One of the most vexing is called the horizon problem.
It's one of those situations where at first, it seems like "where's the problem?" Then you look into it a little more, and kind of go, "... oh." The whole thing has to do with how fast a change can percolate through a system. Amongst the (many) outcomes of the General Theory of Relativity, we are reasonably certain that the upper bound at which disturbances of any kind can propagate is the speed of light.
So if a change of some sort happens in region A, but it is so far away from region B that there hasn't been enough time for light to travel between the two, it is fundamentally impossible for that change to have any effect at all in region B. Such regions are said to be causally disconnected.
So far, so good. The thing is, though, there are plenty of sets of causally disconnected regions in the universe. If at midnight in the middle of winter you were to aim a very powerful telescope straight up into the sky, the farthest objects you could see are on the order of ten billion light years away. Do the same six months later, in midsummer, and you'd be looking at objects ten billion light years away in the other direction. The distance between the two is therefore on the order of twenty billion light years (and this is ignoring the expansion of the universe, which makes the problem even worse). Since the universe is only something like 13.8 billion years old, there hasn't been enough time for light to travel between the objects you saw in winter and those you saw in summer.
Therefore, they can't affect each other in any way. Furthermore, they've always been causally disconnected, at least as far back as we have good information. By our current models, they were already too far apart to communicate three hundred thousand years after the Big Bang, the point at which decoupling occurred and the 2.7 K cosmic microwave background radiation formed.
Herein lies the problem. The cosmic microwave background (CMB for short) is very nearly isotropic -- it's the same no matter which direction you look. There are minor differences in the temperature, thought to be due to quantum fluctuations at the moment of decoupling, but those average out to something very close to uniformity. It seems like some process homogenized it, a bit like stirring the cream into a cup of coffee. But how could that happen, if opposite sides of the universe were already causally disconnected from each other at the point when it formed?
A map of the CMB from the Wilkinson Microwave Anisotrophy Probe [Image is in the Public Domain courtesy of NASA]
It's worse still, however, which I just found out about when I watched a video by the awesome physicist and science educator Sabine Hossenfelder a couple of days ago. Because a 2003 paper found that the CMB isn't isotropic after all.
I'm not talking about the CMB dipole anisotropy -- the fact that one region of the sky has CMB a little warmer than average, and the opposite side of the sky a little cooler than average. That much we understand pretty well. The Milky Way Galaxy is itself moving through space, and that creates a blue shift on one side of the sky and a red shift on the other, accounting for the measurably warmer and cooler regions, respectively.
What Hossenfelder tells us about is that there's an anisotropy in the sizes of the warm and cool patches. It's called the hemispherical power spectrum asymmetry, and simply put, if you sort out the sizes of the patches at different temperatures, you find that one side of the sky is "grainier" than the other. Like I said, we've known about this since 2003, but there was nothing in any of the models that could account for this difference, so cosmologists kind of ignored the issue in the hopes that better data would make the problem go away.
It didn't. A recent paper using newly-collected data from the Planck mission found that the hemispherical power spectrum asymmetry is real.
And we haven't the first idea what could have caused it.
In a way, of course, this is tremendously exciting. A great many scientific discoveries have started with someone looking at something, frowning, and saying, "Okay, hang on a moment." Here we have something we already didn't understand (CMB isotropy and the horizon problem) gaining an added layer of weirdness (it's not completely isotropic after all, but is anisotropic in a really strange way). What this shows us is that our current models of the origins of the universe are still incomplete.
Looks like it's a good time to go into cosmology. In what other field is there a universe-sized problem waiting to be solved?
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