Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Paul Young on 08/02/2011 00:30:02

Title: How can we be sure about the age of the universe?
Post by: Paul Young on 08/02/2011 00:30:02

Paul Young  asked the Naked Scientists:
   
Hi
 
I was just wondering about the age of the Universe
 
If the Universe is expanding faster than the speed of light then that would mean certain objects exist now that will never be seen here on Earth
 
If scientists measure the age of the Universe by observing galaxies/objects that they can see then how can they be sure of the age of the Universe if there are (theoretically) many galaxies they can't and never will see?
 
If you see what I mean...
 
Thanks
 
Paul

What do you think?

Title: How can we be sure about the age of the universe?
Post by: yor_on on 08/02/2011 06:53:26

It's a valid question, and I think the answer must be that the definitions we use only can build on what we know. So as we succeed to put the universe borders further away from us, the universes age become older. There are two factors I can think of regulating it, how red shifted light can become, still being measurable for us, and maybe the expansion. The limit we have now I think is 13.7 billion light years, but maybe we will be able to see further.

What we use is the rate of the expansion (Hubble Constant T = 1/H) but as we may both have had an inflationary period and as we know it's accelerating we can't be sure of that one. Then we know that similar stars are of similar age throughout the universe, of course that is a hypothesis but it seems very reasonable, so called main sequence stars. And then we can use white dwarfs as we think us know how long time it takes for them to become really cool. And that we can see on their spectra (radiation). There might be other ways too, but those are the ones I know of.

==

That is also how we can be sure of that there must be an expansion as I understands it, and most probably an inflation before that. By measuring similar stars, standard candles as they are called (type 1a supernova). They are of the same luminosity, more or less, and exist all over the universe, meaning that the universe seems to be as 'old' everywhere. They may not be perfect, but other evidence has been found in the cosmic microwave background and the large scale structure of the universe, and in gravitational lensing.

Title: How can we be sure about the age of the universe?
Post by: imatfaal on 08/02/2011 18:03:43

Paul - to an extent you are right.

But do remember that when the big bang theory was in its infancy (and being ridiculed) it was predicted that we should be able to observe the very cold glow of the big bang preserved in light from 13.6-7billion years ago. 
1. Physicists calculated that after the big bag the universe was super hot (ie plasma) and at a certain point it would cool enough to transmit light.
2. And that light would still be scudding around the universe
3. And if the expansion of the universe was correctly calculated that light would be red-shifted down to microwave

Well - they found that light, it's the Cosmic Microwave Background Radiation (CMBR).  To this point it has fulfilled the predictions very nicely.   

To get back to your original question, scientists predict the age of the universe through a combination of theory and observation.   Objects that are outside our light cone mean little to us; we can have no contact or relevance to/from them.  But the most distant phenomena/aobjects that we can observe  are over 40Gly away (ie far longer than light would travel in the lifetime of the universe) and we are fairly certain that the theories pan out. 

We are currently observing to the limit - ie we are using light/emr and can see the CMBR (the universe was opaque to light before the period that caused the CMBR) so until we can harness neutrinos or gravity waves to observe early universe we are stuck with a cosmic timelimit that stop at the CMBR