when there are collisions of galaxies, have shown that it is possible for dark matter to move independently of ordinary matter. And as dark matter has not been discovered so far, alternate theories have been proposed to explain the greater gravitational effects within and between galaxies.

A leading alternate theory is Modified Newtonian Dynamics (MOND). This theory suggests that just like relativistic effects kick in when gravitational forces are very strong, there are changes in gravity when the forces become extremely feeble. Working on these lines, one is able explain the anomalies in rotation of galaxies with great accuracy. The same theory, however, needs to fall back on dark matter in some circumstances. The discovery of a galaxy where there is no dark matter would hence embarrass some alternate theories too.

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