Astronomy

Why is the Sky Dark at Night?

woman at night

Currently the question that challenges most astronomers is, ‘why is the sky dark at night despite so many stars?’. Surely the sky becomes dark not because the Sun has set, for it would look different to a space traveler. Over the centuries, several answers have been proposed: absorption of starlight in the interstellar medium, hidden stars, a dark cosmic wall, stars including the Sun being young, expanding Universe, insufficient energy in the stars, etc.

The riddle of the night sky’s darkness can be answered, if the dark unseen matter theory is accepted. The bright quasars, so distant from us, indicate that the young Universe was fiercely bright. As eons rolled by, the light cooled down to its after-glow. There is simply not enough energy available to light up the night sky fully.

If the expansion of the Universe is slowed down by gravity, then the matter we see today is simply not enough to slow down the rate of expansion significantly. There must be 100 times the matter we see in some as yet unknown form to account for the expansion of the Universe.

In 1970 an American scientist, Alan Guth, stated that the whole Universe inflated almost immediately after the Big Bang from a single infinitesimal ‘seed’. It implied that there had to be far more matter in the Universe than anyone had estimated before. The physical phenomenon we see today can be described in terms of four fundamental forces: the gravitational force, the electromagnetic force, the strong force (which holds together the nuclei in an atom) and the weak force (which causes some atoms to break down in radioactive decay). The inflation of the Universe is believed to have been brought about by the separation of the strong and weak forces into distinct types of forces we see now. One assumption of this theory is that 99 per cent of the Universe contains matter we cannot see. Guth’s ideas intensified the search for the missing matter.

All the light from all the stars and galaxies seen by optical telescopes represents only 1 per cent of the Universe. The visible stars themselves contribute only one-third of the gravitational force holding the stars in place. In fact, in the early 1930s the Dutch astronomer, Jan Oort, stated that one should imagine some dark matter in the greater part of space to explain the behaviour of stars we see, studies of the movements of galaxies indicate that they are surrounded by halos of some dark matter that cannot be seen by any telescope. The search for the missing mass has begun.

The IRAS survey of thousands of galaxies indicated that the background radiation in the Universe is uniform. This was later confirmed by the Cosmic Background Explorer Satellite (COBE) in June 1990. If the radiation from all directions is so smooth, then it indicates a smooth distribution of matter in the early Universe. However, this is apparently at odds with another finding, viz. the matter being clumped together in galaxies and clusters of galaxies which are not at all uniform. Only recently, a ‘great wall’ of galaxies spread out over 500 million light years was discovered.

The Harvard Smithsonian Centre for Astrophysics, which published these findings, showed clusters of galaxies separated by long stretches of empty holes. The distances in the Universe are staggering. Hence the question is one of accounting for a vastly different structure. How can a smooth Universe give rise to so different a structure? Some astronomers say that the galaxies which we see are not all there is to see. They are just the tip of the cosmic iceberg. Perhaps, the visible part does not indicate the underlying hidden uniformity and even distribution of matter.

In fact, the slight difference in the background radiation, detected recently was attributed to the motion of the Earth and the solar system about the center of the Milky Way at 230 km/s and the speed of the Milky Way, Virgo Cluster and Hydra Centauras—all moving in unison at 600 km/s. the motion is apparently caused by the gravitational pull of some dark matter. Perhaps the unseen matter consists of subatomic particles with little detectable energy, though capable of being shaped over time into galaxy-size clumps.

The picture of our galaxy is indeed complex. It is a disc of some 200,000 million stars (one of which is our Sun), gas and dust. The disc has a diameter of 100,000 light years (interestingly, a lakh in Indian notation) and the Sun is away from its centre. Our solar system is far out in one of the spiral arms, some 27,000 light years away from the galactic center. The entire system is itself rotating, once every 200 million years. The galaxy itself is surrounded by a spherical halo of globular clusters about 200,000 light years in radius. They seem to be older than most stars. Based on the orbits of stars and gas clouds, modern astronomers have concluded that there is a great deal of dark matter—perhaps 90 per cent of the mass of the galaxy—hidden in the halo of each galaxy.

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