Will Gravitational Waves Ever Be Found?
The BICEP2 gravitational wave announcement may have been premature, but the search has only just begun -- an interview with cosmologist Kendrick Smith.
It's official: data from the Planck satellite has revealed no signs of gravitational waves embedded in the cosmic microwave background, the primordial ‘echo' of the Big Bang that occurred nearly 14 billion years ago.
This landmark result contradicts the now-infamous BICEP2 announcement of the discovery of gravitational waves last March - but this is not the end of gravitational waves, nor the theories behind inflation. In fact, according to cosmologists, we can expect the search to intensify over the coming months and years.
After the details behind the Planck observations were revealed this week, Discovery News was able to speak with cosmologist Kendrick Smith, of the Perimeter Institute for Theoretical Physics in Ontario, Canada, to find out what impact these Planck data will have on our quest to understand what happened when the universe was born.
To recap, in March 2014, researchers of the BICEP2 telescope made a very public announcement that they had discovered the fingerprint of gravitational waves in the most ancient radiation observed in the distant universe - the cosmic microwave background, or simply, the CMB. This radiation is the remnants of the Big Bang and therefore originates from the genesis of our universe.
By studying the CMB, cosmologists are looking into a cosmic time capsule of sorts - the features etched into this radiation were created moments after the Big Bang, so their structure can reveal the conditions (and therefore the physics) of our universe back in the beginning of time.
How the universe began is "one of the biggest open questions cosmologists are trying to answer," said Smith. "There are several different theories on what happened shortly after the Big Bang ... the problem isn't that we don't have a successful theory, it's that we have too many successful theories! We're trying to narrow down the possibilities." Although Smith isn't directly involved in this week's joint BICEP2/Planck publication, he is a member of the international Planck Collaboration.
One theory is that the universe underwent a rapid expansion immediately after the Big Bang and one possible way to detect whether that inflationary period occurred is to look for gravitational waves etched into the CMB.
The BICEP2 telescope, based near the South Pole, is designed to specifically seek out a type of polarization in the ancient CMB radiation called "B-mode polarization." Should B-modes be detected, it's a sign that gravitational waves are present, proving certain inflationary universe theories.
"By searching for these gravitational waves in the cosmic microwave background, we can narrow down the physics of the Big Bang," said Smith.
And that's what the BICEP2 team thought they'd found in their observations of a small patch of sky. There appeared to be a very strong B-mode signal that couldn't (at the time) be explained by any other phenomena.
"In the original BICEP2 result, they saw B-mode polarization in the CMB at a level that roughly corresponded to the largest gravitational wave signal that would be consistent with our observations so far," Smith added.
But just because the signal looked like evidence for gravitational waves, the BICEP2 researchers had underestimated the impact of the magnetized dust that fills our galaxy.
When observing any radiation from beyond the Milky Way, we have to stare through a thin fog of interstellar dust and it just so happens that this dust can also generate B-mode polarized radiation. To compensate for this interference, the European space-based Planck telescope, which is sensitive to frequencies generated by the CMB and galactic dust, was tasked to map the magnetic fingerprint of galactic dust. Planck's mission was completed in 2013, but its huge database is still being processed and interpreted.
Last March, the BICEP2 team only had access to a preliminary Planck dataset and concluded that, in the BICEP2 field of view, the impact of galactic dust was minimal and the B-mode signal they'd detected originated in the CMB.
"They thought it had to be cosmological gravitational waves rather than dust based on a number of statistical analyses, which were mainly driven by very approximate measurements of dust emission in our galaxy," said Smith.
Although no one in the cosmological community disputed the fact that BICEP2 had detected B-mode polarization, they argued that too little was known about the galactic dust and that the signal was just as likely gravitational waves as it was dusty interference, urging caution against concluding that it had to be gravitational waves. And this week's paper, a collaborative effort between Planck and BICEP2 scientists, has shown that there are no detectable traces of gravitational waves in the BICEP2 data.
"What this recent joint Planck/BICEP paper did is take Planck measurements, that are at a different frequency to BICEP2, combine them with the BICEP measurements, so that now with multiple observing frequencies, one can make a clear statistical separation between the cosmological gravitational wave signal and the dust signal," said Smith.
In other words, the B-mode polarization that BICEP2 originally detected was caused by dust and not gravitational waves.
"That's interesting to comment on as often data analysis is very subtle; a paper may have multiple interpretations or loopholes. But this paper is not one of them," he said. "The conclusion is very clear: when you combine the observing frequencies of BICEP and Planck, all of the B-mode (polarization) in the sky can be accounted for by dust and there's no evidence that any of it is gravitational waves."
So what now? Although the March announcement may have been premature, the search for gravitational waves continues. Cosmologists are now armed with a comprehensive map of the obscuring dust in our galaxy. Smith hopes that, over the next 5 years, extensive multifrequency observations may start to root out the elusive B-mode polarization generated by gravitational waves. But they may not.
"There are working models for how the Big Bang might've worked that produce large levels of gravitational waves and there are working models for how the Big Bang might've worked that produced gravitational waves at such tiny levels that they'll never be measured. Either one is a possibility, so it's hard to speculate."
As for the massive interest that surrounded the gravitational wave drama that unfolded in a very public arena, Smith isn't surprised that this particular cosmological study has garnered such public excitement.
"I think it's really great to see," he said. "The questions cosmology is trying to answer are some of the oldest self-motivating questions in science. Like ‘how did the universe begin?' or ‘How did the particles that populate the universe get created by the Big Bang?' Everyone is curious by these questions - often that gets buried in technical detail, so it's really great that this particular study, that is by its nature very technical, was communicated to the public in a way that really inspired people's curiosity."
"We really don't know what the physics were at the time of the Big Bang and we're still exploring ... Whether we see (gravitational waves) or not, we'll narrow down the possibilities of the Big Bang. It's a really exciting time."
This map shows a portrait of our Milky Way galaxy, highlighting several different types of gas, plasma and dust. The composite image comes from the European Space Agency's Planck mission. It is constructed from observations made at microwave and millimeter wavelengths of light, which are longer than what we see with our eyes.
10. Saturn Moon Titan Explored
On Jan. 14, 2005, the European Space Agency's Huygens probe dropped through Titan's atmosphere after a seven-year trek attached to NASA's Cassini spacecraft. Huygens wasn't designed to live for very long after atmospheric reentry, but it unveiled a mysterious outer solar system world to us for the first time. Before this mission, very little was known about Saturn's largest moon, and scientists were unsure whether Huygens would land on a rocky surface or in an ocean. Titan's thick atmosphere -- composed of primarily nitrogen and clouds of methane and ethane, about 50 percent thicker than our atmosphere -- signaled to scientists that Titan was similar to a young Earth. Observations from the Huygens probe and Cassini spacecraft tell us that Titan and Earth share many features, such as sand dunes and lakes. But these features are heavily laced with organic molcules that could support life, leading researchers to speculate about Titan's potential to nurture microbes.
9. Moon Water Confirmed
India's Chandrayaan-1 satellite confirmed the presence of water on the moon in September 2009, building on flyby observations by other probes on their way elsewhere. Although the lunar surface is still drier than Earth's driest desert, evidence of water is there, hinting at a solar wind interaction with the moon's surface that produces water and hydroxyl molecules. It may not be an oasis up there, but future moon colonists could extract and purify the traces of water from the surface to use for drinking, food cultivation, oxygen and fuel. Or, our colonists could take a trek to the moon's poles to mine water from the deepest craters On Oct. 9, 2009, NASA dropped a spent rocket into a crater to produce a 100-foot-wide hole. They found water there too. That rocket produced a massive plume of dust that was analyzed by the Lunar Reconnaissance Orbiter (LRO) and ground-based observatories. At least 25 gallons of water ice was detected in the plume.
8. Organic Chemistry Collected from Comet's Tail
In 2004, the NASA Stardust mission chased after Comet Wild 2 to find out if the icy mass contained the building blocks for life, since meteorites found on Earth contained organic chemistry that originated from space. Sure enough, in August 2009, NASA announced that they had found samples of glycine -- an amino acid -- in Stardust's collection plates. It didn't stop there, there's increasing evidence that exoplanets orbiting distant stars contain organic chemistry in their atmospheres. In 2008, organic chemicals were detected in the disk surrounding a star called HR 4796A, 220 light-years from Earth. And most recently, NASA's Hubble and Spitzer space telescopes detected carbon dioxide, methane and water vapor in the atmosphere of an exoplanet called HD 209458b. These discoveries, sparked by Stardust, have transformed our understanding about how life may have formed on Earth. They also give us a strong hint that life may not be unique to Earth; the universe appears to be manufacturing organic chemistry everywhere.
7. A Supermassive Black Hole on Our Doorstep
There's a monster living in the center of our galaxy, 26,000 light-years from Earth. By 2008, astronomers tracking the behavior of stars orbiting an invisible point confirmed that the monster is a supermassive black hole called Sagittarius A*. A lone star called "S2," with a very fast orbit, has been tracked since 1995 around this invisible point. In 2002, Rainer Schödel and his team at the Max Planck Institute for Extraterrestrial Physics announced that the only explanation for S2's fast orbit was that it was circling a very compact, massive object -- a supermassive black hole -- that was stopping the star from flinging out of its orbit into space. In 2008, after S2 completed one 16-year orbit, it was confirmed that the star was orbiting a black hole with a gargantuan mass of approximately 4.3 million suns. The confirmation of a supermassive black hole in the center of the Milky Way boosted the theory that most galaxies contain a supermassive black hole at their cores.
6. Big Bang "Echo" Mapped for the First Time
In June 2001, NASA set out to find the ancient "echo" of the Big Bang by mapping the cosmic microwave background (CMB) radiation that buzzes like static throughout the cosmos, using the Wilkinson Microwave Anisotropy Probe (WMAP) . When the universe was born, vast amounts of energy were unleashed, which eventually condensed into the stuff that makes up the mass of what we see today. The radiation that was created by the Big Bang still exists, but as faint microwaves. By mapping slight variations in the CMB radiation, the probe has been able to precisely measure the age of the universe (13.73 billion years old) and work out that a huge 96 percent of the mass of the universe is made up of stuff we cannot see. Only 4 percent of the cosmic mass is held in the stars and galaxies we observe; the rest is held in "dark energy" and "dark matter."
5. Hubble Gets to Grips with Dark Energy
In 2002, the Hubble Space Telescope was upgraded with a new instrument, the Advanced Camera for Surveys, that revealed the presence of a mysterious force called "dark energy." The camera was set up to help researchers understand why Type Ia supernovae were dimmer than expected. Hubble's observations of these supernovae discovered that they weren't dimmer because the stars were different (they should all explode with the same brightness). The only explanation was that the universe's expansion was unexpectedly and inexplicably speeding up. This accelerated expansion was making the light dim over vast cosmic distances. Hubble's discovery led to a better understanding of what dark energy is -- an invisible force that opposes gravity, causing the universe's expansion to speed up. WATCH VIDEO about Hubble's most recent upgrade.
4. Eris Discovered; Pluto Demoted
In January 2005, Mike Brown and his team at Palomar Observatory, Calif. discovered 136199 Eris, a minor body that is 27 percent bigger than Pluto. Eris had trumped Pluto and become the 9th largest body known to orbit the sun. In 2006, the International Astronomical Union (IAU) decided that the likelihood of finding more small rocky bodies in the outer solar system was so high that the definition "a planet" needed to be reconsidered. The end result: Pluto was reclassified as a dwarf planet and it acquired a "minor planet designator" in front of its name: "134340 Pluto." WATCH VIDEO about Pluto's demotion to a minor planet. Mike Brown's 2005 discovery of Eris was the trigger that changed the face of our solar system, defining the planets and adding Pluto to a growing family of dwarf planets.
3. Dark Matter Detected
In the summer of 2006, astronomers made an announcement that helped humans understand the cosmos a little better: They had direct evidence confirming the existence of dark matter -- even though they still can't say what exactly the stuff is. The unprecedented evidence came from the careful weighing of gas and stars flung about in the head-on smash-up between two great clusters of galaxies in the Bullet Cluster. Until then, the existence of dark matter was inferred by the fact that galaxies have only one-fifth of the visible matter needed to create the gravity that keeps them intact. So the rest must be invisible to telescopes: That unseen matter is "dark." The observations of the Bullet Cluster, officially known as galaxy cluster 1E0657-56, did not explain what dark matter is. They did, however, give researchers hints that dark matter particles act a certain way, which they can build on. -- Larry O'Hanlon
2. Mars Surface Gives up Signs of Water
In 2008, NASA's Mars Phoenix lander touched down on the Red Planet to confirm the presence of water and seek out signs of organic compounds. Eight years before, the Mars Global Surveyor spotted what appeared to be gullies carved into the landscape by flowing water. More recently, the Mars Expedition Rovers have uncovered minerals that also indicated the presence of ancient water. But proof of modern-day water was illusive. Then Phoenix, planted on the ground near the North Pole, did some digging for samples to analyze. During one dig, the onboard cameras spotted a white powder in the freshly dug soil. In comparison images taken over the coming days, the powder slowly vanished. After intense analysis, the white powder was confirmed as water ice. This discovery not only confirmed the presence of water on the Red Planet, it reenergized the hope that some kind of microbial life might be using this water supply to survive.
1. Alien Planets Spotted Directly
The first alien planets -- called exoplanets -- were being detected in the early 1990s, but not directly. In 2000, astronomers detected a handful by looking for a star's "wobble," or a star's slight dimming as the exoplanet passed in front of it. Today we know of 400 exoplanets. In 2008, astronomers using the Hubble Space Telescope and the infrared Keck and Gemini observatories in Hawaii announced that they had "seen" exoplanets orbiting distant stars. The two observatories had taken images of these alien worlds. The Keck observation was the infrared detection of three exoplanets orbiting a star called HR8799, 150 light-years from Earth. Hubble spotted one massive exoplanet orbiting the star Fomalhaut, 25 light-years from Earth. These finds pose a profound question: How long will it be until we spot an Earth-like world with an extraterrestrial civilization looking back at us?