# Estimating the age of the universe not knowing its size?

1. Aug 31, 2014

### Gerinski

How can we estimate the age of the universe if we don not know how big it is?
It is often said that our visible universe may well be but only a small part of the actual universe, but I guess that the current estimates for the age of the universe come from running backwards the expansion of our visible universe until it would collapse to a single point?
But if it's much bigger beyond the visible horizon, it could have been expanding for much longer and therefore be much older?

2. Aug 31, 2014

### Simon Bridge

Pretty much the same way we can estimate the age of a horse from it's teeth alone - no need for the rest of the horse - we just find a subset of the Universe that has a characteristic that tells us the age.

i.e. the time since the lights came on is the distance to the farthest thing we can see divided by the speed of light.

No - that would suppose that the Universe expanded out from some particular place. That does not work because before the Universe there was no "place" to expand out from ... that is pretty much what "Universe" means.

What you do instead is project the mass/energy density back in time - this has the advantage of not needing to know where some "edge" is since density can be measured "locally" (i.e. inside the visible Universe).

3. Aug 31, 2014

### Bandersnatch

What, Simon, it's 46 billion years old now? You've probably dialed down the complexity to convey the basic idea, but it's bound to bring about confusion sooner or later.

You can't actually look at the farthest objects we see and divide the distance by the speed of light, as the space between those objects is not static. The expansion of the universe means that there is actually more distance between us and the afterglow of the big bang(CMBR - the farthest we can see presently) than 13 something billion ly.

Anyway, to calculate age, you just take the expansion into account(including its changing rate).
You don't need to look at the farthest bits, you can look at any bit that is affected by the expansion(i.e., not our closest neighbourhood) measure its recession velocity, use hubble's law to find distance, and calculate backwards to see when the object ought to coincide with your position, while keeping in mind that velocity is not constant. It will work for any receeding object, and net the same age.

4. Aug 31, 2014

### Simon Bridge

I'm bracing myself yeah ;)

I kinda hope that OP has already taken advantage of the many many reputable online resources dedicated to the age of the Universe and I have interpreted the question accordingly.

But maybe I wasn't clear in the glib starter:
The distance intended in that calculation is not the distance to the object when it's light was received (for which you need to account for the expansion) but the distance to the object when the light was emitted. The distance that is seen. Naturally the object has moved since then.

5. Aug 31, 2014

### Gerinski

TX I think I get it.

6. Sep 1, 2014

### Bandersnatch

Maybe I'm missing your point still, but I don't see how that's correct either. At the time of emission of currently observable CMBR, the emission points were about 40-ish million ly away. At least that's what Jorrie's lightcone calculator tells me.
(for those unfamiliar with it, here's the link: http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html)

Meaning, the expansion not only causes the source to recede while the light is in transit, it also causes the space between the light and the observer to be stretched, so that it always takes more time than D/c to get there(where D is the proper distance at the time of emission).

7. Sep 1, 2014

### mal4mac

From WMAP data we know the current mass/energy density of the universe, and the current rate of expansion of the universe. We use Einstein's General Relativity to compute how fast the universe has been expanding in the past. With that information, we can turn the clock back and determine when the universe had "zero" size, according to Einstein. The time between then and now is the age of the universe.

http://map.gsfc.nasa.gov/universe/uni_age.html
http://map.gsfc.nasa.gov/universe/uni_expansion.html

Last edited: Sep 1, 2014
8. Sep 1, 2014