Philips Atlas of the Universe

THE UNIVERSE

exploding as supernovae and spewing their heavy-element

enriched material into space to be used to form new stars.

One objection to this whole picture was that the back-

ground radiation coming in towards us appeared to be

exactly the same from all directions. It followed that the

expansion of the universe following the inflationary period

would have had to be absolutely smooth – but in that

case how could galaxies form? There would have to be

some irregularities in the background radiation, but for a

long time no trace could be found. Finally, in 1992, tiny

‘ripples’ were discovered.

The next requirement is to decide whether the present

expansion will or will not continue indefinitely, and here

everything depends upon the average density of the materi-

al spread through the universe. If the density is above a cer-

tain critical value, roughly equivalent to one hydrogen

atom per cubic metre, the galaxies will not escape from

each other; they will stop, turn back and come together

again in what may be termed a ‘Big Crunch’. If the density

is below this critical value, the expansion will go on until

all the groups of galaxies have lost contact with each other.

When we look at the material we can see – galaxies,

stars, planets and everything else – it is quite obvious that

there is not nearly enough to pull the galaxies back. Yet

the ways in which the galaxies rotate, and the ways in

which they move with respect to each other, indicate that

there is a vast quantity of material which we cannot see at

all. In the universe as a whole, this ‘missing mass’ may

make up more than 90 per cent of the total. It may be

locked up in black holes; it may be accounted for by

swarms of low-mass stars which are too dim to be detect-

ed; it may be that neutrinos, which are so plentiful in the

universe, have a tiny amount of mass instead of none at

all; it may be that the invisible material is so utterly unlike

ordinary matter that we might not be able to recognize it.

At present we have to admit that we simply do not know.

There is also ‘dark energy’, which seems to be of great

importance, but about which we know virtually nothing.

Assume that the overall density is enough to stop the

expansion of the universe. After perhaps 40,000 million

years following the Big Bang the red shifts will change to

blue shifts as the galaxies begin to rush together again at

ever-increasing speeds. Between ten and a hundred million

years before the Big Crunch, stars will dissolve and the

whole of space will become bright; ten minutes before the

Crunch, and atomic nuclei will disintegrate into protons

and electrons; with one-tenth of a second to go, these will

in turn dissolve into quarks – and then will come the crisis.

It is possible that the Crunch will be followed by a new

Big Bang, and the cycle will begin all over again, though it

is just as likely that the universe will destroy itself. If, on

the other hand, the density is below the critical value, all

that will happen is that the groups of galaxies will go out

of contact with each other; their stars will die, and we will

end with a dead, radiation-filled universe.

Recent research, based upon observations of the bright-

ness of remote supernovae, indicates that instead of

decreasing, as expected, the rate of expansion is increasing

• in fact, the universe is accelerating. Long ago Albert
  Einstein proposed a force acting counter to gravity, and
  called it the ‘cosmological constant’. Later he abandoned it,
  but if the universe really is accelerating it may be that the
  cosmological constant will have to be brought back. This
  points to an open universe, and no return to a Big Crunch.
  Which of these scenarios will prove to be correct – or
  are all of them wrong? Time will tell.

Sky map.New results

came in 2003 from the

MAP probe (Microwave

Anisotropic Probe). The age

of the universe was given

as 13,700 million years, and

this value is certainly more

accurate than any previously

obtained. The microwave

glow of gas from our Galaxy

is coded red. The microwave

glow from the early universe

is revealed in the speckled

grey background.

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