# The age of the universe: easy question - hard answer

1. Feb 22, 2005

### Ordep

Hi all,

I've been struggling with an idea lately and was wondering if anybody could shed some light on it. It seems simple but i don't think the answer is very obvious.

The best guestimate for the age of the universe at the moment is 13.5 billion years old. If we could travel back in time 13 billion years and then measured the age of the universe again what would be the answer? Would it be 0.5 billion years old?

Thank you.

2. Feb 22, 2005

### mathman

I am a bit confused as to why you think the obvious answer (i.e. go back 13 bill yrs and it will be .5 bill yrs old) may not be correct?

3. Feb 22, 2005

### Garth

If the value 13.5 Gyr is correct then yes.

Garth

4. Feb 22, 2005

### Ordep

Ok let me expand a little more on where this question comes from. I believe the general way of looking at the universe today is that we live in a sphere of radius 13.5 billion light years with us in the center. This is because in whatever direction one looks from Earth one sees the microwave background radiation (which is a remnant of the big bang) at that distance. I hope you're all nodding your heads in agreement so far.

Now we also know that for every other point in that sphere their perception of the universe is the same (i.e. in every direction they look they'll think that they are in the center of a sphere of radius 13.5 billion light years long.

So my conclusion to this is that when we talk of a different point in that sphere we're basically talking about travelling back in time since everything we see in the heavens is from the past. So to a point located at 1 million light years away from us that point exists 1 million years in the past. Correct?

So this bring me to my question if we were to travel back in time 13 billion years (in other words to a point 13 billion light years away from us) then the age of the universe would still be 13.5 billion years and not 0.5 billion years.

Where is the flaw (if any) in my logic?

5. Feb 22, 2005

### DB

I like to image a hyperbolic universe, though your bubble (parallel) universe picture is valid and being hypothosised and studied today.
The key about this is that we only see that point (from earth) 1 million light years in the pass, because photons have a finite speed. Traveling to a different point in the universe is not traveling back in time. Though it is possible say your traveling back in time relative to the earth. But your speed will also have an effect of your time perception relative to someone on earth.
We don't know how old the universe is, we only are able to view(collect) photons that took 13.5 billion years to reach earth, obviously in all directions, which is why many perceive the universe as a sphere. So saying the universe is 13.5 billion years old is more of a measure of size, then age. We really dont know the age. But it's no less that 13.5 billion years. And remember that the universe is expanding, so turning back time 13 billion years, is like schrinking the universe to "radius" of 0.5 light years. So now it seems alot easier to reach the end of the universe, I beleive its called the final frontier. What lies at th end of the universe...we don't know. Is there an end....?

6. Feb 22, 2005

### turbo

Basically, that's it in a nutshell.

True. This concept of simultaneity is very important in cosmology. It allows us to visualize the possible extent and geometry of the Universe by extrapolating beyond our visible universe.

Aha! Here's the problem. Above, you are willing to grant that some point near the edge of our visible universe, can have a "now" that is equivalent to ours, and here you equate theoretical travel to that point to "travelling backward in time". This is a relativistic concept that does not coexist well with simultaneity. The location of an observer 13Gly away from us is visible to us due to EM emitted 13Gly ago, but travelling toward him is not equivalent to travelling backward in time. In fact, if you could travel to him at the speed of light and then look around, you would see a universe MUCH older than 26 Billion years old, because in the Big Bang model, the Universe is expanding, and the expansion is accelerating.

If you want to travel to the point 13Gly away and view the universe from there as it existed 13Gy ago, you are proposing two separate forms of travel in one trip. One is spacial (go from place A to place B) and the other is temporal (backward along the timeline toward the Big Bang). If you could travel in both these ways, the universe at point B would look only 1/2 billion years old to you. If you want instead to instantly be transported at the present position of place B NOW (in accordance with the simultaneity concept of most cosmologists), the visible universe there would look just like ours in most respects, including an apparent age of 13.5 billion years. Now to confuse things further, (as above) if you want to travel to the present position of observer B at the speed of light (travelling through an expanding Big Bang universe), it will take you MUCH longer than 13 billion years to get there (and if the expansion is accelerating, you will NEVER get there!), and if you get there, the visible universe will be MUCH older than the 26 billion years that you might expect (without figuring in the expansion).

Edit: another PF denizen has answered you while I was composing my lengthy reply (and having supper), but I'm going to let this stand, as a slightly differing point of view.

Last edited: Feb 22, 2005
7. Feb 22, 2005

### saltydog

Stars in globular clusters surrounding the Milky Way are estimated to be about 15 billion years old.

8. Feb 22, 2005

### turbo

And as we discussed in another post, quasars (even at redshifts z~6.5) exist in hosts that exhibit metallicities in the solar and super-solar range. How can an object less than a billion years old have a metallicity equivalent to that of our own galaxy, which apparently has had 13.7Gy of stellar metal-enrichment to produce the chemical properties of our "neighborhood"? If quasars could condense out of materials in our galactic neighborhood, their high metallicities would be non-controversial, but it we believe that heavy elements (heavier than could be produced by BB Nucleosynthesis) are produced by fusion in stars, it is VERY difficult to explain the high metallicities of these early quasars. By this measure alone, the universe is apparently much older than 13.7Gy - MUCH older.

9. Feb 22, 2005

### Chronos

Actually closer to 13.7 Gly. And this does not have as much to do with the CMB as it does with the Hubble constant - Ho.
Yes, but only in their 'now', which is in our future light cone. We are 'now' at the edge of their observational universe and appear to be much younger than they.
How fast are you traveling? If you travel back in time [i.e., faster than the speed of light], the universe would appear to shrink. If you instantaneously traveled 13 billion light years, the universe would indeed appear to be only .7 billion light years in diameter. Turbo-1 gave a good explanation. I'm just using a slightly different approach.

10. Feb 23, 2005

### Entropy

This is why I think the universe is much older than 13 billion years old. But exactly how old, I can't give even an educated guess.

11. Feb 23, 2005

### Chronos

There is nothing yet suggesting metallicity in the early universe is a significant theoretical problem:

The cosmological evolution of metal enrichment in quasar host galaxies
http://arxiv.org/abs/astro-ph/0309533

In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activity

The Era of Massive Population III Stars: Cosmological Implications and Self-Termination
http://arxiv.org/abs/astro-ph/0310443

Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal-polluted, as well as ionized, by massive Population III stars formed in small-mass (10^6-10^7 Msun) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to 10^{53} ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a redshift of 15-20

12. Feb 24, 2005

### turbo

I italicized the important part of that quote. The authors of that paper predict a slow decline in the metallicity, because if that decline is not observed, the BB model is dead. So far (to z~6.5) no decline is observed. In fact, the scatter of quasar metallicities seems unrelated to redshift (paper cited above).

13. Feb 24, 2005

### Chronos

Agreed, by their argument. Do you have evidence refuting that position?

Last edited: Feb 24, 2005
14. Feb 24, 2005

### A_I_

i've read some replies on that topic and some of you took our Earth as a point of reference to measure.
so the real age of the Universe is not what we calculate because obviously relativity plays its role in everything therefore if we come to a consensus it would be only for us; i mean the age will only be relative to our beliefs in our theories and our calculations.
Talking about the universe and its radius;
well as we r prooving it's expanding
we can not consider a constant radius;
and i believe (i don't know if anyone shares this idea with me)
that each point of the universe is its center with respect to that point.
Basing my judgement on a fact not totally prooven but might be a right answer which is that our Universe is expanding and unlimited.
NO ONE can imagine infinity
thus; no one can imgine the center of an infinite SPHERE (the universe)
this is in the case if our Universe is spherical.
Too many questions.
Hopefully we will reach the speed of light,
maybe by then we can answer a small part of our infinite questions!

15. Feb 24, 2005

### turbo

Yes.

http://cosmos.as.arizona.edu/~thompson/pubdb/docs/barth03a.pdf [Broken]

Last edited by a moderator: May 1, 2017
16. Feb 25, 2005

### Chronos

Insufficient. That does not invalidate the model. From the paper you cited:

The Fe ii/Mg ii ratio is also similar to values found for lower redshift quasars, demonstrating that there is no strong evolution in Fe/a broad-line emission ratios even out to z ~ 6.4. In the context of current models for z p 6.4 iron enrichment from Type Ia supernovae (SNe Ia), this implies that the SN Ia progenitor stars formed at z ~ 10.

This same conclusion has been already been derived numerous times. Until the iron lines are pushed out past z = 8, this is old news.

17. Feb 25, 2005

### turbo

These folks feel that the metallicities (especially iron abundance) of quasars at z~6 are higher than can be explained by the standard model.

http://citebase.eprints.org/cgi-bin/fulltext?format=application/pdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0406630 [Broken]

Last edited by a moderator: May 1, 2017
18. Feb 26, 2005

### RoboSapien

I am an amature but here is a question that is bloging me.

Universe may be 15 billons lightyears wide but to get that wide, did it happen at light speed ? If not then how many years should it take to be so ?

19. Feb 26, 2005

### Chessguy

Another amateur question

let us suppose that we are exactly at the centre of a spherical universe
if we look in one direction and see stars, microwaves or whatever for 6.85 billion light years then nothing
then we turn around and look in the opposite direction and see stuff for 6.85 billion light years then nothing-ness?
If we could see that much stuff then wouldn't that mean that because this light that we can see really eminated from its space 6.85 billion years ago so if it is expanding at the speed of light (I don't know this just asking?) then it has really expanded another 6.85 billion light years in the time it took its light to travel to us? that would make that universe 13.7 + 6.85 = 20.55 billion light years old?

I'm guessing that the people calculating this stuff would have factored that in - so we really only must be able to see stars/microwaves that are 4.57 billion light years away in each direction for the universe to be 13.7 billion years old? assuming it is expanding at the speed of light and we are the centre of the universe of course?

20. Feb 27, 2005

### RoboSapien

Chessguy

Wow, U mean that what we see 15 billion years old has already moved on ahead of that distance since its light reached us.

That means I should ask How long did it take for the universe to be 15+ Billions light years wide depending on the speed of its growth.

I am not an expert but here is my suggestion for the answer of my above question and please correct me because I am sure I am wrong. If we measure the speed of farthest objects in opposite directions of the visible universe then we should get a rough idea of the speed of expansion of universe then we can calculate how long it would have taken to be 15 billion light years wide.

But wait, I also wana know if there any two object that are 15+ billion light years away and in opposite direct of the visible universe because that will make it 30billions light years wide. I am a fool and sorry about being wrong.