Signs of inflation
Physicists are especially interested in gravitational waves because of their link to inflation, which is Planck’s raison d’être. That theory was born in 1980 when a young theorist named Alan Guth, now at MIT, proposed that the universe underwent an unimaginably brief but enormous growth spurt immediately after birth. The baby cosmos ballooned from one billionth of a trillionth of a hydrogen atom’s diameter to that of a soccer ball within just 10-35 seconds. Despite all the evidence supporting the theory, Guth wasn’t sure how exactly inflation began. Nearly three decades later, theorists still don’t know.
Inflation became popular, though, because it solves several problems. One problem is that on the largest scales, space is geometrically flat, even though Einstein’s theory of general relativity would allow it to be curved. Another puzzle: Even though widely separated parts of the universe are too remote to have ever been in contact, they look remarkably similar, with galaxies clustering in the same patterns.
Inflation takes care of the first conundrum by stretching the universe so much that any local curvature of space from a concentration of mass flattens out. And by positing that the cosmos began as a tiny subatomic speck, with all regions initially in contact before inflating, the theory neatly explains why the universe looks so uniform from place to place.
If the idea is correct, inflation was triggered by an energy field called the inflaton that drove the early universe to rapidly expand, stretching out and freezing what otherwise would have been random, short-lived quantum fluctuations. Random quantum fluctuations in the inflaton field itself would have created spatial variations in the density of matter. Inflation would have stretched those variations to observable size.
Regions of the sky that today are separated by twice the apparent diameter of the full moon were once packed into a space much tinier than the diameter of a proton, inflation theory says. “Just the idea that you have the subatomic world projected on the sky today is mind-boggling,” says Turner.
“If you want quantum fluctuations to explain all the structure in the universe, you need something like inflation,” says Planck mission scientist Jean-Loup Puget of the University of Paris’ Institute of Spatial Astrophysics in Orsay. “We want to see if this really works.”
No one knows exactly when inflation started (or why it stopped) and what the earliest moments of the universe were truly like. That’s where the prospect of recording gravitational waves by the Planck mission comes in. The gravitational waves are disturbances in spacetime itself, and are only stretched — not created — by the inflaton field.
Moreover, the soup of electrons and other elementary particles that filled the universe for its first 380,000 years would have posed no obstacle for gravitational waves. Unlike light, which remained trapped by this fog, gravitational waves zipped right through. These waves, some created a mere fraction of a second after the Big Bang, were free to leave their subtle imprint on the cosmic microwave background when that radiation began streaming into space.
As a result, gravitational waves have a simpler relationship to inflation than do fluctuations in the density of matter, says Turner. Finding evidence for gravitational waves would be a more direct indicator that inflation happened and of when it began.
Planck can’t detect gravitational waves directly — that would come with space-based versions of experiments such as LIGO, the Laser Interferometer Gravitational-Wave Observatory (SN: 1/8/00, p. 26). But if their amplitudes are large enough, gravitational waves leave behind a detectable trail, one Planck is built to find.
The waves simultaneously stretch space in one direction while squeezing it in a perpendicular direction. This twisting forces the light to vibrate in specific directions as it journeys into space. In other words, the light is polarized. One of the polarization patterns induced by gravitational waves, called the E mode, is identical to that produced by density variations. But the other pattern, known as the B mode, has a swirling or vortexlike character that can be produced only by gravitational waves.
If Planck detects B mode polarization, it has detected the waves — and recorded something created during the earliest moments of the universe.
Planck “has the potential of making the first serious assault on the B mode polarization,” says Turner.
That polarization pattern can be detected only if the gravitational waves are large enough. Inflation indicates that the strongest gravitational waves are those with the longest wavelengths. Their strength depends on how rapidly the universe expanded during inflation, and that in turn depends critically upon when inflation began.
The earlier inflation kicked in, the larger the waves’ amplitude, says theorist Scott Dodelson of both the Fermi National Accelerator Laboratory in Batavia, Ill., and the University of Chicago. If the waves are amplified enough for Planck to detect, then inflation turned on sooner rather than later.
Timeline for the dawn of time
Trying to understand whether inflation began by 10-35 seconds after the birth of the universe or a tiny fraction of a second later may seem like an arcane endeavor. But finding out when inflation happened could help indicate why it happened. The energies of particles in the universe were higher at these hotter, earlier times, a trend with important implications for the fundamental forces of nature.
The standard model of particle physics insists that, when the universe was hotter than 1015 kelvins, the now-separate weak and electromagnetic forces were a single entity called the electroweak interaction. At earlier times, in the era of grand unification when energies were even higher, the electroweak interaction was united with the strong nuclear force. So whether inflation began at the time of grand unification or slightly later, when only the electromagnetic and weak interactions were unified, could determine how inflation was linked to the energy fields associated with particular elementary particles.
“Once you detect these gravitational waves, you will know with a one-line formula when inflation took place and what the energy scale of inflation is,” says Turner.
Still, he adds, “it’s going to be a very hard slog because there is not a single theorist that will stand out there and say, ‘You’ll have to find the gravitational waves at this level, otherwise I’ll eat my Power Point.’” Planck’s search for gravitational waves “is truly high-reward, high-risk physics,” Turner says.
Although the microwave background was generated by the Big Bang, telescopes see the radiation as it appeared when it first streamed into space some 380,000 years later. Before that time, the universe was too hot for neutral atoms to exist; the radiation was relentlessly scattered by a cloud of ions and free electrons, the way a search light gets lost in a dense fog.
Planck should provide further details on the early universe by detecting the polarization of the microwave background due to the variations in density. These density variations caused free-floating electrons to scatter the microwave background in a way that imparted the specific polarization patterns.
Those free electrons existed during two very separate epochs — the time just before the universe cooled enough for electrons and ions to combine into atoms, and at the much later time when starlight first flooded the universe with ultraviolet light and liberated electrons that had been bound to atoms.
By detecting this polarization, Planck will help pin down exactly not only when the universe first cooled sufficiently for neutral atoms to form, but also the time when the first stars were born.
This facet of the mission “is going to be a huge step forward,” says Bouchet.
Hot (old) sounds
Even if Planck never finds the signature of gravitational waves, it still has a whole cosmic symphony to listen to. That’s because sound waves and the cosmic microwave background go hand in hand.
Soon after the Big Bang, and until the universe cooled 380,000 years later, wherever gravity acted to draw matter in, the photons bound to that matter resisted and exerted an outward pressure. It’s this tug-of-war between gravity’s pull and radiation’s push that generated acoustic oscillations — the cosmic equivalent of sounds.
Like any sound waves, these cosmic acoustic waves consist of a train of compressions and rarefactions — in this case traveling through the hot plasma of electrons and photons glued together within the young universe. The compressions heated the gas while the rarefactions stretched and cooled it. The waves imparted a pattern of temperature fluctuations into the radiation. The pattern persisted even as the universe cooled and the freed radiation journeyed into space.
By measuring peaks in cosmic microwave background temperatures on a host of different spatial scales, Planck will record this primordial symphony as never before. Just as musicians can discern the nature of a violin by listening to the richness of its tones and overtones, cosmologists can examine the highs and lows in the temperatures of the microwave radiation to discern the properties of the universe: its age, how much dark matter and ordinary matter it contains, how fast it’s expanding.
NASA’s WMAP, launched in 2001 and still collecting data, studied the first few of these peaks across the full sky. A slew of ground-based telescopes have examined these peaks over more restricted portions of the sky. But by measuring the higher overtones, Planck will shrink the number of allowed models for the birth of the universe and provide new insight into the physics that debuted during the first billionths of a second after the Big Bang, says Puget.
Good noise
As a bonus, the data Planck collects may also provide new information on dark energy, the mysterious substance that flips gravity from a cosmic pull into a cosmic push and causes the universe to expand at an ever-faster rate (SN: 2/2/08, p. 74).
Once dark energy becomes strong enough to accelerate cosmic expansion, galaxies can no longer congregate into clusters. Understanding how early — and how quickly — galaxies formed clusters could shed light on the nature of dark energy and whether it has remained constant over the lifetime of the universe. And Planck can help answer that question using information that it was designed to discard.
To clearly view the cosmic microwave background, Planck will record and eliminate foreground sources — emissions from the Milky Way and other galaxies. But one man’s noise is another man’s data. For some astrophysicists, the discarded information is a gold mine. It could reveal the magnetic structure of the Milky Way, radio emissions from a host of nearby galaxies and several remote ones, and the evolution of galaxy clusters. The last could be particularly intriguing in shedding light on dark energy.
Still, for many cosmologists, Planck’s main goal — “pinning down the parameters of the physics which produced inflation” —remains paramount, says Dodelson.
“You can’t dream of building an accelerator big enough to detect any of those parameters,” he notes. The world’s biggest atom smasher, the Large Hadron Collider near Geneva, is set to resume operation in the fall, about the same time Planck begins gathering data. Yet the energies available at the LHC are only one-trillionth the level Planck will probe in looking for the signal of gravitational waves, Dodelson says, adding: “That we can hope to learn anything at all about the energy scale when the universe began is unbelievable.”
