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Sunday, 27 February 2011

Dark matter theory challenged by gassy galaxies result, LHC sees no SUSY (yet)

An old friend just sent me this article which discusses new results from gas-rich galaxies in favour of Modified Newtonian Dynamics theory (MOND).
BBC News - Dark matter theory challenged by gassy galaxies result

This coincides nicely with this article I was reading about how the LHC has so far failed to find supersymmetric particles ("sparticles") at the electroweak scale. These particles were thought to be the most likely candidates for dark matter.

Could this be the end for dark matter theory? Well, no. Certainly not at this stage. The author of the gassy galaxies study, Stacy McGraugh, admits that MOND still produces poor results on the scale of galactic clusters. And the LHC results are still a long way from ruling out supersymmetry. It could be that the particles have not been seen because their masses are beyond the range the ATLAS experiment has been looking at - although SUSY models do begin to get complicated if the sparticles involved are very heavy.

It will be interesting to see what results emerge from the LHC in 2011. The collider began to reawaken from its winter shutdown on Feb 19th, when the particle beams started circulating again. The number of collisions is to be stepped up this year, with 100 times more data expected to be collected in 2011 than in 2010.

Tuesday, 22 February 2011

Cycles of Time: An Extraordinary New View of the Universe (Review)

When I came across Roger Penrose's new book, Cycles of Time: An Extraordinary New View of the Universe, I just had to read it, having twice heard him speak on the slightly wacky topic of Conformal Cyclical Cosmology (CCC).  CCC is the idea that the Universe may have undergone several cycles and proposed the controversial idea that information about the time before the Big Bang may still be present in the Cosmic Microwave Background today.  In both of the talks I heard him give - about a year apart - he ended with the tantalising promise that observational evidence testing CCC theory could be just around the corner.

Perhaps the best introduction I can give to this book is to link to a recording of Penrose speaking on the subject.  This talk is from 2005, several years earlier than the talks I heard him give in which he promised observational results.  Alas, I do not have recordings of either of those.

The book expands upon the ideas that Penrose speaks about and is able to present them at greater length, which makes it easier to follow than the talks.  Mathematical details are included, but mostly kept confined to detailed appendices, with references for the reader who desires to pursue the topic in depth.

Penrose's bold idea brings together several topics in physics: entropy, black holes, particle physics and cosmology, all of which are explored in the book.  He closes by explaining exactly how cosmological observations could provide a physical test for his theory.  As I spend my days in an environment in which people play with toy mathematical models or hammer out the details of string theory without too much concern for the real world, I find it refreshing to see a theorist giving serious thought to the testability of his theory.

Monday, 21 February 2011

Theoretical Physics Seminars and Lectures Online

Over the weekend I came across some very helpful resources for keeping up with what's going on in the world of physics research, and for learning physics.

I love listening to physics talks and seminars, but hate the whole experience of going to the department and being crammed into a room with other researchers. Also, how many times have you lost concentration for 30 seconds whilst listening to a talk, only to tune back in to find that you've missed a vital piece of information or step in the reasoning, which makes the rest of the talk impossible to follow?  Wouldn't it be great to be able to pause or rewind the speaker?

There are a few institutions who helpfully publish their seminar series online:

This is not a comprehensive list of institutions, rather a list of those that I found which have significant amounts of up-to-date and interesting material.  I will be updating the list as I find more sources.

Please feel free to add more links in the comments!

Wednesday, 9 February 2011

The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Book Review)

The Hidden Reality: Parallel Universes and the Deep Laws of the CosmosBrian Greene, bestselling author of The Elegant Universe, has recently released a new book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, in which he sets out the arguments for the theory that the universe we inhabit may in fact be just one of many universes.

The concept of this multitude of universes, or “multiverse”, arises in response to many puzzling topics in physics. It has been invoked to explain the apparent fine-tuning of physical constants, as an interpretation of the uncertain nature of quantum mechanics, and as the “braneworld” scenario of string theory - in which our universe is a three-dimensional membrane floating in a higher-dimensional space of alternate universes.

Greene tackles each of these in turn, starting in Chapter Two with the simplest reasoning behind the belief in parallel worlds: if the universe is infinite in extent, it is inevitable that one of the infinite number of planets out there is identical to our own, and that on it there are people exactly like ourselves undergoing an alternate version of our reality. Greene discusses the question of whether or not the physical size of our universe is infinite or finite, which is still very much an open question in modern physics.

Greene, as in his previous books, uses metaphor to make difficult concepts accessible to the general reader. Expect to have to take your time to wrap your brain around the subject matter, but what Greene won't do is blind you with technical jargon or assume specialist knowledge.

I have been a fan of Brian Greene since I read The Elegant Universe at seventeen. If I had to pinpoint a deciding factor in my decision to study theoretical physics, this book would probably be it. This was the book that got me hooked on strings, particles and the quest for a “theory of everything”. It was fantastic to read the author's recent follow-up and have those feelings of curiosity and excitement resparked.

Monday, 7 February 2011

Dark Matter Due for Discovery?

The Large Hadron Collider (LHC) at CERN will be up and running again soon following its winter shutdown. As well as seeking the Higgs Boson, scientists are also hoping that the accelerator will provide insights into the nature of dark matter.

In my last post, Torsion and the Matter-Antimatter Asymmetry, I discussed a recent article which concluded that dark matter might consist of antiparticles left over from the Big Bang.  But this is only one of many theories.  What follows is an introduction to dark matter and a brief summary of the theories that have been proposed to answer to the question: what is dark matter?

Introduction to Dark Matter

The concept of “dark matter” was introduced in 1934 by Fritz Zwicky. By making measurements of the motions of galaxies on the edge of a cluster, he calculated that the total mass of the cluster was 400 times greater than the amount that he observed.

The problem was taken up by astronomers Vera Rubin and Kent Ford at the Carnegie Institute during the 60s and 70s. They measured the velocity curve of spiral galaxies and produced results that also seemed to imply the presence of a large amount of non-visible mass.

The Evidence for Dark Matter

Astronomical observations clearly show that most of the stars in a spiral galaxy lie in a rotating circular plane, with the density of stars being much greater towards the centre of the plane than further out. The stars in the galaxy are kept together by the gravitational attraction between them.
F is the gravitational force,
m1 and m2 are the masses

of the two bodies, r is the

distance between them,
and G is a constant

As stated in Newton's famous gravitational law (shown to the right), the gravitational attraction between two bodies decreases as the distance between them increases. This means that, if most of the mass of the galaxy is located in its centre, then a star orbiting at the edge of the galaxy should experience a weaker gravitational pull than a star that is close to the dense galactic core.

The speed at which a star orbits depends on how much gravitational force is exerted on it by the rest of the galaxy: a star orbiting too slowly will gradually spiral in towards the centre, and a star that orbits too fast will spiral outwards as the gravitational force is not strong enough to keep it reined in. Therefore, we would expect stars that are further out to have lower velocities than those close to the centre. This expectation is plotted on the graph below as curve A. However, Rubin and Kent's results (curve B) show that the velocity of the stars is about the same for all distances from the centre.

There are two possible explanations for this. The first is that the theory of Newtonian gravity, despite its success in predicting the orbits of planets in our own solar system, is flawed. It has been suggested that Newton's law of gravity should be modified to better fit the galaxy rotation curves. This hypothesis is known as MOND – Modified Newtonian Dynamics. Although MOND is very successful in predicting galaxy rotation curves (as one would expect, since this is the situation it was designed to model), it is less good at predicting effects at the larger scale of galactic clusters.

The more popular explanation is dark matter theory: the idea that there is more mass in galaxies than what we can see through our telescopes. The problem is that no-one knows what this dark matter actually is. It was first thought that it might consist of non-luminescent objects such as black holes and dead stars such as brown dwarfs or neutron stars, known collectively as MACHOs – Massive Compact Halo Objects. However, it is now thought that the majority of dark matter is in the form of WIMPs – Weakly Interacting Massive Particles.

But what are WIMPs?

Particle physics has suggested many candidates for WIMPs. Any particle that makes up dark matter would have to be stable – i.e. it must not decay into other particles. Neutrinos, which rarely decay or interact with other particles, have been suggested as a possible candidate.

Another possibility is the theoretical Lightest Supersymmetric Particle (LSP). Supersymmetry is a theory of physics which predicts the existence of a much heavier partner to each known particle. The lightest of these super-particles (nicknamed “sparticles”) is expected to be stable so would be a good candidate for dark matter.

Experiments are currently underway at the LHC to try to produce sparticles, and we could see results as soon as this year. Will dark matter finally reveal its identity?

The discovery of supersymmetry would have many exciting consequences for theoretical physics beyond the identification of dark matter... but that's the subject of a future post.

Friday, 4 February 2011

Torsion and the Matter-Antimatter Asymmetry

Matter-antimatter asymmetry is a mystery which is (almost) as old as the universe:  Why is the amount of matter in the universe - the stuff that makes up you and I and everything familiar - so much greater than the amount of antimatter?

Many scientists have attempted to come up with an explanation for this disconcerting lack of symmetry.  I give a layman's overview of the problem and list the suggested solutions in this article.  The latest suggestion that I want to discuss here is that a variant of General Relativity involving torsion could be responsible for the apparent asymmetry.

Introduction to the problem
Antiparticles have the same mass and spin properties as their particle siblings, but opposite charge - a fact which led to their discovery in 1932, when Carl Anderson noticed a track in a cloud chamber that looked exactly like the path of an electron except for one detail: the fact that it curves anticlockwise instead of clockwise in the magnetic field of the chamber indicates that the particle responsible for it was positively rather than negatively charged.  The track had been left by a new kind of particle - a positive electron, or "positron" - which had been predicted to exist by Dirac four years prior to Anderson's discovery.

The reason that we had never seen one of these particles before is that positrons are rare: the term matter-antimatter asymmetry refers to the fact that the ratio of electrons to positrons has been observed to be staggeringly large.

The dominance of matter over antimatter was established during the first few seconds after the Big Bang.  During this time, the enormous density of the Universe meant that temperatures were very high and particles and antiparticles bubbled in a hugely energetic plasma, undergoing millions of interactions per second.

In an interaction, particles collide and are destroyed, with new particles being born from the energy liberated by the destruction.  But even this seemingly chaotic process follow rules: one such being that Baryon Number, B, defined as the number of particles minus number of antiparticles, is always conserved.  This means that if before the interaction you had two particles and no antiparticles, then afterwards you must have either two particles and no antiparticles, or three particles and one antiparticle, or any other possibility such that B=2 just as it did before the interaction took place.

The problem currently faced is that the value of B in our universe today is observed to be a very large positive number, and there is no explanation as to why this should be the case.  In a universe displaying an astonishing degree of symmetry, it feels surprising and unnatural to have such a blatant asymmetry built into the universe's initial conditions.

Torsion as a possible solution
A recent paper by Nikodem J. Poplawski of Indiana University is the latest to suggest a solution - not only to the conundrum of matter-antimatter asymmetry, but also to the puzzling nature of dark matter and the origin of dark energy.

The approach is to modify Einstein's theory of General Relativity to include a non-zero torsion tensor.  The torsion tensor expresses the amount of 'twist' which exists in the fabric of space-time.  The action of this term is to interact with the fields in Einstein's equations which represent fermions, causing their masses to change.  It acts unequally on particles and antiparticles, creating an asymmetry.  The result is that particles in the early universe decay into normal matter and antiparticles into dark matter.  Therefore the authors suggest that the overall Baryon number of the Universe is in fact zero - there is exactly the same amount of matter as antimatter - but that the antimatter is present in the form of dark matter, which lurks in great clouds in galaxies, undetectable except by the gravitational effects of its mass.

The authors also claim that torsion leads to the conclusion that the cosmological constant is non-zero.  This means that torsion provides an explanation for the presence of dark energy, the mysterious force which is causing the universe to expand at an ever-increasing rate.

The authors make no quantitative predictions of what ratio of matter to antimatter we could expect their theory to yield, so we cannot compare yet the universe that would result from such a theory to the one we observe in order to test the theory.