If the CMB is in all of space, how does the Earth not get cooked in it?
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Because the CMB is very, very cold relative to Earth. Only a couple of Kelvin above absolute zero.
Cosmic microwave background is much less energetic than microwave you have at home. As the other commenter said, it is a few degrees higher than absolute zero.
Ill hijack this to ask a question of my own: if the cmb shows the universe as it was when it first existed and tiny then why cant we use it to figure out how large the universe is outside the observable part?
The CMB heats the entire universe to 2 kelvin. That is 2 degrees above the coldest it is physically possible to be. It is also heating the Earth, but not by any noticeable amount.
So a slightly different way to answer than most: Yes. The earth is being continuously heated by the CMB and given no other mechanisms, the earth would turn back into a molten ball. But there are other mechanisms, the most important being the earth’s own radiation. The Earth radiates away heat as infrared light far faster than the CMB is heating it up, and if that was the only things affecting the earth we’d stabilize at a few Kelvin.
Of course, the sun dumps far more heat into the earth than the CMB does so we stabilize just under 300K which is rather cozy all in all.
The sun hits us with an incredible 1700 Watts per square meter of power. The CMB hits us with a paltry 0.000002 W/m². ~~That’s probably not even measurable after the Earth’s magnetic field blocks it.~~ In fact, the reason that it’s everywhere is part of what makes us safe. It started a lot more powerful, but as the universe grew, the energy was spread thinner and thinner. Now there’s just not enough in one place to do anything significant. It took us until 1964 to even have equipment sensitive enough to pick it up.
Edit, I wasn’t even thinking about it, but they didn’t have Hubble on 1964, so it has to get through.
The important thing to understand here is that the Big Bang was not an explosion, at least in the sense we typically think of explosions. It was the very beginning of a long cooling-off process, which we are still undergoing today.bI bring this up because it has been a very long time since the CMB was hot enough to cook anything.
There was a period of about 7 million years, starting about 10 million years after the Big Bang, when the whole universe was between 373 and 273 Kelvin: the temperature range where liquid water can exist. But that was more than 13 trillion years ago, and there weren’t even any stars yet. By the time the first stars began to form, the background temperature of the universe was down to about 60 Kelvin -colder than liquid nitrogen- and that was still more than 13 trillion years ago. Nowadays it’s down to about 3 Kelvin: colder than liquid helium.
The CMB is extremely diffuse. Don’t let the “microwave” part of the cosmic microwave background confuse you; that’s just indicating the range of the wavelengths. The strength of the energy coming in as microwaves is super weak. Earth is far more likely to get cooked in the energy we receive from the sun due to greenhouse gases retaining that energy than we do from the cosmic microwave background radiation. The difference between the two is so overwhelmingly massive it’s probably misleading to even compare them.
The CMBR is extremely low magnitude noise in relation to all of the other cosmic energy sources, like the sun.
People have all had digital TV for like 20 years now, so using a TV on a channel you don’t get wouldn’t be a very good analogy, but it’s essentially like having a TV on white noise at your lowest volume setting. You can still have a conversation easily in the same room without even noticing it. While if you cranked the volume to max the noise would drown out anything that was said
If you still listen to FM radio in your car or wherever, then it’s like when you start to lose signal and start hearing that slight crackle as the station gets out of tune. It’s slight enough that you can still listen to the music without being annoyed for a good while until you get further out of range.
That’s because the magnitude of the noise is so slight.
Same deal with CMBR. It’s barely perceptible.
The actual answer is: earth is much hotter than the CMB. Thus earth radiates more heat radiation away than is incoming from the CMB.
Heat only goes from hot to cold.
Like imagine sitting in a room full of ice. Does the ice heat you up? There‘s thermal radiation coming from the ice that‘s muuuch hotter than the CMB.
And thermal radiation from your body that is very muuuch more than that coming from the ice.
Hence you heat up the ice walls, not the other way around.
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