Physics Blogs

Squelching Boltzmann Brains (And Maybe Eternal Inflation)

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Posted on May 5, 2014 by Sean Carroll

horizonThere’s no question that quantum fluctuations play a crucial role in modern cosmology, as the recent BICEP2 observations have reminded us. According to inflation, all of the structures we see in the universe, from galaxies up to superclusters and beyond, originated as tiny quantum fluctuations in the very early universe, as did the gravitational waves seen by BICEP2. But quantum fluctuations are a bit of a mixed blessing: in addition to providing an origin for density perturbations and gravitational waves (good!), they are also supposed to give rise to Boltzmann brains (bad) and eternal inflation (good or bad, depending on taste). Nobody would deny that it behooves cosmologists to understand quantum fluctuations as well as they can, especially since our theories involve mysterious aspects of physics operating at absurdly high energies.

Kim Boddy, Jason Pollack and I have been re-examining how quantum fluctuations work in cosmology, and in a new paper we’ve come to a surprising conclusion: cosmologists have been getting it wrong for decades now. In an expanding universe that has nothing in it but vacuum energy, there simply aren’t any quantum fluctuations at all. Our approach shows that the conventional understanding of inflationary perturbations gets the right answer, although the perturbations aren’t due to “fluctuations”; they’re due to an effective measurement of the quantum state of the inflaton field when the universe reheats at the end of inflation. In contrast, less empirically-grounded ideas such as Boltzmann brains and eternal inflation both rely crucially on treating fluctuations as true dynamical events, occurring in real time — and we say that’s just wrong.



All very dramatically at odds with the conventional wisdom, if we’re right. Which means, of course, that there’s always a chance we’re wrong (although we don’t think it’s a big chance). This paper is pretty conceptual, which a skeptic might take as a euphemism for “hand-waving”; we’re planning on digging into some of the mathematical details in future work, but for the time being our paper should be mostly understandable to anyone who knows undergraduate quantum mechanics.

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Physicists confirm existence of new type of meson

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syracuseunivPhysicists in the College of Arts and Sciences at Syracuse University have made several important discoveries regarding the basic structure of mesons—subatomic particles long thought to be composed of one quark and one antiquark and bound together by a strong interaction.

Recently, Professor Tomasz Skwarnicki and a team of researchers proved the existence of a meson named Z(4430), with two quarks and two antiquarks, using data from the Large Hadron Collidor beauty (LHCb) Collaboration at CERN in Geneva, Switzerland. This tetraquark state was first discovered in Japan in 2007 but was later disputed by a team of researchers at Stanford University. Skwarnicki's finding was published earlier this month and has since garnered international publicity.
Quarks are hard, point-like objects that are found inside protons and neutrons and form the nucleus of an atom.

Now, another analysis by Syracuse University physicists—this one led by Distinguished Professor Sheldon Stone and his research associate Liming Zhang—shows two lighter, well-known mesons, originally thought to be composed of tetraquarks, that are structured like normal mesons.
Stone says that one of the particles, uniquely named the f0(980), was assumed to have four quarks because it seemed to be the only way for its mass to "make sense."
"The four-quark states cannot be classified within the traditional quark model, where strongly interacting particles [hadrons] are formed from either quark-antiquarks pairs [mesons] or three quarks [baryons]," says Stone, who also heads up Syracuse University's High-Energy Physics Group. "They are, therefore, called 'exotic particles.'"
Stone points out that his and Skwarnicki's analyses are not contradictory and, together, increase what physicists know about the strong interaction that forms the basis of what holds all matter together.

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Is time quantized? In other words, is there a fundamental unit of time that could not be divided into a briefer unit?

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John Baez is a member of the mathematics faculty at the University of California at Riverside and one of the moderators of the on-line sci.physics.research newsgroup. He responds:

"The brief answer to this question is, 'Nobody knows.' Certainly there is no experimental evidence in favor of such a minimal unit. On the other hand, there is no evidence against it, except that we have not yet found it. There are no well-worked-out physics theories incorporating a fundamental unit of time, and there are substantial obstacles to doing so in a way that is compatible with the principles of General Relativity. Recent work on a theory of quantum gravity in which gravity is represented using loops in space suggests that there might be a way to do something roughly along these lines--not involving a minimum unit of time but rather a minimum amount of area for any two-dimensional surface, a minimum volume for any three-dimensional region in space and perhaps also a minimum 'hypervolume' for any four-dimensional region of space-time."


Time: Is it real or an illusion of the mind?

Time: Is it real or an illusion of the mind?

William G. Unruh is a professor in the department of physics and astronomy at the University of British Columbia. He offers this reply:

"There is certainly no experimental evidence that time--or space for that matter--is quantized, so the question becomes one of whether there exists a theory in which time is quantized. Although researchers have considered some theories in which there is a strict quantization of time (meaning that all times are an integer multiple of some smallest unit), none that I know of has ever been seriously regarded as a viable theory of reality--at least, not by more people that the original proponent of the theory.

"One could, however, ask the question in a slightly different way. By putting together G (Newton's constant of gravity), h (Planck's constant) and c (the velocity of light), one can derive a minimum meaningful amount of time, about 10-44 second. At this temporal scale, one would expect quantum effects to dominate gravity and hence, because Einstein's theory links gravity and time, to dominate the ordinary notion of time. In other words, for time intervals smaller than this one, the whole notion of 'time' would be expected to lose its meaning.

"The biggest obstacle to answering the question definitively is that there exists no really believable theory to describe this regime where quantum mechanics and gravity come together. Over the past 10 years, a branch of theoretical physics called string theory has held forth the greatest hope, but it is as yet far from a state where one could use it to describe the nature of time in such a brief interval."

Another, somewhat iconoclastic perspective on this question comes from William G. Tifft, a professor of astronomy at the University of Arizona:

"There are several ways to answer this question. 1) There is no conclusive evidence that time is quantized, but 2) certain theoretical studies suggest that in order to unify general relativity (gravitation) with the theories of quantum physics that describe fundamental particles and forces, it may be necessary to quantize space and perhaps time as well. Time is always a 1-dimensional quantity in this case. 3) My own work, which combines new theoretical ideas with observations of the properties of galaxies, fundamental particles and forces, does suggest that in a certain sense time may indeed be quantized. To see this we need some background information; in this scenario, time is no longer 1-dimensional!

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Inside Black Holes

Three recent black hole events and how they shape our universe

By Helen Thompson  May 17, 2014

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A combination of infrared and X-ray observations indicates that a surplus of massive stars has formed from a large disk of gas around Sagittarius A*. (X-ray: NASA/CXC/MIT/F.K.Baganoff)

222689main_sagittariusa_20080415_hi.jpg__800x600_q85_cropNear the middle of the Milky Way, there lives a supermassive black hole called Sagittarius A* (Sgr A) that any day now might eat a gas cloud (called G2) that’s floating towards it at 5 million miles per hour.

Here on earth, we have a front row seat to this extremely rare galactic event that will have ripple effects throughout our galaxy.

“As it veers toward the black hole, the doomed cloud will shred and stretch into a piece of string over 100 billion miles long,” explains Dan Evans, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Boston. Evans spoke at Smithsonian’s Future Is Here conference today and gave attendees a tour of black hole events in our galaxy.

First, what exactly is a black hole? It’s a spot in space where a huge amount matter is extremely compressed and the gravitational forces around this spot of matter are so strong that they trap light, hence the name. “Black holes are extremely simple and extremely powerful,” he says. Astronomers characterize black holes based on three key factors: mass, spin, and electrical charge. On the other hand, a black hole feeding on matter emits the same amount of energy as one billion trillion hydrogen bombs per second. This is called accretion, and here’s a simulation of what that might look like:

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Radiation from early universe may help prove Albert Einstein's theory of general relativity

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236175-albert-einsteinResearchers have measured the minute gravitational distortions in polarized radiation from the early universe and discovered that these ancient microwaves can provide a 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 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.

The findings are set to be published in the journal Physical Review Letters.

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