Since I will be trying to write sciencey stuff, here is an article to get me back in gear for writing. Your comments are very much appreciated!
Every successful science relies on a handful of basic assumptions. The fewer and the simpler, the better. If you ask most scientists, one assumption is always better than two. Since its beginnings as an observational science near the middle of the past century, modern Cosmology has contributed great strides to human knowledge. Cosmology, the study of the Universe as a whole, is one science that scientists are in love with. It boasts only one basic assumption.
“Let’s assume that the Universe is homogenous and isotropic on the large scales. In other words, no position is special,” an Astronomy professor would tell her students in Cosmology 101. This is the so-called Cosmological Principle. Cosmologists have a not so romantic definition of the word “special”. They use it in regards to the average amount of matter, temperature, and other physical quantities. With the help of increasingly powerful telescopes, this principle has been verified over and over again for the past seventy years or so.
Recently, this assumption has been facing increasingly difficult challenges. If these challenges stand the test of further analyses, then our Universe might be closer to an Orwellian animal farm than some people think, a Universe where all positions are (apparently) equal yet some are more equal than others.
First, what does it mean that the Universe is homogenous and isotropic? Homogenous means that it is the same everywhere. Isotropic means that it looks the same in all directions. If you try to test this statement by taking a look around you here on planet Earth, we will (hopefully) very quickly discover its speculator failure! Yet it turns out to be true on the large scales, by large I mean distances between clusters of galaxies large.
As you can see, the Cosmological Principle is by no means obvious. Actually, I can’t think of a less obvious scientific principle. Every time it was believed (sometimes religiously so) that one place in the Universe is more unique than others, observations force the opposite point of view. To disprove that the Earth is at the center of the Universe, it took great thinkers in the likes of Aristarchus, Al Biruni, Nicolaus Copernicus, and finally Isaac Newton himself. With the onset of modern Cosmology and as more stars and galaxies were regularly observed, it was much easier to disprove that neither the Sun nor the center of our Milky Way galaxy, however interesting, is a unique cosmic position. In some regions of our world, however, the force of these observations are yet to be successful.
All you need in order to check the Cosmological Principle is a map of the Universe at extragalactic scales. The disagreements to it emerge from the clearest map scientists have today for the early Universe. Since its launch in 2001, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) has been collecting data to perfect this cosmic map.
It is a map of electromagnetic waves (like light) that left the Universe when it was at the infant age of 380,000 years young after the Big Bang. At this time, the Universe is one big hot body, a “blackbody” glowing the brightest in the microwave frequency range, so this map imaginatively goes by the cosmic microwave background (CMB) map!
The cosmic microwave temperature fluctuations from the 5-year WMAP data seen over the full sky. Red regions are warmer and blue regions are colder by about 0.0002 degrees Kelvin or Celsius.
Using the WMAP data, scientists create a temperature map of the night sky confirming the homogeneity of the Universe to one part in 100,000. The small amounts of deviation from the cosmic average of 3 Kelvin (270 degrees below zero in Celsius) are the seeds of the rich cosmic structure we see around us today. Another 13 billion years later and with gravity pulling everything together, these tiny inhomogeneities evolve to give us planets, stars, nebulae, galaxies, clusters of galaxies, and superclusters of galaxies. To give you a feeling of how grand this large scale structure is, it is enough to mention that an average galaxy contains a few billion stars (as our own Sun) while an average galaxy cluster contains a few thousand galaxies. In our observable Universe, each galaxy cluster enjoys the often quite distant company of about another one billion clusters.
Is the Universe also isotropic as well as homogenous? Does it look the same in all directions or is it hotter one way than the other? The temperatures calculated along each direction in the sky should also match which they do. Almost. Understanding these inhomogeneities and anisotropies is one task of current Cosmology.
Work on answering this puzzle earned John Mather and George Smoot the 2006 Nobel Prize in Physics “for their discovery of the blackbody form and anisotropy of the cosmic microwave background.” The CMB data, being one indisputable evidence for the Big Bang, is a huge scientific hit.
Four anomalies have been observed in the CMB. First, there is one plane on the sky where the temperature fluctuations are unusually concentrated. The axis perpendicular to this plane has been cleverly dubbed “the axis of evil.” Then, in the southern hemisphere, there is a suspiciously large colder than average area. These two anomalies have been known for quite a well, and explaining them away as mere observational effects due to the probe’s position in our Galaxy has not been very successful.
The more recent third and fourth anomalies are even stranger. Adrienne Erickcek, Marc Kamionkowski, and Sean Carroll at the California Institute of Technology discovered a hemispherical power asymmetry in the CMB. In other words, they found that the “total size of the fluctuations on one half of the sky seems to be slightly larger than on the other half.” This effect, they hypothesize, is an imprint from an asymmetric inflationary era, when shortly after the Big Bang our Universe went through a short period of very rapid expansion called inflation. The reason some parts of the Universe would go through more inflation than others is a best a matter of speculation. One imaginative, though not impossible, speculation is that this asymmetry is a signature of events before the Big Bang, an era where current physics breaks down and new physics is awaiting discovery.
If this doesn’t seem outrageously unexpected enough, a new paper by Nicolaas Groeneboom and Hans Kristian Eriksen both at the University of Oslo suggests that our Universe might be “elongated” meaning that there are bigger temperature fluctuations along one axis than on the plane perpendicular to the axis. The signal found is 3.8 times as what would be expected from normal correlated noise, a borderline confidence indicator. The authors describe this result as “highly intriguing” but call for more understanding of the noise in the data before it can be given a cosmological interruption.
So what are cosmologists making out of all of this? The new insights studying these cosmic anomalies bring will definitely be interesting. Our Universe is full of surprises and when a scientific principle falls, new science emerges forcing us to stretch our imagination. The best way to describe a scientist’s reaction to these finds is to borrow the words of Sean Carroll from his popular blog Cosmic Variance. “We don’t know yet. That’s what makes it fun.”
