Here I talk about a story from my favorite Domain “Physics” (No computation this time but the next blog will relate this story to Computational Physics)
|Image Source: http://planetquest.jpl.nasa.gov/news/darkMatterPie_image.html
Although they partially share the names, dark energy and dark matter are otherwise different. The former is a force that makes the universe expand at an increasing speed while, later is a type of matter that outweighs ordinary stars and galaxies 5 to 1 (see Figure above). The dark energy is named so because we hardly know about and the dark matter is dark as it is utterly invisible. We know it’s there because its gravity pulls galaxies and stars around, but it neither emits nor reflects any light. The concept of Dark Energy won the Nobel recently. Well my current story on dark matter is not an advocacy for the nobel committee rather an attempt to tell you something about it. In the next blog I would bring out the how scientific computing is used or can be used for related research.
For the historical facts regarding previous research and history of discovery I would recommend to look at the wiki page for dark matter as I would like to keep the story close to the scientific analysis. Lets start with a simple question why does matter or better say two bodies separate. If you know about Newton laws of gravity you would come with with the answer that says because the force of attraction acting between two bodies is not enough to hold them together. You are right. So if they expand the force of attraction should get even smaller as the distance has increased. Well done. So we conclude that the masses of the bodies in the universe are somehow important. Great, now I put the same question in a different way.
In terms of orbital motion. Will there be a difference in the orbital velocity of a body orbiting close to the orbital center and the body far from the centre? From what we saw till now the answer should be yes. The body closer to the orbital centre should have a higher speed of revolution as compared to the body far from the orbital centre (Kepler’s 3rd Law). This should happen because the acting force on the orbiting body differs with the distance to the orbital centre. Here is the catch, in the galaxies observed so far, that goes against. The orbital speed at the edge of galaxies is not very different from those at the centre. This made people pause and ponder, looking for a possible explanation. So if our knowledge about physics of motion is correct there should be something around that makes this uniform speed at different distances possible. If there is some distributed matter that compensates for this loss in gravity and makes the orbital speed almost uniform across the galactic radii, it would explain the deviation of our observation from expectation.
Let’s talk about another unusual phenomenon. When you see an object, light from the object comes to you which produce undistorted image given there in nothing deviating this light between the object and the observer. Now if we place a glass piece (slab, curved or spherical) between the object and observer we get a distorted image of the object. This distortion will depend on the shape of the glass object. This phenomenon has been observed with some of the galactic clusters rendering their distorted image. So we should expect some sort of glass piece between us and those clusters. But there can be other things than glass that produce similar effects, e.g. gravity. With gravity acting as a lens the phenomenon is called “Gravitational Lensing”. The observed images have suggest for a clustered object between the light source and the observers (us).
This somehow gives us a clue that there is something in the universe that is there in abundance but we cant really see that. This gives rise to the concept of “Dark Matter”. There are some other proposed evidences in favor of their existence, but I prefer not to get into further details, rather we try try to see what really is (or can be) dark matter. Answering this question is somewhat difficult but let me try.
As of our current knowledge, dark matter:
>> Has gravitational effect
>> Does not interact with light
>> Does not emit light
>> They should be enough in amount to account for our observation
>> They should be retained within galaxy (else they wold escape the galaxies and our observations will be different)
Based on this we take a look at the ordinary matter or known bodies as possible candidates for constituting dark matter. For most of the heavier hidden objects like MACHOs their estimated amount is not enough to be considered absolute for dark matter. The sub-atomic candidate ‘neutrino’ for this has a speed enough to escape galaxies gravitational pull. So dark matter can consist of a completely new kind of matter (or particle) or some known matter (or matter) in unknown state.
As per our current hypothesis dark matter particles interact with each other only through gravity, moving past each other without collision. Thus there can be numerous dark matter particles around us (even within). A class of hypothetical (rather unknown) particle called WIMPs (Weakly Interacting Massive Particles) is another candidate for constituting the dark matter. But all these are just candidates, yet to be proved and established. Scientist are setting traps to capture and know more about the dark matter but it is yet to be realized. There are theories about cold, warm and hot dark matter based on their speeds (slow, relativistic, fast). But just the theories and at times, competing theories. Currently, the cold dark matter forms the more accepted theory than other candidates and is more agreed upon with our observation. It explains the galactic phenomenon like large structure formation (accumulation of matter) but there are arguments against it as well.