̽��ֱ�� of Cambridge - string theory

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

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Taming the multiverse: Stephen Hawking’s final theory about the big bang

sc604 — Wed, 02 May 2018 10:47:29 +0000

̽��ֱ��theory, which was submitted for publication before Hawking’s death earlier this year, is based on string theory and predicts the universe is finite and far simpler than many current theories about the big bang say.

Professor Hertog, whose work has been supported by the European Research Council, first announced the new theory at a conference at the ̽��ֱ�� of Cambridge in July of last year, organised on the occasion of Professor Hawking’s 75^th birthday.

Modern theories of the big bang predict that our local universe came into existence with a brief burst of inflation – in other words, a tiny fraction of a second after the big bang itself, the universe expanded at an exponential rate. It is widely believed, however, that once inflation starts, there are regions where it never stops. It is thought that quantum effects can keep inflation going forever in some regions of the universe so that globally, inflation is eternal. ̽��ֱ��observable part of our universe would then be just a hospitable pocket universe, a region in which inflation has ended and stars and galaxies formed.

“ ̽��ֱ��usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean,” said Hawking in an interview last autumn. “ ̽��ֱ��local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested. ”

In their new paper, Hawking and Hertog say this account of eternal inflation as a theory of the big bang is wrong. “ ̽��ֱ��problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats the quantum effects as small fluctuations around this,” said Hertog. “However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down in eternal inflation.”

“We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure,” said Hawking.

̽��ֱ��theory of eternal inflation that Hawking and Hertog put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the universe as tiny vibrating strings. Their approach uses the string theory concept of holography, which postulates that the universe is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.

Hawking and Hertog developed a variation of this concept of holography to project out the time dimension in eternal inflation. This enabled them to describe eternal inflation without having to rely on Einstein’ theory. In the new theory, eternal inflation is reduced to a timeless state defined on a spatial surface at the beginning of time.

“When we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning,” said Hertog.

Hawking’s earlier ‘no boundary theory’ predicted that if you go back in time to the beginning of the universe, the universe shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work. “Now we’re saying that there is a boundary in our past,” said Hertog.

Hertog and Hawking used their new theory to derive more reliable predictions about the global structure of the universe. They predicted the universe that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.

Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm. “We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes,” said Hawking.

This makes the theory more predictive and testable.

Hertog now plans to study the implications of the new theory on smaller scales that are within reach of our space telescopes. He believes that primordial gravitational waves – ripples in spacetime – generated at the exit from eternal inflation constitute the most promising “smoking gun” to test the model. ̽��ֱ��expansion of our universe since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors. But they might be heard by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.

Reference:
S.W. Hawking and Thomas Hertog. ‘A Smooth Exit from Eternal Inflation?’’ Journal of High-Energy Physics (2018). DOI: 10.1007/JHEP04(2018)147

Professor Stephen Hawking’s final theory on the origin of the universe, which he worked on in collaboration with Professor Thomas Hertog from KU Leuven, has been published in the Journal of High Energy Physics.

We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes.

Stephen Hawking

Andre Pattenden

Stephen Hawking

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Strings that surprise: how a theory scaled up

amb206 — Tue, 04 Mar 2014 09:44:10 +0000

In December 2013 Professor Michael Green of Cambridge ̽��ֱ�� and Professor John Schwarz of California Institute of Technology were awarded the 2014 Fundamental Physics Prize, one of a series of annual 'Breakthrough Prizes' set up to raise the profile of the physical and biological sciences. Their shared $3 mn prize was given for “opening new perspectives on quantum gravity and the unification of forces”.

Green and Schwarz are known for their pioneering work in string theory, postulated as a way of explaining the fundamental constituents of the universe as tiny vibrating strings. Different types of elementary particles arise in this theory as different vibrational harmonics (or ‘notes’). ̽��ֱ��scope of string theory has broadened over the past few years and is currently being applied to a far wider field than that for which it was first devised, which has taken those who research into it in unexpected directions.

Although the term ‘string theory’ was not coined till 1971, it had its genesis in a paper by the Italian physicist Gabriele Veneziano in 1968, published when Green was a research student in Cambridge. Green was rapidly impressed by its potential and began working seriously on it in the early 1970s. As he explains in the accompanying film, he stuck with string theory during a period when it was overshadowed by other developments in elementary particle physics.

As a result of a chance meeting at the CERN accelerator laboratory in Switzerland in the summer of 1979, Green (then a researcher at Queen Mary, London) began to work on string theory with Schwarz. Green says that the relative absence of interest in string theory during the 1970s and early 1980s was actually helpful: it allowed him and a small number of colleagues to focus on their research well away from the limelight.

“Initially we were not sure that the theory would be consistent, but as we understood it better we became more and more convinced that the theory had something valuable to say about the fundamental particles and their forces,” he says.

In August 1984 the two researchers, while working at the Aspen Center for Physics in Colorado, famously understood how string theory avoids certain inconsistencies (known as ‘anomalies’) that plague more conventional theories in which the fundamental particles are points rather than strings. This convinced other researchers of the potential of string theory as an elegant unified description of fundamental physics.

“Suddenly our world changed - and we were called on to give lectures and attend meetings and workshops,” remembers Green.

String theory was back on track as a construct that offered a compelling explanation for the fundamental building blocks of the universe: many researchers shifted the focus of their work into this newly-promising field and, as a result of this upturn in interest, developments in string theory began to take new and unexpected directions.

Ideas formulated in the past few years, indicate that string theory has an overarching mathematical structure that may be useful for understanding a much wider variety of problems in theoretical physics that the theory was originally supposed to explain – this includes problems in condensed matter, superconductivity, plasma physics and the physics of fluids.

Green is a passionate believer in the exchange of ideas and he values immensely his interaction with the latest generation of researchers to be tackling some of the knottiest problems in particle physics and associated fields.

“ ̽��ֱ��best ideas come from the young people entering the field and we need to make sure we continue to attract them into research. It is particularly evident that at present we fail to encourage sufficient numbers of young women to think about careers in physics,” he says. “Scientific research is by its nature competitive and there are, of course, professional jealousies - but there’s also a strong tradition of collaboration in theoretical physics and advances in the subject feel like a communal activity.”

In 2009 Green was appointed Lucasian Professor of Mathematics at Cambridge. It comes with a legacy that Green describes as daunting: his immediate predecessor was Professor Stephen Hawking and in its 350-year history the chair has been held by a series of formidable names in the history of mathematical sciences.

̽��ֱ��challenges of pushing forward the boundaries in a field that demands thinking in not three dimensions but as many as 11 are tremendous. ̽��ֱ��explanation of the basic building blocks of nature as different harmonics of a string is only a small part of string theory – and is the feature that is easiest to put across to the general public as it is relatively straightforward to visualise.

“Far harder to articulate in words are concepts to do with explaining how time and space might emerge from the theory,” says Green. “Sometimes you hit a problem that you just can’t get out of your head and carry round with you wherever you are. It’s almost a cliché that it’s often when you’re relaxing that a solution will spontaneously present itself.”

Like his colleagues Green is motivated by wonderment at the world and the excitement of being part of a close community grappling with fundamental questions. He is often asked to justify the cost of research that can seem so remote from everyday life, and that cannot be tested in any conventional sense. In response he gives the example of the way in which quantum mechanics has revolutionised the way in which many of us live.

In terms of developments that may come from advances in string theory, he says: “We can’t predict what the eventual outcomes of our research will be. But, if we are successful, they will certainly be huge - and in the meantime, string theory provides a constant stream of unexpected surprises.”

Michael Green will be giving a lecture – ‘ ̽��ֱ��pointless Universe’ – as part of Cambridge Science Festival on Thursday 13 March, 5pm-6pm, at Lady Mitchell Hall, Sidgwick Site, Cambridge. ̽��ֱ��event is free but requires pre-booking.

For more information about this story contact Alexandra Buxton, Office of Communications, ̽��ֱ�� of Cambridge, amb206@admin.cam.ac.uk 01223 761673

In August 1984 two physicists arrived at a formula that transformed our understanding of string theory, an achievement now recognised by a major award. Professor Michael Green of the Department of Applied Mathematics and Theoretical Physics explains how string theory has taken unexpected directions.

We can’t predict what the eventual outcomes of our research will be. But, if we are successful, they will certainly be huge.

Michael Green

Strings that surprise: how a theory progressed

Johnny Settle

Strings

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