Hot topics in Modern Cosmology

Spontaneous Workshop (SW) IX brings together specialists on recent insights in Particle Physics, Astrophysics and Cosmology. The aim is to stimulate debate on common topics in views of providing us with innovating ideas.

Beyond ΛCDM

The aim of this conference is to survey the successes and shortcomings of the standard ΛCDM cosmological model, and to decide which theoretical and observational directions beyond ΛCDM will be the most fruitful over the coming decade.

International Conference on Quantum Gravity, Cosmology and String Theory

The Center for Fundamental Physics (CFP) at Zewail City of Science and Technology is organizing its first international conference on Quantum Gravity, Cosmology and String Theory (QGCS15) in Giza, Egypt from 11 to 15 January 2016. 

Pubblicità

Understanding the early Universe

Our understanding of the early Universe has greatly increased in the past few years due to the high-quality observational data on the cosmic microwave background and large scale structure.

The Primordial Universe after Planck

It is anticipated that the new results from the Planck satellite, based on the full mission data and including the Planck analyses in polarisation, will have been released some time before the meeting. It will therefore be timely to discuss the physics of the primordial universe, both from a theoretical and observational points of view.

Lost in a multiverse

I recenti risultati pubblicati dal gruppo di Harvard sull’esperimento BICEP2 sono stati interpretati come la “prima evidenza diretta” dell’inflazione cosmica dovuta al passaggio delle onde gravitazionali primordiali le cui tracce sono state impresse nella radiazione cosmica di fondo (post). Non solo, ma c’è chi dice che questi dati potrebbero essere collegati a qualcosa ancora di più strano che gli astronomi chiamano multiverso. Secondo questa ipotesi affascinante, il nostro Universo non sarebbe l’unico ma sarebbe parte di una grande vastità di universi che formano una sorta di gigantesco “albero cosmico” il cui numero potrebbe essere dell’ordine di

quasi impronunciabile. Ma se questi universi esistono davvero, non li vediamo perchè dal momento in cui è nato il nostro Universo non c’è stato abbastanza tempo affinchè la luce si propagasse per raggiungere il nostro orizzonte cosmico. Dunque, essi si troverebbero al di là del nostro limite osservativo e perciò non potranno mai essere rivelati, almeno in linea di principio. Come fanno allora i cosmologi ad affermare che esistono invece delle evidenze a favore della loro esistenza?

Recent BICEP2 results could be an experimental artifact

Swirling controversy. The purported swirls in the cosmic microwave background could in fact be a spurious signal from within our galaxy, a rumor suggests.

Qualche settimana fa, la notizia relativa alla scoperta della “prima evidenza diretta” dell’inflazione cosmica, annunciata dai ricercatori dell’Harvard CMB Group che lavorano all’esperimento BICEP2, potrebbe rivelarsi un “artefatto sperimentale”. E’ oggi quanto circola tra gli addetti ai lavori nonostante i ricercatori di Harvard abbiano già sottolineato che i risultati saranno soggetti ad una serie di controlli.

More at Science: Blockbuster Big Bang Result May Fizzle, Rumor Suggests

See update(s) here: Follow up on BICEP

The Conversation: Has dust clouded the discovery of gravitational waves? [UPDATE]

Secrets of the Universe’s First Light

The  first definite proof that the Universe underwent an almost unimaginably fast expansion when it was only a trillionth of a trillionth of a trillionth of a second old has taken the world by storm. This sudden growth spurt was first theorized more than three decades ago. Yet only last month did astrophysicists reveal what may be “smoking gun” evidence that the Universe swelled from microscopic to cosmic size in an instant, an announcement that’s being compared to the discovery of the Higgs boson.

More at The Kavli Foundation: Secrets of the Universe’s First Light

Some physicists doubt on inflation evidence

Circa una settimana fa, la notizia della presunta scoperta delle “tracce” impresse nella radiazione cosmica di fondo dovute al passaggio delle onde gravitazionali primordiali ha preso d’impatto come una tempesta improvvisa il mondo dei media (post). Non solo, ma si vocifera già la nomination per il Premio Nobel al gruppo di ricercatori di Harvard che non solo avrebbero trovato la “prima evidenza diretta” del processo dell’inflazione cosmica ma anche degli indizi a supporto dell’esistenza di altri universi. Nonostante ciò, alcuni fisici stanno suggerendo di “rimettere nel frigo lo champagne”, almeno per ora.

Theoretical physicists and cosmologists James Dent, Lawrence Krauss, and Harsh Mathur have submitted a brief paper (arXiv:1403.5166 [astro-ph.CO]) stating that, while groundbreaking, the BICEP2 Collaboration findings have yet to rule out all possible non-inflation sources of the observed B-mode polarization patterns and the “surprisingly large value of r, the ratio of power in tensor modes to scalar density perturbations.”

“However, while there is little doubt that inflation at the Grand Unified Scale is the best motivated source of such primordial waves, it is important to demonstrate that other possible sources cannot account for the current BICEP2 data before definitely claiming inflation has been proved”, as stated by the authors in the paper. 

Inflation may very well be the cause, and Dent and company state right off the bat that “there is little doubt that inflation at the Grand Unified Scale is the best motivated source of such primordial waves“, but there’s also a possibility, however remote, that some other, later cosmic event is responsible for at least some if not all of the BICEP2 measurements. (Hence the name of the paper: “Killing the Straw Man: Does BICEP Prove Inflation?”). Not intending to entirely rain out the celebration, Dent, Krauss, and Mathur do laud the BICEP2 findings as invaluable to physics, stating that they “will be very important for constraining physics beyond the standard model, whether or not inflation is responsible for the entire BICEP2 signal, even though existing data from cosmology is strongly suggestive that it does“.

The Physics arXiv Blog: Cosmologists Say Last Week’s Announcement About Gravitational Waves and Inflation May Be Wrong

arXiv: Killing the Straw Man: Does BICEP Prove Inflation?

 

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’