( CNN ) The world 's biggest and most powerful physics experiment is taking place as you read this . The Large Hadron Collider ( LHC ) , a particle accelerator and the largest machine in the world , is ready for action following a two-year shutdown . After problems that delayed the restart in March , scientists at the European Organization for Nuclear Research ( CERN ) completed final tests , enabling the first beams to start circulating Sunday inside the LHC 's 17 mile ( 27 km ) ring . `` Operating accelerators for the benefit of the physics community is what CERN 's here for , '' CERN Director-General Rolf Heuer said on the organization 's website . `` Today , CERN 's heart beats once more to the rhythm of the LHC . '' The LHC generates up to 600 million particles per second , with a beam circulating for 10 hours , traveling more than 6 billion miles ( more than 10 billion kilometers ) -- the distance from Earth to Neptune and back again . At near light-speed , a proton in the LHC makes 11,245 circuits per second . It took thousands of scientists , engineers and technicians decades to devise and build the particle accelerator , housed in a tunnel between Lake Geneva and the Jura mountain range . The purpose of the lengthy project is to recreate the conditions that existed moments after the `` Big Bang '' -- the scientific theory said to explain the creation of the universe . By replicating the energy density and temperature , scientists hope to uncover how the universe evolved . Our current , limited , knowledge is based on what 's called The Standard Model of particle physics . `` But we know that this model is not complete , '' Dr. Mike Lamont , operations group leader at the LHC , told CNN in March . The burning questions that remain include the origin of mass and why some particles are very heavy , while others have no mass at all ; a unified description of all the fundamental forces such as gravity ; and uncovering dark matter and dark energy , since visible matter accounts for only 4 percent of the universe . The LHC could also question the idea that the universe is only made of matter , despite the theory that antimatter must have been produced in the same amounts at the time of the Big Bang . CERN says the energies achievable by the LHC have only ever been found in nature . The machine alone costs approximately three billion euros ( about $ 3.3 billion ) , paid for by member countries of CERN and contributions by non-member nations . The organization also asserts that its guidelines for the protection of the environment and personnel comply with standards set by Swiss and French laws and a European Council Directive . Scientists and physics enthusiasts will be waiting with bated breath as the LHC ventures into the great unknown . `` After two years of effort , the LHC is in great shape , '' said CERN Director for Accelerators and Technology , Frédérick Bordry . `` But the most important step is still to come when we increase the energy of the beams to new record levels . '' Peter Shadbolt contributed to this report .
The Large Hadron Collider ( LHC ) begins again after a two-year shutdown . It's the largest and most powerful particle accelerator in the world, which is why it was shut down for two years to upgrade its equipment.