Ask Ethan No. 10: Why the Universe is the Same Everywhere

Original author: Ethan Siegel
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One of the most tragic human traits is the tendency to save life. We all dream of a magical rose garden beyond the horizon, instead of enjoying the roses blooming outside our window.

Dale Carnegie.


The reader asks:
I did not really understand cosmic inflation and the horizon problem. I think you've already talked about this, but I would like to hear more details.

Let's get back to the beginning.

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This is your universe. It extends as far as the most powerful telescopes can peer in all directions. They see tens of thousands of galaxies scattered over distances of tens of billions of light-years in all directions, wherever we look. On a large scale, everything is about the same - densities, temperatures, types of stars and galaxies, metallicity of stars (the relative concentration of elements is heavier than helium), etc. All that we see is the further we look, the more young things we see, and the faster they move away from us.

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All this is connected with the expansion and evolution of the universe. Our three main proofs in the context of the General Theory of Relativity: the

Hubble expansion of the Universe - the redshift is related to the distance to the galaxy, the
existence and properties of cosmic microwave background radiation (KMPI) - an almost perfectly homogeneous sea of ​​radiation of a black body in all directions, with a temperature slightly above absolute zero
the abundance of light elements - hydrogen, deuterium, helium (helium 3 and 4) and lithium in the young universe, before the formation of stars,

tells us that the universe evolved and expanded from a hotter and denser state, and that it has been in its current state for about 13.8 billion years. Long, but not infinite. This paradigm is known as the Big Bang.

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But there is one problem. 13.8 billion years, the Universe expanded according to the laws of general relativity, i.e. the speed of expansion of space was determined by a set of initial conditions and the energy content of the universe (ordinary matter, dark matter, radiation, neutrinos, dark energy, spatial curvature, etc.). Everything is good with this - the problem is that the Universe is homogeneous and has approximately the same properties in all directions, wherever we look.

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The densities and properties of galaxy clusters on one side of the Universe are identical to those on the other - no matter how you divide the Universe into parts. That should be weird, isn't it?

Indeed, since the Big Bang, information could not be transmitted faster than the speed of light. We can see a 46 billion light-years-long piece of the Universe because light has traveled this distance in the expanding Universe over the past 13.8 billion years. Therefore, if we look in one direction at 23 billion light-years, and then in the other direction, we can expect that these two sections will not be connected with each other.

Still not clear? I will give an analogy.

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Imagine boiling water in a pan. You warm it from below, and water boils in all places at the same time. There is no temperature difference between the upper and lower parts of the water. Why is that?

You heat the pan from below, but the water heats up everywhere. This is because its molecules move, collide with each other and share their energy. Water from below exchanges energy and information with water from above, and the time required for this exchange is much less than the time required for boiling.

But it's not always the case.

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In this photo, lava flows from an active volcano into the ocean. When a lava heated above 1000 ° C comes into contact with water, it immediately boils. But the ocean is large, and the speed of heat propagation is finite. Not too far from this place, the water temperature will practically not change due to lava. From a practical point of view, these regions are divided with each other; they do not exchange information or properties.

It would be strange if regions separated from each other, which do not come into contact with each other and do not exchange information, have the same temperature. But in the Universe this is exactly what happens.

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Cosmic microwave background radiation (KMFI) was emitted when the Universe was only 380,000 years old, and at that time light could travel no more than a million light-years in any direction. If you filled the microwave sky with circles of a million light-years radius, you would need more than ten billion independent regions to fill everything that we see. Nevertheless, in these regions that are not related to each other, the temperature, spectrum and density are the same with an accuracy of 99.99%.

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This problem, when regions that are not in contact with each other, for some reason, have identical properties, is called the horizon problem.

It can be solved as follows: maybe these regions did not have time to exchange information with each other, but what if, after the Big Bang, they already had the same properties?

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In general, this is an extension: what happened before the Big Bang, which not only prepared it, but also prepared the initial conditions that our Universe has. They took a tiny region (possibly infinitely small), expanded it exponentially, and it stretched to sizes larger than the observed Universe, while setting the following rules:

any matter, particles, energy or topological defects that existed in this region before expansion, are so reduced in density that there will be no more than one such particle in the entire Universe (the problem of monopoles is solved)
no matter how curvature space has before expansion, its expansion is so it stretches so that it becomes indistinguishable from a flat space (the problem of a flat universe is solved),
whatever temperature and density variations in different regions of space exist before expansion, our observable Universe came from one small region. Therefore, the temperature and density of the Universe are the same everywhere (the problem of the horizon is solved)
the quantum fluctuations that occurred during the expansion led to a clear set of predictions of temperature and density fluctuations in the visible part of the Universe.

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If we discard the inflation theory, then we have to substitute the other three problems (monopoles, a flat universe and the horizon) and one prediction (about the spectrum of temperature fluctuations and density of the Universe) under the carpet, and say: “These are just the initial conditions of the Big Bang” to make your model work.

Or you can recognize the inflationary model - as a simple, elegant and immediate way to solve all these problems.

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That is why our Universe, as far as we know, is the same everywhere and in all directions.

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