Galaxies, Lenses, Globulars, What More Do You Want?

Welcome to my first post for threesigmaresult! I've chosen to redo a post that previously appeared on my other blog, theobservershunch, I've done this because it was one of my favourites, partly because it has a very pretty pictures but mostly because it describes some research done by some of my colleagues. Enjoy.

Click for VERY Big.

The picture above which was released by the Hubble Heritage Project shows a cluster of galaxies called Abell S0740, the data used to make the picture was collected by a team of astronomers including two of my collaborators/friends at Durham, John Lucey and Russell Smith. There are two versions of the image, one without annotations and one showing zoomed regions of interest. See them both here.

Milky Way Behaving Badly

Head over to Space.comfor a story about the dark side of our well behaved galaxy's nature, its being throwing its weight around and generally being a bad neighbour. The story is all about the streams of stars that are being found trailing around the Milky Way, these trails are thought to be the shredded remains of dwarf galaxies or globular clusters that wandered too close to the MW. Below you can see some of the streams uncovered by the Sloan Digital Sky Survey. The streams are detected by looking at the colours and positions of a huge number of stars over as large an area as possible, doing this it is possible to pick out groups of stars with the same sort of colour, implying that they probably formed at the same time from the same material.


Some of the streams have been associated with known GCs or dwarf galaxies, basically these objects lie right in the middle of the stream, in the pictorial representation at the top you can see the original dwarf with its tails of stars which spread out both in front and behind it in its orbit of the MW. Over time the streams will stretch further and further, getting progressively thinner and more tangled, until they form a diffuse halo of stars around the MW.

This kind of research is interesting because the current theories for the formation of galaxies predict that there should be many more dwarf galaxies around the MW than we see at present, one solution is that many of them have simply been torn apart by the MW and their stars spread into the halo of the galaxy. If enough of these streams are found this could help solve this so called "missing satellite problem".

Friday Lunchtime Talks - MOdified Gravity

On Friday we had the latest of the Durham Astronomy groups Friday Lunchtime Talks, these usually consist of two members of the group (which consists of over 50 members including students) giving a half hour talk, either about their own work or a paper that has appeared recently in which they have some interest. Last Fridays was something different however, it was an hour long talk by Professor Emeritus John Moffat of the Perimeter Institute on the subject of his own version of modified gravity, MOG.

I always have a lot of respect to someone that comes to Durham to talk about modified gravity, it really can be like entering the Lions den. Tinkering with gravity is usually done in an attempt to explain astrophysical phenomena without the need to demand that most of the mass in the Universe is invisible and can only be detected through its gravitational influence (the so called Dark Matter). Durham is world renowned as a centre of research into Dark Matter, as such some members of the group could be expected to be particularly partisan on the issue. I think its a great credit to the people involved that things never seem to get too heated, in fact in this Fridays talk, most of the difficult questions were directed from the one Professor in the group who has never been too happy with the current Cosmological Paradigm. He's generally good value for entertainment, and important in keeping everything honest.

Anyway, onto the talk itself. John Moffat has been working on modifying the gravitational laws for many years and has produced several different models. These models differ from most of the work on MOND (MOdified Netwonian Dynamics) in that from the beginning they were specifically chosen to be relativistic, that is that they are modifications to Einsteins General Relativity and not Newtons law of gravity. Essentially in MOdified Gravity (or if you prefer, MOffat Gravity, MOG) the gravitational constant G (also called Newtons Constant) is not in fact a constant, but can vary both in time and in space, so that the G that relates the force between two masses separated by one distance R1 is not the same G that relates the force between the same masses if they are separated by distance R2, similarly for time T1 and T2. These changes in G have to be negligible for objects within the solar system otherwise we should be able to detect deviations from the GR predictions which so far have not been observed, but they can become significant over galactic scales.

The speaker showed several examples of where his proposed changes would allow us to fit astronomical observations without the need to invoke Dark Matter. In particular he mentioned fitting the rotation curves of dwarf galaxies, spiral galaxies and clusters of galaxies (for clusters its technically not a rotation curve but the principle is the same). This in particular interested me as rotation curves are something I have personal experience with, both in my MSci project and the first paper I have published. Below is an example of a spiral galaxy rotation curve, it simply measures the speed at which stars at different radii in the disk orbit the galaxy, the data is the black circles (with error bars) the various dashed lines show the amount of velocity provided by the mass of various components of the galaxy including a DM halo. MOG allows you to explain the observed rotation of the galaxy without this DM component, by assuming the influence of the other two components is stronger than you would naively expect using Newtonian (or GR) gravity.

For me the first and largest problem that appeared during the talk appeared when JM was talking about these fits to the rotation curves. It seems it is possible using his model to fit all spiral galaxies using the same values for two parameters, as far as I could tell these parameters deal with the scale over which G begins to diverge from a constant value and some sort of normalisation of the size of the divergence. The problem was that when he fit the rotation curves of dwarf galaxies, or of clusters of galaxies the values of these two free parameters were different, all dwarfs had the same values, all clusters had the same values but different from the dwarfs or the spirals. This would appear to mean that there has to be yet another effect going on, meaning we need more free parameters to explain everything in terms of one unified gravitational theory. If this is true it would seem to be a problem. I have to go and look at the actual papers and see if in fact this is the case, or if I simply misunderstood something, it could be that the values he was quoting were actually telling you something about the scales involved, i.e that dwarf galaxies are smaller than spirals which are smaller than clusters.

The second major part of the talk dealt with JM's attempts to explain the results from the Bullet Cluster without the need for Dark Matter. This cluster is so far unique and very exiting. Its unique because it is actually two clusters, one of which has passed through the other, during this interaction the hot gas that resides in the clusters hit each other and slowed down, the gas from the smaller cluster having shocked and formed a bullet like shape as seen as the red triangle on the right of the image below. In the image below you can see the hot intracluster gas as the red regions. This is interesting because the hot intracluster gas has been separated from the galaxies of the clusters (seen in the blue regions), normally there is around 10x as much mass in this hot gas as there is in the galaxies of a cluster. By crashing through each other the two clusters have managed to separate the collisional material (the gas) from the collision less material (the galaxies and any Dark Matter).


It is possible to use gravitational lensing of background galaxies by the clusters of galaxies to work out the mass that must be contained within the galaxy clusters. This cluster is so important because it can be convincingly shown that even though the vast majority of the visible mass (the intracluster gas) has been removed the effect of gravity is still very strong, considerably stronger than can possibly be explained by the visible mass contained in the galaxies in the cluster. The traditional explanation is therefore that there must be some invisible mass (DM), which from this cluster we can see must be collision less, otherwise it would have piled up where the intracluster gas is. In MOG the explanation for the extra lensing, above what is predicted by GR for just the normal mass is that the gravity from the galaxies is stronger at larger distances, and also I believe that there is some lensing caused by the gravity from the intracluster gas.

If this is the case then there is happily a way to test which approach is correct, MOG or DM, the intracluster gas is much more massive than the mass contained in the luminous matter of the galaxies, but it is also located on one side of the cluster. Therefore it would seem that if MOG is correct there should be an increase in lensing on the side of the cluster nearest to the gas, of course the size of the effect would depend on many factors, but may in principle be measurable. If MOG is incorrect and DM is really at work, then the lensing should be more symmetric around the cluster, both because the DM is by far the largest mass contribution but also because the gravity of the gas is much less at larger distances than predicted by MOG.

Whatever the result of work such as this, we are really going to learn something fundamental about the Universe. Either most of the mass of the Universe is in some really exotic form, or else the force of gravity is even stranger than we have ever dreamt.

Unfortunately time caught up with us and we didn't get to find out much on the implications of this modified gravity on Cosmology, for example whether it could explain the mysterious Dark Energy at the same time that does away with Dark Matter. Many other interesting questions were raised however, it appears that MOG doesn't allow for singularities, so no black holes, though objects observationally indistinguishable from them probably can exist. This led one prominent member of the group to spend the time to see if the Metric for MOG is compatible with these condensed objects having a "last stable orbit", apparently it doesn't seem like it can, and as these are thought to be observed around BHs this is probably a problem for the theory.

All in all a very interesting talk, a perfect example of the kind of research that is ongoing in Astronomy. I don't think anyone is going to be packing in the DM work anytime soon, but if the DM particles continue to remain so illusive, its good to see that there are concrete alternatives being formulated.

Cranks Emails - 3 - Or William C. Mitchell Knows Nothing About Cosmology/Astronomy

Yesterday I asked for some more examples of crank emails, this morning I found about 15 in my inbox, a big shout out to CMB for sending his collection. In particular one stood out, it is an author attempting to hawk his Anti Big Bang book called - Bye Bye Big Bang - Hello Reality, or as I would have called it Bye Bye Scholarly Study - Hello Page After Page Of Embarrasingly Poor Arguments. NOTE: I haven't changed anything except to tidy up the lines a bit, all those strange symbols were in the original email. Oh and I removed the links to the book at Amazon because they don't fit on the page, the spelling mistakes are all the original authors. If this is a Google search then welcome, let's just have a quick look at how little William C. Mitchell knows about Cosmology, and research, and writing legibly.

Subject: COSMOLOGY

If you aren't interested in cosmology, please don't read any further. I
don't want to annoy anyone.

BIG BANG FRAUD © 2004 William C. Mitchell
The following is a review of some of the many items that are part of an
elaborate fraud that is perpetrated by the Big Bang cosmology establishment.

Some Necessary Background Information.
The solutions to Einstein's Special Relativity equations, as solved by Alexander Friedmann, provided for three possible cosmological ”cases” on which Big Bang Theory (BBT) is based. Those cases are: a “closed“ universe having positively curved space that would eventually collapse; a “flat” universe having uncurved Euclidean space that would expand forever; or an “open” universe having negatively curved space that also would expand forever, but at a slowly increasing rate. (Before Friedmann's work, I don’t believe that Einstein had ever given any consideration to negatively curved space.)

The Velocity of Matter in Space.
The velocity of matter departing from us in space, that is calculated by an equation [V/c = (Z+1)2 -1)/(Z+1)2 +1)], that is derived from the Einstein-Lorentz transformations, results in a rate of expansion that reaches c for large redshifts. However, that is incompatible with the Friedmann closed, flat or open universe cases mentioned above; and it is also incompatible with a universe of relatively slowly increasing expansion that has recently gained considerable acceptance. Regardless of those incompatibilities, that equation continues to be used to determine the velocity of matter in space as a function of redshift.

Lets look at this claim, that cosmologists use the above equation to "determine the velocity of matter in space as a function of redshift", this is of course rubbish, we don't need to know the velocities of objects, we can measure it using spectroscopy. We use the measured velocities to determine the physical distance to the object, using Hubble's Law.

The second claim appears to be claiming that it is impossible for objects to appear to recede at greater than the speed of light, this is down to Mitchell's utter lack of understanding of the expansion of the Universe. It is entirely possible for space to appear to expand faster than the speed of light, individual regions of space expand at a fixed rate, so that if you look over a long enough distance the individual expansions add up to produce an expansion that is faster than light. There is absolutely no problem with reconciling faster than light expansion of space with General Relativity.

The third claim is either a misstatement or a flat out lie, he is essentially claiming that when working out distances to objects in the distant Universe cosmologists neglect the effect of the extra expansion due to Dark Energy. This very easily disproven, many people use Ned Wrights handy Cosmology calculator to convert between things like redshift, physical distance, comoving distance etc, this program can be found here. Try it out, you input the value of the Hubble Constant, the fraction of the energy of the Universe that is in matter (both luminous and dark), the redshift to the object you want to know the distance to, and, drum roll, the fraction of the energy of the Universe that is due to Dark Energy. Try changing the value of the DE while leaving everything else the same, what happens? that's right all of the calculated numbers change. So there you have it cosmologists do include the effect of Dark Energy.

Some More Background Information.
Hubble time, which is the time that the BB would have occurred if the universe had a fixed rate of expansion at the Hubble rate (the Hubble constant) ever since the BB. The consensus of working astronomers seems to be that rate should be about 65 km/sec/MPc, but, because it appears to make the BB universe older, BBers prefer a value of about 50 km/sec/MPc (equal to about 15 km/sec/MLYs) putting the Hubble Time at about 20 billion years,

This book is now quite out of date so I will forgive most of this, even though it is all wrong now, the currently accepted value of the Hubble Constant is around 72 km/s /Mpc, on its own this would infer an age of the Universe that was too short, when compared to the ages of stars. Clearly you can't have stars older than the Universe. However when you include the effects of Dark Energy and Dark Matter, and do the full calculation you get and age of the Universe that is between 13 and 14 Billion Years (Gyr), more than old enough to accommodate even the oldest objects in the Universe.

The Age of the Universe.
The age of the BB universe is based on the “flat” universe case that became important as a result of Inflation Theory. According to that theory, the age of that flat universe is equal to 2/3 of the Hubble time, or about 13.3 billion years. Lately, that approximate age is most often presented as the true BB universe age.
A very much “open” universe of accelerating expansion, has recently become the favored new theory of many BBer. However, the age of the universe that is presented in the media is still based on the flat universe case of inflation theory; that is, at 2/3 of the 20 billion year Hubble time.

As my two previous points have explained this is simply wrong, use the cosmology calculator you will see that changing the value of any of the parameters, including the Dark Energy gives a different age for the Universe, (its the part that says: "It is now XXX Gyr since the Big Bang.").

The Size of the Universe.
Using the above erroneously derived age of the universe, and based on no factual data, BBers have also erroneously assumed that the expansion of the outer edge of the universe has expanded at the speed of light. That allows them to determine its radius, in billions of light years, to be equal to its age in billions of years, or about 13.3 BLYs.
(A plot of the Friedmann's solutions to Einstein’s SR equations would show that, although the rate of expansion of a fixed rate universe would ever be at c, the initial rate of expansion of a flat rate BB universe would be higher, perhaps at about 2c; and the initial rate of expansion of a closed BB universe would be even higher, perhaps at about 3c or more; and the initial rate of expansion of the newly popular BB universe of accelerating expansion might be as low as 1/2c, or even lower.)

This whole statement is garbage, even the wikipedia article gets the facts right on this. The fact that space/time is curved and that the Universe is expanding means that the radius of the observable Universe is not simply just the speed of light divided by the age of the Universe. The actual observable Universe calculated using all of the parameters in the cosmology calculator is about 46 Billion Light years in radius, you can see this in the cosmology calculator if you put a very high redshift, say 2000, this corresponds to a time only 65 thousand years after the Big Bang, now look at the Comoving radial distance, this is how far light can have travelled from 65 thousand years after the Big Bang till now. Again Mitchell reveals he understands nothing about Cosmology, certainly not anything to do with the implications of an expanding Universe in General Relativity.

The Distance to Matter in Space.
Based on that erroneously derived present size of the universe, BBers have concluded that the distance to remote matter in space is proportional to its erroneously derived departing velocity.
Although many BBers still believe that, in accordance with inflation theory, the radius of the universe is many times larger than 13.3 BRYs, they continue to report the distance to high redshift matter in space in accordance with the above; and although many of them have recently accepted the very open universe of accelerating expansion, they continue to report the distance to matter in the same manner.
Thus the distance of matter in space and its velocity as determined by BBers are, not only erroneous, but inconsistent with any currently accepted variety of BB.

I'm not really sure what he means here, but as it is entirely based on his erroneous premises, it can clearly be seen to be nonsense.

Resulting Media Reports.
By the use of such fallacious logic, the distances and departing speeds of matter in the space of the universe, as determined and disseminated by the comological establishment, all that is read, seen or heard in the media, and accepted by all the world regarding those figures, is based on those compounded errors.

The only errors we have seen so far have been yours, to paraphrase a favourite film of mine, "you are a poor scientist Mr Mitchell".

More Background Information.
The so-called “Age Paradox” has plagued BBT since day one. Some stars are known by astronomers to be considerably older than the reputed age of the BB universe; and far worse than that, a number of astronomers have estimated that it might have taken more than 100 billion years for the formation of the giant galactic structures that are observed in space.
That is not a “paradox,” but a disastrous problem that BBers have struggled for decades to overcome, meanwhile attempting to hide it, or dismiss it as a trivial, soon to be solved problem.

There is no age paradox its all a load of bullshit, mostly spread by Creationists who think that if they can disprove BBT then more people will believe the world is 6000 years old. I will state this clearly so it can be understood by anyone, including hopefully the incompetent like Mr Mitchell. There are no known objects in the Universe that have measured ages greater than the age of the Universe within the errors of the measurement. I work on the study of Globular Cluster systems, cranks often claim that the ages of GCs are older than the Universe, they are wrong, correctly determining the ages of these systems is difficult and requires a lot of telescope time, usually you don't get enough to do it very accurately, so what you do is try to measure many at once, when you do this equal numbers appear to be younger or older than their actual ages, this makes some of the objects which are actually 10-12Gyr old appear to be 15 Gyr old. This is simply to do with signal-to-noise, statistics and fitting of templates to determine ages. Similarly with objects like white dwarfs, or certain low mass stars which we know could continue to shine for 100 Billion years, yet we never see any of them that are older than 13 Gyr, why is this? Because there are none older than 13Gyr.

The point he makes about galaxy structures would actually appear to be true if you don't actually think about the issues for a moment. Lets face it what crank actually wants to do some thinking and then some difficult maths? For one, the early Universe was much more dense than it is now, as gravity acts as an inverse square of the distance between mass this means that if the Universe was half the size the average force of gravity between clumps of matter would be 4 times larger. Clearly earlier in the Universe it was much easier for gravity to pull matter together to from stars/galaxies/clusters of galaxies. A second point is that the simulations he talks about do not include the effects of dark matter, which contains many times as much mass as luminous matter, adding this in means you can easily form large structures in as little as a few Gyr. If you don't believe me here is a picture of a simulation showing exactly that. Every dot in the image is a huge clump of dark matter (and stars).

Solutions to the Big Bang Age Problem.
A recent new scheme to help BBers solve their age-old age paradox is the invention of “dark energy” or “quintessence.” They have decided that those imaginary entities cause the rate of expansion of the universe to be accelerating. The universe would therefore have expanded more slowly in the past. It would therefore be older than previously thought, thus helping to solve their age problem. (Incidentally, some Bbers of long ago had proposed a universe of accelerating expansion, but their schemes failed to survive.)

Not much to say here, he provides no proof that the cosmological constant or quintessence is impossible, mostly because it is far beyond his intellect to grapple with the maths I'm sure.

The Two Biggest Frauds of All:
The Big Bang creation of the universe was “out of nothing.” Until Inflation theory came along in the early 1980s, BBers believed that, before the Big Bang, there was no matter, no energy, no time, no space.
An equally ridiculous solution to that fraud was the ”inflation” resulting from a “vacuum fluctuation” of the energy of empty space as the source of the Big Bang. Somehow the space and energy, that hadn’t perviously existed, was now said to have been there all the time, and it contained unimaginable amounts of energy.

This is a vacuous and also incorrect statement. The BBT makes no real claims about what happens before the BB in fact it makes no real claims at present about what happened in the very smallest fraction of a second after the BB, why? Because when the matter reaches the incredibly high energies found just after the BB the laws of physics break down, we cannot use the laws we have now to explain all the way back. We know that the laws of physics break down because gravity is not yet reconciled with quantum mechanics and until we have a quantum theory of gravity we can't know what happens earlier with any certainty. Does this matter? Not really, the theory as a whole does not depend on the mechanism, as long as some process can produce the initial expansion and then the inflationary phase everything is fine.

The above are just a few of the items of fraud that continue to be disseminated. However, leading BB cosmologists are well aware of those; and many more problems and inconsistencies of BBT. While attempting to appear unconcerned, minimize their importance, or avoid mention of them, they struggle to invent new schemes to circumvent them; while making big money as professors, lecturers, TV personalities, and authors of articles and books with intriguing new titles. However, most of the innovations they produce (like inflation and acceleration), that are intended to solve BBT problems, produce only new problems.
Meanwhile, those intelligent and educated establishment folks choose to ignore the problems of BBT, and to perpetrate the fraud that continues to be disseminated in the media.

I'd love to know who makes big money from being an astronomer, I certainly don't know of any rich astronomers. If I wanted to be rich the easiest way would be to take up televangelism, or to write a really poor book about how scientists are frauds and the BBT is wrong and then try and sell it to the wingnut creationists. I mean you wouldn't even need to do any hard work then right? No need to check facts, or test theories, you know those people will believe anything as long as it makes BBT or evolution look bad.

If you would like to gain a thorough understanding of Big Bang Theory and its many flaws; and then learn the truth about cosmology based, not on wild ideas and fantasies, but on vast amounts of astronomical data, true science, logic and common sense, you must read my book, BYE BYE BIG BANG - Hello Reality.
This inexpensive, 446 page, paperback - that includes a bibliography, 5 appendixes, and name and subject indexes - can be found in most large US city and large US university libraries; or order it from your book dealer (at $19.95 - available to them from THE DISTRIBUTORS*), or click on:

Best wishes,
William C. (Bill) Mitchell
Carson City, Nevada, USA

*Conact Patty Walsh at THE DISTRIBUTORS, 702 S. Michigan, South Bend, Indiana
46601, USA. Email: pwalsh01@ameritech.net, Telephone: 574-232-8500

In conclusion I can't help but feel terribly embarrassed for the poor guy, it really is that bad, he hasn't even checked the simplest of his arguments, I would expect better from a high school essay. There are plenty of problems with the BBT, none of which are major, if there weren't there wouldn't be anything left for Astronomers to do, but he has decided to attack problems that don't exist. Very sad. I think we can see from the short email that reading the book is probably going to be pointless. If he can fit that many distortions, lies, out of date data into just that few lines how many will the book have?

The Universe, All Of It On Your Screen

Here is an image that the ICCmofos had on their door a while back, showing distances to objects stretching from the Core of the Earth to the edge of the observable Universe. Using a logarithmic scale it shows the distances to objects such as Earth orbiting satellites, the planets, nearby stars, galaxies, clusters of galaxies, SDSS galaxies and the Cosmic Microwave Background. It is very cool in a very nerdy way. Unfortunately I can't seem to track down who made it right now.

Of particular note are the SDSS galaxies shown at the top of the page as blue dots, the filamentary structure of the Universe on large scales is immediately obvious. I still think it is pretty amazing that we're smart enough as a species to measure such things and then are able to plot the distances to anything in the Universe on only 3-4 sheets of A4.

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.

Hobbit Galaxies

According to this story from space.com the SDSS has found 8 more dwarf galaxies in the local group. Seven of which appear to be satellites of the Milky Way, these seven galaxies, all of which are very faint and diffuse appear to be essentially old. The one new galaxy which is not thought to be bound to the MW appears to still be forming stars and to have a significant reservoir of neutral hydrogen with which to form more stars.

This is all very interesting because it has implications for one of the big problems of modern cosmology, the so called "missing satellite" problem. This is the observation that cosmological simulations of structure formation in a dark-energy, dark matter dominated Universe seem to predict many more small satellite galaxies around larger galaxies like the MW than we see. If however these galaxies do exist and they are simply very faint and hard to see as the SDSS results imply, then there isn't any problem at all. Seeing as the SDSS only covers about an eighth of the sky, and doesn't see very faint things, it does imply that there should be many more of these galaxies left out there to be found. I think a cottage industry of trying to find these things is probably about to spring up.

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.