http://mp.weixin.qq.com/s/1LrQk4B9D8KoSwiG1zvnpg
【听力】What If Dark Matter Doesn’t Exist?
2018-03-13 Seeker 英文巴士
若暗物质不存在……
来自英文巴士
02:5602:56
NB: This may not be a word-for-word transcript.
What If Dark Matter Doesn’t Exist?
Dark matter has been the go-to explanation for many scientists as to why there seems to be more gravitational influence in our universe than the gravity measured by visible matter. But what if there’s no dark matter at all?
The idea of dark matter was first proposed by Henri Poincaré in 1906. Then in 1933, evidence of dark matter was found by Fritz Zwicky while studying galaxy clusters. And then more evidence of dark matter was found based on the calculations of rotational velocities in galaxies made by Vera Rubin in 1978.
Our universe is made up of less than 5% of visible matter – like stars, planets, galaxies, you, me, and everything we can see. Everything else is stuff we can’t see, which we call dark matter. And scientists think 95% of the matter in our universe is dark matter.
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Right now, dark matter is used to explain a few pesky things in astrophysics, like – Why hot galaxy clusters don’t blow themselves apart. Why stars don’t get flung out into space because of galaxy rotations. And also, the temperature fluctuations of the CMB – or cosmic microwave background radiation.
That said, there are theories that are trying to explain all of this without assuming it’s dark matter. These are: scale invariance and emergent gravity.
The empty space scale invariance theory says that properties of empty space should not change based on scale. An astrophysicist looked back at the early equations of Einstein, and Newtonian Dynamics, and modified an acceleration term in Newton’s Law. With this change, the law could explain a new force. And this force can explain things that were explained by dark matter, such as holding a galaxy together that’s spinning at high rotational velocities. The only problem is, on Earth, this force can be up to a billion times smaller than the force of gravity, making it pretty difficult to measure. It doesn’t explain gravitational lensing, which is the bending of light – or the temperature fluctuations of the CMB.
Now, the craziest, but to me, the most interesting, is the holographic principle, which says that our entire universe can be described on an imaginary sphere that surrounds ordinary matter. It led to the theory of emergent gravity. And that’s the idea that gravity arises (or emerges) through the presence of matter. It suggests that this happens the same way temperature arises from the movement of microscopic particles of matter.
So, if gravity is formed this way, then it can be used to explain gravitational phenomena again! Because something is keeping galaxies gravitationally bound.
It’s possible that the force created in the emergent gravity theory does this, but it still doesn’t explain the CMB temperatures.
Scientists are still developing theories like these, so they can explain things like the reason our universe hasn’t completely blown apart because of invisible matter. And they think it might be because of a gravitational field that developed through the movement of visible matter in our universe. It’s all very complicated, and yet, epic. For the moment, arguments for dark matter still stand tall above many of these other theories, but hey, maybe not forever.
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