Philips Atlas of the Universe

The Expanding Universe

ATLAS OF THE UNIVERSE

B

efore we can make any attempt to trace the history

of the universe, we must look carefully at the situation

we find today. As we have seen, each group of galaxies is

receding from each other group, so that the entire universe

is expanding. We are in no special position, and the only

reasonable analogy – not a good one, admittedly – is to

picture what happens when spots of paint are put on to a

balloon, and the balloon is then blown up. Each paint

spot will move away from all the other paint spots,

because the balloon is expanding. Similarly, the universe

is expanding and carrying all the groups of galaxies along

as it does so.

For example, we may say that one particular galaxy

is receding at a rate of 2000 kilometres (1250 miles) per

second. Anyone living on a planet in that system would

maintain that it is the Milky Way Galaxy which is moving

away at 2000 kilometres per second; there is no absolute

standard of reference.

The distribution of galaxies is not random. They tend

to congregate in groups or clusters, and our own Local

Group is very far from being exceptional; for instance the

Virgo Cluster, at a distance of around 50 million light-

years, contains thousands of members, some of which

(such as M87, the giant elliptical) are much more massive

than our Galaxy. Moreover, there is a definite large-scale

structure; there are vast sheets of galaxies – such as the

so-called Great Wall, which is 300 million light-years long

and joins two populous clusters of galaxies, those in Coma

and in Hercules. It seems that the overall pattern of the

distribution of the galaxies is cellular, with vast ‘voids’

containing few or no systems.

If the speed of recession increases with distance, there

must come a time when an object will be receding at the

full speed of light. We will then be unable to see it, and we

will have reached the boundary of the observable universe,

though not necessarily of the universe itself. It has gener-

ally been assumed that this limit must be somewhere

between 14,000 million and 15,000 million light-years, but

we have not yet been able to penetrate to such a distance,

though the remotest objects known to us are moving away

at well over 90 per cent of the speed of light. The best cur-

rent estimate for the age of the universe as we know it is

13,700 million years.

Much depends upon what is termed the Hubble

Constant, which is a measure of the increase of recessional

velocity with distance. Measurements made in 1999, with

the Hubble Space Telescope, give a value of 70 kilometres

(45 miles) per second per megaparsec (a parsec, as we

have already noted, is equal to 3.25 light-years, and a

megaparsec is one million times this figure). At one time

there was a curious situation – it seemed that the universe

might be much younger than had been believed (no more

than 10,000 million years) and this would indicate that

some known stars are older than the universe itself, which

is clearly absurd. However, the Hubble team observed

galaxies out to 64 million light-years, and identified 800

Cepheid variables, so that the result seems to be much

more reliable than any previous estimate.

This may be the moment to mention what is called

Olbers’ Paradox, named after the 19th-century German

astronomer Heinrich Olbers, who drew attention to it

(though in fact he was not the first to describe it). Olbers

asked, ‘Why is it dark at night?’ If the universe is infinite,

then sooner or later we will see a star in whichever

direction we look and the whole sky ought to be bright.

This is not true, partly because the light from very remote

objects is so red shifted that much of it is shifted out of the

visible range, and partly because we are now sure that the

The Hubble deep field

picture:this is the deepest

optical image ever obtained.

It was taken with the Hubble

Space Telescope in 1995–6;

images were obtained

with four different filters

(ultra-violet, blue, orange

and infra-red) and then

combined. Only about a

dozen stars are shown –

the brightest is of magnitude

20 – but there are 1500

galaxies of all kinds. The

field covers a sky area

of 2.5 arc minutes. At this

density, the entire sky

would contain 50,000 million

galaxies, and we are seeing

regions as they used to be

when the universe was only

one-fifth of its present age.

The region of the deep field

photograph lies in Ursa

Major, chosen specifically

because it seemed to

contain no notable objects.

observable universe is not infinite. But how big is the

entire universe as opposed to the observable part of it? If

the universe is finite, we are entitled to ask what is outside

it; and to say ‘nothing’ is to beg the question, because

‘nothing’ is simply space. But if the universe is infinite,

we have to visualize something which goes on for ever,

and our brains are unequal to the task. All we can really

do is say that the universe may be ‘infinite, but unbound-

ed’. An ant crawling round a school-room globe will be

able to continue indefinitely while covering a limited

range; this is a poor analogy, but it is not easy to think of

anything better.

New information has come from Hipparcos, the astro-

metric satellite, which was launched in 1989 to provide a

new, much more accurate catalogue of the stars within

around 200 light-years of the Sun. The catalogue, finally

issued in 1997, provides new data about the luminosities

and proper motions of the stars, and this extra knowledge

can be extended to further parts of the Galaxy. It has even

been suggested that the observable universe may be

around 10 per cent larger than has been believed, but final

analyses have yet to be made.

Certainly the Hipparcos catalogue will be invaluable

as a standard reference for centuries to come. An even

more ambitious catalogue, Gaia, is tentatively planned for

the early 21st century.

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