This probably has the wrong tag on it, for which I apologize. If I’m not mistaken, this is cosmology not just physics.
Anyways, how exactly does the background radiation suggest a universe with a beginning? Couldn’t the same kind of radiation exist in a more static one?
In: Physics
No. The cosmic background radiation is basically saying: in every direction, *everything* was insanely boiling hot at some point in the past, and it was all basically the same temperature everywhere (with tiny fluctuations). Since we’ve established beyond reasonable doubt that the universe is expanding, the only way that makes any sense is if all the matter was closer together at some point in the past.
Put the two together, and you have a very simple conclusion: the universe used to be really hot, dense soup, and now it’s very not-dense, cold soup.
The cosmic microwave background has a *lot* to say about the past and current state of the universe. This is only the tip of the iceberg.
Imagine the universe was like a giant balloon, and a long time ago, it was really, really small and super hot. Then, it started to grow bigger, like when you blow up a balloon. As it grew, it cooled down.
Now, just like how when you bake cookies, the smell spreads out everywhere in the house, the heat from when the universe was tiny spread out everywhere. That heat is still around today, but it’s not hot anymore—it’s cooled down so much that we can’t feel it, but special tools can “see” it, like a kind of glowing light.
This glowing light is called cosmic background radiation, and it tells us the universe started from something small and hot, just like blowing up a balloon starts from something small. If the universe had always been the same size, we wouldn’t have this leftover heat spread everywhere. That’s why scientists think the universe started with a “Bang” and has been growing ever since!
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