Researchers have studied cosmic radiation spare from the Big Bang to put an end to a long-standing discussion over whether the entire Universe is the equal or same in all directions, or if it is associated with some kind of spin axis. It results that in our quickly expanding Universe, there is no 'wished' direction of expansion, the Universe is expanding in each direction at just the same rate. And while that is good news for our present planetary models and its bad news for Einstein’s popular field equations. But before we come to the new evidence, let’s run over some related info first. For years, researchers have debated that the Universe can work in one of two methods: it can either be homogeneous and isotropic, which means it is necessarily the same in all directions, or it can be anisotropic, meaning it might appear uniform on the outside, but there is, in fact, a ‘wished’ direction waiting deep inside its fibres.
To understand the theory of anisotropy, consider a diamond crystal. As Adrian Cho describes for Science magazine, a diamond crystal has unvarying density, but its atoms are all lined up in exact directions.
Consider a piece of wood, all the superficial bumps and creases on the outside as well; it is only one substance, one unvarying block of wood. But the truth is it is, in fact, stronger along the grain than through it. In other words, if something is anisotropic, it has a definite physical property that is of a different importance or value when measured in different directions. The idea of the Universe becoming anisotropic was suggested in answer to certain clues that the Universe might not be as homogeneous and isotropic as we have expected. But it now looks like the anisotropic Universe theory has far more problems to worry about. Back in 1543, Nicolaus Copernicus showed that Earth is not the center of the Universe by directing out that our planet, in fact, revolve around the Sun, not the other way around.
Adrian Cho says, "That observation gave birth to the Copernican principle, which holds that we have no extraordinary place in the infinite, Centre-less Universe. In the early 20th century, with the dawn of Albert Einstein's general theory of relativity and the observation that the Universe is growing in all directions, that idea changed into the cosmological principle, which accepts that the Universe is the same everywhere and in each direction."
It is a solid hypothesis, and we have since created every existing cosmological model, which explain the Big Bang, the expansion of the Universe, and the extents of everything in it, on the hypothesis that the Universe is isotropic. But over the past decade or so, certain facts have to produce doubt over this idea. As Cho clarifies, matter is not spread equally across the Universe when you study it on a small scale. For instance, star systems, galaxies, and galaxy collections are distributed all over the Universe in casual clumps, and researchers have proposed that this means some kind of force or directional movement has pushed them into the spot.
Cho says, "This, they assume, arises because the Universe was born as a homogeneous soup of sub-atomic elements in the Big Bang. As the Universe suffered an exponential growth spurt called inflation, tiny quantum fluctuations in that soup expanded to gargantuan sizes, providing density variations that would seed the galaxies."
Our normal model of cosmology is constructed on the hypothesis that these differences are only important on a very small measure, and on the largest measures, they are unimportant. But what if the Universe was like a diamond crystal, and there is a preferred or wished direction that is intrinsic to its whole structure, regardless of just how far you zoom out?
That is where the anisotropic theory comes in, and the occasion for it was only made stronger in the early 2000s when NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft discovered weird 'bumps' in the Cosmic Microwave Background (CMB) that no one is been able to describe. In fact, there is one area in our Universe that is so confusing, researchers have exactly called it the Axis of Evil, but many have dismissed it as a statistical accident.
To understand once and for all which choice best reveals the reality of our Universe, scientists from University College London in the UK decided to look at the eldest form of radiation in the Universe, the Cosmic Microwave Background (CMB), called the 'afterglow' of the Big Bang. Instead of searching for disproportions in the CMB like the Axis of Evil, they tried to discover evidence of a preferred direction of enlargement.
As one of the team member, Daniela Saadeh, told Matt Williams at Universe Today:
"We analyzed the temperature and polarization of the cosmic microwave background (CMB), a relic radiation from the Big Bang, with the help of data and information from the Planck mission. We related the real CMB against our predictions for what it would look like in an anisotropic universe. After this search, we concluded that there is no sign for these patterns and that the hypothesis that the Cosmos is isotropic on large scales is a good one".
The team finished estimating that there is a 1-in-121,000 chance of a preferred direction of Universal expansion, which is the best validation of the isotropic Universe theory we have ever had.
Saadeh told Cho at Science, "For the first time, we really exclude anisotropy. Before, it was only that it had not been probed".
An anisotropic Universe would leave sure patterns in the CMB like that in the bottom image, but the CMB in fact looks like the top image, which is completely random. Credit: (top) ESA and the Planck Collaboration, (bottom) D. Saadeh ET. Al.
As Universe Today indicates, this is a bit unsatisfactory, because a Universe that is not homogeneous and isotropic would support the lone real answers we have to Einstein’s field equations, a set of 10 equations in his general theory of relativity that explains the important interaction of gravitation as a cause of space-time being bent by matter and energy. These answers, suggested by Italian mathematician Luigi Bianchi in the late 19th century, led them a view of anisotropic Universe, but if that hypothesis does not hold true, we are perhaps going to have to understand a whole new manner to explain the results of Einstein’s field equations.
But that is a far less complex prospect than if the evidence overwhelmingly directed to an anisotropic Universe, and we had to re-think our whole standard model of cosmology. So this is absolutely a win, all things considered.
Saadeh said, "In the last 10 years there has been considerable discussion around whether there were signs of large-scale anisotropy lurking in the CMB. If the Universe were anisotropic, we would need to study again many of our calculations about its history and content. Planck excelllent data came with a golden opportunity to perform this health check on the standard model of cosmology and the good news is that it is safe".