Blue Shift

As Kea commented yesterday, the Blue Mountains appear blue because of eucalyptus mist in the atmosphere. This is an example of blueshift? Instead of being redshifted by expansion of the Universe, light here is shifted into blue. Today's physicists would say it is caused by dark energy...

Here you can also visit the spectacular Jenolan Caves. Last month scientists here announced their discovery that these caves are 340 million years old! That is far before the age of dinosaurs, the Carboniferous era. They are by far the oldest open caves on Earth. Previously Carlsbad Caverns in the US were thought to be the oldest at 90 million years.

As we learned from Special Relativity, Space and Time are both part of the same phenomenon. They are related by factor c, commonly experienced as the speed of light. Within these local conditions, c may be considered constant. Special Relativity does not allow for the curvature of gravity. We must consider the curved Universe of General Relativity.

The circle below represents our entire Universe, with spatial dimensions x, y, and z compressed into the screen. There is no centre in Space, for every bit resembles every other bit. There is a centre in Time, commonly called a "Big Bang." Near that initial singularity, total mass M occupied a tiny Universe. We are separated from that origin by age t. As Isaac Newton showed 400 years ago, gravitational pull from a spherical mass is the same as if all that mass were concentrated in the centre.

Newton also realised that the Moon's path and the trajectory of a cannon shell are both orbits around Earth's centre. Photon paths may be considered as orbits around the Universe. Radius R of this path increases with expansion. When a satellite moves into a higher orbit, its velocity slows. Mass M bends light paths into a circle, and slows the velocity of light.

Scale R of the Universe is its age t multiplied by c, or R = ct. From this one can derive GM = tc^3. To answer Q9's question about horizons, the maximum distance light can travel is given by int{c(t)dt} = (3/2)ct. The horizon distance is exactly proportional to scale R. The most distant visible objects are about halfway around that circle. That is why we don't see new structures appear in the CMB.

There is news on Universe Today that observations may require revision of the "Hubble constant," and therefore the estimated age. We can take measurements forever, and keep adding epicycles to explain these results. It is much better to explain the world from first principles. Then we can put the Universe on a T-shirt!