Cosmology Q&A: Understanding the 15.5 Billion Year Figure

[eighteyes]

A layperson question...

I've taken a cosmology class, but I don't have a strong foundation in physics outside AP high school classes. So don't take too much time to explain to me,

we're basing our understanding on the universe in how long we can see back in a giant time sphere around us
how exactly are we arriving at the 15.5 billion year figure? red shift? it takes that long for light to reduce in frequency to a microwave state as a byproduct of space expansion?

and now some nonsense questions...
how did our matter manage to get ahead of light itself from the big bang?
is it possible that the light we participated in our nuclear fusion may in fact, through a trickery of time and space, be a star in the sky, or perhaps a burst of microwave radiation? A gravity revolving door if you will.
if Jupiter and saturn had been closer together in the accretion of proto-solar gasses, would they have reached enough mass to ignite, thus rendering life on earth, fairly unlikely?

and finally, i want someone to recognize how funny it is that ~almost 500 years ago copernicus considering ourselves no longer at the center of the known universe, and it was a huge deal. even he would find it amusing that today, again. we are at the center of the known universe.

 

[WannabeNewton]

To answer your first question: what is done to determine the age of the universe (which is what I think your question was about even though the age of the universe is about 14 billion years) is that one takes the inverse of the present day value of the Hubble constant, t_{0} = \frac{1}{H_{0}} which is determined, essentially, through red shift like you said: http://en.wikipedia.org/wiki/Hubble's_constant#Determining_the_Hubble_constant

 

[Chalnoth]

To answer your first question: what is done to determine the age of the universe (which is what I think your question was about even though the age of the universe is about 14 billion years) is that one takes the inverse of the present day value of the Hubble constant, t_{0} = \frac{1}{H_{0}} which is determined, essentially, through red shift like you said: http://en.wikipedia.org/wiki/Hubble's_constant#Determining_the_Hubble_constant

This isn't correct. While this does get close to the current age of the universe, this is largely an accident of the recent expansion history.

Instead, what we do is we measure, with a variety of observational methods, how our universe has expanded over time. This measurement of how our universe has expanded over time gives us an estimate of how old it is.

 

[Chalnoth]

Iand now some nonsense questions...
how did our matter manage to get ahead of light itself from the big bang?

It didn't. Why would you think it did?

is it possible that the light we participated in our nuclear fusion may in fact, through a trickery of time and space, be a star in the sky, or perhaps a burst of microwave radiation? A gravity revolving door if you will.

That light is long gone.

if Jupiter and saturn had been closer together in the accretion of proto-solar gasses, would they have reached enough mass to ignite, thus rendering life on earth, fairly unlikely?

You need about 13 Jupiter masses to fuse deuterium, and around 75-80 Jupiter masses to fuse normal hydrogen into helium.

and finally, i want someone to recognize how funny it is that ~almost 500 years ago copernicus considering ourselves no longer at the center of the known universe, and it was a huge deal. even he would find it amusing that today, again. we are at the center of the known universe.

Well, everywhere is at the center of their known universe. This is just due to the fact that light travels a the same speed in every direction. Science has continued to show that our place in the universe is less and less special than we previously thought.

For example, one thing a lot of people aren't aware of is that before around 1920 or so, we didn't know any other galaxies than the Milky Way even existed. Now we know that there are over 100,000,000,000 galaxies that we can see, around 100,000,000,000 stars in each of those galaxies, and there are likely vastly more galaxies out there that we cannot see (either because they are too far away or too dim).

 

[WannabeNewton]

This isn't correct. While this does get close to the current age of the universe, this is largely an accident of the recent expansion history.

Instead, what we do is we measure, with a variety of observational methods, how our universe has expanded over time. This measurement of how our universe has expanded over time gives us an estimate of how old it is.

So the Hubble time is not something that gives, in general, the correct age of the universe for some value of the Hubble parameter that is not necessarily the current value?

 

[Chalnoth]

So the Hubble time is not something that gives, in general, the correct age of the universe for some value of the Hubble parameter that is not necessarily the current value?

The Hubble parameter is, after all, just the current rate of expansion. The fact that the age of the universe is pretty close to 1/H_0 is more or less an accident of how fast our universe has expanded in the past. A universe with only normal and dark matter and no dark energy, for instance, would be much younger at the same current expansion rate. Dark energy has slowed the rate of reduction of the Hubble parameter, which makes the universe older for the same current expansion rate.

 

[WannabeNewton]

The Hubble parameter is, after all, just the current rate of expansion. The fact that the age of the universe is pretty close to 1/H_0 is more or less an accident of how fast our universe has expanded in the past. A universe with only normal and dark matter and no dark energy, for instance, would be much younger at the same current expansion rate. Dark energy has slowed the rate of reduction of the Hubble parameter, which makes the universe older for the same current expansion rate.

Good to know. Thank you very much.

 

[JonDE]

how did our matter manage to get ahead of light itself from the big bang?

What he is saying is since the light is now reaching us that he thinks that we moved faster then the light and that it is now catching us.
As Chalnoth said above we didn't move faster then the light, I guess an easier way to explain it is the light took a longer path to get where we are today. I used to have a link that explained it very well with pictures but I am unable to find it as this moment.