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Most people confess to being utterly confused by the big bang. This
is no surprise. After all, the big bang was a one-off event and there is
nothing in our everyday experience that helps us to visualise it. Words
and pictures attempting to explain such an event are bound to be inadequate.
Who, for instance, can really imagine an explosion which occurred everywhere
at once and from which everything – space, time, matter and energy – suddenly
burst into existence?

But just because the big bang is unimaginable does not mean that it
is impossible to grasp its essential features. We asked three scientists
who really know about the big bang – particle physicists Steven Weinberg
and astrophysicists Jayant Narlikar and Martin Rees – to answer questions
readers ask most often.

Astronomers say all the galaxies are rushing away from us in the aftermath
of the big bang. Surely this means we are at the centre of the Universe
and that the big bang happened in our back yard?

‘Not at all,’ says Weinberg. ‘The Universe is in a state of universal
expansion. Every bit of the Universe is rushing away from every other bit.
It’s true that all the galaxies are racing away from us, but if you could
sit on any one of those galaxies and observe the sky from it, you would
see us racing away. In fact, you would see all the galaxies racing away
just as we do from our own Milky Way.’

Rees visualises universal expansion this way: ‘Think of the galaxies
as sitting at the vertices of a rectangular lattice. No vertex is any way
special. Now imagine that the entire rectangular lattice is expanding. The
view from every vertex will be precisely the same.’ Narlikar prefers another
picture. ‘Imagine the galaxies as dots painted on a balloon being inflated,’
he says. ‘All the dots move away from one another: no particular dot has
any special status. Similarly, the Universe looks the same from every galaxy
and so, viewed from any galaxy, the rest of the galaxies would appear to
move away.’

The idea that the Universe looks the same from every place is incorporated
into modern theories, says Weinberg. ‘It’s called the Copernican principle.
Copernicus discovered that there was nothing special about the position
of the Earth. It was just one of a number of planets. The Copernican principle
says there is nothing special about our Galaxy – that the pattern of the
Universe’s expansion looks the same from any galaxy.’

Why is it that the further away a galaxy is from us, the faster it is
moving?

‘It’s a direct consequence of the Copernican principle,’ says Weinberg.
‘If the expansion of the Universe is assumed to look the same wherever you
are, then you can deduce that the speed of expansion has to be proportional
to distance – in other words, that the speed of galaxies equals some constant
times its distance.’

This constant is known as the Hubble constant and the law as Hubble’s
law, after the American astronomer Edwin Hubble who discovered it in 1929.
Hubble’s law states that a galaxy which is twice as far away as another
is moving away from us at twice the speed. Similarly, a galaxy three times
as far away is travelling three times as fast, and so on. Today, this law
is known to hold true for galaxies at a distance of at least several billion
light years.

‘The reason that the speed of a galaxy is proportional to its distance’,
says Weinberg, ‘is precisely because of the Copernican principle that things
look the same wherever you are – whatever typical galaxy you are on.

‘Imagine looking at a galaxy which is, say, a billion light years away.
Then imagine a fellow in that galaxy looking at a galaxy along the line
of sight which is another billion light years further away. Imagine, the
galaxy you are looking at is travelling at 10 000 kilometres per second.
But the fellow in the second galaxy must also see the third galaxy travelling
at 10 000 kilometres per second. So the third galaxy, which is twice as
far away from you as the second galaxy, is travelling away at 20 000 kilometres
per second – twice as fast from you.

‘You can build this up into a proof that the speed of a galaxy is proportional
to its distance,’ says Weinberg. ‘And it’s all derived from the principle
that the Universe looks the same from all positions.’

Popular accounts, and even astronomers, talk about expanding space.
But how is it possible for space, which is utterly empty, to expand? How
can ‘nothing’ expand?

‘Good question,’ says Weinberg. ‘The answer is: space does not expand.
Cosmologists sometimes talk about expanding space – but they should know
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Rees agrees wholeheartedly. ‘Expanding space is a very unhelpful concept,’
he says. ‘Think of the Universe in a Newtonian way – that is simply, in
terms of galaxies exploding away from each other.’

Narlikar puts it differently. ‘Space is not utterly empty: it has visible
matter in the form of galaxies and also a lot of dark matter.’ Weinberg
elaborates further. ‘If you sit on a galaxy and wait for your ruler to expand,’
he says, ‘you’ll have a long wait – it’s not going to happen. Even our Galaxy
doesn’t expand. You shouldn’t think of galaxies as being pulled apart by
some kind of expanding space. Rather, the galaxies are simply rushing apart
in the way that any cloud of particles will rush apart if they are set in
motion away from each other.’ The matter inside individual galaxies does
not take part in the general expansion because it is held together by gravity.

The dark matter, which gives out no light, makes its presence known
because its gravity pulls on visible stars and galaxies. No one knows what
it is, but it accounts for at least 90 per cent of the mass of the Universe.

Last year, in a blaze of publicity, astronomers in the US announced
that they had discovered ‘ripples’ in the cosmic background radiation, the
cooled remnant of the fireball in which the Universe was born about 15
billion years ago. The discovery triggered a stream of readers’ letters
about the big bang radiation . . .

The matter the Earth is made of was in the big bang, and mixed in with
that matter was the fireball radiation. So why is the radiation from the
Big Bang still arriving at the Earth today? Shouldn’t it have passed us
long ago?

‘Well, radiation that was emitted from our immediate neighbourhood in
the big bang did pass us,’ says Weinberg. ‘It’s slightly complicated because
the Universe is expanding, but leaving that aside, radiation that was emitted
10 billion light years away passed us 10 billion years after the big bang.
Radiation that was emitted 15 billion light years away is just passing us
now.’ This is because the Universe is 15 billion years old.

‘The complication occurs because the Universe was smaller in the past,’
says Weinberg. ‘Light that may have been, at one time, just a few inches
away from us has taken this long to catch up because of the expansion of
the Universe since it was emitted.’ For light from the big bang, it has
been rather like running a 100-metre race with the track lengthening during
the race. The Universe has been expanding while the big bang radiation
has been struggling to get to Earth – the finishing post.

‘Despite this complication, it’s true to say that the radiation we are
seeing now is radiation that was emitted from far away. And the radiation
that has passed us was emitted not so far away,’ says Weinberg.

The wavelength of the fireball radiation has been stretched by the expanding
Universe in the past 15 billion years and now appears as microwaves. ‘What
microwave radiation we measure today came from far corners of the Universe,’
says Narlikar.

How is it that when we look at the cosmic background radiation we see
imprinted on the whole sky a picture of the Universe as it was shortly
after the big bang?

‘When you look at the sky, you don’t see a picture of the whole Universe
as it was 15 billion years ago,’ says Weinberg. ‘It’s a picture of that
part of the Universe that was sufficiently far away from us that the radiation
is just reaching us now.’

‘The Universe has a horizon, just like the horizon around a ship,’ says
Rees. ‘As time goes on the (Universe’s) horizon gets further away, just
as the horizon gets bigger if you climb up the mast of a ship.’

‘When we look at the sky we see a picture of a spherical shell of the
early Universe,’ says Weinberg. ‘It’s a sphere which, if the Universe hadn’t
been expanding, would have been 15 billion light years in radius (actually,
it was much smaller then because the Universe was much smaller).’

‘In the early epochs, radiation was well mixed with matter,’ says Narlikar.
‘Subsequently (about 100 000 years after the big bang), it got ‘decoupled’
from matter and started travelling freely. So the picture it brings today
carries the imprint of what it was like when the Universe was about a thousand
times smaller.’

Steven Weinberg, now at the University of Texas at Austin, won the Nobel
Prize for Physics in 1979. His book, The First Three Minutes, is a classic
popular account of the big bang. Jayant Narlikar is at the Inter-University
Centre for Astronomy and Astrophysics in Pune, India. He has written many
technical books and is best known for his collaborative work with Fred Hoyle.
Martin Rees of the Institute of Astronomy in Cambridge is a world-renowned
astrophysicist who recently collaborated on a popular acount of cosmology.