The Expansion of the Universe

One of the most commonly held theories of the universe’s origin is known as the big bang theory. Observations made by Edwin Hubble in the 1920s revealed a "red-shift" in the spectra of far away galaxies. Although the speed of light emitted by a moving object is always constant (c), the wavelenth of the light appears to an observer to have shifted. The farther the galaxy was from Earth, Hubble found, the greater the degree of red-shift. Hubble accounted for this divergence from the expected spectra by hypothesizing that the galaxies were moving, and, in fact, receding from each other. This phenomenon is analogous to the Doppler effect, in which the wavelengths emitted by objects moving away from an observer, such as sound, become longer, while those of objects moving toward an observer decrease. Moreover, the wavelength shift of all the light observed by Hubble appeared to have a redshift, not a blueshift, indicating that all matter was moving away from the Earth. This discovery represented a monument al development in our understanding of the universe’s origin. If galaxies are receding at velocities directly proportional to their distance, scientists came to believe that the universe itself was expanding. In other words, the universe is not constant in size. The fact that all light appeared to be redshifted, not blueshifted, would seem to indicate that the Earth is the center of the universe and that all expansion is occuring around it. However, Einstein's theory of general relativity explains that the expansion can be viewed as identical from all points in space provided tha t the "center" of expansion occurs at a point outside our four dimensional spacetime but within the multi-dimensional relativistic universe, which is non-euclidean or curved by the gravitational warping of matter along the dimension of time. The question of when this expansion began and what preceded it then arose.

We now believe that the universe began to expand some 15 billion years ago after an initial explosive expansion termed the "big bang". At the birth of our universe, all energy and matter (equivalent by Einstein's famous equation, E=MC^2) existed in a single point that expanded outwards. The remains of this primordial fireball may have left a residual amount of kinetic energy, which astronomers observed evenly distributed throughout the universe as blackbody radiation of about 3 Kelvin.

If the universe is expanding now, when or will it stop? Scientists haven’t been able to arrive at a satisfactory answer to this questions, but have developed several hypotheses. All matter in the universe exerts gravity on itself, which creates an attractive force between planets, stars, galaxies, comets and all other celestial material. The big bang set all this matter into motion, in opposition to gravity. Since the big bang, gravity has countered the force exerted by the explosion, slowing the velocities of the universe’s expanding matter. One hypothesis presents itself if the energy of the big bang is insufficient to overcome the gravitational attraction. In this case, the matter in the universe will continue to slow until its kinetic energy is equal to the gravitational force. At this point, the matter in the universe will begin to reverse its direction and accelerate toward a central point of ultimate convergence. A "big crunch" will occur as the matter falls into itself into a dense, hot mass (a). If this were to occur, the universe would undergo a collapse that may resemble its expansion in the reverse direction. Possibly, the frictional energy generated as the universe's matter and the universe itself contract into a dense volume will create another explosive expansion that could replicate the universe's initial expansion. However, the initial energy contained in the big bang may be able to overcome the gravitational forces, which decline as the distance between galaxies increases. If so, the there will be no big crunch. Instead the universe will expand so it is infinitely far apart. If there is enough kinetic energy for this to happen, the universe might have just enough energy to expand to infinity and then its velocity will go to zero and expansion will halt (b). If there is more than enough kinetic energy to overcome gravity, the universe will expand to infinity and continue to expand at a constant speed (c).

To better understand the nature of the universe's expansion, a basic knowledge of the relation between spacetime and the big bang theory is useful.

Tests of Relativity | Gravitational Redshift