How Old is the CMB? Big Bang Cool Down Explained

[HarryWertM]

In other words, when did the big bang cool down to 3 degrees? Like 13.7 billion years ago, or in recent millenia? If it only cooled that much near current time - which is what I understood - then the CMB must be "coming from" the space right around the Milky way - which is not what I understood.

 

[mgb_phys]

The CMB is an image of the universe when it was only about 350,000 years old - we can date this reasonably accurately because we know hoe long it would take for the initial conditions to cool to it's very well known temperature.

So essentially the age of the universe old.

 

[bapowell]

The CMB reflects the current temperature of the universe, around 3K. But it was created, as mgb says, much earlier when the universe was much hotter. The CMB photons have been moving through the universe since that time, and they have been cooling with the expansion. The CMB photons that we are receiving today on Earth originated billions of light years away.

 

[russ_watters]

Just to clarify, the CMB has never stopped cooling down but follows a predictable curve...which is of use in calculating its age.

 

[Chalnoth]

In other words, when did the big bang cool down to 3 degrees? Like 13.7 billion years ago, or in recent millenia? If it only cooled that much near current time - which is what I understood - then the CMB must be "coming from" the space right around the Milky way - which is not what I understood.

The CMB was emitted when our universe condensed from a plasma to a gas. This phase change happens at around 3000K (about 5000 degrees Fahrenheit). Since then, our universe has expanded around a thousandfold, which in turn has cooled the CMB by a factor of about a thousand, leading to the current temperature of around 3K.

And as for where the CMB was emitted, it was emitted everywhere. It's just that the part of it that we see is the part that has had photons in flight for the last 13.7 billion years, which turns out to be from a part of the universe that was, at the time, around 45 million light years away or so. The rapid early expansion of our universe has caused the light to take this long for it to reach us.

 

[bapowell]

The rapid early expansion of our universe has caused the light to take this long for it to reach us.

Or not. If the universe is infinite, no rapid early expansion is necessary.

 

[Chalnoth]

Or not. If the universe is infinite, no rapid early expansion is necessary.

Infinite or not, rapid expansion is very much required for light that starts out around 48 million light years away to take 13.7 billion light years to get here.