Is the 14 billion year age of the universe based on observation or assumption?

[Vexa]

So, supposedly the age of the universe is about 14 billion years. But the only reason we believe that is because we can only see something 14 billions years away. Do we just assume that just because we can't see past the 14 billion mark, there's nothing there? And by that assumption, shouldn't we be able to see the actual "Big Bang" if we look 14 billion years into the past? For all we know, the universe is 900 billion years old and some far away light just never reaches us.

 

[zhermes]

So, supposedly the age of the universe is about 14 billion years. But the only reason we believe that is because we can only see something 14 billions years away.

No. There are other reasons; the temperature and scale-size of variations in the cosmic microwave background (CMB) are consistent with this age. Similarly the oldest stars and galaxies are about this age (both things close and far away). Also the level of structure of galaxies and galaxy-clusters (even more so) are consistent with this number.

Do we just assume that just because we can't see past the 14 billion mark, there's nothing there?

Not at all. The visible universe only extends to 14 giga-lightyears, but there should be lots more universe out there (its hard to make/find an actual estimate).

And by that assumption, shouldn't we be able to see the actual "Big Bang" if we look 14 billion years into the past?.

Yes we should. And we do. http://en.wikipedia.org/wiki/CMB"

 

[nicksauce]

No, that's wrong. We take it to be 14 billion years old, because the equations of the standard model of cosmology (the LCDM model) give an age of 14 billion years when you use the accepted present day values of the dark matter density, matter density, dark energy density, and Hubble constant.

The fact that the furthest thing we can see is 14 billion light years is just a coincidence. The observable universe is actually larger than 14 billion light years, and in principle we could see things as far away as 46 or so billion light years, or so. (46 > 14 comes from the fact that the universe is expanding, so its size in light years is larger than its age in years)

 

[Chalnoth]

As others have said, it is quite clear that our observable region is 14 billion years old. Note that this is not a statement that everything began then, just that our region did. There are likely things further away, and likely other regions which we cannot observe that began before or will begin in the future. But ours is 14 billion years old.

 

[zhermes]

Note that this is not a statement that everything began then, just that our region did. There are likely things further away, and likely other regions which we cannot observe that began before or will begin in the future.

All standard cosmologies say that everything began 14 gyrs ago; why do you think otherwise?

 

[Chalnoth]

All standard cosmologies say that everything began 14 gyrs ago; why do you think otherwise?

The standard cosmologies make assumptions which nobody seriously believes extend infinitely in all directions, such as spatial homogeneity. The various hypothetical models that we do have for the birth of a region of space-time like our own do not predict ours to be a unique event.

More generically, mathematically speaking it is vastly easier for an entire class of things to exist than for a single member of the class to exist.

 

[zhermes]

The standard cosmologies make assumptions which nobody seriously believes extend infinitely in all directions, such as spatial homogeneity. The various hypothetical models that we do have for the birth of a region of space-time like our own do not predict ours to be a unique event.

Interesting. Can you point me towards some papers on the subject?

 

[Chalnoth]

Well, I'm not aware of anybody that seriously publishes regarding default assumptions in cosmology.

You can verify what I've said about hypothetical models for the birth of a region of space-time by reading up on loop quantum cosmology, eternal inflation, the string theory landscape, or Sean Carrol's arrow of time stuff. There are sure to be many other ideas out there as well.

As for the homogeneity point, just consider that our current proposed explanation, inflation, is supposed to explain homogeneity by growing a very small region into a very big one, meaning that any observable section of the original will be so vastly smaller than the whole that it cannot help but be homogeneous. This doesn't entirely work in detail, because inflation can't start with a region that isn't already pretty homogeneous, but the fact remains that we don't expect inflation to have started everywhere, just in a spot that happened to have the right conditions.