Tick off the Higgs - so what comes next?

This Cern image shows a proton-proton collision with features expected from decay of a Higgs boson. Photo / AP

The Higgs Boson Explained from PHD Comics on Vimeo.

- University of California at Irvine and phdcomics.com

The discovery of the Higgs boson was hailed as one of the great breakthroughs of the 21st century, explaining some of the fundamental physics of the universe. Yet in many ways the achievement has only highlighted how much we still do not know. The coming years will see humankind embark on new missions that will seek to advance our understanding: both into the limitless depths of space and the subatomic world within. Here are four questions that still baffle science.

1. What is dark matter?

Space is not empty and it is also growing. Modern science suggests that "normal" matter - everything on Earth and all the stars and planets ever observed - constitute just 5 per cent of that space. The rest is made up of dark energy (accounting for 70 per cent) and dark matter - of which very little is known. Invisible because it does not emit or absorb light, we suspect dark matter is there because scientists have detected its gravitational pull. But although it was first hypothesised in the 1930s, describing its make-up has become the subject of intense scientific debate.

The leading theory being studied at the Cryogenic Dark Matter Search detector at the Soudan Mine in Minnesota is that it comprises massive sub-atomic particles formed during the Big Bang which have unique properties and are capable of passing through galaxies without causing any observable effects. The other mainstream theory is that it is very large clumps of ordinary matter, ranging in size from black holes to neutron stars. The debate moved forward last week when researchers in Germany said they had discovered filaments of what they believe to be dark matter connecting two galaxy clusters 2.7 billion light years away.

2. What are gravitational waves?

They are the universe's most elusive waveforms, created by unfathomably huge events far out in the universe - the collision of neutron stars or the convergence of black holes. Yet despite the cataclysms which spawned them, it has long been held that these "ripples on the face of time" happened so far away that they would be too weak to be recorded when they reached Earth. But scientists at the Anglo-German Geo600 project near Hannover believe they could be on the brink of measuring their first gravitational waves. If they do, it is believed it will usher in a new era of astronomy.

At present radio astronomy relies on other forms of electromagnetic radiation to peer into the universe. While these forms of energy are far stronger than gravitational waves, they are also much more easily corrupted by other matter. In contrast, gravitational waves pass through the universe as if it is transparent, allowing humans to glimpse back into the origins of the Big Bang - and possibly explaining how the cosmos was born. They could allow scientists to describe the creation of black holes and delve deep into phenomena such as supernovae.The instruments used in the hunt are highly sensitive, and the search has so far been fruitless, but scientists are convinced the waves are there - as predicted by Einstein in 1916 and strongly suggested by later observations. It is just a matter of finding them.

3. Can we travel faster than light?

It is an immutable fact that nothing can travel faster than light - or at least it was an immutable fact for most of the 20th century. Yet the possibility of travelling at more than 186,282 miles a second (299,792km/s) has long intrigued scientists. To be able to do so would provide the key to true inter-galactic travel. It might also open the door to time travel.

Hence the excitement which surrounded the claim last year that neutrino particles had travelled 724km through the earth, from the Cern laboratory in Geneva to the Gran Sasso National Laboratory in Italy, in three milliseconds, some 60 nanoseconds faster than light. Overturning Einstein's 1905 Special Theory of Relativity sent shockwaves through the scientific community, resulting in a retest and the finding that the neutrinos had equalled, not surpassed, light. The quest continues.

4. Is there a theory of everything?

Finding a theory that unifies all particles and forces in the universe would certainly be a tidy way of ordering our understanding. Scientists spent much of the 20th century bringing different theories together - most notably for particle physics in the Standard Model. For three decades the model has unified three of the four fundamental forces: the electromagnetic force; the strong force binding quarks together in atomic nuclei; and the weak force controlling radioactive decay.

Yet the standard model fails to incorporate gravity, something we have been familiar with since Newton's apple. Perhaps the best-publicised attempt to incorporate everything came from the freelance physicist Garrett Lisi, who unveiled his ideas in 2007.

Lisi bases his "simple" theory on a bafflingly complex shape known as E8, plotting all known particles plus 20 notional ones on its 248 points. Although discovered in 1887, the eight-dimensional figure was only recently understood, requiring calculations that if written on paper would cover Manhattan. Lisi claims it could be the answer to everything.

- Independent