Archive for the 'Mathematical & Physical Sciences' Category

The Dark Matter of Gravitational Lensing

By Tom D Kitching, on 27 March 2015

Hello, I’m Tom Kitching, a cosmologist at UCL’s Mullard Space Science Laboratory. My research involves using the distorting effect of curved spacetime on the images of galaxies to map dark matter in the Universe. In some new research, my collaborators and I have pinned down the nature of dark matter to an unprecedented level of accuracy.

Dark matter is a substance that fills the Universe, and accounts for nearly 95% of all matter that exists – but we have no idea what it is. The name dark matter is however a bit of a misnomer. Dark matter is in fact transparent and doesn’t emit or absorb ordinary light at all. However it does interact with light in a very special way: its gravitational pull bends the light rays around and through it, in a similar way to how a magnifying glass does.

Our new research out now uses that bending effect, known as gravitational lensing, to make maps of dark matter around galaxy clusters.

Maps of dark matter locations around galaxy clusters. Credit: NASA, ESA, D. Harvey, R. Massey

Maps of dark matter locations around galaxy clusters from the research. Credit: NASA, ESA, D. Harvey, R. Massey

Galaxy clusters are particularly interesting places because dark matter, galaxies, and hot X-ray gas are all being smashed together as the Universe evolves. What we observe is a snapshot of these collisions, which are the biggest most energetic collisions in the Universe.

By mapping the different components of these collisions we can determine the physical properties of the dark matter. To date this approach had only ever been applied to only a few clusters of galaxies. In our research we applied to this to 72.

With this very large sample we determined that dark matter has to exist in galaxy clusters with a probability of 100,000,000,000,000:1. This is the most definitive detection of dark matter ever.

Using this very information rich data set we managed to measure how dark matter interacts with its self: a property known as its cross-section.

The cross-section measures what happens when two particles bump into each other. From the Earth we can test whether dark matter bumps into ordinary matter and what happens, which gives us clues to what dark matter is. But it’s only in space, around galaxy clusters that we can test what happens when dark matter bumps into itself, a crucial piece in the puzzle. By using this data we measured the cross-section to be smaller than previous experiments.

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Solar Orbiter: here comes the Sun

By Oli Usher, on 25 March 2015

The Solar Orbiter project has moved a step closer to reality, with the completion of its structural thermal model (STM) this month. UCL is heavily involved with this European Space Agency mission, with the university’s Mullard Space Science Laboratory leading the development of parts of its scientific payload.

The spacecraft is being built at Airbus Defence & Space in Stevenage, just north of London.

The Solar Orbiter Structural Thermal Model at Airbus Defence & Space in Stevenage. Photo: O. Usher (UCL MAPS)

The Solar Orbiter Structural Thermal Model at Airbus Defence & Space in Stevenage. Photo: O. Usher (UCL MAPS)

The STM is a full-scale model of the spacecraft, which is used for extensive pre-flight testing. Its appearance, structure and design are virtually identical to what will launch in 2018 – the main difference is that the delicate (and costly) electronics are replaced by dummy components that mimic their physical size, weight and properties.

The extensive pre-flight testing, covering potential hazards such as vibration, G-forces and heat resistance (crucial for a solar mission) will be carried out on the STM, to identify potential issues with the flight model before it’s too late.

UCL leads the development of two of the scientific instruments that will fly on board Solar Orbiter.

Louise Harra and Chris Owen, both of UCL Mullard Space Science Laboratory, at the Airbus facility in Stevenage. Photo: O. Usher (UCL MAPS)

Louise Harra and Chris Owen, both of UCL Mullard Space Science Laboratory, at the Airbus facility in Stevenage. Photo: O. Usher (UCL MAPS)

The Extreme Ultraviolet Imager, with Louise Harra as a co-PI, will image the atmosphere of the Sun, which is heated by our star’s magnetic field. The solar atmosphere is far hotter than the Sun’s surface – an apparent paradox – and Solar Orbiter will aid scientists in their efforts to shed light on precisely why this is.

The Solar Wind Analyser, led by Chris Owen, will study the particles that make up the solar wind. The solar wind is a constant stream of charged particles churned out by the Sun and blown out into the far reaches of the Solar System. It cannot be studied from the ground, as the particles are deflected by the Earth’s magnetic field.

The scientific instruments UCL is contributing to. Top left: the Solar Wind Analyser PAS Top right: the Solar Wind Analyser HIS Bottom left: the Solar Wind Analyser EAS (on the boom being attached to the spacecraft) Bottom right: the apertures in the heat shield behind which the Extreme Ultraviolet Imager is located

Dummy models of the scientific instruments UCL is contributing to on the Solar Orbiter STM.
Top left: the Solar Wind Analyser Proton-Alpha Sensor
Top right: the Solar Wind Analyser Heavy Ion Sensor
Bottom left: the Solar Wind Analyser Electron Analyser System (on the boom being attached to the spacecraft)
Bottom right: the apertures in the heat shield behind which the Extreme Ultraviolet Imager is located

One of the unique features of Solar Orbiter, over all other solar observation missions to date, is that it will give an unprecedented perspective on the Sun. During its mission, it will use Venus fly-bys, each time getting a small nudge from the planet. This will gradually move Solar Orbiter out of the plane of the Solar System – where Earth and all the other planets lie – and by about six years into the mission, it will be in a position to look down onto the poles of the Sun. These have never been seen before, and they are crucial to understanding the Sun’s magnetic field.

Solar Orbiter will also fly telescopes closer to the Sun than any spacecraft before and be able to observe features such as sunspots evolve for weeks at a time, thanks to an orbit that follows the rotation of the Sun.

Solar Orbiter will fly inside the orbit of Mercury, at barely over a quarter of the Earth’s distance from the Sun. This means extremely powerful Solar radiation, bright light and intense heat. Part of the engineering challenge of building the spacecraft is its heat shield, which protects Solar Orbiter’s delicate electronics, shielding them from temperatures of 600 degrees and keeping the internal temperature down to a much more manageable 60 degrees.

The Solar Orbiter STM tilted to show the heat shield. Photo: O. Usher (UCL MAPS)

The Solar Orbiter STM tilted to show the heat shield. Photo: O. Usher (UCL MAPS)

The instruments can poke through small openings in the heat shield so as to observe the Sun’s surface without being fried.

Solar Orbiter is scheduled for launch in 2018. The launch vehicle will be provided by the United States. Like many space missions, this is a project of international cooperation.




Darkness falls across the land

By Dewi W Lewis, on 24 March 2015



These two photographs were taken in the quad during the eclipse. The first at 9:25 the second at 9:31am.

During this time the light dropped by a factor of 16: dropping from an Exposure Value (EV) value of 11 to 7, or 4 stops, to the photographers amongst you.

My eyes told me it got “a bit darker”, the reality was far more!

Solar eclipse

By Oli Usher, on 20 March 2015


The clouds didn’t part, but there was a great turnout for the eclipse party in the quad this morning. (So great that all the smartphones made eduroam a bit unreliable – sorry for the occasional breaks in the stream.)


Thanks everyone for coming – and see you for the next big one in 2026!

An Antarctic answer to a cosmic question

By Oli Usher, on 16 March 2015

It’s a setup worthy of Heath Robinson. In a faraway galaxy, a high energy cosmic ray interacts with a photon, producing a series of particle decays eventually releasing a neutrino. The neutrino then crosses billions of light years of space, punching through dust, gas and nebulae without effect. Eventually it reaches Earth, plunges through our atmosphere and finally meets something it cannot pass through: our planet.


Our planet. Photo: NASA

When it reaches Earth, scientists hope to detect it. But the complications are only beginning.

High energy neutrino impacts are rare. A cubic kilometre of matter should typically see one roughly once every century. When it does, it triggers a brief and not particularly bright flash of microwave radiation – at the same wavelength and intensity as the flashes from piezoelectric cigarette lighters.

Ryan Nichol

Ryan Nichol

Detect these rare flashes, and you can detect the impact of neutrinos.

Scientists are patient people, and science is no stranger to large and expensive experiments. But building a detector of a cubic kilometre in size, far away from any sources of accidental interference – then waiting potentially a century or more to see a single result – is not reasonable.

Scientists need, somehow, to use the Earth itself as a giant detector, to increase the odds of seeing one.

Searching for somewhere remote enough that the faint signal doesn’t get drowned out, big enough to act as a natural detector, and dry enough not to absorb the microwaves, has taken UCL’s Ryan Nichol to Antarctica three times in the last decade.

There, ANITA (Antarctic Impulsive Transient Antenna), the instrument he works on, circles the South Pole, suspended from a balloon, monitoring a million cubic kilometres of ice for the telltale flash of microwaves that would signal a neutrino detection.

Anita, shortly after takeoff. © Brian Hill (University of Hawaii-Manoa)


Scientists are targeting neutrinos because of what they tell us about cosmic rays.

Cosmic rays are fast-moving particles – usually protons and ions – that bombard the Earth’s upper atmosphere. Some of these move at exceptionally high speeds, up to 99.99999999999999999999% of the speed of light, giving them vast amounts of kinetic energy. A single one of these particles can have the same energy as a cricket ball travelling at 55 miles per hour. But frustratingly, their origins are shrouded in mystery. They must come from some kind of high-energy phenomenon – such as supernovae, black holes or the centres of galaxies – but we can’t figure out what.

Ultra-high energy cosmic rays can travel a long way, but they can’t travel infinitely far. They can interact with photons (light particles), which are everywhere in the Universe, even the vacuum between galaxies. This means that cosmic rays eventually – after about a hundred million light years – come to a halt.

As a result, all the cosmic rays detected in Earth’s upper atmosphere come from a region that includes our galaxy and a few neighbouring ones, but not from the hundreds of billions of galaxies that lie further away. This is by no means a small volume of space, and yet astronomers have come up empty handed when trying to pinpoint the origins of high energy cosmic rays within our cosmic neighbourhood. They need a way of broadening the search.

By looking for high energy neutrinos instead – which are launched like billiard balls by cosmic rays when they interact with photons, but which, unlike them, can cross the void of space without interacting – particle physicists like Nichol hope to extend the search radius from a few tens of millions of light years to several billion.

This should give them a large enough sample to begin to plot where the highest energy cosmic rays are coming from, and see if they can match this up with known objects in the sky that could be causing them.

But first, they need to develop a reliable way of spotting the neutrinos, which is where ANITA comes in.

And for ANITA to do its job, Nichol needs to get to Antarctica. Even in this age of easy international travel, this is not simple. Without any permanent inhabitants, Antarctica has no towns, isn’t visited by scheduled airlines, and even in the brief Antarctic Summer, is still cold and battered by extreme weather.


Nichol’s base of operations in Antarctica is McMurdo Station, the largest scientific research station on the continent. It looks like a cross between a mining town and a scruffy university campus – with halls of residence, cafeterias serving mediocre food, and social life centred on a few slightly disreputable bars.

Located on the coast, and served by several runways, McMurdo is one of the gateways to the constellation of smaller research stations dotted around Antarctica. Its summer population tops 1000 – not just researchers, but engineers, cooks, cleaners, drivers – but most of the scientists are just passing through, waiting for connecting flights to more remote stations where they will carry out their research.

For one group of scientists, though McMurdo is perfectly placed: those carrying out research using balloons. High in the sky above the base, a constant, predictable wind blows. Release a balloon into this, and it will gently float in a giant circle around Antarctica, before returning to where it started about two weeks later.

ANITA prior to being attached to the balloon. In the background, Mount Erberus, the second-highest volcano in Antarctica

ANITA prior to being attached to the balloon. In the background, Mount Erberus, the second-highest volcano in Antarctica. Photo: Ryan Nichol

For the first couple of days after ANITA is released, McMurdo base is still in range. ANITA floats high in the atmosphere, around three times the altitude of commercial airliners, and so the base, with all the radio interference that comes from it, remains above the horizon for a long time. But once out of range, the only interference is the occasional chirp from communications satellites high above. ANITA silently watches the ice, recording vast amounts of data, and storing it for analysis later on.

The first two flights have not conclusively detected any high energy neutrinos yet. There has been one tentative detection, but the scientists would want to see several before being confident that they hadn’t just seen a false positive.

The flight of ANITA 3 in December 2014. The path in red shows the balloon veering off course and ending its journey near Davis base.

The flight of ANITA 3 in December 2014/January 2015. The path in red shows the balloon veering off course and ending its journey near Davis base.

This may all change soon. The third flight’s results are on their way home now – and taking a suitably convoluted route back. The December 2014 observations had to be cut short after the balloon blew off course, and the ANITA experiment was jettisoned onto the ice.

ANITA after crash-landing on the ice. Photo: © Josh F/Australian Antarctic Division

ANITA after crash-landing on the ice. Photo: © Josh F/Australian Antarctic Division

Rescued by helicopter and returned to an Australian base, ANITA is now waiting to board the icebreaker Aurora Australis for the journey to Tasmania. The research team hope to get it back next month.

The most recent ANITA mission featured more sensitive instrumentation than the previous two. Given this, the scientists are hopeful – though not certain – that they will get some more tangible results than previous years. They estimate that they should detect somewhere between 0 and 5 high energy neutrino events in this year’s data. So, barring bad luck, the planning, the cold and the years of hard work may just pay off.

The Aurora Australis. Photo: Alex Kozyr

The Aurora Australis. Photo: Alex Kozyr/public domain


So if you’re part of the ANITA team, that’s how you detect the highest energy cosmic ray in the Universe. All you need is for it to interact with a photon, emitting a neutrino that flies across billions of years of space before colliding with a water molecule, emitting a flash of microwaves. And for the flash of microwaves to be detected by a sensor suspended from a balloon floating 35km over the most remote and inhospitable region of the world, for the detection to be recorded onto a hard disk, for the hard disk to be dropped onto the ice, then rescued by helicopter, shipped by icebreaker and flown back to the lab, to be decoded by a physicist.

It’s simple, really.

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Hiding in plain sight

By Oli Usher, on 12 March 2015

UCL has played a key role in the development of the Dark Energy Survey’s instrumentation, including building a series of lenses that form a crucial component of the Dark Energy Camera. UCL’s Ofer Lahav also serves as co-chair of the Dark Energy Survey science committee.

The Dark Energy Survey team published a new result this week that shows the power of the instrument.

The Milky Way is surrounded by a few dozen dwarf galaxies. Some of these are easy to spot, like the Large Magellanic Cloud, which is visible even to the naked eye as a pale blob in the Southern Hemisphere’s sky, clearly separate from the Milky Way.

Others are much harder to spot, as it is virtually impossible to distinguish their stars from the ones in the foreground, which are part of the Milky Way. Now, the Dark Energy Survey has discovered what appears to be several more. This image shows the challenge in identifying them:

Credit: Fermilab/Dark Energy Survey

Credit: Fermilab/Dark Energy Survey

The image shows one of the candidate galaxies, known as DES J0335.6-5403. It sits around 100,000 light years from Earth, and has very few stars, only around 300 could be detected, compared to an estimated 200 billion stars in the Milky Way.

The team were, however, able to analyse the image and mask all the stars that belong to our Milky Way. And when you do that, the dwarf galaxy suddenly jumps out at us:

Credit: Fermilab/Dark Energy Survey

Credit: Fermilab/Dark Energy Survey

Although UCL’s involvement in this study was only minor, it shows the power of the instrument and its contribution to the science over the past decade.

Where noble gases won UCL a Nobel Prize

By Oli Usher, on 2 March 2015


Readers familiar with UCL will immediately recognise the Slade School of Fine Art – the North Wing on UCL’s historic (and grade I listed) quadrangle. What they may not know is that the building, which dates back to the 1870s, was originally also home to UCL’s Department of Chemistry.

It was in this building that some of UCL’s most famous contributions to chemistry were made: William Ramsay and his successor J Norman Collie both worked here.

In fact, as far as we can work out, it is the building in the world which has seen the discovery of the greatest number of chemical elements. Ramsay discovered argon, neon, krypton and xenon while working here, as well as isolating helium (which had been seen in the spectrum of the Sun, but not observed here on Earth) and radon. These elements are together known as the noble gases, and Ramsay’s discoveries secured for him the 1904 Nobel Prize in Chemistry – UCL’s first.

In 1913, the year of Ramsay’s retirement, UCL’s Chemistry department moved to a new purpose-built lab immediately behind its old home. The building – which still houses some of the department’s laboratories and offices – is now named after another towering figure in chemistry: Kathleen Lonsdale.

After a century of heavy use and piecemeal remodelling, the building is looking a little tired . A multi-million pound complete refurbishment of the building is planned to begin later this year.


High resolution image


Exoplanets and education at 11 Downing Street

By Oli Usher, on 19 February 2015

Guest post by Anita Heward

Artist's impression of exoplanet Gliese 876b. Credit: NASA, ESA, G. Bacon (STScI)

Artist’s impression of exoplanet Gliese 876b. Credit: NASA, ESA, G. Bacon (STScI)

The potential for exoplanet research to inspire students and widen participation at university from under-represented groups will be highlighted today at 11 Downing Street.

Clara Sousa Silva

Clara Sousa Silva

Dr Clara Sousa Silva, who completed her PhD at UCL last year, has been invited to give a four-minute presentation during an event hosted by the Chancellor for Your Life and Researchers in Schools (RIS). Clara is currently participating in the RIS teacher training and professional development programme.

The programme builds links between universities and schools by training researchers that have completed PhDs to become classroom teachers, while supporting them for 20% of their time to pursue academic interests. Clara is training as a physics teacher at Isleworth and Syon School for Boys and, through the RIS programme, is continuing aspects of her research into atmospheric characterisation of cool stars and exoplanets.

Clara also heads up the education programme for Twinkle, a UK space mission that will characterise exoplanet atmospheres using an instrument led by UCL.

EduTwinkle will draw on Clara’s experiences with RIS and the Brilliant Club to foster a productive relationship between space exploration and British schools, increase girls’ uptake of STEM subjects at A-level and higher education and to widen participation at universities from under-represented communities.

  • Anita Heward is press officer for the Twinkle Space Mission

The art of unseeing

By Oli Usher, on 16 February 2015

While astronomers expend a lot of effort trying to see things better – building ever more powerful telescopes that can detect even the faintest, most distant objects – they are occasionally faced with the opposite problem: how to unsee things that they don’t want to see.

A group of scientists led by UCL’s Emma Chapman is working on methods to solve this problem for a new radiotelescope that is currently under development. In so doing, they could help give us our first pictures of a crucial early phase in cosmic history.

How to avoid seeing things you don’t want to see is a particular problem for cosmologists – the scientists who study the most distant parts of the Universe. Many of the objects and phenomena that interest them are faint, and lie hidden beyond billions of light years of gas, dust and galaxies. To make matters even more difficult, telescopes are flawed too – the data from them is not perfectly clean, which is manageable when you’re looking at relatively bright and clear objects – but a serious problem when you’re looking at the faint signature of something very far away.

Observing these phenomena is much like looking at a distant mountain range through a combination of a filthy window, a chain-link fence, some rain, clouds… and a scratched pair of glasses. In other words, you’re unlikely to see very much at all, unless you can somehow find a way to filter out all the things in the foreground.


The Square Kilometre Array (SKA) is a new radiotelescope, soon to begin construction in South Africa and Australia. The SKA will use hundreds of thousands of interconnected radio telescopes spread across Africa and Australia to monitor the sky in unprecedented detail and survey it thousands of times faster than any current system.

Artist's impression of the South African site of the Square Kilometer Array. Credit: SKA Organisation (CC BY)

Artist’s impression of the South African site of the Square Kilometer Array. Credit: SKA Organisation (CC BY)

One of its objectives is to make the first direct observations of a brief phase of a few hundred million years in cosmic history known as the ‘era of reionisation’. This technical term conceals something quite dramatic: a profound and relatively sudden transformation of the whole Universe, which led to the space between galaxies being fully transparent to light as it is today.


Murphy’s law

By Jonathan Oppenheim, on 9 February 2015

Jonathan Oppenheim has co-authored a new paper on quantum thermodynamics, which can be read here.

You’re probably familiar with the second law of thermodynamics in one of its many forms:

Anything that can possibly go wrong, does.
— Murphy’s law

“Happy families are all alike; every unhappy family is unhappy in its own way.”
— Leo Tolstoy in Anna Karenina, almost 20 years before Boltzmann’s Kinetic Theory of Gases

Shit happens
— ancient proverb

Because every day we feel the consequences of the second law. Even Homer Simpson has been known to admonish his children: “In this house, we obey the laws of thermodamynics!” Not that Bart or Lisa would have much choice. The second law of thermodynamics governs much of the world around us – it tells us that a hot cup of tea in a cold room will not spontaneously heat up; it tells us that unless we are vigilant, our homes will become untidy rather than tidy; it tells us how efficient the best engines can be and even helps us distinguish the direction of time – we see vases shatter, but unless we watch movies backwards, never see the time-reverse – a shattered vase reforming with just a nudge.

Vases do not spontaneously unbreak themselves. Photo: Drew Bandy (CC BY NC SA)

Vases do not spontaneously unbreak themselves. Photo: Drew Bandy (CC BY NC SA)

The second law tells us that order tends towards disorder, something we are all very familiar with — trying to achieve a very specific state of affairs can be very difficult, because there are many different ways things can go wrong. Murphy’s Law (anything that can go wrong, will go wrong), is a reasonable statement of the second law of thermodynamics. As is its less precise version “Shit happens”.

More concretely, the second law tells us that for isolated systems, the entropy, a measure of disorder, can only increase. I like to think of the second law as constraining what can happen to a system — left on its own, things don’t get more ordered.

But the laws of thermodynamics only apply to large classical objects, when many particles are involved. What do the laws of thermodynamics look like for microscopic systems composed of just a few atoms? That laws of thermodynamics might exist at the level of individual atoms was once thought to be an oxymoron, since the laws were derived on the assumption that systems are composed of many atoms.

Are there even laws of thermodynamics at such a small scale?

The question is becoming increasingly important, as we probe the laws of physics at smaller and smaller scales.

Statistical laws apply when we consider large numbers. For example, imagine we toss a coin thousands of times. In this case, we expect to see roughly equal numbers of heads as tails, while the chance that we find all the coins landing heads is vanishingly small. If we imagine tossing a larger and larger number of coins, the chance of having an anomalous coin tossing such as all tails goes to zero and our confidence that we’ll have roughly half heads, half tails, increases until we are virtually certain of it.

However, this is not true when tossing the coin just a few times. There’s a reasonable chance we will find all the coins landing tails. So, can we say anything reasonable in such a case? Similar phenomena occur when considering systems made out of very few particles, instead of very many particles. Can we make reasonable thermodynamical predictions, about systems which are only made up of a few particles.

Can we even talk about the laws of thermodynamics when we're only looking at a handful of particles? Picture credit: Stef Simmons (CC BY)

Can we even talk about the laws of thermodynamics when we’re only looking at a handful of particles? Picture credit: Stef Simmons (CC BY)

Surprisingly, the answer is yes, and the mathematical tools from a field known as quantum information theory help us to understand the case when we don’t have a large number of particles. What we find, is that not only does the second law of thermodynamics hold for quantum systems, and those at the nano-scale, but there are even additional second laws of thermodynamics. In fact, there is an entire family of second laws. So, while Murphy’s law is still true at the quantum scale — things will still go wrong; the ways in which things go wrong is further constrained by additional second laws. Because remember, the second law is a constraint, telling us that a system can’t get more ordered. These additional second laws, can be thought of as saying that there are many different kinds of disorder at small scales, and they all tend to increase as time goes on. What we find is a family of other measures of disorder, all different to the standard entropy, and they must all increase.

This means that fundamentally, there are many second laws, all of which tell us that things become more disordered, but each one constrains the way in which things become more disordered. Why then does there only appear to be one second law for large classical systems? That’s because all the second laws, although different at microscope scales, become similar at larger scales. At the scale of the ordinary objects we are used to, all the quantum second laws are equal to the one we know and love.

What’s more, it can sometimes happen that the traditional second law can appear to get violated – quantum system can spontaneously become more ordered, while interacting with another system which barely seems to change. That means some rooms in the quantum house may spontaneously become much tidier, while others only become imperceptibly messier.

What do these additional second laws look like? Well, first let’s get a bit more technically dirty. If you already know your thermodynamics fairly well, this is a good point to join us. For those who’ve had enough, here is a picture of Watt’s steam engine. It’s big enough that none of these additional second laws matter.