Hot Topics in General Relativity and Gravitation

In this centennial anniversary of General Relativity, we will hold a conference on Hot Topics in General Relativity and Gravitation with the motivation to emphasize the tremendous progresses that have been made in Astrophysics and in Cosmology since Einstein’s discovery of General Relativity (GR) in 1915. This international conference will be held at ICISE as part of the Rencontres du Vietnam.

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9th International Conference on Interconnections between Particle Physics and Cosmology

Recent advances in observational astronomy and the discovery of 125-GeV Higgs boson have brought paradigm shifts on the potential connections between new fundamental particles and our understanding of their impact on the early universe and its evolution. With the content of the universe well known from astrophysical observations, a key aspect is that 27% of the universe appears to consist of dark matter. If current theories are correct, the particle physics candidate for this matter may well be observed in ongoing direct and/or indirect dark matter detection experiments or at the LHC. In addition, about 69% of the universe, the dark energy, still remains a significant mystery that major theoretical attempts are trying to understand.

Symmetries and Phases in the Universe

Cosmology is a broad interdisciplinary research discipline involving different fields of physics. The main goal of the Excellence Cluster Universe is to face the fundamental challenges of modern cosmology by bridging the gap between the astrophysics and the particle/nuclear physics communities.

Cosmology and Structure Formation

Our understanding of the cosmos has been making rapid progress thanks to an interplay of recent precision cosmological observations of the CMB and LSS, and advancement in theoretical understanding. The aim of this 6th KIAS workshop is to review the recent progress in the observational results and our current state of understanding of them from a theoretical perspective, in particular, within the framework of the standard Lambda CDM model and inflation.

Cosmological Quests for the Next Decade

The inaugural cosmology conference in April 2014 will celebrate 40th anniversary of KASI. The workshop will cover recent progresses in observational and theoretical cosmology including the galaxies and large-scale structures, peculiar velocities, cosmic microwave background radiation, type Ia supernovae and gravitational lensing on the observational side, and the early universe, inflation, dark energy, dark matter, non-Gaussianity and numerical simulation in the theoretical side. Through close assessment of the present data and our current understanding we will be able to make plans for opening future windows in studying and describing our universe.

Using the Universe as ‘tool’ to detect gravitons

La gravità è l’unica tra le quattro forze fondamentali per la quale gli scienziati non hanno ancora rivelato la sua unità fondamentale. Infatti, secondo il modello standard delle particelle elementari si ritiene che l’interazione gravitazionale venga trasmessa attraverso il gravitone, allo stesso modo con cui l’interazione elettromagnetica viene trasmessa dai fotoni. Oggi, nonostante esistano delle basi teoriche a favore dell’esistenza dei gravitoni rimane, però, il problema di rivelarli, almeno sulla Terra dove le possibilità sono estremamente basse se non quasi nulle.

For example, the conventional way of measuring gravitational forces, by bouncing light off a set of mirrors to measure tiny shifts in their separation, would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they’d collapse and form a black hole. Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of, in this case the Universe, to search for the telltale effects of gravitons. That is what two physicists are proposing. In the paper, “Using cosmology to establish the quantization of gravity”, published in Physical Review D (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the Universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early Universe.

“This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons”, Krauss said. “Moreover, what we find most remarkable is that the Universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth”. It is generally believed that in the first fraction of a second after the Big Bang, the Universe underwent rapid and dramatic growth during a period called “inflation.” If gravitons exist, they would be generated as “quantum fluctuations” during inflation. Ultimately, these would evolve, as the Universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency’s Planck satellite are searching for this “imprint” of inflation in the polarization of the CMB.

Krauss said his and Wilczek’s paper combines what already is known with some new wrinkles.

“While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new”, he said. “Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future”. Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon. This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint. “I’m delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity”, said Wilczek. “It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background”, Krauss added.

ASU: Researchers propose a new way to detect the elusive graviton

arXiv: Using Cosmology to Establish the Quantization of Gravity

Towards understanding the Big Bang

Le leggi della fisica non sono in grado di descrivere cosa accadde durante il Big Bang. Infatti, sia la teoria dei quanti che la relatività generale non permettono di spiegare lo stato fisico singolare, infinitamente denso e caldo che caratterizzava le fasi iniziali della storia dell’Universo. Forse un giorno, la formulazione di una teoria che permetta di descrivere la gravità su scale quantistiche potrebbe fornirci una risposta (vedasi Idee sull’Universo). Oggi, alcuni scienziati del  Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) a Golm/Potsdam e del Perimeter Institute in Canada hanno fatto una scoperta importante in questo contesto. La loro idea è quella di assumere che lo spazio consista di piccolissime unità chiamate “mattoni fondamentali”. Partendo da questo concetto, gli scienziati arrivano alla formulazione delle equazioni più importanti della cosmologia, e cioè le equazioni di Friedmann, che permettono di descrivere l’Universo. Il risultato è che questo processo mostra, in definitiva, che la meccanica quantistica e la relatività possono essere effettivamente unificate.

For almost a century, the two major theories of physics have coexisted but have been irreconcilable: while Einstein’s General Theory of Relativity describes gravity and thus the world at large, quantum physics describes the world of atoms and elementary particles. Both theories work extremely well within their own boundaries; however, they break down, as currently formulated, in certain extreme regions, at extremely tiny distances, the so-called Planck scale, for example. Space and time thus have no meaning in black holes or, most notably, during the Big Bang. Daniele Oriti from the Albert Einstein Institute uses a fluid to illustrate this situation: “We can describe the behaviour of flowing water with the long-known classical theory of hydrodynamics. But if we advance to smaller and smaller scales and eventually come across individual atoms, it no longer applies. Then we need quantum physics“. Just as a liquid consists of atoms, Oriti imagines space to be made up of tiny cells or “atoms of space”, and a new theory is required to describe them: quantum gravity.

In Einstein’s relativity theory, space is a continuum. Oriti now breaks down this space into tiny elementary cells and applies the principles of quantum physics to them, thus to space itself and to the theory of relativity describing it. This is the unification idea.

A fundamental problem of all approaches to quantum gravity consists in bridging the huge dimensional scales from the space atoms to the dimensions of the Universe. This is where Oriti, his colleague Lorenzo Sindoni and Steffen Gielen, a former postdoc at the AEI who is now a researcher at the Perimeter Institute in Canada, have succeeded. Their approach is based on so-called group field theory. This is closely related to loop quantum gravity, which the AEI has been developing for some time. The task now consisted in describing how the space of the Universe evolves from the elementary cells. Staying with the idea of fluids: How can the hydrodynamics for the flowing water be derived from a theory for the atoms? This extremely demanding mathematical task recently led to a surprising success. “Under special assumptions, space is created from these building blocks, and evolves like an expanding Universe“, explains Oriti. “For the first time, we were thus able to derive the Friedmann equation directly as part of our complete theory of the structure of space“, he adds. This fundamental equation, which describes the expanding Universe, was derived by the Russian mathematician Alexander Friedmann in the 1920s on the basis of the General Theory of Relativity. The scientists have therefore succeeded in bridging the gap from the microworld to the macroworld, and thus from quantum mechanics to the General Theory of Relativity: they show that space emerges as the condensate of these elementary cells and evolves into a Universe which resembles our own. Oriti and his colleagues thus see themselves at the start of a difficult but promising journey. Their current solution is valid only for a homogeneous Universe, but our real world is much more complex. It contains inhomogeneities, such as planets, stars and galaxies. The physicists are currently working on including them in their theory. And they have planned something really big as their ultimate goal.

On the one hand, they want to investigate whether it is possible to describe space even during the Big Bang.

A few years ago, former AEI researcher Martin Bojowald found clues, as part of a simplified version of loop quantum gravity, that time and space can possibly be traced back through the Big Bang. With their theory, Oriti and his colleagues are hoping to confirm or improve this result. If it continues to prove successful, the researchers could perhaps use it to explain also the assumed inflationary expansion of the Universe shortly after the Big Bang as well, and the nature of the mysterious dark energy. This energy field causes the Universe to expand at an ever-increasing rate. Oriti’s colleague Lorenzo Sindoni therefore adds: “We will only be able to really understand the evolution of the Universe when we have a theory of quantum gravity“. The AEI researchers are in good company here: Einstein and his successors, who have been searching for this for almost one hundred years.

Max Planck Institute: Quantum steps towards the Big Bang

arXiv: Cosmology from Group Field Theory Formalism for Quantum Gravity

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

La 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

Related articles

• B-mode polarization spotted in cosmic microwave background (physicsworld.com)

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)