# The age of the Universe?

1. Jan 17, 2014

### Baggins101

Hi Great Ones (!)

Just wondering if someone could help me understand something:

Time is relative. If our planet orbited close to a black hole where the effect of gravity was very strong we would look out into the night sky and see stars formed and die, possibly in a single night. We would be writing that a Sun sized star has a stable life of hours or days rather than 8-10 billion years. Would we also be writing that the Universe was just a few centuries old?

Also, presumably, the effect of gravity in the Universe would be greater the denser it is, therefore time would have passed more slowly in the dense early Universe.. close to the singularity time would hardly move at all (although relative to what I do not know!)

So...... how can we possibly claim the Universe is 13.7 billion years old? Relative to what??

Is the Universe 13.7 billion LIGHT years "old"?

Last edited: Jan 17, 2014
2. Jan 17, 2014

### phinds

The age of the universe is taken as the amount of time that would have been experienced by a body that is at rest relative to the CMB, which the Earth is reasonably close to doing. You are right that the time experience by something orbiting a black hole would be different.

How do you figure something can have an age that is measured in units of distance? Is the universe 3 feet old?

3. Jan 17, 2014

### Baggins101

Thanks. My reference to light was confusing as I didn't intend to mean the unit "light years", rather that the age of the Universe is determined by the time it has taken light from the earliest state of the Universe to reach us. This would remove the need for any other frame of reference.

4. Jan 17, 2014

### phinds

Well, no, it would not, at least theoretically. In practical terms, it so close as to hardly matter, but that's not the point. The Earth should not be taken as a universal reference. Co-moving with the CMB is the only thing I've ever heard of that is taken as a universal standard for the age of the universe. In terms of relativistic speeds, we are very close to co-moving, we aren't exactly. From Earth, there are hot and cold spots in the CMB that show this (we are moving slightly in one direction)

5. Jan 17, 2014

### jcsd

13.7 billion years is the age of the Universe for an isotropic observer (i.e. one who is it at rest relative to the CMB), and is also the maximum age of the Universe for any observer in the present time (as defined by cosmological time).

If an observer started at the big bang and traveled at speeds arbitrarily close to c relative to the isotropic observers he passed and arrived at present day Earth, the amount of proper time experienced by them since the big bang would be arbitrarily small.

6. Jan 18, 2014

### Chalnoth

To put this in even more concrete terms, 13.7 billion years is the maximum age of the universe for an observer who sees the CMB at 2.725K. The observer who sees this age is one that has always seen no dipole in the CMB.