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Garth

Science Advisor

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## Main Question or Discussion Point

I have always stressed the need, when talking about cosmological concepts, such as length, mass, energy and time, to define how such entities are

In particular, when talking about time we need to relate the unit 'second' to a physical clock by which that second is measured. Problems arise in the very early universe when such clocks might not yet exist.

A paper on today's arXiv makes the same point: On the physical basis of cosmic time.

We define the two time systems by sampling two photons, one emitted by a caesium atom the other sampled from the CMB radiation.

The first, an "atomic" second, is defined as the duration of exactly 9.19263177x10

The second, a "photonic" second, is defined as the duration of exactly 1.604x10

Both systems of time measurement are physically significant and agree with each other in the present era, although they will diverge from each other at other times.

The “photonic” second retains a physical basis even at very early times when particles and particle masses do not.

Garth

*measured*.In particular, when talking about time we need to relate the unit 'second' to a physical clock by which that second is measured. Problems arise in the very early universe when such clocks might not yet exist.

A paper on today's arXiv makes the same point: On the physical basis of cosmic time.

My own work defines two gauges in which time is measured, one in which fundamental particle masses are constant and the other in which the energy of an individual photon in the CMB is defined to be constant.In this manuscript we initiate a systematic examination of the physical basis for the time concept in cosmology. We discuss and defend the idea that the physical basis of the time concept is necessarily related to physical processes which could conceivably take place among the material constituents available in the universe. It is common practice to link the concept of cosmic time with a space-time metric set up to describe the universe at large scales, and then define a cosmic timetas what is measured by a comoving standard clock. We want to examine, however, the physical basis for setting up a comoving reference frame and, in particular, what could be meant by a standard clock. For this purpose we introduce the concept of a `core' of a clock (which, for a standard clock in cosmology, is a scale-setting physical process) and we ask if such a core can--in principle--be found in the available physics contemplated in the various `stages' of the early universe. We find that a first problem arises above the quark-gluon phase transition (which roughly occurs when the cosmological model is extrapolated back to 10^{-5}seconds) where there might be no bound systems left, and the concept of a physical length scale to a certain extent disappears. A more serious problem appears above the electroweak phase transition believed to occur at 10^{-11}seconds. At this point the property of mass (almost) disappears and it becomes difficult to identify a physical basis for concepts like length scale, energy scale and temperature -- which are all intimately linked to the concept of time in modern cosmology. This situation suggests that the concept of a time scale in `very early' universe cosmology lacks a physical basis or, at least, that the time scale will have to be based on speculative new physics.

We define the two time systems by sampling two photons, one emitted by a caesium atom the other sampled from the CMB radiation.

The first, an "atomic" second, is defined as the duration of exactly 9.19263177x10

^{9}periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.The second, a "photonic" second, is defined as the duration of exactly 1.604x10

^{11}periods of the radiation corresponding to the peak of the CMB black body spectrum.Both systems of time measurement are physically significant and agree with each other in the present era, although they will diverge from each other at other times.

The “photonic” second retains a physical basis even at very early times when particles and particle masses do not.

Garth

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