Hubble’s Law of the Universe Expanding

Although widely attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from the general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if this was the case. Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of and distance to distant bodies, and suggested an estimated value of the proportionality constant, which when corrected by Hubble became known as the Hubble constant. Though the Hubble constant H0{displaystyle H_{0}} is roughly constant in the velocity-distance space at any given moment in time, the Hubble parameter H{displaystyle H}, which the Hubble constant is the current value of, varies with time, so the term ‘constant’ is sometimes thought of as somewhat of a misnomer. Moreover, two years later Edwin Hubble confirmed the existence of cosmic expansion, and determined a more accurate value for the constant that now bears his name.
Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917.


In addition, the newly collected data derived from the GRAIL mission helps astronomers redefine the late heavy bombardment--a proposed episode that occurred about 4 billion years ago, during which a heavy shower of projectiles pelted the bodies of the inner Solar System, including Earth and its beloved Moon, creating heavy lunar cratering in the process. The concept of the late heavy bombardment is primarily based on the ages of massive near-side craters that are either within, or adjacent to, dark, lava-flooded basins (lunar maria), that are named Oceanus Procellarum and Mare Imbrium. However, the composition of the material existing on and below the surface of the lunar near-side indicates that the temperatures beneath this area are not representative of Earth's Moon as a whole at the time of the late heavy bombardment. The difference in the temperature profiles may have caused scientists to overestimate the amount of crater-excavating projectiles that characterized the late heavy bombardment. New studies by GRAIL scientists indicate that the size distribution of impact craters on the lunar far-side is a more accurate reflection of the crater-forming history of the inner Solar System than those pock-marking the near-side. GRAIL has also generated new maps showing lunar crustal thickness. These maps have managed to uncover still more large impact basins on the near-side hemisphere of Earth's Moon--revealing that there are fewer such basins on the far-side, which is the side that is always turned away from Earth. This observation begs the question: How could this be if both hemispheres were on the receiving end of the same number of crashing, impacting, crater-excavating projectiles? According to GRAIL data, the answer to this riddle is that most of the volcanic eruptions on Earth's Moon occurred on its near-side hemisphere. HST's detection of a site, which appears to show persistent, intermittent plume activity on Europa, provides a promising target for the Europa mission to investigate. Equipped with its new and sophisticated suite of science instruments, the mission can detect whatever may potentially be swimming around in the hidden global ocean sloshing around beneath its secretive crust of ice.