A Short History Of The Dark Side - Part 3

The Hubble Ultra Deep Field, a view of the distant Universe, at a time before the influence of Dark Energy began to be felt, we think.

I think its time to finally round off the long running "Dark Side" trilogy, and like all final chapters it has to be bigger better and more exciting. So sorry but this one is pretty long.

When we left off we had been investigating the properties of the mysterious Dark Matter that permeates the Universe, despite being strange (only interacting through the force of gravity) DM is nevertheless a physical entity, in most theories some form of elementary particle. The next constituent of the Dark Universe is much weirder, being some strange form of energy which exerts a negative pressure on the Universe causing it to expand. Now I think many Astronomers are happy to admit that they are not happy about Dark Energy, everyone seems reasonably happy to admit the existence of Dark Matter, it simply turns up too often on too many scales, but the properties of Dark Energy at present are known from only one or two methods and even then not too accurately, making naturally cautious Astronomers worry about conclusions being drawn from them. Nevertheless the history and implications of a Dark Energy dominated Universe are interesting, so lets have a look at them.

Hubble's observation that essentially all galaxies are receding from us with a velocity that is proportional to the distance between the Milky Way and them was a vital discovery that provided the first evidence that the Universe was expanding. Naturally the idea of a finite age for the Universe (revolutionary at the time) intrigued people, what had happened in the past and what would happen in the future? For a long period of time the belief in the astronomical community was that the Universe started in a Big Bang and that over time the force of gravity would begin to counteract the expansion and slow it.


Hubble's original plot showing that distance to a galaxy and recessional velocity (or redshift) are related.

The mathematical formalism that determines the behaviour of the Universal Expansion shows that in this picture there are 3 possibilities for the fate of the Universe:

1. The Universe is not dense enough to halt the expansion and the Universe expands forever.
2. The Universe is exactly dense enough to overcome the expansion when the Universe reaches infinite size. It has the so called "critical density".
3. The Universe is more than dense enough to counteract the expansion and the Universe re-collapses.

Astronomers were therefore keen to determine which of these fates awaited the Universe. To do that they could make use of one of the best standard candles: Type 1a supernovae. In Type 1a supernovae a dense white dwarf that has been accreting matter from a companion suddenly passes over a limiting mass: the Chandrasekhar mass. At this mass the star becomes unstable, undergoes rapid runaway fusion and blows itself apart. Because the Chandrasekhar limit is so precisely defined it means that all Type 1a supernovae have almost exactly the same intrinsic brightness, they also have unique signatures in there spectra that mean they can be separated from other non-uniform SN, hence they can be used as standard candles. Put simply, one supernovae that is observed to be a quarter as bright as another must be twice as far away. To make this technique even more useful these things are bright, as in bright enough to be seen across billions of light years.

In the mid 1990's two groups were using these Type 1a supernovae as standard candles, in attempt to measure how much the expansion of the Universe had slowed since the Big Bang. To do this they combined the physical distance information from the supernovae with the redshift of the galaxy in which the supernovae occurred, this redshift through Hubble's law is also a distance but one that depends on the expansion of the Universe. Hence by plotting one against the other you get a plot of how the expansion of the Universe has changed over time, in essence you are looking for how Hubble's linear relation changes or curves over larger times/distances. When both groups plotted their results they both found the same puzzling result: instead of the rate of the expansion slowing over time it has actually been increasing. Damn it, I'm sure both teams thought as they contemplated all the extra work involved. But very rapidly it probably occurred to them that there's a Nobel prize in it for someone, hence a healthy dose of rivalry between the two teams.

You can see this in the top half of the figure above, which shows the results from the two surveys, what you see is the magnitude of the supernovae (a distance indicator) plotted against its redshift (a measure of the expansion of the Universe). The three lines show three predictions for the constituents of the Universe. Two dashes are for a Universe where the entire critical energy density is made up by mass (matter and dark matter), the three dashes are for a Universe where only 30% of the critical density exists in mass, and the solid line (which is best fit by the data) is for one where the Universe has the critical density, 30% being due to mass and 70% due to dark energy. New data on more and more supernovae at larger and larger distances has all agreed very well with the original results, meaning the Dark Energy has slowly become accepted as just another constituent of the Universe.

So what could be this strange Dark Energy? To date there are two main contenders, A cosmological constant and quintessence.

The cosmological constant can be thought of as a pressure of a vacuum, particle physics in fact predicts that empty space should have vacuum fluctuations that provide exactly the type of negative pressure required, unfortunately the predictions from particle physics for the level of this pressure are out by up to 120 orders of magnitude, often called the most incorrect prediction in history, oops. The problem is how to cancel out most of this pressure but not all of it, to date no one is sure how to do this. The implications of a cosmological constant are that the expansion will necessarily increase without end, as space is what is causing the expansion the more space there is the more expansion there is. Eventually all structures not gravitationally bound will be separated by so much space light will never be able to pass between them, turning the Universe Dark. In other words in a Milky Way in the distant future, all of the galaxies beyond out local group will slip beyond this distance and disappear forever. Not a very cheerful thought, but hey, who said existence had to be cheerful?

Quintessence is thought to be some sort of particle-like excitation with a possibly dynamical nature. In other words it need not be a constant value per area of space as the cosmological constant, it could vary in strength over time and possibly have different strengths in different areas of the Universe. This is similar to the behaviour of the field that is thought to have caused the intense period of expansion in the early Universe known as inflation. Quintessence could even reverse and cause a contraction of the Universe at some point.

So where does that leave us? What is the eventual fate of the Universe? The truth is that we really don't know for sure, our theoretical knowledge of what is causing the accelerating expansion is not good enough to allow us to determine with certainty what the Universe in the very distant future will be like. The best we can do at present is to try to determine if either of the two cases above can be ruled out. The trick to determining which of these two cases is correct (if either is) is to extend the observations of Type 1a SN to higher redshift and track how the speed of expansion changes. There are many studies ongoing to try to do this to higher and higher precision, expect more interesting results in the coming years.

A "Short" History Of The Dark Side - Part 2

In the previous post we saw how in certain circumstances it is possible to explain supposedly missing mass in terms of standard physics. I'm now moving onto a case where it appears that this isn't possible, the strange "Dark Matter" that seems to permeate space.


Its been known since the 1930s that if you add up the mass seen in clusters of galaxies (like that shown above) there isn't enough mass to account for the motion of the galaxies in the cluster. This is exactly analogous to the situation in GCs, where instead of individual stars appearing to be moving to fast its now entire galaxies moving too quickly. Over the years some of this missing mass has been found, in the form of very tenuous hot gas that resides between the galaxies in the cluster. The image below shows the Centaurus cluster as seen in X-Rays, the X-Rays are produced by the hot gas in between galaxies in the cluster. The gas itself is also of great use in determining the mass in the cluster, this is because we can measure the temperature and density of the gas and from this infer the gravity that must be present to stop the gas expanding out of the cluster and into intercluster space. Although this gas is very diffuse when its mass is added up it still adds up to more than the mass contained in the galaxies in the cluster but still it only makes up a small fraction of the total mass we know must be in the cluster from the motions of the galaxies.


Over time other manifestations of this missing mass has been seen, it was observed that spiral galaxies rotated too quickly to be explained by just the visible mass, then it was noticed that the stars in elliptical galaxies where also speeding around too quickly, finally that the GCs and dwarf galaxies around normal galaxies were themselves moving too quickly to be explained by the luminous mass of the parent galaxy. Other effects were noticed that do not rely on the kinematics of objects, it was observed that the bending of light due to the gravity of galaxy clusters and individual galaxies was too severe to be explained by the visible mass.

As it became clear that this invisible mass was a real phenomenon and not due to some problems with our models people attempted to explain this missing mass. Initially people attempted to explain this missing mass in terms of stellar remnants (like in the GCs) and/or gas and dust that doesn't emit light strongly. With our increasing ability to observe at different wavelengths of light where we would expect gas and dust to be emitting radiation it became clear that this gas and dust could only explain a small fraction of the missing mass. Similarly stellar population modeling showed that it was almost impossible to explain the missing mass as being due to stellar remnants, it would require far too many stars to have already died by now, this would only be possible if initially most stars that formed in a galaxy were very massive. This is not observed in nearby galaxies and from what we know of star formation is not expected to be the case in most situations.

So what is the solution to the puzzle? Well the one that is most popular is inclusion of some matter which is not made of the same material as normal atoms, this Dark Matter is non-baryonic and only interacts with luminous matter through the force of gravity, if it did interact in any other way we would be able to see it. This solution seems like a fudge, except that it can be used to fit all of the problems I have listed above and more, something which the other contending model has difficulty with. This other approach (called Modified Newtonian Dynamics or MOND) is to assume that gravity behaves differently on different scales, this approach can reproduce many of the observed effects but not all, and is generally distrusted because gravity in general and general relativity in particular is seen to work so well in every observation we have to date. At the present much work is being done to investigate and try and detect a Dark Matter particle, if one is found it will be the crowning achievement of modern astronomy, if it is not observed then we have major problems.

However just as the astronomical community was reaching consensus on the existence of DM another set of observations appeared that has led to another dark substance. We will look at this in my next post in the series.

A "Short" History Of The Dark Side - Part 1

I am often asked what an astronomer does and what the main problems facing astronomy are today. The first question is easily answered, an astronomer spends most of their time in front of a PC trying to make sense of confused data that is never enough for the task. Occasionally you get to go observing to out of the way places like Hawaii or Chile, or to conferences in equally exotic locations where you argue over minor points inside a lecture theatre from dawn till dusk, avoiding the always lovely weather and interesting locals. Somedays it seems to be hard work, but on others you find something that no one has ever known before and on others still you get to sit 4 and a half kilometers in the air on top of a huge volcano and watch the sun set over the Pacific in absolute quiet. Its probably the best job in the world on days like those.


The other question is more difficult, it depends on which sub-field you work on, but I would guess that most people would agree that the most pressing area of research at the moment is investigating the so called "Dark Sector". That part of the Universe that is due to exotic particles or strange forces of nature. In this and the following posts in the series I'm going to try to pull together what I understand about Dark Matter and Dark Energy, perhaps even offer a few opinions. This first post does not actually deal with unusual objects or forces but explains a similar set of observations that can be explained using standard physics, I am doing this so that in the later posts I can explain the fundamental differences between the two cases, so onto the main post.

As an astronomer I am used to the fact that we are rarely able to see everything we need to to understand a given object completely. This is simply a by-product of the fact that we don't have infinitely sensitive instruments, so there will always be objects that are difficult or impossible to detect, objects like brown dwarfs and isolated neutron stars or black holes.

In many systems we cannot see these objects directly but we can observe the influence in other ways, in particular through the effects of their gravity, for example in the cores of Globular Clusters the velocity of the stars is so high that it can't be explained by all the mass we can see, if there wasn't some unseen mass whose gravity was holding the GC together the cluster would simply blow apart. This is fairly strong evidence that there must be something else at work here. Happily the amount of missing mass in GCs is consistent with what we would expect of the type of stellar populations that make up a GC, so we would expect some fraction of stars that are not large enough to make it to main sequence (Brown Dwarfs) and some stars to have already expended all of their fuel and to have died by now (white dwarfs, neutron stars and black holes). These objects are simply too faint to be seen in the GC which is why their mass is "missing". When you add up all the mass that should be in these stars its about enough to explain the mass deficit in GCs entirely in terms of normal (baryonic) matter.


In the second post in this series we will look at observations of galaxies and galaxy clusters and examine why the approach used for GCs cannot explain missing mass in these systems.