Dark Forces

Hubble Space Telescope image of the galaxy cluster Cl 0024+17 superimposed with a blue map showing the cluster's dark matter distribution.Image Credits: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University).




The puzzles of the universe seem to be never ending. The universe is sprinting and the dark energy is forcing the galaxies apart to accelerate the expansion. The dark matter outweighs the ordinary matter. To make matters worse, scientists have no idea about the origins of the dark energy or dark matter.HST, with its determined eyes scanning space had some clues, if not a complete answer. Dark energy was discovered a decade ago by two independent studies on the expansion of the universe. One study was led by Adam Riess of The Space Telescope Institute and John Hopkins University. It is mathematically equivalent to cosmological constant, introduced by Albert Einstein to balance the universe from collapsing under the pull of gravity (However, Einstein removed the constant once the expansion of the universe was discovered by Edwin Hubble). Riess used the HST data to improve the value of the expansion rate of the universe (Hubble constant) to an accuracy of three percent. Ironically, this implies that the dark energy, as Einstein assumed is steadily pushing the fabric of the universe. Riess and other researchers would eventually like to see the Hubble constant refined to a value with an error of no more than one percent, to put even tighter constraints on solutions to dark energy. Science is closing in to comprehend one of the most baffling concepts of modern day Astronomy.








Scientists estimate that 74% of the universe is dark energy, 22% is dark matter leaving the remaining 4% to account for all the galaxies and the intergalactic medium, known as normal matter. In the absence of the dark matter, this ordinary matter would fly apart, because the gravity of the normal matter cannot withstand the never ending assault of the dark energy. The search for the dark matter is a “ghost hunt’ as it is not detectable directly. However, the effect of dark matter is measurable through something called gravitational lensing. This is a technique that utilizes the fact that the light bends in the presence of a strong gravitational field. The HST has observed strong gravitational lensing in different galaxy clusters, which underlines the presence of dark matter as predicted theoretically by the Astrophysicist Fritz Zwicky in 1933. While studying the dwarf galaxies, HST provided strong evidences for the presence of dark matter. Hubble’s sharp view penetrated the cosmos to see that large number of small galaxies remain intact, even as the bigger galaxies around them are being ripped apart by the gravitational force of other galaxies in the cluster. The halo of the dark matter protect them like an invisible shield from the assault of gravitational tug of war going on inside the clusters for several billions of years.
“We were surprised to find so many dwarf galaxies in the core of this cluster that were so smooth and round and had no evidence at all of any kind of disturbance. They must be very, very dark-matter-dominated galaxies.” says Astronomer Christopher Conselice, currently the principal investigator of the HST survey. The halo of the dark matter protects the dwarf galaxies like a shield, from the sword of gravitational forces that emerge from the big galaxies.