# Age of the Universe

## Summary:

Age of the Universe

## Main Question or Discussion Point

Summary: Age of the universe

Just a bit of a fun here, not sure how this equates into anything useful.

So we believe the "age of the universe" to be around 13.8 billion years, it seems to me that this is a relative time frame based on the rate of flow of time on earth. I find myself wondering how other observers would measure the "age of the universe."

Assuming the earliest black hole appeared around 400 million years after the big bang and still existed. An observer very near to it would measure the age of the universe slightly more than 400 million years old. Am I correct in this assumption? Or would the observer still think the universe is 13.8 billion years old and it's just as far as we are concerned that observer has only experienced 400 million years?

Likewise if an observer appeared shortly after the big bang and never experienced any time dialation effects from their own velocity or proximity to matter, how old would the universe appear to that observer? I guess this is the same as asking if there is a fundamental rate of flow of time?

I have been told before that we can draw a graph with velocity on 1 axis and time on the other and we travel through this "spacetime" at the speed of light. As we approach the speed of light in velocity our time slows down, if our velocity was zero, what would the flow of time be? This is where I get confused as "travelling through time at the speed of light," does not make sense to me. It does not seem a valid measurement.

I fully appreciate / comprehend that there is no such thing as zero velocity in the "real universe," we are always moving. It's just a hypothetical question.

Like i say not sure there is any point to this question apart from curiosity.

phinds
Gold Member
2019 Award
I find myself wondering how other observers would measure the "age of the universe."
Google "co-moving observer" and/or do a forum search for same. A good place to start is the set of links at the bottom of this page (I just checked them. They are not what you are looking for)

I have read https://en.wikipedia.org/wiki/Comoving_and_proper_distances but clearly i must be not understanding something. I still do not have a suitable answer to my question. Does the observer by the black hole measure the age of the universe differently to the observer who has existed since the big bang?

Bandersnatch
Does the observer by the black hole measure the age of the universe differently to the observer who has existed since the big bang?
Yes, he does. The age you quoted in the opening post is the age as measured by a comoving observer (stationary w/r to the local space), far away from any gravitational sources. I.e. it's the maximum age any observer can measure. For any observer not travelling at extreme velocities w/r to locally comoving observers, or not in extreme gravitational environments (such as very close to black holes) the difference in perceived age will be negligible. This includes us on Earth.

So we believe the "age of the universe" to be around 13.8 billion years, it seems to me that this is a relative time frame based on the rate of flow of time on earth.
Indeed. A year is defined as the period of time it takes for the Earth to orbit the Sun. Before the Earth existed, there was no definition of a year.

Measuring anything in years that pre-dates the formation of the Earth is simply an extrapolation from our current methods of time measurement. If the Earth is 4.54 billion years old, then by definition there have only been 4.54 billion years. Nothing can be 13.8 billion years old.

PeterDonis
Mentor
2019 Award
A year is defined as the period of time it takes for the Earth to orbit the Sun.
That's how that period of time was originally defined, but when cosmologists use the term they just mean a particular number of seconds (using the SI definition of the second, which does not depend on Earth's orbit or even on Earth's existence).

davenn and PeroK
PeroK