The features of a universe without matter

This is a situation that is actually taken seriously, since this is most likely what our universe will become when it finally reaches equilibrium with thermal death, and will be filled almost exclusively with photons.

One of the characteristics of such a universe (no matter, only photons) is that it is impossible to determine its size.

Imagine our universe, but devoid of matter. Now imagine another identical one, but a billion times smaller. The light passing through them makes the exact same physically indistinguishable journeys, except for the time it takes the photons to complete them.

But photons do not ‘feel’ time, and in the universes described there is no matter with which clocks or observers can be made.

This makes the two universes equivalent to each other, characterized by scale invariance.

In our universe, everything will eventually decay, and only the lightest quarks and the electron will remain. But their masses depend on interaction with the Higgs field, whose value can and probably will vary over time.

The energy of a particle is the sum of its kinetic energy plus its rest mass energy. Shortly after the Big Bang, the kinetic energy they possessed was so high that mass was negligible, causing matter particles to behave virtually like photons. Moreover, with the extreme temperatures of the time, the Higgs field would not have been able to generate mass.