Did the Big Bang actually occur?

Planckton

First the Higgs, now the B-modes. This discovery lets us look further into the past of space than ever before. But what exactly was measured by Bicep2 and how reliable are the results?

At lunch yesterday with my astrophysical postdoc colleagues, the question was not long in coming: “So, what do you think about the bicep2 discovery?”. Since we are all researching phenomena a long way away from the Big Bang, we quickly discovered that not all details of the discovery and its consequences were completely clear to us. After all, astrophysics is such a broad field that it is anything but trivial for non-cosmologists what their colleagues do who deal with the origins of our universe. Thankfully, astrophysical institutes offer enough opportunities to intercept those colleagues and to ask them all the questions that remained open after reading the publication describing the discovery. Accordingly, yesterday I not only spent lunch discussing the early phase of the universe, but also the afternoon tea break to work through the cosmological news that was brought out into the world from Cambridge on Monday.

© F.A.Z. Announcement of the press conference to announce a “major discovery”, screenshot

That something special was coming was already clear last Friday when the announcement of a declaration to be held on Monday at the Harvard-Smithonian Center for Astrophysics on a “major discovery” made the rounds on the Internet. What this discovery would be was admirably kept secret. Not even scientists in close proximity to the responsible research team had any idea what it might be. On Monday at 2:45 p.m. the time had come: “First Direct Evidence of Cosmic Inflation”, so the promising title of the presentation. However, the transmission of the presentation in the web stream broke down immediately. The title of the paper published at the same time indicated that the Bicep2 collaboration reported the detection of B-mode polarization in the cosmic background radiation.

In order to understand what that means, one must first deal with the concept of cosmic inflation. Cosmic inflation describes the idea that very shortly after the Big Bang, at an age of about 10 to the power of minus 38 seconds to about 10 to the power of minus 32 seconds, the universe expanded exponentially at a speed faster than light. Dieter Lüst illustrates this expansion in his book “Quantenfische” with the fact that the growth of the universe during inflation corresponds to the growth of one centimeter to ten million times the size of the Milky Way within a trillionth of a trillionth of a trillionth of a second. That something like this should have happened shortly after the creation of the universe does not sound obvious, especially since it is unclear what caused the exponentially rapid expansion and what could have stopped it.

© dpaZeitpfeil: From the Big Bang to today.

The relatively solidly secured cosmological knowledge only begins after the inflation phase, when the universe had already cooled down to an energy scale that can be described by the standard model of particle physics. At that time, the universe consisted of a hot mixture of different elementary particles and expanded at a speed that corresponds to Einstein’s field equations. The further development of the universe can be reconstructed surprisingly well on the basis of the expiring expansion together with the known laws of microphysics. This reconstruction is consistent with empirical findings, such as the observed number of neutrino families and the abundance of helium in the universe. The so-called “baby photo” of the universe, the cosmic background radiation, comes from a much later phase, around 380,000 years after the Big Bang. The portrayal of the early phase of the universe in many textbooks and illustrations therefore often only begins when the universe is just under a second old, leaving out the stages of development that were empirically on a rather uncertain basis up to now.
This naturally raises the question of why such an exotic concept as inflation should be brought into play at all if modern physics should not introduce misunderstood quantities that are not absolutely necessary. One of the reasons why inflation is needed is because it can solve two massive problems of cosmology. The first is the flatness problem: cosmological observations indicate that the universe is geometrically flat. Its energy density thus corresponds to a critical value that must have been realized in the early phase of the universe with an accuracy that seemed absurd. Such coincidences and fine-tuning without a deeper explanation are unacceptable within physics.

The second prominent problem is the horizon problem. The cosmic background radiation, as it was recently measured by the Planck satellite, for example, shows a young universe that (apart from minor fluctuations) looks the same in all directions. If the universe had always expanded evenly according to Einstein's field equations, this area would be made up of many small sub-areas that, due to the finite speed of light, could not causally interact before the cosmic background radiation arose: There was no information between these sub-areas , Radiation or matter are exchanged. The uniformity of the cosmic background radiation would therefore also be a mystery. Inflation can solve both of these problems. Due to the exponentially fast expansion, deviations from a flat geometry are "ironed out" and a causally interacting area of ​​space is inflated in such a way that it can occupy the entire area of ​​the universe that is visible to us. A third, important reason is that without inflation one would not be able to explain how structures such as our Milky Way, for example, arose from a homogeneous universe, as is assumed in the classical cosmological models.

The existence of several maxima in the power spectrum of the cosmic background radiation is one of the predictions of the inflation theory.

Despite all the theoretical elegance, the problem remained that there was no really reliable empirical evidence for the existence of inflation. A first, important success of the inflation model was when WMAP was able to detect the second acoustic peak in the power spectrum of temperature fluctuations in the measurement of the anisotropy of the cosmic background radiation in accordance with the inflation model. With regard to cosmic background radiation, inflation makes another prediction, namely that it should be somewhat more uniform on large scales than on small scales - this, too, is in line with the data available so far. Another very central consequence of the inflation model is that the phase of inflation must have led to the emission of gravitational waves. The ultimate success of the inflation model is closely linked to the possibility of detecting these gravitational waves. Such a detection can be carried out indirectly based on the effect of the gravitational waves on the cosmic background radiation. “The most immediate prospect [to detect primordial gravitational waves] is based on the cosmic background radiation, and this has become the core of all future background radiation experiments”, reads accordingly in the European “Science Vision” of 2007, in which the most pressing questions of Astrophysics and possible experiments to answer them are presented.

This is exactly what the BICEP2 experiment succeeded in doing. In order to understand the results, one has to answer the question of how inflation affected the cosmic background radiation (a nice summary of this can be found on Sean Carroll's blog). Inflation has not only smoothed out spacetime, it has also inflated microscopic disturbances, specifically the quantum mechanical fluctuations of existing quantum fields, in the exponential expansion of spacetime. Two different quantum fields are decisive here: On the one hand, the inflation field itself fluctuated, which was converted into radiation and matter after the inflation phase. These fluctuations are therefore preserved as density fluctuations and are visible in the cosmic background radiation as temperature fluctuations. On the other hand, fluctuations in the gravitational field were increased, which - reinforced by inflation - represent the gravitational waves already mentioned. These gravitational waves lead to a polarization of the cosmic background radiation, that means they cause an alignment of the oscillation plane of the electromagnetic radiation.

© Sky & Telesope The polarization patterns of E-modes and B-modes

That the cosmic background radiation actually has a slight polarization was shown for the first time in 2002 by a group around John Kovac, who is now also responsible for the Bicep2 observations. However, the polarization detected at the time was not due to gravitational waves, because the cosmic background radiation is polarized not only by the fluctuations in the gravitational field, but also by fluctuations in the density field such as those caused by fluctuations in the inflation field. Both types of polarization (roughly speaking: scalar E-modes for polarization due to density fluctuations, tensor B-modes due to gravitational waves) can be differentiated in principle by their spatial shape: E-modes are star or circular while B-modes are Contain a twist by comparison. So if you want to learn about the gravitational waves caused by inflation, you have to look for B-modes in the polarization of the cosmic background radiation. The problem is made more complicated by the fact that E-modes can be converted into B-modes by gravitational lens effects, an effect that has also already been observed. The fact that one can learn more about inflation from B-modes than from E-modes is ultimately due to the fact that one does not know the connection between density fluctuations and fluctuations in the inflation field, while the strength of the gravitational waves depends directly on the energy scale of inflation.

© Reuters The Bicep Telescope and the 10m South Pole Telescope.

The Bicep2 experiment stationed at the South Pole observed a small area of ​​cosmic background radiation in the direction of the galactic South Pole at a frequency of 150 gigahertz (almost 2 millimeters) from 2010 to 2012. This region of the sky has been chosen particularly favorably because there is very little interference from the Milky Way. The measured polarization was broken down into E and B modes and plotted as a function of the angle scale of the observed area (roughly speaking, the angles indicate the distance between those points in the sky that are compared with one another in the analysis). B-modes, which can be traced back to gravitational waves, show up in particular on large angular scales, which are relatively little affected by the polarization by E-modes. In fact, the data show significantly higher polarization values ​​on these scales than would be expected based solely on density fluctuations. Modeling the data results in a ratio (the so-called “r-factor”) between gravitational waves (“tensor”) and density fluctuations (“scalar”) of 0.2 with a statistical significance of 7 sigma. The influence of gravitational waves caused by inflation on the cosmic background radiation would have been demonstrated for the first time, and this with an unexpectedly high amplitude: previously an r-factor close to zero was assumed, i.e. a much lower proportion of tensor modes in comparison to scalar modes (At this point the discovery appears to be in contradiction to the previous results of the Planck mission. However, there seem to be ways of alleviating or resolving this contradiction, which are now to be worked out). The paper ends with the sentence: “The long search for tensor B-modes is apparently over and a new era of B-mode cosmology has begun”.

© BICEP2 B-mode pattern found by Bicep2

In fact, this result now brings many approaches to weed out theories of inflation and further concretize them. The r-value of 0.2 is in line with the simplest theories of inflation, such as those developed by Andrei Linde in the 1980s (here you can see Linde's reaction to the discovery). The strength of the gravitational waves makes it possible to set the energy scale of inflation. The value of the r-factor sets this energy scale very high: it is about 13 orders of magnitude above the energy range that the LHC can reach near Geneva. With this observation, it seems to have actually been possible to get indications of how the universe behaved unimaginably shortly after its creation: our view of the universe's past has expanded decisively. This information is especially valuable in understanding what a unified theory of quantum gravity might look like.

Despite all the euphoria, an important question is of course how robust the Bicep2 results are. The measurement results were first checked for experimental errors in so-called “jackknife tests” by subtracting subsets of the data that were exposed to various instrumental influences. The influence of synchrotron emission, which is also polarized and emitted by accelerated electrons, was tested on the basis of WMAP measurements. A disruptive effect that is very critical for the result is the polarized radiation from interstellar dust particles, which has so far been relatively poorly understood. Here the team looked at various models and checked the measurement data for a possible correlation with the modeled dust radiation. Such a correlation was not found, but even if a possible polarization due to dust is subtracted from the signal, the B-mode excess persists, although somewhat weakened at an r-value of 0.16. The results are particularly convincing, not least because the correlation of the new data with the data from the previous Bicep1 experiment shows that the detected signal can also be found in the old data, not only at the frequency observed by Bicep2, but also at one another (100 GHz). The ratio of the signals at these two frequencies does not seem compatible with the fact that they were generated by the emission of dust. So overall, the discovery seems impressively robust. More will be seen when other experiments try to reproduce the result in a targeted manner.

© ESA The Planck telescope - soon confirmation of the Bicep2 discovery?

A candidate for this would of course be the Planck satellite, which had actually been discussed for the discovery of B-modes. Its polarization data are still being evaluated. Since Planck's observations cover the entire sky and are therefore not primarily aimed at discovering B-modes, it may be that the sensitivity of the observations is not sufficient to detect B-modes. In any case, the Bicep2 discovery will mean that the design of planned missions can now be tailored to the physical requirements in a much more targeted manner. A colleague from Harvard, whose office is right next to the room in which the presentation took place on Monday, stated on Monday evening that she should perhaps have chosen cosmology or particle physics as her research area instead of galactic astrophysics. I don't think she wanted to exchange her comparatively solid observations of galactic phenomena for complicated polarization analyzes, but she is right: first the Higgs and now the B modes. The times could hardly be more exciting for exploring the high-energy origins of our universe.

Many thanks to Nicolas Ponthieu for patiently answering my numerous questions and giving me valuable advice.

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Up close and personal with the Big Bang

From Sibylle Anderl

First the Higgs, now the B-modes. This discovery lets us look further into the past of space than ever before. But what exactly was measured by Bicep2 and how reliable are the results?

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