with regard to the theory of dark matter, it seems to me that common sense would be to assume that we can only detect a small amount of the substance of the universe. individual particles, light, dust etc strewn over infinite distances and that larger concentrations to create stellar bodies make up the rare but visible parts of the universe. why has it ever been assumed that planets and stars are the bulk of the matter in the universe? also, when we speak of looking back in time nearer to the origins of the universe say 14 billion years ago, is that a distance beyond the centre of the universe. how far can we look in the opposite direction. why assume nothing exists 50 billion light years away?
Because, any mass, at any temperature gives off some form of EM radiation. Even neutral Hydrogen in interstellar spaces radiates at about the 10cm line. A fact used by cosmologists to investigate the early Universe, walking the Lyman Alpha Forest as its called. Lyman Alpha being one of the allowed transitions in H. Dark Matter is just that, Dark. It does not give off measurable radiation. This is in total contradiction to all known behaviour of matter. Whatever it is, it's not the normal matter we 'see' every day. On the grounds there is 9 times more dark matter than 'light' then for every H atom radiating there 9 something elses not radiating. Wierd stuff.
yes, but if light and therefore radiation would possibly have mass, would not nearly all such emmissions throughout time be dark from our perspective. how accurately could we surmise sampling the light etc. detectible from earth over a limited time. if we calculate the mass of galaxies with a knowledge of gravitational forces and momentum, why would the discovery of blackholes at the centres of galaxies be of a surprise given a measurement of mass via em radiation
thed ... Re. "It does not give off measurable radiation." Or could it be that the radiation level is such that it is 'lost' against the CBR? Or is that what you mean by not giving off 'measurable' radiation? Curious.
Chagur It is not that the radiation is lost it simply is not there. Bruce I am not sure what you mean. The logic of the argument for measuring a galaxies mass goes like this. We know there is a relationship between the mass of a star and its luminosity. The Mass-Luminosity relationship. Given that a Galaxy is made of lots of stars (and gas and dust) there should be a similar relationship. The details are not known as there are many factors to take into account. One relationship that is used in the Tully-Fisher law. If the galaxy is a spiral you can measure the velocity of the rotation of the arms through red/blue doppler shifts (NB: This is not the Hubble red shift). This can be used to find the mass of the galaxy. Simple Newtonian mechanics. The value for this mass is hugely more than the value derived from the light. Not 10 times more but more like hundreds of times more. As I said, even neutral Hydrogen at 3.5 K in interstellar space gives off some radiation (light) and this is taken into account. Simply put, there is a mass present that does not radiate, give off EM radiation, light. It is 'dark'. If you do the maths for galactic clusters, their individual velocities are larger than the masses derived from the light output. Yet they are gravitationally bound. Again, mass is present that does not give off light.
