Of dark energy, matter and cosmological parameters

03 June 2013

Dr Obinna UmehFinding the Holy Grail: Obinna Umeh's PhD thesis, The influence of structure formation on the evolution of the Universe, was described by an examiner from Oxford as "a remarkable piece of work and one of the most impressive theses I have read".

Obinna Umeh lives in a world of dark energy, "spatial averaging" and "quantifying cosmological parameters", in a Universe filled with mystery, darkness and vast empty spaces.

His PhD thesis, titled The influence of structure formation on the evolution of the Universe, was described by an examiner from Oxford as "a remarkable piece of work and one of the most impressive theses I have read".

One of his two UCT supervisors, Dr Chris Clarkson (the other was Emeritus Professor George Ellis), echoed these sentiments, describing the thesis as "excellent ... one of his key results concerns substantial calculations which now allow us to estimate distances and brightnesses to distant galaxies to high precision".

Umeh says in cosmology there is a working assumption known as the Cosmological Principle (CP).

"This principle states that observers on Earth do not occupy a special place in the Universe. One could re-state this assumption by saying that the distribution of all forms of matter in the Universe is homogeneous (independent of position) and isotropic (independent of direction) on a scale much greater than the scale of our galaxy, the Milky Way."

Mathematically, he adds, the form of space time metric that satisfies this assumption is known as Friedmann-Lamatre-Robertson-Walker metric (FLRW).

"When we solve Einstein field equations (these equations describe interaction between massive objects of the kinds seen in the Universe) based on this metric and fit its prediction against some of the well-understood cosmological observations, we immediately notice that things do not add up if we consider only forms of matter visible to us.

"Hence, in order to reproduce the Universe seen when we point our telescopes at the sky, we are compelled to add two additional forms of matter, namely dark energy and dark matter."

His PhD considers what he calls "a mild breaking of the Cosmological Principle".

"We considered a situation where spatial averaging over the type of structures we see in the night sky (for example galaxies and clusters of galaxies) could reveal the type of Universe we observe on a large scale, thereby eliminating the need for dark energy."

Umeh has a strong background in theoretical physics/string theory, and says he is trying to carve a global niche in quantifying non-linear effects on all cosmological parameters, in readiness for the SKA (Square Kilometre Array) project.

Apart from being able to answer some key questions of great academic importance about backreaction in cosmology, the essence of Umeh's research is to establish a formalism for quantifying cosmological parameters in a Universe filled with all kinds of structures.

"My PhD results show that the dynamics of structures in the Universe have a non-negligible effect on how the Universe evolves and that if such an effect is not properly taken into account, it could jeopardise precision measurements of some of the cosmological parameters by the next generation of radio telescopes, like MeerKAT and SKA."

The Universe is not merely expanding, he says, but expanding at an ever-accelerating pace.

"About 70% of its rate of expansion is determined by dark energy and the larger percentage of the rest is determined by dark matter."

The thing that most fascinates him in cosmology is the existence of Cosmic Microwave Background radiation. This is the radiation that fills every corner of the Universe, left behind by events that took place in the early stages of its development.

"It opens our eyes accurately to what happened when the Universe was about 10 to 37 seconds old," says Umeh. "It is fentoseconds away from the Holy Grail of how we got here."

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