On the physical basis of cosmic time

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 time $t$ as 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 $\sim 10^{-5}$ 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 $\sim 10^{-11}$ 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.Comment: 44 pages. Submitted to Studies in History and Philosophy of Modern Physic

Association between CMD signs and symptoms, oral parafunctions, race and sex, in 4–6-year-old African-American and Caucasian children

The associations between oral parafunctions, signs and symptoms of craniomandibular disorders (CMD), race, and sex were analysed in recordings from 203 4-6-year-old African-American and Caucasian children. Significant correlations were found between bruxism, nail biting, thumb sucking and most of the CMD signs and symptoms. There were also significant associations between most of the signs and symptoms and race, while significant association with sex was found only regarding headache, TMJ sounds and chewing pain. Significant associations were found between most CMD signs and TMJ sounds supporting the view that joint sound recordings have diagnostic value. There were also significant associations between the pain variables recorded by questionnaire and those recorded by palpation, which indicates that reliable data can be obtained by interviewing children as young as five. The results of this study support the concept that oral parafunctions have a significant role in the aetiology of CMD. The results also show that race and sex need to be considered when analysing the possible aetiological role of oral parafunctions in CMD. Longitudinal studies, beginning with low age groups are needed to better determine the role of childhood oral parafunctions in CMD aetiology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75673/1/j.1365-2842.1995.tb00241.x.pd

Prevalence of signs and symptoms of craniomandibular disorders and orofacial parafunction in 4-6-year-old African-American and Caucasian children

Children, 4-6 years old, 153 Caucasian and 50 African-American, from a pre-school and kindergarten programme in a low income industrial area, who participated in a voluntary oral health examination, were questioned and examined for signs and symptoms of craniomandibular disorders (CMD) and of oral parafunctions. Most of the CMD signs and symptoms were mild. Eight per cent had recurrent (at least 1-2 times per week) TMJ pain, and 5% had recurrent neck pain, African-American chil- dren more often than Caucasian children ( P < 0.05). Seventeen per cent had recurrent headache. Three percent had recurrent earache. Pain or tiredness in the jaws during chewing was reported by 25% of the children, more often by African-American than by Caucasian children ( P < 0.001) and more often by girls than by boys ( p < 0.05). Pain at jaw opening occurred in 10% of the children, more often in the African-American than in the Caucasian group ( P < 0.001). Thirteen per cent of the children had problems in opening the mouth. Deviation during opening was observed in 17% and reduced opening in 2%. Reduced lateral movements, locking or luxation were not observed in any child. Palpation pain was found in the lateral TMJ area in 16%, in the posterior TMJ area in 25%, in the temporalis and masseter areas in 10%, and pain for all regions was found more often in the African-American than in the Caucasian children ( P < 0.01). Thirty-four per cent of the African-American, and 15% of the Caucasian children admitted to having ear noises ( P < 0.01). TMJ sounds, as recorded by auscultation, occured in 48% of the children, more often in the African- American than in the Caucasin children ( P < 0.001) and more often in girls than in boys ( P < 0.05). Bruxism was noted in 31% of the African-American and in 17% of the Caucasin children ( P < 0.05). Thumb sucking was reported in 56% of the children, more in the girls than in the boys ( P < 0.01). The habit was still present in 29% of the children. Fifty-five per cent had the parafunction nail biting. The results of this study showed that mild but distinct signs and symptoms of CMD already occur by the age 4-6 with slight differences in distribution observed between the sexes and strong differences noted between the African-American and the Caucasian races.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71853/1/j.1365-2842.1995.tb00240.x.pd