Thursday, 20 October 2011

Dark Matter For Non- Experts and Non- Physicists

The matter we are made of and see around us, like,  fire, rain, chairs, planets, stars etc. are made of electrons, protons and neutrons. Since they emit/reflect light,  heat or other radiations (electromagnetic waves) we can ``see'' them either with naked eye or telescopes or other detectors. Thus, though the air in the room is invisible, it is also a normal matter because we can detect the heat emanating from it by a thermometer or an Infrared Detector. Since most of the mass of such matter is due to protons and neutrons (rather than light electrons) which are collectively called ``Baryons'', normal matter is also referred to as ``Baryonic Matter''.

On the other hand if there would be any matter which do not emit/reflect light, heat (in any form), we would not be able to ``see'', i.e., detect them with either eye, or any telescope. And such a form of HYPOTHETICAL matter would be called as ``DARK MATTER'' (DM). 

However all forms of matter, visible or non-visible obey the same laws of Gravitation, they exert mutual gravitation the same way. This simply means that an APPLE will hurtle towards the ground the same way even if it were invisible. Thus even though DM is not visible by eye or telescopes, ITS EFFECT CAN BE FELT through its gravitational interaction. In other words ``Gravitational Mass'' of both Normal Matter and Dark Matter can be found on equal footings.

And when astronomers do their measurement of say a distant Galaxy Cluster, they try to measure both the (i) Amount of Visible Matter, by direct observations as well as (ii) Gravitational Mass, by indirect ways, in the framework of certain theories .
And they often find that:

``The Gravitational Mass is Much LARGER Than The Visible Mass''

In this way, astronomers have inferred the presence of UNSEEN Matter or ``Dark Matter'' in many Galaxy Clusters and even in the outer reaches of our own galaxy called ``Milky Way''.



DM could comprise some elementary particles which hardly interact with normal matter. One such particle is ``Neutrino''; every moment thousands of neutrinos are passing through our body, yet we do not feel any pain or any effect because they do not interact with our eye, nerve, or tissues made of electron, proton and neutron. However, originally, it was believed that neutrinos have no mass just like light quanta (photons) have no mass. Then it was hypothesized that neutrinos may have a tiny mass may be 0.001% of electron mass. With such a tiny mass, neutrinos would still be very agile and move with a speed close to that of light.

Such FAST & AGILE neutrinos would constitute what is known as


In fact even up to 1980, most of the cosmologists thought that they require just such HDM, and may be just neutrinos.

But soon it became clear that in order that various cosmic structures, like Galaxy Clusters, Super Clusters etc. could form in a relatively short span of the Universe (as per the Big Bang HYPOTHESIS/ASSUMPTION of the origin of the Universe, the DM particles should be heavy & sluggish. And thus came the concept of


Here the elementary particles constituting DM should be much more massive, say 10-100 times as massive as Protons and move sluggishly with a speed negligible compared to the speed of light. Simultaneously, such particles should be entirely different from protons or neutrons. And scores of big experiments world over have FAILED TO DETECT ANY SUCH FICTITIOUS PARTICLE! It was hoped that the Large Hadron Collider (LHC: Hadron =Heavy Particle) should finally be able to create and detect them. But LHC TOO HAS FAILED TO FIND ANY SUCH EXOTIC PARTICLES!

And now observations on two close by small galaxies, which were supposed to be the best hosts for such CDM show that they may not have any such CDM made of exotic elementary particles!

The finding goes completely against the current cosmological models, which suggest that dark matter should be clustered at the centers of galaxies.

"After completing this study, we know less about dark matter than we did before. Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies," said Matt Walker, the study's lead author.

This may be considered as a big set back for the Big Bang Paradigm as its predictions go haywire.


The two above forms of DM are non- baryonic because the elementary particles constituting them are intrinsically different from the social/interacting normal matter.

But, in principle, CDM may comprise, planets and very tiny cold and faint stars called ``Brown Dwarfs''. This is so because they have no light of their own and astronomers are rarely able to ``see'' them. For example, the maiden planet outside the solar system got detected (not seen) may be in 1994!

Also, some physicists claim that CDM is nothing but extremely cold interstellar clouds of Hydrogen Molecules, which hardly emit any radiation. In fact, such cold hydrogen could be so cold that it would be in SOLID form!

Note, although such form of CDM would be invisible, they are made of Normal Matter; i.e., electrons, protons and neutrons.However Big Bang Model requires Exotic Non-normal Cold Dark Matter.


Some physicists claim that actual law of gravity used to infer the ``Gravitational Mass'' could in subtle error; i.e., as if one overestimates the ``mass'' of the falling APPLE by an incredibly small amount. Although such claimed overestimation must be negligible for the apple, it can be significant on galactic or cosmic scales. Thus as per some physicists, there is actually no unseen/dark matter.

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