While many physics articles are freely available at arXiv, many are not; they're tucked away in subscription journals. If you work at a government or university lab, you probably have access to most of the physics articles you need.
High-energy physics is a global effort, with many international collaborations. There are about a dozen journals dedicated to high-energy physics. When the Open Access movement started, the researchers at CERN decided to try shifting those journals to Open Access. This would let anyone in the world read the articles they produced. Students and scholars at small colleges, even high schools, would be able to access their research papers.
The CERN researchers spent several years talking with libraries and the publishers. In the end they negotiated an agreement that let the libraries of the world move the money that would have been spent on subscriptions to SCOAP3 (Sponsoring Consortium for Open Access Publishing in Particle Physics). SCOAP3 then pays the publishers and the publishers make the articles freely available to the world.
JHU signed on to the SCOAP3 initiative in 2008. You can see JHU listed with the other US partners. There have been bumps in the road, but this month the journals make their content freely available. It's a great milestone!
Now go read an article about high-energy physics. Just for giggles.