Gravitational waves from early universe remain elusive

The color scale in this image from the Planck mission represents the emission from dust, a minor but crucial component that pervades our Milky Way galaxy. The texture indicates the orientation of the galactic magnetic field. It is based on measurements of the direction of the polarized light emitted by the dust.

A joint analysis of data from the Planck space mission and the ground-based experiment BICEP2 has found no conclusive evidence of gravitational waves from the birth of our universe, despite earlier reports of a possible detection. The collaboration between the teams has resulted in the most precise knowledge yet of what signals from the ancient gravitational waves should look like, aiding future searches.

Planck is a European Space Agency mission with significant NASA contributions. BICEP2 and its sister project, the Keck Array, are based at the South Pole and funded by the National Science Foundation, also with NASA contributions.
"By analyzing both sets of data together, we could get a more definitive picture of what's going on than we could with either dataset alone," said Charles Lawrence, the U.S. project scientist for Planck at NASA's Jet Propulsion Laboratory, Pasadena, California. "The joint analysis shows that much of the signal detected by BICEP2/Keck is coming from dust in the Milky Way, but we cannot rule out a gravitational wave signal at a low level. This is a good example of how progress is made in science, one step at a time."
Planck and BICEP/Keck were both designed to measure relic radiation emitted from our universe shortly after its birth 13.8 billion years ago. An extraordinary source of information about the universe's history lies in this "fossil" radiation, called the cosmic microwave background (CMB). Planck mapped the CMB over the entire sky from space, while BICEP2/Keck focused on one patch of crisp sky over the South Pole.
In March of 2014, astronomers presented intriguing data from the BICEP2/Keck experiments, finding what appeared to be a possible signal from our universe when it was just born. If the signal were indeed from the early cosmos, then it would have confirmed the presence of ancient gravitational waves. It is hypothesized that these waves were generated by an explosive and very rapid period of growth in our universe, called inflation, which took place when the universe was only a tiny of a fraction of one second old.
Specifically, the BICEP/Keck experiments found evidence for a "curly" pattern of polarized light called B-modes. These patterns would have been imprinted on the CMB light as the gravitational waves slightly squeezed and stretched the fabric of space. Polarization describes a particular property of light. Usually, the electric and magnetic fields carried by light vibrate at all orientations equally, but when they vibrate preferentially in a certain direction, the light is polarized.
"The swirly polarization pattern, reported by BICEP2, was also clearly seen with new data from the Keck Array," said Jamie Bock of the California Institute of Technology in Pasadena, and JPL, a member of both the BICEP2/Keck and Planck teams.
"Searching for this unique record of the very early universe is as difficult as it is exciting, since this subtle signal is hidden in the polarization of the CMB, which itself only represents only a feeble few percent of the total light," said Jan Tauber, the European Space Agency's project scientist for Planck.
One of the trickiest aspects of identifying the primordial B-modes is separating them from those that can be generated much closer to us by interstellar dust in our Milky Way galaxy.
The Milky Way is pervaded by a mixture of gas and dust shining at similar frequencies to those of the CMB, and this closer, or foreground, emission affects the observation of the oldest cosmic light. Very careful data analysis is needed to separate the foreground emission from that of the CMB.
"When we first detected this signal in our data, we relied on models for galactic dust emission that were available at the time," said John Kovac, a co-principal investigator of the BICEP2/Keck collaboration at Harvard University, Cambridge, Massachusetts. "These seemed to indicate that the region of the sky chosen for our observations was relatively devoid of dust."
The BICEP2/Keck experiments collected data at a single microwave frequency, making it difficult to separate the emissions coming from the dust in the Milky Way and the CMB. On the other hand, Planck observed the sky in nine microwave and sub-millimeter frequency channels, seven of which were also equipped with polarization-sensitive detectors. Some of these frequencies were chosen to make measurements of dust in the Milky Way. By careful analysis, these multi-frequency data can be used to separate the various contributions of emissions.
The Planck and BICEP2/Keck teams joined forces, combining the space satellite's ability to deal with foregrounds using observations at several frequencies, with the greater sensitivity of the ground-based experiments over limited areas of the sky.
"The noise in the instruments limits how deeply we can search for a signal from inflation," said Bock. "BICEP2/Keck measured the sky at one wavelength. To answer how much of the signal comes from the galaxy, we used Planck's measurements in multiple wavelengths. We get a big boost by combining BICEP2/Keck and Planck measurements together, the best data currently available."
The final results showed that most of the original BICEP2/Keck B-mode signal, but not necessarily all of it, could be explained by dust in our Milky Way. As for signs of the universe's inflationary period, the question remains open.
The joint Planck/BICEP/Keck study sets an upper limit on the amount of gravitational waves from inflation, which might have been generated at the time but at a level too low to be confirmed by the present analysis.
"The new upper limit on the signal due to gravitational waves agrees well with the upper limit that we obtained earlier with Planck using the temperature fluctuations of the CMB. The gravitational wave signal could still be there, and the search is definitely on," said Brendan Crill, a member of both the BICEP2 and Planck teams from JPL.
A paper on the findings is still under peer review.
NASA and JPL developed detector technology for both the BICEP and Keck Array experiments, as well as for the Planck space telescope. JPL is managed by Caltech for NASA.

---

Story Source:
The above story is based on materials provided by NASA/Jet Propulsion Laboratory. Note: Materials may be edited for content and length.

Decoding the gravitational evolution of dark matter halos

Researchers at Kavli IPMU and their collaborators have revealed that considering environmental effects such as a gravitational tidal force spread over a scale much larger than a galaxy cluster is indispensable to explain the distribution and evolution of dark matter halos around galaxies. A detailed comparison between theory and simulations made this work possible. The results of this study, which are published in Physical Review D as an Editors' Suggestion, contribute to a better understanding of fundamental physics of the universe.

In the standard scenario for the formation of a cosmic structure, dark matter, which has an energy budget in the universe that is approximately five times greater than ordinary matter (e.g., atoms), first gathers gravitationally to form a crowded region, the so-called dark matter halos. Then these dark matter halos attract atomic gas and eventually form stars and galaxies. Hence, to extract cosmological information from a three-dimensional galaxy map observed in SDSS BOSS, the SuMIRe project, etc., it is important to understand how clustering of dark matter halos has gravitationally evolved throughout cosmic history. (This is referred to as the halo bias problem.)
"Various studies have described the halo bias theoretically," said Teppei Okumura, a project researcher involved in the study from Kavli IPMU. "However, none of them reproduced simulation results well. So, we extended prior studies motivated by a mathematical symmetry argument and examined if our extension works."
The authors demonstrate that higher-order nonlocal terms originating from environmental effects such as gravitational tidal force must be taken into account to explain the halo bias in simulations. They also confirm that the size of the effect agrees well with a simple theoretical prediction.
"The results of our study allow the distribution of dark matter halos to be more accurately predicted by properly taking into account higher-order terms missed in the literature," said Shun Saito, the principal investigator of the study from Kavli IPMU. "Our refined model has been already applied to actual data analysis in the BOSS project. This study certainly improves the measurement of the nature of dark energy or neutrino masses. Hence, it has led to a better understanding of the fundamental physics of the universe."
This study is supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) No. 25887012.

---

Story Source:
The above story is based on materials provided by Kavli Institute for the Physics and Mathematics of the Universe. Note: Materials may be edited for content and length.

---

Journal Reference:

1. Shun Saito, Tobias Baldauf, Zvonimir Vlah, Uroš Seljak, Teppei Okumura, Patrick McDonald. Understanding higher-order nonlocal halo bias at large scales by combining the power spectrum with the bispectrum. Physical Review D, 2014; 90 (12) DOI: 10.1103/PhysRevD.90.123522

---

Is gravity the force driving time forwards? | sci-english.blogspot.com

Is gravity the force driving time forwards? |sci-english.blogspot.com

A new theory seeks to explain the so-called arrow of time and why it travels in one direction. Dan Falk reports.

It’s obvious that time flows in one direction. A teacup falls to the floor and shatters but we never see the broken shards reassemble; scramble an egg and it will never unscramble; we get older not younger with each passing day.
It might seem less obvious to ask: why does our Universe have an arrow of time?  But physicists and philosophers have asked, and struggled with this question, at least since the days of Aristotle. So whenever a new theory comes along, the scientific community greets it with great caution. Nevertheless, a paper in Physical Review Letters last October, by Flavio Mercati at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and his colleagues, has made physicists sit up and take notice. Mercati believes he can demonstrate that the most familiar of nature’s forces – gravity – is what sets time ticking in a certain direction.
At first glance, gravity isn’t an obvious place to look for clues about time. There’s nothing in the mathematics of gravity to suggest why time would flow in a particular direction.  Instead, physicists have looked to the second law of thermodynamics to explain why breaking a teacup or scrambling an egg are irreversible processes. The second law says that the amount of disorder in a closed system – what physicists call entropy – steadily increases over time. It tells you that if you have a highly ordered “low-entropy” system now, you can expect to have a high-entropy system in the future. Tidy your house on Sunday, and you can be sure it will be messy by the end of the week.
But how did our Universe end up in its low-entropy, highly ordered state?
“The question we might have made some tiny progress on is: Why was the teacup, or the egg, in this low entropy state in the first place?” says Mercati.

'To understand the arrow of time, we don’t need
to worry about the initial conditions of the Universe'

It was the British physicist Roger Penrose, back in the 1970s, who first drew attention to the second law’s shortcomings as a way to define time’s arrow. According to the law, our Universe ought to appear more and more ordered as we turn back the clock – but our understanding of the early Universe shows the opposite. Immediately after the Big Bang it was a hot plasma of fundamental particles like protons and electrons – a very scrambled egg, so to speak.
Mercati and his colleagues, Tim Kowlowski of the University of New Brunswick and independent British physicist Julian Barbour, offer new thinking on the problem. Their paper “Identification of a Gravitational Arrow of Time,” argues that to understand the arrow of time, we don’t need to worry about the initial conditions of the Universe; instead, they believe gravity can explain the phenomenon by itself. Few scientists had pursued this path because gravity says nothing about time; its equations are time-symmetric. To get a feel for this, imagine a film of a planet orbiting the sun.  Now imagine playing the film backwards. Unlike a scrambled egg unscrambling itself, which shows a change from disorder to order, the planet-film would look much the same projected backwards as forwards – except that the planet would move in the opposite direction.
To explore how gravity might explain the problem of time, they modelled its effects on a simple version of the Universe, a “toy model”, in which an array of 1,000 particles are arranged randomly in virtual space, and allowed to move in response to gravity alone. They found that the particles inevitably reach a point where they’re tightly clumped together; after that, they move further apart.  It’s a one-way process – there’s no going back to the clumped state.
Interestingly, the complexity of the system – defined in a precise way – grows, even as the system becomes less tightly clumped. Their definition of complexity was related to the space between the particles: roughly the ratio between the maximum and minimum  distances. Defined in this way, the complexity is lowest when  the particles are most tightly clumped, and grows as the system evolves and spreads out.
The toy model achieves a fair facsimile of the way our Universe moved spontaneously from a state of low complexity – the plasma – to a state of high complexity: galaxies, stars, planets, and so forth.  The arrow of time, they argue, follows from this natural increase in complexity.
The team’s argument is not an alternative to the second law of thermodynamics, but a complement to it. Gravity, says Mercati, “creates the conditions for having eggs that you can scramble in the first place”. So the second law explains the irreversible shattering of teacups, but the clumping power of gravity explains the creation of the ordered conditions in which complex structures – eggs, teacups, human beings – can form at all.  

'Overall, it’s no surprise to learn that the Universe gets more complex with time.'

Mercati and his colleagues’ work certainly won’t be the last word on time’s arrow – but it’s a welcome step forward, according to physicist Paul Davies, author of About Time and many other popular physics books. “Overall, it’s no surprise to learn that the Universe gets more complex with time,” he says. “But it’s very hard to pin down in any mathematical way that that is in fact the case. So this model is very welcome. One toy model that demonstrates something very clearly is worth a thousand hand-wavy descriptions.”
Physicist Sean Carroll of Caltech, who tackled the arrow of time by a different method with colleague Jennifer Chen about a decade ago, is also pleased with Mercati’s result. “They worked out an explicit model, where you can solve the equations, which is always a good thing to do,” he says.
The team’s model still has room for refinement. The model uses only simple Newtonian gravity; it ignores Einstein’s more complete theory, known as general relativity. It also ignores quantum theory. Further insight will come, Mercati speculates, when we have a framework that combines these two approaches – the long-sought quantum theory of gravity.
“The feeling that we’re missing something – that’s the real drive behind physics,” says Mercati. “If we felt that everything was in place, and that there were only a few details to fill in, I’d be much less interested in it.”

Dan Falk is a science journalist based in Toronto.

Extreme gravity effects revealed by oxygen for the first time in neutron star that 'eats' white dwarf | sci-english.blogspot.com

Neutron star 'eats' oxygen-rich white dwarf in a peculiar binary system | sci-english.blogspot.com

Astronomers from SRON Netherlands Institute for Space Research and Utrecht University have found blurred oxygen signatures in the X-rays from a neutron star that 'eats' a white dwarf. For the first time the effects of extreme gravity are revealed by oxygen instead of iron atoms.

Although strong gravity near neutron stars and black holes has been studied before in a similar way, this result is unique. Until now, only blurred X-ray signatures of iron atoms have been observed in the X-rays from a neutron star. However, the characteristics of these so called 'iron lines' are disputed, which makes them less suited for extreme gravity field measurements.
The neutron star has been studied before but now Oliwia Madej, PhD student at Utrecht University and SRON Netherlands Institute for Space Research, has found blurred oxygen signatures in the X-rays from the star. She made this discovery in an archival observation performed by ESA's XMM-Newton observatory, which is equipped with the SRON reflection grating spectrometer (RGS) that is extemely sensitive in these particular wavelenghts. The research was carried out under supervision of SRON-researcher Peter Jonker.
The neutron star that the astronomers observed is part of a binary system called 4U 0614+091. In the binary, the neutron star and a white dwarf closely orbit each other in roughly 50 minutes. The white dwarf -- basically a burnt out star -- orbits at such a small distance from the neutron star that the oxygen-rich gas is pulled off the dwarf and starts closely swirling around the neutron star in a disk.
Extreme gravity
"Normally, hot oxygen atoms emit X-rays at a specific energy," Madej explains. "But because of the extreme gravity and the hot gas in the disk around the neutron star, this oxygen signature in the X-ray data is blurred." From the shape of the blur Madej tried to estimate the inner radius of the oxygen-rich disk around the neutron star, which should give an idea of the maximum radius that the neutron star could possibly have.
"Unfortunately, the current data are not yet good enough to give a definitive answer on the size of a neutron star," Peter Jonker admits. "To determine this in greater detail we need more observation time. And when we find the signature of iron molecules as well, we can now compare the characteristics of the two emission lines. Measured together, uncertainties about the measurements of the iron line can be taken away, which will guide the interpretation in other systems where only iron has been seen. All in all our observations are definitely an important step on the way towards a better understanding of the extreme conditions around and inside a neutron star."
Neutron stars -- shaped out of the collapsing cores of massive stars -- are the most compact objects with a surface in the universe. A neutron star has a slightly higher mass compared to a white dwarf, but the matter is squeezed into a ball of only 10-20 km in diameter. At these high densities, normal atoms cannot exist anymore. Anything denser would collapse into a black hole. Therefore, astronomers are very interested in the state of the matter inside a neutron star.
The results of the research appear in the Monthly Notices of the Royal Astronomical Society.

---

Story Source:
The above story is based on materials provided by SRON Netherlands Institute for Space Research. Note: Materials may be edited for content and length.

---

Journal Reference:

1. O.K. Madej, P.G. Jonker, A.C. Fabian, C. Pinto, F. Verbunt, J. de Plaa. A relativistically broadened O VIII Lyalpha line in the ultra-compact X-ray binary 4U 0614 091. Monthly Notices of the Royal Astronomical Society, 2010; (accepted for publication) [link]

---