## Bll Cosmic coordinate systems The Hubble transformation

We will use cosmic coordinate systems that fill up spacetime. Given one system x, there is another one x' that differs from the original one by a Hubble transformation x' x + t v, ti constant, (B.l) where v is a velocity parameter, and y and z are kept unchanged. A third system will be given by another Hubble transformation, x x' + t2v x + (ii + t2)v. (B. 2) The cosmic coordinate systems are similar to the inertial coordinate systems, but now the velocity parameter takes over the time parameter...

## Relative cosmic time

In cosmology one is not interested in comparing quantities at two reference frames moving with a constant velocity with respect to each other. Rather, one is interested in comparing quantities at two different cosmic times. For example, one often asks what was the density of matter or the temperature of the Universe at an earlier cosmic time t as compared to the values of these quantities at our present time now (t 0). The backward time t is the relative cosmic time with respect to our present...

## Postulates of special relativity

In the following we give the basic principles of the special theory of relativity. These principles are needed to describe the electromagnetic field and other physical phenomena, and they constitute their spacetime symmetry background 1-8 . The special theory of relativity was developed by Einstein in 1905 in order to overcome and correct certain basic concepts that were in use at that time, such as asymmetries in relative motion of bodies. Examples of relative motion in electrodynamics, and...

## Four dimensions in classical mechanics

It is well known that classical mechanics is based on a four-dimensional manifold of three-dimensional space and the time. However, there is an essential difference between the concepts of space and time in classical mechanics and the four-dimensional spacetime of special relativity. In classical mechanics the three-dimensional subspace with constant t is absolute and is independent of the inertial coordinate system. This means one has a separate three-dimensional space, along with a...

## B41 The geodesic equation

As usual the equations of motion are obtained in general relativity theory from the covariant conservation law of the energy-momentum tensor (which is a consequence of the restricted Bianchi identities), and the result, as is well known, is the geodesic equation that describes the motion of a spherically symmetric test particle. In our five-dimensional cosmological theory we have five equations of motion. They are given by We now change the independent parameter s into an arbitrary new...

## Info

In general a set of four quantities Va, which transform like the components of the coordinates xa under the Lorentz transformation, is called a four-vector. One can extend the definition of vectors to tensors of any order in the Minkowskian spacetime. The event described by the position four-vector xa is just an example of such quantities. A scalar is then a tensor of order 0, whereas a vector is a tensor of order 1. Under a Lorentz transformation a scalar is left invariant, a four-vector Va...

## Inadequacy of the classical transformation

We first do it classically, and for simplicity it is assumed that the motion is one-dimensional. Denoting the coordinates and velocities in the systems K and K' by x, v and x', v', respectively, then x x tv, v' v, y' y, z' z, where v was assumed to be constant. The x's and v's in these equations represent the coordinates and velocities not for just one particle but for as many as one wishes, with t the same for all of them. The above transformation does not satisfy the equation of expansion of...

## Cosmology and special relativity

It thus seems that not only light propagates in a constant velocity in vacuum but the Universe also expands with a constant fashion whose proportionality constant is the Hubble constant. But there is an essential difference between them light propagation is expressed in terms of space and time, whereas the Universe expansion is by space and velocity at a fixed and every specific cosmic time. Thus we are dealing with what might be called the dual space in cosmology as compared to the ordinary...

## Problems

5.1 Given two inertial systems K and Kwhere the latter is moving relative to K with the speed v. As judged by an observer in K, the clocks in the system K' go slower than those in K. On the other hand, as viewed by an observer in K', the clocks in the system K go more slowly than those in K'. Show that there is really no contradiction between the above two observations. Solution The solution of the problem is left for the reader. 5.2 Find the timelike component of the acceleration four-vector...

## Proper time

When a particle moves in the ordinary three-dimensional space, it describes a path in the Minkowskian spacetime, called a world line. The four-vector dxa represents the infinitesimal change in the position four-vector xa, and it is a tangent vector to the world line. The square of dxa, namely r)apdxadxP, is a scalar, and it is denoted by ds2. Accordingly we have ds2 - r apdxadx13 c2dt2 - dx2 - dy2 - dz2. The physical meaning of ds can best be understood if we evaluate it in an inertial...