Astronomer Dr. 'Joe' Liske explores questions about Dark Energy
Dark energy is a relatively new topic in Cosmology but there’s a long history of science behind it. It begins
with an effect you can notice when a fast-moving vehicle whizzes past. The pitch of its sound is higher as it
approaches then drops lower when it passes and moves away. This is called the Doppler effect, named after
Christian Doppler and discovered in the 1840s. When a source of energy waves moves towards you, the frequency of
the waves (sound, radio, light…) you receive is faster than the frequency being emitted from the source because
its forward motion puts them closer together. The opposite is true in the same way.
Using a similar concept, astronomers determined that the universe was expanding because they saw the spectral
light output from galaxies being "redshifted", meaning that all galaxies must be receding from each other.
If the universe was flying apart, sort of like the burst of a skyrocket, then they theorized it might have
originated from a Big Bang – still the most popular theory of origin. But this expansion would be expected to
gradually slow, due to the attractive gravitational force of the bodies counteracting the outward momentum from
the Bang. In 1998 some observations suggested otherwise
and things got a lot more complicated. The new observations showed an accelerating expansion – not a slowing one.
Something is at work – a dark energy.
“Dark” means that we can’t see it and don’t know what it is. Only its effect is being observed, but there are
interesting theories, and astronomer Dr. ‘Joe’ Liske was willing to explore some questions with us.
When people think of dark energy should they think of it as a repulsive energy, in the way that like-charged
particles repel each other?
No, that would not be a good way of thinking about dark energy, for
several reasons, but I can see why one might be tempted to think like
that: since dark energy is supposed to be responsible for the
accelerated expansion of the Universe it's easy to assume that it must
exert some kind of "anti-gravity". But that is not the case: a blob of
dark energy would still gravitationally attract any matter outside of it.
The reason dark energy makes the expansion of the universe accelerate is
that it is extremely smoothly distributed (hence it's called "energy" and
not "matter") and that it does not dilute away as the Universe expands.
The densities of ordinary matter, dark matter and even of light, as well
as their associated energy densities, all dilute away as the Universe
expands, but the density of dark energy remains constant (at least to
the limits to which we have been able to measure it so far). It is this
persistent energy density that leads to an ever-increasing "speed" at
which two very distant objects separate from each other.
Do you think dark energy may be a type of energy unlike any we currently know of?
It is definitely exotic stuff! Whatever it is, it is definitely unlike
anything we are used to in our day-to-day lives. However, in physics the
properties of dark energy are not so unheard of. For example, the
concept of "vacuum energy" has been around for a long time and this is
indeed the simplest explanation for what dark energy might actually be.
The only problem with this explanation is that if you try to predict how
much vacuum energy there should be around, based only on some pretty
fundamental principles of physics, then your prediction is off by a
factor of 10 to the power of 120 (i.e. a 1 followed by 120 zeros)
compared to what we actually observe in cosmology. It is this
spectacular disagreement that makes the observed accelerated expansion
of the Universe so difficult to reconcile with fundamental physics. This
is arguably one of the greatest challenges in all of physics.
Returning to the original question, theoretical physicists have come up
with several alternatives to vacuum energy as candidates for dark
energy, all of them even more exotic. However, so far it has proven very
difficult to underpin any form of dark energy with a viable physical
theory, i.e. to understand its origin and nature within the standard
model of (particle) physics.
As I understand it, the initial discovery of accelerated expansion came
from the red-shift method and then observations of the movement of
gravitational lensing showed supporting evidence. Is this accurate, and
are there still other methods sensitive enough to give additional
The original discovery of the accelerated expansion of the Universe was
made by measuring the relation between distance and redshift using
supernovae. This relation is predicted by General Relativity (the best
theory of gravity that we have available) as long as one knows the
mass-energy content of the Universe. By comparing the predictions of
General Relativity with the actual measurements it became clear that
there was absolutely no way to explain the observed distance-redshift
relation if the Universe only contained "normal" things like ordinary
matter, photons or even dark matter. The observations could only be
explained, to a high degree of confidence, if one invoked the existence
of an additional energy component in the Universe with the exotic
property of having negative pressure - that's what we now call "dark
So far, the only "smoking gun" for the existence of dark energy is that
there is something funny going on with the expansion history of the
Universe. Hence any observation that somehow depends on the expansion
history will in principle be sensitive to dark energy. Above I mentioned
the use of supernovae to map out the distance-redshift relation. This
works because supernovae can be used as so-called "standard candles" -
their true luminosity can be inferred from other observations so that
comparison with their apparent brightness on the sky yields a distance.
Similarly, one can also use a "standard ruler" method to map out the
distance-redshift relation: the 3D distribution of galaxies is highly
non-random, and in particular it has imprinted on it a feature for which
we can calculate its absolute scale (in lightyears). Measuring the
angular size of this feature on the sky as a function of redshift again
allows us to map out the distance-redshift relation.
In addition to the above, there are still other methods to constrain the
expansion history of the Universe. Each of these has its own set of
problems which might cause you to remain skeptical of the results of
each individual method. However, the remarkable thing is that all of the
different methods obtain results that are compatible with one another -
which inspires confidence that the overall result of an accelerated
expansion of the Universe is indeed real.
Assuming the expansion of the universe is indeed accelerating, is there
an explanation, other than energy, that you can imagine?
Yes, in fact there are two alternative explanations (I hasten to add
that neither was invented by myself):
Instead of introducing an exotic form of energy to explain the observed
acceleration of the Universe, one can also modify the underlying
theory of gravity - i.e. one can postulate that General Relativity is
incomplete and that it must be modified in such a way as to be able to
explain the observed acceleration without the introduction of some
exotic energy component. This is an approach that is being actively
pursued by many researchers, with some success. The difficulty here lies
in finding a physical motivation for the necessarily more complicated
form of the new theory of gravity. However, note that if this
explanation is correct, then we are also confronted with entirely new
When making predictions from General Relativity for the the evolution of
the Universe then we usually assume that the Universe in perfectly
homegeneous and isotropic, i.e. that it looks the same everywhere and in
every direction. This assumption is usually referred to as the
"Cosmological Principle". However, we know that this assumption is not
strictly true. On very large cosmological scales the Universe looks
indeed very smooth, but on smaller scales matter clumps together into
filaments, walls and clusters of galaxies. The second alternative
explanation for the observed acceleration is that it is due to the
inhomogeneity of matter on relatively small scales. The simplest
versions of this approach do not seem to hold up against the data, but
the more complicated versions have not yet been ruled out.
What kind of experiments do you feel need to be done to further resolve
the question of dark energy or the accelerating expansion?
As I have explained above the only hint of something strange going on is
the expansion history of the Universe. There are currently no theories
explaining the accelerated expansion that make some other observable
prediction that is not to do with the expansion. Hence, the only thing
we can do right now to make progress is to try to map out the expansion
history of the Universe as accurately as we can. As I have alluded to
above, there are several ways of doing this and most are already being
pursued right now. There are also many plans to pursue them even more
rigorously in the future.
However, there is one method that has not yet been tried for lack of
suitable instrumentation: the most direct way of measuring whether an
object is accelerating or decelerating is to measure its velocity at two
different times and to see whether there is any difference. Although
this sounds a little far-fetched, it is indeed possible to apply the
same principle to the Universe: by making extremely precise measurements
of the redshifts (or recession velocities) of distant galaxies or gas
clouds now, and again in, say, 10 years' time. The deceleration or
acceleration of the expansion of the Universe will cause the redshifts
to change over this time period - although only by a tiny amount
(because 10 years is a ludicrously short time period compared to the age
of the Universe - 13.75 billion years).
This direct, "real-time" observation of the Universe's changing
expansion speed hence requires extreme precision as well as some
patience. However, I have good reasons to believe that it will indeed be
possible to perform this "experiment" with the next-generation
40-m-class European Extremely Large Telescope that is currently being
developed here at ESO.
Dr Joe Liske is a staff astronomer at the European Southern Observatory
(ESO) in Germany. Joe obtained his PhD from the University of New South
Wales in Sydney, Australia. He then moved to Scotland to work as a
postdoctoral fellow at the Universities of St Andrews and Ediburgh
before joining ESO in 2003.
He now spends half of his time trying to understand how galaxies like
our own Milky Way formed by studying huge sky surveys. The other half of
his time is dedicated to the science of the European Extremely Large
Joe is also the host of the Hubblecast
, two popular video podcasts featuring the latest science, news and images
from the Hubble Space Telescope and the European Southern Observatory.