The Great London:
Astrophysics

Scientists behind a theory that the speed of light is variable - and not constant as Einstein suggested - have made a prediction that could be tested.

Einstein observed that the speed of light remains the same in any situation, and this meant that space and time could be different in different situations.

The assumption that the speed of light is constant, and always has been, underpins many theories in physics, such as Einstein's theory of general relativity. In particular, it plays a role in models of what happened in the very early universe, seconds after the Big Bang.

But some researchers have suggested that the speed of light could have been much higher in this early universe. Now, one of this theory's originators, Professor Joao Magueijo from Imperial College London, working with Dr Niayesh Afshordi at the Perimeter Institute in Canada, has made a prediction that could be used to test the theory's validity.

Structures in the universe, for example galaxies, all formed from fluctuations in the early universe – tiny differences in density from one region to another. A record of these early fluctuations is imprinted on the cosmic microwave background – a map of the oldest light in the universe – in the form of a 'spectral index'.

Working with their theory that the fluctuations were influenced by a varying speed of light in the early universe, Professor Magueijo and Dr Afshordi have now used a model to put an exact figure on the spectral index. The predicted figure and the model it is based on are published in the journal >Physical Review D.

Cosmologists are currently getting ever more precise readings of this figure, so that prediction could soon be tested – either confirming or ruling out the team's model of the early universe. Their figure is a very precise 0.96478. This is close to the current estimate of readings of the cosmic microwave background, which puts it around 0.968, with some margin of error.

Radical Idea

Professor Magueijo said: "The theory, which we first proposed in the late-1990s, has now reached a maturity point – it has produced a testable prediction. If observations in the near future do find this number to be accurate, it could lead to a modification of Einstein's theory of gravity.

"The idea that the speed of light could be variable was radical when first proposed, but with a numerical prediction, it becomes something physicists can actually test. If true, it would mean that the laws of nature were not always the same as they are today."

The testability of the varying speed of light theory sets it apart from the more mainstream rival theory: inflation. Inflation says that the early universe went through an extremely rapid expansion phase, much faster than the current rate of expansion of the universe.

The Horizontal Problem

These theories are necessary to overcome what physicists call the 'horizon problem'. The universe as we see it today appears to be everywhere broadly the same, for example it has a relatively homogenous density.

This could only be true if all regions of the universe were able to influence each other. However, if the speed of light has always been the same, then not enough time has passed for light to have travelled to the edge of the universe, and 'even out' the energy.

As an analogy, to heat up a room evenly, the warm air from radiators at either end has to travel across the room and mix fully. The problem for the universe is that the 'room' – the observed size of the universe – appears to be too large for this to have happened in the time since it was formed.

The varying speed of light theory suggests that the speed of light was much higher in the early universe, allowing the distant edges to be connected as the universe expanded. The speed of light would have then dropped in a predictable way as the density of the universe changed. This variability led the team to the prediction published today.

The alternative theory is inflation, which attempts to solve this problem by saying that the very early universe evened out while incredibly small, and then suddenly expanded, with the uniformity already imprinted on it. While this means the speed of light and the other laws of physics as we know them are preserved, it requires the invention of an 'inflation field' – a set of conditions that only existed at the time.

Author: Hayley Dunning | Source: Imperial College London [November 25, 2016]

A team of international scientists, led by astronomers from Cardiff University's School of Physics and Astronomy, has shown for the first time that galaxies can change their structure over the course of their lifetime.

By observing the sky as it is today, and peering back in time using the Hubble and Herschel telescopes, the team have shown that a large proportion of galaxies have undergone a major 'metamorphosis' since they were initially formed after the Big Bang.

By providing the first direct evidence of the extent of this transformation, the team hope to shed light on the processes that caused these dramatic changes, and therefore gain a greater understanding of the appearance and properties of the Universe as we know it today.

In their study, which has been published in the Monthly Notices of the Royal Astronomical Society, the researchers observed around 10,000 galaxies currently present in the Universe using a survey of the sky created by the Herschel ATLAS and GAMA projects.

The researchers then classified the galaxies into the two main types: flat, rotating, disc-shaped galaxies (much like our own galaxy, the Milky Way); and large, oval-shaped galaxies with a swarm of disordered stars.

Using the Hubble and Herschel telescopes, the researchers then looked further out into the Universe, and thus further back in time, to observe the galaxies that formed shortly after the Big Bang.

The researchers showed that 83 per cent of all the stars formed since the Big Bang were initially located in a disc-shaped galaxy.

However, only 49 per cent of stars that exist in the Universe today are located in these disc-shaped galaxies -- the remainder are located in oval-shaped galaxies.

The results suggest a massive transformation in which disc-shaped galaxies became oval-shaped galaxies.

A popular theory is that this transformation was caused by many cosmic catastrophes, in which two disk-dominated galaxies, straying too close to each other, were forced by gravity to merge into a single galaxy, with the merger destroying the disks and producing a huge pileup of stars. An opposing theory is that the transformation was a more gentle process, with stars formed in a disk gradually moving to the centre of a disk and producing a central pile-up of stars.

Lead author of the study Professor Steve Eales, from Cardiff University's School of Physics and Astronomy, said: "Many people have claimed before that this metamorphosis has occurred, but by combining Herschel and Hubble, we have for the first time been able to accurately measure the extent of this transformation.

"Galaxies are the basic building blocks of the Universe, so this metamorphosis really does represent one of the most significant changes in its appearance and properties in the last 8 billion years."

Professor Asantha Cooray, a co-author of the study from the University of California, said: "This study is important as it establishes statistics showing that almost all stars formed in spiral galaxies in the past, but a large fraction of these now appear as large, dead, elliptical galaxies today. This study will require us to refine the models and computer simulations that attempt to explain how galaxies formed and behaved over the last 13 billion years."

Dr David Clements, a co-author of the study from Imperial College London, said: "Up to now we've seen individual cases in the local universe where galaxy collisions convert spirals into ellipticals. This study shows that this kind of transformation is not exceptional, but is part of the normal history of galaxy evolution."

Matthew Allen, a Ph.D. student at Cardiff University and a member of the team, said: "This is a huge step in understanding how the galactic population has evolved over billions of years. Using some of the most cutting edge data and techniques, we are finally beginning to understand the processes that have shaped our Universe."

Source: Cardiff University [August 27, 2015]

The universe is expanding uniformly according to research led by UCL which reports that space isn't stretching in a preferred direction or spinning.

The new study, published in >Physical Review Letters, studied the cosmic microwave background (CMB) which is the remnant radiation from the Big Bang. It shows the universe expands the same way in all directions, supporting the assumptions made in cosmologists' standard model of the universe.

First author, Daniela Saadeh (UCL Physics & Astronomy), said: "The finding is the best evidence yet that the universe is the same in all directions. Our current understanding of the universe is built on the assumption that it doesn't prefer one direction over another, but there are actually a huge number of ways that Einstein's theory of relativity would allow for space to be imbalanced. Universes that spin and stretch are entirely possible, so it's important that we've shown ours is fair to all its directions."

The team from UCL and Imperial College London used measurements of the CMB taken between 2009 and 2013 by the European Space Agency's Planck satellite. The spacecraft recently released information about the polarisation of CMB across the whole sky for the first time, providing a complementary view of the early universe that the team was able to exploit.

The researchers modelled a comprehensive variety of spinning and stretching scenarios and how these might manifest in the CMB, including its polarisation. They then compared their findings with the real map of the cosmos from Planck, searching for specific signs in the data.

Daniela Saadeh, explained: "We calculated the different patterns that would be seen in the cosmic microwave background if space has different properties in different directions. Signs might include hot and cold spots from stretching along a particular axis, or even spiral distortions."

Collaborating author Dr Stephen Feeney (Imperial College London) added: "We then compare these predictions to reality. This is a serious challenge, as we found an enormous number of ways the Universe can be anisotropic. It's extremely easy to become lost in this myriad of possible universes -- we need to tune 32 dials to find the correct one."

Previous studies only looked at how the universe might rotate, whereas this study is the first to test the widest possible range of geometries of space. Additionally, using the wealth of new data collected from Planck allowed the team to achieve vastly tighter bounds than the previous study. "You can never rule it out completely, but we now calculate the odds that the universe prefers one direction over another at just one in 121,000," said Daniela Saadeh.

Most current cosmological studies assume that the Universe behaves identically in every direction. If this assumption were to fail, a large number of analyses of the cosmos and its content would be flawed.

Daniela Saadeh, added: "We're very glad that our work vindicates what most cosmologists assume. For now, cosmology is safe."

Source: University College London [September 22, 2016]

A new study of the early universe reveals how it could have been formed from an older collapsing universe, rather than being brand new.

The universe is currently expanding and it is a common theory that this is the result of the 'Big Bang' – the universe bursting into existence from a point of infinitely dense and hot material.

However, physicists have long debated this idea as it means the universe began in a state of complete breakdown of physics as we know it. Instead, some have suggested that the universe has alternated between periods of expansion and contraction, and the current expansion is just one phase of this.

This so-called 'Big Bounce' idea has been around since 1922, but has been held back by an inability to explain how the universe transitions from a contracting to an expanding state, and vice versa, without leading to an infinite point.

Now, in a new study published today in >Physical Review Letters, Dr Steffen Gielen from Imperial College London and Dr Neil Turok, Director of the Perimeter Institute for Theoretical Physics in Canada, have shown how the Big Bounce might be possible.

Broken Symmetry

Cosmological observations suggest that during its very early life, the universe may have looked the same at all scales – meaning that the physical laws that that worked for the whole structure of the universe also worked at the scale of the very small, smaller than individual atoms. This phenomenon is known as conformal symmetry.

In today's universe, this is not the case – particles smaller than atoms behave very differently to larger matter and the symmetry is broken. Subatomic particle behaviour is governed by what is called quantum mechanics, which produces different rules of physics for the very small.

For example, without quantum mechanics, atoms would not exist. The electrons, as they whizz around the nucleus, would lose energy and collapse into the centre, destroying the atom. However, quantum mechanics prevents this from happening.

In the early universe, as everything was incredibly small, it may have been governed solely by the principles of quantum mechanics, rather than the large-scale physics we also see today.

In the new study, the researchers suggest that the effects of quantum mechanics could prevent the universe from collapsing and destroying itself at end of a period of contraction, known as the Big Crunch. Instead, the universe would transition from a contracting state to an expanding one without collapsing completely.

Dr Gielen said: "Quantum mechanics saves us when things break down. It saves electrons from falling in and destroying atoms, so maybe it could also save the early universe from such violent beginnings and endings as the Big Bang and Big Crunch."

Simple Ingredients

Using the idea that the universe had conformal symmetry at its beginning, and that this was governed by the rules of quantum mechanics, Dr Gielen and Dr Turok built a mathematical model of how the universe might evolve.

The model contains a few simple ingredients that are most likely to have formed the early universe, such as the fact that it was filled with radiation, with almost no normal matter. With these, the model predicts that the effect of quantum mechanics would allow the universe to spring from a previous universe that was contracting, rather than from a single point of broken physics.

Dr Turok said: "The big surprise in our work is that we could describe the earliest moments of the hot Big Bang quantum mechanically, under very reasonable and minimal assumptions about the matter present in the universe. Under these assumptions, the Big Bang was a 'bounce', in which contraction reversed to expansion."

The researchers are now investigating how this simple model can be extended to explain the origin of perturbations to the simple structure of the universe, such as galaxies. "Our model's ability to give a possible solution to the problem of the Big Bang opens the way to new explanations for the formation of the universe," said Dr Gielen.

Author: Hayley Dunning | Source: Imperial College London [July 12, 2016]

A quick method for making accurate, virtual universes to help understand the effects of dark matter and dark energy has been developed by UCL and CEFCA scientists. Making up 95% of our universe, these substances have profound effects on the birth and lives of galaxies and stars and yet almost nothing is known about their physical nature.

The new approach, published today in >Monthly Notices of the Royal Astronomical Society and funded by the Royal Society, is twenty-five times faster than current methods but is just as accurate, allowing scientists more computer power to focus on understanding why the universe is accelerating and galaxies are positioned where they are.

"To uncover the nature of dark energy and the origin of our 14 billion year old accelerating universe, we have to compare the results from big studies to computational models of the universe," explained Dr Andrew Pontzen, UCL Physics & Astronomy.

"Exciting new ventures, including the Large Synoptic Survey Telescope and the Javalambre Physics of the Accelerating Universe survey, are on the horizon, and we want to be ready to do the best possible job of understanding them", added joint author Dr Raul Angulo, CEFCA, Spain.

Dr Pontzen continued: "But every computer simulation we run gives a slightly different answer. We end up needing to take an average over hundreds of simulations to get a 'gold standard' prediction. We've shown it's possible to achieve the same model accuracy by using only two carefully-constructed virtual universes, so a process that would take weeks on a superfast computer, can now be done in a day."

The scientists say their method will speed up research into the unseen forces in the universe by allowing many model universes to be rapidly developed to test alternate versions of dark energy and dark matter.

"Our method allows cosmologists to run more creative experiments which weren't feasible before due to the large amount of computer time needed. For example, scientists can now generate lots of different models of dark energy to find the one which best explains real-world survey data. We could also use this approach to see how individual galaxies look and fit inside the overall structure of the universe by spending the freed-up time on computing the virtual universes in much greater detail," said Dr Pontzen.

The new method removes the biggest uncertainties in the model universe by comparing its properties with an 'inverted' version. In the inverted model universe, galaxies are replaced by empty voids, and the empty voids of space with galaxies. The scientists tried this approach after noticing a mathematical symmetry linking the two seemingly different pictures.

When they compared the output of the paired universes to that of the gold standard method - which averages 300 virtual universes to remove uncertainties - they found the results to be very similar. The new approach showed less than 1% deviation from the gold standard, suggesting the new approach makes predictions that are accurate enough to use in forthcoming experiments.

"In addition to the reversal process, we also adjust the ripples of the early universe to carefully-chosen values, to further eliminate inaccuracies" added Dr Angulo.

The team now plan on using the new method to investigate how different forms of dark energy affect the distribution of galaxies through the universe. "Because we can get a more accurate prediction in a single shot, we don't need to spend so much computer time on existing ideas and can instead investigate a much wider range of possibilities for what this weird dark energy might really be made from," said Dr Pontzen.

Source: University College London [July 06, 2016]