Hello


Mr Alex Vilenkin in his book " Many Worlds in One: The Search for Other Universes " propose an elegant demostration for the origin of the universe

A spatial voyager is travelling of "Inertia"
The engines of his starcraft are switched off

He is passing by galaxi "A" heading to galaxi "B"
His speed relative to an observer on galaxi "A" is for example v = 100.000 km\hour
Galaxi "A" and galaxi "B" are movimg away , due to the expansion of universe, at a relative speed, for example ,
v = 20.000 km\hour
When the voyager reaches galaxi "B" , an observer on B will claim that his speed is v = 100.000 - 20.000 = 80.000 km\hour

Let's assume that universe has never had a start

One observer , in the past, should have measured a speed of the voyager, close to the speed of light
According to this observer the time on board of the starcraft was " frozen "
The voyager however does not agree
Also from his point of view there was not a start

There is no symmetry between the observers on the galxies and the voyager but this is not possible thus universe must have had a start

The demostration is based only on the theory of the relativity and on the evidence of the expansion of the universe consequently it is valid regardless of future development of atomic physics which could , on the contrary, invalidate the theory of big bang

I do not know this book, but the above example seems to be just wrong.

Disregarding relativistic effects, the voyager will reach galaxy B still with a speed of 100.000 km\hour. The expansion of the universe has no influence on his speed relative to objects he's passing. Light rays that travel over immense distances still reach us with the speed of light.

The expansion of the universe, and therefore the existence of a start, can be determined not through the speed but through the expansion of the light rays from faraway galaxies. The light expansion causes a red shift and a slow motion effect.

Two galactic observers see an ship moving at non-relativistic speeds but someone in the "far" past sees it moving at relativistic speeds.

So what?

Sounds like Relativity 101 to me, not any groundbreaking discovery on the nature of our universe.

What am I missing?

Yes, it has
For this reason the speed of remote stars is, from our point of view . higher than the speed of light without being in conflict with the theory of relativity



I also supposed that the expansion of the universe by itself entails a "start" but the author of the book disagrees on this point

According to the author this is the definitive demostration that the universe has had a start since it makes use of well proven assumptions, only
In other words the demostration will remain valid even in case the present cosmological
theories should be refuted in the future