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As the most mysterious celestial body in the universe, black holes are both familiar and strange to us. It is familiar because it has and has only three basic quantities (mass, angular momentum, electric charge), which can be said to be the simplest celestial body in the universe, while it is strange because all this is based on a theoretical model, and we always know nothing about what it really looks like, especially what it looks like inside.

As the event horizon, the surface of the black hole is like a boundary, which physically separates the inner and outer worlds of the black hole. On the outside, we can never imagine the world inside the black hole, and what happens outside the black hole can never be known to the inside world of the black hole, just as we can never know what is outside the universe. In this way, could our universe be such an isolated black hole, and are we in this black hole?

Last time, I talked about why the universe didn't directly collapse into a big black hole at the high density in the early days of the Big Bang. Although the universe has been expanding, many people have asked whether our universe itself is a big black hole.

Black hole cosmology is indeed a very imaginative idea. First of all, the black hole and the universe look very much alike, although so far we do not know the true shape of the universe, but the sphere is currently the most imaginable. And from an observational point of view, the observable universe is a spherical region centered around us and with a radius of 46.5 billion light-years. Of course, just looking like is not enough, the universe is much more round, "who is not a baby ah", the key is whether you have the ability to "boundary" on the surface of the ball.

We know that our universe is not eternal static, on the contrary, it is expanding all the time. Especially driven by dark energy, most galaxies in the universe are fleeing us quickly, and the farther away the celestial bodies are, the faster they escape. In this way, there will always be a place where the celestial bodies away from us will reach the speed of light, which means that the light they emit now will never reach us, and we will never be able to see what they and the celestial bodies behind them will look like in the future. This place is like that boundary, and things outside the boundary have nothing to do with us from now on. Astronomically, this boundary is called the "Hubble radius", which depends on the Hubble constant, which is currently more than 10 billion light-years.

From this point of view, the Hubble radius is somewhat similar to the Schwarzschild radius of the event horizon of the black hole. But it is not surprising that there is only a similar visual interface. If we simply calculate the mass of the observable universe into the Schwarzschild radius, we will find that if the universe is really a black hole, its radius is more than 10 billion light-years! On such a large scale, the orders of magnitude are all right. Is this some kind of coincidence, or is there another mystery inside?

If you think about it in this way, you can also find many similarities between the universe and black holes. For example, the size of a black hole basically depends on its mass, so the black hole itself gets bigger by gobbling up the matter around it. Is it possible that our universe has been expanding because it is frantically devouring "matter" outside the universe?

For example, as the black hole gets bigger, its surface area increases, but its temperature decreases. Yes, the larger the black hole, the cooler it is. The temperature of our universe seems to follow this law, and with the continuous expansion of space, the temperature of the cosmic microwave background radiation is decreasing. At present, it has been reduced from nearly 3000K in the decoupling period to less than 3K today.

Given the similarities, some people (including scientists) doubt whether there is a connection between the black hole and the universe. But more people still think it should be a coincidence.

For example, the Schwarzschild radius formula for calculating the size of static black holes cannot be used directly in this way. As we said last time, according to general relativity, a black hole is just a gravitational anomaly in space, and it needs a relatively flat space around it. The universe already contains all the space. If it is a black hole, what is around it?

In addition, the event horizon can not be used as the only criterion for judging black holes. Even for some theoretical black holes (such as an overspinning Kerr black hole or an over-charged Rmurn black hole), their event horizon may disappear in extreme cases, and then the internal singularity will be directly exposed as a naked singularity. So it doesn't matter whether there is a visual interface for a black hole, but whether there is a singularity or not.

If the universe is really a black hole, where is its singularity? After all, there is no so-called center in the universe. If there is a central singularity similar to a black hole in the universe, then all matter in the universe should not have any stable orbits, and they will have one and only one end-crashing into the singularity.

But then again, there could be a singularity in the universe, yes, the singularity of the Big Bang. It's just that the singularity is obviously different from the singularity of a black hole. Because the singularity of the black hole is in the black hole, but the singularity of the Big Bang is not in the universe, but it is the universe itself. In other words, our universe does not explode from the singularity, but expands from the singularity.

In addition, the singularity of a black hole is a future singularity, where time will end; but the singularity of the Big Bang is more like the singularity of a white hole, a singularity of the past, whether in time or space, where everything begins. Or how to say that the Big Bang is the only event closest to the white hole.

So the universe is definitely not a black hole, at least not the kind of black hole we think it is. Unless we expand the meaning of the word "black hole" so that it is no longer a black hole in the special sense of general relativity, but represents an "independent" region. For example, the region where inflation stops in the eternal inflation theory can be seen as a "black hole", a relatively "independent" universe.

Our universe is such an independent "black hole universe", its "outer" space is still in endless inflation. There is nothing we are familiar with, only an inflation field filled with primitive inflationary material. Occasionally there are similar areas where inflation stops, and they can be seen as parallel universes of our universe. Because these universes are separated by inflationary regions much faster than the speed of light, they are like black holes forever isolated from the outside world.

In addition to this black hole universe in eternal inflation, there is another theory about the universe in the scientific community that is more in line with the conjecture of the black hole universe, which is the holographic universe.

The concept of holographic universe originates from an important conjecture in theoretical physics-AdS / CFT duality. What this conjecture says is that, in some cases, a high-dimensional theory containing gravity is equivalent to a low-dimensional field theory without gravity. Take the black hole we are familiar with. In black hole thermodynamics, it is not the volume of the black hole, but the surface area that is proportional to the entropy of the black hole. In other words, all the information of a black hole may be recorded on its two-dimensional surface, and the three-dimensional space inside it is just a projection of the two-dimensional surface.

Extending to the entire universe, this view is tantamount to saying that the three-dimensional universe we live in may be just a projection of a two-dimensional information structure. The three-dimensional world we perceive with gravity may actually be just a hologram of the two-dimensional surface of the universe without gravity. This is the source of the term holographic universe.

The concept of the holographic universe is so amazing that nowadays there are various views on the holographic universe. For example, some people think that our universe may be the three-dimensional surface of a four-dimensional black hole. Beyond our universe is a higher-dimensional four-dimensional universe in which a four-dimensional star collapses to form a four-dimensional black hole, and the three-dimensional surface of this four-dimensional black hole is the three-dimensional universe in which we live.

Although the view of the holographic universe is too "unorthodox", it is so reasonable in mathematics. So now some theoretical physicists and astronomers are working together to find clues to the existence of the holographic universe.

references

[1] https://en.wikipedia.org/wiki/Observable_universe

[2] https://en.wikipedia.org/wiki/Black_hole_cosmology

[3] https://www.nature.com/articles/240298a0

[4] https://www.zhihu.com/question/23337053/answer/2385039554

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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