Radiation from early universe found key to answer major questions in physics

The UC San Diego astrophysicists employed the HuanTran Telescope in Chile to measure the polarization of the cosmic microwave background. Credit: POLARBEAR

Astrophysicists at UC San Diego have measured the minute gravitational distortions in polarized radiation from the early universe and discovered that these ancient microwaves can provide an important cosmological test of Einstein's theory of general relativity. These measurements have the potential to narrow down the estimates for the mass of ghostly subatomic particles known as neutrinos.

The radiation could even provide physicists with clues to another outstanding problem about our universe: how the invisible "dark matter" and "dark energy," which has been undetectable through modern telescopes, may be distributed throughout the universe.

The scientists are publishing details of their achievement in the June issue of the journal Physical Review Letters.

The UC San Diego scientists measured variations in the polarization of microwaves emanating from the Cosmic Microwave Background -- or CMB -- of the early universe. Like polarized light (which vibrates in one direction and is produced by the scattering of visible light off the surface of the ocean, for example), the polarized "B-mode" microwaves the scientists discovered were produced when CMB radiation from the early universe scattered off electrons 380,000 years after the Big Bang, when the cosmos cooled enough to allow protons and electrons to combine into atoms.

Astronomers had hoped the unique B-mode polarization signature from the early cosmos would allow them to effective "see" portions of the universe that are invisible to optical telescopes as gravity from denser portions of the universe tug on the polarized light, slightly deflecting its passage through the cosmos during its 13.8 billion year trip to Earth. Through a process called "weak gravitational lensing," the distortions in the B-mode polarization pattern, they hoped, would allow astronomers to map regions of the universe filled with invisible "dark matter" and "dark energy" and well as provide a test for general relativity on cosmological scales.

The recent discovery confirms both hunches. By measuring the CMB polarization data provided by POLARBEAR, a collaboration of astronomers working on a telescope in the high-altitude desert of northern Chile designed specifically to detect "B-mode" polarization, the UC San Diego astrophysicists discovered weak gravitational lensing in their data that, they conclude, permit astronomers to make detailed maps of the structure of the universe, constrain estimates of neutrino mass and provide a firm test for general relativity.

"This is the first time we've made these kinds of measurements using CMB polarization data," said Chang Feng, the lead author of the paper and a physics graduate student at UC San Diego who conducted his study with Brian Keating, an associate professor of physics at the university and a co-leader of the POLARBEAR experiment. "This was the first direct measurement of CMB polarization lensing. And the amazing thing is that the amount of lensing that we found through these calculations is consistent with what Einstein's general relativity theory predicted. So we now have a way to verify general relativity on cosmological scales."

The POLARBEAR experiment examined a small (30 degree square) region of the sky to produce high resolution maps of B-mode polarization, which enabled the team to determine that the amplitude of gravitational fluctuations they measured was consistent with the leading theoretical model of the universe, known as the Lambda Cold Dark Matter cosmological model. Another team Keating's group collaborates with, based at the Harvard-Smithsonian Center for Astrophysics, called BICEP2, used a telescope at the South Pole to examine B-mode polarization across wide swaths of the sky. In March, it announced it had found evidence for a brief and very rapid expansion of the early universe, called inflation.

One of the most important questions in physics that can be addressed from these data is the mass of the weakly interacting neutrino, which was thought to have no mass, but current limits indicate that neutrinos have masses below 1.5 electron volts. Feng said the B-mode polarization data in his study, while consistent with the predictions of general relativity, are not statistically significant enough yet to make any firm claims about neutrino masses. But over the next year, he and Keating hope to analyze enough data from POLARBEAR, and its successor instrument -- the Simons Array -- to provide more certainty about the masses of neutrinos.

"This study is a first step toward using polarization lensing as a probe to measure the mass of neutrinos, using the whole universe as a laboratory," Feng said.

"Eventually we will be able to put enough neutrinos on a 'scale' to weigh them -- precisely measuring their mass," Keating says. "Using the tools Chang has developed, it's only a matter of time before we can weigh the neutrino, the only fundamental elementary particle whose mass is unknown. That would be an astounding achievement for astronomy, cosmology and physics itself."

The study was supported by grants from the National Science Foundation, National Aeronautics and Space Administration, the Simmons Foundation, and Irwin and Joan Jacobs.

Source: University of California - San Diego

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Plugging the hole in Hawking's black hole theory

Recently physicists have been poking holes again in Stephen Hawking's black hole theory -- including Hawking himself. For decades physicists across the globe have been trying to figure out the mysteries of black holes -- those fascinating monstrous entities that have such intense gravitational pull that nothing -- not even light -- can escape from them. Credit: Still image from video courtesy of Michigan State University

Recently physicists have been poking holes again in Stephen Hawking's black hole theory -- including Hawking himself. For decades physicists across the globe have been trying to figure out the mysteries of black holes -- those fascinating monstrous entities that have such intense gravitational pull that nothing -- not even light -- can escape from them. Now Professor Chris Adami, Michigan State University, has jumped into the fray.

The debate about the behavior of black holes, which has been ongoing since 1975, was reignited when Hawking posted a blog on Jan. 22, 2014, stating that event horizons -- the invisible boundaries of black holes -- do not exist.

Hawking, considered to be the foremost expert on black holes, has over the years revised his theory and continues to work on understanding these cosmic puzzles.

One of the many perplexities is a decades-old debate about what happens to information -- matter or energy and their characteristics at the atomic and subatomic level -- in black holes.

"In 1975, Hawking discovered that black holes aren't all black. They actually radiate a featureless glow, now called Hawking radiation," Adami said. "In his original theory, 

Hawking stated that the radiation slowly consumes the black hole and it eventually evaporates and disappears, concluding that information and anything that enters the black hole would be irretrievably lost."

But this theory created a fundamental problem, dubbed the information paradox.

Now Adami believes he's solved it.

"According to the laws of quantum physics, information can't disappear," Adami said. "A loss of information would imply that the universe itself would suddenly become unpredictable every time the black hole swallows a particle. That is just inconceivable. No law of physics that we know allows this to happen."

So if the black hole sucks in information with its intense gravitational pull, then later disappears entirely, information and all, how can the laws of quantum physics be preserved?

The solution, Adami says, is that the information is contained in the stimulated emission of radiation, which must accompany the Hawking radiation -- the glow that makes a black hole not so black. Stimulated emission makes the black hole glow in the information that it swallowed.

"Stimulated emission is the physical process behind LASERS (Light Amplification by Stimulated Emission of Radiation). Basically, it works like a copy machine: you throw something into the machine, and two identical somethings come out.

"If you throw information at a black hole, just before it is swallowed, the black hole first makes a copy that is left outside. This copying mechanism was discovered by Albert Einstein in 1917, and without it, physics cannot be consistent," Adami said.

Do others agree with Adami's theory that stimulated emission is the missing piece that solves the information paradox?

According to Paul Davies, cosmologist, astrobiologist and theoretical physicist at Arizona State University, "In my view Chris Adami has correctly identified the solution to the so-called black hole information paradox. Ironically, it has been hiding in plain sight for years. Hawking's famous black hole radiation is an example of so-called spontaneous emission of radiation, but it is only part of the story. There must also be the possibility of stimulated emission -- the process that puts the S in LASER."

With so many researchers trying to fix Hawking's theory, why did it take so long if it was hiding in plain sight?

"While a few people did realize that the stimulated emission effect was missing in Hawking's calculation, they could not resolve the paradox without a deep understanding of quantum communication theory," Adami said. Quantum communication theory was designed to understand how information interacts with quantum systems, and Adami was one of the pioneers of quantum information theory back in the '90s.

Trying to solve this information paradox has kept Adami awake many nights as demonstrated by his thick notebooks filled with 10 years of mathematical calculations.

So where does this leave us, according to Adami?

"Stephen Hawking's wonderful theory is now complete in my opinion. The hole in the black hole theory is plugged, and I can now sleep at night," he said.

Adami may now sleep well at night, but his theory is sure to keep other physicists up trying to confirm whether he has actually solved the mystery.

The study was co-authored by Greg Ver Steeg, University of Southern California and is published online in the journal Classical and Quantum Gravity.

Source: Michigan State University

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Pulsars with black holes could hold the 'Holy Grail' of gravity

Discovering a pulsar orbiting a black hole could be the ‘holy grail’ for testing gravity.
Credit: SKA Organisation/Swinburne Astronomy Productions

The intermittent light emitted by pulsars, the most precise timekeepers in the universe, allows scientists to verify Einstein's theory of relativity, especially when these objects are paired up with another neutron star or white dwarf that interferes with their gravity. However, this theory could be analysed much more effectively if a pulsar with a black hole were found, except in two particular cases, according to researchers from Spain and India.

Pulsars are very dense neutron stars that are the size of a city (their radius approaches ten kilometres), which, like lighthouses for the universe, emit gamma radiation beams or X-rays when they rotate up to hundreds of times per second. These characteristics make them ideal for testing the validity of the theory of general relativity, published by Einstein between 1915 and 1916.

"Pulsars act as very precise timekeepers, such that any deviation in their pulses can be detected," Diego F. Torres, ICREA researcher from the Institute of Space Sciences (IEEC-CSIC), explains. "If we compare the actual measurements with the corrections to the model that we have to use in order for the predictions to be correct, we can set limits or directly detect the deviation from the base theory."

These deviations can occur if there is a massive object close to the pulsar, such as another neutron star or a white dwarf. A white dwarf can be defined as the stellar remnant left when stars such as our Sun use up all of their nuclear fuel. The binary systems, composed of a pulsar and a neutron star (including double pulsar systems) or a white dwarf, have been very successfully used to verify the theory of gravity.

Last year, the very rare presence of a pulsar (named SGR J1745-2900) was also detected in the proximity of a supermassive black hole (Sgr A*, made up of millions of solar masses), but there is a combination that is still yet to be discovered: that of a pulsar orbiting a 'normal' black hole; that is, one with a similar mass to that of stars.

Until now scientists had considered this strange pair to be an authentic 'holy grail' for examining gravity, but there exist at least two cases where other pairings can be more effective. This is what is stated in the study that Torres and the physicist Manjari Bagchi, from the International Centre of Theoretical Sciences (India) and now postdoc at the IEEC-CSIC, have published in the Journal of Cosmology and Astroparticle Physics. The work also received an Honourable Mention in the 2014 Essays of Gravitation prize.

The first case occurs when the so-called principle of strong equivalence is violated. This principle of the theory of relativity indicates that the gravitational movement of a body that we test only depends on its position in space-time and not on what it is made up of, which means that the result of any experiment in a free fall laboratory is independent of the speed of the laboratory and where it is found in space and time.

The other possibility is if one considers a potential variation in the gravitational constant that determines the intensity of the gravitational pull between bodies. Its value is G = 6.67384(80) x 10-11 N m2/kg2. Despite it being a constant, it is one of those that is known with the least accuracy, with a precision of only one in 10,000.

In these two specific cases, the pulsar-black hole combination would not be the perfect 'holy grail', but in any case scientists are anxious to find this pair, because it could be used to analyse the majority of deviations. In fact, it is one of the desired objectives of X-ray and gamma ray space telescopes (such as Chandra, NuStar or Swift), as well as that of large radio telescopes that are currently being built, such as the enormous 'Square Kilometre Array' (SKA) in Australia and South Africa.

Source: Plataforma SINC

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