Mysteries are delightful, perhaps diabolical, insistent and obsessive nags on the curious human imagination. What is the true nature of the Universe and what is our place in the inscrutable cosmic scheme of things? Can we answer these questions, or do they lie beyond our reach, perhaps hidden in secretive exotic corners somewhere beyond the cosmological horizon of our visibility? Indeed, domains that exist beyond our cosmological horizon are so remote that the light traveling to us from those regions has not had the time to reach us since the inflationary Big Bang birth of the Universe almost 14 billion years ago because of the expansion of Space. Wandering to us throughout the incredibly vast swath of Space and Time, the Cosmic Microwave Background (CMB) radiation carries bewitching clues about what happened long ago and far away in the first magnificent instants of the baby Universe’s mysterious birth. This background radiation of ancient light is the relic thermal radiation left over from the primordial era of recombination in Big Bang cosmology, and it is a tattle-tale–it gives away the most profound secrets of our Universe to those who live in our cosmic Wonderland. In September 2016, a team of astrophysicists revealed that their study of the CMB radiation shows that the Universe expands the same way in all directions–it has no preferred direction at all.
This new research, published in the September 22, 2016 issue of Physical Review Letters, supports assumptions made in the cosmological Standard Model of the Universe. The lead author of the study, Dr. Daniela Saadeh, commented in a September 22, 2016 University College London Press Release that “The finding is the best evidence yet that the Universe is the same in all directions. Our current understanding of the Universe is built on the assumption that it doesn’t prefer one direction over another, but there are actually a huge number of ways that Einstein’s Theory of Relativity would allow for space to be imbalanced. Universes that spin and stretch are entirely possible, so it’s important that we’ve shown ours is fair to all its directions.” Dr. Saadeh is of the University College London’s Department of Physics and Astronomy in England.
The CMB is a ghostly, gentle glow of very ancient light that pervades the entire Universe. It streams softly through Space and Time with an almost unvarying intensity from all directions–and it is the relic afterglow of the Big Bang itself. This primordial light that lingers whispers to us some very haunting long-lost secrets about an extremely ancient era that existed long before there were observers to witness it. The CMB is the oldest light that we are able to observe. It began its long journey to us 13.8 billion years ago–billions of years before our Solar System had formed, and even before our barred spiral Milky Way Galaxy had formed, spinning like a starlit pin-wheel in Space. The CMB comes to us from a vanished era when all that existed was a turbulent sea of fiery, dazzling radiation and a wild, rushing, screaming flood of elementary particles. The ancient Universe was not the comparatively cold and quiet place that it is now, and the more or less familiar inhabitants of the Universe–stars, planets, moons, and galaxies–all eventually formed from this newborn flood of elementary particles, as the Universe greatly expanded and became increasingly colder and colder. We now look upon the Universe’s dying glow–the lingering ashes of its mysterious fiery formation–as it rushes ever faster and faster to its unknown fate.
The CMB is an almost-uniform background of radio waves that floods the entire Cosmos. It was released when the Universe had finally cooled off enough to grow transparent to light and other forms of electromagnetic radiation–about 380,000 years after its Big Bang birth. The primordial Universe was then brimming with searing-hot ionized gas. This gas was almost entirely uniform, but it did possess some exquisitely tiny deviations from this ancient uniformity–strange spots that were only very slightly (1 part in 100,000) more or less dense than their surroundings. These very small deviations from complete uniformity provide astrophysicists with a gift of sorts–a map of the primordial Universe–the CMB radiation. This precious, beaming afterglow of our Universe’s vanished babyhood contains the lingering fossil imprints left as a legacy of those ancient particles–the pattern of very, very small primordial intensity variations from which scientific cosmologists can try to determine the attributes of the Universe.
When the CMB radiation first embarked on its incredible journey billions of years ago, it was as beautiful and brilliant as the surface of a dazzling star–and it was just as seething-hot. However, the continuing expansion of Spacetime stretched it a thousand times over since then. This caused the wavelength of that ancient light to be stretched along with the expansion, and now the CMB is an almost unimaginably frigid 2.73 degrees above absolute zero.
As the Universe stretched in its expansion, its matter and energy stretched along with it–and very quickly cooled. The radiation shot out by the glaring Cosmic fireball which suffused the entire neonatal Universe, evolved through the entire electromagnetic spectrum–from gamma-rays, to X-rays, to ultraviolet light–and ultimately through the beautiful rainbow of colors that we see in the spectrum of visible light. Visible light is the light that human beings can see. The primordial light was then stretched even further into the infrared and radio wavelengths of the electromagnetic spectrum. The afterglow of that ancient fireball, the CMB, traveling around from literally all regions of the sky can be detected by radio telescopes. In the ancient Universe, Space itself glared fiercely with the fires of its formation, but as time passed, the fabric of Space continued to expand and stretch, and the radiation cooled. For the first time, the Universe grew dark in ordinary visible light–just as we see it today.
In the Universe’s childhood, before the stars and planets were born, it was hot, dense, and brimming with a uniform glow emanating from a white-hot fog of hydrogen plasma. As the young Universe stretched, both the plasma and the radiation filling it cooled off. When the Universe was finally cool enough, protons and electrons combined to create neutral hydrogen atoms. These first atoms could no longer absorb the thermal radiation, and so the Cosmos became transparent–no longer a glaring and opaque fog. This era is what scientific cosmologists term the recombination epoch–the time period when neutral atoms first formed. The event that occurred soon after recombination, referred to as the era of photon decoupling, is the time when photons were at last free to travel through Space instead of being constantly scattered by protons and electrons in the plasma. The photons that existed at the early era of photon decoupling have been propagating and dancing their way through Space ever since, becoming ever fainter and less energetic as they wander through the Universe. This is because the expansion of Space causes their wavelength to increase as time goes by–and wavelength is inversely proportional to energy, according to Planck’s relation. The surface of the last scattering refers to the set of points in Space at the right distance from us so that we can now receive photons originally emitted from those points at the very ancient time of photon decoupling in the early Universe.
Exact measurements of the CMB are of great importance in scientific cosmology. That is because any proposed model of the Universe must be able to explain this radiation. This ancient gentle glow of primordial light is almost uniform in all directions. However, the extremely small lingering variations reveal a very specific, tattle-tale pattern, the same as that expected of a uniformly distributed searing-hot gas–one that has expanded to the current size of the Universe. In particular, the spectral radiance at varying angles of observation in the sky shows tiny anisotropies (irregularities), which differ with the size of the region that is being observed. They have been carefully measured, and they match what would be expected if tiny thermal variations, generated by quantum fluctuations of matter in a very, very small space, had expanded and stretched to the size of the observable Universe we see today.
George Gamow, Ralph Alper, and Robert Herman were the first scientists to predict the existence of the CMB back in 1948. Alpher and Herman were also able to predict that the temperature of the CMB would be approximately what scientists now know it to be.
Serendipity is the term used to describe the experience of someone who is searching for one thing, but finds something else instead. Scientific serendipity is not an uncommon occurrence. Indeed, the discovery of the CMB is one of the most famous examples of this particular phenomenon. Discovered back in the 1960s by Dr. Arlo Penzias and Dr. Robert W. Wilson of the Murray Hill facility of Bell Telephone Laboratories in New Jersey, the CMB revealed itself in the form of background “noise” in their radio dish. At first the two scientists attributed this mysterious “noise” to pigeon droppings–there were a lot of pigeons at Murray Hill–but this proved not to be the case. The “noise” that Penzias and Wilson picked up in their radio dish was the first cry of our newborn Universe–the tattle tale CMB radiation. Dr. Robert Dicke of nearby Princeton University, and his colleagues, also made important contributions to the discovery of the CMB. Penzias and Wilson were awarded the 1978 Nobel Prize in Physics for their serendipitous discovery.
Currently, no model other than the Big Bang has managed to explain the CMB fluctuations. Because of this, most scientific cosmologists favor the Big Bang model of the Universe as the best explanation for the CMB. The almost complete uniformity throughout the observable Universe and its faint but measured anisotropy provide powerful support for the Big Bang model.
A Mysterious Universe With No Sense Of Direction
The team of astronomers from UCL and Imperial College London, England, used measurements of the CMB that were obtained between 2009 and 2013 by the European Space Agency’s (ESA’s) Planck satellite. Recently, the spacecraft had released important information concerning the polarization of the CMB across the entire sky, for the first time. This newly acquired data provided a complementary view of the ancient Universe that the scientists were able to make good use of.
The cosmologists then modeled a comprehensive variety of stretching and spinning scenarios and how these varying models might reveal themselves in the CMB–including its polarization. They then went on to compare their findings with the real map of the Universe obtained from the Planck satellite, and they began a careful hunt for specific clues buried in the tattle-tale data.
Dr. Saadeh explained in the September 22, 2016 UCL Press Release: “We calculated the different patterns that would be seen in the Cosmic Microwave Background if Space has different properties in different directions. Signs might include hot and cold spots from stretching along a particular axis, or even spiral distortions.”
Study co-author Dr. Stephen Feeney, of Imperial College London, added in the same Press Release that “We then compare these predictions to reality. This is a serious challenge, as we found an enormous number of ways the Universe can be anisotropic. It’s extremely easy to become lost in this myriad of possible universes–we need to tune 32 dials to find the correct one.”
Earlier studies only investigated how the Universe might rotate. However, this new study is the first to test the widest possible range of geometries of Space. In addition, using the treasure trove of new data obtained from Planck enabled the scientists to achieve much tighter bounds than the previous study. “You can never rule it out completely, but we now calculate the odds that the Universe prefers one direction over another at just one in 121,000,” Dr. Saadeh continued to explain to the press.
The majority of current cosmological studies assume that the Universe behaves the same in every direction–it has no preferred direction. If this assumption were to fail, many analyses of the Universe and its contents would be flawed.
“We’re very glad that our work vindicates what most cosmologists assume. For now, cosmology is safe,” Dr. Saadeh added.