Big Bang, beginning of the Universe
The universe, as we see it today, is expanding from a widely accepted theoretical event in spacetime called the Big Bang that occurred approximately 13.7 billion years ago.
We have observed that galaxy clusters, including our own, have been receding from each other. A common analogy applied to our expanding universe is of a spotted balloon being blown up. As the balloon expands, so too does the distance between the spots. This increased distance is obviously and evidently true when applied to the many galaxy clusters within our universe.
We watch the galaxies recede from us and believe ourselves to be stationary; however, this is just our relative view of the universe. For example, a galaxy receding from us at a rate of 'x' km/sec would see our galaxy moving away from itself at that same speed. Some galaxies do not recede from each other because their gravity holds them together. These are the galactic groups called 'clusters'.
If another galaxy's speed is increasing with respect to its distance from our own Milky Way, the other galaxy will inevitably reach 'lightspeed' and in effect will no longer be observable. This distance, the boundary of the observable universe, is not the end of the universe itself but is assumed to be somewhere in the region of 15 to 20 billion light-years away, a distance we have yet to penetrate.
In 1929 Edwin Hubble, an American astronomer, discovered that galaxies all around us were receding, because light analyzed from each galaxy was red-shifted (the absorption lines were shifted to the red side of the spectrum, an effect known as the Doppler Effect, indicating that the light sources were moving away our galaxy). Edwin hypothesized that the further the galaxy, the faster its recession from earth; this became known as the Hubble Constant (Ho).
The equation for the Hubble Constant (Ho = v/d) is simple in form but extremely hard to specify because the figures are largely inaccurate (v is a galaxy's radial outward velocity i.e. the motion of the galaxy from our line-of-sight, and d is that galaxy's distance from the Earth). An accurate result relies on the precision of the values obtained for v and d (d is the more difficult of the two since reliable distance markers - such as variable stars and supernovae - must be found in galaxies to determine their distances).
Even today an exact figure for the Hubble Constant cannot be agreed upon. Two teams of researchers assigned to finding the Hubble constant have conflicting results. The first team - associated with Allan Sandage of the Carnegie Institutions - has obtained a value of 57 km/sec/Mpc using Type 1a supernovae. The second team - associated with Wendy Freedman of the same Institution - has obtained a value of ~70 km/sec/Mpc using Cepheids and the Hubble Space Telescope.
The structure of the universe is not fully known. Does it have any boundaries? Stephen Hawking believes the universe is boundless yet finite in size. One could keep on moving in one direction and eventually end up in the same place. As an analogy, if you walked in a straight line around Earth, you would eventually return to the same point at which you started.
Author: Astronomy Today Staff