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Does Dark Energy Really Exist? [SciAm 2009 Octubre]



Hola, les dejo el artículo de "¿Realmente existe la energía oscura?" Extraído de Scientific American, edición de octubre, 2009. Traducción próximamente.

In science, the grandest revolutions are often
triggered by the smallest discrepancies. In the
16th century, based on what struck many of
his contemporaries as the esoteric minutiae of celestial
motions, Copernicus suggested that Earth
was not, in fact, at the center of the universe. In
our own era, another revolution began to unfold
11 years ago with the discovery of the accelerating
universe. A tiny deviation in the brightness of
exploding stars led astronomers to conclude that
they had no idea what 70 percent of the cosmos
consists of. All they could tell was that space is
filled with a substance unlike any other—one
that pushes along the expansion of the universe
rather than holding it back. This substance became
known as dark energy.
It is now over a decade later, and the existence
of dark energy is still so puzzling that some cosmologists
are revisiting the fundamental postulates
that led them to deduce its existence in the
first place. One of these is the product of that
earlier revolution: the Copernican principle, that
Earth is not in a central or otherwise special position
in the universe. If we discard this basic
principle, a surprisingly different picture of what
could account for the observations emerges.
Most of us are very familiar with the idea that
our planet is nothing more than a tiny speck orbiting
a typical star, somewhere near the edge of
an otherwise unnoteworthy galaxy. In the midst
of a universe populated by billions of galaxies
that stretch out to our cosmic horizon, we are led
to believe that there is nothing special or unique
about our location. But what is the evidence for
this cosmic humility? And how would we be able
to tell if we were in a special place? Astronomers
typically gloss over these questions, assuming
our own typicality sufficiently obvious to warrant
no further discussion. To entertain the notion
that we may, in fact, have a special location
in the universe is, for many, unthinkable. Nevertheless,
that is exactly what some small groups of
physicists around the world have recently been
considering.
Ironically, assuming ourselves to be insignificant
has granted cosmologists great explanatory
power. It has allowed us to extrapolate from
what we see in our own cosmic neighborhood to
the universe at large. Huge efforts have been
made in constructing state-of-the-art models of
the universe based on the cosmological principle—
a generalization of the Copernican principle
that states that at any moment in time all points
and directions in space look the same. Combined
with our modern understanding of space, time
and matter, the cosmological principle implies
that space is expanding, that the universe is getting
cooler and that it is populated by relics from
its hot beginning—predictions that are all borne
out by observations.
Astronomers find, for example, that the light
from distant galaxies is redder than that of nearby
galaxies. This phenomenon, known as redshift,
is neatly explained as a stretching of light
waves by the expansion of space. Also, microwave
detectors reveal an almost perfectly smooth
curtain of radiation emanating from very early
times: the cosmic microwave background, a relic
of the primordial fireball. It is fair to say that
these successes are in part a result of our own humility—
the less we assume about our own significance,
the more we can say about the universe.
Darkness Closes In
So why rock the boat? If the cosmological principle
is so successful, why should we question it?
The trouble is that recent astronomical observations
have been producing some very strange
results. Over the past decade astronomers have
found that for a given redshift, distant supernova
explosions look dimmer than expected. Redshift
measures the amount that space has
expanded. By measuring how much the light
from distant supernovae has redshifted, cosmologists
can then infer how much smaller the universe
was at the time of the explosion as compared
with its size today. The larger the redshift,
the smaller the universe was when the supernova
occurred and hence the more the universe has
expanded between then and now.
The observed brightness of a supernova provides
a measure of its distance from us, which in
turn reveals how much time has elapsed since it
occurred. If a supernova with a given redshift
looks dimmer than expected, then that supernova
must be farther away than astronomers
thought. Its light has taken longer to reach us,
and hence the universe must have taken longer to
grow to its current size [see box on opposite
page]. Consequently, the expansion rate of the
universe must have been slower in the past than
previously expected. In fact, the distant supernovae
are dim enough that the expansion of the universe
must have accelerated to have caught up
with its current expansion rate [see “Surveying
Spacetime with Supernovae,” by Craig J. Hogan,
Robert P. Kirshner and Nicholas B. Suntzeff; Scientific
American, January 1999].
This accelerating expansion is the big surprise
that fired the current revolution in cosmology.
Matter in the universe should tug at the fabric
of spacetime, slowing down the expansion,
but the supernova data suggest otherwise. If cosmologists
accept the cosmological principle and
assume that this acceleration happens every
where, we are led to the conclusion that the universe
must be permeated by an exotic form of energy,
dark energy, that exerts a repulsive force.
Nothing meeting the description of dark energy
appears in physicists’ Standard Model of
fundamental particles and forces. It is a substance
that has not as yet been measured directly,
has properties unlike anything we have ever seen
and has an energy density some 10120 times less
than we may have naively expected. Physicists
have ideas for what it might be, but they remain
speculative [see “The Quintessential Universe,”
by Jeremiah P. Ostriker and Paul J. Steinhardt;
Scientific American, January 2001]. In short,
we are very much in the dark about dark energy.
Researchers are working on a number of ambitious
and expensive ground- and space-based
missions to find and characterize dark energy,
whatever it may be. To many, it is the greatest
challenge facing modern cosmology.
A Lighter Alternative
Confronted with something so strange and
seemingly so improbable, some researchers are
revisiting the reasoning that led them to it. One
of the primary assumptions they are questioning
is whether we live in a representative part of the
universe. Could the evidence for dark energy be
accounted for in other ways if we were to do
away with the cosmological principle?
In the conventional picture, we talk about the
expansion of the universe on the whole. It is very
much like when we talk about a balloon blowing
up: we discuss how big the entire balloon gets,
not how much each individual patch of the balloon
inflates. But we all have had experience with
those annoying party balloons that inflate unevenly.
One ring stretches quickly, and the end
takes a while to catch up. In an alternative view
of the universe, one that jettisons the cosmological
principle, space, too, expands unevenly. A
more complex picture of the cosmos emerges.
Consider the following scenario, first suggested
by George Ellis, Charles Hellaby and Nazeem
Mustapha, all at the University of Cape Town in
South Africa, and subsequently followed up by
Marie-Noëlle Célérier of the Paris-Meudon Observatory
in France. Suppose that the expansion
rate is decelerating everywhere, as matter tugs on
spacetime and slows it down. Suppose, further,
that we live in a gargantuan cosmic void—not a
completely empty region, but one in which the
average density of matter is only a half or maybe
a third of the density elsewhere. The emptier a
patch of space is, the less matter it contains to
Febslow
down the expansion of space; accordingly,
the local expansion rate is faster within the void
than it is elsewhere. The expansion rate is fastest
at the very center of the void and diminishes toward
the edge, where the higher-density exterior
begins to make itself felt. At any given time different
parts of space will expand at different
rates, like the unevenly inflated party balloon.
Now imagine supernovae exploding in different
parts of this inhomogeneous universe, some
close to the center of the void, others nearer the
edge and some outside the void. If we are near
the center of the void and a supernova is farther
out, space expands faster in our vicinity than it
does at the location of the supernova. As light
from the supernova travels toward us, it passes
through regions that are expanding at ever faster
rates. Each region stretches the light by a certain
amount as it passes though, and the cumulative
effect produces the redshift we observe.
Light traveling a given distance is redshifted by
less than it would be if the whole universe expanded
at our local rate. Conversely, to achieve
a certain redshift in such a universe, the light has
to travel a greater distance than it would in a
uniformly expanding universe, in which case the
supernova has to be farther away and therefore
appear dimmer.
Another way to put it is that a variation of expansion
rate with position mimics a variation in
time. In this way, cosmologists can explain the
unexpected supernova observations without invoking
dark energy. For such an alternative explanation
to work, we would have to live in a
void of truly cosmic proportions. The supernova
observations extend out to billions of light-years,
a significant fraction of the entire observable
universe. A void would have to be of similar size.
Enormous by (almost) anyone’s standards.
A Far-fetched Possibility
So how outlandish is this cosmic void? At first
glance, very. It would seem to fly in the face of
the cosmic microwave background, which is
uniform to one part in 100,000, not to mention
the apparently uniform distribution of galaxies
[see “Reading the Blueprints of Creation,” by
Michael A. Strauss; Scientific American, Febslow
ruary 2004]. On closer inspection, however, this
evidence may not be so conclusive.
The uniformity of the relic radiation merely
requires the universe to look nearly the same in
every direction. If a void is roughly spherical and
if we lie reasonably close to its center, these observations
do not necessarily preclude it. In addition,
the cosmic microwave background has
some anomalous features that could potentially
be explained by large-scale inhomogeneity [see
box on next page].
As for the galaxy distribution, existing surveys
do not extend far enough to rule out a void
of the size that would mimic dark energy. They
identify smaller voids, filaments of matter and
other structures hundreds of millions of lightyears
in size, but the putative void is an order of
magnitude larger. A lively debate is now under
way in astronomy as to whether galaxy surveys
corroborate the cosmological principle. A recent
analysis by David Hogg of New York University
and his collaborators indicates that the largest
structures in the universe are about 200 million
light-years in size; on larger scales, matter appears
smoothly distributed, in accordance with
the principle. But Francesco Sylos Labini of the
Enrico Fermi Center in Rome and his colleagues
argue that the largest structures discovered so far
are limited only by the size of the galaxy surveys
that found them. Still larger structures might
stretch beyond the scope of the surveys.
By analogy, suppose you had a map showing
a region 10 miles wide, on which a road stretched
from one side to the other. It would be a mistake
to conclude that the longest possible road is 10
miles long. To determine the length of the longest
road, you would need a map that clearly showed
the end points of all roads, so that you would
know their full extent. Similarly, astronomers
need a galaxy survey that is larger than the biggest
structures in the universe if they are to prove
the cosmological principle. Whether surveys are
big enough yet is the subject of the debate.
For theorists, too, a colossal void is difficult
to stomach. All available evidence suggests that
galaxies and larger structures such as filaments
and voids grew from microscopic quantum seeds
that cosmic expansion enlarged to astronomical
proportions, and cosmological theory makes
firm predictions for how many structures should
exist with a certain size. The larger a structure
is, the rarer it should be. The probability of a
void big enough to mimic dark energy is less
than one part in 10100. Giant voids may well exist
out there, but the chance of our finding one in
our observable universe would seem to be tiny.
Still, there is a possible loophole. In the early
1990s one of the authors of what is now the standard
model of the early universe, Andrei Linde,
and his collaborators at Stanford University
showed that although giant voids are rare, they
expand faster early on and come to dominate the
volume of the universe. The probability of observers
finding themselves in such a structure
may not be so tiny after all. This result shows that
the cosmological principle (that we do not live in
a special place) is not always the same thing as the
principle of mediocrity (that we are typical observers).
One can, it seems, be both typical and
live in a special place.
Testing the Void
What observations could tell whether the expansion
of the universe is driven by dark energy or
whether we are living in a special place, such as
at the center of a giant void? To test for the presence
of a void, cosmologists need a working
model of how space, time and matter should
behave in its vicinity. Just such a model was formulated
in 1933 by Abbé Georges Lemaître,
independently rediscovered a year later by Richard
Tolman and further developed after World
War II by Hermann Bondi. The universe they
envisaged had expansion rates that depended
not only on time but also on distance from a specific
point, just as we now hypothesize.
With the Lemaître-Tolman-Bondi model in
hand, cosmologists can make predictions for a
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