The Pharos of Alexandria, one of the Seven Wonders of the ancient world, was the tallest building on Earth (120m). Its mysterious mirror, whose reflection could be seen more than 55 km off-shore, has fascinated scientists for centuries. (Image courtesy All Posters.com)

PHAROS Distant beacons as cosmological probes

At high redshift most of the baryons are confined in the so called Lyman-alpha clouds. HST observations showed that the number of these clouds strongly decreases with the redshift, opening one of the major problem of Astrophysics and Cosmology:

What is the fate of the majority of the baryons in the local Universe?

Numerical work suggests that at z<1 30-40% of the baryons should be in a tenuous warm phase, shock-heated to temperatures of 0.1-10 million K, in the filaments connecting galaxy clusters and in clusters outskirts. Despite a large investment of Chandra and XMM time observation of the warm intergalactic matter has yielded so far only limited information, because of the insufficient throughput and resolution of the present generation of X-ray spectrometers.

Pharos will increase by an order of magnitude both throughput and resolution, opening a new window to "X-ray the Universe" and solve the missing baryon problem.