This column is about the recent discovery of gravitational waves created in the very early stages of the Big Bang, during what is called the inflationary era. It is a discovery of unprecedented importance in cosmology, because it enables us to visualize and gain information about the universe all the way back to the time when it had a diameter of 10-26 meters and was doubling in size every 10-37 seconds.
Our universe is a very strange place. It is expanding at an ever-increasing rate, driven by some mysterious "dark energy" that makes up about 2/3 of the total mass-energy in the universe. Dark energy is the intrinsic energy of space itself, which we do not understand and we cannot explain. (See my AV Column #95 in the May-1999 issue of Analog). Another 2/7 of the mass of the universe is in the form of "dark matter", another form of mass-energy that we do not understand. Dark matter, which is definitely not the normal matter of atoms and known particles, is concentrated around galaxies and is largely responsible for their formation in the early universe. But perhaps the universe's deepest mystery is cosmological inflation, a process that started immediately after the Big Bang and caused the universe to expand exponentially fast, much faster than the speed of light, and then mysteriously "switched off", allowing the universe to expand at the much more leisurely rate that continues to this day.
Inflation was first suggested by Alan Guth in 1980 in an effort to solve problems with the then-developing Big Bang model of cosmology. Guth was confronting three serious problems inherent in the naive Big Bang model: the problems of horizon, flatness, and monopoles. The horizon problem arises from the fact that separate parts of the universe go out of speed-of-light contact very early in the Bang and have no further contact, yet we find that they have the same temperature with only small variations, as evidenced by measurements of the cosmic microwave background radiation. The flatness problem is related to the observation, made with ever-increasing accuracy, that the curvature parameter W (omega) of the universe is precisely W=1. This means that the energy content of the universe is just right to fit in the crack between positive curvature of excess mass leading to eventual re-collapse from the pull of gravity and negative curvature created by excess kinetic energy of expansion. The monopole problem arises from the prediction by most particle-physics models that the extreme temperatures of the early Big Bang should have produced floods of massive exotic particles, including magnetic monopoles. While most of the other exotics have long since decayed into more normal particles (electrons, protons, neutrinos, photons, ...), magnetic monopoles cannot do that. They have a single magnetic charge that is either an isolated "north" or "south" magnetic pole. Because of that magnetic charge, the monopoles are "stuck" in this configuration and cannot decay to lighter particles, since there are no lighter magnetically-charged particles available, so they should be around today. Nevertheless, all experimental searches for magnetic monopoles have been negative.
Guth's inflation concept, after some refinement, was able to deal with all of these problems, explaining the uniformity, flatness, and absence of magnetic monopoles in our universe as resulting from the smoothing effects of very rapid expansion in the initial stages of the Big Bang. However, there were critics of the inflation concept. It was argued that inflation was not a part of Einstein's equations describing the universe, that it was "put in by hand" in an unsatisfactory way, that the switching on and off seemed quite arbitrary, and that it made no testable predictions.
The latter criticism, as it turns out, is not true. While inflation smoothes and homogenizes the early universe, quantum mechanics tells us that no physical process can be completely smooth because of quantum fluctuations. Those fluctuations, during the inflation era with its ultra high energies, should have produced vast quantities of gravitational waves that are, in principle, detectable.
What are gravitational waves? A gravitational field, which, following Einstein, can be viewed as a distortion of local space, surrounds every massive object. Gravitational waves can be viewed as spreading ripples in this space distortion that arise when a massive object is shaken or moved so that its gravitational field is disturbed. Like light waves, gravity waves travel at the speed of light and obey the inverse-square law [intensity proportional to 1/(distance)2]. Gravitational waves induce a kind of "kneading" distortion in the space through which they move, making local distances alternately larger and smaller. In one direction perpendicular to the wave's direction of travel space is stretched, while in the other direction space is compressed, with the stretch and compression exchanging places after half a period of the wave.
The gravitational wave can be visualized as a long sausage with its sides alternately pinched in side-to-side and top-to-bottom, with the pinches along the length of the sausage repeating with each wavelength and the entire sausage moving forward at the speed of light. It is particularly interesting to consider that the pinch in the sausage can spiral either clockwise or counterclockwise as the wave travels. These twists in the gravity wave correspond to two circular polarization modes carrying spins of ±2, and these are related to the polarization swirls in the microwave background that we will describe below.
An enormous amount of energy should have gone into these gravitational waves during the inflation era, and they should have still been very strong about 400,000 years later, when neutral atoms of hydrogen formed, the universe went from opaque to transparent to light, and the cosmic background radiation was released. In 1996 it was pointed out that these primordial gravitational waves, if they are present, should have left their mark on the cosmic microwave background radiation that we have been detecting with ever increasing precision. In particular, they should have an effect on the pattern of polarization in the microwaves.
Electromagnetic waves, whether they be microwaves, light, or gamma rays, travel through space with the electric and magnetic fields oscillating in directions perpendicular to each other and to the direction of motion. The plane in which the electric field oscillates is said to be the direction of polarization of the wave. For example, the light that bounces from the roadway tends to be polarized horizontal in the plane of the pavement. If you wear sunglasses that block the horizontally polarized light, the road glare is greatly reduced.
The microwaves of the cosmic background tend to be slightly polarized, and one can map the sky by measuring their polarization direction at each detection pixel. This polarization map carries the signal of the primordial gravity waves from inflation, because the gravity wave imprinted themselves on the cosmic background radiation by creating pinwheel-like swirl patterns of the microwaves. These swirls can be in circles or tilted 360° fans, but in any case, they have a definite twist that demonstrates the imprint of gravity waves.
The swirling polarization modes are called "B-modes" because they resemble magnetic (B) field lines, while the symmetric polarization patterns that lack a swirl direction are called "E-modes" because they resemble electric (E) field lines. Detailed calculations have shown that while many types of phenomena can create E-mode patterns in the cosmic microwave background, gravity waves can make both E and B modes, the latter principally because of their spin ±2 circular polarization modes mentioned above. A few other phenomena, e.g., gravitational lensing, can make weak B modes, but these can be removed by careful analysis.
When it was realized in the late 1990s that the key to testing the inflation model lay in the careful mapping of the cosmic background polarization, the race was on. The space missions WMAP, launched in 2001, and PLANCK, launched in 2009, were both equipped with polarization sensing instrumentation. However, the WMAP polarization capabilities were only able to place an upper limit on the B modes, and the PLANCK group is still analyzing their polarization data. The experiment that has provided the first results, to 5-sigma precision is BICEP2, a ground-based cosmic microwave detector located at the South Pole. It is specifically designed to look for B-mode polarization patterns in the cosmic microwave background. From 2010 to 2012, BICEP2 observed a strip of sky over the South Pole about 15° wide in declination by ±45° long in right ascension using an array of superconducting transition-edge sensors operating at 150 GHz.
Because of the BICEP2 measurements, we now have some detailed information on the primordial gravity waves produced by cosmological inflation. The first fact is that the gravity wave signatures are there, confirming the inflation scenario and falsifying a number of other prominent cosmological models, including the recycling "clapping brane" model of Steinhart and Turok (see my AV Column #115 in the January-2003 issue of Analog). The second fact is that they are strong. The standard of comparison is the relative strengths of microwave fluctuations to gravity wave fluctuations in the cosmic microwave background, the so called scalar-to-tensor ratio rT. which is expected by current models to be about 10%. Rather surprisingly, the preliminary BICEP2 results give a value of about 20% for this ratio (rT=0.2+0.07-0.05). Because of this surprisingly large signal strength, BICEP2 was able to report a 5-standard-deviation confidence level in their results, rendering them quite credible. However, their results appear somewhat inconsistent with the previous preliminary estimates from PLANCK, which gave rT<0.11.
The information that remains to be extracted from the BICEP2 data is the "tilt" or spectral index nT of the gravitational frequency spectrum, which quantifies how the strength of the gravity waves depends on frequency. This has an expected value of about nT = -0.025, but this quantity has not yet been reported by BICEP2. We also note that the PLANCK data is still being analyzed to extract similar B-mode information, and other observations are currently in progress. This offers the possibility of a confirmation (or not) of the BICEP2 results very soon.
The next year will be an interesting one in cosmology, as old models are falsified by these new results and new models are invented to accommodate them. The old data on the cosmic microwave background from COBE, WMAP, and PLANCK enabled us to study the state of the universe about 400,000 years after the Big Bang. The new BICEP2 data should enable us to study the state of the universe about 10-37 seconds or so after the Big Bang. That's a very significant improvement.
Followup note (11/25/2014): After the release of the BICEP2 analysis described above, the PLANCK group released their data on galactic dust and its effects on microwave polarization patterns (due to the ellipsoidal shapes of many dust particles). The conclusion is that the observed B-mode polarization patterns could be the result of interactions with galactic dust rather than an inflation signature. The BICEP2 group agrees that the issue is not settled and is modifying their installation at the South Pole for greatly improved sensitivity and to measure new data in several frequency windows. They are seeking to provide a definitive answer to the origin of the B-mode patterns.
"The BICEP2 CMB polarization experiment", R. W. Ogburn IV, et al., Proc. of SPIE 7741, 77411G (2014); URL: http://circle.ubc.ca/bitstream/handle/2429/37248/Halpern_SPIE_7741_77411G.pdf
"BICEP2 2014 Results Release"; URL: http://bicepkeck.org .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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