It’s a scenario London commuters are all too familiar with: a muffled announcement, ‘signal failure’ or ‘passenger action’; a station closed and thousands of passengers’ journeys interrupted.

Closed. Photo: Oatsy40 (CC-BY)

London’s transport network is extensive, but fragile.

Everyday disruptions to the service have knock-on effects across the system.

Some are easy to predict – a closure at Mornington Crescent will mean more people exiting at Camden Town, on the same line and only a few minutes’ walk away.

Mornington Crescent: quiet, only on one line, and located near to another station. Photo: Diamond Geezer (CC-BY-NC-ND)

Others are far more complex – how would a closure at a busy hub like Euston affect traffic across the network?

Two innovations of the past few decades mean that travel chaos is far more predictable than it was. One is the wealth of data captured by the Oyster Card system, which has been recording passengers touching in and out of the Underground since 2003. The other is the advance in computing power which makes statistical analysis of millions of journeys easy.

Ricardo Silva

One statistician who has looked at Oyster data to chart the impacts of disruption on the underground is UCL’s Ricardo Silva. He has recently built a statistical model that predicts the knock-on effects of unplanned station and track closures across London’s urban rail network.

His work could help transport operators react more effectively to disruption. It can potentially be used to identify where bus services need to be beefed up, as well as identify bad decisions passengers make when reacting to disruption, which can help station staff make more useful announcements about alternative routes.

At the heart of Silva’s model is a database of every journey taken on the Tube, Overground and DLR using Oyster Cards, over 70 randomly chosen weekdays in 2012 and 2013 – covering tens of millions of passenger journeys.

The Oyster Card system is used for over a billion journeys per year on the Tube alone – generating a wealth of data in the process. Photo © TfL Press Office, all rights reserved

Transport for London (TfL) strip out all personal information, such as the passenger’s name or Oyster Card serial number before supplying the data, and give each passenger a randomly allocated ID number. This means that Silva can track individual journeys across the network – including which stations the passengers travel between, and at what time – without invading their privacy.

Alongside the passenger data, he has TfL’s log of all incidents on those days, so he can tease out the difference between passenger behaviour when the network is running smoothly, and when it is being disrupted by a partial closure.

London’s urban rail network has 374 stations, which means there are almost 140,000 possible paths a passenger can take as they navigate their way across the capital. (There are a handful of pairs of stations, Silva says, that nobody travelled between in any of the 70 days he studied.)

Silva’s model predicts minute by minute how many journeys are being made between each pair of stations, how many passengers will enter and leave the system at each station, and how many will be inside the system at any given time.

In its most basic form, the model is simply a description of where passengers are entering and leaving the system at any given time; a reflection of how the network is when it’s working normally. But unlike the real network, he can experiment, closing stations or lines and seeing how the virtual passengers adapt to the disruption.

In principle, the model can give staff immediate feedback about what passengers are likely to be doing at that point, when an unplanned service disruption takes place. However, implementing real time feedback will require further work as the existing technical facilities are not ready for that yet.

Silva’s model takes a lot of computing power – it takes a few days to run a simulation on an ordinary desktop computer – but it is not unmanageable. Extending the model to take account of passenger flows through the transport network, or to run the simulation quicker would require supercomputer facilities such as UCL’s Legion Cluster.

UCL’s Legion Cluster supercomputing facility. Photo: Tony Slade, © UCL Creative Media Services (all rights reserved)

The model doesn’t only apply to London – it is applicable to any transport network where passengers’ entry and exit points are tracked, meaning it could be useful for transport authorities around the world.

Related links

• Ricardo Silva
• UCL Statistical Science
• Transport for London

The planet Mercury is approximately 48 million miles away, but this summer I’m bringing Mercury to the SouthBank!

My name is Amy Edgington, I am a PhD student in the Earth Sciences department here at UCL, and I have been lucky enough to be selected as one of the speakers for Soapbox Science in London on 30 May.

Soapbox Science is a series of public outreach events happening all over the country this summer, promoting female scientists and the incredible work they are doing. It’s a great opportunity to share my research with the general public, answer questions, and engage in exciting debate!

Soapbox Science in London, sponsored by L’Oreal, ZSL, STFC and NERC, will transform the SouthBank into an arena for scientific discussion and learning. The variety of topics on show is massive- so there will definitely be something for everyone!

I will be taking to the soapbox to discuss my PhD research so far, investigating the interior of the planet Mercury. The large uncompressed density of the innermost planet suggests it is highly enriched in metallic iron, however, there are ground based measurements1 that imply a liquid layer remains in its interior.

Even just these two clues start to build an intriguing picture of the structure and dynamics hidden deep beneath Mercury’s cratered surface. As part of the Earth Sciences department here at UCL I use ab initio molecular dynamics to study the behaviour and thermodynamics properties of the materials that might form Mercury’s core, namely liquid iron and liquid iron alloys on the atomic scale.

A better knowledge of these materials may lead to a greater understanding of the interior of the solar systems smallest planet, and hopefully unlock insights into its evolution. I will be discussing all of this and more with the help of some iron bolts and a giant polystyrene Mercury on 30 May, from 2-5pm on the Southbank.

Follow @SoapboxScience and @ES_UCL to keep updated with this event, plus visit  soapscience.org for more information on all the SoapBox Science events this summer, and to read some of the amazing Women in Science blogs.

Related Links:

• What is Soapbox Science?
• Investigating Mercury – PhD Amy Edgington is talking about her research and life at UCL.
• Amy Edgington, PhD student at Earth Sciences, UCL
• Amy’s Women in Science Blog: High heels, big computers and planetary size cakes
• Soapbox Science Founders & Sponsors: For Women in Science, Fondation L’OREAL, ZSL, Science &Technology Facilities Council, Natural Environment Research Council
  

Quantum refrigerator. Photo: O. Usher (UCL MAPS)

This photo shows a specialised refrigerator, used for cooling objects to within a fraction of absolute zero, located in the Physics building on UCL’s main campus.

When in operation, the refrigerator is entirely enclosed in a sealed and insulated housing, which has been removed here for maintenance.

As with a household fridge, the temperature drops as you go down – with the highest of the four shelves being at a temperature of about 50 Kelvins (-223 Celsius) and the bottom one at just 0.03 K (-273 C).

The refrigerator is used to cool small objects, such as transistors, down to levels where thermal effects (such as the vibration of the atoms in them) are eliminated, allowing quantum effects to be observed.

Samples can be raised or lowered through each shelf via a circular hole in the centre of the refrigerator.

The refrigerator is cooled by a mixture of liquid Helium-3 and Helium-4. The lowest shelf of this refrigerator has a strong claim to being the coldest place in London (alongside a handful of similar facilities at other London universities).

High resolution images

• High resolution images of the equipment are available on Flickr

Related links

• UCL Physics & Astronomy
• UCL Quantum

Sagittarius A* region, as observed by NASA’s Chandra X-Ray Observatory. Credit: NASA/CXO

This image shows the centre of the Milky Way galaxy, as seen in X-rays by the Chandra X-Ray Observatory. X-rays are produced by high-temperature, high-energy phenomena.

Within this region lies Sagittarius A*, the Milky Way’s central black hole.

An international team of astronomers, including UCL’s Silvia Zane, has just published new research on a magnetar (a type of super-magnetic neutron star) that is orbiting Sagittarius A*, explaining why it is cooling far slower than theories suggest.

• Read about the research on the UCL Mathematical & Physical Sciences news pages

Messier 51, from the University of London Observatory. Credit: UCL/ULO/Ian Howarth

The University of London Observatory – UCL’s astronomical observatory in Mill Hill, North London – has to deal with England’s murky skies and London’s bright lights, but it can still make some impressive images. Messier 51, seen in the picture above, is actually not one galaxy but two – a large spiral galaxy (Messier 51a) interacting with a smaller dwarf galaxy (NGC 5195). Over the next few hundred million years, they will merge together into one larger galaxy.

Such mergers are quite common. Large spiral galaxies can absorb dwarf galaxies without major disruption to their shapes, though the (rarer) mergers between similarly-sized galaxies tend to destroy all structure, leaving a largely featureless elliptical galaxy. This will be the fate of the Milky Way when it merges with the Andromeda Galaxy in a few billion years time.

The Messier 51 pair are a popular target for amateur astronomers – on a dark night, even relatively basic telescopes can pick out the very faint comma-shape of the galaxy pair, visible near one end of the Plough (Ursa Major).

The picture is one of several newly processed images just published by UCL’s observatory, based on data gathered by astronomy students. The observatory now routinely archives all the digital data gathered with its Celestron telescopes, which are used intensively for undergraduate teaching. This growing archive of data means that multiple observations can be easily combined into a single image, improving contrast and revealing faint details that would otherwise be invisible.

A selection of several dozen of these images from the observatory, with multiple observations processed and combined to form colour composites, is available online to the public. They are free to reuse and reproduce.

• View and download high-resolution ULO images from our Flickr site.

This is the third and final in a series of posts marking the 25th anniversary of the Hubble Space Telescope. Read the first here, and the second here.

Hubble won’t last forever – electrical faults could render any of its instruments inoperable at any time. Though this has happened before, there is no longer a Space Shuttle that can be launched to send astronauts on a repair mission, so any future instrument failures are likely to be permanent.

Moreover, the telescope needs to reliably and steadily lock onto the position of the astronomical objects it is observing. It does this thanks to gyroscopes dotted around the spacecraft – but these will eventually wear out and fail too. Engineers are quietly confident that Hubble can last till at least 2015, but beyond that, the observatory’s future is unclear.

Artist’s impression of the James Webb Space Telescope. Credit: ESA/C. Carreau

By the end of 2018, the James Webb Space Telescope should join Hubble in orbit – with hardware built at UCL on board. The Webb telescope is not a like-for-like replacement. Webb will have far more powerful capabilities in infrared light, allowing it to peer deep into dust clouds, observe planetary systems being formed, and the see distant redshifted light of the first galaxies.

UCL’s contribution to the James Webb Space Telescope: the NIRSpec (Near Infrared Spectrograph) calibration assembly. This helps maintain accurate scientific observations. Photo credit: UCL MSSL

But it will not have Hubble’s abilities in ultraviolet and visible light. A new breed of telescopes on the ground, such as the European Extremely Large Telescope and the Thirty Meter Telescope will partly replace Hubble’s visible light capabilities (although not with Hubble’s sharpness).

But when Hubble fails, no telescope in operation or in development will replace its ability to observe ultraviolet light, which is blocked by the Earth’s atmosphere.

This is the second in a series of posts marking the 25th anniversary of the Hubble Space Telescope. Read the first here.

UCL astronomers have been involved with the full range of Hubble science over the years.

Here are just a few highlights.

Detecting the first organic molecule on an extrasolar planet

Artist’s impression of HD 189733b passing in front of its star. The small amount of starlight that passes through the planet’s atmosphere carries with it the fingerprint of the gases present there. Credit: ESA/Hubble (Martin Kornmesser)

In 2008, a team including UCL’s Giovanna Tinetti (now working on the proposed Twinkle mission) made the first detection of an organic molecule on a planet outside the Solar System, using Hubble. Organic molecules – ones based on carbon – are thought to play a key role in the emergence of complex chemistry and the appearance of life.

Although this planet, known as HD 189733b, is so hot that it is almost certainly sterile, the team proved it has traces of methane in its atmosphere. This shows not only that organic molecules exist outside our Solar System, but that they can be detected from Earth – and that one day we might detect signs of life on another planet using the same methods.

Hubble observed the light coming from HD 189733b’s parent star as the planet passed between Hubble and the star. The gases in the atmosphere leave a faint fingerprint in the light that passes through the atmosphere, letting the scientists deduce what gases were present.

Understanding the superwinds of Messier 82

Composite image of Hubble and WIYN observations of M 82. Credit: Mark Westmoquette (University College London), Jay Gallagher (University of Wisconsin-Madison), Linda Smith (University College London), WIYN//NSF, NASA/ESA

Messier 82 (or M 82 for short) is a peculiar-looking galaxy – in the news last year thanks to a UCL lecturer and his students discovering a bright supernova there. It is in the midst of a sustained period of star formation known to astronomers as a ‘starburst’ – and this has dramatic effects on the appearance of the galaxy. It shines brightly with bright blue newly-formed stars, with noticeable regions of disrupted gas and dust clouds. It is an easy and popular target for amateur astronomers to find with mid-sized telescopes.

But the most dramatic aspect of M 82’s appearance has to be the powerul winds of glowing gas ejected out of the galaxy.

In 2004, a team of scientists including Linda Smith and Mark Westmoquette (both then at UCL) used archival Hubble images, alongside images from the WIYN telescope, to trace these ‘superwinds’. They found multiple streams of gas expanding at different rates, creating a shower of hot gas expelled from the galaxy. Some of these were travelling at more than a million miles per hour.

The team believes that the burst of star formation was triggered by a near-miss with nearby spiral galaxy M 81, which disrupted the gas clouds in M 82.

Probing the dark universe using galaxy clusters

The discovery of the accelerating expansion of the Universe was one of the most startling scientific breakthroughs of the past century. Astronomers had long known that the universe was expanding, but assumed that this expansion was gradually slowing over time. Instead, in 1998, two teams of astronomers discovered that the universe’s expansion is speeding up, with an unknown force they call dark energy driving it.

The amount of energy involved in this mysterious process is enormous – calculations show it makes up around three quarters of the total mass and energy content of the universe. This is on top of dark matter – matter whose gravity can be seen, but which is totally invisible to telescopes – which also outweighs all the ordinary matter in the universe. This means that the matter we can actually see – stars, planets, nebulae, dust clouds, galaxies – only makes up about one part in 20 of the whole universe. The rest is dark; it neither emits, reflects nor absorbs light.

A team of astronomers including UCL’s Ofer Lahav proposed to carry out a huge programme of observations called the Cluster Lensing and Supernova Survey with Hubble (CLASH), in order to gather as much data as possible about the mysteries of the dark universe.

Cluster MACSJ1206, one of 25 observed by the CLASH programme. The distorted, magnified shapes of background galaxies can be faintly seen in the image. Credit: NASA, ESA, the CLASH team and Marc Postman

Between 2010 and 2013, the survey made detailed images of 25 massive galaxy clusters. These clusters, which are among the largest structures in the universe, have so much gravity – largely thanks to the dark matter they contain – that they warp space-time. This means that they bend the path of light that passes through them, in a similar way to a lens.

The lensing of light reveals the location of dark matter, which is otherwise invisible. It also amplifies the light coming from galaxies in the background, enabling astronomers to see distant supernovae that would otherwise be too faint to observe. The apparent brightness of these supernovae is one of astronomers’ key tools for measuring the expansion rate of the universe, and hence the nature of dark energy.

UCL’s work in CLASH has focused on ‘photometric redshifts’, a means of deducing the distance of faraway galaxies from the colour profile they have. This work was led by Ofer Lahav, along with researchers Stephanie Jouvel and Ole Host.

On Friday, the UCL Science blog will explain what comes next for the ageing space telescope – and how UCL is helping to build its successor.

This week marks the NASA/ESA Hubble Space Telescope’s 25th birthday. Since its launch on 24 April 1990, it has revolutionised astronomy, playing a role in huge scientific events including the first images of exoplanets and the measurement of the rate of expansion of the universe.

Along the way, it has taken stunning, sharp images of space that are now icons of popular culture.

One of Hubble’s famous images, the ‘Pillars of Creation’ in the Eagle Nebula. Credit: NASA, ESA and the Hubble Heritage Team

At 25, the telescope has lasted far longer than was ever planned. But thanks to regular servicing over the years, most recently in 2009, NASA’s engineers calculate the telescope still has a few years of operation left before its hardware begins to wear out. It should even last long enough to see the first few years of operations of its successor, the James Webb Space Telescope, which launches in 2018.

Hubble had a long and difficult gestation – the idea dates back to the 1940s, with design work beginning in the ‘70s. By the early 1980s, amid rising costs and political controversy, it was clear that the US couldn’t deliver Hubble alone, so the European Space Agency was brought in as a partner.

ESA astronaut Claude Nicollier during the third Hubble servicing mission in 1999. Credit: NASA/ESA

Since then, Hubble has been an international project, with staff from around the world working on the telescope, and observing time awarded to astronomers from around the world in annual competitions. Among them have been a fair few from UCL.

Hubble science comes in different flavours.

Most of the telescope’s time is devoted to observations carried out on behalf of small teams of astronomers. If their proposal is considered to be scientifically interesting, they will get to observe their chosen targets and use the data for their research. After a year, the data is uploaded to an online archive for anyone – other scientists, or even members of the public – to view. Surprisingly, perhaps, these archival observations are still incredibly useful, and they end up being used in huge numbers of studies, often making important discoveries that are totally unrelated to the original plan.

As well as these small projects, a proportion of Hubble’s time is set aside for bigger projects. These might be a systematic survey of dozens of galaxy clusters, or a highly detailed map of a large, nearby galaxy. These surveys are carried out in order to create large, comprehensive archives that can be used for a wide range of different scientific goals.

UCL astronomers have been involved with the full range of Hubble science over the years.

On Wednesday, the UCL Science blog will cover a few of the research highlights from UCL’s work with Hubble over the years.

The first dark matter map to come out of the Dark Energy Survey. Credit: Fermilab/Dark Energy Survey

The Dark Energy Survey, an international collaboration to probe the history, evolution and large-scale structure of the cosmos, has produced its first dark matter map. Dark matter is a transparent form of matter that is distributed in vast filaments throughout the known universe. Galaxies are located along these structures, and galaxy clusters lie where they meet. Because dark matter is fully transparent, it cannot be observed directly – its presence must be inferred from the gravitational effects it has on light, in particular, the way it distorts the shape of galaxies which lie in the background.

The map is the first of a series of dark matter maps that will be published by the survey team, and is part of a batch of research that is being released to coincide with the April meeting of the American Physical Society this week.

UCL is heavily involved with the Dark Energy Survey, and has several researchers involved in the newly-published research. The university is also involved in the project through the scientific instrumentation (UCL built some of the lenses that are used by the Dark Energy Camera), and UCL’s Ofer Lahav is co-chair of the DES science board.

Related links

• Dark Energy Survey
• Press release from Fermilab

UCL’s Mullard Space Science Laboratory has hosted a group of postgraduate Fine Art students from UCL’s Slade School. The students set up sculptures in the laboratory’s parkland campus in rural Surrey, inspired by the laboratory’s grounds and research.