Questions & Answers

• Taking into account the mass-energy equivalence, could dark matter be the cosmic background radiation?

In fact, the cosmic background radiation does contribute, but only in a small way. It is estimated that the critical density of the Universe is ~ 2 x 10^-29 g/cm³, while the energy density of the cosmic background radiation corresponds to ~ 4.6 x 10-^-34 g/cm³. The contribution of this residual energy from the Big Bang was very important in the first moments of the Universe; today it is not so important because its energy density has diminished much quicker than that of ordinary matter.

• The amount of both visible and dark barionic matter in the Universe can be estimated from the relative amount of deuterium and helium; but how can we measure how much deuterium and helium there is in the Universe?

Each atom can be excited by heating the matter that contains it. An excited atom de-excites quickly, emitting photons of well-defined energy; these are seen as specific bright lines emitted (or absorbed) by each celestial body. Our sun emits (absorbs) a large quantity of bright lines: from the wavelength of the line we can establish which element has emitted it, and from the intensity of the line we can then deduce, for example, how many atoms of helium or deuterium are found in the outer parts of the sun. The measurements are made with very precise spectroscopes. It is worth remembering that helium was discovered by analysing solar light, before it was observed directly on earth.
The same method can be used to estimate the amount of deuterium and helium in the stars.
Helium and deuterium can also be found in masses of cosmic gas, but they can be seen only if there is a source to heat the said gas.
 

• What are the limits to the validity of Newton's universal law of gravity? Do we understand the force of gravity at very great distances?

In astrophysics we must often extrapolate the laws of physics to distances and masses which are much bigger than those that can be verified directly in the laboratory.
In particular, we can ask ourselves if Newton's universal law of gravity is really valid at stellar, galactic and extragalactic distances. Newton himself extrapolated his law (verified for our solar system) to the distance of stars, and now we must extend it to much greater distances.
At very great distances, gravitation might behave differently from how, with extreme precision, we have seen it behaves at the distance scale of the solar system; for example, it could have a dependence on distance R different from 1/R². Until now, however, there has been no indication of any deviation and we can therefore use it with confidence. Nevertheless, even if it is very unlikely, we should still remember that we need to carry out even better experimental verifications of the law of gravity at very great distances.
A very-massive celestial body introduces local variations in time-space. It is here, i.e. in the presence of large masses on relatively modest dimensions and therefore strong gravitational fields, that it is necessary to modify classic mechanics by introducing Einstein's theory of General Relativity. A light ray that propagates in space bends when it passes close to very-massive celestial bodies; this is because the ray follows the distorsion in time-space introduced by the presence of large masses.

• Why is it that a large cloud of hot gas in a mass of galaxies emits X-rays?   

The dark matter made up of heavy particles present in the group of galaxies heats the cloud, particularly free electrons; when these are subjected to accelerations they radiate X-rays. We do not observe directly the cloud of dark matter, but the effects on the electrons.