[astro] Is Earth at the heart of a giant cosmic void?

[Eugen Leitl]

http://www.newscientist.com/article/mg20026821.200-is-earth-at-the-heart-of-a-giant-cosmic-void.html?full=true&print=true

 Is Earth at the heart of a giant cosmic void?

    * 12 November 2008 by Marcus Chown

    * Magazine issue 2682. Subscribe and get 4 free issues.

(Image: Daniel Sicolo / Design Pics Inc / Rex Features)

IT WAS the evolutionary theory of its age. A revolutionary hypothesis that
undermined the cherished notion that we humans are somehow special, driving a
deep wedge between science and religion. The philosopher Giordano Bruno was
burned at the stake for espousing it; Galileo Galilei, the most brilliant
scientist of his age, was silenced. But Nicolaus Copernicus's idea that Earth
was just one of many planets orbiting the sun - and so occupied no
exceptional position in the cosmos - has endured and become a foundation
stone of our understanding of the universe.

Could it actually be wrong, though? At first glance, that question might seem
heretical, or downright silly. But as our cosmic horizons have expanded over
the centuries so too has the scope of Copernicus's idea. It has morphed into
the Copernican, or cosmological, principle: that nothing distinguishes the
position of Earth's galaxy from any other place in the entire universe. And
that idea, some cosmologists point out, has not been tested beyond all doubt
- yet.  Copernicus's principle has not been tested beyond doubt — yet

That could be about to change. A new generation of experiments might shore up
the cosmic orthodoxy - or blow it out of the water. That unexpected
alternative, some people go so far as to say, might be no bad thing at all.

The modern-day Copernican principle amounts to two assumptions. First, that
averaged over large enough scales the universe is homogeneous, having
essentially the same properties in all locations. Second, that the universe
is isotropic, or appears to have the same properties when viewed in any
direction from every location. These two ideas are intimately related, but
logically separate (see diagram). They were introduced into cosmology not
because of any observational evidence, but to save face. In 1917, Albert
Einstein had applied his theory of gravity - general relativity - to the
dynamics of the universe. Without the simplifying assumptions of homogeneity
and isotropy, Einstein's fiendishly complex equations proved impossible to
solve.

Even with those assumptions, Einstein's initial insistence that we live in an
unchanging universe led him to the wrong solutions. By dropping the
"unchanging universe" requirement a few years later, cosmologists created the
picture that became the kernel of today's phenomenally successful big bang
model. In this picture, the universe started out as a single, infinitely hot
and dense point in space, and has since been expanding - initially rapidly,
but gradually more slowly as gravity has exerted its pull on the mass of the
cosmos.

All seemed well, with evidence in support of the big bang model piling up
throughout the 20th century. Then, in 1998, astronomers studying stellar
explosions known as type 1a supernovae made a sensational discovery. These
supernovae are thought to be uniformly bright, so that the fainter they
appear to us, the farther they must be away. But measurements showed that the
most distant supernovae did not fit in: they were a lot fainter than they
should have been, and seemed impossibly far away. Some time over the past few
billion years, they must have begun to race away from us ever faster. Rather
than the universe's expansion slowing down, it looked like it was speeding
up.

This startling possibility can be accommodated by the standard cosmological
equations, but only at a price. That price is introducing dark energy - an
unseen energy pervading space that overwhelms gravity and drives an
accelerating expansion. Dark energy is problematic. No one really knows what
it is. We can make an educated guess, and use quantum theory to estimate how
much of it there might be, but then we overshoot by an astounding factor of
10120.

That is grounds enough, says George Ellis, a leading cosmology theorist based
at the University of Cape Town in South Africa, to take a hard look at our
assumptions about the universe and our place in it. "If we analyse the
supernova data by assuming the Copernican principle is correct and get out
something unphysical, I think we should start questioning the Copernican
principle."

On the face of it, homogeneity and isotropy are unlikely assumptions. Just
take a look at the night sky. It is anything but uniform, with most stars
concentrated in a band across the sky - the Milky Way.

Of course, that's not the full picture. In 1924, Edwin Hubble discovered that
certain diffuse sources of light in the night sky, called spiral nebulae, are
actually groups of stars far beyond the Milky Way. The realisation came that
the broad swath of the Milky Way is just our own galaxy - the bright lights
of downtown seen from our distant suburb - and that it is merely one among
countless others splashed across the heavens.

Since then, surveys have shown how galaxies are distributed more or less
isotropically - evenly in whichever direction we choose to look. What's more,
the cosmic background radiation - the afterglow of the big bang fireball,
discovered in 1964 - has pretty much the same intensity and temperature
whatever direction we look in.

So while the case for isotropy seems virtually sewn up, the evidence for
homogeneity is much less convincing. It is also harder to come by. To create
a three-dimensional picture of how matter is distributed in the universe, we
need to know how far away different galaxies are. That would mean identifying
galaxies that, like type 1a supernovae, are uniformly bright at all distances
- a near impossible task, as most galaxies are dynamic, ever-changing bodies.

According to Ellis and others, our uncertainty about galaxy distances allows
an interesting possibility. The distribution of matter could look the same in
all directions, but vary with distance from us. In particular, we might be
sitting in the middle of a "void" - a vast spherical bubble in an otherwise
homogeneous universe. This bubble is not devoid of matter. In fact, most of
the stars and galaxies we can see from Earth would be contained within it.
It's just that everywhere beyond it, which is too far away to see, the
density of stars and galaxies is much higher.

How would such a bubble help? In such a low-density region, the braking pull
of gravity is weaker, and so the region would quite naturally be expanding
faster than the more dense area enveloping it. A bubble surrounding us,
covering the volume from which light emitted over the past few billion years
is just reaching us, would be just the thing to explain the supernova
observations. Observing from within such a bubble, but using distant
supernovae as yardsticks, we would see a universe whose expansion seems to be
occurring faster than it used to - without the need to invoke dark energy
(see diagram).

"Dark energy is a necessity if we assume the supernova acceleration is due to
a change in the entire universe's expansion rate over time," says Ellis. "But
it's equally possible, and no more radical, to say that it reflects a change
in the universe's expansion - in space."

But here's the rub. For things such as the cosmic background radiation to
appear isotropic to us from within a void, we would have to be at or
extremely close to its centre - which is not only anti-Copernican, but also
highly unlikely. Ellis is unperturbed. "We live in an improbable universe,"
he says. "You can shift around the improbability - for instance, substituting
an Earth-centred void for dark energy - but you can't remove the
improbability." The problem is to find ways to tell a homogeneous from an
inhomogeneous universe. "Without being able to move from our location, that's
very hard."

Robert Caldwell of Dartmouth College in New Hampshire agrees. "It would great
if there were someone out there who could look back at us and tell us if
we're in a void," he says. "Or if we could look in a distant cosmic mirror
and see ourselves."

Remarkably, that just might be possible. Caldwell and his colleague Albert
Stebbins have been on the case, developing a void-testing idea dreamed up by
Jeremy Goodman of Princeton University in 1995 (Physical Review D, vol 52, p
1821). Their scheme involves exploiting the effect that a void would have on
the well-travelled photons of the cosmic background radiation (Physical
Review Letters, vol 100, p 191302).

The story of these photons starts about 400,000 years after the big bang,
when the universe, previously a dense ionised soup of charged nuclei and
electrons, had cooled down enough for neutral, uncharged atoms to form.
Photons had got stuck in the charged soup, but could now suddenly travel
unimpeded through the neutralised cosmos.

Obstacles to the photons' progress began to reappear after some 200 million
years, as the first stars or quasars began to re-ionise neutral atoms.
Nevertheless, most of these photons continued on untroubled, slowly losing
energy as they journeyed through the expanding cosmos. In some cases, they
made their way into our telescopes more than 13 billion years down the line.

So what happens to these photons in a void? When they pass by matter, they
receive an energy boost. In a void, this gravity assist is less pronounced,
and the photons lose energy. They regain the energy on leaving the void
again. In fact, because the void itself is expanding and becoming emptier
while the photons cross it, they gain a tiny bit more energy on crossing back
into a denser region than they lost on entering the void.

As a result of this energy boost, a void would stand out like a sore thumb to
most observers, as a colossal "hot" patch in a cosmic background distributed
otherwise uniformly across the sky. The only observers not able to see the
void in this way would be us earthlings living in the centre of the bubble:
all the photons that come our way will have passed through the same amount of
void, and so our cosmic background will look completely isotropic. As indeed
it does.  The only people not to see the void would be us earthlings

But here comes the clever part, say Caldwell and Stebbins. Some of those
hotter photons that have passed completely through our void will scatter off
ionised gas floating about on the other side and be reflected back our way,
as off a mirror. So what we would actually see is a mixture of photons, most
of which have come to us directly, but with a smattering of these hotter,
reflected photons. As a result, the hmatter-and-photon fluid sloshing about
in the early universe created regions of higher density, whose greater
gravity in turn dragged in yet more matter as the universe expanded.

The researchers propose measuring how the ripples vary in size with distance
from us, and therefore at different periods of the universe's evolution.
Combined with supernova results, which tell us the rate of expansion of the
universe at a particular time, this will tell us the universe's geometry - a
property known as curvature - at different epochs. The standard, homogeneous
cosmological model predicts that this curvature evolves smoothly, making it
an easy matter to calculate the geometry of space today from measurements
taken at any distance.

This results in a simple consistency check: take measurements at two or more
distances, and see what value for today's curvature pops out in each case. If
the values do not agree, something is wrong with the homogeneous model. "If
the measurements at different distances imply different curvatures today,
then the assumption on which the standard cosmological model is based - the
Copernican principle - is wrong," says Ellis.

Ellis himself, together with Clarkson and Jean-Philippe Uzan of the Institute
of Astrophysics in Paris, France, has developed a variation on this type of
consistency check. It involves taking measurements over about a decade of how
fast single cosmological objects, such as quasars, are moving away from us
with the universe's expansion, and how that motion changes at different
distances - and therefore at different epochs. The results will tell us how
the universe's rate of expansion has changed over time, and this can be
checked against the predictions of the homogeneous cosmological model
(Physical Review Letters, vol 100, p 191303).

The observational sensitivity required to record such tiny motions - the
change in distance over 10 years is typically less than a billionth of the
distance of the objects from us - is currently beyond astronomers'
capabilities. But it should become feasible, the researchets or completely
different tests. So what would it actually mean if, against the expectation
of Caldwell and the majority of cosmologists, the outcome were that the
Copernican principle is wrong?

It would certainly require a seismic reassessment of what we know about the
universe. Our big bang model is particularly simple, characterised by
universal quantities such as matter and energy densities and the rates of
expansion and acceleration, whether negative or positive. If the Copernican
principle fails, all that goes out of the window too. In that case,
quantities we measure in our own rather special corner of the universe will
turn out to have only parochial significance, with no deeper universal
meaning. We would no longer be sure what, if anything, we can conclude about
the wider universe, its origin, evolution and fate. Cosmology would be back
at the drawing board.

If we are in a void, answering how we came to be in such a privileged spot in
the universe would be even trickier. But regardless of how uncomfortable the
philosophical implications might be, for Ellis it is a matter of scientific
principle to test cherished but untested assumptions. "Whatever our
theoretical predilections, they will in the end have to give way to the
observational evidence," he says.

The Copernican principle might survive the tests, leaving us with the known
unknown of dark energy. Or it might fall, leaving us with the unknown unknown
of an entirely new cosmological model. Either way, cosmologists will still
have plenty of explaining to do.

Marcus Chown is the author of Quantum Theory Cannot Hurt You (Faber, 2008)