Slipstream space

Slipspace travel has been the UNSC's main way of traveling from planet to planet, and star system to star system. Before this technology was discovered by Shaw and Fujikawa (the scientists who built the first Slipspace engine) in 2291, mankind was never able to leave their home star system of Sol. But now the UNSC traverses the galaxy in spacecraft to whole new worlds.

Mechanics
This new engine allowed ships to tunnel into "the Slipstream" (also called "Slipspace"). Technically called "Shaw-Fujikawa Space" (after the scientists who proved its existence), Slipspace is a domain with alternate physical laws, allowing faster-than-light travel without relativistic side-effects (much like hyperspace from the popular Star Wars movies). Faster-than-light travel is not instantaneous; "short" jumps routinely take up to two months, and "long" jumps can last six months or more, which is why most UNSC ships have cryo chambers.

Slipspace can be thought of as our Universe (which, technically, it is) but with a greater number of dimensions. Our plane of existence is thought to have four dimensions (up-down, front-back, side-to-side, and time), but Cortana states in Halo: First Strike that Slipspace is an eleven-dimensional spacetime. Slipspace is currently theorised (in 2552) as a "tangle" of our plane's dimensions, rather like taking the classic "flat sheet" used to represent gravity and crumpling it up into a ball, therby creating extra dimensions and shorter spaces between points. As such, the physical laws of our plane (eg. relativity) are accentuated and distorted.

The SFTE (Shaw-Fujikawa Translight Engine) generated a resonance field, which when coupled with the unusual physics of the Slipstream, allowed for dramatically shorter transit times between stars; however, scientists noted an odd "flexibility" to temporal flow while inside the Slipstream. Though no human scientist is sure why travel time between stars is not constant, many theorize that there are "eddies" or "currents" within the Slipstream—there is generally a five to ten percent variance in travel times between stars. This temporal inconsistency has given military tacticians and strategists fits—hampering many coordinated attacks.

On Page 53 of Halo: Ghosts of Onyx, more detail is provided about the Shaw-Fujikawa engines. "Shaw-Fujikawa engines allowed UNSC ships to leave normal space and plow through a dimensional subdomain colloquially known as "Slipstream space." ... The drive used particle accelerators to rip apart normal space-time by generating micro black holes. Those holes evaporated via Hawking radiation in a nanosecond. The real quantum mechanical "magic" of the drive was how it manipulated those holes in space-time, squeezing a hundred-thousand-ton cruiser into Slipspace."

The Covenant have a very finely tuned version of this technology, and it is far superior to the UNSC's. Instead of simply tearing a hole into slipspace, it cuts a very fine slit and slips into slipspace with precision (much like a scalpel compared to a chainsaw). It exits the same way, and can have pinpoint accuracy. In one instance, the Ascendant Justice is able to jump into slipspace with Cortana's understanding of Covenant technology in the presence of a gravitational field.

In Halo: First Strike, while inside the Ascendant Justice, Cortana manages to figure out how to make a slipspace jump from within the atmosphere of Threshold. Later, a Covenant AI manages to leak the data out to the rest of the Covenant. In Halo 2, the Prophet of Regret uses this knowledge to jump while directly over New Mombasa, destroying the city during The First Battle of Earth.

Slipspace travel is dangerous because of the high level of radiation encountered during the trip, which is dangerous to the health of crew. This is negated by the introduction of Lead Foil to UNSC ships, which absorbs the ratiation

Communications
The Forerunner were somehow capable of advanced message carrier waves in slipspace. This way, messages could travel and great speeds throughout their empire. Cortana was able to use this technology to declare codes Bandersnatch and Hydra during The First Battle of Earth and The Battle of Installation 05.

The closest technology the UNSC has been able to create like this is the Slipspace COM Launcher.