GENEVA — Researchers at the world's biggest particle accelerator in Switzerland have submitted proposals for a new, much larger, supercollider.

Its aim is to discover new particles that would revolutionize physics and lead to a more complete understanding of how the Universe works.

If approved, it will be three times larger than the current giant machine.

But its £12bn price tag has raised some eyebrows, with one critic describing the expenditure as "reckless".

That money -- which is only the initial construction cost -- would come from member nations of the European Organization for Nuclear Research (Cern) including the UK, and some experts have questioned whether it makes economic sense.

The biggest achievement of the Large Hadron Collider (LHC) was the detection of a new particle called the Higgs Boson in 2012. But since then its ambition to track down two holy grails of physics -- dark matter and dark energy -- have proved elusive and some researchers believe there are cheaper options.

The new machine is called the Future Circular Collider (FCC). Cern's director general, Prof Fabiola Gianotti, told BBC News that, if approved, it will be a "beautiful machine".

"It is a tool that will allow humanity to make enormous steps forward in answering questions in fundamental physics about our knowledge of the Universe. And to do that we need a more powerful instrument to address these questions," she said.

Pallab Ghosh and Kate Stephens go inside the biggest particle accelerator in the world to find out why scientists want an even larger one.

The LHC consists of an underground circular tunnel 27km in circumference. It accelerates the inside of atoms (hadrons) both clockwise and anticlockwise to speeds close to the speed of light and at certain points crashes them together harder than any other atom-smasher in the world can.

The smaller, sub-atomic particles left over from the collisions help scientists work out what atoms are made of and how they interact with each other.

The supercollider's detection of the Higgs Boson particle more than 10 years ago was ground-breaking.

The existence of a building block that gives all other particles in the Universe their form was predicted in 1964 by the British physicist, Peter Higgs, but was only discovered at the LHC in 2012. It was the final piece of the jigsaw of the current theory of sub-atomic physics, which is called the Standard Model.

The proposal is for the larger FCC to be built in two stages. The first will begin operating in the mid 2040s and will collide electrons together. It is hoped the increased energy will produce large numbers of Higgs particles for scientists to study in detail.

The second phase will begin in the 2070s and require more powerful magnets, so advanced that they have not yet been invented. Instead of electrons, heavier protons will be used in the search for brand new particles.

The FCC will be nearly three times the circumference of the LHC, a whopping 91km and twice as deep.

It is because the LHC, which cost £3.75 billion to build, and started operating in 2008, has not yet been able to find particles that will help to explain 95% of the cosmos.

Scientists are still searching for two big unknowns -- a force called dark energy which acts like the opposite to gravity, and drives objects in the Universe such as galaxies apart. The other is dark matter, which can't be detected but its presence is felt through gravity.

"We are missing something big,'' Prof Gianotti tells us.

She says the FCC is needed because the discovery of these dark particles would lead to a new more complete theory of how the Universe works.

More than 20 years ago many researchers at Cern predicted that the LHC would find these mysterious particles. It didn't.

Critics, such as Dr Sabine Hossenfelder of the Frankfurt Institute of Advanced Studies, say there is no guarantee that the new collider will succeed.

''Particle physics is a research area that is large and well-funded for historical reasons, having grown out of nuclear physics and it needs to shrink back to a reasonable size, maybe a tenth of its current size," she said.

A former UK government chief scientific advisor, Prof Sir David King, told BBC News he believed that spending £12bn on the project would be ''reckless''.

''When the world is faced with threats from the climate emergency, would it not be wiser to channel these research funds into the endeavors to create a manageable future?''

And there is also a debate among particle physicists themselves about whether a giant circular collider is the best option.

Prof Aidan Robson of Glasgow University, told BBC News that a collider built in a straight line would be cheaper.

"There are three main advantages. First of all, a linear machine could be done stage by stage. Second, the cost profile would be rather different -- so the initial stage would cost less, and third the tunnel is shorter, and you could do it quicker," he said.

But the FCC is Cern's preferred option. It is the result of a widespread consultation among physicists in Europe and worldwide and it is in the process of gauging reaction to its proposal from its member nations, who will have to pay for the new machine. — BBC