EXPANSION OF THE UNIVERSE

In the first half of the 20^th century, Physicists and Mathematicians who were working on Einstein's theory of relativity discovered that the equations had some solutions that described an expanding universe. To prove this, Scientists decided to study the spectrum of light emitted by the elements in the distant space. Atoms absorb or emit light in characteristic wavelengths: hydrogen, helium, and all the other atomic elements have their own spectrum signatures. An astrophysicist, Vesto Slipher was studying the spectra of light emitted from nearby galaxies. He noticed that the light coming from many galaxies shifted toward the red or longer wavelength, end of the spectrum. The obvious interpretation of this "red-shift" was that the galaxies were moving away from each other.

In 1929 Edwin Hubble measured the red-shifts of a number of distant galaxies. He also measured their relative distances by assessing the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted red-shift against relative distance, he found that the red-shift of distant galaxies increased as a linear function of their distance. The only logical explanation for this observation is that the universe was expanding.

Once scientists understood that the universe was expanding, they instantly realized that it would have been smaller in the past. At some point in the past, the entire universe should have been a single point. This point, later called the singularity, was the beginning of the universe as we understand it today.

Properties of the Expanding Universe

The equations of the expanding universe imply three possible solutions, each of which predicts a different eventual fate for the universe as a whole. What fate will ultimately befall our universe can be determined by measuring how fast the universe expands relative to how much matter the universe contains.

The three possible expanding universes are called open, flat, and closed universes. If the universe were open, it would expand forever. If the universe were flat, then again, it would expand forever, but the expansion rate would slow to zero after an infinite amount of time. If the universe were closed, it would eventually cease expansion and re-collapse on itself, possibly leading to another big bang (or Big Crunch). In all three cases, the expansion slows down, and the force that causes the slowing is gravity.

A simple analogy to visualize these three types of universes is to consider a spaceship launched from the surface of the Earth. If the spaceship doesn’t have enough speed to escape the Earth's gravity, it will eventually fall back to Earth. This is analogous with a closed universe that re-collapses. If the spaceship is given just enough speed so that it has just enough energy to escape, then at an infinite distance away from the Earth, it will come to a stop (this is the flat universe). And lastly, if the ship is launched with more energy to escape, it will always have some speed, even when it is an infinite distance away (the open universe).

Questions to Ponder:

• How fast is the Universe expanding (the rate or velocity of expansion)?
• Do you think the expansion of the Universe faster than the speed of light?
• How can observations of the distant space prove that the expansion is accelerating?
• How are galaxy distances inferred from their relative velocities?