̽��ֱ�� of Cambridge - dark matter

̽��ֱ��big question - Cambridge ̽��ֱ�� team joins ALPHA hunt for dark matter

sb726 — Wed, 07 Feb 2024 11:48:36 +0000

A team of Cambridge scientists are working to help identify the mysterious and invisible material believed to make up 85 per cent of all the matter in the Universe.

Mission to map the dark Universe sets off on space journey

sc604 — Sat, 01 Jul 2023 15:16:27 +0000

̽��ֱ��Euclid space telescope will map the 'dark Universe' by observing billions of galaxies out to 10 billion light-years, across more than a third of the sky, to gather data on how its structure has formed over its cosmic history.

Led by the European Space Agency (ESA) and a consortium of 2,000 scientists, including from the ̽��ֱ�� of Cambridge, Euclid will spend six years venturing through space with two scientific instruments: a UK-built visible imager (VIS) that will become one of the largest cameras ever sent into space, and a near-infrared spectrometer and photometer, developed in France. ̽��ֱ��mission is supported by funding from the UK Space Agency.

“Watching the launch of Euclid, I feel inspired by the years of hard work from thousands of people that go into space science missions, and the fundamental importance of discovery – how we set out to understand and explore the Universe,” said Chief Executive of the UK Space Agency, Dr Paul Bate. “ ̽��ֱ��UK Space Agency’s investment in Euclid has supported world-class science on this journey, from the development of the ground segment to the build of the crucial visible imager instrument, which will help humanity begin to uncover the mysteries of dark matter and dark energy.”

Euclid took off on board a SpaceX spacecraft from Cape Canaveral in Florida at 4.11pm (BST) on 1 July.

Cambridge’s Institute of Astronomy team has been involved in Euclid since 2010, supporting development of the astrometric calibration pipeline for the optical image data from Euclid, ensuring that the positions of the billions of sources to be imaged by Euclid can be determined to exquisite accuracy.

“Dark energy and dark matter fundamentally govern the formation and evolution of our Universe,” said Dr Nicholas Walton from the Institute of Astronomy. “ ̽��ֱ��Euclid mission will finally uncover the mysteries of how these ‘dark’ forces have shaped the cosmos that we see today, from life here on Earth, to our Sun, our Milky Way, our nearby galaxy neighbours, and the wider Universe beyond.”

̽��ֱ��Science and Technology Facilities Council (STFC) also contributed to design and development work on Euclid instrumentation and provided funding to UK astronomy teams who will analyse the data returned from the mission about the physics responsible for the observed accelerated expansion of the Universe.

“This is a fantastic example of close collaboration between scientists, engineers, technicians, and astronomers across Europe working together to tackle some of the biggest questions in science,” said Mark Thomson, Executive Chair at STFC.

UK Space Agency funding for the Euclid mission is divided between teams at ̽��ֱ�� College London, ̽��ֱ��Open ̽��ֱ��, ̽��ֱ�� of Cambridge, ̽��ֱ�� of Edinburgh, ̽��ֱ�� of Oxford, ̽��ֱ�� of Portsmouth and Durham ̽��ֱ��.

̽��ֱ��wider Euclid Consortium includes experts from 300 organisations across 13 European countries, the US, Canada and Japan.

A European mission to explore how gravity, dark energy and dark matter shaped the evolution of the Universe soared into space from Cape Canaveral on 1 July.

̽��ֱ��Euclid mission will finally uncover the mysteries of how these ‘dark’ forces have shaped the cosmos that we see today, from life here on Earth, to our Sun, our Milky Way, our nearby galaxy neighbours, and the wider Universe beyond

Nicholas Walton

ESA

Last glimpse of Euclid on Earth

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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New findings that map the universe’s cosmic growth support Einstein’s theory of gravity

sc604 — Tue, 11 Apr 2023 14:00:00 +0000

̽��ֱ��findings, from the Atacama Cosmology Telescope collaboration involving researchers from the ̽��ֱ�� of Cambridge, provide further support to Einstein’s theory of general relativity, which has been the foundation of the standard model of cosmology for more than a century. ̽��ֱ��results offer new methods to demystify dark matter, the unseen mass thought to account for 85% of the matter in the universe.

For millennia, humans have been fascinated by the mysteries of the cosmos. From ancient civilisations such as the Babylonians, Greeks, and Egyptians to modern-day astronomers, the allure of the starry sky has inspired countless quests to unravel the secrets of the universe.

And although models that explain the cosmos have existed for centuries, the field of cosmology, where scientists use quantitative methods to understand the evolution and structure of the universe, is relatively new—having only formed in the early 20th century with the development of Albert Einstein’s theory of general relativity.

Now, a set of papers submitted to ̽��ֱ��Astrophysical Journal by researchers from the Atacama Cosmology Telescope (ACT) collaboration has produced a new image that reveals the most detailed map of matter distributed across a quarter of the entire sky, reaching deep into the cosmos. It confirms Einstein’s theory about how massive structures grow and bend light, with a test that spans the entire age of the universe.

“We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across,” said co-author Professor Blake Sherwin from Cambridge’s Department of Applied Mathematics and Theoretical Physics, where he leads a group of ACT researchers. “It looks just as our theories predict.”

Although dark matter makes up a large chunk of the universe and shaped its evolution, it has remained hard to detect because it doesn’t interact with light or other forms of electromagnetic radiation. As far as we know, dark matter only interacts with gravity.

To track it down, the more than 160 collaborators who have built and gathered data from the National Science Foundation’s Atacama Cosmology Telescope in the high Chilean Andes observe light emanating following the dawn of the universe’s formation, the Big Bang—when the universe was only 380,000 years old. Cosmologists often refer to this diffuse light that fills our entire universe as the “baby picture of the universe,” but formally, it is known as the cosmic microwave background radiation (CMB).

̽��ֱ��team tracks how the gravitational pull of large, heavy structures including dark matter warps the CMB on its 14-billion year journey to us, like how a magnifying glass bends light as it passes through its lens.

“We’ve made a new mass map using distortions of light left over from the Big Bang,” said Mathew Madhavacheril from the ̽��ֱ�� of Pennsylvania, lead author of one of the papers. “Remarkably, it provides measurements that show that both the ‘lumpiness’ of the universe, and the rate at which it is growing after 14 billion years of evolution, are just what you’d expect from our standard model of cosmology based on Einstein's theory of gravity.”

“Our results also provide new insights into an ongoing debate some have called ‘ ̽��ֱ��Crisis in Cosmology’,” said Sherwin. This crisis stems from recent measurements that use a different background light, one emitted from stars in galaxies rather than the CMB. These have produced results that suggest the dark matter was not lumpy enough under the standard model of cosmology and led to concerns that the model may be broken. However, the team’s latest results from ACT were able to precisely assess that the vast lumps seen in this image are the exact right size.

“When I first saw them, our measurements were in such good agreement with the underlying theory that it took me a moment to process the results,” said Cambridge PhD candidate Frank Qu, lead author of one of the new papers. “But we still don’t know what the dark matter is, so it will be interesting to see how this possible discrepancy between different measurements will be resolved.”

“ ̽��ֱ��CMB lensing data rivals more conventional surveys of the visible light from galaxies in their ability to trace the sum of what is out there,” said Suzanne Staggs from Princeton ̽��ֱ��, Director of ACT. “Together, the CMB lensing and the best optical surveys are clarifying the evolution of all the mass in the universe.”

“When we proposed this experiment in 2003, this measurement wasn’t even on our agenda; we had no idea the full extent of information that could be extracted from our telescope,” said Mark Devlin, from the ̽��ֱ�� of Pennsylvania, Deputy Director of ACT. “We owe this to the cleverness of the theorists, the many people who built new instruments to make our telescope more sensitive, and the new analysis techniques our team came up with.”

With ACT having been decommissioned in late 2022, further papers highlighting some of the other final results are slated for submission in the coming year. Observations will continue at the site with the Simons Observatory, including a new telescope due to begin in 2024 that can map the sky almost ten times faster.

̽��ֱ��pre-print articles highlighted in this release are available on act.princeton.edu and will appear on the open-access arXiv.org. They have been submitted to ̽��ֱ��Astrophysical Journal.

This work was supported by the U.S. National Science Foundation, Princeton ̽��ֱ��, the ̽��ֱ�� of Pennsylvania, and a Canada Foundation for Innovation award. Team members at the ̽��ֱ�� of Cambridge were supported by the European Research Council.

A new image reveals the most detailed map of dark matter distributed across a quarter of the entire sky, reaching deep into the cosmos.

We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across

Blake Sherwin

ACT Collaboration

A new map of the dark matter made by the Atacama Cosmology Telescope. ̽��ֱ��orange regions show where there is more mass; purple where there is less.

Yes

Have we detected dark energy? Cambridge scientists say it’s a possibility

sc604 — Wed, 15 Sep 2021 15:36:10 +0000

A new study, led by researchers at the ̽��ֱ�� of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. ̽��ֱ��researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world – from tiny moons to massive galaxies, from ants to blue whales – makes up less than five percent of the universe. ̽��ֱ��rest is dark. About 27% is dark matter – the invisible force holding galaxies and the cosmic web together – while 68% is dark energy, which causes the universe to expand at an accelerated rate.

“Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn’t discovered until 1998,” said Dr Sunny Vagnozzi from Cambridge’s Kavli Institute for Cosmology, the paper’s first author. “Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter ‘hitting’ ordinary matter, but dark energy is even more elusive.”

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the universe’s expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. “These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said co-author Dr Luca Visinelli, from Frascati National Laboratories in Italy. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”

At the time, the most popular explanation for the excess were axions – hypothetical, extremely light particles – produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can’t be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein’s theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be hidden, or screened, when it comes to small scales, and can only operate on the largest scales where Einstein's theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the XENON1T excess.

“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said Vagnozzi. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”

̽��ֱ��researchers used their model to show what would happen in the detector if the dark energy was produced in a region of the Sun called the tachocline, where the magnetic fields are particularly strong.

“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. “We first need to know that this wasn’t simply a fluke,” said Visinelli. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”

If the excess was the result of dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect dark energy within the next decade.

Reference:
Sunny Vagnozzi et al. ‘Direct detection of dark energy: the XENON1T excess and future prospects.’ Physical Review D (2021). DOI: 10.1103/PhysRevD.104.063023

Dark energy, the mysterious force that causes the universe to accelerate, may have been responsible for unexpected results from the XENON1T experiment, deep below Italy’s Apennine Mountains.

It was surprising that this excess could in principle have been caused by dark energy rather than dark matter. When things click together like that, it’s really special.

Sunny Vagnozzi

betmari

Sun

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Attribution-Noncommerical

New data tests 'theory of everything'

sc604 — Thu, 19 Mar 2020 16:36:44 +0000

Despite having many different versions of string theory circulating throughout the physics community for decades, there have been very few experimental tests. Astronomers using NASA’s Chandra X-ray Observatory, however, have now made a significant step forward in this area.

By searching through galaxy clusters, the largest structures in the universe held together by gravity, researchers were able to hunt for a specific particle that string theory predicts should exist. While the resulting non-detection does not rule out string theory altogether, it does deliver a blow to certain models within that family of ideas.

“Until recently I had no idea just how much X-ray astronomers bring to the table when it comes to string theory, but we could play a major role,” said Professor Christopher Reynolds of Cambridge's Institute of Astronomy, who led the study. “If these particles are eventually detected it would change physics forever.”

̽��ֱ��particle that Reynolds and his colleagues were searching for is called an axion. These as-yet-undetected particles should have extraordinarily low masses. Scientists do not know the precise mass range, but many theories feature axion masses ranging from about a millionth of the mass of an electron down to zero mass. Some scientists think that axions could explain the mystery of dark matter, which accounts for the vast majority of matter in the universe.

One unusual property of these ultra-low-mass particles would be that they might sometimes convert into photons, or particles of light, as they pass through magnetic fields. ̽��ֱ��opposite may also hold true: photons may also be converted into axions under certain conditions. How often this switch occurs depends on how easily they make this conversion, in other words on their 'convertibility.'

Some scientists have proposed the existence of a broader class of ultra-low-mass particles with similar properties to axions. Axions would have a single convertibility value at each mass, but 'axion-like particles' would have a range of convertibility at the same mass.

“While it may sound like a long shot to look for tiny particles like axions in gigantic structures like galaxy clusters, they are actually great places to look,” said co-author David Marsh of Stockholm ̽��ֱ�� in Sweden. “Galaxy clusters contain magnetic fields over giant distances, and they also often contain bright X-ray sources. Together these properties enhance the chances that conversion of axion-like particles would be detectable.”

To look for signs of conversion by axion-like particles, the team of astronomers examined over five days of Chandra observations of X-rays from material falling towards the supermassive black hole in the centre of the Perseus galaxy cluster. They studied the Chandra spectrum, or the amount of X-ray emission observed at different energies, of this source. ̽��ֱ��long observation and the bright X-ray source gave a spectrum with enough sensitivity to have shown distortions that scientists expected if axion-like particles were present.

̽��ֱ��lack of detection of such distortions allowed the researchers to rule out the presence of most types of axion-like particles in the mass range their observations were sensitive to, below about a millionth of a billionth of an electron's mass.

“Our research doesn’t completely rule out the existence of these particles, but it definitely doesn’t help their case,” said co-author Helen Russell of the ̽��ֱ�� of Nottingham. “These constraints dig into the range of properties suggested by string theory, and may help string theorists weed their theories.”

̽��ֱ��latest result was about three to four times more sensitive than the previous best search for axion-like particles, which came from Chandra observations of the supermassive black hole in M87. This Perseus study is also about a hundred times more powerful than current measurements that can be performed in laboratories here on Earth for the range of masses that they have considered.

Clearly, one possible interpretation of this work is that axion-like particles do not exist. Another explanation is that the particles have even lower convertibility values than this observation’s detection limit, and lower than some particle physicists have expected. They also could have higher masses than probed with the Chandra data.

The results are reported in ̽��ֱ��Astrophysical Journal.

NASA's Marshall Space Flight Center manages the Chandra program. ̽��ֱ��Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.

Adapted from a NASA press release.

One of the biggest ideas in physics is the possibility that all known forces, particles, and interactions can be connected in one framework. String theory is arguably the best-known proposal for a 'theory of everything' that would tie together our understanding of the physical universe.

If these particles are eventually detected it would change physics forever

Christopher Reynolds

NASA/CXC/Univ. of Cambridge/C. Reynolds et al

Perseus: A galaxy cluster located about 240 million light years from Earth

Yes

Women in STEM: Dr Francesca Chadha-Day

sc604 — Thu, 06 Feb 2020 07:00:00 +0000

I can’t remember a time when I didn’t want to be a physicist. That’s always been my ambition, because it’s the most fundamental thing in the universe, so that’s what I wanted to study. I went through school knowing that I wanted to be a physicist, so I had a lot of drive. I read about quantum physics and bought books by Richard Feynman, which I found really beautiful and inspiring.

I’ve never had a problem seeing a woman as a scientist. I think I’m quite lucky in that my Mum is a biologist, so, while that’s a fairly different subject area, I’ve always seen that you can have a family and a career. My Mum was the main breadwinner when I was growing up and my Dad did most of the childcare, so I think that always made it very clear that you didn’t have to go by traditional gender roles.

My interest in particle physics was cemented when I attended the CERN Summer Student Programme in Geneva. ̽��ֱ��whole experience was inspirational, and I was lucky enough to be there when the Higgs boson was discovered. That was a really amazing moment. We’d had a theory for decades that predicted this particle existed, and then they managed to build a machine that actually showed, unambiguously, that the theory was correct.

I can only hope that there will be more huge discoveries in my lifetime. I applied to read Natural Sciences for my undergraduate degree, and when I started I wasn’t quite sure what kind of physics I wanted to do. Going to Geneva helped me to decide the path I wanted to follow with my research. Before then I’d been told by a number of people that ‘theoretical particle physics is very hard’ and ‘it might not have a future’, and ‘maybe you should do something easier’. But then going to CERN really showed me that it does have a future, and it was something that I really wanted to do. I completed my PhD in Theoretical Particle Physics at Oxford ̽��ֱ��, and was then awarded a junior research fellowship at Peterhouse.

I work on the boundary between theoretical physics and x-ray astronomy. Cambridge is one of the leading universities in the world for physics and has really good research groups for both of these disciplines. And the college system is really great because it means you bump into people from all kinds of different subjects in college and have really interesting discussions that I wouldn’t have if I just hung around the department.

I work on particles called axions, mostly. We don’t currently know whether axions exist but they are motivated to exist by a number of different problems. One of these is string theory, which is the main candidate for a theory that explains both quantum physics and gravity. A problem with string theory is that it doesn’t have a lot of other predictions so it’s ‘mathematically nice’ but it’s hard to know if it’s true or not. One of the predictions is that you would get a lot of axions, so searching for those helps. If we found them it would provide some evidence for string theory but wouldn't prove it.

̽��ֱ��other main motivation for axions is dark matter. So dark matter is matter that we know exists, because we can see its gravitational pull on other matter, by looking at, for example, the velocities of stars in the Milky Way. They are going faster than we expect, which means there must be more mass in the middle than we can see. But we don’t know what it is, and axions can also act as dark matter. So they’re motivated from a number of different angles. People are trying lots of different ways of searching for them, and I’m using an interdisciplinary approach that’s on the boundary between particle physics and astrophysics. I’m looking at analysing astrophysical spectra to try and work out whether it matches what we think it should just from the particles that we definitely know exist. Or whether there are other effects that might be signatures from new particles.

My advice to others who are thinking about studying a STEM subject is; absolutely, go for it. It’s likely that people will tell you that you can’t do it. That happened to me at every stage, from applying to Cambridge, applying for my PhD, applying for fellowships, people have always advised against it, and they’ve always been wrong. But you definitely won’t get anything if you don’t try, so it’s always worth just going for it. More specifically, for those who want a career in physics, study further maths. Do as much maths as possible, and also experiment with the conditions that your brain works best. So I have different places where I work in different ways and soundtracks for working on different problems, and so it’s quite important to curate how you’re working, that’s probably more important than putting in 12 hours a day.

My daily work involves a lot of reading papers, keeping up to date with the literature, programming is a big part of my job, to do simulations of, for example, what effects we might expect axions to have, so I’m asking the question, if axions existed, what would we expect this spectrum to have. And normally the way to answer that is to write some code. Talking to colleagues about different ideas for projects, different things we could study or look at, writing papers, there’s a lot of working out the different conversions between different units and minus signs and so on.

Away from work, I perform as a science comedian. I used to be quite bad at public speaking, and I wanted to get better because it’s really important for any career in science. Even if you don’t do public engagement, you give a lot of seminars and talks, so I challenged myself to take up every speaking opportunity that came my way for a while, and then I’d get better by practising. We got an email around the department from an organisation that facilitate academics to do stand-up comedy about their research. So according to my self-imposed rule, I had to sign up for it. So I thought, what have I got to lose, and it went from there. I find it really enjoyable, when you can make a room full of people laugh hysterically it’s such a high. Most of my material is about physics, so it’s a public engagement talk, but it’s funny, it’s interesting and people learn something as well.

Dr Francesca Chadha-Day is a theoretical physicist, a research fellow at Peterhouse, and a science comedian. Here, she tells us about her lifelong love of physics, her work on dark matter and particles called axions, and the high that comes with making a roomful of people laugh.

Yes

Massive holes ‘punched’ through a trail of stars likely caused by dark matter

sc604 — Wed, 07 Sep 2016 08:00:50 +0000

Researchers have detected two massive holes which have been ‘punched’ through a stream of stars just outside the Milky Way, and found that they were likely caused by clumps of dark matter, the invisible substance which holds galaxies together and makes up a quarter of all matter and energy in the universe.

̽��ֱ��scientists, from the ̽��ֱ�� of Cambridge, found the holes by studying the distribution of stars in the Milky Way. While the clumps of dark matter that likely made the holes are gigantic in comparison to our Solar System – with a mass between one million and 100 million times that of the Sun – they are actually the tiniest clumps of dark matter detected to date.

̽��ֱ��results, which have been submitted to the Monthly Notices of the Royal Astronomical Society, could help researchers understand the properties of dark matter, by inferring what type of particle this mysterious substance could be made of. According to their calculations and simulations, dark matter is likely made up of particles more massive and more sluggish than previously thought, although such a particle has yet to be discovered.

“While we do not yet understand what dark matter is formed of, we know that it is everywhere,” said Dr Denis Erkal from Cambridge’s Institute of Astronomy, the paper’s lead author. “It permeates the universe and acts as scaffolding around which astrophysical objects made of ordinary matter – such as galaxies – are assembled.”

Current theory on how the universe was formed predicts that many of these dark matter building blocks have been left unused, and there are possibly tens of thousands of small clumps of dark matter swarming in and around the Milky Way. These small clumps, known as dark matter sub-haloes, are completely dark, and don’t contain any stars, gas or dust.

Dark matter cannot be directly measured, and so its existence is usually inferred by the gravitational pull it exerts on other objects, such as by observing the movement of stars in a galaxy. But since sub-haloes don’t contain any ordinary matter, researchers need to develop alternative techniques in order to observe them.

̽��ֱ��technique the Cambridge researchers developed was to essentially look for giant holes punched through a stream of stars. These streams are the remnants of small satellites, either dwarf galaxies or globular clusters, which were once in orbit around our own galaxy, but the strong tidal forces of the Milky Way have torn them apart. ̽��ֱ��remnants of these former satellites are often stretched out into long and narrow tails of stars, known as stellar streams.

“Stellar streams are actually simple and fragile structures,” said co-author Dr Sergey Koposov. “ ̽��ֱ��stars in a stellar stream closely follow one another since their orbits all started from the same place. But they don’t actually feel each other’s presence, and so the apparent coherence of the stream can be fractured if a massive body passes nearby. If a dark matter sub-halo passes through a stellar stream, the result will be a gap in the stream which is proportional to the mass of the body that created it.”

̽��ֱ��researchers used data from the stellar streams in the Palomar 5 globular cluster to look for evidence of a sub-halo fly-by. Using a new modelling technique, they were able to observe the stream with greater precision than ever before. What they found was a pair of wrinkled tidal tails, with two gaps of different widths.

By running thousands of computer simulations, the researchers determined that the gaps were consistent with a fly-by of a dark matter sub-halo. If confirmed, these would be the smallest dark matter clumps detected to date.

“If dark matter can exist in clumps smaller than the smallest dwarf galaxy, then it also tells us something about the nature of the particles which dark matter is made of – namely that it must be made of very massive particles,” said co-author Dr Vasily Belokurov. “This would be a breakthrough in our understanding of dark matter.”

̽��ֱ��reason that researchers can make this connection is that the mass of the smallest clump of dark matter is closely linked to the mass of the yet unknown particle that dark matter is composed of. More precisely, the smaller the clumps of dark matter, the higher the mass of the particle.

Since we do not yet know what dark matter is made of, the simplest way to characterise the particles is to assign them a particular energy or mass. If the particles are very light, then they can move and disperse into very large clumps. But if the particles are very massive, then they can’t move very fast, causing them to condense – in the first instance – into very small clumps.

“Mass is related to how fast these particles can move, and how fast they can move tells you about their size,” said Belokurov. “So that’s why it’s so interesting to detect very small clumps of dark matter, because it tells you that the dark matter particle itself must be very massive.”

“If our technique works as predicted, in the near future we will be able to use it to discover even smaller clumps of dark matter,” said Erkal. “It’s like putting dark matter goggles on and seeing thousands of dark clumps each more massive than a million suns whizzing around.”

Reference:
Denis Erkal et al. ‘A sharper view of Pal 5’s tails: Discovery of stream perturbations with a novel non-parametric technique.’ arXiv:1609.01282

̽��ֱ��discovery of two massive holes punched through a stream of stars could help answer questions about the nature of dark matter, the mysterious substance holding galaxies together.

While we do not yet understand what dark matter is formed of, we know that it is everywhere.

Denis Erkal

V. Belokurov, D. Erkal, S.E. Koposov (IoA, Cambridge). Photo: Colour image of M31 from Adam Evans.

Artist's impression of dark matter clumps around a Milky Way-like galaxy

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Opinion: Large Hadron Collider sees tantalising hints of a new particle that could revolutionise physics

Anonymous — Thu, 17 Dec 2015 11:38:32 +0000

At the start of December a rumour swirled around the internet and physics lab coffee rooms that researchers at the Large Hadron Collider had spotted a new particle. After a three-year drought that followed the discovery of the Higgs boson, could this be the first sign of new physics that particle physicists have all been desperately hoping for?

Researchers working on the LHC experiments remained tight-lipped until December 14 when physicists packed out CERN’s main auditorium to hear presentations from the scientists working on CMS and ATLAS experiments, the two gargantuan particle detectors that discovered the Higgs boson in 2012. Even watching the online webcast, the excitement was palpable.

Everybody was wondering if we would witness the beginning of a new age of discovery. ̽��ֱ��answer is … maybe.

Baffling bump

̽��ֱ��CMS results were revealed first. At first the story was familiar, an impressive range of measurements that again and again showed no signs of new particles. But in the last few minutes of the presentation a subtle but intriguing bump on a graph was revealed that hinted at a new heavy particle decaying into two photons (particles of light). ̽��ֱ��bump appeared at a mass of around 760GeV (the unit of mass and energy used in particle physics – the Higgs boson has a mass of about 125 GeV) but was far too weak a signal to be conclusive on its own. ̽��ֱ��question was, would ATLAS see a similar bump in the same place?

̽��ֱ��ATLAS presentation mirrored the one from CMS, another list of non-discoveries. But, saving the best for last, a bump was unveiled towards the end, close to where CMS saw theirs at 750GeV – but bigger. It was still too weak to reach the statistical threshold to be considered solid evidence, but the fact that both experiments saw evidence in the same place is exciting.

̽��ֱ��discovery of the Higgs back in 2012 completed the Standard Model, our current best theory of particle physics, but left many unsolved mysteries. These include the nature of “dark matter”, an invisible substance that makes up around 85% of the matter in the universe, the weakness of gravity and the way that the laws of physics appear fine-tuned to allow life to exist, to name but a few.

Could supersymmetry one day crack the mystery of all the dark matter lurking in galaxy clusters? NASA/wikimedia

A number of theories have been proposed to solve these problems. ̽��ֱ��most popular is an idea called supersymmetry, which proposes that there is a heavier super-partner for every particle in the Standard Model. This theory provides an explanation for the fine-tuning of the laws of physics and one of the super-partners could also account for dark matter.

Supersymmetry predicts the existence of new particles that should be in reach of the LHC. But despite high hopes the first run of the machine from 2009-2013 revealed a barren subatomic wilderness, populated only by a solitary Higgs boson. Many of the theoretical physicists working on supersymmetry have found the recent results from the LHC rather depressing. Some had begun to worry that answers to the outstanding questions in physics might lie forever beyond our reach.

This summer the 27km LHC restarted operation after a two-year upgrade that almost doubled its collision energy. Physicists are eagerly waiting to see what these collisions reveal, as higher energy makes it possible to create heavy particles that were out of reach during the first run. So this hint of a new particle is very welcome indeed.

A cousin of Higgs?

Andy Parker, head of Cambridge’s Cavendish Laboratory and senior member of the ATLAS experiment, told me: “If the bump is real, and it decays into two photons as seen, then it must be a boson, most likely another Higgs boson. Extra Higgs are predicted by many models, including supersymmetry”.

Perhaps even more exciting, it could be a type of graviton, a hypothesised particle associated with the force of gravity. Crucially, gravitons exist in theories with additional dimensions of space to the three (height, width and depth) we experience.

For now, physicists will remain sceptical – more data is needed to rule this intriguing hint in or out. Parker described the results as “preliminary and inconclusive” but added, “should it turn out to be the first sign of physics beyond the standard model, with hindsight, this will be seen as historic science.”

Whether this new particle turns out to be real or not, one thing that everyone agrees on is that 2016 is going to be an exciting year for particle physics.

Harry Cliff, Particle physicist and Science Museum fellow, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Harry Cliff (Cavendish Laboratory) discusses the potential discovery of a new particle at the Large Hadron Collider and its implications for particle physics.

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̽��ֱ��Large Hadron Collider/ATLAS at CERN

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