# The age old age of the universe question

Igottaknow
I have always been confused why it is said that the universe is 13.7 billion years old it just does not make sense to me. Why do we not say that the stuff that we can observe is 13.7 billion years old? We base this on the most distant things that we can see from our perspective here on Earth right? Let's say that light traveled here from the most distant galaxy and then passed the Earth and was observed by a civilization with a billion year head start on us and is a billion light years the other way. They would say that the universe is 14.7 billion years old. (I know that this is a very crude thought experiment by the way)

I know that we look in all directions and get a pretty good idea of the formation of galaxies and a time frame for when stars started forming and all that but, we observe a far off galaxy and we see it in the position it was in all that time ago. From that galaxy's perspective it is nowhere near where it was when the light left it so many billions of years ago and in an expanding universe (Which is expanding faster than light according to some theories) should be another "x" amount of distance from the position that we see it in today farther out.

I could be totally wrong about how we have put an age on the universe or misunderstood or even be using very bad logic. I am no astronomer for sure. Can someone clear this up for me?

## Answers and Replies

gplayle

1 person
Gold Member
I think your problem is that you are thinking the age is set by the most distant light sources we can see. That would not be enough by itself-- the age is set by the dynamics of the universe that we infer from looking at the redshifts as a function of distance. The interesting point is that the light we see that has been traveling for 13.7 billion years is very highly redshifted, so there is, in some sense, not much redshift left there that would be possible. That's how we know we are seeing almost the whole age of the universe, not just the distance away it is or how long the light has been traveling.

Igottaknow
I think your problem is that you are thinking the age is set by the most distant light sources we can see. That would not be enough by itself-- the age is set by the dynamics of the universe that we infer from looking at the redshifts as a function of distance. The interesting point is that the light we see that has been traveling for 13.7 billion years is very highly redshifted, so there is, in some sense, not much redshift left there that would be possible. That's how we know we are seeing almost the whole age of the universe, not just the distance away it is or how long the light has been traveling.

Thank you. I did think that we were using redshift measurements to infer or confirm that the universe is in fact expanding (similar to the doplar effect) but was not aware that this was telling us anything about the universes age.

Am I wrong in thinking that we observe light from the position that distant object was in 13.7 billion years ago and it is in fact not farther away than we see its position from here?

Gold Member
Dearly Missed
I have always been confused why it is said that the universe is 13.7 billion years old ...
I could be totally wrong about how we have put an age on the universe ... Can someone clear this up for me?

Look at this picture:
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg
It can help get some idea of how people go about estimating HOW LONG DISTANCES HAVE BEEN EXPANDING.

Our basic model of geometry (and gravity) is the 1915 GR equation and the 1923 Friedmann simplification derived from it. These have been checked in every possible way people can think of and have always proven to give a good approximation to observations. GR has been checked at earthly scale, solar system scale, and astronomical scale with many sorts of phenomena--it is our basic law both of gravity and of geometry, i.e. how distances and angles etc behave over time.

The (gravity/geometry) law says that if distances somehow get started expanding they will tend to continue but the percentage growth rate will gradually decline because of gravity, depending on how much matter there is. The different curves you see in Figure 14 are the results of varying two basic parameters (matter density and Lambda the cosmo constant.)

That much is the math model--well verified in many different ways. It generates curves. You can see that each curve tells you a different age of expansion because each one gets down to zero size a different length of time back into the past.

At some point in past a generic distances (one unit at present) was essentially zero size and that defines the START OF EXPANSION. How far back in time you have to go to find that start, depends on which curve you follow.

Observations are what allow us to tell which curve we are on.

We can tell that the one we are on is most like the curve labeled (0.27, 0.73). In Figure 14, that is the heavy solid line. You can study the effects of different amounts of matter, and different cosmo constant, by looking at the other curves, dotted, thin solid etc.

The crosshairs labeled (NOW, redshift 0.0) mark the present. All the curves are together at the present and they go back in time and hit the distance scale 0.0 at different times in past.

From observations we know we are on (0.27, 0.73) curve. So this tells us expansion has been occurring for 13.7 billion years.

Universe could of course be older. It might have been contracting, and had a rebound 13.7 billion years ago. What we are talking about is the age of the expansion which we are witnessing now, we don;t know yet what got that started or what happened before.

Last edited:
1 person
Igottaknow
Ok thank you. I am still curious about the current position of this far off object vs the position that we observe it at based on the light that has traveled from it. Here is kinda where i am confused. We observe that the farthest galaxy from us is about 13.3 billion light years away http://www.space.com/18502-farthest-galaxy-discovery-hubble-photos.html and formed about 420 million years after the big bang (sum of 13.7b btw) and we see this thing based on light that has been traveling that long.

Here is kinda how i am envisioning this.

A spaceship 15 light minutes from us is sitting in space not moving. It decides to take off heading away from an observer at a high rate of speed. The observer of the spaceship had his back turned when the spaceship started moving so he never got a good idea where it started at but he can see the progress of this thing as it moves through space pretty much 15 minutes ago as it travels away from him. Wouldn't this mean that from the moment he observed it it will always be 15 minutes after the fact and the spaceship is in fact farther away than what he sees?

That's kind of why I asked the basic question "Why do we not say that the stuff that we can OBSERVE is 13.7 billion years old?" vs saying "The universe IS 13.7 billion years old"?

Gold Member
That's kind of why I asked the basic question "Why do we not say that the stuff that we can OBSERVE is 13.7 billion years old?" vs saying "The universe IS 13.7 billion years old"?
We say both of those things. The stuff we observe is 13.8 billion years old, but it is also extremely redshifted, so fits into a model of the universal dynamics that says we are seeing close to the beginning. It is also consistent with very high density gas, when the universe was quite young. It is only the combination of these facts that tell us that we are seeing very close to the beginning. When we see various different sources at different distances, it is like taking snapshots at various points in the history of the universe. The snapshots tell the story of a universe that goes from very young (the most distant sources) to rather old (the nearby ones). The only way we can tell the age of the universe is by playing out that story, there is inference involved.

1 person
Gold Member
Dearly Missed
...Am I wrong in thinking that we observe light from the position that distant object was in 13.7 billion years ago and it is in fact not farther away than we see its position from here?

Ok thank you. I am still curious about the current position of this far off object vs the position that we observe it at based on the light that has traveled from it. Here is kinda where i am confused. We observe that the farthest galaxy from us is about 13.3 billion light years away http://www.space.com/18502-farthest-galaxy-discovery-hubble-photos.html and formed about 420 million years after the big bang (sum of 13.7b btw) and we see this thing based on light that has been traveling that long.
...

The farthest stuff we observe is not the farthest galaxy we observe. The farthest is the cloud of hot (3000 degree Kelvin) gas that we see as it was around year 0.0004 billion. More precisely year 0.00038 billion or in more familiar terms year 380,000.

It was around that year that the hot gas cooled enough, by expansion, to become sufficiently transparent to
let the light continue freely. The gas stopped being so dazzling that it scattered and blocked the light. So what we call the CMB was the orange-ish glow of partially ionized 3000 K hot gas just when it became transparent. The light has been traveling for 13.7 billion years.

If you are interested in this kind of thing then I suggest you open this
http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
and learn how to read the table.

The top row tells about the CMB. "T" is the year the light was emitted, namely year 0.0004 billion.
S = 1090 is the factor its wavelengths have been stretched out by expansion during the 13.7 billion years it has been traveling.

Dthen = 0.042 billion ly = 42 million ly is how far the source hot gas was from us when it emitted the light we are now getting today.

(freeze-frame distance, if you could have frozen expansion process to give yourself time to measure, that was how far it was back then)

Dnow = 45 billion ly is how far that material is from us NOW (13.7 billion years later during which distances have expanded by factor of 1090 and that material which was the hot gas has cooled and collected into stars and galaxies just as our material has done during the same period of time.)

We see it as it was. And if its galaxies have beings with telescopes THEY see us as we were, namely as 3000 K hot gas that glowed to them making the CMB they see. It is reciprocal. We each were 42 million ly close to each other when the light was released. We each are 45 BILLION ly far from each other now. And in the meantime our matter has condensed to form stars and planets.

The factor between 42 million and 45 billion is the 1090 which you see in the top row of the table.

The table also gives "recession speeds" as multiples of the speed of light. these are rates the distances were then (are now) growing. It is not like ordinary motion, it is geometry change as required by GR, nobody gets anywhere by it, everybody just becomes farther apart. Once started, geometry change tends to continue but the rate can gradually change. I mentioned that in previous post.

See if you can get something out of the table. It is interactive, you can change the parameters of the table, control the range, control how many steps, select which columns to display etc. It also can draw graphs. a guy here at PF created the program. blue dots give you popup information explaining some things.

Gold Member
Dearly Missed
Igotta, watch out for sources like "space.com"! I see from your quote they may be doing wide-audience readers a disservice by exclusively using LIGHT TRAVEL TIME to indicate distance.
" kinda where i am confused. We observe that the farthest galaxy from us is about 13.3 billion light years away http://www.space.com/18502-farthest-...le-photos.html [Broken]..."

13.3 billion years is the travel time. In terms of freeze-frame distance (cosmologists call it "proper distance" that galaxy would have been much closer when it emitted the light, and much farther away NOW, when we are receiving the light it emitted 13.3 billion years ago. It is now something over 30 billion ly (if you could freeze expansion to measure its instantaneous actual distance.) Because expansion rates have varied over course of history, there is no simple correspondence between light travel time and proper distance when one is working a cosmological scale over cosmological timespans.

To get the proper distances Dthen and Dnow, an easy way is to find out the redshift z and plug the redshift into one of the several available online calculators.
Two examples of high redshift galaxies:
http://en.wikipedia.org/wiki/UDFy-38135539
http://en.wikipedia.org/wiki/UDFj-39546284

Last edited by a moderator:
1 person
Igottaknow
Ok i totally get this now. Thank you so much Marcus. Just doing a search, after reading your response, on the diameter of the observable universe tells you a diameter of 93 billion light years.
Really cool stuff. Thank you for the link to that chart too that is super helpful.

goldust
In my opinion, time is not absolute but is relative or in other words personal. Maybe to Earth the universe seems 13.7 billion years old. But maybe to some other parts of the universe the age could be much different. Different parts of the universe could be moving at different speeds through space as indicated by differences in the cosmological background microwave radiation. Also, it is claimed there's been a single big bang event, but even that is not determined to be certain. We cannot be sure what happens if parts of the universe we cannot observe. The age of the universe must for now remain a question.

Gold Member
Certainly, those who hold to the concept of "universal inflation" would imagine that the uber-universe is indeed infinitely old, in a striking return to classical thinking.