Dark matter : science or fiction

As scientists began to unravel more information about our galaxies they were intrigued by more questions than answers. One of the many questions was that of the eluding Dark matter. It so happens that as scientists began studying about the rotation of galaxies and their temperature distribution they were able to predict the amount of material present in particular parts of space and that is what they exactly did. But Scientists were stunned with what they had found. Amazingly it turned out that there was 5 times more material in clusters of galaxies than they had expected by observing the hot gases. Thus it led scientists to believe that most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then concluded that most of the matter in the entire Universe is invisible. This invisible stuff is aptly called 'dark matter’. Scientist use the term to describe any matter that is not detectable by our usual x-ray, optical or even satellite telescopes.Dark matter does not reveal its presence by emitting any type of electromagnetic radiation. It emits no infrared radiation, nor does it give off radio waves, ultraviolet radiation, X-rays or gamma rays. It is truly "dark." leading Cosmologists to believe that we can only see about 10 percent of the matter in the universe.Until they can accurately determine the mass of the universe, they will not know for sure whether it is expanding infinitely or will stop expanding at some point and collapse. So, how in the world do scientists know that dark matter really exists?

Well we have to hand it to our scientists they have found a way to detect dark matter too. The key to detect dark matter lies in the ever so reliable gravitational forces. The most obvious of these gravitational effects has to be the rotation of galaxies. This begs us to ask the question of how that is to be done. No worries to study galactic rotation, astronomers look at the emission line spectra of stars in each part of the galaxy. When the light from a star is observed using a diffraction grating or a prism, the starlight is separated into its true colors, in much the same way ordinary sunlight is separated by rain drops during the formation of a rainbow.The true colors constituting starlight separate into a series of light and dark lines in the visible spectrum, with each colored line corresponding to a specific wavelength of light. The specific wavelengths at which these lines occur are characteristic of the elements the stars Thus, they can be used as an elemental "fingerprint" to identify a star's composition.When a star emitting these line spectra is moving away from us, all of the wavelengths of the spectral lines are shifted to higher values than they would have been were the star stationary or moving side to side (neither towards nor away from us). All of the spectral lines are thus shifted towards the long wavelength part of the spectrum, or to the red end of the spectrum.

Similarly if the galaxies are moving towards us they spectral lines would shift to the short wavelength part of the spectrum, or to the blue end. These phenomenons are respectively called the ‘red shift’ and the ‘blue shift’. By measuring the shift in wavelength, researchers can calculate the precise speed of a star, either towards us or away from us.

When a galaxy is rotating, the starlight from stars on the side of the galaxy that is moving towards are blueshifted, while the starlight from the stars on the other side of the galaxy are redshifted. Thus, we can tell how fast and in what direction each individual star in the galaxy is orbiting about the center of the galaxy.
When stars orbit the center of a galaxy, their orbital speed is determined by the distribution of the mass contained within the galaxy. A graph showing the orbital speeds of the stars versus their distances from the center of the galaxy is known as the "rotation curve" for the stars in the galaxy. If one takes all the luminous matter that can be seen in the galaxy (stars, gas and dust) and predicts the rotation curve using the well-known laws of gravitational physics discovered by Newton, the speed of stars should decrease in a predictable manner the father away they are from the center of the galaxy. Scientists however noticed some hair raising data from the curves. They saw that the rotational speed didn’t fall off with distance as expected. Instead, the curves level off, and star far away from the center of the galaxy move faster than expected. The only way to account for this observation is that a large quantity of matter which cannot be seen--dark matter--exists in the galaxies. To explain the astronomical observations, this dark matter must surround the spherical distribution known as a galactic halo.Theoretical candidates for dark matter have been divided into two groups, dubbed MACHOs and WIMPs. The existence of MACHOs (Massive Astrophysical Compact Halo Objects) has been confirmed experimentally--recently in our own Milky Way galaxy. The nature and origin of MACHOs are currently a matter of great speculation and debate, but their masses and distributions have been measured by their gravitational effects. Proposals for candidate MACHOs include primordial black holes, as well as some types of new, exotic astrophysical objects whose properties have yet to be properly described.WIMPs (Weakly Interacting Massive Particles) are exotic, massive elementary particles that do not interact strongly with matter. (Hence they have not been interacting with our detectors so we have not detected them yet). Because WIMPs do have mass, and there would be great numbers of them, their individually weak but collectively strong gravitational effects account for part of the impact that dark matter has on the rotation curves of galaxies.Dark matter is known to exist through the gravitational effect it exerts on visible matter in the universe. Scientists have made discoveries that point very strongly to their presence. But even than a lot more work still needs to be done if we truly are to understand more about dark matter and how it behaves.
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By Tahsin Uddin Mullick
North South University, Dhaka, Bangladesh.
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The Aftermath Publications, Issue 4
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