One of the most important achievements of the 20th century was the precise definition of how large, vast and massive our universe is. With roughly two trillion galaxies enclosed in a volume of 46 billion light-years in radius, our observable universe allows us to reconstruct the entire history of our cosmos, right up to the Big Bang and maybe even a little earlier. But what about the future? What will the universe be like? Will it?
Someone says that the expansion of the universe is slowing down. The Nobel Prize was awarded for the "discovery" that the expansion of the universe is increasing. But who is right? Could the universe collapse one day in the process of the so-called Big Compression (inverse to the Big Bang)?
Future behavior is best predicted based on past behavior. But just as humans can sometimes surprise us, the Universe can too.
The expansion rate of the Universe at a given moment depends only on two factors: the total energy density that exists in space-time and the amount of space curvature present. If we understand the laws of gravity and how different types of energy evolve over time, we can reconstruct everything that happened at a certain point in the past. We can also look at different distant objects at different distances and measure how light is stretched out due to the expansion of space. Every galaxy, supernova, molecular gas cloud, and the like - anything that absorbs or emits light - will tell the cosmic story of how the expansion of space stretched it out from the moment light was born to the moment we observed it.
From a variety of independent observations, we were able to conclude what the universe itself consists of. We made three large independent observation chains:
- In the cosmic microwave background, there are temperature fluctuations that encode information about the curvature of the universe, normal matter, dark matter, neutrinos and total density.
- Correlations between galaxies at the largest scales - known as baryonic acoustic vibrations - provide very rigorous measurements of the total density of matter, the ratio of normal matter to dark matter, and how the expansion rate has changed over time.
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“And the most distant, glowing standard candles in the Universe, type Ia supernovae, tell us about the expansion rate and dark energy, how they have changed over time.
These chains of evidence, taken together, paint us a coherent picture of the universe. They tell us what is in the modern Universe and give us a cosmology in which:
- 4.9% of the energy of the Universe is represented by normal matter (protons, neutrons and electrons);
- 0.1% of the energy of the Universe exists in the form of massive neutrinos (which act as matter in recent times and acted as radiation in early times);
- 0.01% of the energy of the Universe exists in the form of radiation (like photons);
- 27% of the energy of the Universe exists in the form of dark matter;
- 68% of the energy is inherent in space itself: dark energy.
All this gives us a flat Universe (with a curvature of 0%), a Universe without topological defects (magnetic monopoles, cosmic strings, domain walls or cosmic textures), a Universe with a known expansion history.
The equations of general relativity are very deterministic in this sense: if we know what the Universe is made of today, and the laws of gravity, we know exactly how important each component was at each individual interval in the past. In the beginning, radiation and neutrinos dominated. For billions of years, the most important components were dark matter and normal matter. Over the past several billion years - and this will get worse over time - dark energy has become the dominant factor in the expansion of the universe. This makes the universe accelerate, and from that moment on, many people cease to understand what is happening.
There are two things we can measure when it comes to the expansion of the universe: the rate of expansion and the rate at which individual galaxies, from our point of view, go into perspective. They are related, but they remain different. The expansion rate, on the one hand, speaks of how the fabric of space itself stretches over time. It is always defined as speed per unit distance, usually given in kilometers per second (speed) per megaparsec (distance), where a megaparsec is about 3.26 million light years.
If there were no dark energy, the expansion rate would fall with time, approaching zero, since the density of matter and radiation would fall to zero as the volume expanded. But with dark energy, this expansion rate remains dependent on the dark energy density. If dark energy, for example, were a cosmological constant, the expansion rate would flatten out to a constant value. But in this case, individual galaxies moving away from us would accelerate.
Imagine the expansion velocity of a certain magnitude: 50 km / s / Mpc. If the galaxy is at a distance of 20 Mpc from us, it appears to be receding from us at a speed of 1000 km / s. But give it time, and as the fabric of space expands, this galaxy will eventually be further from us. Over time, it will be twice as far: 40 Mpc, and the removal speed will be 2000 km / s. It will take more time, and it will be 10 times farther: 200 Mpc, and the removal speed of 10,000 km / s. Over time, it will move away at a distance of 6000 Mpc from us and will move away at a speed of 300,000 km / s, which is faster than the speed of light. The further time passes, the faster the galaxy will move away from us. That is why the Universe is "accelerating": the rate of expansion is decreasing, but the speed of separation of individual galaxies from us only grows.
All of this is consistent with our best measurements: dark energy is a constant energy density inherent in space itself. As space stretches, the density of dark energy remains constant, and the Universe will end in a "Great Freeze", when everything that is not tied together by gravity (like our local group, galaxy, solar system) will diverge and diverge. If dark energy is truly a cosmological constant, this expansion will continue indefinitely until the universe becomes cold and empty.
But if dark energy is dynamic - which is theoretically possible, but without observable evidence - it could end in a Big Squeeze or Big Rip. In the Big Compression, dark energy will weaken and gradually reverse the expansion of the universe so that it begins to contract. There may even be a cyclical universe, where "compression" gives rise to a new Big Bang. If dark energy grows stronger, a different fate awaits us, when the connected structures will be torn apart by the gradually increasing rate of expansion. However, today everything indicates that the Great Freeze awaits us, when the Universe will expand forever.
The main scientific goals for future observatories like ESA's Euclid or NASA's WFIRST include measuring whether dark energy is a cosmological constant. And while the leading theory speaks in favor of constant dark energy, it is important to understand that there may be possibilities not excluded by measurements and observations. Roughly speaking, the universe can still collapse, and this is possible. More data needed.
ILYA KHEL