Still in the dark

C Sivaram sheds some light on dark matter believed to dominate the masses of all galaxies.

A  conundrum crucially confronting current cosmology and astronomy is the nature of the all-pervading dark matter believed to dominate the masses and clusters of galaxies apart from accounting for one-fourth of all matter in the universe.

Although experiments to detect this ubiquitous dark matter have been going on for the past two decades, there is still no positive clue as to what it consists of.

Let us first look at the observational evidence – that such matter should dominate large cosmic structures such as galaxies and clusters of galaxies. Curiously, the evidence for the existence of such non-radiating matter or dark matter (DM) goes back to over 80 years ago. The Swiss-American astronomer Fritz Zwicky was trying to estimate the mass of large clusters of galaxies.

As we know, a large galaxy like our Milky Way is a gravitationally-bound large cosmic structure containing some hundred billion stars. There are many galaxies, smaller than ours, having a billion stars or less. Just as large numbers of stars cluster to form galaxies, galaxies themselves group to form large galactic clusters which may contain 30,000 or more galaxies.

The clusters themselves can group to form super clusters. Just as the Earth’s orbital velocity and distance from the Sun enables us to estimate the mass of the Sun using Kepler’s laws and Newton’s gravitational force law, the motion of the Sun around the galaxy enables us to estimate the total mass of the galaxy.

Similarly, the motion of the galaxies in a large cluster of galaxies and the size of the clusters enables us to deduce the total mass of the cluster. This mass is called ‘dynamical mass’. Thus Zwicky (and others) estimated the dynamical mass of several rich galaxy clusters. We can also estimate the mass of the cluster in another way.

Luminous mass

We know the mass of a typical galaxy and its luminosity (the power emitted in watts, due to all the stars, nebulae). So, by estimating the luminosity of the galactic cluster and comparing it with an individual galaxy, we can estimate the cluster mass.

Thus, if it is thousand times more luminous than a typical galaxy, the mass must be 1,000 times that of a galaxy and so on. The mass measured in this manner is called the ‘luminous mass’.

Zwicky learnt that rich galaxy clusters like the Coma cluster have dynamical masses at least 100 times their luminous mass. He concluded that the matter in such clusters is not made up of luminous objects like stars, or clusters of stars, but consists of matter which does not radiate – it is dark and this was dubbed Dark Matter (DM).

Zwicky’s inference

Zwicky’s observations were later confirmed by others and although he had overestimated the amount of DM, it is now accepted as an established paradigm. Later observations revealed unmistakably that even individual galaxies like the Milky way are dominated by DM.

Indeed, as much as 90 per cent of the galaxy mass is due to DM. How do we know this for galaxies? Briefly most of the light from the galaxy comes from the central regions, where most of the stars are concentrated. However, if most of the mass is also in the central part, the velocities of objects orbiting the galaxy, far from its center, should fall off with distance according to Kepler’s Law.

For example, since most of the mass of the solar system is concentrated in the Sun, a distant planet like Pluto moves at only 3 km per sec, whereas the Earth orbits the Sun at 30 km per sec. Surprisingly, it turns out that objects orbiting the galaxy at larger distances from the galactic center move around more or less at the same velocity as objects much closer to the center.

This can only be accounted for, if the mass progressively increases with radius as we move out further away from the central region. But this matter does not radiate as most of the light is from the central region. So, the conclusion is that 90 per cent of the galaxy is DM. This also seems to be universally true for all types of galaxies. It was first thought that the DM may be very faint. But sensitive measurements over all wavelengths suggest that such objects can hardly account for five per cent of the DM.

The consensus now is that it is some new kind of particle which does not couple with radiation. For some time, many people thought that a familiar particle like the neutrino (produced in beta-decay and nuclear reactions) known to have been copiously present in the early phase of the Universe could have clustered around galaxies and galaxy clusters and is perhaps the DM.

But the mass deduced of neutrinos is too low and may account for less than one per cent of DM. So, many new particle candidates have been proposed.

Popular candidates are WIMPS (Weakly interacting massive particles), axions, gravitinos, glue balls, gluinos, Q-balls and wimpzillas. They should have been produced in extreme energy conditions in the early Universe and should now hang around as DM. That is why the LHC (which mimics the energy when the universe was one picosecond old) is expected to produce such particles.

Again, as such particles are supposedly ubiquitously present as DM, they could be detectable in clever laboratory experiments. They should brush shoulders with normal atoms and nuclei producing detectable signals. They can impart their momentum to a nucleus or electron causing small recoil which can be measured.

So, many sophisticated underground experiments are going on like the DAMA experiments, the Iodine, Xenon, Cd-Te, experiments. These experiments are all underground so as to stop buff of other particles like cosmic rays.

Experiments

The DAMA experiment some time back reported evidence for AM particles about thirty times the proton mass, the signal also had annual variation expected from the about three per cent variation in the Earth-Sun distance over a yearly period! But this was not confirmed by other experiments.

The axons experiment also provided no positive clue. The experiments are sensitive to certain mass ranges. Although much money has been spent on the experiments for years, we are still in the dark as to what type of particle (or particles) the DM is.

Meanwhile some scientists have pointed out that unusual gamma rays of energy, around 50-60 Giga electron volts coming from the galactic center could be evidence of annihilation of DM particles with their anti-particles.

They would be expected to be concentrated at the galactic center, so that their annihilation rate would be much higher. Of course, this could be also explained as radiation coming, for example, from high energy processes from pulsars.

But the result so far is that we are still in the dark about what really is dark matter. Indeed, we are at the crisis point in the hunt which has been likened to looking for a black cat in a dark room.