The age of the universe certainly depends on the reference frame, but it's probably not sensical to think of time as something that can start at a certain time. In the big bang model, everything in our observable universe goes to zero volume at some point in spacetime. Since it's just a single...
It's because the form of the equation includes a scale factor parameter "a". A sphere twice as big has half the curvature, but you can just normalize the curvature to 1 and just put all the length dependence into the parameter a. The value of a will change over time.
There are some non-isotropic stresses in the fluid near the surfaces due to surface tension. Consider two bubbles on the left and the right, with some fluid between them. The fluid in the center between the bubbles feels some stress up and down. Normally, in a stationary fluid the stresses are...
If you look carefully at a movie in reverse, you will see that gravity still attracts. When a ball jumps off the floor on to a table, the ball slows down as it rises to the table due to the attraction of gravity.
Now, the laws of the universe are not all time symmetric, but the force of gravity...
I would add to the above that an angle has a numerical scalar value, and when two angles are aligned such that the vertices intersect, and one ray of each angle intersects, then the value of the angle formed by the common vertex and the other two rays is the sum or difference of the values of...
Speed and velocity depend on your choice of coordinates, and you can easily choose coordinates where speed > c. However, if you create a plot of the worldline of any object, you will find that it stays within the lightcone which expands outward at the speed of light relative to a local inertial...
This is a bit like observers arguing over which shadows are real in Plato's cave. We only see what we measure, and all of our measurements project what is measured into some simpler space.
When we make a measurement, there is some contribution from all the possible paths up to the measurement...
They are all just our models for predicting reality. Ultimately, if two models give the same result, then the only grounds to choose between them is if one is simpler or more elegant than the other. The only thing real is the universe itself.
Formulating physics in terms of equations of motion...
You don't need any concept of effective nuclear charge here. Just use the actual nuclear charge (the atomic number). The higher the nuclear charge, the more tightly it will bind to electrons.
That's why I chose the term radiation here. The near field that surrounds charges has energy, but this energy can be considered to be part of the mass energy of the charges itself, and so it can be considered matter. So I would say some part of the EM field is matter and some part is not.
Ionized matter is plasma, and plasmas are certainly not dark. Fully ionized plasma will not emit atomic line radiation, but it will still emit free-free bremsstrahlung, and if there is a magnetic field, then also synchrotron radiation. Also, depending on the density of the plasma, light above...