Gravity Beam

The unofficially named Gravity Beam is a newly introduced weapon that Covenant ships use. While a Gravity Lift is usually used for ferrying troops and supplies, it can also be used as the interesting yet devastating gravity beam.

This shipboard weapon is first seen in the last cutscene of Regret as the Master Chief escapes a Covenant Assault Carrier opening fire on the temple with it's grav beam.

To prove that the Gravity Lift plays a role in transforming itself into the gravity beam as a weapon, look at the circumference, which is the outer wall of the beam, and you'll see those same oscillating purple lines on the wall of the beam. As a matter of fact the outer wall, the circumference of the grav beam, has the same purple color of the ordinary gravity lift.

Gravity Beam Theories
It is very possible that the gravity beam is merely a gravity lift put into maximum power on reverse as most people realize that as the grav beam hits the floor of the temple the debri appears to be pushed downward as the grav beam hits it. As masterchief falls deeper into the water there is a bright circle over him. That was the gravity beam directly over him.If that were an energy projector than Master Chief would be dead in an instant.The beam also emitted and formed itself on the belly of the ship exactly where the gravity lift should be.Unlike gravity beams an energy projector is entirely white and also seem to thin. They are also mounted in different parts of covenant ships,unlike the gravity beam which is placed in one specific spot which is exactly where the gravity lift is located.

Other Theories
It is also very possible that the covenant have immatated the gravity technology of the forerunner.If a halo fan were to study the flowing gravity in Halo Installation 05's controll room as it is active,they will notice it looks almost exactly the same as the gravity beam from the covenant ship that destroyed regrets temple.The only differences are that the shipboard gravity beam is purple and seems to be more powerful since it is used for weapons purposes which is very unlike the gravity flow in Halo Installation 05's controll room which is white and seems to be weaker.