The direct observation of gravitational waves originating from the birth of the Universe is a station for Physics. It confirms inflation theory and opens new ways for a "theory of All»
Gravitational waves are the last of the forecasts της Γενικής Θεωρίας της Σχετικότητας που δεν έχει ακόμη επαληθευθεί πειραματικά και ο πληθωρισμός είναι μια θεωρία που ερμηνεύει την ομοιομορφία του Σύμπαντος με την υπόθεση ότι λίγο μετά τη δημιουργία του αυτό διαστελλόταν με ταχύτητες μεγαλύτερες από την ταχύτητα του φωτός. Την περασμένη Δευτέρα αμερικανοί αστρονόμοι ανακοίνωσαν ότι παρατήρησαν τα ίχνη της εκπομπής βαρυτικών κυμάτων κατά τη διάρκεια της πληθωριστικής φάσης του Σύμπαντος, αμέσως μετά τη στιγμή του Big Bang. This discovery is of historical significance, not so much because it confirms the existence of gravitational waves as because it is the first tangible indication - if not proof - of the theory of the inflationary universe. And it may also have opened a window of control for theories that try to unify three of the four basic forces of nature, as well as those that try to link gravity to quantum mechanics.
At the last Monday's press conference at the Harvard-Smithsonian Center for Astrophysics, the scientists (from left) Klem Pike, Jaymi Bok, Chao-Lin Coo and John Cowkay
Last Monday, the BICEP2 research team announced a discovery of great importance: the detection of gravitational waves at the first moments of the creation of the Universe. The news provoked a great feeling because, as American cosmologist Sean Carroll wrote: "Beyond discovering life on other planets or direct dark matter detection, I can not think of another discovery of astronomical nature more important in understanding the Universe from what has just been announced. " It is therefore natural to reward the reward of this discovery with the Nobel Prize in Physics. We will then examine in detail what exactly has been observed and what importance it has in the context of General Theory of Relativity and Cosmology and how exactly this detection has been achieved, ie the instrument used and the way in which the observations were analyzed.
What are the gravitational waves
From the early 1930 it was understood that the General Theory of Relativity, formulated by Einstein 15 years earlier, predicted the existence of gravitational waves, that is, space and time disorders that spread to the Universe at the speed of light. The effort to detect these waves has begun since the 1960 decade, but to date they have not been directly observed despite the construction of increasingly sensitive detectors because their amplitude is very small. Direct observation of gravitational waves will surely be rewarded with a Nobel Prize, but there is hardly any doubt about their existence, since there are too many indirect observations that confirm their existence. So their direct observation will not change anything important in our perception of the Universe. On the contrary, the first moments of the Universe are now the subject of intense research effort and many alternative theories, because there is not enough experimental data to help us reject some of them for the benefit of others.
Lightning inflation
Διάγραμμα της addressς της πόλωσης συναρτήσει της θέσεως που δείχνει τη μορφή πόλωσης του τρόπου-Β
The prevailing theory of the early stages of the Universe is that of inflation, which was formulated in 1980 to explain the fact that for distances much greater than the distances between galaxies the Universe is homogeneous and isotropic. This means that the Universe has the same properties in all its parts, and that there is no direction in it that has particular properties. Furthermore, the theory of inflation explains why the Universe is practically flat, a property that has many consequences, one of which is Euclid's well-known conclusion that the sum of the angles of a triangle is 180 degrees. So according to the theory of the inflationary Universe, just a moment after the Big Bang and for a very short period of time, only 10-32 seconds (that is, ten billionths of a trillionth of a trillionth of a second), the Universe expanded at an unimaginable speed, very faster than the speed of light. The cause and details of this rapid expansion, called inflation, are not clear, as there are many different theories. So it would be very important to ascertain, firstly, whether there was an era of inflation in the early age of the Universe in the first place and, in the event that the answer is positive, to see which of the theories that propose inflation agree with the observations.
The trace of frosted flash
The aim of the observations of the BICEP2 group was to record with unparalleled detail the background microwave radiation, as is called the "echo" of the Great Explosion in the present day. The "glow" of that era was cooled by the expansion of the Universe, and by the minimum wavelength of the g-rays it has now reached a wavelength of one centimeter in the microwave region. This radiation is the only information we have of the beginnings of the universe, but unfortunately it has been emitted 380.000 years after the Great Explosion, so one would say that it does not allow us to "see" the first moments of the Universe. But this is not correct because at the first moments there were intense gravitational waves beyond the photons, causing the photons to "collide" with the gravitational waves and to acquire properties characteristic of these "conflicts".
The most important of these properties is the polarization of light, i.e. the oscillation of the light waves in a preferred direction. If we record the polarization of background microwave radiation, we will be able to see if it agrees with the existence of an inflationary period in the Universe, as well as its quantitative characteristics, for example, the rate of expansion and the duration of the phenomenon. Today there are several research teams working in this direction using instruments such as the BICEP telescope family, the South Pole telescope, the Polarbear telescope and the Planck spacecraft. All these instruments have found that microwave background radiation is weakly polarized and in a special way called "B-mode", in which the polarization direction changes with the viewing direction so as to give the image a "turbine". But the existence of B-mode may be due to other causes other than the presence of gravitational waves. The first research team to announce polarization due to gravitational waves is the BICEP2 team.
Why in the South Pole?
The BICEP research program began observations in 2006 with the first-generation telescope, which operated until 2008. Then BICEP2, ten times more sensitive, took over and operated for three years, from the beginning of 2010 to the end of 2012. Two other instruments of the same group are currently in operation, with a sensitivity ten times better than that of BICEP2. All instruments are installed in American base Amundsen-Scott, που βρίσκεται πάνω στον γεωγραφικό Νότιο Πόλο. Η επιλογή της τοποθεσίας έγινε κατ' αρχάς επειδή στη θέση αυτή η υγρασία της ατμόσφαιρας είναι ελάχιστη, αφού όλο το water exists in the form of ice. Thus the absorption of microwaves caused by atmospheric water vapor is negligible and the quality of observation is comparable to that from a spacecraft such as the Planck mission. But there are three other important advantages: (a) the atmospheric conditions are extremely stable, which minimizes the sources of possible observational errors, (b) the observation area does not change with the time of day, since the visible stars neither rise nor two, and (c) the low temperature prevailing there makes it easier to maintain the observing instruments at the extremely low temperature required, only 0,25 of a degree above absolute zero, i.e. -272,9 degrees Celsius.
BICEP2: hypersensitive and flexible
The BICEP2 telescope has a lens diameter of only 20 centimeters, minimal compared to the observation instruments of other research teams working on the same object. For example, the SPT telescope has a diameter of 10 meters, that of the Polarbear 2,5 meter, while the Planck Space Telescope has a diameter of 1,7 meters. But due to the small diameter of the BICEP2 telescope, it has the ability to observe a much larger portion of the sky than the rest of the sky observes. So it was able to detect fluctuations that are long wavelength and extend over long distances in the sky, such as those due to the interaction of gravitational waves and inflationary expansion. But I would say that the extra sensitivity of the polarization detectors, which are developed on the telescope's focus and record the polarization of the radiation at two frequencies, 100 and 150 MHz (that is, about the frequency of the radio broadcast FM stations).
Polarization with a "gravitational" signature
The data from the three years of observations were analyzed with great care and it was found that polarization occurs at the expected angular distance in the sky and has a pronounced "turbulence", the combination of which is the sign of the gravitational-wave interaction. Next, the necessary control of the significance of this result was made, which concluded that the probability of the observed result due to statistical fluctuations rather than the desired natural phenomenon is less than one in 2.000.000. This in the philosophy of science is considered certainty.
For what inflation are we talking about?
However, apart from the fact of the first statistically confirmed detection of the imprint of gravitational waves in the polarization of microwave radiation, which I emphasize confirms the hypothesis that the Universe went through an inflationary expansion phase, the quantitative Results of the experiment. In other words, how big is the amplitude of gravitational waves and how does that depend on their wavelength? The latter in particular is of particular interest because it is linked to the details of the 'kind' of inflation for which many theoretical models exist today. Here we should note that traditionally the amplitude of gravitational waves, AT, is measured by the value of a parameter r = AT/AS , where the amplitude of the density perturbations of the Universe, AS, has already been measured by the Planck space mission.
The unexpectedly high price
The price r = 0,2 measured by the BICEP2 group is considered to be extremely high and does not agree with the observations so far or with the theoretical models of inflation that are now considered more acceptable. For example, the Planck mission had set r = 0,11 as the upper limit of the width of the gravitational waves. In spite of this discrepancy between observations, cosmologists were surprised by the announcement of the r = 0,2 price for another theoretical reason. Most inflation models give r values lower than 0,01! Therefore, the r = 0,2 value measured is a great challenge for the theorists, in the sense that they should proceed to a bold revision of their inflation models, since of course the results of BICEP2 are confirmed by other observations.
Alma on the way to "Theory of All"
Beyond the above, BICEP2's observations have provided two important elements to the physics of elementary particles. The first is that the new observation alone provides data on the interconnection of gravity with quantum mechanics, which still seeks a correct theoretical approach. The second is that gravitational waves of such a wide amplitude involve actions in the area of which the integration of electricity with patients and strong nuclear forces is expected. So we have a natural lab in front of us that allows us to study this unification without the need to build a much larger accelerator than CERN. The state of gravity wavelength dependence on wavelength appears more complex at this time, and rather requires a more detailed study of the BICEP2 group's announcement.
An announcement, many questions
Announcing the results of an experiment through a press conference is a completely unusual practice in the international scientific community. An observation of this importance would be expected to be sent for publication in one of the two most prestigious scientific journals, either in Nature or in Science. These journals have very strict rules for the publication of scientific papers and in particular require the approval of the publication by three independent reviewers and the prohibition of any official announcement of the results before the publication of the issue of the journal in which it is published. Therefore, one would say that by announcing the results in a press conference, the participants of the experiment "burned" their discovery, in the sense that it could not be published in any of them. The reason they followed this path is not obvious at this time, but we should note three points. The first is that the head of the experiment, John Kovac, a professor at Harvard University, is not a random scientist in the field. His doctorate, 12 years ago, focused on exactly the first polarization detection in background microwave radiation. The international scientific community therefore takes its results and opinions very seriously. The second is that last Tuesday the magazine "Nature", in a completely unusual move, published a "package" of videos and printed information with which it warmly supports the announcements of the BICEP2 team. It is therefore possible for a topic of this importance for the magazine to take second place the rules of publication that it traditionally follows. The third is that there are currently about a dozen research teams working on gravitational wave detection through the study of the polarization of background microwave radiation. It is possible that one of them has reached the same result or even has already sent a similar discovery for publication in a magazine, so it is a matter of priority in the case of a future Nobel Prize.
Haris Varvoglis is Professor of the Department of Physics of the Aristotle University of Thessaloniki.
