El Caracol (The Snake) at Chichen Itza was once thought to be just some kind of pavilion. Now we know that it was a sophisticated astronomical observatory. During solstice days, sunlight entering certain windows would produce a snake image on the inner walls. With its circular dome it even looks like an observatory.
Yesterday we saw the tallest structure, called Kukulkan's Pyramid. During Vernal and Autumnal equinox days, the stairway's shadow would appear as a snake descending the pyramid steps. This symbolised the serpent-god Kukulkan's descent to Earth. Arranging this display required advanced knowledge of astronomy and geometry.
The Mayans believed that Space and Time were a unity. More than 1000 years later Einstein would codify this idea into Special Relativity. Humans are hardwired to think this way. Few people know exactly how many kilometres are between work and home, but nearly everyone knows the time it takes to get to work, down to the minute. The conversion factor is the speed of your vehicle. Before accurate maps existed, the distances between towns was measured in days travel.
Special Relativity can be reduced to this simple principle: Space and Time are one phenomenon related by c. The light cone in 3 dimensions looks like the expanding spherical waves from a radio transmitter. If you are outside the light cone our separation is spacelike and you can't read this anyway. If you are within the sound of my voice, our separation is timelike. Our separation is then literally a matter of time.
The conversion factor between Space/Time is c. Factor c is more than just the speed of light, it is the speed of any massless particle. The effects of Special Relativity apply even in a dark room with the lights out. To convert Time into Space, just multiply by c.
Applied to our Universe, there is no centre in Space. Every bit resembles every other bit. There is a centre in Time, what we call a "Big Bang." Near the Big Bang, the Universe was a tiny volume. Scale R of the Universe is our timelike separation t from that centre, multiplied by today's conversion factor c. In equations, R = ct.
That is all you really need to understand the expanding Universe. The Universe can't expand at the same rate c forever, for gravity slows it down. From a simple principle come GM = tc^3 and predictions about expansion, redshift and structure formation. It also produces invitations to the most interesting places.