Thursday, March 15, 2018

Dark energy again

Albert Einstein
In a previous article I mentioned that Einstein introduced a third term in the right side of his cosmological equation, to force this equation to have as solution a stationary cosmos, that would not expand or contract. The attempt was unsuccessful, for such a cosmos would have been in unstable equilibrium, and the smallest variation would have pushed it to either expanding or contracting. The term in question depends on a constant (L, the cosmological constant), which we don’t really know what it is.
Einstein's cosmological equation
For most of the twentieth century, it was assumed that the value of the cosmological constant must be zero. In other words, the third term of the Einstein equation would not exist, wouldn’t be necessary. However, in 1998 it was discovered that the universe seems to be expanding rapidly. At least, this seems to be indicated by the study of supernovas in very distant galaxies, about one billion light-years away from us. To explain this discovery, the cosmological constant term was resurrected, but giving it a sign opposite to that proposed by Einstein, so that rather than the expansion being counteracted, it would be accelerated. This proposal has become the standard cosmological model, in which the first term of the equation, which represents the effect of the mass, currently counts as 31%, while the third, that of the cosmological constant, counts as 69%. In this model, the second is assumed to be zero. I leave apart the question that the mass term does not match, so it has been necessary to assume that there is also a dark matter, that we don’t know what it is.
Some cosmologists, however, do not agree with the standard model and try to offer alternatives:
       A few think that dark energy is not necessary, because the effect of apparent acceleration can be explained in terms of the granularity of the cosmos (the fact that there are huge empty spaces, together with others full of galaxies). According to Einstein’s equation, empty spaces expand more quickly than those that contain a lot of mass. This phenomenon is called backreaction, but other cosmologists, faithful to the standard model, argue that the effect, although true, is too small to explain the accelerated expansion. Since we cannot experiment with real galaxies, all these studies are based on simulations, which, having to work with a model of the entire universe, require enormous computer resources.
      Another proposal asserts that accelerated expansion is not something real, but the result of an optical mirage. When very distant supernovae are observed, their light has to cross intermediate regions with groups of galaxies alternating with voids. This trip could lead to changes in the direction of the light beam (gravitational lens effects) that, when reaching us, would cause an optical illusion, giving the impression of an acceleration where there is none. These effects are also studied through simulations, but they are not realistic, for they must be simplified to reduce the computer resources, which also reduces their reliability.
Gravitational lens
Some of the simulations reduce the magnitude of the problem by placing the galaxies on the vertices of a three-dimensional grid. These simulations seem to suggest that dark energy might not be necessary, but they are not considered reliable, because the universe is not a grid. Others apply Newton’s equations rather than Einstein's, because they are easier to solve, but the effects we seek could be precisely in the equations of General Relativity. Others, finally, simplify the space itself, simulating toroid universes, where whatever gets out at the left comes in from the right. There are estimates that the simplifications could lead to significant errors, of the order of 10%, which would affect the results by eliminating or significantly reducing the accelerated expansion of the universe. In spite of all this, most cosmologists remain faithful to the standard model and believe in the existence of dark matter and dark energy, although we haven’t the least idea what they are.
And then we have the pending problem that the calculations of the Hubble constant made by two different procedures do not agree. But that is another story.

The same post in Spanish
Manuel Alfonseca

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