Perhaps the largest discovery about the Universe we made at the end of the last century, when we discovered one of the strangest cosmic truths: distant galaxies not only fly away from us as time moves forward, but also fly away faster and faster. The discovery of the accelerating expansion of the Universe as part of the Supernova Cosmology Project with the help of the High-z Supernova Search Team earned scientists the Nobel Prize in physics. While this is one of the strangest and most unusual phenomena in the universe.
The fact is that the Universe did not always accelerate, flying away from us. For billions of years, the expansion has slowed down, and to someone living ten billion years ago, it might seem that it is contracting. What happened?
In the 1920s, four pieces of evidence were presented - three observables and one theoretical - that the universe was expanding. Here they are:
1. Discovered that the spiral nebulae in the night sky were real galaxies, or "island universes", containing billions of stars and located far beyond the Milky Way.
2. Measurement of reds and blues shifts of these galaxies by Vesto Slifer showed how quickly these galaxies either move away from us (redshift) or approach us (blueshift), and the vast majority followed the first scenario.
3. Distance measurements to each of these galaxies were carried out by Edwin Hubble and his assistant Milton Humason. Combined with Slipher's observations, they revealed a clear relationship: the further the galaxy was, the faster it seemed to move away from us.
4. Finally, a powerful theoretical leap made by Einstein's general theory of relativity: the realization that the Universe, which is filled with galaxies of roughly the same density in all directions, must be unstable unless it expands or contracts.
This led to a picture of the 1929 universe: it was hotter, denser, and expanded faster in the past, and then became colder, less dense, and expanded more slowly over time.
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This is quite logical from the point of view of the Big Bang. Imagine the Big Bang as the starting pistol of a great space race, a race between the initial expansion on one side, which was very fast at first, and gravity on the other side, which pulls everything together. It is easy to imagine three different options, each of which results in a different pace of the universe:
1. Large Compression. Perhaps the initial expansion rate was quite high, but the force of gravity was stronger. The expansion should slow down and stop. The universe must reach its maximum size and begin to shrink. And finally, it must collapse again, returning to the state prior to the Big Bang.
2. Great Freezing. This is the opposite scenario to the previous one: in which expansion starts quickly and gravity slows it down, but not enough. Expansion lasts forever, gravity slows it down all the time, but cannot stop it. This scenario is known as the Heat Death of the Universe: The Great Freeze.
3. Critical Universe. There is also the possibility that you will find yourself in the middle, when the expansion rate and gravity equalize each other, and the expansion rate will slow down over time. One particle less, one more particle in the Universe - and you get the first or second scenario. But this particle does not exist. The "critical universe" scenario would lead to the slowest possible heat death.
For billions of years, it seemed that the critical option would win. You see, when you live in the universe and look at different galaxies, you can not only measure the current rate of expansion, but by looking at the most distant galaxies, you can also measure the rate of expansion at the beginning of the history of the universe.
This image shows galaxies already unattainable for us.
For billions of years - about seven billion to be exact - it seemed like we were living in a critical universe. The expansion began in the era of radiation (photons and neutrinos), and then everything cooled down enough for the era of matter (both ordinary and dark) to begin. As the universe continued to expand, the density of matter fell and fell as the volume of matter increased and the mass remained the same.
But at some point, the density of matter dropped to such a low value that another, more subtle contributor to the energy density of the Universe emerged: dark energy. In about seven billion years, the magnitude of dark matter reached a few percent of the total energy density, and by the time the universe was 7.8 billion years old, the density of dark energy had reached an important value: 33% of the total energy density in the universe. This is important because that amount of dark energy is needed for the rate of expansion to begin to rise.
Since then, about 6 billion years ago, the density of matter began to decline, while dark energy remained constant. Currently, dark matter accounts for about 68% of the total energy of the Universe, and matter has dropped to 32% in total (27% dark matter and 5% ordinary matter). Over time, in the future, the density of matter will continue to fall, while the density of dark energy will remain constant, dark energy will be more and more prevalent.
Energy density in the Universe at different times in its past
For individual galaxies, this will mean that a galaxy that began to move away from us at the time of the Big Bang faster than others will demonstrate an obvious decline in velocity (from our point of view) in the first 7.8 billion years. Then the deceleration rate will stop falling and will remain unchanged for some time. Then it will begin to grow, and the galaxy will begin to move away from us even faster than before, since the space between us and distant galaxies is expanding at a tremendous speed. At some point - and this is scary because it applies to 97% of the galaxies in our visible universe - every galaxy outside our local group will move away at a speed faster than the speed of light, thus becoming out of our reach due to physical limitations.
Outlined in yellow is the current size of the visible universe: 46 billion light years; The size we can achieve is in pink: 14.5 billion light years
As far as we can tell, the universe has always had the amount of dark energy it now has inherent in the cosmos itself. But it took 7.8 billion years, or the entire history of the Universe one and a half billion years before our solar system formed, for the density of matter to drop to such a level that dark energy took over the expansion of the universe. Since then, all galaxies outside our local group have been receding from us and will continue to recede until the last one disappears. The universe has been expanding over the past six billion years, and if we had appeared earlier, we might not have gone beyond these three options offered by our intuition. At best, we could only guess what exactly the universe is. And that would be our biggest reward.
