## Wednesday, December 10, 2014

### Recommended by us: Interview with John Ellis

To come across John Ellis at CERN is actually not a rare event: at seminars, at coffee breaks... But probably in those situations you do not have so much courage or time to ask him what is his position about the role of science in the society, or even more when he decided to become a theoretical physicist!

If you have a kind of curiosity about these topics and other related questions, take a look at this interview: past CERN Summer Students with the kind help of the PH Newsletter have had the fortune to ask to him directly

http://ph-news.web.cern.ch/content/interview-john-ellis

Have a good read and, if you will have the patience to go until the end, you will find a final question to you.

## Saturday, December 6, 2014

### Pre-celeberating 2015 for 100 years of Einstein's General Relativity and maybe more...

In a few weeks we will all land in the exciting year of 2015! Many people will be celebrating New Year, as usual! (but still exciting) But, unusually, gravity physicists will be celebrating 100 years of General Relativity after Albert Einstein.

in 2015, gravitational-wave physicists, although, may have one extra celebration to do on the top of others, which is celebrating the first direct detection of gravitational waves! Advanced version of gravitational-wave detectors will start to take data in a few month, hunting wild celestial gravitational-wave sources such as black-holes and neutron-stars. I'll write more about this here some time soon. But for now, you may check out my earlier post in Gravity Room.

Anyways, just before entering in to 2015, Princeton University has recently released The Digital Einstein Papers in an open-access website which is an awesome collection and an excellent holiday reading! All the papers are in both English and German languages.

## Friday, December 5, 2014

### Pullback: "Che cos’è che non va?" di Enrico Persico

Di seguito pubblichiamo un pezzo di Prof. Guido Pegna (che ho avuto la fortuna di avere come professore di Elettronica all'Universita' di Cagliari) che a sua volta riporta un articolo di Enrico Persico sul Giornale di Fisica.

A questo link trovate una serie di esperimenti realizzati da Prof. Pegna, mentre a questo link ci sono tutti gli esperimenti esposti e utilizzabili al Museo di Fisica dell'Universita' di Cagliari.

Buona lettura!

 Emilio Segré, Enrico Persico e Enrico Fermi sulla spiaggia di Ostia nel 1927

## Saturday, November 15, 2014

### Interstellar

 "Honestly, Interstellar really sucks" -- this is not quite true, but I couldn't help thinking of this scene.

When I asked my spouse whether she would come to watch latest Christopher Nolan' movie Interstellar, she replied: "No way!".

"But it's about black holes.", I said.
"Exactly." - she replied.

"But Kip Thorne, a world-famous physicist, was involved in the production.", I said.
"Even more so." - she replied.

"But the director is Christopher Nolan!", I replied.
"Indeed."

"But it's gonna be a Hollywood Blockbuster!", I continued

"But it's the movie of the year!"
"Exactly."

That was the end of the conversation. As a matter of fact, she went to watch the movie without (and even before!) me, but I guess this is normal within women logic.

Anyway, together with part of the Lisbon gang, yesterday we finally went to watch Interstellar in its iMAX curved-spacetime, relativistic glory, so now we too are entitled to talk about this movie.

## Friday, October 24, 2014

### Tullio Regge passed away

 Tullio Regge (July 11, 1931 in Turin - October 23, 2014 in Orbassano)

Theoretical physicist and mathematician Tullio Regge, aged 83, passed away yesterday. He gave fundamental contributions to scattering theory (the theory of Regge poles is named after him) and to General Relativity among many other fields. His "Regge calculus" -based on a discretization of spacetime- is still widely used in Loop Quantum Gravity.

The relevance of Regge's heritage in modern physics is well shown by the fact that something like half of my papers cite the so-called Regge-Wheeler equation which describes how a Schwarzschild black hole vibrates after a perturbation and how it emits gravitational waves. The Regge-Wheeler equation was found in the late 1950s, even before the very concept of "black hole" (a name coined by Wheeler only in the 1960s) was formulated.

The Italian Institute for Nuclear Physics has dedicated its homepage to this news, more details on Regge's work can be found here.

## Friday, October 17, 2014

### The brightest pulsar ever

Last week the astrophysics community was hit by an exciting news. A very luminous X-ray source located in the "Cigar" galaxy M82 was discovered to be a spinning neutron star (a pulsar) rather than a black hole, as all models have assumed so far. This research was lead by astrophysicist Matteo Bachetti, who also happens to be a writer of this blog. Thus, we take the opportunity to ask Matteo a couple of questions whose answers you will not find in the excellent press (and radio) coverage that has followed the publication of this discovery.

Q: Matteo, first of all congratulations for what sounds like a great scientific achievement! How would you explain this discovery to my grandma?

## Thursday, September 18, 2014

### Running Citizenship

Today's historical referendum in Scotland gives me the opportunity to talk about something I have in mind since I came back from the U.S. earlier this year. It's actually a very trivial concept that originates from the fact that:

When outside Europe, I feel European. When within Europe, I feel Southern European/ Mediterranean. When in Southern Europe, I feel Italian and, finally, when in Italy I definitely feel Sardinian.

There is actually no contradiction in this Matrioska-like sense of belonging and I'm sure most people who happen to live abroad (which is already a rather subjective concept....) share the same feeling.

Anyway, this is interesting because in physics there is a much deeper and far-reaching concept that is (vaguely) related to the one above: that of the running couplings. In a quantum field theory, the coupling "constants" that define the strength of the couplings among quantum fields are not really constant, but actually their values depend on the energy scales.

As an example, take the most famous coupling constant, Newton's gravitational constant G that appears in the gravitational force law

F= G*mass1*mass2 / distance^2

which simply means that the intensity of the gravitational force between two masses (mass1 and mass2) is proportional to the masses and inversely proportional to the square of the distance. The proportionality factor is nothing but what we use to call Newton's constant and its value is G~6.673×10^{-11} N·(m/kg)^2. Now, Newtonian gravity is a classical theory and there is no such thing like a running G (although the situation might be different in alternative theories of gravity, such as scalar-tensor theories, but this is another story...). Indeed, G is a constant no matter how close the two masses are or how massive the objects are. In a quantum field theory, G would depend on the energy involved, for example on the typical distance of the interaction.

If you think about that, this is a beautiful and elegant concept: it teaches us there's no such thing like "the ultimate theory", but each theory (if consistently quantized) would be different at different energy scales. For example, a theory like electromagnetism (or QED in its quantum version) becomes more strongly coupled as the energy increases. The QED coupling constant (the fine structure constant α) is about α ≈ 1/137 at low energies, whereas one measures α ≈ 1/127 at the scale of the Z boson, about 90 GeV. A theory like quantum chromodynamics (QCD) behaves exactly in the opposite way and it becomes more weakly coupled at high energies. This phenomenon is called asymptotic freedom (because the interaction between particles becomes zero at infinite energy) and its discovery  (by Frank Wilczek, David Gross and David Politzer) was worth the Nobel prize in 2004.

What does this have to do with Scotland? (if anything...) The idea that can be borrowed from particle physics is that of a "running citizenship". In other words, each person changes her/his sense of belonging to some country/community accordingly to the "characteristic scale of the problem". It's something that European people are already experiencing given that economy in Europe is mainly governed on European scales, whereas local regulations are governed by state laws. Something similar also happens for federal countries, although the idea of running citizenship that i'm trying to describe has more to do with sense of belonging than with politics (politics often tries to change the sense of belonging and to tailor it accordingly, though).

In physics, the theory that studies the running couplings is called renormalization group and the running is usually called "flow". In most theories, the coupling either grows or decreases with energy, but for some special theories it asymptotes a constant value, a "fixed point".

As for the running citizenship, my impression is that we better try to have a fixed point for that, because the other two cases are quite catastrophic. The analog of a theory like QED would be a sense of citizenship that becomes smaller and smaller as smaller scales are approached, eventually terminating in individuals that do not belong any community. This would imply a sort of total isolation for each individual. The other case would be funny: an individual would become more and more aware of the global nature of Mankind and would feel like more and more part of the entire Universe as smaller scales are approached (this sounds like a nice outcome but perhaps a bit too Hippie for these times....). The most natural solution would be approaching a limit, a minimum size of the community (which we can perhaps identify with the family or hopefully with something larger than that) and then having each person feeling as a part of a larger community dependending on the context and the environment.

In which category does the running citizenship flow of the Scottish people fall? We shall discover this quite soon and the outcome has probably very little to do with physics.....

## Sunday, August 31, 2014

### Greetings from Mons

 Some of the participants of the Mons Meetings 14 held this week at Mons, Belgium, and greatly organized by my colleague and friend Terence Delsate (the guy wearing glasses in the group picture...)

## Tuesday, July 22, 2014

### Jordi on Dark Matter @ FameLab 2014

Fame Lab is a competition to find new voices of science across the world. In these events various speakers compete for the best popular science short talk. The events are hosted in various countries and there are competitions at regional, national and international level.

This year Jordi Casanellas (former PhD student in IST-Lisbon and now postdoc at the Albert Einstein Institute near Berlin) attended Fame Lab Germany. As the video below shows, in the first stages of the competition speakers cannot use blackboards (not to mention slides or projectors) and their talk has to be ~3 mins long.

Jordi talked about his field of research, Dark Matter, and in fact did a great job: he passed the regional competition and went to the national finals (see video below), where he placed second!

So, if you want to have an idea of what Dark Matter exactly is (or what scientists actually think it should be...) you don't have to do anything but listen to Jordi's speeches. Enjoy!

## Sunday, July 13, 2014

### Pull back: "Scientist: Four golden lessons" by Steven Weinberg

Just came across this brilliant essay by Nobel Laureate Steven Weinberg, writing about his experience as a young scholar, his first steps in research and the connection between science and epistemology.

The essay is so short and clear that I could have copied it here in its entirety, but I'll just give you one inspirational paragraph:

[...]
Look back 100 years, to 1903. How important is it now who was Prime Minister of Great Britain in 1903, or President of the United States? What stands out as really important is that at McGill University, Ernest Rutherford and Frederick Soddy were working out the nature of radioactivity. This work (of course!) had practical applications, but much more important were its cultural implications.
[...]

and leave the rest for the original

## Sunday, June 8, 2014

### Vita dura per i neutrini sterili e non solo

Si è appena conclusa a Boston la conferenza NEUTRINO2014 dedicata, così come vuole il nome, ai nuovi risultati sperimentali e teorici che provengono dal mondo di queste elusive e misteriose particelle, per l’appunto, i neutrini.

Ci sono parecchie novità interessanti e volevo quindi fare un piccolo riassunto sulle cose più sfiziose.

Partiamo dalla ricerca dei cosiddetti neutrini sterili, e cioè di quelle particelle ipotetiche che sono state introdotte per spiegare alcune anomalie riscontrate nel corso degli anni da alcuni esperimenti che non si inquadravano nel modello delle oscillazioni a tre neutrini. Il primo esperimento a riscontrare un’anomalia è stato LSND (Liquid Scintillator Neutrino Detector) a Los Alamos, in cui è stato registrato un eccesso di antineutrini elettronici, con significatività di 3.8 sigma, su un fascio pressoché puro di antineutrini muonici. Se interpretati in uno schema di oscillazione a due neutrini, per la particolare configurazione della baseline, L, (la distanza tra sorgente di neutrini e rivelatore) e l’energia, E, da cui dipendono la probabilità di oscillazione di un neutrino muonico ad uno elettronico $P_{\nu_{\mu}\rightarrow\nu_{e}}\left(L,\, E\right)=\sin^{2}2\vartheta_{e\mu}\sin^{2}\left(1.267\:\frac{\Delta m_{41\,}^{2}L}{E}\right)$ questo eccesso sarebbe indicativo di un'oscillazione con una piccola ampiezza e un grande $$\Delta m^{2}\sim1$$ $$\textrm{eV}{}^{2}$$. Appare chiaro che un $$\Delta m^{2}\sim1$$ $$\textrm{eV}{}^{2}$$, non può essere incorporato in un modello a tre soli neutrini (elettronico, muonico e tauonico) in cui esistono solamente due differenze di masse al quadrato indipendenti. Infatti deve valere la relazione $\Delta m_{21}^{2}+\Delta m_{32}^{2}+\Delta m_{31}^{2}=m_{2}^{2}-m_{1}^{2}+m_{3}^{2}-m_{2}^{2}-m_{1}^{2}-m_{3}^{3}=0\,$ e dal momento che la differenza di massa al quadrato dei “neutrini solari” è $$\Delta m_{SOL}^{2}=\Delta m_{21}^{2}=7.58_{-0.26}^{+0.22}\times10^{-5}$$ $$\textrm{eV}{}^{2}$$ e di “quelli atmosferici” è $$\Delta m_{ATM}^{2}=\left|\Delta m_{31}^{2}\right|\simeq\left|\Delta m_{32}^{2}\right|\simeq2.35_{-0.09}^{+0.12}\times10^{-3}$$ $$\textrm{eV}{}^{2}$$, non vi è spazio per una differenza di massa al quadrato dell'ordine dell'$$\textrm{eV}{}^{2}$$, se non in uno schema in cui sia presente almeno un nuovo stato di neutrino massivo $$\nu_{4}$$ sterile, per cui possa essere interpretata la differenza di massa al quadrato come $$\Delta m_{\textrm{new}}^{2}\equiv m_{4}^{2}-m_{1}^{2}=\Delta m_{41}^{2}$$. Questo neutrino però deve essere sterile, e quindi non deve partecipare a nessuna delle interazioni (tranne quella gravitazionale) in quanto esistono delle misure effettuate al LEP sul decadimento del bosone Z in neutrini, che indicano che il numero di neutrini “attivi” (e cioè quelli che interagiscono per forza debole) deve essere esattamente tre.

## Wednesday, June 4, 2014

### "Science in the era of Facebook and Twitter – get used to it" by Heino Falcke

Astronomer Heino Falcke has recently wrote a nice post on the relation between science, dissemination of scientific results and social media.

The post was triggered by the recent debate on the BICEP2 results and contains interesting advices on the new role that scientists and journalists should learn to have in the era of Facebook.

“Science is wrong, most of the time” – I am not sure who said that first, but I am sure someone did so well before me. This is a banality for those who do science at the forefront of our knowledge, yet sometimes it seems very difficult to also accept that view in the public discourse. Well, in the days of Facebook and Twitter it is plain obvious to everyone.

Continue reading on Heino Falcke's blog

“

# Blockbuster Big Bang Result May Fizzle, Rumor Suggests

The biggest discovery in cosmology in a decade could turn out to be an experimental artifact—at least according to an Internet rumor. The team that reported the discovery is sticking by its work, however.
Eight weeks ago, researchers working with a specialized telescope at the South Pole reported the observation of pinwheel-like swirls in the polarization of the afterglow of the big bang, or cosmic microwave background (CMB). Those swirls are traces of gravitational waves rippling through the fabric of spacetime a sliver of a second after the big bang, argue researchers working with the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) telescope. Such waves fulfilled a prediction of a wild theory called inflation, which says that in the first 10-32 seconds, the universe underwent a mind-boggling exponential growth spurt. Many scientists hailed the result as a "smoking gun" for inflation.
However, scientists cautioned that the result would have to be scrutinized thoroughly. And now a potential problem with the BICEP analysis has emerged, says Adam Falkowski, a theoretical particle physicist at the Laboratory of Theoretical Physics of Orsay in France and author of the Résonaances blog. The BICEP researchers mapped the polarization of the CMB across a patch of sky measuring 15° by 60°. To study the CMB signal, however, they first had to subtract the "foreground" of microwaves generated by dust within our galaxy, and the BICEP team may have done that incorrectly, Falkowski reports on his blog today.
To subtract the galactic foreground, BICEP researchers relied on a particular map of it generated by the European Space Agency's spacecraft Planck, which mapped the CMB across the entire sky from 2009 until last year. However, the BICEP team apparently interpreted the map as showing only the galactic emissions. In reality, it may also contain the largely unpolarized hazy glow from other galaxies, which has the effect of making the galactic microwaves coming from any particular point of the sky look less thoroughly polarized than they actually are. So using the map to strip out the galactic foreground may actually leave some of that foreground in the data where it could produce a spurious signal, Falkowski explains. "Apparently, there is something that needs to be corrected, so at this point the BICEP result cannot be taken at face value," he tells Science.

## Monday, May 5, 2014

### Radiazione Cherenkov: non si smette mai di imparare! La svista nel programma di divulgazione scientifica Cosmos.

Recentemente ho scritto un articolo riguardo al programma televisivo di divulgazione scientifica “Cosmos: a spacetime odyssey” trasmesso su National Geographic Channel (in Italia è trasmesso sul canale 403 di Sky) e da FOX. Nella sesta puntata della serie intitolata “Dove tutto si crea” (“Deeper, Deeper, Deeper Still” invece il titolo inglese [1]) si parla, tra le altre cose, di supernovae, e dei neutrini ed antineutrini emessi in queste spettacolari esplosioni stellari (per rivedere lo spezzone relativo alle supernovae clicca qui e guarda dal minuto 30 in poi, ma vale la pena di guardare tutta la puntata). In questo precedente articolo, di cui consiglio la lettura prima di continuare la lettura di quest’ultimo, mi sono soffermato nell’analisi dell’incredibile ricostruzione, realizzata con l’ausilio della computer grafica [2], della rivelazione di queste particelle emesse a seguito dell’esplosione di supernova.

## Thursday, April 24, 2014

### The first observed SMBHB?

 An artistic illustration of black hole
SMBHB stands for Super Massive Black Hole Binary. If the results get confirmed, this shows that we have observed inactive SMBHBs for the first time ever! SMBHs can be 10^7 times more massive than our own Sun. The Sun is one million times more massive than the Earth. The mass of Earth is about 10^24 kg, by the way.

SMBHBs can be used as excellent natural labs for testing many aspects of gravitational and high-energy physics. As you might know already or might just guess from the term "black hole", a black hole cannot be seen by our usual optical telescopes (not even by other common types of telescopes in other ranges of frequencies of electromagnetic waves such as radio-telescopes and x-ray telescopes). A black hole is such a massive object that even light cannot scape from its gravitational field. That’s also why you can not see a black hole, simply because neither light nor other electromagnetic waves can scape from the gravitational field of a black hole and reach your eyes. The only way to observe black holes is studying their gravitational field effect on the motion of nearby stars. An even better way to study and observe these inconspicuous giants is listening to them! (check out this note to see how) They are pretty loud!

To check out more details about this first serious candidate of SMBHB at the galaxy SDSS J120136.02+300305.5, see the original article, published recently at The Astrophysical Journal: F. K. Liu et al. 2014 ApJ 786 103.

## Tuesday, April 15, 2014

### I neutrini da supernova come non li avete mai (s)visti...

Recentemente mi è capitato di seguire il documentario televisivo statunitense Cosmos: Odissea nello spazio (si tratta del seguito dell’omonimo programma condotto dal defunto Carl Segan) ora presentato dall’astrofisico e divulgatore scientifico Neil deGrasse Tyson. Curiosamente uno dei produttori esecutivi del programma è Seth MacFarlane noto per essere il creatore (e doppiatore di molti personaggi) delle serie animate i Griffin, American Dad! e The Cleveland Show (pare che MacFarlane da bambino fu impressionato dal programma “Cosmo” di Carl Segan, maturando la convinzione che il programma servisse "[per ridurre] la distanza che separa la comunità accademica dal grande pubblico" e così abbia deciso di investire sulla nuova produzione dello stesso programma di divulgazione scientifica, decisione stigmatizzata dalla frase pronunciata dallo stesso all’attuale presentatore del programma Tyson: "I'm at a point in my career where I have some disposable income ... and I’d like to spend it on something worthwhile.").

Curiosità a parte, nell’episodio 6 di questa nuova edizione di Cosmos, andato in onda il 13 Aprile su National Geographic Channel e contemporaneamente su Fox Network nella notte, tra i vari argomenti si parla anche di neutrini e in particolare di neutrini provenienti dall’esplosioni di supernova (potete vedere una replica dell’episodio a questo link, oppure in quest'altro link per la versione in italiano. La parte sui neutrini è visibile circa al minuto 28, momento in cui si inizia a parlare delle supernova e dei relativi neutrini ed antineutrini emessi in questa eccezionale esplosione).

## Universe is a Jungle and Gravitational Waves are sounds of the animals in it

Close your eyes and imagine you are in a helicopter flying over a very big beautiful jungle in the heart of Africa. Open your eyes now! What you see is like the following pictures: a lot of trees and plants which you see them as a big, green picture and call it “jungle”!

What you see from your helicopter above a huge jungle in Africa

The ground is so covered by trees and plants such that there is no way to see the animals who are living in this jungle from your helicopter flying ~500m above the ground. You also can’t hear their voices. Even though you do believe that lions, tigers, elephants, monkeys, and etc are living down there and make some sounds some times naturally, based of what Mr. Einstein have told you. You have never seen these animals before but according to Mr. Einstein, they should sound like followings: tigers [listen], lions [listen], elephants [listen], monkeys [listen].

There are some animals under the jungle trees that there is no way to see them from your helicopter, you just can hear their voices

However you can not see the lions in the jungle because they have hidden by many trees, but you are able to see if some birds are flying around your helicopter or even very far away, but above the trees.

You are able to see if some birds are flying around your helicopter or even far away, but above the jungle trees.

## Unwanted sounds from your helicopter: noises!

Unfortunately, you can not hear any animal voice neither birds nor lions. Noise of your helicopter engine [listen] is the only sound that you can hear (however you probably can hear your friend beside you when he is shouting in your ear). With the assumption of a completely silent helicopter, not all the animals have strong voices which can reach you at the helicopter. Even if the animals make strong enough voices, in reality, what you hear is helicopter noise plus an extremely weak voice of an animal. Therefore it would be so difficult to recognize the animal voice in presence of such a high level of noise. In the best circumstances, you will need a super-duper high-tech artificial ears to filter out animal voice from helicopter noise. To do this, indeed, it will be required to know how does the animal voice that you are looking for look like. You can not use voice of monkey and look for voice of lion in the data! Fortunately, by experience, we do know what does the voice of a typical lion look like, however the African lion voice might be a little different. But it doesn’t matter that much, it perfectly works for our purpose.

In fact, voice, or basically sound wave, is not more than some simple mechanical waves in the air. When you speak, your vocal cords shake the air around and make some mechanical waves in air which can be heard by your friend’s ears. Waves, in all forms, need an environment to travel through. If you speak in vacuum [suppose your body doesn’t implode in vacuum!] nobody can hear you. Gravitational waves, in the other hand, are some kind of waves which are produced by moving super-massive sky-objects like black holes, and neutron stars. Instead of air in the case of sound waves, gravitational waves travel trough the fabric of spacetime and affect masses in the field. Gravitational waves are prediction of Einstein’s general theory of relativity. If strong gravitational waves go through your body, you will experience a situation like the following picture. Although, effects are super exaggerated in this picture.

When strong gravitational waves go through your body. (different polarization)

When you speak your vocal cords shake the air around and make sound waves

## Getting back to the analogy of Gravitational Waves

Above described circumstances in jungle is very much similar to what we study in the field of gravitational waves. Universe is the jungle, and Earth is your helicopter in this case. You can look at the sky and are able to see some celestial objects like planets, stars, comets and etc with naked eye or even with modern telescopes. They are the birds above the trees that we can see them but can’t hear them. But this is not everything which exists in the jungle. There are some animals hidden by trees that there is no way to see them even with modern telescopes [Gravitational Wave Sources]. The only way to detect them is listening to their voices [Gravitational Wave Signals]; however they are extremely weak compared to helicopter noise [Detector Noise]. You need to use your super-duper high-tech artificial ears [Gravitational Wave’s Detectors] to get some data, which is basically background noise plus some signal. People use Matched Filtering methods to filter out the signal from data. Post-Newtonian theory, for example, is your knowledge about the voice of a typical lion and if you want to know exactly the model of African lion voice you should use Numerical Relativity.

Universe as a jungle

Celestial objects as jungle animals including birds: stars, supernovas, galaxies, and lions [can’t see but can hear]: black holes and neutron star binaries

We actually use Post-Newtonian formalism to study the sound of a particular source: two angry lions fighting! [compact binary systems, e.g. two extremely massive black holes orbiting each other]. This is the most promising source of gravitational waves that ground current detectors on the earth can detect. One of our motivations to study this system is to find out if Mr. Einstein was right [2]. Gravitational wave astronomy will open a new window to the universe. In addition to the animals that we expect to detect their voices, some strange creatures, might be heard by this new generation of astronomy that we never expect. Somebody may has been “LOST” in spacetime from many years ago. Who knows?

We might hear voice of strange creatures (like super-massive BHs) in the jungle (universe) that we have never been aware of them before Gravitational Waves Astronomy

A good source: two angry lions fighting! [compact binaries]

Imagine in 3014, when kids will be learning Quantum Mechanics at kindergartens, they will learn the sounds of cosmic creatures at elementary schools. Latest modelings show that the voice of some sky animals are expected to be as followings.

• Tigers: Equal mass binary gravitational waves: Two black holes, each of 50 solar masses [Listen
• Lions: Extreme mass ratio binary gravitational waves: Initially circular orbit, into rapidly spinning BHs [Listen
• Elephants: Extreme mass ratio binary gravitational waves: Initially circular orbit, into slowly spinning BHs [Listen
• Monkeys: Extreme mass ratio binary : Initially highly eccentric orbit, into rapidly spinning BH [Listen]

• Next generation of astronomers will “listen” to the sky instead of looking. Photo: My imaginary son which is an old fashion astronomer at right, and his son which is a modern astronomer at left, looking and listening to the sky from MIR-III space station. ©Photo by: my lovely great-granddaughter, who is not an astronomer but a poet.

## References

[1] The title is borrowed from a Bernard Shutz's talk, "Gravitational Waves: Listening to the Music of the Spheres", public talk at Washington University in St. Louis, 2010.
[2] Title of a book by C.M. Will, “Was Einstein Right?”, 2nd Edition, Basic Books, New York, 1993.

## Thursday, March 20, 2014

### Paper of the day: "Damn it! Why wasn't me to write this??"

One of my favorite songwriters, now retired Francesco Guccini, wasn't used to sing pieces written by other authors. One of the rare occasions in which he decided to do so is this one:

where he sings Roberto Vecchioni's "Luci a San Siro". Guccini's incipit starts by saying some like

"The song I am going to sing is titled: - Damn it! Why wasn't me to write this song?

... Well, the paper I am going to review today is titled"

"Damn it! Why wasn't me to write this paper?"

The paper I am referring to appeared some days ago on the arXiv,

it is written by Carlos Herdeiro and Eugen Radu from the University of Aveiro. I have to admit it, this paper is just beautiful. Seriously. Not only the result circumvents one of the classical theorems of General Relativity [the black hole no-hair theorem, see below] but, in doing so, it also connects elegantly two solutions which were previously thought to be very different. As if that was not enough, it is beautifully written in such a way that the overall feeling is the one that only great papers can give -- a feeling that only scientists have the privilege to appreciate [and possibly artists can do so too, while watching/listening to//performing other colleagues' pieces of arts as in the video above].

## Tuesday, March 18, 2014

### How to describe today's discovery to your grandma!

Figure.1: BICEP2 telescope at South Pole
Today, a major discovery in astrophysics has been announced by a research collaboration named BICEP2 which raised lots of interest, attention, and discussions in the science community as well as in public. Paolo already wrote a great note on this at The Gravity Room but here I would like to add another note to show you how I summarize the whole story to my grandma (or whoever else not in the field):
1. Question: What they have discovered?
Answer: A strong evidence for the existence of gravitational waves (GWs) coming all the way from the very early stages of the universe i.e. just after Big Bang explosion!
More details: Using a modern telescope located at south pole, they have found another strong evidence for existence of GWs by studying the polarization of CMB (Cosmic Microwave Background) data. This is another indirect detection of GWs but not the first one. The first (indirect) detection was made by Hulse and Taylor (Nobel Prize winners of 1993) from a binary pulsar source while this recent experiment has detected the signature of GWs from the very early stages of the universe i.e. just after the Big Bang. Compared to the Hulse-Tayor's Binary Pulsar, in this case GWs are coming from completely different sources! Remember, we have not detected GW directly yet. This is what LIGO/VIRGO and other interferometric GW detectors are supposed to do in the next few years: the first direct detection of GWs...

Figure.2: Look grandma! This diagram is the heart of the story which compares the previous results (red) to BICEP2 results (blue) on measuring two parameters of CMBpolarization (vertical and horizontal axises). In another word, the red and blue areas show the allowed values of CMB polarization parameters based on the old and new experiments. New results (blue) clearly exclude the possibility of parameter "r" to be zero! This means a smoking gun for GWs coming all the way from Big Bang!
2. Question: Why is it important?
Answer: This is the first time that we have observed the signature of GWs from the Big Bang! It's quite exciting, isn't it? It also supports our early scenarios of the early universe and quantum gravity specially the Big Bang and Cosmic Inflation... In addition, if the results get confirmed by future experiments and the result holds up, it gives us an unprecedented view of the earliest moments in the history of the universe.
3. Question: Should we celebrate now?
Answer: Yes, but not too much! Because (1) Looking at the same thing (polarization of CMB in B-modes), soon, several similar experiments will come out with new data/results. So it's better to wait a bit and see the confirmations/disconfirmations... This is how science works! (2) A much bigger party is on the way: LIGO! In which we will be able to directly detect GWs from compact binary systems for the first time. LIGO will not only directly detect GWs but also will open a completely new branch of astronomy i.e. GW Astronomy; in which we can measure the physical parameters of the astronomical objects like sky-position, mass, spin, etc that for some cases we might not even be able to measure by other instruments such as optical-, radio-, x-ray-, and gamma-ray-telescopes.

Figure.3: The frequency spectrum of Gravitational Waves and the sensitive range of different detectors. So, grandma! Focus on the "red" lines/notes. Looking at this figure, this is all they are talking about: using "Cosmic CMB polarization" as a detector to detect those GWs emitted from "quantum fluctuations in early universe" in the low frequency band of the spectrum. Notice that the primordial GWs coming from the early universe can be seen in a broad range of frequencies but CMB polarization experiments can only detect a small range of lower frequencies. Now you may ask "what do you need LIGO for while BICEP2 has already detected GWs!" Grandma! Look! Now focus on purple lines/notes and notice that the GWs that LIGO (one among other Terrestrial Interferometers) is supposed to detect is in the other side of the frequency spectrum for higher frequencies and for different sources including quantum fluctuations of early universe at much higher frequencies plus others sources such as compact binary systems and supernova explosions.
Grandma! You should fight against gravity and stay tuned until the first detection announcement of LIGO in the next few years... Then we can totally celebrate testing the last untested piece of Einstein's General Theory of Relativity: Gravitational Waves!

## Monday, March 17, 2014

### BICEP2: three birds with one stone?

There is nothing like the amazing news that was announced today at the Harvard-Smithsonian Center for Astrophysics in Boston to restart this blog after one-month silence. Researchers from the BICEP2 collaboration organized a press release that was supposed to be broadcast live. However, and this already tells something about the expectation mounting around the event, too many people tried to watch the streaming, the Harvard server collapsed and was unable to broadcast the event live. It was a great pity for me that I left the institute 2 days ago to come back to Lisbon, as for a couple of days I couldn't attend this historical event in person...

 The BICEP2 observatory is located at the South Pole. The reward for 6-month darkness and absurd temperatures is the kind of landscape shown in this picture (not to mention the scientific reward for this recent discovery)

So what's all this hype about? The BICEP2 collaboration detected for the first time the primordial gravitational waves produced during the first instants of the Universe, right after (meaning something like 0.000000000000000000000000000001 seconds after) the Big Bang. This is going to be a historical discovery, for at least three reasons:

1) Gravitational waves are one of the main predictions of Einstein's General Relativity and they were still waiting for a direct detection. Evidence for gravitational waves come from the inspiral of a binary system, whose orbit shrinks because of the emission of gravitational waves. Scientists have observed the shrinking of the orbit (an observation that was awarded the Nobel Prize in 1993) but did not detect the emitted gravitational waves directly. Actually, not even the BICEP2 experiment detects primordial gravitational waves directly, but it can detect their effect on the CMB, which is somehow more direct (or, if you wish, less indirect) than what is now routinely done with binary pulsars. [Actually, I'm having an ongoing discussion with various colleagues about what a direct detection actually is. This discussion can easily becomes philosophical and I'd rather skip it here... I'll just tell you that this detection is definitely the most direct evidence of gravitational waves that we have so far]. Most importantly, the gravitational waves detected by BICEP2 are totally different from those emitted by neutron stars and black holes. Thus, this result can be seen as yet another confirmation of Einstein's gravity in a region which was completely unexplored to date.

2) Primordial gravitational waves need to be enormously amplified if they are to be detected at the present epoch (remember they were produced some 14 billions years ago...). Essentially, the majority of scientists believe that the only mechanism to explain such amplification is cosmological inflation. For this reason, today's results are often quoted as the first evidence for inflation, something that cosmologists and particle physicists were after since the late 70s. In very few and simple words, the theory of inflation assumes that the Universe has undergone a phase transition right after the Big Bang, in which it started expanding exponentially for an extremely short time (10^{-32} seconds). This exponential growth was predicted to explain other characteristics of the Universe that were already observed in the past, like its flatness, homogeneity and isotropy. Essentially, inflation provides a dynamical mechanism that, starting from generic initial conditions, makes the universe homogeneous and isotropic right to the level that we know observe.

3) Had enough? No, there's even more. Gravitational waves and inflation alone are not enough to explain why an experiment like BICEP2 would today detect such signal. Indeed, intrinsic with the idea of inflation is that fact that such spacetime perturbations were produced by quantum effects. The energy scale of such effects (some 10^16 GeV) is way larger than the energy currently produced in particle accelerators (and most likely larger than the energy that we could ever produce on Earth!) Therefore, the very fact that we can detect such effect and make sense of it (in fact, these results seem to favor one of the simplest theories of inflation, which was proposed back in the 80s) is already a confirmation of the quantum nature of gravity, whose fully understanding is the Holy Grail of all theoretical physics.

What's next? As one of the spokesmen of BICEP put it during the press release: "an exceptional discovery requires exceptional confirmation". Likely enough, these outstanding results could be confirmed soon by the Planck collaboration and by other experiments that are measuring the properties of the CMB. If confirmed, not only they would ease the decision for one of the next Nobel prizes in Physics, but they would immediately open a new era in cosmology, gravity and even particle physics.

 One of the main results of the BICEP2 experiment [taken from bicepkeck.org]. The axis represent the "spectral index" n_s and the tensor-scalar ration, r. The blue area represents the region which is favored by the new observations. A value of r larger than zero implies emission of primordial gravitational waves. Before the results from BICEP2, scientists considered r~0.1 a quite large number and previous experiments were compatible with r=0 (that is, they did not observe any gravitational waves, although previous experiments, unlike BICEP2, were not specifically designed for this purpose). The fact that BICEP2 measured r~0.2 not only confirms the existence of gravitational waves, but it also means that inflation must have been quite strong to amplify the quantum perturbations to that level. Only some inflationary model can explain a value r~0.2. Funny enough, one of the most favored models is also one of the easiest and it was proposed in the 80s. After the Higgs bosons, Nature seems really to appreciate the simplest solutions to explain fundamental physics....
For people like me, it is really hard to go back and do our scientist-of-the-street job after such overwhelming discoveries. This is probably why I stopped working and decided to write here...

Added: this is the reaction of Andrei Linde, one of the fathers of the theory of inflation (and indeed the scientist who proposed the theory that it seems now favored by BICEP2 data):

Hilarious!

## Wednesday, February 12, 2014

### You can sleep soundly: we will not be devoured by a black hole

Today Público, one of the main Portuguese daily newspaper, published a very nice piece on our group in Lisbon, talking about our recent paper and the group's supercomputer Baltasar Sete-Sóis.

 Overused pictorial description of a black hole #4

PS:
Why the long silence? Four (4) requests to referee received in 3 days (2 reports submitted, 2 to go), moved back to Lisbon 1 week ago, found new apartment, wrote 3 financial and scientific reports for my past fellowship, did paperwork for a new contract, prepared a talk i'm giving in 30 mins, read a Master thesis, trying to write a proposal and at the same time also trying to work (for real) and...live!

## Saturday, January 4, 2014

### Back to Mississippi

First time I went to Oxford Mississippi was 7 years ago, as a visiting master student at The University of Mississippi or, as it is more commonly known, OleMiss. I arrived there around May and, as in most college towns like Oxford, the city emptied right after the term finished, few days after my arrival. This is to say, nothing much was going on in Oxford at that time; nonetheless during my six-month visit there I managed to meet very interesting people, live curious experiences, have fun, and specially work hard in what has later become my field of research. Looking back, I had really a great time.

This is why i'm so excited to come back in Oxford MS tomorrow to attend the Workshop "Testing General Relativity with Astrophysical Observations".

 The beautiful logo of the workshop recalls the iconographic Southern magnolia with the trajectory of an extreme mass-ratio inspiral, i.e. the evolution of a stellar-mass compact object orbiting a supermassive black hole like the one at the center of the Milky Way. [credits: Ana Sousa]

This workshop will bring together experts in tests of general relativity, modified theories of gravity and astrophysics. The aim is to foster informal discussions on the current status of experimental constraints on Einstein’s theory and their prospects for the near future, when advanced gravitational-wave observatories will be operational.

The list of participants is impressive and the program promises this will be a fun and fruitful meeting. For me this will also be a special experience, as at Olemiss I met some of my scientific advisors, some closest collaborators and also good friends.

Oxford has been named by USA Today as one of the top six college towns in the U.S. and it is also known as the hometown of  Nobel-prize winning author William Faulkner, as well as residence of novelist and politician John Grisham. However, as a 23-yr old student, I like to remember myself walking around the Olemiss campus while listening to Afroman's hits, which are definitely less literate but undeniably fun. Here is an example, particularly apt for this occasion:

OK, the lyrics is bitter and somehow sexist but, hell, Mississippi is not only Oxford!

I better stop here and continue preparing my slides for this meeting....