loop_qg

La strada verso l’unificazione delle due più grandi teorie della fisica moderna

Albert Einstein e Niels Bohr
Sung-Sik Lee, un professore associato di fisica del Perimeter Institute, ritiene che uno dei metodi utilizzati per lo studio della materia potrebbe costituire la chiave verso l’unificazione della relatività generale e della meccanica quantistica. Sappiamo che da un lato la relatività generale descrive il moto dei pianeti attorno al Sole mentre la meccanica quantistica ci spiega come si muovono gli elettroni attorno al nucleo atomico. Ad oggi, possiamo affermare senza alcun dubbio che entrambe le teorie rappresentano due grandi trionfi della fisica moderna e le loro previsioni sono state varie volte verificate sperimentalmente. Ma c’è un problema: non possiamo utilizzarle insieme.

La relatività ci spiega che lo spaziotempo è “liscio” e “regolare” e solo oggetti di grande massa possono “piegarlo” esattamente nel modo che conosciamo. La teoria quantistica, invece, ci dice che i costituenti più piccoli dell’Universo sono in continua “fluttuazione” e si trovano sempre in uno stato di indeterminazione. La domanda è: come è possibile che qualcosa possa essere contemporaneamente ‘ferma’ e ‘fluttuante’ ma anche trovarsi ‘esattamente’ e con un certo ‘grado di incertezza’, ad esempio, in una determinata posizione? In altre parole, come facciamo ad elaborare una teoria che comprenda le regole della relatività generale e della meccanica quantistica, permettendoci cioè di descrivere la gravità su scale quantistiche? Oggi non lo sappiamo. Certamente, i fisici che si occupano di stringhe o di gravità quantistica a loop hanno fatto grandi progressi ma è anche vero che da quasi un secolo non abbiamo ancora una teoria unificata che sia in grado di descrivere completamente le leggi della fisica.

Perimeter Institute: MAPPING THE ROAD TO QUANTUM GRAVITY
arXiv: Quantum renormalization group and holography


Per approfondire questo ed altri argomenti: Idee sull’Universo

darkenergy_universe

BOSS quasars yield a precise determination of cosmic expansion

An artist’s conception of how BOSS uses quasars to measure the distant universe. Light from distant quasars is partly absorbed by intervening gas, which is imprinted with a subtle ring-like pattern of known physical scale. Astronomers have now measured this scale with an accuracy of two percent, precisely measuring how fast the universe was expanding when it was just 3 billion years old. (Illustration by Zosia Rostomian, Lawrence Berkeley National Laboratory, and Andreu Font-Ribera, BOSS Lyman-alpha team, Berkeley Lab.)
La survey del cielo denominata Baryon Oscillation Spectroscopic Survey (BOSS), che rappresenta la parte più grande della terza survey Sloan Digital Sky Survey (SDSS-III), ha osservato i quasar distanti per realizzare una mappatura delle variazioni di densità del gas intergalattico a redshift elevati permettendo così di tracciare la struttura dell’Universo primordiale. BOSS ci fornisce da un lato una carta temporale della storia evolutiva dell’Universo al fine di avere maggiori indizi sulla natura dell’energia scura e dall’altro ci permette di realizzare nuove misure della struttura su larga scala, le più precise mai ottenute sull’espansione cosmica sin dall’epoca in cui si sono formate le prime galassie.

The latest quasar results combine two separate analytical techniques. A new kind of analysis, led by physicist Andreu Font-Ribera of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and his team, was published late last year. Analysis using a tested approach, but with far more data than before, has just been published by Timothée Delubac, of EPFL Switzerland and France’s Centre de Saclay, and his team.

The two analyses together establish the expansion rate at 68 kilometers per second per million light years at redshift 2.34, with an unprecedented accuracy of 2.2 percent.

This means if we look back to the universe when it was less than a quarter of its present age, we’d see that a pair of galaxies separated by a million light years would be drifting apart at a velocity of 68 kilometers a second as the Universe expands”, says Font-Ribera, a postdoctoral fellow in Berkeley Lab’s Physics Division. “The uncertainty is plus or minus only a kilometer and a half per second”. BOSS employs both galaxies and distant quasars to measure baryon acoustic oscillations (BAO), a signature imprint in the way matter is distributed, resulting from conditions in the early Universe. While also present in the distribution of invisible dark matter, the imprint is evident in the distribution of ordinary matter, including galaxies, quasars, and intergalactic hydrogen. “Three years ago BOSS used 14,000 quasars to demonstrate we could make the biggest 3D maps of the universe”, says Berkeley Lab’s David Schlegel, principal investigator of BOSS. “Two years ago, with 48,000 quasars, we first detected baryon acoustic oscillations in these maps. Now, with more than 150,000 quasars, we’ve made extremely precise measures of BAO”. The BAO imprint corresponds to an excess of about five percent in the clustering of matter at a separation known as the BAO scale.

Recent experiments including BOSS and the Planck satellite study of the cosmic microwave background put the BAO scale, as measured in today’s Universe, at very close to 450 million light years, a “standard ruler” for measuring expansion.

BAO directly descends from pressure waves (sound waves) moving through the early Universe, when particles of light and matter were inextricably entangled; 380,000 years after the Big Bang, the Universe had cooled enough for light to go free. The cosmic microwave background radiation preserves a record of the early acoustic density peaks; these were the seeds of the subsequent BAO imprint on the distribution of matter.

Previous work from BOSS used the spectra of over a million galaxies to measure the BAO scale with a remarkable one percent accuracy. But beyond redshift 0.7 (roughly six billion light years distant), galaxies become fainter and more difficult to see. For much higher redshifts like those in the present studies, averaging 2.34, BOSS pioneered the “Lyman-alpha forest” method of using spectra from distant quasars to calculate the density of intergalactic hydrogen. As the light from a distant quasar passes through intervening hydrogen gas, patches of greater density absorb more light. The absorption lines of neutral hydrogen in the spectrum (Lyman-alpha lines) pinpoint each dense patch by how much they are redshifted. There are so many lines in such a spectrum, in fact, that it resembles a forest, the Lyman-alpha forest. With enough good quasar spectra, close enough together, the position of the gas clouds can be mapped in three dimensions, both along the line of sight for each quasar and transversely among dense patches revealed by other quasar spectra. From these maps the BAO signal is extracted. Although introduced by BOSS only a few years ago, this method of using Lyman-alpha forest data, called autocorrelation, by now seems almost traditional. The just-published autocorrelation results by Delubac and his colleagues employ the spectra of almost 140,000 carefully selected BOSS quasars. Font-Ribera and his colleagues determine BAO using even more BOSS quasars in a different way.

Quasars are young galaxies powered by massive black holes, extremely bright, extremely distant, and thus highly redshifted.

Instead of comparing spectra to other spectra, Font-Ribera’s team correlated quasars themselves to the spectra of other quasars, a method called cross-correlation. “Quasars are massive galaxies, and we expect them to be in the denser parts of the Universe, where the density of the intergalactic gas should also be higher”, says Font-Ribera. “Therefore we expect to find more of the absorbing gas than average when we look near quasars”. The question was whether the correlation would be good enough to see the BAO imprint. Indeed the BAO imprint in cross-correlation was strong. Delubac and his team combined their autocorrelation results with the cross-correlation results of Font-Ribera and his team, and they converged on narrow constraints for the BAO scale.

Autocorrelation and cross-correlation also converged in the precision of their measures of the Universe’s expansion rate, called the Hubble parameter. At redshift 2.34, the combined measure was equivalent to 68 plus or minus 1.5 kilometers per second per million light years.

It’s the most precise measurement of the Hubble parameter at any redshift, even better than the measurement we have from the local universe at redshift zero”, says Font-Ribera. “These results allow us to study the geometry of the Universe when it was only a fourth its current age. Combined with other cosmological experiments, we can learn about dark energy and put tight constraints on the curvature of the Universe, it’s very flat!” David Schlegel remarks that when BOSS was first getting underway, the cross-correlation technique had been suggested, but “Some of us were afraid it wouldn’t work. We were wrong. Our precision measures are even better than we optimistically hoped for”.

LBL: BOSS Quasars Track the Expanding Universe – the Most Precise Measurement Yet
arXiv: Baryon Acoustic Oscillations in the Lyα forest of BOSS DR11 quasars
arXiv: Quasar-Lyman α Forest Cross-Correlation from BOSS DR11: Baryon Acoustic Oscillations
kepler_186f

La scoperta di Kepler-186f conferma l’esistenza di pianeti terrestri nella zona abitabile

The artist’s concept depicts Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth. The artistic concept of Kepler-186f is the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. Credit: Danielle Futselaar
Nonostante la recente scoperta di un pianeta extraterrestre delle dimensioni della Terra abbia fatto il giro del web, siamo ancora lontani dall’affermare che possa essere potenzialmente abitabile e perciò ospitare qualche forma di vita intelligente.

Denominato con la sigla Kepler-186f, si tratta del primo corpo celeste in orbita attorno ad una stella nana rossa ad una distanza ideale, nota come zona abitabile, dove l’acqua potrebbe esistere allo stato liquido, una condizione necessaria per lo sviluppo di qualche forma di vita, sia essa primitiva o complessa, almeno come noi la conosciamo. Inoltre, dobbiamo ammettere che potremo non essere in grado di saperlo poichè il pianeta non solo è distante dalla Terra, trovandosi a circa 500 anni-luce, ma risulta estremamente debole per effettuare delle eventuali “osservazioni dirette”. Non sappiamo di certo se la sua superficie sia rocciosa, nè se esista una atmosfera o di cosa sia fatta o se c’è in definitiva acqua sulla superficie.

Oggi, con l’attuale tecnologia non siamo ancora in grado di analizzare lo spettro dell’atmosfera di un esopianeta e non siamo nemmeno vicini a realizzare queste misure estremamente complesse. Forse dovremo aspettare la prossima generazione di telescopi spaziali, diciamo tra 10 o 20 anni. Nel frattempo qui sulla Terra si prepara ad entrare in funzione NESSI, cioè New Mexico Exoplanet Spectroscopic Survey Instrument, che avrà lo scopo di “assaggiare”, per così dire, gli esopianeti fornendo preziosi indizi sulla loro composizione chimica.

Insomma, stiamo parlando del primo corpo celeste delle dimensioni dei pianeti terrestri che si trova proprio nella zona abitabile ma è anche vero che le cose potrebbero cambiare presto. La ricerca di un pianeta che possa ospitare forme di vita intelligenti rimane ancora un obiettivo molto lontano, pura ricerca accademica, poichè non esiste un sistema solare alieno che si trovi abbastanza vicino da permettere una sorta di viaggio spaziale all’umanità, a meno che non avremo inventato tra qualche decina di anni i viaggi nel tempo. Viaggiando ad una velocità prossima a quella della luce, per raggiungere Kepler-186f  e fare ritorno a terra occorrerebbero più di 1000 anni.

NASA: NASA's Kepler Discovers First Earth-Size Planet In The 'Habitable Zone' of Another Star
AFP: Quest for extraterrestrial life not over: experts say
Gemini Observatory: FIRST POTENTIALLY HABITABLE EARTH-SIZED PLANET CONFIRMED BY GEMINI AND KECK OBSERVATORIES
Digital press-kit: Kepler-186f: The First Earth-size Habitable Zone Planet of Another Star
Video: Animation depicting Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone
einstein_board

Testing different aspects of the twin paradox using a physical system

Immaginiamo due gemelli, identici, tranne per il fatto che uno dei due possieda una navicella spaziale. Quest’ultimo decide di fare un viaggio verso una stella distante, diciamo qualche decina di anni-luce, mentre l’altro gemello rimane a terra. Viaggiando con una velocità pari a circa il 75% -85% rispetto a quella della luce, il gemello raggiunge la stella e fa ritorno a terra, incontrando il proprio gemello decisamente più vecchio di lui. Questo fenomeno, noto come paradosso dei due gemelli, è dovuto alla dilatazione dei tempi come viene descritto nella teoria della relatività speciale. Di fatto, Albert Einstein predisse che orologi soggetti ad accelerazioni diverse misurano il tempo in maniera differente. Per quanto strano possa sembrare, l’effetto della dilatazione dei tempi è stato verificato più volte in laboratorio e viene continuamente utilizzato nei sistemi GPS.

The GPS is able to provide you with your position by timing very precisely the signals emitted by satellites, and to this end it needs to take into account the time dilation due to the different accelerations of the satellites. While GPS is one of the most precise systems we have, it can locate your smartphone with an error margin of a few metres. The precision could be improved by using the most precise clocks that we know on Earth, known as quantum clocks because they are ruled by the laws of quantum mechanics. There are plans funded by space agencies to launch these clocks into orbit. It is natural to think that a GPS consisting of quantum clocks would also need to take into account relativistic effects.

However, we do not fully understand how to combine quantum mechanics and relativity. The inability of unifying both theories remains as one of the biggest challenges of modern science.

Predictions in the 1970s said that there is a physical phenomenon that is both quantum and relativistic called the Dynamical Casimir Effect. But it wasn’t until 2011 that an experimental setup could be developed to test the prediction. Here is what theory predicted: if light is trapped between mirrors that move at velocities close to the speed of light, then they will generate more light than there is in the system. Even if initially there is no light between the mirrors, just vacuum, light shows up because the mirror turns the quantum vacuum into particles. This is supposed to happen because vacuum at the quantum level is like a sea of pairs of particles that are constantly emitting and absorbing light. They do this at incredible speeds, but if the mirror moves that fast too some of these particles are reflected by the mirror before disappearing and can be observed. But setting up such a system has proved difficult. In 2011, this difficulty was circumvented in the experiment conducted by Per Delsing at Chalmers University of Technology in Sweden. In this case the mirrors were different. They were magnetic fields inside a Superconducting Quantum Interferometric Device (SQUID), but they behaved exactly like mirrors, making light bounce back and forth. Unlike physical mirrors, these magnetic fields could be moved at incredible speeds. Einstein used to think of clocks as light going back and forth between mirrors. Time can be inferred from the distance between the mirrors divided by the speed of light, which remains constant no matter what. But he never thought about particles being created by motion, a prediction that was made many years after his death.

In a recent work, with colleagues at the University of Nottingham, Chalmers University and University of Warsaw, we have taken inspiration from the 2011 experiment.

We propose using a similar setup to test different aspects of the twin paradox using a physical system, which haven’t been tested so far.

Although it won’t involve human twins, the possibility of achieving enormous speeds and acceleration allows the observation of time dilation in a very short distance. Also, all previous experiments that have tested the theory have involved atomic clocks, which are “point-clocks”, that is, what measures time in these atomic clocks is confined to a tiny point in space. Our experiment would instead use something that has finite length. This is important because, along with time, Einstein’s theory predicts that length of the object changes too. We believe our experiment would test that aspect of the theory for the first time. We have found that particle creation by motion, which was observed in 2011, has an effect on the difference in time between the clock that is moving and the one that is static. Using this setup, while we can reconfirm that time dilation occurs, the more interesting application would be to help build better quantum clocks, by means of a better understanding of the interplay between quantum and relativistic effects.

The Conversation: How to test the twin paradox without using a spaceship

arXiv: The twin paradox with macroscopic clocks in superconducting circuits
astronomy_meeting

Observing the Universe with the Cosmic Microwave Background

The Planck satellite mission has provided a multifrequency detailed view of the Universe at millimeter waves, exploring the cosmic microwave background (CMB) and the relevant foregrounds with an unprecedented combination of sensitivity, angular resolution and frequency coverage. Meanwhile, a number of ground based and balloon-borne experiments are exploring the tiniest details of the CMB (anisotropy, polarization, spectral anisotropy, etc.) providing a wealth of new knowledge on our universe. New space mission concepts have also been proposed, involving significant technology improvements, and are actively investigated.

This school will provide an up to date review of the latest results and of their impact on cosmology and on fundamental physics. Experimental, interpretation and theoretical activities will be weighted to provide a well balanced understanding of the current status and of the forthcoming efforts in this field.

Le idee degli scienziati sull'Universo | Scientists' ideas about the Universe

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