Wednesday, 26 March 2014

Gravitational Waves Reveal Our Universe's Birth

Judith E. Braffman-Miller sheds some light on the recent observation of gravitational waves and what that means for the early universe.

NASA image

She writes: According to the inflationary Big Bang theory, our Universe was born 13.8 billion years ago when all of Space emerged from an exquisitely small Patch and then--in the tiniest fraction of a second--expanded exponentially to attain macroscopic size. In March 2014, researchers announced their historic discovery that they had at last observed the long-sought "smoking gun" suggesting Inflation had followed closely on the heels of the Big Bang. The Big Bang plus Inflation model is an audacious and common-sense defying theory proposing that the extremely dense and hot neonatal Universe started out smaller than an elementary particle. According to Inflation theory, the Universe experienced such a monumental burst of wild growth during the first tiny fraction of a second of its existence, that all that we know and all that we are, emerged from it. By using a radio telescope at the Earth's South Pole, the US-led team of scientists detected the first evidence of primordial gravitational waves, the ripples in Space that Inflation is thought to have generated almost 14 billion years ago when the Universe first started its expansion at the dawn of Time.
As Dr. Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics (CfA), in Cambridge, Massachusetts, has put it: "Weird things might be true things."
The telescope used for this discovery captured an image of the gravitational waves as they continued to ripple through the Cosmos about 380,000 years after its Big Bang birth. Stars did not exist as yet to light up the primordial Universe with their sparkling fires--and matter itself was still chaotically strewn across all of Space in the form of a wild soup of plasma. The snapshot showed the Cosmic Microwave Background (CMB)radiation--which is the afterglow of the Big Bang itself--that radiated from unimaginably seething white-hot plasma, and then over the passage of billions upon billions of years, cooled to frigid microwave energies, as a result of the expansion of the Universe.
The scientists involved in this "smoking gun" of a historic discovery are with the BICEP2 experiment. About 10 teams of scientists all over the globe have been searching for this signal indicating that Inflation had actually occurred. This signal, called primordial B-modes, are a specific pattern of polarization. As a wavelength of polarized light wanders through Space, it jitters at an angle to its direction of movement. If Inflation had really occurred, it would have sent out gravitational waves rippling through Space and Time--and these waves would have imprinted the B-mode polarization pattern on the CMB.
Our Wonderland
According to the inflationary Big Bang theory, our Universe started out as a tiny Patch smaller than a proton--and then in the tiniest fraction of a second experienced runaway Inflation. That tiny, tiny Patch, far too small for a human being to see, was so extremely small that it was almost, but not exactly, nothing. That little Patch was, in fact, so searing-hot and dense that everything we know sprung from it. Space and Time were born together in the madly expanding fireball of the inflationary Big Bang. The neonatal Universe brimmed and danced with extremely energetic radiation; a seething, turbulent sea of dazzling particles of light (photons). The entire newborn Cosmos sparkled with a blinding brilliance of wonderful light. What we now observe almost 14 billion years later is the greatly expanded and expanding, fading aftermath of that primordial bursting of neonatal brilliance. And now we watch helplessly from our obscure little rocky blue world, as the flames of Universal formation fade and cool, and our Cosmos expands darkly into Eternity--like the eerie grin of the Cheshire Cat in a nightmare of a Wonderland.
Almost 14 billion years ago, all of Spacetime sprung into existence from a tiny ancient soup of densely packed, searing-hot particles. Spacetime has been relentlessly expanding from this initial brilliant state, and cooling off, ever since. All of the galaxies are rushing away from one another and away from our own large barred-spiral Galaxy, the star-blasted Milky Way. But it is a mistake to imagine that our Universe has a center. Rather, everything is traveling away from everything else, carried by the now-accelerating expansion of Spacetime. The expansion of the Universe is frequently compared to a rising loaf of leavening raisin bread. The expanding dough rises, carrying the raisins along with it for the ride. The raisins become increasingly more widely separated from each other due to the expanding dough.
On the largest scales, the Cosmos appears to be the same wherever we observe it. The Big Bang plus Inflation model has, for decades, been the strongest theory explaining this strange observation, which suggests that in the earliest instant of our Universe's history, everything was in contact with everything else. This tantalizingly hints that the primordial Universe must have been very, very small, indeed.
The Big Bang theory alone does explain some of the observed features of the Universe. The major suggestions of the Big Bang model--the unimaginably dense and searing-hot condition of the primordial Cosmos, the birth of galactic structures, the formation of helium, and the expansion of Spacetime itself--are all derived from a large number of observations independent of any cosmological model.
Because the distance between galaxy clusters is increasing today, the Big Bang model suggests that everything was considerably closer together in the remote past. This idea has been diligently worked out all the way back to that ancient time when the Universe was extremely hot, dense, and possibly even smaller than a proton!
However, in spite of its many successes, the Big Bang model by itself is lacking. A theory like Inflation is very badly needed for two very good reasons. The first is called the horizon problem--the weird observation that the Universe looks the same on opposite sides of the sky (opposite horizons). This very troubling mystery exists because there has not been sufficient time since the birth of our Cosmos almost 14 billion years ago for light, or any other signal, to make the long journey all the way across the entire Universe and back again. So, how could the opposite sides of the horizon possibly know how to look identical? That is the question. The second problem with the Big Bang theory is the flatness problem--the observation that our Cosmos resides dangerously well-balanced at exactly the dividing line between eternal expansion and eventual re-collapse back to its original tiny, hot, dense condition.
Dr. Alan Guth of the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, first proposed the Inflation model in 1980. Dr. Guth commented on the new observations in the March 17, 2014 Nature News, saying that "This is a totally new, independent piece of cosmological evidence that the inflationary picture fits together."
Basically, Dr. Guth's idea suggests that the Universe expanded at an exponential rate for a few tens of trillionths of trillionths of trillionths of seconds after the Big Bang--expanding like a balloon or bubble from subatomic to football size. Inflation solves some long-perplexing cosmic problems, such as the horizon problem and the flatness problem.
Although the theory of Inflation has proved to be consistent with all cosmological data gathered so far, conclusive evidence for it has been missing.
The "Smoking Gun"
"This is not just a home run, it is a grand slam. It is the smoking gun for inflation," said Dr. Mark Kamionkowski to the press on March 17, 2014. Dr. Kamionkowski is a physicist at Johns Hopkins University in Baltimore, Maryland.
Scientific cosmologists knew that Inflation would leave a distinctive fingerprint--that this very brief but violent episode of exponential expansion would have generated gravitational waves, which stretch Space in one direction while squeezing it in another. Although these very ancient waves, or ripples, would still be propagating across the Cosmos, by now they would be much too weak to observe directly. However, they would have left their distinctive tattle-tale signature on the CMB, because they would have polarized the ancient radiation in a vortex-like, curling pattern--the B mode, sometimes called the Cosmic Curl.
In 2013, a different telescope located in Antarctica--the South Pole Telescope (SPT), was the first observatory to spot a Cosmic Curl in the CMB radiation from so very long ago and far away. That cosmic fingerprint, however, was over angular scales of less than one degree--which is approximately twice the size of Earth's Moon in the sky. It was therefore attributed to the way foreground galaxies curve the Space through which theCMB makes its long and treacherous journey. But the signal emanating from the primordial gravitational waves is thought to peak at angular scales somewhere between one and five degrees.
That is precisely what Dr. John Kovac of CfA and his team have detected--using the BICEP2 instrument situated mere meters away from its competitor, the SPT.
In order to detect the elusive--and very small--B mode, the CMB needs to be measured with a precision of one ten-millionth of a Kelvin in order to separate that primordial effect from other possible sources as, for example, galactic dust.
"The key question is whether there could be a foreground that masquerades like this signal," Dr. Daniel Eisenstein explained in the March 17, 2014 Nature News. Dr. Eisenstein is an astrophysicst at the CfA. However, the team of scientists has almost entirely ruled out that possibility, he added. In the first place, the astrophysicists were very careful to aim BICEP2 at what is called the Southern Hole, which is a patch of sky that is known to carry only small quantities of such emissions. BICEP2 is an array of 512 superconducting microwave detectors. The team of scientists also compared their data with data taken by the previous experiment, BICEP1, and showed that a dust-generated signal would have displayed a differing spectrum and color from what they found.
In addition, data taken with a more sensitive and newer polarization experiment, called the Keck array, revealed the same identical characteristics. The team finished installing the Keck array at the South Pole in 2012, and it will continue to operate for another two years.
"To see this same signal emerge from two other, different telescopes was for us very convincing," Dr. Kovac told the press on March 17, 2014.
Until now, astronomers have only had one lone line of evidence to use to investigate whether Inflation really occurred--the CMB's pattern of temperature variations which do, in fact, support this version of Inflation theory.
But having the B modes available brings this investigation up to a whole new level.
"The details have to be worked out, but from what I know it's highly likely this is what we've been waiting for. This is the discovery of Inflationary gravitational waves," commented Dr. John Carlstrom in the March 17, 2014 Nature News. Dr. Carlstrom is an astronomer of the University of Chicago, Illinois, who is the lead researcher on the SPT.
The Inflation occurred in the realm of quantum physics, and observing gravitational waves generated from that remote and weird epoch provides the "first-ever experimental evidence for quantum gravity," commented Dr. Max Tegmark in the same issue of Nature News. Dr. Tegmark is a cosmologist at MIT. In other words, it reveals that the force of gravity is at the heart of the weird world of the quantum --just like the other three known forces of Nature: the strong nuclear force, the weak nuclear force, and electromagnetism.
Physicists depend on two separate theories to explain the Cosmos. The first is Einstein's General Relativity, which applies to large macroscopic objects such as galaxies and stars. The second is quantum mechanics which explains things very well on the subatomic level.
Together, the two models cover the four known forces--General Relativity explains gravity, while quantum mechanics deals with the other three. Alas, the two theories are inherently incompatible, breaking down in extreme domains such as those found within the secretive hearts of black holes or in the instants just after the Big Bang. Therefore, physicists are searching for a single framework that can encompass all four known fundamental forces and works at all levels in all domains. Physicists call this all-encompassing theory The Theory of Everything (TOE).
Dr. Abraham (Avi) Loeb of CfA noted in the March 21, 2014 that the new discovery "will give additional motivation, and also additional constraints, on models of Inflation and, perhaps, a Theory of Everything. But, of course, it will take time." Dr. Loeb was not a part of the discovery team.
The discovery team reported their findings on March 17, 2014 at a press conference at the CfA--held after they had described their results to other scientists in a more technical discussion.
The new results do not explain what triggered the Inflation--only that it happened! Neither do the new results answer the haunting question of whether Inflation is eternal, setting into motion an endless sea of big bangs and the eternal formation of pocket universes. This Cosmological landscape is usually referred to as the Multiverse. However, it is difficult to tune Inflation in such a way that an endless sea of pocket universes do not bubble into existence, Dr. Guth pointed out to the press on March 17, 2014.
"This discovery probes new physics, and that's why it's of such fundamental importance, to physics as well as Cosmology. Since we can't really do the experiments in the laboratory, we better rely on the Universe to give us some clues about what happens at these energy scales," Dr. Loeb told the press.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.