Marc Kamionkowski is a theoretical physicist, with a special interest in dark matter, inflation and the cosmic microwave background. He is a professor of Physics and Astronomy at Johns Hopkins University in the United States.
Bruce Partridge was among the first to search for fluctuations in the cosmic microwave background, the remnant ‘heat’ from the Big Bang. He is part of the science team around ESA’s Planck satellite. Bruce is a Professor of Astronomy at Haverford College in Pennsylvania.
Benjamin D Wandelt has studied a variety of problems in cosmology, and is an internationally acclaimed expert in the analysis of cosmic microwave background. He is Professor and Chair of Theoretical Cosmology at Institut d’Astrophysique de Paris.
Marc Kamionkowski“These gravitational waves that we’re seeing were emitted one trillionth of a trillionth of a trillionth of a second after the Big Bang. That’s pretty damn cool!”
What are gravitational waves and why is this result important?
Marc Kamionkowski: Gravitational waves are propagating ripples in spacetime that are predicted in Einstein’s theory of relativity. We know that they exist, because we have evidence (which was recognized with a Nobel prize in 1993) that gravitational waves are emitted from binary stars. Still, we have never detected the effects of gravitational waves on matter. In other words, we have seen them “transmitted” but we’ve never seen them “detected”.
The new results from BICEP2 at the South Pole are significant in several ways. First of all, we’ve now “detected” the gravitational waves; i.e., we’ve seen them put matter in motion.
Second of all, these gravitational waves that we’re seeing were emitted one trillionth of a trillionth of a trillionth of a second after the Big Bang. That’s pretty damn cool.
And since we have little in terms of information from such early times in the universe, any piece of information from that epoch is extremely valuable.
Bruce Partridge: Recall that in Einstein’s formulation, space-time can be deformed. Mass, for instance, curves space. Gravitational waves are periodic undulations in space time. They can be generated by certain kinds of motion of matter. They travel at the speed of light, and produce slight stretchings of space-time as they sweep by.
Why is that important? If the idea of inflation, that the early universe went through a period of extremely rapid expansion, is correct, then the cosmic structure we see today, in galaxies and clusters and even larger structures, arose from initially microscopic fluctuations that got expanded up by the inflation to astronomical scales.
These initial fluctuations consisted of both changes in density from one place to another, and the stretching of space-time, which equals gravitational waves.
So if inflation is right, the universe ought to have a background of gravitational waves.
The quantity “r” that the BICEP experiment at the South Pole measured is in effect the ratio of gravitational waves to density fluctuations. If inflation is right, it ought to be something like 0.01 to 1.
Finding evidence for gravitational waves is the cleanest test we have of inflation; hence the fevered search for such evidence.
The gravitational waves could be either detected directly, as in the joint ESA-NASALISA project or indirectly, by the polarization imprint they make on the CMB on angular scales of degrees (the so-called B mode of polarization). BICEP detected these B modes.
Bruce Partridge“Finding evidence for gravitational waves is the cleanest test we have of inflation; hence the fevered search for such evidence.”
Benjamin Wandelt: Gravitational waves are a key feature of Einstein’s theory of gravity. They are normally created whenever anything accelerates – so just by walking around you create gravitational waves that “shear” space-time and anything in it – only very, very slightly. “Shear” is what happens when you squeeze a hoola-hoop between your hands, bringing them together from left and right. The circle becomes an ellipse – it shortens between your hands and becomes larger in the vertical direction. The same would happen to the hoola-hoop if a gravitational wave traveled through its center, in the direction you are looking through it.
If the BICEP2 result holds up, this is the first detection of gravitational waves. There is a Nobel prize waiting just for that and major international efforts in direct GW detection are ongoing. One could argue that effects of gravitational waves have been seen in the past, e.g. the slowing rotation frequency of millisecond pulsars is thought to be partially due to gravitational wave emission. But this is the first time the effect of gravitational waves has been observed on something else, in this case on the photons in the primordial plasma – this effect in turn causes polarized emission from the electrons these photons scatter with.
On top of that if the signal is due to primordial gravitational waves, we are seeing the first glimpse of quantum gravity. The waves are excited by quantum mechanical fluctuations of space-time so this measurement reaches deep into the frontier of physics.
How was the measurement made? Gravitational waves create all three types of fluctuations in the cosmic microwave background: temperature, E-mode polarization, and B-mode polarization. But ordinary density perturbations only produce temperature and E-mode polarization so that’s why the discovery of B-modes is exciting. Unfortunately observational systematics, foreground emissions (the Galaxy) and higher-order effects can produce B-modes, too. Of these the higher order effects are the least likely culprit since they can be calculated theoretically and give a small contribution at the range of scales that BICEP2 probes.
A large fraction of the real estate in the BICEP2 papers addresses the issues of systematics. The main issue is that spurious foreground signals cannot be fully excluded yet based on their analyses. The way to distinguish such foregrounds from the cosmic microwave background (CMB) is by using their variation with microwave frequencies (think color!) while the CMB remains constant.
BICEP 2 measured only at one frequency (while Planck measured at 9 different frequencies). They cross-correlated with BICEP1 which was at a different frequency and found consistency but the BICEP1 data was far noisier than the BICEP2 data so this check is not fully convincing. But there will be follow-up data from other ground-based and balloon based experiments, and of course, Planck.
Does this result change our understanding of the Big Bang?
Marc Kamionkowski: This certainly adds greatly to our understanding of the Big Bang. We have an idea, called “inflation,” for what may have set the Big Bang in motion. Many observations we’ve collected over the past decade and a half support the idea of inflation.
Still, there are many different models for inflation which may have worked. This new gravitational-wave signal, though, helps us to greatly narrow the range of models.
Bruce Partridge: Only in that it confirms the idea of a very early (10^-30 sec) period of inflation. There were many other reasons to favor inflation— but the B modes were the “smoking gun.”
Benjamin D Wandelt“If the result holds up, it gives us a direct view of what happened during inflation, so yes, the discovery of gravitational waves from inflation would be a revolutionary result.”
Benjamin Wandelt: If the result holds up, it gives us a direct view of what happened during inflation, so yes, the discovery of gravitational waves from inflation would be a revolutionary result. At the level suggested by BICEP2, the signal is strong enough that a follow-up space mission such as COrE (Cosmic Origins Explorer) can learn about the dynamics of inflation in more detail. Taken at face value, the BICEP2 data excludes inflationary models that were highly favored by the Planck data (such as the Starobinsky model), since they do not predict a sufficiently large gravitational wave amplitude. All inflationary models predict some level of primordial gravitational waves (though the level depends on the model), so this signal has been called the “smoking gun” of inflation.
The power in the gravitational wave signal directly measures the energy scale of inflation independently of the details of the inflationary model. A value as high as r=0.2 implies that inflation occurred at energies a million million times higher than those probed at CERN’s Large Hadron collider!
How does it relate to the Planck satellite data and results?
Marc Kamionkowski: I think that BICEP2 complements Planck very well. Planck is a spectacular instrument that has broad capabilities and can tell us many important things about the Universe.
BICEP2, on the other hand, is a smaller experiment that was targeted exclusively on this gravitational-wave signal, a signal that will probably be difficult (though not impossible; it still remains to be decided) to tease out of Planck data.
Planck also has an important trove of information about Galactic foregrounds that might provide some small contamination to BICEP2, and it will thus be valuable to further confirm and characterize BICEP2’s findings.
Bruce Partridge: Interesting. One goal of the Planck mission was to detect or set limits on r. The most direct way to do so is to measure the B modes, as BICEP did. Planck’s polarization results won’t be available until later in the year, so we can’t yet comment on the B modes.
However, there are other means of limiting r that also come from cosmic microwave background measurements, and those Planck did do, and published some limits.
These can be interpreted as being in tension with the BICEP results. The most general fitting to the Planck results gives an upper limit on r of 0.26 (compatible with BICeP’s r = 0.2). A slightly more restricted analysis including all the Planck data and some other measurements pushes the upper limit on r down to 0.11, in disagreement with BICEP.
We won’t know for sure until Planck polarization results emerge.
Benjamin Wandelt: BICEP2 is a very targeted experiment – it focused hundreds of detectors on a small, clean patch of the sky, mapping over a narrow range of scales (about 1 degree) at a single frequency. By contrast, Planck made maps at 9 frequencies, across the entire sky at twelve times higher resolution.
Aside from the potential sensitivity to foreground contamination, the BICEP 2 result is in a moderate degree of tension with the Planck constraint. Planck had set an indirect upper limit on the perturbation power from gravitational waves and the power found by BICEP2 exceeds this by up to a factor of 2. Now, both of these results have error bars, but the statistical chance for the tension to appear to be this big by chance is a bit less than 1 in 20. Planck did not directly measure B-modes so the tension can be resolved by allowing more freedom in the range of models considered in a joint analysis. This is what BICEP2 did in the analysis that appears in their discovery paper.
As I hinted at before, the BICEP2 data taken at face value will cause a re-evaluation of the entire zoo of inflationary models – this is what our colleagues and us are currently studying.
What kinds of space missions or experiments do we need in order to take this further?
Marc Kamionkowski: Good question. There has been plenty of discussion in Europe, the US, and Japan about a space-based mission that is aimed at studying the so-called “B modes” that BICEP2 has found.
I think that the fact that BICEP2 has now found them bolsters the case for such a satellite, as we now know with far greater confidence that there is a treasure trove of information in this cosmic microwave background signal that can be exploited.
The detailed design of the experiment may differ from what people have considered now that the goal is to study what has turned out to be a relatively strong signal rather than try to simply detect what may have been a very faint signal.
Bruce Partridge: Planck is a space mission, and I suspect has the sensitivity to confirm/deny r = 0.2. Many other ground-based experiments will be going after the B modes as well, including the South Pole Telescope and the Atacama Cosmology Telescope.
To really nail the B modes a satellite equipped with far more detectors than Planck carries would need to be launched. There are at least a couple of competing proposals. But I emphasize that checking r = 0.2 can, and will, be done before any new space mission is planned or launched.
Benjamin Wandelt: I think it’s clear that we need confirmation at different microwave frequencies (colors) and by an independent experiment. There are several competitors of BICEP2 and follow-up experiments such as the Keck array with even more detectors from the South Pole.
From the ground only a few frequencies are accessible since the atmosphere only transmits microwaves in a few atmospheric frequency windows. A next generation CMB polarization space mission, such as the Cosmic Origins Explorer (COrE) concept, would be designed to limit instrumental systematics and have the ability to make an unambiguous measurement across all frequencies, thereby decisively excluding foreground effects. If the level of gravitational wave power seen by BICEP is really there, such a space mission could get high enough data quality to use the gravitational wave signal to tease out even more information about inflation, such as whether only a single field was involved; and to probe (using gravitational waves!) models that have been proposed to explain the Planck anomalies.
See our report- Planck, Higgs and the Big Bang
Also watch: Planck maps the dawn of time
Other useful links:
LISA pathfinder overview
LISA Project Office
Wayne Hu, University of Chicago
Institut d'Astrophysique de Paris
HFI Planck (in French)