Archivi tag: Polarization

Alan Guth commenta i risultati di BICEP2

E’ ancora vivo il fermento che ha generato in questi giorni l’annuncio dei ricercatori dell’Harvard CMB Group sull’esperimento BICEP2 in merito alla rivelazione di un segnale presente nella radiazione cosmica di fondo associato al passaggio di onde gravitazionali primordiali, una forte evidenza indiretta dell’inflazione cosmica (post). Il modello inflazionistico fu inizialmente proposto negli anni ’80 da Alan Guth, oggi Victor F. Weisskopf Professor of Physics presso il MIT, che commenta qui di seguito il significato scientifico dei dati ottenuti da BICEP2.

Q: Can you explain the theory of cosmic inflation that you first put forth in 1980?

A: I usually describe inflation as a theory of the “bang” of the Big Bang: It describes the propulsion mechanism that drove the universe into the period of tremendous expansion that we call the Big Bang. In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged. The original Big Bang theory was really a theory of the aftermath of the bang. The universe was already hot and dense, and already expanding at a fantastic rate. The theory described how the universe was cooled by the expansion, and how the expansion was slowed by the attractive force of gravity. Inflation proposes that the expansion of the universe was driven by a repulsive form of gravity. According to Newton, gravity is a purely attractive force, but this changed with Einstein and the discovery of general relativity. General relativity describes gravity as a distortion of spacetime, and allows for the possibility of repulsive gravity. Modern particle theories strongly suggest that at very high energies, there should exist forms of matter that create repulsive gravity. Inflation, in turn, proposes that at least a very small patch of the early universe was filled with this repulsive-gravity material. The initial patch could have been incredibly small, perhaps as small as 10-24 centimeter, about 100 billion times smaller than a single proton. The small patch would then start to exponentially expand under the influence of the repulsive gravity, doubling in size approximately every 10-37 second. To successfully describe our visible universe, the region would need to undergo at least 80 doublings, increasing its size to about 1 centimeter. It could have undergone significantly more doublings, but at least this number is needed. During the period of exponential expansion, any ordinary material would thin out, with the density diminishing to almost nothing. The behavior in this case, however, is very different: The repulsive-gravity material actually maintains a constant density as it expands, no matter how much it expands! While this appears to be a blatant violation of the principle of the conservation of energy, it is actually perfectly consistent. This loophole hinges on a peculiar feature of gravity: The energy of a gravitational field is negative. As the patch expands at constant density, more and more energy, in the form of matter, is created. But at the same time, more and more negative energy appears in the form of the gravitational field that is filling the region. The total energy remains constant, as it must, and therefore remains very small. It is possible that the total energy of the entire universe is exactly zero, with the positive energy of matter completely canceled by the negative energy of gravity. I often say that the universe is the ultimate free lunch, since it actually requires no energy to produce a universe. At some point the inflation ends because the repulsive-gravity material becomes metastable. The repulsive-gravity material decays into ordinary particles, producing a very hot soup of particles that form the starting point of the conventional Big Bang. At this point the repulsive gravity turns off, but the region continues to expand in a coasting pattern for billions of years to come. Thus, inflation is a prequel to the era that cosmologists call the Big Bang, although it of course occurred after the origin of the universe, which is often also called the Big Bang.

Q: What is the new result announced this week, and how does it provide critical support for your theory?

A: The stretching effect caused by the fantastic expansion of inflation tends to smooth things out — which is great for cosmology, because an ordinary explosion would presumably have left the universe very splotchy and irregular. The early universe, as we can see from the afterglow of the cosmic microwave background (CMB) radiation, was incredibly uniform, with a mass density that was constant to about one part in 100,000. The tiny nonuniformities that did exist were then amplified by gravity: In places where the mass density was slightly higher than average, a stronger-than-average gravitational field was created, which pulled in still more matter, creating a yet stronger gravitational field. But to have structure form at all, there needed to be small nonuniformities at the end of inflation. In inflationary models, these nonuniformities — which later produce stars, galaxies, and all the structure of the universe — are attributed to quantum theory. Quantum field theory implies that, on very short distance scales, everything is in a state of constant agitation. If we observed empty space with a hypothetical, and powerful, magnifying glass, we would see the electric and magnetic fields undergoing wild oscillations, with even electrons and positrons popping out of the vacuum and then rapidly disappearing. The effect of inflation, with its fantastic expansion, is to stretch these quantum fluctuations to macroscopic proportions. The temperature nonuniformities in the cosmic microwave background were first measured in 1992 by the COBE satellite, and have since been measured with greater and greater precision by a long and spectacular series of ground-based, balloon-based, and satellite experiments. They have agreed very well with the predictions of inflation. These results, however, have not generally been seen as proof of inflation, in part because it is not clear that inflation is the only possible way that these fluctuations could have been produced. The stretching effect of inflation, however, also acts on the geometry of space itself, which according to general relativity is flexible. Space can be compressed, stretched, or even twisted. The geometry of space also fluctuates on small scales, due to the physics of quantum theory, and inflation also stretches these fluctuations, producing gravity waves in the early universe. The new result, by John Kovac and the BICEP2 collaboration, is a measurement of these gravity waves, at a very high level of confidence. They do not see the gravity waves directly, but instead they have constructed a very detailed map of the polarization of the CMB in a patch of the sky. They have observed a swirling pattern in the polarization (called “B modes”) that can be created only by gravity waves in the early universe, or by the gravitational lensing effect of matter in the late universe. But the primordial gravity waves can be separated, because they tend to be on larger angular scales, so the BICEP2 team has decisively isolated their contribution. This is the first time that even a hint of these primordial gravity waves has been detected, and it is also the first time that any quantum properties of gravity have been directly observed.

Q: How would you describe the significance of these new findings, and your reaction to them?

A: The significance of these new findings is enormous. First of all, they help tremendously in confirming the picture of inflation. As far as we know, there is nothing other than inflation that can produce these gravity waves. Second, it tells us a lot about the details of inflation that we did not already know. In particular, it determines the energy density of the universe at the time of inflation, which is something that previously had a wide range of possibilities. By determining the energy density of the universe at the time of inflation, the new result also tells us a lot about which detailed versions of inflation are still viable, and which are no longer viable. The current result is not by itself conclusive, but it points in the direction of the very simplest inflationary models that can be constructed. Finally, and perhaps most importantly, the new result is not the final story, but is more like the opening of a new window. Now that these B modes have been found, the BICEP2 collaboration and many other groups will continue to study them. They provide a new tool to study the behavior of the early universe, including the process of inflation. When I (and others) started working on the effect of quantum fluctuations in the early 1980s, I never thought that anybody would ever be able to measure these effects. To me it was really just a game, to see if my colleagues and I could agree on what the fluctuations would theoretically look like. So I am just astounded by the progress that astronomers have made in measuring these minute effects, and particularly by the new result of the BICEP2 team. Like all experimental results, we should wait for it to be confirmed by other groups before taking it as truth, but the group seems to have been very careful, and the result is very clean, so I think it is very likely that it will hold up.

Courtesy MIT: 3 Questions: Alan Guth on new insights into the ‘Big Bang’

A subtle distortion in the oldest light of the Universe

La rivista Physics World ha pubblicato le 10 notizie più importanti del 2013 nel campo della fisica tra le quali viene menzionata la scoperta ottenuta dagli scienziati che hanno rivelato per la prima volta delle piccole distorsioni relative alla luce più antica dell’Universo, un risultato che potrebbe fornire nuovi indizi sulle fasi primordiali della storia dell’Universo (post). Le misure si riferiscono alla polarizzazione della radiazione cosmica di fondo, all’epoca durante la quale cioè la luce interagì per l’ultima volta con la materia, circa 380.000 anni dopo il Big Bang. Queste distorsioni, note come “modi-B“, sono dovute al fenomeno della lente gravitazionale che si ha quando la luce viene deflessa da oggetti di grande massa. Nel 2014, si attendono con grande interesse i nuovi dati dal satellite Planck che ci darà delle informazioni ancora più dettagliate sulla polarizzazione della radiazione cosmica di fondo e quindi sulla presenza dei modi-B (vedasi L’Universo Infante).

A multi-institutional collaboration of researchers led by John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics at the University of Chicago, made the discovery. They announced their findings in a paper published in the journal Physical Review Lettersusing the first data from SPTpol, a polarization-sensitive camera installed on the telescope in January 2012. “The detection of B-mode polarization by South Pole Telescope is a major milestone, a technical achievement that indicates exciting physics to come”, said Carlstrom, who also is deputy director of the Kavli Institute for Cosmological Physics. The cosmic microwave background is a sea of photons (light particles) left over from the Big Bang that pervades all of space, at a temperature of minus 270 degrees Celsius, a mere 3 degrees above absolute zero.

Measurements of this ancient light have already given physicists a wealth of knowledge about the properties of the Universe. Tiny variations in temperature of the light have been painstakingly mapped across the sky by multiple experiments, and scientists are gleaning even more information from polarized light.

Light is polarized when its electromagnetic waves are preferentially oriented in a particular direction. Light from the cosmic microwave background is polarized mainly due to the scattering of photons off of electrons in the early Universe, through the same process by which light is polarized as it reflects off the surface of a lake or the hood of a car. The polarization patterns that result are of a swirl-free type, known as “E modes,” which have proven easier to detect than the fainter B modes, and were first measured a decade ago by a collaboration of researchers using the Degree Angular Scale Interferometer, another UChicago-led experiment. Simple scattering can’t generate B modes, which instead emerge through a more complex process, hence scientists’ interest in measuring them.

Gravitational lensing, it has long been predicted, can twist E modes into B modes as photons pass by galaxies and other massive objects on their way toward earth. This expectation has now been confirmed.

To tease out the B modes in their data, the scientists used a previously measured map of the distribution of mass in the Universe to determine where the gravitational lensing should occur. They combined their measurement of E modes with the mass distribution to provide a template of the expected twisting into B modes. The scientists are currently working with another year of data to further refine their measurement of B modes. The careful study of such B modes will help physicists better understand the Universe. The patterns can be used to map out the distribution of mass, thereby more accurately defining cosmologically important properties like the masses of neutrinos, tiny elementary particles prevalent throughout the cosmos. Similar, more elusive B modes would provide dramatic evidence of inflation, the theorized turbulent period in the moments after the Big Bang when the Universe expanded extremely rapidly. Inflation is a well-regarded theory among cosmologists because its predictions agree with observations, but thus far there is not a definitive confirmation of the theory. Measuring B modes generated by inflation is a possible way to alleviate lingering doubt. “The detection of a primordial B-mode polarization signal in the microwave background would amount to finding the first tremors of the Big Bang”, said the study’s lead author, Duncan Hanson, a postdoctoral scientist at McGill University in Canada.

B modes from inflation are caused by gravitational waves. These ripples in space-time are generated by intense gravitational turmoil, conditions that would have existed during inflation. These waves, stretching and squeezing the fabric of the Universe, would give rise to the telltale twisted polarization patterns of B modes.

Measuring the resulting polarization would not only confirm the theory of inflation, a huge scientific achievement in itself, but would also give scientists information about physics at very high energies, much higher than can be achieved with particle accelerators. The measurement of B modes from gravitational lensing is an important first step in the quest to measure inflationary B modes. In inflationary B mode searches, lensing B modes show up as noise. “The new result shows that this noise can be accounted for and subtracted off so that scientists can search for and hopefully measure the inflationary B modes underneath”, Hanson said. “The lensing signal itself can also be used by itself to learn about the distribution of mass in the Universe”.

University of Chicago: Swirls in remnants of Big Bang may hold clues to universe’s infancy

physicsworld.com: Cosmic neutrinos named Physics World 2013 Breakthrough of the Year

SPT detects ‘weak signs’ of primordial gravitational waves in the CMB

spt_polarizationLa ricerca dei modi-B, relativi alla componente della polarizzazione della radiazione cosmica di fondo associata alla propagazione delle onde gravitazionali nella mappa della radiazione cosmica, rappresenta una prova fondamentale che potrebbe dare credito al modello dell’inflazione cosmica. Nonostante i calcoli prevedono una intensità del segnale molto debole, oggi alcuni ricercatori del South Pole Telescope (SPT) hanno pubblicato i dati di uno studio in cui dichiarano di aver rivelato deboli fluttuazioni associate ai modi-B della componente di polarizzazione.

Scientists believe that approximately half a million years after the Big Bang, the Universe began switching from a state of plasma and energy to one where temperatures had dropped to a point where the universe became transparent enough for light to pass through. That light is known as cosmic microwave background (CMB) and is still visible today. Cosmologists studying it have formed the basis of a theory known as inflation, where the Universe came to exist as it does today through a process of very rapid expansion just after the Big Bang. In order to prove that the inflation theory is correct, scientists have been studying minute fluctuations in the temperature of the CMB, they revel fluctuations in density of the early Universe. They also study fluctuations of the polarization of the CMB which is due, it is believed, to radiation being scattered across the Universe by the energy of the Big Bang. Fluctuations in polarization were for a time merely theory, but in 2002, they were actually detected, giving credence to inflation theory. Those fluctuations were given the name E-mode polarizations. Theory has also suggested that there are also B-mode fluctuations in polarization, which are far more subtle, they are thought to describe the rotation of CMB polarization.

Finding evidence of them has been extremely difficult, however, as they exist as just one part in ten million in the CMB temperature distribution. But now it appears the team at SPT has done just that, adding further credence to the inflation theory.

The researchers report that they were able to detect E-mode polarization due mostly to improvements in detector technology. Adding credence isn’t the same as finding proof of a theory, of course, and that’s why scientists believe the detection of E-mode polarizations is so important. Many believe it will ultimately lead to the detection of primordial gravitational waves, immense ripples in spacetime that theory suggests should have come about as a result of the force of inflation. If they can be detected, the theory of inflation would likely become the accepted theory regarding the early formation of the Universe.

Nature: Polarization detected in Big Bang's echo
arXiv: Detection of B-mode Polarization in the Cosmic Microwave Background with Data from the South Pole Telescope

Cosmology in the Planck Era

The Rencontres du Vietnam on Cosmology in the Planck Era will review the most recent status of the field. The conference will consist of plenary sessions for invited indepth oral presentations (review talks and talks on specific specialised topics), and contributed papers, in the form of relatively short oral papers (sollicitated or selected from abstract submission). Special emphasis is being placed on active participation by young researchers and post-docs.

The main topics of the conference are:

  • Planck Cosmology Results
  • Cosmology with wide-field SZ cluster surveys
  • Polarization
  • Supernovae
  • BAO
  • Inflation
  • Modified Gravity
  • Dark matter candidates
  • Dark matter searches

Exascale Radio Astronomy

The topics covered by the next-generation of radio telescopes will touch upon cosmology, dark ages, epoch of reionization, galaxy evolution, dark energy, dark matter, general relativity, cosmic magnetism, and pulsars etc.

Why Exascale Radio Astronomy? (PDF)

Exploring the physics of inflation

The year 2013 will be exciting and full of new results based on the measurement and analysis of CMB temperature fluctuations, polarization, and LSS data. The Planck satellite is expected to provide new cosmological information and release all-sky CMB maps with unprecedented quality in the early part of 2013. These new results, along with the information that will be produced by on-going and planned CMB and LSS experiments will be a very important contribution to improving our knowledge on the inflationary period of the universe. The meeting will focus on the physics of inflation from an observational, theoretical, and instrumental perspective. Special emphasis will be given to results derived from the forthcoming polarization data. The conference takes place in the context of the Spanish Consolider-Ingenio Project known as EPI: Exploring the Physics of Inflation. There will be a limited number of invited talks in order to allow the majority of the time for contributed discussion. The conference will be in Santander, which is located on the northern coast of Spain. Santander is a popular summer destination surrounded by some of the most popular beaches in the north coast. The dates of the conference were chosen to avoid the peak of the tourist season, in order to offer a more relaxing (and affordable) environment.

The innermost regions of relativistic jets and their magnetic fields

The study of relativistic jets in active galactic nuclei and other sources involving the accretion onto compact objects, like gamma-ray bursts and microquasars, has gained added interest thanks to the recent simultaneous multi spectral-range observations and the advances in the theoretical and numerical modeling. During June 10-14th, 2013, the Instituto de Astrofísica de Andalucía-CSIC in Granada, Spain, will host a meeting (poster) aimed to discuss the recent results in the study of relativistic jets. The meeting will focus on the study of the innermost regions of AGN jets to obtain a better understanding of the jet formation mechanisms and determine the origin and location of the high energy emission, as well as the role played by the magnetic field.

Topics to be discussed include:

  • Jet formation
  • Black hole, accretion disk, jet connection
  • Multi-spectral-range emission
  • Magnetic fields and polarization
  • Jet dynamics and stability
  • Unification models, microphysics, particle acceleration
  • Relativistic stellar jets

Cosmic Microwave Background 2013

Detection of primordial gravitational waves would be one of the most significant scientific discoveries of all time. Such a detection would be a direct evidence for inflation and shed light on fundamental physics such as quantum gravity behind inflation. The accurate measurement of CMB polarization is the best way to discover the primordial gravitational waves. It is thus one of the greatest challenges in experimental cosmology in the next decade. Thanks to remarkable technological development in recent years, the detection is not a mere dream anymore but becoming factual. The primary objective of the conference is to exchange ideas on how to reach the precision at the level of r=0.001 or even better, where r is the tensor-to-scalar ratio. Another important goal of the conference is to evaluate scientific values of such a measurement, where relations to other observations such as CMB temperature anisotropies are also important. To these ends, the conference will have talks from all the CMB projects in operation, in preparation or in planning.

The conference consists of plenary sessions and poster sessions. Topics in the plenary sessions will include the following:

– Planck results/instruments
– WMAP9
– On-going ground/balloon CMB experiments
– CMB experiments in preparation
– Future CMB experiments including satellites
– Theory overviews
– Nature of foregrounds and their separation

Talks in the plenary sessions are by invitation only. Poster sessions will be based on abstract submissions to cover a broad range of topics on CMB research, from inflationary model building to superconducting sensor development, which will stimulate vigorous and open discussions among participants. Posters on a wider range of topics related to CMB are also welcomed; examples of such topics are:

– Synergy between CMB and other observations, such as CIRB, 21cm, dark energy and dark matter.
– Astrophysics/astronomy with foregrounds.