- Have you detected B-modes from inflation?
We have detected B-mode polarization at precisely the angular scales where the inflationary signal is expected to peak with very high significance (> 5 sigma). We have extensively studied possible contamination from instrumental effects and feel confident we can limit them to much smaller than the observed signal. Inflationary gravitational waves appear to be by far the most likely explanation for the signal we see.
- Couldn’t it just be galactic emission or polarized dust?
The data disfavor this. The best current models of polarized galactic emission in our observing region show it to be much fainter than the signal we see. Also, there is little evidence for correlation between our B-mode maps and the predicted pattern from the galaxy. Finally, within our own data, the “color” of the B-modes found by comparing different frequencies is consistent with CMB but disfavors galactic contamination.
- Have you detected a gravitational wave?
The frequency of the cosmic gravitational waves is very low, so we are not able to follow the temporal modulation. However, we are indeed directly observing a snapshot of gravitational waves through their imprints on matter and radiation over space. Ordinary density perturbations cannot create the pattern we observe. The presence of a water wave can be detected by feeling its up-and-down motion or by taking a picture of it. We are doing the latter.
About the science
- What is B-mode polarization and how is it generated by inflation?
Measuring the polarization of the Cosmic Microwave Background at different points on the sky determines a direction and polarized intensity (the polarized intensity of the CMB is less than 1/1,000,000 its total brightness). This can be visualized as a map of little line segments at every spot on the sky, the patterns of which we analyze. B-mode polarization is essentially the swirly part of that pattern (known mathematically as the ‘curl’). For the density fluctuations that generate most of the polarization of the CMB this part of the primordial pattern is exactly zero.[This is because density flows in the early universe go into or out of dense regions, and the polarization lines up with these flows in a way that doesn’t swirl, producing only so-called E-mode polarization. To generate a B-mode pattern in the early universe you need gravitational waves.]Inflation magnifies quantum fluctuations, which exist even in vacuum. The quantum fluctuations in the inflation field itself (“inflaton”) become the density fluctuations seen in the CMB and at much later times in galaxy distributions. During inflation, the quantum fluctuations in gravity (“graviton”) become long wavelength gravitational waves that produced the B-mode we see.
- Wasn’t B-mode polarization detected last year?
Yes – but a different kind. Last summer the 10-meter South Pole Telescope announced the first evidence for B-mode polarization in the CMB on arcminute scales which arises from thelensing, or bending of light, by gravitational attraction of structures formed in the relatively recent universe. Recently a second telescope, Polarbear, has also detected this effect. Our data sees this lensing effect too, but what is critical is that we see strong B-mode polarization at the much larger angular scales–2 to 4 degrees on the sky–where lensing is a tiny effect but where inflationary gravitational waves are expected to peak.
- Didn’t Planck find that r < 0.11? Do your experiments disagree?
Our measurements don’t disagree. Constraints on the gravitational-wave background level r reported from Planck and previous experiments are not from measurements of B-mode polarization. Instead, they come from the CMB temperature measurements which show surprisingly low power at the largest scales, implying little room for an additional contribution from tensors in the context of the simplest models. B-mode measurements like ours aim to directly measure the inflationary gravitational-wave pattern itself at the degree angular scales where it should peak. The tension between the high level of B-mode polarization we see and the apparent low power at large scales may be a statistical fluke, but many possible extensions to the simplest model could also relieve this apparent tension.
- Are you claiming you’ve detected running of the spectral index?
No. Running is a commonly-studied modification to the simplest Lambda Cold Dark Matter (LCDM) model in which the slope of primordial spectrum is allowed to change from large to small scales, but it is only one of the possible model extensions that could relieve tension between a high value of r and the low large-scale temperature spectrum. We only choose it as an example because it is simple and familiar from analyses by Planck and other teams. We anticipate that cosmologists will think broadly about possible parameter shifts or model extensions.
- Will your data/maps be made public?
Yes. The papers describing the results we present today are available for download on our webpage right now (and will be on arXiv tomorrow morning). The B-mode results and the information needed to do cosmological analyses with them are available for download now. The maps we’ve shown, including the high signal-to-noise images of B-mode polarization, will eventually also be released in digital form with the information needed to re-analyze and compare them to independent observations (though this will take us a while).
About the project
- Why the South Pole?
Water vapor in the atmosphere absorbs microwaves, making detailed studies of the CMB impossible from most places on earth. The South Pole is near the middle of the Antarctic plateau, the driest environment on the planet. At an effective altitude of over 10,000 feet (~3000 meters), stable weather patterns and winter temperatures averaging -72F (-58C), the South Pole is the closest a ground-based telescope can get to being in space. The patch of sky we study is visible from the South Pole continuously, 24h each day for the whole year. The National Science Foundation’s US Antarctic Program, which operates the South Pole Station, provides excellent infrastructure, communications, and support for the small team needed to run our telescopes, including our winterover scientists Steffen Richter and Robert Schwarz.
- Who funds BICEP2 / this project?
BICEP2 is funded by the National Science Foundation. Like the other telescopes in our BICEP and Keck Array series of experiments, NSF has supported its construction and operation, and also runs the South Pole Station where all our telescopes observe. The Keck Foundation has also contributed major funding for construction of our telescopes. NASA, JPL, and the Moore Foundation have generously supported the development of the ultra-sensitive detector array technology which make these measurements possible.
- Who leads BICEP2?
John Kovac has led the BICEP2 experiment. Clem Pryke has led the analysis that produced today’s results. Jamie Bock contributed the optical concept for the experiment and developed the detector array technology. Chao-Lin Kuo designed the polarization sensitive detectors used in BICEP2 and is leading the next major telescope in this series, BICEP3. BICEP2 is the second stage of a coordinated program, the BICEP and Keck Array series of experiments, developed for the South Pole. This coordinated program has a co-PI structure. The four PIs of this series are John Kovac (Harvard), Clem Pryke (UMN), Jamie Bock (Caltech/JPL), and Chao-Lin Kuo (Stanford/SLAC). All have worked together on the present result, along with talented teams of students and scientists. Other major collaborating institutions for BICEP2 include UCSD, UBC, NIST, University of Toronto, Cardiff, and CEA.
- What does “BICEP2” stand for?
Officially, “BICEP2” is not an acronym. It’s simply a name.
- How big is your team?
Although we have twelve collaborating institutions (see above) the core team working on this result has been relatively small for a major science project–a few dozen people. We’d like to particularly highlight the contribution of our dedicated graduate students Randol Aikin, Justus Brevik, Kirit Karkare, Jon Kaufman, Sarah Kernasovskiy, Chris Sheehy, Grant Teply, Jamie Tolan, and Chin Lin Wong. Our project postdocs have also included Colin Bischoff, Immanuel Buder, Jeff Filippini, Stefan Fliescher, Martin Lueker, Roger O’Brient, Walt Ogburn, Angiola Orlando, Zak Staniszewski, and Abigail Vieregg.
- Are there competing experiments?
Indeed yes, this is a highly competitive field. There are currently about a dozen ground-based and balloon-borne telescopes that are targeting the goal of measuring B-mode polarization. Many of them are described in this recent review. In addition, polarization data from ESA’s Planck satellite are eagerly anticipated. We look forward to results from these experiments which we hope will confirm and further extend the detection we report today.
- What comes next?
In the coming months our team plans to release improved data from the Keck Array, which will further test the BICEP2 detection at 150 GHz and add improved sensitivity at 95 GHz to further constrain foregrounds. By the end of this year the Planck satellite will release its polarization results. Of course we look forward to hearing results from other experiments which can test these results, and extend the frequency and angular coverage. We expect rapid progress. On a somewhat longer timescale, a detected signal at this amplitude raises exciting possibilities for studying inflation through more precise CMB polarization measurements over the entire sky which will spur the community to develop a new generation of ground-based and space-borne experiments.
How to see quantum gravity in Big Bang traces
Ron Cowen – nature.com
Can a quantum of gravity ever be detected? Two physicists suggest that it can — using the entire Universe as a detector.
Researchers think that the gravitational force is transmitted by an elementary particle called the graviton, just as the electromagnetic force is carried by photons. But most of them despair about ever recording individual gravitons. That is because gravity is so weak that any interactions between gravitons and matter are thought to be beyond human ability to detect in the foreseeable future.
Some physicists, including Freeman Dyson at the Institute for Advanced Study in Princeton, New Jersey, have gone further and claimed that building a graviton detector may actually be physically impossible. Several kinds of detectors have been proposed, but they would fail owing to a combination of instrumental and quantum noise, Dyson said at a conference in Singapore last month in honour of his 90th birthday.
Einstein’s general theory of relativity predicts the existence of ripples in space-time, known as gravitational waves, and physicists assume that these waves would be made of gravitons, just as electromagnetic waves are made of photons. But Dyson argued that the standard approach to searching for gravitational waves — by bouncing light off a set of mirrors to measure tiny shifts in their separation — would be hopeless for detecting gravitons: To be sensitive enough to detect the miniscule distance change due to an individual graviton, the mirrors would have to be so heavy that they would collapse to form a black hole.
But in a paper posted on the arXiv preprint server on 20 September1, Lawrence Krauss, a cosmologist at Arizona State University in Tempe, and Frank Wilczek, a Nobel-prizewinning physicist at the Massachusetts Institute of Technology (MIT) in Cambridge, consider whether the existence of gravitons — and the quantum nature of gravity — could be proved through some expected, but yet-to-be detected, features of the early Universe.
In the standard theory of cosmic birth, the universe underwent a dramatic growth spurt known as inflation during the first fraction of a second of its existence. Based on Einstein’s general theory of relativity, inflation would generate gravitational waves, which stretch space-time along one direction while contracting it along the other. That would affect how electromagnetic radiation — including the cosmic microwave background (CMB) left behind by the Big Bang — travels through space, causing it to become polarized. Researchers analysing results from the European Space Agency’s Planck satellite are searching for this imprint of inflation in the polarization of the CMB.
The process by which inflation generates gravitational waves is assumed to be quantum-mechanical in nature, and due to gravitons popping in and out of existence in the vacuum of space. Using a standard analytic tool known as dimensional analysis, Krauss and Wilczek show that the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that arises only in quantum theory. That means that gravitational waves are indeed an entirely quantum-mechanical phenomenon, they say.
If that is so, finding the fingerprint of gravitational waves in the polarization of the cosmic microwave background “may be the first empirical test that gravity must be quantized”, says Krauss.
The result, although implicit in previous studies, “is not generally recognized”, says Krauss. He and Wilczek emphasize that rather than making any new predictions or novel calculations, “we are merely bringing to the fore an implication of existing results that seems particularly noteworthy”.
Because the inflation-derived gravitational waves can be traced back to individual gravitons, “what we finally hope to detect is the signal from a single graviton amplified by the [expansion of the] Universe into something detectable”, says Wilczek. “The Universe is acting as our experimental device.”
Alan Guth, a cosmologist at MIT who originated the idea of inflation, says that Krauss and Wilczek’s main contribution “is to clarify an important point: the expected gravitational waves arise from the quantum properties of the gravitational field itself, and are not merely a by-product of the gravitational field interacting with the quantum fluctuations of other fields”.
“Thus, if we see convincing observational evidence from the cosmic microwave background polarization measurements that these gravitational waves exist, then we will have strong evidence that gravity really is quantized,” he says.
The work does not convince Stephen Boughn, a theoretical physicist at Haverford College in Pennsylvania who has co-authored a report2 agreeing with Dyson’s conjecture that gravitons may be impossible to detect. “The authors point out that the magnitude of the effect depends on Planck’s constant; however, it depends on other inflationary parameters as well,” he says. “It’s my impression that there is certainly enough slack in inflationary theory and quantum gravity so that, whatever the observed level of polarization, these theories can be made to fit the observations.”
Still, says Wilczek, “for all the noise and the smoke” about trying to unify quantum theory and gravity, “there’s been very little fire sighted”. The polarization imprint “might be a chance to see a few sparks”.
- Krauss, L. M. & Wilczek, F. Preprint at http://arxiv.org/abs/1309.5343 (2013).
- Rothman, T. & Stephen Boughn, S. Found. Phys. 36, 1801–1825 (2006).
This animation illustrates the painstaking work performed by cosmologists in the Planck Collaboration to extract the Cosmic Microwave Background from the data collected by Planck. The first image in the sequence shows the sources of emission detected on the whole sky at the microwave and sub-millimetre wavelengths probed by Planck, which range from 11.1 mm to 0.3 mm (corresponding to frequencies between 27 GHz and 1 THz).
The different sources of emission include:
– discrete emission from individual galactic and extragalactic sources;
– diffuse radio emission from interstellar material in the Milky Way, which is mostly due to synchrotron radiation emitted by electrons that spiral along the lines of the Galactic magnetic field, but also comprises bremsstrahlung radiation, emitted by electrons that are slowed down in the presence of protons, as well as emission from spinning dust grains;
– diffuse emission due to the thermal emission from interstellar dust in the Milky Way;
– and, finally, the Cosmic Microwave Background.
The cosmologists had to remove all possible contamination due to emission by foreground sources before they could fully explore the Cosmic Microwave Background data and compare them to cosmological models.
The latest results from the Planck space telescope have confirmed the presence of a microwave haze at the centre of the Milky Way. However, the haze appears to be more elongated than originally thought, which casts doubt over previous claims that annihilating dark matter is the cause of the emissions.
A roughly spherical haze of radiation at the heart of our galaxy was identified as far back as 2004 by the Wilkinson Microwave Anisotropy Probe (WMAP). Since then, some astrophysicists have suggested that this haze is produced by annihilating dark-matter particles.
However, some researchers have questioned whether the haze actually exists at all, suggesting that it could be an artefact of how the WMAP data were analysed. Doubts were raised as to whether WMAP was capable of picking out this weak signal buried deep in emissions from galactic dust, the cosmic microwave background (CMB) and other noise from hectic regions of the galaxy.
It is definitely there
The argument now seems to have been settled by the latest results from Planck, a European Space Agency mission launched in May 2009. “Crudely speaking, we agree with all the WMAP results,” explains Krzysztof Gorksi of NASA’s Jet Propulsion Laboratory in California, who is a member of the Planck team. “Planck is more sensitive, and has a greater frequency range, taking us into a realm that WMAP couldn’t even see,” he told physicsworld.com. One of the telescope’s main objectives is to accurately map fluctuations in the CMB, so it is well suited to subtracting that radiation to reveal the haze.
With the presence of the haze independently verified, focus has returned to determining its origin. After its original discovery, some researchers, including Dan Hooper of Fermilab near Chicago, US, argued that annihilating dark matter could explain the galactic haze. Dark matter has long been thought to bind galaxies together, but detecting it directly has remained elusive. In Hooper’s mechanism, dark-matter particles annihilate to produce conventional electrons and positrons. These particles then spiral around the Milky Way’s magnetic field to produce the radiation we see as the microwave haze……….
Read more: physicsworld.com
Theories of the primordial Universe predict the existence of knots in the fabric of space – known as cosmic textures – which could be identified by looking at light from the cosmic microwave background (CMB), the relic radiation left over from the Big Bang.
Using data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) satellite, researchers from UCL, Imperial College London and the Perimeter Institute have performed the first search for textures on the full sky, finding no evidence for such knots in space.
As the Universe cooled it underwent a series of phase transitions, analogous to water freezing into ice. Many transitions cannot occur consistently throughout space, giving rise in some theories to imperfections in the structure of the cooling material known as cosmic textures.
If produced in the early Universe, textures would interact with light from the CMB to leave a set of characteristic hot and cold spots. If detected, such signatures would yield invaluable insight into the types of phase transitions that occurred when the Universe was a fraction of a second old, with drastic implications for particle physics.
A previous study, published in Science in 2007, provided a tantalising hint that a CMB feature known as the “Cold Spot” could be due to a cosmic texture. However, the CMB Cold Spot only comprises around 3% of the available sky area, and an analysis using the full microwave sky had not been performed.
The new study, published today in Physical Review Letters, places the best limits available on theories that produce textures, ruling out at 95% confidence theories that produce more than six detectable textures on our sky.
Stephen Feeney, from the UCL Department of Physics and Astronomy and lead author, said: “If textures were observed, they would provide invaluable insight into the way nature works at tremendous energies, shedding light on the unification of the physical forces. The tantalizing hints found in a previous small-scale search meant it was extremely important to carry out this full-sky analysis.”
Co-author Matt Johnson, from the Perimeter Institute, Canada, said: “Although there is no evidence for these objects in the WMAP data, this is not the last word: in a few months we will have access to much better data from the Planck satellite. Whether we find textures in the Planck data or further constrain the theories that produce them, only time will tell!”
Read more: phys.org and arxiv.org
A scientist in the US is arguing that the vacuum should behave as a metamaterial at high magnetic fields. Such magnetic fields were probably present in the early universe, and therefore he suggests that it may be possible to test the prediction by observing the cosmic microwave background (CMB) radiation – a relic of the early universe that can be observed today.
One of 2011’s strangest predictions in physics was the suggestion by Maxim Chernodub of the French National Centre for Scientific Research that, at incredibly high magnetic fields, superconducting states can emerge from the vacuum. This was particularly interesting because one of the main difficulties facing scientists working on traditional superconductivity is preventing superconducting states disappearing in the presence of even moderate magnetic fields….
Read more: physicsworld.com