Expert: Courtney Seligman - 1/28/2010

Question
QUESTION: Hi, As far as i am aware we have dated the universe to be approximately 14 billion years old,depending on what methods we use to date this. I believe these calculations involve mass/expansion rates etc and extrapulating backwards. That all seems logical to me. However, do any of the calculations allow for time-dilation? The density of mass in a given area affects the flow of time. So as the universe expands 'universal' time would slow down. Conversly, if we go back to when matter was in a much smaller universe,the gravity density would mean a much faster 'universal' time. The expansion of space causes time to change its flow/rate and so 14billion years is a subjective term we would use NOW. So, do any of our universe-dating techniques allow for any time dilation or is time taken as purely linear ie a year measured now is the same as a year measured billions of years ago?
Regards Richard

ANSWER: Until the last decade, the age was estimated by assuming a rate of expansion equal to the current rate, and theoretical adjustments based on assumptions about the change in mass density over time. Under those circumstances, the age obtained depended upon the assumptions, and was very uncertain.

Current calculations, however, accurately take changes in the rate into account. It turns out that type Ia supernovae, which result from the explosive destruction of white dwarfs in binary star systems, have a brightness which is directly related to the shape of their light curves (how long it takes them to fade away). Using that relationship, it is possible to determine whether the expansion rate was increasing or decreasing at various times in the past, and with that additional information, "nail down" the exact age, to remarkable accuracy.

In case you want to know how the rate has changed, the Universe expanded very rapidly early on, but the expansion rate slowed (because of the excess of mass, compared to the empty space containing that mass) for the first half of its current age. By that time, the tendency of space to expand had become equal to the tendency of mass to slow the expansion, and it "coasted" for a while. As it became still larger, the excess of space compared to mass caused it to expand faster, and for the rest of eternity, it will expand at a gradually increasing rate. Eventually, the rate of expansion will be equal to the rate of expansion of totally empty space (doubling in size every few billions of years).

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QUESTION: Thankyou very much for your reply but either your missing my point or im not reading your answer very well. I understand how the mass density must effect the expansion rate. What im specifically asking is presumably the mass density must effect time too-as we go back in time, mass density increases so effecting the rate at which time passes. Do the calculations we currently use allow for TIME itself to have been variable caused by the changing mass density. Expansion rate is distance divided by time. If time has 'flowed' at a different rate in the past this would effect the age of the universe because time as we measure it now would not be the same as if we had measured it in the past. Simply put,would time be 'flowing' at a different rate a year after big bang (mass density high) compared to the present. If that is the case,which it seems to me it should be,this would need to be incorporated into any calculation of the universes age. regards Richard

Answer
The "age" is the time it took the light from the furthest reaches of the Universe to reach us, taking into account all factors which might affect it.

I think you may be confusing time as an observer would perceive it at a particular place and time, which is called "proper" time, and the apparent slowing of time as seen by a distant observer. For instance, an object falling toward a black hole would appear to a distant observer to slow down as it approaches the event horizon, because of the time dilation caused by the gravity at the event horizon. Similarly, in very distant parts of the Universe, the recessional velocity of those regions would make their time appear (to us) to be moving very slowly. However, the "proper" time, as observed by a person at a given location, is unaffected by motion and gravity. That is, wherever you are in the Universe, however you are moving relative to the rest of the Universe, and whatever the local mass density is, time appears to you to pass at the same rate as anywhere else.