A recent paper published in Physical Review Letters puts new limits to the isotropy observed in the Universe, taking Copernicus' principle to be valid with more resolution than ever before.
In my last year as an undergrad in college I took a general relativity course, I remember we spent two months studying cosmology. One of the first principles we were told of, was that the Universe is considered to be homogeneous and isotropic. This principle was assumed by Friedman, by Lemaitre, by Robertson and by Walker in what became known as the FLRW cosmological model.
General relativity implies complex equations, 10 indeed, that are coupled to each other, and they cannot be solved in an analytical way unless we do some assumptions of symmetry to simplify them. Homogeneity and isotropy are two very reasonable ones. Although we see stars in particular positions and directions, at a great scale, all points in the universe and all directions seem equal, there is no preferred direction or placed in the Universe.
This is not easy so verify observationally, or at least not in the 1930s, when cosmology was appearing to stage. But, nevertheless, assuming a homogeneous and isotropic universe has given a great success in describing it. Predictions based in FLRW cosmological models, on the composition and evolution of the universe, have been a great test to the assumptions.
In the last two decades, thanks to the new observational data from the COBE, WMAP, and Planck space telescopes, some previous results have been verified with higher accuracy than ever before, such as the Cosmic Microwave Background radiation, or the composition of the universe. But these observations have also given birth to new questions, not easy to respond. Cosmology has incorporated new ingredients such as dark matter and dark energy. These are accepted by an important portion of the cosmology community, but there are other scientists that believe that there are no dark matter and/or dark energy, and the answer is in a different focus.
There are important scientists trying to modify Einstein's equation of relativity to explain why the universe seems to be expending an an increasing rate, for example. Other researchers were assuming that the universe does not contain dark energy, instead, the observations have been ill-interpreted due to assuming that the universe is homogeneous and isotropic. Having some preferred directions could help explain the observations without needing to include the dark sector.
In this recent publication, a research group in London, has analyzed data from the space telescopes concluding that the universe is highly isotropic. They have assumed that the universe, while still homogeneous, be non-isotropic. These assumptions lead to solutions of Einstein's equations known as Bianchi cosmologies. There are several of these posible solutions depending on the types of non-isotropy considered. Using data from the cosmic microwave background temperature and the polarization from the Planck satellite, the researchers conclude that the universe is very unlikely anisotropic.
In the figure they represent the inhomogeneity pattern in the cosmic microwave background that would arise due to small anisotropic expansion of the universe. The authors constrain all model of the anisotropic expansion. Each map of the universe (each elipse) shows the temperature, the electric polarization (E), and magnetic polarization (B), these last magnified 30 times to see the inhomogeneities.
Their results, after comparing the numerical simulations with the observations, point that the probability of having a non-isotropic universe is of 1 in more than one hundred thousand.
Black holes, the most exotic bodies in the Universe, an absolutely abstract concept in our daily basis, might be objects of study in our laboratories.
Jeff Steinhauer, from Israel Institute of Technology, has simulated in his lab the Hawking radiation that takes place in black holes. This allows to study and better understand the quantum nature around a black hole, which might help in the unification of quantum mechanics and general relativity. His work is published in Nature Physics.
Black holes wars
Black holes are exotic cosmic objects that arise after the cataclysmic death of vey massive stars. The extreme gravitational field in them doesn't allow neither light to scape from them. This makes it difficult to observe and study them. But General Relativity is a very powerful tool that allows theoreticians to explore them.
Stephen Hawking, in 1974, studying some quantum properties around black holes, predicted that they are not indeed really black, but gray. Black holes should emit radiation due to quantum fluctuations that can make appear and disappear virtual elementary particles around the black hole (very near to its event horizon). It happens that these virtual particles come in pairs of particle and antiparticle that appear and annihilate in a very small fraction of time. But it can happen that one of them falls into the hole while the other can scape. Hawking also predicted that this radiation should be random, thermal radiation, although recently this is not that clear, and trying to explain it has become the information paradox, an open, hot and passionate debate between theoreticians (Hawking, Maldacena, Suskind).
Quantum mechanics applies to very small scales, to elementary particles, while general gravity applies to high gravitational fields, stars, black holes, galaxies, the Universe. The fact that we don't find accesible in Nature situations where we have elementary particles and high gravitational fields at the same time, makes it almost impossible to have observational data to test and build the theory. That is why in the last decade some efforts have been driven to find alternative ways to explore black holes in the lab.
Simulating General Relativity in the Lab.
There are some works in the field of photonics that simulate the behavior of light around a black hole. Also, using a Bose-Einstein condensate (BEC), a ultra-cold gas of atoms (one billionth of a degree above absolute zero) that can show macroscopic quantum behavior, Jeff Steinhauer, is doing experiments to create a sound-wave analog to a black hole. In his work, Steinhauer creates phonons, instead of photons, which are mechanical excitations and can be considered the equivalent of what photons are to light waves, but in sound waves. He stimulates the BEC with lasers to make them move at about 1 millimeter per second. This velocity is twice the speed of sound in the BEC. By doing this, he can trap phonons in the BEC just as photons are trapped in black holes. As the phonons are created spontaneously, just as virtual photons around the black hole, some of them can be trapped while their pair can scape, just mimicking Hawking radiation.
Steinhauer has found that phonons falling inside the acoustic black hole where entangled to phonons escaping, indicating that they were cerated from the same perturbation, just as pair of virtual particles. Also, Steinhauer found that the sound radiation observed was compatible with a noise radiation, as would be the case of thermal radiation from a black hole.
There are still some open questions to this work, such as the frequency analysis. The entanglement was clear for high energy phonons (high frequency), but was not that clear for low energy ones. Also, even though it could be though that these experiments could clarify the information paradox (unravel the characteristics of Hawking radiation to see if it can be extracted information from inside the black hole by using this radiation), it is not completely clear from this work.
In any case, a very interesting door has been open that can seed light into the darkness of black holes.
Physicist, working in quantum optics and nonlinear dynamics in optical systems. Loves to communicate science.