
#1
Sep2704, 06:55 AM

P: 343

As you go back in time,the gravitating bodies come closer and closer.So the gravitational field gets more and more intense.We know that clocks move slower in the presence of a gravitational fieldso as you go back in time the clocks become slower and slower.So how can we say,using today's time, that the age of the universe is 13.5 billion years?We assume that the time would move at the same rate(as it is today) as we go back in timebut this is wrong.
Now if I follow the observation that clocks get slower and slower as we go back in time,I never really get to the origin of universeat best I'm approaching it asymptoticallyso is the age of the universe infinite? Kindly explain. Jagmeet Singh 



#2
Sep2704, 07:57 AM

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PF Gold
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Welcome to Physics Forums Wave's_Hand_Particle!




#3
Sep2704, 03:05 PM

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gp, although I have never been able to find reference to it, i am sure this time dilation must have been taken into account in the calculations of the age of universe. It would simply be too huge of an oversight for the entire Cosmology community to commit in unison. 



#4
Sep2704, 03:32 PM

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Question on the age of universe
My understanding: this effect of time dilation due to gravitation takes place e.g. in a Schwarzschild spacetime in which the term dt of the metric is multiplied by a factor which is not constant (it depends on the radial distance to the mass). But the universe can be assumed to be a RobertsonWalker spacetime for nearly the whole cosmological history. In a RW spacetime, dt is multiplied by a constant (ds^2 = c^2 dt^2 + ...) and thus no time distortion takes place for comoving observers in any age of the universe.




#5
Sep2704, 04:34 PM

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But let's see how important this effect might be, in terms of what we humans here on Earth (or in orbit) can see, with photons. First, I'm sure you'd agree that, whether we need to think in comoving terms or not (per hellfire), the average gravitational redshift of the present universe is utterly tiny, and we have no hope of ever measuring it! Why? Because the average mass density of the present universe is something like a few H atoms ... per cubic metre. And if we can't observe a gravitational redshift, could we observe a time dilation? Second, the earliest we can see, using photons, is the surface of last scattering, at a redshift of ~1000. Way back then, ~370,000 years after the Big Bang, the average mass density of the universe was certainly higher than it is today ... how much higher? Well, instead of working that out (see if you can do it), let's ask what's the minimum gravitational redshift we could detect? (Hint: Rebka & Pound) When did the universe have an average mass density that would give rise to such a gravitational redshift? Third, there are indeed 'gravitational redshift effects' expected in the cosmic background microwave radiation, and they may soon be detected; they're not quite the same as the 'time dilation effect' you two have mentioned. Google on 'SachsWolfe effect' or 'integrated SachsWolfe effect'. 



#6
Sep2704, 05:37 PM

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#7
Sep2704, 06:07 PM

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The overall gravitational field in the early universe was stronger than at present, therefore we might worry about having to include a cosmological gravitational red shift on top of the cosmological recession red shift. However in fact these two red shifts are the same phenomenon, the nullgeodesics diverge over curved spacetime, but interpreted in two different ways. As you cannot generally parallel transport a vector, the energymomentum vector, across spacetime curvature you cannot prove that masses remain constant; they have to be defined to be so by a measurement convention. If atomic masses are assumed constant then the red shift is recessional, but if the measurement convention is that energy is conserved then the energy, i.e. frequency, of the photon conveying the information from the distant object is assumed constant and the red shift is interpreted as gravitational in origin. These two interpretations are related by a conformal transformation. It depends on how you choose to define the standard by which ancient observations are compared with those of the present epoch. Garth 



#8
Sep2704, 11:51 PM

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The notion of a variable time scale, whether due to matter density, or intrinsic to expansion, can lead to significant cosmologically alternatives  such as those mentioned by the originator of this topic. Specifically, a time rate that asympotically approaches zero translates to an infinite age for the universe  there being no beginning in the temporal sense. This also fits with the idea of "matter from nothing" as part of an ongoing process (e.g., perpetual inflation as once suggest by Guth). In this line of inquiry, there is no need to ponder the origin of the big bang or the actuality of a singularity.




#9
Sep2804, 05:14 AM

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Garth 



#10
Sep2804, 05:23 AM

P: 343

Is this what some of you are saying: 13.5 billion years is the age measured by a comoving observer(i.e. an observer who has been a witness and a part of the expanding universe right from the beginning)?If this is so,is the 't' in RW metric(or whatever) the proper time?Even then, using today's rate of flow of time the age would be infinite .
Is this what some others are saying:the time dilation caused by gravitational effects would be very small and we can very well ignore it? Jagmeet Singh 



#11
Sep2804, 06:53 AM

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Jagmeet thank you for your searching questions.
The cosmological solution of the GR field equation assumes the universe is homogeneous and isotropic on 'large enough' scales to obtain a model universe. Cosmology is the business of comparing these model universes with the real one by observations. In the model universe the discrete masses of galaxies and stars are smeared out into a representative gas. A comoving observer is one for whom that gas is stationary for whom the cosmological principle, isotropy and homogeneity, hold  this idealised definition can now be made more exact by defining the comoving observer as one for whom the CMB is globally isotropic. (Our galaxy is moving at 0.2%c relative to this frame) In my post above I explained another measurement convention in which the age of the universe is infinite, but generally recession with constant atomic masses is the normal/standard interpretation of Hubble red shift. There are of course also local gravitational red shifts caused by the concentration of mass in stars etc., which generally can be ignored. However, in the case of a quasar, at whose centre lays a black hole (we think), this might become significant. Garth 



#12
Sep2804, 07:19 AM

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#13
Sep2804, 08:57 AM

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Surely you answer your own question, the comoving observer using a 'physical' clock constructed from atoms will measure the lapsed time since the big bang to be 13  14 billion years. However, and this may be 'where you are coming from', do we not have to question the concept of such time at an epoch in the universe's history, close to the big bang, when there were no atoms (it was too hot), let alone clocks constructed from them?  Garth 



#14
Sep2804, 11:03 AM

P: 343

Garth,what I meant by rate of flow of time was (/delta t) recorded
by an observer in some epoch of the universe per second of the observer in today's world.That could well be .001 seconds per second. 



#15
Sep2804, 04:28 PM

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Hmmm. If you measure time by counting the 'beat' of a photon, sampled from the peak intensity of the CMB Planck spectrum, then as you go back in time the present microwave photon is blue shifted and its frequency or beat increases. As we approach the big bang that increase becomes infinitely large as the blueshift (going back in time) becomes infinite. [We observe this as the red shift of distant objects becoming infinite as their epoch approaches t = 0.]
Consequently if time is measured by the number of such beats, as there has been an infinite number of them, the universe is infinitely old, as measured by that 'photon clock'. Is that what you mean? Garth 



#16
Sep2904, 02:11 AM

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Jagmeet 



#17
Sep2904, 02:38 AM

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Interesting. This reminds me of
"TIME WITHOUT END: PHYSICS AND BIOLOGY IN AN OPEN UNIVERSE" by Freeman Dyson http://www.aleph.se/Trans/Global/Omega/dyson.txt but run backwards, towards the begining, instead of forwards, towards the end. The basic assumption is that energy and time scale inversely, if you scale up the hamiltonian (energy) by some factor lambda, you scale down "time" by the same factor lambda, so that [tex] \Delta E \Delta t [/tex] is constant. A higher energy means things happen "faster". This measure of time Dyson calls "biological time". Dyson raises some other points, I'd have to think about some more, as I rescan the article (about energy dissipation limits and such). 



#18
Sep2904, 04:10 AM

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