A golden age for precision observational cosmology

Yin-Zhe Ma gave a nice physics colloquium at UQ last week, A Golden Age for Cosmology

I learnt a lot. Too often, colloquia are too specialised and technical for a general audience.

There are three pillars of experimental evidence for the Big Bang model: Hubble expansion of the universe, relative abundance of light nuclei due to nucleosynthesis in the first few minutes, and the Cosmic Microwave Background.

Ma showed Hubble's original data from 1929 for redshift versus distance of galaxies. There was a lot of noise in the data. Nevertheless, Hubble was right.

Big Bang Nucleosynthesis

This was first proposed in 1948 by Ralph Alpher and George Gamow. (Hans Bethe was an honorary author of the paper as a joke so that the author list would sound like the first three letters of the Greek alphabet. Gamow had a mischievous sense of humour.)

The chain of nuclear reactions that will produce the lightest elements and isotopes is shown below.

Because the binding energy of 4He is so large, it could have only been formed at an extremely high temperature of about 10^10 K. (Or is the issue activation energy for formation, not binding energy?)

Detailed calculations using parameters from terrestrial nuclear physics give the observed relative abundances of the elements. In particular, the universe is 74% hydrogen and 24 per cent helium.

The astrophysicist's periodic table showing the origin of the different chemical elements is rather cute.

Giving credit to George Gamow

Gamow, who died in 1968, made impressive contributions to theoretical physics. His Wikipedia page is worth reading. He claimed that he predicted the Cosmic Microwave Background in the late 1940s and did not receive sufficient credit when it was discovered in 1964. The 2019 Nobel Prize citation for James Peebles also minimises Gamow's early contributions. Whether this is fair or not can be debated.

Anisotropies in the Cosmic Microwave Background.

The past two decades have seen amazing advances in precision measurements of these anisotropies. The radiation is isotropic to one part in 25000, with a temperature of 2.72548±0.00057 K.

Measurements of the anisotropies have allowed precise determinations of key cosmological parameters by fitting theoretical predictions to the data shown below from the 2018 Planck collaboration. Different peaks have different physical origins. 

The level of precision in the data is truly amazing.

The solid line is a fit to theory involving six parameters. What would Enrico Fermi say? This is not "making the tale of an elephant wiggle" because the fit parameters are all consistent with independent determination of the cosmological parameters from Hubble expansion and the relative abundance of the light elements.

Aside. The paper from the Planck 2 collaboration has been cited 19000 times, but has almost 200 authors. How does one use that information in evaluating individual authors in job and promotion applications? How are they to be compared to a single-author paper with 100 citations or a five-author paper with 500 citations?

Is this a golden age for cosmology? 

Yes, in terms of precision measurements. 

On the theoretical side, the golden age may have passed. It is not clear that new concepts or theories will emerge. The outstanding questions are:

What is the nature and origin of dark matter? of dark energy? 

Why is the cosmological constant so small? Why is it so fine-tuned?

Can the validity of inflation be pinned down?

Does quantum gravity matter?

A lot of smart people have spent decades on these problems and made little progress. That fact does not preclude the possibility of a theoretical breakthrough. However, it does not make me optimistic. I hope I am wrong.

Colloquium on 2021 Nobel Prize in Physics

 Every year the UQ Physics Department has a colloquium where someone describes the science behind the latest Nobel Prize. This year I am going to talk about Parisi and the spin glass problem. My colleague Henry Nourse will talk about the climate modelling part.

In preparation, I have found the book, Spin Glasses and Complexity by Daniel L. Stein and Charles M. Newman, very helpful. It is at the level of a colloquium and has a nice chapter on applications to other areas of science (e.g. proteins, simulated annealing, optimisation, computer science, ...) It enabled me to finally "understand" the background and significance of Hopfield's famous paper from 1982, "Neural networks and physical systems with emergent collective computational abilities".

Thinking about replica symmetry breaking has brought back memories of when I was a graduate student at Princeton. When I started Anderson was thinking about spin glasses a lot and had people working on it. I heard lots of talks about spin glasses, replica symmetry breaking, travelling salesmen, ultrametricity, ... Even David Gross gave a colloquium about work he did on spin glasses, with a very warm introduction by Phil. ["I introduce David Gross the condensed matter theorist"] However, once the cuprates happened at the end of 1986, Anderson seemed to largely drop the spin-glass work. Except for Dan Stein, everyone started working on cuprates. In hindsight, I wonder if that was a mistake. In particular, it might have been better for many of his students if they had worked on complexity rather than cuprates.

Next week I will post a draft of my slides. In the meantime, two questions for readers:

1. What are some specific questions you might like answered in such a colloquium?

2. What are some specific resources you may have come across about this year's prize that you found helpful or interesting?

Here are the slides.

Mathematics, biology, and emergence

Last night I heard a model public lecture about science. The School of Mathematics and Physics at UQ hosted a public lecture at the Queensland State Library. Holly Krieger, a pure mathematician at Cambridge, spoke on the Mathematics of Life. This is part of a biannual lecture series endowed by Kurt Mahler.

The lecture was amazing, both in content and presentation. It was engaging for high school students, and stimulating for experts. I wish I had a video or a copy of the slides. Krieger is well known to some through her Numberphile videos on YouTube. Here are a few things I learned in the lecture.

Mathematics is the language of relationships and patterns.

We forget how even the concept of numbers is abstract. The notion of functions is even more so.

An underlying theme of the lecture was that of emergence: a simple rule describing the interactions between the components of a system lead to collective behaviour (complexity) of the whole system.

Examples were given from biological systems that raise the question: how does the system know to do this?

Swarms of starlings were shown in the short film, The art of flying by Jan van IJken.
How do they move in concert when there is no leader?



Other examples included ant bridges, an experiment with a slime mould that was able to replicate the Japanese transport network (here is the Canadian version), stripes and spots on animals (pattern formation explained with coupled reaction-diffusion equations by Alan Turing).

To illustrate how simple rules lead to complex behaviour, several cellular automata were demonstrated starting with Pascal's triangle and Sierpinski triangle. The latter was connected to biology through the pattern on the shell of a (poisonous) cone snail.

Rule 30 produces patterns similar to those found on the shell. It has periodic patterns such as stripes and aperiodic chaotic patterns.
It seems the new Cambridge train station also has this pattern!

Rule 184 can describe traffic including jamming for medium traffic densities.
The occurrence of a traffic jam does not depend on the initial state or a particular car, but only depends on the density of cars and the interaction (rule) between cars.

A nice video was shown of a traffic shockwave.
When water flows from a tap (faucet) and hits a flat sink bottom at right angles it may produce a "hydraulic jump" such as that shown below.

That is just the first half of the lecture. I may blog later about the second half which concerned chaos, defined as small initial changes leading to significant changes in outcome.

One of the most interesting things for me about the lecture was Krieger's claim that "Emergent complexity isn't everywhere. It can be hard to detect or confirm.'' i.e., just because we see complex behaviour (patterns) does not mean that it is due to emergence. In question time she said that this was in response to some of Wolfram's grand claims in A New Kind of Science, along the lines that everything (consciousness, gravity, continuity, free will, ...) could be explained in terms of discrete computational models such as cellular automata.

I think a more nuanced view is necessary. I agree, along with many others, that Wolfram's grand claims are not justified. But, I do not equate emergent complexity solely with simple rule-based computational models such as cellular automata. Different people do define emergence differently. For example, Sophia Kivelson and Steve Kivelson propose the following definition.

An emergent behavior of a physical system is a qualitative property that can only occur in the limit that the number of microscopic constituents tends to infinity.

This would rule out classifying most of the phenomena described in the lecture as emergent. I disagree with this definition. On the other hand, I am not sure I agree with Krieger's claim. I do think almost anything interesting is emergent: consciousness, critical phenomena, the vacuum in quantum field theory, superconductivity, ...

Universality, probability, and the growth of rough surfaces

On Friday there was a nice UQ Maths and Physics Colloquium, Beyond the Gaussian Universality Class, given by Ivan Corwin,
The talk was a very nice example of synergy between fundamental physics and maths research.
There are interesting connections with simple one-dimensional models for surface growth, the Kardan-Parisi-Zhang equation, the KPZ universality class, traffic models, random matrix theory, directed polymers in random media, ....

KITP seminars online

A wonderful thing about the web is that now there is so much material online. A pioneer in putting all their seminars and colloquia online is the KITP at Santa Barbara. I know some people who regularly watch seminars (both old and recent). Others do not know it exist. This is a particularly valuable resource for students and those of us in distant countries.

I have to confess that until yesterday I have never actually watched a talk; just occasionally skimmed some slides. Generally I find I don't have the patience to watch talks online. I just seem to prefer to look at papers. However, yesterday I was forced to do this because at the weekly UQ condensed matter theory group meeting we watched a nice talk by Antoine Georges on Hund's metals. Although, I have read and blogged about some of the relevant papers, I really found it helpful seeing what was highlighted and going through the material at a "slow pace". Hopefully, I will do this more often.

What do you think about online talks or lectures? How often do you watch them? Are there any that you would particularly recommend?

Rebutting the historical conflict thesis

Last friday we had Peter Harrison give the Physics colloquium on "The Progress of Science and the Decline of Religion?"
He is a historian, who prior to coming back to UQ, held a chair at Oxford, and in 2011 gave The Gifford Lectures, which were recently published.
He is probably best known for arguing that changing approaches to Biblical interpretation, associated with the Reformation [moving away from an emphasis on allegorical interpretations towards more literal and historical interpretations] changed peoples conception of "nature" and had a significant influence on the development of modern science.
Peter is director of the Centre for the History of European Discourses at UQ and attracts many stimulating and distinguished seminar speakers, some of whom I have blogged about before.

One issue Peter addressed head on is the "conflict thesis" which claims that science and religion have always been in conflict and particularly that religion has impeded the progress of science. This view is popular in the public realm but not among historians of science. [The Wikipedia page is worth reading]. Like most issues the reality is much more complex.

Peter mentioned several widely cited historical "conflict" incidents such as Galileo and Darwin. In both cases there were people who opposed them and who supported them using religious and scientific arguments. For the Galileo affair the main contention was about competing scientific models and different philosophical perspectives. Furthermore, Galileo's scientific case was hardly solid; no stellar parallax had been observed and his argument using the tides was (in hindsight) wrong. In Darwin's case he was opposed by Lord Kelvin (who had miscalculated the age of the earth) and supported by Asa Gray and some conservative theologians.
John Heilbron's book, The Sun in the Church "illuminates the niches protected and financed by the Catholic Church in which science and mathematics thrived."
Stephen Gaukroger's book, The Emergence of a Scientific Culture: Science and the Shaping of Modernity emphasised how science had religious sanctions.

If you have a few hours (and $200!) you can watch some very nice lectures on the above historical issues. I highly recommend a course given by Lawrence Principe (Johns Hopkins) and sold by The Great Courses [The Learning Company in Australia]. [Peter Harrison recommended these to me and I bought them on special for A$52 including shipping].

Peter also discussed more recent history including some sociological studies, which seemed to attract the most questions from the audience.
He gave a similar talk at BrisScience a few years ago and can be viewed here.

An alternative to cosmic inflation

On Friday Robert Mann gave a very nice colloquium at UQ, The Black Hole at the Beginning of Time. The video is below.

The (end of) the talk is based on the recent paper
Out of the white hole: a holographic origin for the Big Bang 
Razieh Pourhasan, Niayesh Afshordi, and Robert B. Mann

The key idea is to consider our universe as the 4-dimensional boundary (brane or hologram) of a 5-dimensional space-time in which there is a black hole.
In our universe one then has not just 4D gravity and matter, but also induced gravity and an effective fluid from the 5D "bulk".

(For better or worse) this work was recently featured on the cover of Scientific American.

Robert covered a massive amount of material moving through special relativity, general relativity, black holes, big bang, cosmology, recent results from the Planck satellite,  inflation, the multiverse,... and finally his alternative model.
I took several pages of notes.
He went overtime. I think this was one of the rare cases where I did not mind the speaker doing it.

Besides learning some interesting physics, what was most interesting to me was the refreshing way the work was presented. The tone was something like, "cosmology has some amazing successes but there are a few paradoxes, inflation is an interesting idea but also presents some problems, ...fine tuning is a challenge, ... so let me throw out a different idea.... it is a bit weird... but lets see where it goes ... it also has some strengths and weaknesses .... I am not sure this is better than inflation, but it is worth looking at." There was no hype or sweeping things under the rug.

Many in the audience were undergrads. I thought it was a great talk for them to hear. It was largely tutorial, there was some fascinating physics, connections to experiment were emphasised, healthy skepticism was modelled, and there was no hype.

I also liked the talk because it confirms my prejudice that people need to work harder, more creatively, and more critically, on foundational problems in cosmology. Dark matter, dark energy, inflation, and fine tuning are all really weird. They may be right. But they may not be. I think just accepting them as the only option and regressing to even weirder ideas like the multiverse is a mistake. [Of course, it is easy as an outsider to tell colleagues to work harder and more creatively.]

The physical model of the early universe that was presented was completely different to inflation. Yet it solves most of the same problems (horizon, flatness, and no monopoles). Its biggest problem is that it does not predict the observed 4 per cent deviation from scale invariance.

The most important and interesting bits are from about 52:00 to 58:00.

The art and discipline of a good colloquium

There is a helpful and challenging article The Physics of Physics Colloquia by James Kakalios on The Back Page of the APS News. It is based around old notes Suggestions for giving talks by Robert Geroch.

Both Kakalios and Geroch are worth reading in full, but here are a few random things that stood out to me. [Things I need to keep working on].

"What is the key take-away point that you want to impress on everyone when they leave your talk?"

Divide the talk up, centred around 3 or 4 key messages.

"Figures are easier to understand than words."

"You have been staring at these data and plots for years, but many in the audience have not."

Don't include more than five non-trivial equations.

"It is almost always a disaster to run over time".

Much of this may seem "common sense". However, as management guru Steven Covey said, "Common sense is not common practise." Preparing and giving a good talk requires a lot of discipline, particularly with regard to cutting out material.

Should cats be herded?

Everyone agrees that you need to herd cattle and sheep. But what about cats?
Cats are best enjoyed and fulfil their purpose if they are left alone and allowed to be what they are.

What is the relevance of this? It is sometimes claimed that academic researchers are like cats. They are fiercely independent and groups of them are very difficult to manage. Hence, books such as Herding Cats: Being advice to aspiring academic and research leaders by Geoff Garrett and Graeme Davies.

I became aware of the existence of the book because Geoff Garrett, who is now Queensland's Chief Scientist, gave the UQ Physics Colloquium on friday. I have not read the book. Afterwards a colleague expressed reservations about the ideas presented, saying, "this is relevant to engineers, not physicists!"

One idea that was presented was the importance of having a "Big Hairy Audacious Goal" which creates team spirit. Although laudable on some level, I am hard pressed to think of examples in science that have been fruitful or that I personally find inspiring. Maybe I am jaded but ones such as "nuclear fusion in our lifetime", "build a quantum computer", "discover a room temperature superconductor", "cheap organic solar cells to save the planet", or "lets make our university number one" just don't seem that achievable via highly managed research teams. 

Furthermore, it seems that most Nobel Prize discoveries did not result from such programs, but rather from curiousity driven research by "independent" research groups. One obvious exception are Nobel Prizes for discoveries in elementary particle physics.

What do you think?

Is space-time emergent?

On Friday at UQ we had a very stimulating colloquium A new view on quantum gravity and the origin of the Universe by Bei-Lok Hu (University of Maryland). A key aspect of this new view is that general relativity and space time should be viewed as emergent phenomena (more below).

There are six main points of experimental evidence in cosmology:
1. Hubble expansion of the universe.
2. Cosmic microwave background radiation (isotropy and uniformity).
3. Element abundance (+ nucleosynthesis)
4. Ratio of baryon/photon (entropy content of universe)
5. Structure: galaxy, clusters,...   hierarchy of scales
6. Cosmological constant ~ 0,  vacuum energy density

The fact that the night sky is dark implies a finite universe, and expansion or a hierarichial structure  (Olber's paradox).

Hu contrasted Two views of quantum gravity.

1. Bottom up view
Quantum gravity = quantisation of general relativity
This is the more traditional view and has been dominant.

2. Top down view
Gravity is emergent  and general relativity should be viewed as the "hydrodynamics" of some underlying "microscopic" theory.
This means that one must deal with a micro-macro transition as well as a quantum-classical one. This view has become more popular in the last 5 years.

Hu's advocacy of 2. is summarised in detail in a conference paper.

It should be pointed out that Bob Laughlin has also advocated such a perspective. His book, A Different Universe, has a chapter, The Fabric of Space-Time, which ends with the claim that if Einstein were alive today he would,

conclude that his beloved principle of relativity was not fundamental at all but emergent - a collective property of the matter constituting space-time that becomes increasingly exact at long length scales but fails at short ones. This is a  different idea from his original one but something fully compatible with it logically, and even more exciting and potentially important. It would mean that the fabric of space-time was not simply the stage on which life played out but an organizational phenomenon, and that there might be something beyond.

R.B. Laughlin, A Different Universe, p. 126

[See also his 2004 Perspective, The Cup of the Hand, in Science].

On monday we will have another colloquium, this one by Thanu Padmanabhan (IUCAA, Pune University), and advocating a similar view, and summarised in this conference paper.

What is wrong with these colloquia?

Are you preparing a talk? There was a provocative article What's Wrong with Those Talks by David Mermin, published by Physics Today back in 1992. It is worth digesting, even if you do not agree with it. He does practice what he preaches. I once remember him reading a referee report from PRL once in a talk on quasi-crystals. (He claimed the referee was Linus Pauling). One of his main points is we need to be very modest about what we hope we can achieve in a talk, particularly a colloquium. People will rarely complain if the talk is too basic and they understand most of it. The primary purpose is to help people understand why you thought the project was so interesting that you embarked on it.

Should debatable data generate theoretical hyper-activity?

At the physics colloquium today recent experimental data was highlighted that has been interpreted as evidence for dark matter. I thought this looked familiar and recalled I wrote an earlier blog post Trust but verify, urging caution. If one does a search on the arxiv with the words "dark matter AND positron AND FERMI" one finds more than one hundred papers, many proposing exotic theoretical scenarios.
It will be interesting to see in a decade whether all this theoretical hyper-activity was justified.

There is still a Kondo problem

It was nice having Peter Wolfle visit UQ the past few days and give a colloquium style talk on the Kondo effect.
A nice accessible article which discusses the basics of the Kondo effect and how it occurs in quantum dots, carbon nanotubes, and "quantum corrals" is this 2001 Physics World article by Leo Kouwenhoven and Leonid Glazman.
In his talk Peter gave a nice discussion of the problem of the Kondo lattice (a lattice of localised spins interacting with a band of itinerant electrons). There is still an open question (originally posed by Doniach) of what happens in between the two limits of weak coupling (one expects magnetic ordering of the spins due to the RKKY interaction mediated by the itinerant electrons) and strong coupling (where the individual spins are Kondo screened by the itinerant electrons). Is there a quantum critical point? Does a non-Fermi liquid occur near it?
All of this is discussed in a nice review article Peter co-authored, Fermi liquid instabilities at magnetic phase transitions. It was very useful to Michael Smith and I when we wrote a PRL about possible Weidemann-Franz violations at a quantum critical point.
If you want a discussion of the RKKY-Kondo competition from the point of view of quantum entanglement, see this PRA paper  Sam Cho and I wrote.

What did Escher know?

In the interesting physics colloquium that Marcelo Gleiser gave on friday at UQ he used the Escher print above to illustrate CP symmetry in a system which violates both C and P symmetry.
[C is charge conjugation and P is parity].
I found the image here.
Another example of breaking of a symmetry while conserving a combined symmetry concerns vibronic transitions in molecules. For molecules which have inversion symmetry selection rules suggest that electronic transitions that involve no change in parity should not be observed. However, they sometimes are because they are combined with a vibrational transition. Hence, vibronic [= vib(rational)-(elect)ronic] transitions. I think the first observed case of this may have been in benzene.

A turbulent claim?

On Friday we had a nice clear and stimulating physics colloquium, Turbulent times in quantum physics from Brian Anderson.

What are unique characteristics of turbulence?

A beautiful video of a dragon fly in fluid flow was shown to illustrate this.

1. continuous flow

2. unpredictable flow details

3. eddy formation, interaction

4. rapid mixing

5. energy input at one length scale and energy dissipation at another length scale.

The latter is described in a landmark paper from 1941 by Kolmorgorov. He used dimensional analysis to show that the kinetic energy spectrum

E(k) ~ k^-5/3 where k is the wave vector.

A superfluid has no viscosity. But turbulence is still possible. Feynman suggested in 1955 that this could arise as a disordered tangle of vortices.

Three features of quantum turbulence

1. dynamics is described by a quantum dynamical equation (e.g., a non-linear Schrodinger equation) rather than the Navier-Stokes equation.

2. Kolmogorov scaling (this was observed in 1998)

3. disordered tangled arrangement of vortices

BECs have "high potential" for step-by-step construction of a quantum turbulent state.

There are only a million atoms in the BECs studied here.

[But isnt this just 100^3? What is the max. no of vortices one could put in such a small system, 100?]

Spontaneous vortices can be produced with a temperature quench.

It was claimed that dissociation of vortex-antivortex pairs is related to quantum turbulence. However, in two dimensions this dissociation is just the Kosterlitz-Thouless transition which I doubt this has anything to do with quantum turbulence.

Quantum vs. classical turbulence in two dimensions was discussed.

Jupiter's great red spot is considered to be an example of the latter.

Two dimensions leads to different kinetic energy scaling for quantum turbulence, E(k) ~ k^ -3 for large k

Numerical simulations claim to see a crossover to this scaling [However, the graph shown did not appear to have a horizontal scale and so one could not see how may decades of k this covered].

The take home point of the talk was meant to be:

Atomic quantum fluids are enabling advances in difficult physics problems that are relevant beyond quantum physics labs.

However, I failed to see these advances from the talk. The experiments are beautiful and fascinating. But, I could not see how the experiments or simulations have led to any new insights or advances beyond those from Kolmogorov in 1941 and Feynman in 1955. To me this is another example of how people in the BEC community oversell the significance of their work. Potential advances and hoped for insights are not the same as real advances and insights.

For an example of a real advance in a difficult problem which spread across disciplines consider the case of the Hopfield net, which was influenced by ideas from spin glasses in condensed matter physics. This had a large influence on neural networks in computer science and biology. The fact that Hopfield is now a Professor of Molecular Biology at Princeton is a testimony to the advances he made.

Chemical Engineering departments now regularly hire faculty who do research using density functional theory (DFT). This is testimony to the advances that have been made in modelling real materials and chemical processes using quantum chemical methods.

When departments of Aeronautical and Mechanical Engineering hire people to work on quantum turbulence will be a real sign of a significant contribution.

Boulder school lectures online

The 2010 Boulder School for Condensed and Materials Physics (July 6-30, at the University of Colorado, Boulder) is entitled "Computational and Conceptual Approaches to Quantum Many-Body Systems".
This year the School will again employ a webcast system that allows for real-time streaming of video/audio of its proceedings, including all the lectures, questions, answers, and discussions.

This is a great resource for people who cannot attend this great event.

From a Physics Ph.D to climate change analyst

Today Jenny Riesz from ROAM consulting gave a really nice physics colloquium. Jenny was an outstanding physics undergraduate at UQ and then did a Ph.D here on the spectroscopy of melanin. We collaborated on this paper on the Transition Dipole Strength of Melanin.

Jenny is currently the Climate Change Manager at ROAM consulting. Here are a few things I learnt today.

After Jenny finished here Ph.D she wanted to get a job in industry. She sent out lots of applications and got not one single response. Eventually, through personal connections she got an interview.

One thing she learnt was to rewrite here CV. She should not have listed her publications at the beginning, but rather her skill set.

People who can define and solve problems are valuable and rare.

A Ph.D (particularly in a multi-disciplinary research area) can teach the value of networking and collaboration.

Australia has the longest electricity grid in the world.

Biggest expense is distribution of electricity (wires) rather than generation.

Australian Government has set a 20% Renewable Energy Target by 2020.

But this will be 80% of future growth in supply.

A key issue in meeting this goal and in reducing carbon emissions is where the resource is relative to the grid.

Small island nations are the most vulnerable to climate change. A 2 degree temperature increase could destroy 30 nations. Jenny has been involved with Project Survival Pacific helping these nations have a voice, including being part of the Solomon Islands delegation at the UN climate change summit.

Galaxy formation as a condensation phenomena

Yesterday there was an interesting colloquium, Simulating star cluster evolution on high-end graphics cards, by a new UQ staff member, Holger Baumgardt.

The fact I am writing a blog post about it should be taken as a compliment and the comments below need to be taken with a large grain of salt, since the subject goes way beyond my expertise. But they may be interesting in terms of how an outsider sees things...

First, a few things I learnt.

Globular clusters are 10-12 billion years old, much older than typical galaxies.

Black holes may be at the centre of most galaxies.

UCDs are ultra-compact dwarf galaxies somewhere between star clusters and galaxies.

The figure below shows a plot of the radii of different objects versus their mass. It is taken from this paper.

Scaling relations for low-mass, hot stellar systems: half-light radius plotted against total mass.The dashed line shows the fitted relation for elliptical galaxies, while the solid lines indicates the median for Galactic globular clusters ( pc) that do not follow a mass-radius relation.

The claim appears to be that there is a qualitatively different behaviour between galaxies (thousands of stars) and globular clusters (millions of stars).

Ben Powell asked an important question: does the data shown in the talk (similar to that above) justify this claim?

It is not clear to me that it does.

If there is a qualitative change when one has more than a million solar masses, than an important theoretical question to answer is why?

A challenge to computer simulations is to then try and reproduce this change. On a regular PC one can simulate the classical dynamics of thousands of stars. But what about a million? Hence, the desire to use graphics cards...

A couple of questions I had (as an ignorant condensed matter physicist):

What happens if one attacks this problem in the continuum limit? (i.e., rather than having a million point like particles one has a continuous mass distribution).

Can one write down a "density functional" type theory?

Are there any analogies to other problems concerning condensation of liquid droplets whether in nuclear physics or low density gases?

Alan Mark asked a good question about the dependence of the simulation results on the choice of initial configuration. No doubt inspired by his experience with molecular dynamic simulations of large biomolecular systems, where I believe this can be problematic.

Quantum limited detectors

Yesterday John Clarke (University of California, Berkeley) gave a nice

colloquium at UQ on Applications of SQUIDs (Superconducting Quantum Intereference Devices).

Clarke is arguably the "father" of the development of SQUIDs in both science and technology. Many of the people currently leading the development of superconducting qubits were at one time his students or postdocs. Tim Duty is to be thanked for bringing Clarke to UQ to give this fascinating talk.

The two key physical effects on which the SQUID

is based are Josephson tunneling and magnetic flux quantisation.

In a DC SQUID the I-V curve is modulated by magnetic flux, and so the device is basically a flux to voltage transducer with noise that can approach the quantum limit. They can detect magnetic fields as small as a femtotesla!

Roger Highfield's book, The Science of Harry Potter describes how the sorting hat that Harry used in the first book is based on SQUIDs. [When I told my son that, he said this is not correct because the hat is based on magic and that is the whole point!]

Clarke described two nice projects he has been involved in building highly sensitive detectors to answer fundamental questions in Cosmology.

The first involves the search for dark matter, specifically looking for clusters of galaxies, largest bound objects in the universe. This uses Sunyarev-Zeldovich effect involving shifts in the Cosmic Microwave Background because CMB photos are scattered by hot gas bound to clusters.

This invovles a Transition edge sensor which is limited by photon shot noise. 100 squid array, multiplexer, current summing circuit, comb of frequencies....

This is now installed at the South pole telescope.

The second project involves the search for cold dark matter, specifically the axion, a particle proposed in 1978 to explain the magnitude of the electric dipole moment of neutron, which is three orders of magnitude smaller than standard model predicts. [This was news to me. I thought the standard model explained everything!, more or less].

How can axions be detected? Primakoff conversion (Sikivie, 1983) of an axion to photon. I did not follow how this worked.

Noise temperature of amplifiers needs to be reduced. Want quieter ones than HEMTs (High electron mobility transistors (based on GaAs).

With SQUID noise temperature becomes about 30-40 times lower than HEMT.

Then the required measurement time decreased from 270 years to 100 days, so graduate students may be interested in working on this project!

Clarke then discussed Microtesla magnetic resonance imaging.

A standard clinical MRI machine uses 1.5 Tesla, costs 2$M, and you sometimes need to reinforce floor. Inspired by Michael Crichton novel (1999) to use earths magnetic field for imaging. [Was this tongue in cheek?]

Clarke showed results from a PNAS paper in 2004, which imaged a Red Pepper [capsicum to some?], with a resolution of 0.7mm. He also showed 3D images of arm with a field of 132 micro tesla. He went on to discuss T1 weighted contrast imaging (contrast agent is Gd salt). I then had to leave to go to my son's futsal game....

And to think that all this technology and science has come about because a young Ph.D student about 40 years ago was challenged to think about physical signatures of spontaneous symmetry breaking in superconductors....