Gravitational waves from early universe remain elusive

A joint analysis of data from the Planck space mission and the ground-based experiment BICEP2 has found no conclusive evidence of gravitational waves from the birth of our universe, despite earlier reports of a possible detection. The collaboration between the teams has resulted in the most precise knowledge yet of what signals from the ancient gravitational waves should look like, aiding future searches.

The color scale in this image from the Planck mission represents the emission from dust,
 a minor but crucial component that pervades our Milky Way galaxy. The texture indicates
 the orientation of the galactic magnetic field. It is based on measurements 
of the direction of the polarized light emitted by the dust
 [Credit: ESA/Planck Collaboration]

Planck is a European Space Agency mission with significant NASA contributions. BICEP2 and its sister project, the Keck Array, are based at the South Pole and funded by the National Science Foundation, also with NASA contributions.

"By analyzing both sets of data together, we could get a more definitive picture of what's going on than we could with either dataset alone," said Charles Lawrence, the U.S. project scientist for Planck at NASA's Jet Propulsion Laboratory, Pasadena, California. "The joint analysis shows that much of the signal detected by BICEP2/Keck is coming from dust in the Milky Way, but we cannot rule out a gravitational wave signal at a low level. This is a good example of how progress is made in science, one step at a time."

Planck and BICEP/Keck were both designed to measure relic radiation emitted from our universe shortly after its birth 13.8 billion years ago. An extraordinary source of information about the universe's history lies in this "fossil" radiation, called the cosmic microwave background (CMB). Planck mapped the CMB over the entire sky from space, while BICEP2/Keck focused on one patch of crisp sky over the South Pole.

In March of 2014, astronomers presented intriguing data from the BICEP2/Keck experiments, finding what appeared to be a possible signal from our universe when it was just born. If the signal were indeed from the early cosmos, then it would have confirmed the presence of ancient gravitational waves. It is hypothesized that these waves were generated by an explosive and very rapid period of growth in our universe, called inflation, which took place when the universe was only a tiny of a fraction of one second old.

The anisotropies of the Cosmic Microwave Background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380 000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today. The highest resolution version of this image [12 572 px ? 6286 px] is available upon request. Please make inquiries using the "Contact Us" link in the left-hand menu 
[Credit: ESA/Planck Collaboration]

Specifically, the BICEP/Keck experiments found evidence for a "curly" pattern of polarized light called B-modes. These patterns would have been imprinted on the CMB light as the gravitational waves slightly squeezed and stretched the fabric of space. Polarization describes a particular property of light. Usually, the electric and magnetic fields carried by light vibrate at all orientations equally, but when they vibrate preferentially in a certain direction, the light is polarized.

"The swirly polarization pattern, reported by BICEP2, was also clearly seen with new data from the Keck Array," said Jamie Bock of the California Institute of Technology in Pasadena, and JPL, a member of both the BICEP2/Keck and Planck teams.

"Searching for this unique record of the very early universe is as difficult as it is exciting, since this subtle signal is hidden in the polarization of the CMB, which itself only represents only a feeble few percent of the total light," said Jan Tauber, the European Space Agency's project scientist for Planck.

One of the trickiest aspects of identifying the primordial B-modes is separating them from those that can be generated much closer to us by interstellar dust in our Milky Way galaxy.

The Milky Way is pervaded by a mixture of gas and dust shining at similar frequencies to those of the CMB, and this closer, or foreground, emission affects the observation of the oldest cosmic light. Very careful data analysis is needed to separate the foreground emission from that of the CMB.

This image shows the all-sky maps recorded by Planck at nine frequencies during its first 15.5 months of observations. These were collected using the two instruments on board Planck: the Low Frequency Instrument (LFI), which probes the frequency bands between 30 and 70 GHz, and the High Frequency Instrument (HFI), which probes the frequency bands between 100 and 857 GHz. The Cosmic Microwave Background is most evident in the frequency bands between 70 and 217 GHz. Observations at the lowest frequencies are affected by foreground radio emission from the interstellar material in the Milky Way, which is mostly due to synchrotron radiation emitted by electrons that spiral along the lines of the Galactic magnetic field, but also comprises bremsstrahlung radiation, emitted by electrons that are slowed down in the presence of protons, as well as emission from spinning dust grains. Observations at the highest frequencies are affected by foreground emission from interstellar dust in the Milky Way. The combination of data collected at all of Planck's nine frequencies is crucial to achieve an optimal reconstruction of the foreground signals, in order to subtract them and reveal the underlying Cosmic Microwave Background [Credit: ESA/
Planck Collaboration]

"When we first detected this signal in our data, we relied on models for galactic dust emission that were available at the time," said John Kovac, a co-principal investigator of the BICEP2/Keck collaboration at Harvard University, Cambridge, Massachusetts. "These seemed to indicate that the region of the sky chosen for our observations was relatively devoid of dust."

The BICEP2/Keck experiments collected data at a single microwave frequency, making it difficult to separate the emissions coming from the dust in the Milky Way and the CMB. On the other hand, Planck observed the sky in nine microwave and sub-millimeter frequency channels, seven of which were also equipped with polarization-sensitive detectors. Some of these frequencies were chosen to make measurements of dust in the Milky Way. By careful analysis, these multi-frequency data can be used to separate the various contributions of emissions.

The Planck and BICEP2/Keck teams joined forces, combining the space satellite's ability to deal with foregrounds using observations at several frequencies, with the greater sensitivity of the ground-based experiments over limited areas of the sky.

"The noise in the instruments limits how deeply we can search for a signal from inflation," said Bock. "BICEP2/Keck measured the sky at one wavelength. To answer how much of the signal comes from the galaxy, we used Planck's measurements in multiple wavelengths. We get a big boost by combining BICEP2/Keck and Planck measurements together, the best data currently available."

This illustration shows how photons in the Cosmic Microwave Background (CMB) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the Universe. Using data from ESA's Planck satellite, cosmologists have been able to measure this gravitational lensing of the CMB over the whole sky for the first time. The CMB consists of the most ancient photons in the history of the Universe, which were emitted only 380,000 years after the Big Bang. Since then, CMB photons have travelled for over 13 billion years across the Universe, witnessing the dramatic changes that took place during the various cosmic epochs such as the formation of stars, galaxies and galaxy clusters. Several observable effects arise from the interaction between CMB photons and the large-scale structure they crossed during their journey. One of the most intriguing is gravitational lensing, the deflection of light as it travels in the vicinity of massive objects such as galaxies and galaxy clusters. Gravitational lensing creates tiny, additional distortions to the mottled pattern of the CMB temperature fluctuations. On their way to the Solar System, where they are eventually detected by the sensors on board Planck, photons from the CMB may cross many different massive systems as well as empty spaces. The photons encounter structures in different evolutionary stages, since the massive structures grow denser and the cosmic voids become less dense as time goes by. All of the structures from the different cosmic epochs contribute to bending the path of CMB photons. The total effect of these multiple deflections is a modification to the pattern of CMB temperature fluctuations, thus changing the typical 'shapes' of hot and cold spots in the CMB. As a result of the detection of gravitational lensing experienced by CMB photons, cosmologists can use Planck to explore 13 billion years of the formation of structure in the Universe [Credit: ESA/Planck Collaboration]

The final results showed that most of the original BICEP2/Keck B-mode signal, but not necessarily all of it, could be explained by dust in our Milky Way. As for signs of the universe's inflationary period, the question remains open.

The joint Planck/BICEP/Keck study sets an upper limit on the amount of gravitational waves from inflation, which might have been generated at the time but at a level too low to be confirmed by the present analysis.

"The new upper limit on the signal due to gravitational waves agrees well with the upper limit that we obtained earlier with Planck using the temperature fluctuations of the CMB. The gravitational wave signal could still be there, and the search is definitely on," said Brendan Crill, a member of both the BICEP2 and Planck teams from JPL.

A paper on the findings is still under peer review.

Source: NASA/Jet Propulsion Laboratory [January 31, 2015]