How Old is the CMB? Big Bang Cool Down Explained

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Discussion Overview

The discussion centers on the age of the Cosmic Microwave Background (CMB) radiation and its cooling process since the Big Bang. Participants explore the implications of the CMB's temperature, its origins, and the expansion of the universe in relation to the CMB's current state.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question when the universe cooled to 3 degrees, suggesting it could have occurred either around 13.7 billion years ago or in more recent times.
  • One participant states that the CMB represents the universe at about 350,000 years old, with its temperature cooling over time from much higher initial conditions.
  • Another participant clarifies that the CMB photons have been traveling through the universe since their emission, which occurred when the universe was much hotter, and that they originated from billions of light years away.
  • A participant notes that the CMB has continuously cooled down following a predictable curve, which is relevant for calculating its age.
  • There is a discussion about the emission of the CMB, with one participant stating it was emitted everywhere and that the photons we observe today have traveled for 13.7 billion years from a region that was approximately 45 million light years away at the time of emission.
  • Some participants introduce the idea that if the universe is infinite, the necessity for rapid early expansion may be questioned.
  • Others argue that rapid expansion is still required for light from a certain distance to reach us over the vast time frame discussed.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the cooling timeline of the CMB and the implications of the universe's expansion, indicating that the discussion remains unresolved.

Contextual Notes

There are limitations in the assumptions made about the universe's expansion and the definitions of distances involved, which remain unresolved in the discussion.

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

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