Fifty Years After Cosmic Discovery, BICEP2 Observations Make a Big Bang
On March 17, 2014, I was in the middle of the Pacific Ocean, as a guest speaker on astronomy for cruise-ship passengers on a world voyage. After my talk on the nature of the universe, a member of the audience asked my thoughts on “today’s news story that surely will mean a Nobel Prize for the person responsible for the theory of the inflationary universe.” So that’s how I first learned about the greatest discovery in astronomy for half a century. Its importance for fundamental physics is as significant as the finding of the Higgs Boson announced in July 2012.
A team of astronomers using the BICEP (Background Imaging of Cosmic Extragalactic Polarization) instrument on a small telescope located at the South Pole have detected and measured circular polarization in the cosmic microwave background (CMB). These ripples in the thermal sea of fossil radiation from the early universe make a strong case for the existence of primordial gravitational waves, and there’s a hint of evidence for the era of cosmic inflation.
To place the BICEP2 claims in a historical perspective, ScienceWatch searched Thomson Reuters Web of Science to find the key papers on the “Cosmic Microwave Background” that marked the pathway to the BICEP2 results.
The Cosmic Microwave Background: 50 Years of Hot Papers
(Listed in reverse chronological order, with notes on each paper’s findings and significance)
|1||Planck Collaboration (P.A.R. Ade, et al.), “Planck 2013 results. XVI. Cosmological parameters,” arXiv: 1303.5076, March 2014.
Principal results: Planck is the successor to WMAP. The first results determine the cosmological parameters to high precision, and they are in accord with standard spatially-flat-six parameter ΛCDM (Lambda Cold Dark Matter) cosmology, and in excellent agreement with constraints from baryon oscillation surveys.
|2||G. Hinshaw, et al., “Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological parameter results,” Astrophysical J. Suppl. Ser., 208(2): No. 19, October 2013.
Principal results: The final presentation of WMAP results combines WMAP data from many cosmological datasets to obtain values for the cosmological parameters to a precision of 1.5%. The values in this paper are the ones now being used in observational cosmology.
|3||E. Komatsu, et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological interpretation, “ Astrophysical J. Suppl. Ser., 192(2): No 18, February 2011.
Principal results: The seven-year data are combined with astrophysical data to determine the six parameters of the ΛCDM universe. The paper reports significant improvements in the measurement of E-mode and B-mode polarization.
|4||K. Komatsu, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation,” Astrophysical J. Suppl. Ser., 180(2): 330-76, February 2009.
Principal results: This paper builds on and consolidates the results from #5 [below]. Astronomers who observe the extragalactic universe use the values of the cosmological constants extensively.
|5||D.N. Spergel, et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Determination of cosmological parameters,” Astrophysical J. Suppl. Ser., 148(1): 175-94, September 2003.
Principal results: This paper opens the era in which cosmology becomes a precision science. Values for cosmological parameters are given with unprecedented accuracy. The concordance model begins here, too: the ΛCDM universe. The paper is highly cited because it removed all of the uncertainties about the age of the universe and its composition.
|6||P. de Bernardis, et al., “A flat universe from high-resolution maps of the cosmic microwave background radiation,” Nature, 404(6781): 955-9, 27 April 2000.
Principal results: The BOOMERanG (Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetics) experiment used a microwave telescope that was flown to an altitude of 38 km by a balloon. This was the first CMB mission conducted in Antarctica. The high-resolution maps provided evidence for a flat universe (no curvature); importantly, this paper demonstrated that high-resolution maps would allow precision measurements of cosmological parameters.
|7||D.J. Schlegel, et al., “Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds,” Astrophysical J., 500(2): 525-53, 20 June 1998.
Principal results: Astrophysical papers that require estimates of Galactic extinction cite the maps derived from COBE data of the infrared emission from dust in the Milky Way. They were also used to remove the foreground contamination from the CMB experiments that followed COBA.
|8||G.F. Smoot, et al., “Structure in the COBE differential microwave radiometer 1st-year maps,” Astrophysical J., 396(1): L1, 1 September 1992.
Principal results: The Cosmic Background Explorer (COBE) had a long gestation period prior to launch in late 1989. On January 14, 1990, the COBE team announced a temperature of 2.73 K for the CMB. Key paper #8 publishes the map of the anisotropies in the CMB that showed the presence of structure in the early universe.
|9||R.H. Dicke, et al., “Cosmic black-body radiation,” Astrophysical J., 142(1): 414-9, 1 July 1965.
Principal results: This paper examines the thermal history of an expanding universe that starts from a singularity and contains black body radiation. The first part describes a work in progress: Dicke and Roll had constructed a radiometer and receiving horn for the detection of the fossil radiation at a wavelength of 30 mm. On hearing of the results that were to be announced in paper #10 [below], Peebles pointed out the cosmological significance of the “excess antenna temperature.” The history of modelling the Hot Big Bang begins with this paper.
|10||A.A. Penzias, R.W. Wilson, “A measurement of excess antenna temperature at 4080 MC/S,” Astrophysical J., 142(1): 419-21, 1 July 1965.
Principal results: This is the first announcement of the discovery of excess microwave radiation from the whole of the sky, 4.08 GHz (73 mm).
|SOURCE: Thomson Reuters Web of Science *Unless stated otherwise, citations are from the Web of Science. For paper #1, which was not published at this writing, the citation record is taken from the SAO/NASA Astrophysics Data System (ADS).|
Fifty years of the cosmic microwave background
It’s 50 years since the serendipitous discovery in 1964 of the CMB transformed observational cosmology, setting it on a path that has led to its becoming a precision science. The CMB is the oldest light in the universe, released 380,000 years after the Big Bang when the universe was cold enough for electrons and protons to stick together as hydrogen atoms.
This fossil radiation is a rich source of information about the early universe. The Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, together with the Atacama Cosmology Telescope and the South Pole Telescope have given us remarkable results on the structure and evolution of the universe. Considered alongside other cosmological tests, the case for Big Bang cosmology is now overwhelming.
During the past two decades a concordance model of cosmology has flourished, underpinned by exquisite measurements of key parameters such as the cosmic proportions of baryonic matter, cold dark matter, and dark energy. The LCDM (Lambda Cold Dark Matter) model is frequently referred to as the Standard Model. It describes the parameters of a universe with a cosmological constant (denoted by L) associated with the dark energy that drives the acceleration of the universe, and containing the cold dark matter the made the universe lumpy, so that galaxies formed.
We have come a long way in half a century. Instead of cosmology being a matter of belief in which practitioners would back either the Steady State Model or the Big Bang concept, it has blossomed as a precision science, with the capacity to probe the earliest universe.
BICEP2 at the South Pole
The astronomical facilities at the Amundsen-Scott South Pole Station benefit from the high altitude of 2,835 m and an extremely dry atmosphere: this is as close as a microwave telescope can get to space with leaving the ground. The BICEP2 observing runs were in the midst of the Antarctic winter and required 72 hours of continuous operation. The three-year data set was taken in the years 2010–12.
The BICEP2 instrument measures the so-called B-mode circular polarization of the CMB on angular scales of 1 – 5 degrees. It is one of a family of experiments, the BICEP/Keck Array series, all of which demand extreme sensitivity and rigorous control of systematic errors. The focal plane of the 26-cm telescope is where the BICEP2 detector resides on a superconductor printed circuit board. It uses a completely new technology: an array of 512 transition-edge sensor bolometers operating at 150 GHz. The detectors operate at 270 mK to reduce photon noise. The set-up filters, processes, images, and measures polarized radiation.
BICEP2 scanned a carefully selected field of 380 square degrees centered at RA = 0 hr, dec = -57.5°, less than 1% of the entire sky. The chosen direction is well away from the plane of the Milky Way, so the line-of-sight to the depths of the universe has a very low level of contamination by interstellar dust, and that’s why it was selected.
B-mode Circular Polarization
The CMB temperature anisotropy has E-mode polarization (produced by scalar density fluctuations) and B-mode polarization (produced by primordial gravitational waves). Since 2002 there have been many measurements of the E-mode pattern, including those of WMAP and Bicep1. B-modes are more difficult to detect, thus requiring greater sensitivity.
The figure above shows the E signal and B signal as released by the BICEP2 Collaboration. The polarization maps are the deepest ever made at degree angular scales: the noise level is 87 nanoKelvin degrees. The open access downloadable version of the paper is at http://arxiv.org/abs/1403.3985. Currently it is still in peer-review stage, which is being conducted openly via social media.
The distorted curling patterns in the B-mode map are just as expected from strong gravitational waves pummeling the very fabric of spacetime. These giant waves are a by-product of the inflation era of the universe.
Why Cosmic Inflation is Important
In 1981, Alan Guth (then at Stanford, now at MIT) proposed that the universe went through a phase of exponential expansion driven by positive vacuum energy density. In 2006 WMAP results provided a strong hint for inflation. The BICEP2 results take us beyond a hint: they are confirmation that the universe really did have an inflation era. (For his theories on cosmic inflation, by the way, Guth has been designated a Citation Laureate, recognized by Thomson Reuters as a likely winner of the Nobel Prize.)
Inflation posits that the entire universe started as a small region of spacetime that expanded exponentially a mere 10-35 seconds after the onset of the Big Bang. In this “outrageous idea” the universe starts as a tiny speck, less than a billionth the diameter of a proton, with a mass no more than a generous glass of wine. It repeatedly doubles in size and mass: 1, 2, 4 , 8 …. This is a reminder of the centuries-old wheat chessboard problem in which one grain is placed on the first square, then 2 , 4, 8 …: how much wheat when the board is complete? The 63 doublings stack up to almost 1,020 grains.
The universe did far better than that! About 275 doublings between 10-35 and 10-32 seconds. At which point it was roughly the size of a basketball, containing all the mass that we now find in the billions of galaxies in the observable universe. And it was flying apart in a universal expansion that continues today 13.8 billion years later.
As the universe raised its game by doubling the stakes, so little ripples of quantum uncertainly became magnified into enormous gravitational waves. And they distorted light in the manner picked up by BICEP2. Furthermore, the irregularities of density they caused eventually led to the condensation of galaxies in the expanding universe.
Inflation explains why the universe is so flat and accounts for why it is roughly homogeneous. It solves two enormous intellectual puzzles, and provides the means of explaining how structure—protons, atoms, planets, stars, galaxies, and clusters of galaxies—arose in the universe.
The Cosmic Frontier: What to Expect
BICEP2 has given us a glimpse of conditions in the universe at the energy scale of 1016 GeV, which is impressively close to the Planck scale of 1018 GeV. Compare that to the Large Hadron Collider which probes conditions at ~104 GeV. Literally, we have to observe the early universe in order to understand it; we’ll never be able to create those conditions in a laboratory, any more than we could make a black hole or a neutron star in a lab.
BICEP2 results need confirmation from Planck, which is part of the reason why the results are still in the peer review phase. Next we can look forward to Bicep3, which will work through 2014-15 at the South Pole. It has 2,560 detectors operating at 100 GHz. Expect improved data too from the Keck Array of five polarization receivers at the pole.
The field is highly competitive: currently about a dozen ground-based and balloon-borne telescopes are targeting the goal of measuring B-mode polarization. Looking further ahead, all-sky polarization measurements will stimulate a new generation of experiments that will be capable of dealing with foreground contamination from the Milky Way and other sources.
Dr. Simon Mitton researches the history of cosmology at the University of Cambridge
The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.