Other physicists have expressed skepticism over the Harvard group's claims of making metallic hydrogen. Importantly, the claim is made on the basis of one single experiment that has not yet been replicated by the group reporting the claims. From a news article published in Nature
Other researchers aren't convinced. It’s far from clear that the shiny material the researchers see is actually hydrogen, says geophysicist Alexander Goncharov of the Carnegie Institution for Science in Washington DC. Goncharov has criticized the Silvera lab’s methods before. He suggests that the shiny material may be alumina (aluminium oxide), which coats the tips of the diamonds in the anvil, and may behave differently under pressure.
Loubeyre and others think that Silvera and Dias are overestimating the pressure that they reached, by relying on an imprecise calibration between turns of the screw and pressure inside the anvil. Eugene Gregoryanz, a physicist at the University of Edinburgh, UK, adds that part of the problem is that the researchers took only a single detailed measurement of their sample at the highest pressure — making it hard to see how pressure shifted during the experiment.