New theories of gravity fail to handle gravitational waves’ impact on Milky Way.
Ars Technica said:Late in the 20th century, scientists discovered something amazing: gravity doesn't just suck, it also blows. This knowledge comes to us by looking at distant supernovae and determining how fast they are receding from us. It turns out that the rate at which objects are receding from us is accelerating. The Universe isn't just expanding; it is expanding faster each day.
General relativity can cope with that—sort of—by adding a cosmological constant. This constant turns up naturally from the math, but neither the math nor the physics tells us what its value should be. Explaining what this constant might mean physically also turns out to be a bit tricky. There are many models, but the big obstacle to most of them is that they don't just add a constant term to general relativity. Instead, most of them have additional physical consequences.
Now, many of these models that seemed to fit with all the data may not fit after all. That's because these models predict unreasonable gravitational wave distortions within galaxies.
The accelerating expansion of the Universe has consequences, which might be best highlighted by looking at the alternatives. If the rate of expansion of the Universe was slowing, that would mean gravity sucks, and there is enough mass in the Universe to draw it all back together (let us prepare for our fiery gravitational death). If the rate of the expansion of the Universe was constant, we could say: gravity sucks, but there simply isn't enough mass in the Universe to draw it all back together. Our destiny is boredom, and nothing special is going on.
To be accelerating, something has to be pushing. At some length scale, gravity has to push, not pull. The tricky part is that general relativity makes some pretty accurate predictions about the Universe at scales ranging from the orbit and wobble of Mercury right up to the formation of galaxies and galaxy clusters and beyond. Any new version of gravity that includes a push has to match all of that data.
This is best summed up by saying that reality has a nasty habit of slapping people who try to modify general relativity. After a couple of black eyes, even the most hard-headed scientist gets the message. Yet, a modification has to be made. So a popular approach is to build in a modification that pushes at very large scales and dies away at shorter scales through some kind of screening mechanism. This should preserve general relativity as we observe it and still give the Universe the acceleration it needs.
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