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!

BICEP2 telescope in South Pole
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]. 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):