Archive for the 'Mathematical & Physical Sciences' Category

The Euclid Satellite and Gravitational Lensing

By Peter L Taylor, on 30 January 2018

In 1999 the physics community was stunned when it was discovered that far from slowing down under the pull of its own gravity, the universe is actually expanding at an accelerating rate. This is the cosmic acceleration problem and it poses one of the largest challenges in physics today. Here, at UCL’s Mullard Space Science Laboratory (MSSL) we are leading the effort to find a solution.

Parts for the Euclid Satellite, which will conduct a massive astronomical survey, are being built on site. Meanwhile we prepare for the arrival of the groundbreaking Euclid data by developing and testing techniques to distinguish between theories of cosmic acceleration.

We take advantage of an effect of Einstein’s theory of general relativity called gravitational lensing. As light from distant sources travels towards Earth, it is deflected by the gravitational attraction of intervening mass slightly distorting the shape of the observed image. This effect is normally very small, but by measuring the shape of billions of galaxies, amazingly we can statistically differentiate between theories.  This type of analysis has been conducted before, but to get the most out of next generation experiments we must hone our methods. When comparing the two leading statistical techniques, we found both have serious drawbacks. Nevertheless we have shown that the two methods are just special cases of a more general theory: narrowing our search for a better approach.

Irrespective of the statistical technique that is ultimately used, we must always accurately predict the lensing signal to compare to the real universe. An intermediate step involves determining the power spectrum that describes the ‘clumpiness’ of matter on different length scales, at different times. We are working on a publicly available code that will help researches who generate these spectra determine whether their output is accurate enough to meet the needs of upcoming lensing experiments like Euclid.

Gravitational lensing is very a delicate business. Large collaborations must identify, understand and mitigate annoying instrumental and astrophysical effects that contaminate the lensing signal. UCL is leading the charge in preparation for Euclid’s launch in the early 2020s.


Image credit to ESA

From summer interns to cubesats in space

By Charlotte E Choudhry, on 7 October 2016


Many students send emails requesting summer internships at UCL Physics & Astronomy, but one particularly caught Dr Anasuya Aruliah’s eye. Vishal Ray was a 2nd year Aerospace Engineering undergraduate at the prestigious Indian Institute of Technology Bombay, India (IIT). He belonged to a student team building their own miniature satellite. Vishal was just the student that Dr Aruliah needed for her new direction of research: satellite drag. After a successful application to the International Students Dean’s Summer Student Scholarships, he was awarded a 2 month internship in summer 2015.
Dr Aruliah’s group, the Atmospheric Physics Laboratory (APL), is a subgroup of the Astrophysics Group. It has a long history of researching the upper atmosphere using a global circulation atmospheric model. They also operate a network of Fabry-Perot Interferometers (FPIs) in Arctic Scandinavia to observe the aurora. The Earth’s atmosphere is like an onion skin, with the troposphere (the domain of weather forecasters), stratosphere and mesosphere as layers on top of each other. The thermosphere is the final layer of the Earth’s atmosphere, and is the altitude region between 90-400km. Low Earth Orbit (LEO) satellites occupy the top of the thermosphere, and rely on upper atmospheric models to predict their orbits.

Recently the APL group found a discrepancy between measurements of thermospheric winds calculated from Doppler shifts of airglow photons, and winds determined from atmospheric drag on the Challenging Minisatellite Payload (CHAMP) satellite. This is an important puzzle to solve because satellite drag measurements are put into atmospheric models to bring them as close to reality as possible. If the ground and satellite measurements do not agree, then which is correct?

The IIT miniature satellite, commonly called a cubesat, is composed of a single cube, only 30 cm in length, width and breadth, and weighing only 10 kg, as much as a few bags of sugar. Their cubesat is called Pratham. This fits perfectly with UCL’s involvement in the European Union FP7-funded QB50 project, in which fifty cubesats carrying miniaturised sensors will be launched nearly simultaneously. This is an international collaboration involving many universities, academic institutes and the space industry. It is an unprecedented science operation, with potential for future Space Weather monitoring campaigns. The QB50 cubesats will be carried by rocket into the upper thermosphere, and fall to Earth in decaying orbits while sampling regions of the thermosphere and ionosphere that were previously poorly understood owing to the lack of detailed measurements.

“The simplicity and low cost of cubesats has spurred much excitement and creativity amongst young (and old) engineers and scientists over the last few years. There are new frontiers being opened by this miniaturised space technology,” said Dr Aruliah.

The UCL Mullard Space Science Laboratory (MSSL) designed and built one of the three key sensors: the Ion Neutral Mass Spectrometer, which will be carried on several of the cubesats, as well as their own cubesat called UCLSat. The QB50 cubesats are scheduled for launch in three batches over the winter period of 2016-2017. Two batches from a Ukrainian-Russian Dnepr rocket, and a third from the International Space Station. During Vishal’s internship at UCL he met with the MSSL cubesat and sensor team, led by Mr Dhiren Kataria and Dr Rob Wicks; and with Dr Stuart Grey in the UCL Department of Civil, Environmental & Geomatic Engineering.

Dhiren Kataria holding the UCLSat designed and built at MSSL

Dhiren Kataria holding the UCLSat designed and built at MSSL

Vishal used his experience to write several sophisticated computer programs to calculate drag coefficients from simulations of a cubesat orbiting in our 3-dimensional atmospheric model called CMAT2. This work was subsequently built upon by Dr Aruliah’s 4th year project student, Jennifer Hall. Jennifer wrote her own programs to derive and compare satellite drag coefficients from CMAT2 simulations and EISCAT radar measurements. Jennifer’s project won the UCL Physics & Astronomy Tessela prize for best use of computer technology in a 4th year project.

IIT Pratham group

Pratham team at the Indian Institute of Technology Bombay. Vishal Ray is 2nd from the right in the top row.

One year on, after a busy 3rd year of studies, Vishal has written up his summer project as a journal paper, and “Pratham” was scheduled for launch at 0530 UTC on the 26 September 2016. The IIT Bombay student team installed their cubesat on the launch vehicle PSLV C-35 on the remote island of Sriharikota in South India. Vishal said that he “…had goosebumps when we actually placed the satellite on the launch vehicle module and completed the testing for one last time!”. “Pratham” was successfully launched and will measure the total electron count from 800 km altitude in a Sun Sychronous Orbit. MSSL were the first to receive Pratham’s beacon signal, which the students were incredibly excited to hear. You can hear the cubesat from 4:20 onwards as it passes within range of the detector at MSSL. The signal is decipered as “Pratham IIT Bombay Student Satellite”. The accompanying image is of Theo Brochant De Viliers (MSSL) beside the MSSL receiver.

The prospect of finally being launched is very exciting, with both projects having been nearly 10 years in the making. Once launched, the missions will change from the technical challenges of the innovation of miniature sensor devices to the scientific challenges of collecting, analysing and interpreting the measurements. The rewards will be great: from the new technologies surrounding cubesats; to the training of future space scientists and engineers, and to the Space Weather community.

Hidden in the archives: Finding the first-ever evidence of exoplanetary system

By Oli Usher, on 13 April 2016

You never know what hidden treasures can be uncovered in the archives.

And this was certainly the case at Carnegie Observatories’ collection when research for an article led to the unexpected discovery of a 1917 glass plate showing the first-ever evidence of a planetary system beyond our own Sun.

It all started last year when UCL astrophysicist Dr Jay Farihi contacted Carnegie Observatories’ Director, John Mulchaey, whilst researching an article on planetary systems surrounding white dwarf stars. Farihi was searching for a glass plate that contained a stellar spectrum of van Maanen’s star – a white dwarf discovered by Dutch-American astronomer Adriaan van Maanen.

The 1917 photographic plate spectrum of van Maanen's star from the Carnegie Observatories’ archive.

The 1917 photographic plate spectrum of van Maanen’s star from the Carnegie Observatories’ archive.


Giotto at Halley: 30 years ago!

By Oli Usher, on 14 March 2016

pencil-iconWritten by Professor Andrew Coates, UCL Mullard Space Science Laboratory

It was the year of the tragic Challenger disaster – but UCL-MSSL was making good news in space and making history too. The Giotto spacecraft carried 10 instruments, including one led by UCL-MSSL just 596 km (MSSL-ESOC!) from comet Halley on the night of 13th/14th March, with some spectacular results.

Giotto was ESA’s first solo interplanetary space mission, launched in 1985 on the penultimate Ariane 1 rocket. In many ways ESA itself can be thought of as ‘coming of age’ with this first bold step on its own out of Earth orbit. To date, Giotto collected the most complete set of data we have from a comet – the famous comet Halley.

Giotto approaching Comet Halley

Giotto approaching Comet Halley


Why is the universe speeding up?

By Oli Usher, on 27 October 2015

The Dark Energy Survey aims to answer that question, and UCL astrophysicists are heavily involved in the collaboration.

This film explains the project – and features Prof Ofer Lahav (UCL Physics & Astronomy), one of the leading figures of the DES science programme.

Solar scientist sculpted

By Oli Usher, on 21 October 2015

The Royal Society has recently unveiled a bust of Lucie Green to join their collection of eminent scientists.

Lucie is Professor of Physics at UCL’s Mullard Space Science Laboratory, an expert on the sun’s magnetic field and frequently appears on TV as a presenter or guest on space and astronomy programmes.

This is all maths. Honest.

By Pietro Servini, on 12 October 2015

On the Sunday and Monday of the August Bank Holiday, UCL held its inaugural Spark Festival at the Olympic Park in Stratford. Filled with stalls representing research from across the STEM subjects (Science, Technology, Engineering and Maths), it was designed to inspire children to become the scientists of tomorrow.

Seven mathematics PhD students – Anna Lambert, Huda Ramli, Matthew Scroggs, Matthew Wright, Oliver Southwick, Pietro Servini and Rafael Prieto Curiel –  went along to see whether they could set up some experiments…

UCL PhD students at the Spark Festival

UCL PhD students at the Spark Festival

In 2012, athletes from across the world dived into the 50 m long swimming pool nestled within the streamlined Aquatic Centre of Stratford’s Olympic Park. The British charge, led largely by Rebecca Adlington, stuttered and sank, with only three medals and none of them gold.

But even they have an easier time swimming than the smallest organisms that inhabit our planet, for whom swimming in water is exactly like us trying to move in corn syrup. Hence their development of long tails (or flagella) that they move in a corkscrew movement in order to propel themselves forwards: were they simply to flick their tails one way and then the other, they would end up right back where they started, never going anywhere.

Bacteria with flagella

Not the best way to win in the race called life.

But why swim in a liquid in the first place, when you could just run across it?

We’re mathematicians! We don’t really do experiments – unless you count the simulations we run on computers or the thought processes we carry out in the giant hallways of our minds. For sure, we don’t spend our days in the office filling up inflatable paddling pools with dozens of litres of water and a hundred kilos of cornflour, going slowly crazy as we croak out “Corn!” in the manner of Lord Mormont’s raven in Game of Thrones. Which means that we’re not very good when it comes to optimising the area around the pool that should be covered with a tarpaulin, especially when there are kids around. Or, for that matter, figuring out how to pipette food colouring into a cylinder full of corn syrup – probably the stickiest substance known to humanity[citation needed] – and then take out the pipette without removing the food colouring too, when the whole purpose of the experiment is to show that the flow is reversible and that that is exactly what will happen.

But how else were we to entertain the hundreds of children who attended UCL’s Spark Festival at the Olympic Park over the August Bank Holiday weekend, a two day extravaganza of stalls showcasing an eclectic collection of research being done in the STEM subjects?

And so there we were, surrounded (hopefully) by the young scientists of the future, who were excitedly mixing vials (plastic cups) of sticky food dye together; transferring the semi-solid, semi-liquid mixture of cornflour and water from one bucket to another (and, obviously, to the ground as well); excitedly jumping up and down on the surface of this non-Newtonian fluid in our paddling pool (or falling down in it); whilst we were trying desperately to convince them and their parents that this was indeed all maths.

This is all maths

This is all maths

Which, of course, it is. Our mixture of cornflour and water (also known as oobleck) is an example of a non-Newtonian fluid: where the viscosity (the fluid’s “thickness”) is not constant as it is in air or water, but either increases (as in our case) or decreases (paint, ketchup, toothpaste, blood) as you apply a force to it. The study of these substances is important (or so we tell ourselves) and mathematicians play a key role in modelling their flow to predict what they might do in certain situations.

Shear thickening substances such as ours, for example, are being used to develop the body armour of the future.

Taylor-Couette experiment

Taylor-Couette experiment

Our Taylor-Couette experiment involved a highly viscous fluid (corn syrup) filling the space between an inner rotating cylinder and an outer cylindrical wall and some blobs of differently coloured food dye placed within the gooiness. When the fluid is rotated, the food colouring is mixed together; but rotate now in the opposite direction and the indistinct mess separates out into the original distinct blobs. We have reversible flow, where time doesn’t really matter and frictional forces dominate; and all because the Reynolds number,


is very low. In our Taylor-Couette experiment, this was because the viscosity was very high; but mathematicians use the same equations to model the flow of lava (also high viscosity) or the motion of glaciers (low typical speed) or, yes, the swimming of microbes in water (small length scale).

So how much did we succeed in proving that splashing around in oobleck or getting covered in sticky corn syrup was maths? Who knows! But hopefully we planted that seed of thought that maths is more than what you study in school; that it’s exciting, fun and beautiful; that it can provide stunning insights into the mysteries of nature; and that it can link together things that seem completely different. And at least we succeeding in helping to ensure, as one young girl breathlessly shouted as she frantically tried to stay afloat on our cornflour, that this engineering festival was “way better than Imperial’s!”

Even if we did have to spend Tuesday morning on our hands and knees, scrubbing the ground with a broken broom and chipping away at an inch-thick layer of cornflour on the grass, cursing our inability to properly site a tarpaulin.

Total eclipse of the Moon

By Oli Usher, on 28 September 2015


Last night saw both a supermoon (the Moon’s closest approach to Earth, in which it appears about 14% bigger than it does at its most distant), and a lunar eclipse, in which the full Moon passes through the Earth’s shadow.

During a lunar eclipse, the disc of the Moon progressively goes from bright white to a deep red: when in the Earth’s shadow, the only light illuminating its surface is the light that is bent through Earth’s atmosphere. This light – effectively, the light of all the sunrises and sunsets on Earth – is red because blue light is scattered in Earth’s atmosphere.

This sequence of photos was produced by Theo Schlichter, Computing and Instrumentation Officer at UCL’s observatory, using a Canon EOS450D camera and a 200mm lens. The composite was produced by Dr Steve Fossey.

First person: 8.3 magnitude earthquake hits Chile

By Ofer Lahav, on 23 September 2015

A combination of high mountains, clear skies and bone-dry deserts makes the north of Chile one of the world’s best places to observe the sky. Numerous international observatories are located there, and astronomers from around the world frequently travel there to carry out their research. Ofer Lahav, Perren Professor of Astronomy at UCL, was there during the magnitude 8.3 earthquake of 16 September. This is his personal account of the events.

Cerro Tololo Inter-American Observatory. Photo: Ofer Lahav

The Blanco 4-metre telescope at Cerro Tololo Inter-American Observatory. Photo: Ofer Lahav

The Earthquake started on Wed 16 Sep at 19:54 (local time), just as we were preparing for the start of DES (Dark Energy Survey) observations.

We left the telescope immediately, and moved to the shaky ground outside the dome. Eventually we were evacuated from there to the CTIO dining hall.

The team was evacuated to the observatory's dining hall - Ofer Lahav is leftmost.

The team was evacuated to the observatory’s dining hall – Ofer Lahav is leftmost.

After-shocks continued throughout the night, but the oscillations finally decayed.

The following morning was sunny and quiet. Observations resumed the following night.

Boulders dislodged from the mountainside and fell onto the road near the observatory. Photo: Ofer Lahav

Boulders dislodged from the mountainside and fell onto the road near the observatory. Photo: Ofer Lahav

I left next morning as planned, flights were on schedule.

We were all impressed by the way the observatory staff handled the situation efficiently and calmly.

It is re-assuring that the system, in part assembled at UCL, is working so well – the only obstacles between the DECam instrument and the galaxies are the Earth’s atmosphere and quakes…

The astronomers who didn’t stop for a missile strike (or a flat battery)

By Oli Usher, on 18 September 2015

This remarkable document comes from the observing log of the University of London Observatory. The observatory is now a UCL teaching facility and part of the Department of Physics & Astronomy, but at the time was an important research facility.

It reveals the effect of the Nazi bombing of London during the second world war.


UCL’s Bloomsbury campus was severely damaged in raids early in the war, with the Library’s dome destroyed and the historic buildings around the main quadrangle all gutted by fire.


Relentless bombing from aircraft early in the war, then by V1 and V2 missiles later in the war brought the conflict close to Londoners, and the unperturbed tone of the log makes clear how commonplace it had become – even in the quiet suburban environment of Mill Hill, where the observatory is located.

The log reads:

1944 August 3

Opened 21h. Sky clear.

Found red light on spectrograph not workingm, owing to plug having been left in and battery run down. Set on γ Cas with the help of dark room red light.

I started exposure at 21h 44m GMT.

Flying bomb exploded very close and shifted star in declination out of the field.

Star recovered and exposure restarted at 21h 47m GMT.

Just after starting the second time, a second flying bomb exploded. This was more distant and though it shifted image from the [spectrograph] slit, star did not go out of field and was quickly recovered.

Exposure ended 22h 07m GMT.

Exposure time = 20m.

Plate developed.

Closed 22h ½.


The event was a V1 flying bomb strike on nearby Hendon, which killed eight and destroyed 193 houses.

The signatures on the end of the document are of historical note too – alongside CCL Gregory, the director of the observatory, was EM Peachey. She is better known today under her married name of Margaret Burbidge. At the time, she was a young astronomer who had just passed her PhD, but she went on to become one of the preeminent astrophysicists of the twentieth century.

The observatory will be holding a one-day astronomy event in the Quad on 2 October, and an exhibition of astronomical images produced by UCL staff and students using its telescopes will be held along the walls of the North Cloisters throughout the term.