Light has great but finite speed. As we look out into space we look back in time. The light from the moon left there 2 seconds earlier. The light from the sun left 8 minutes earlier. The light from the nearest star is 4.2 light years away. The light from stars near the center of our Milky Way Galaxy left about 25 thousand years earlier. The light from the Andromeda Galaxy the largest in our local neighborhood of galaxies traveled through empty space for 2.3 million light years.
Distance = Speed X Time. Therefore a Light Year is the product of the speed of light multiplied by the time of travel and is a distance of 9.5 Trillion Kilometers
Open the Tour of the Universe link at the top of this section.
By Science we know that the unverse is still growing and evolving.
Space is expanding. The current rate of expansion is called Hubble’s constant and is 70 kilometers per second for each megaparsec of distance in space between galaxies. (A megaparsec is one million parsecs or about 3.3 million light years).
Therefore: A cubic volume of space one megaparsec on a side grows at the rate of 2X10E41 cubic kilometers every second. This is new space that did not exist before. A side note is that this does not happen in space bound my matter or our rulers would grow at the same rate and we would not notice the expansion as Edwin Hubble first observed.
This observation of the expansion of space has been repeated over and over by astronomers and by those studying the history and structure of the universe. Here are some consequences of these observations.
Not only is space expanding but the energy of empty space (yes there is energy in empty space) is being added so the energy density of empty space is constant.
Not only is space expanding but the rate of expansion is increasing. We do not know why this is happening but we can observe it.
Light has a speed, it is not instaneous, so it takes light time to travel in space. As it travels through expanding space its wavelength gets longer. This is the cosmological redshift.
Looking out into the solar system, the Milky Way Galaxy and the Universe is looking back in time. We can use the cosmological redshift to tell us how far back in time we are looking.
Einstein postulated that light speed is the same for all observers independent of their relative velocities. The consequence of this postulate is that space and time are not constants. At velocities close to the speed of light space contracts and time slows.