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 (video) 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.

redshift and blueshift

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 confirmation?

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 energy".

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 physics.

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 Telescope project.
Joe is also the host of the Hubblecast and ESOcast , two popular video podcasts featuring the latest science, news and images from the Hubble Space Telescope and the European Southern Observatory.