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More generally, there seems to be a substantial number of high school bullshit artists (HSBSA's) who thought they could get through with vague generalities and reuse of words from the prompt.

 

We are acquainted with the phenomenon.

 

Welcome, Mr/Ms HSBSA Bug, to the college science class windshield.

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36 minutes ago, Pariah said:

Cancer, you should Spoiler some of these so as not to cause inadvertent migraines in your readers. 

 

Just trying to make clear that teaching at a university is not all sweetness, light, and admiring the attractive young people.

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That physical conditions in parts of the Universe now so widely separated that they cannot ever have exchanged information ... are nevertheless very similar.  Without inflation, that requires insanely fine tuning and impossible similarity in the initial conditions in parts of the Universe that have no knowledge of each other.  That's the horizon problem.

 

The inflation hypothesis solves the problem by positing an era (from 10^-38 to 10^-33 seconds after the Big Bang) during which vast amounts of energy were injected into the early Universe, and space was created rapidly between every pair of initial points.  This stretched out to  absurd smoothness all the pre-inflation irregularities, and separated the initial points from each other so that post-inflation they were no longer in communication (they were outside each other's horizon, that is, the time needed for light to go from one to the other was rather larger than the age of the Universe).  Given the present age of the Universe (about 14 Gyr) all points we can see now (everything within about 14 Billion light-years) are from a tiny part of the pre-inflation fireball that got inflated to smoothness, so it all looks very similar now; and this will be true for an observer no matter that observer now is in the Universe.  Their 14billion year horizon will be different from ours, but inflation will have smoothed things out so that they will see remarkably uniform conditions.  (Widely separated observers might see different sets of uniform conditions, but there's no way for the different observers to compare notes since information travels no faster than light.)

 

The inflation hypothesis requires certain things to be true (that the Universe geometry is flat, so the Universe is at critical density) that seem to be true from our observations now.  There is a plausible source for the requisite energy that caused the inflation, but the details are deep in particle physics and not known.

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13 hours ago, Cancer said:

I should have recited the question about the "horizon problem" in cosmology that I was grading at the time.  Only a third of the 18 responses actually had some notion in the right direction.

 

Sorry, I initially read this as 'horizontal problem' and came to quite a different conclusion.

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I am not really in that racket, but my understanding is that in their deep interior, neutron stars don't have many neutrons.  It's more of a weird quark lattice where everything is shifting what's there continuously.  "Cold nuclear matter" is very peculiar stuff.

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