One of the most important achievements of the 20th century wasan accurate definition of how large, vast, and massive our universe is. With approximately two trillion galaxies contained in a radius of 46 billion light-years, our observable Universe allows us to reconstruct the full history of our cosmos, right up to the Big Bang, and maybe even a little earlier. But what about the future? What will be the universe? Will it?
Expansion rate of the universe at a certainthe moment depends on only two factors: the total energy density existing in space-time, and the amount of space curvature present. If we understand the laws of gravity and how various types of energy evolve over time, we can restore everything that happened at a certain point in the past. We can also look at various distant objects at different distances and measure how the light is stretched due to the expansion of space. Each galaxy, supernova, molecular gas cloud, etc. - everything that absorbs or emits light - will tell the cosmic history of how the expansion of space stretched it from the moment of birth of light until the moment we observed it.
From many independent observations, we were able to conclude what the Universe directly consists of. We made three large independent chains of observations:
- In the cosmic microwave background there are temperature fluctuations that encode information about the curvature of the Universe, normal matter, dark matter, neutrinos and the total density.
- Correlations between galaxies on the largestscales - known as baryonic acoustic vibrations - provide very stringent measurements of the total density of matter, the ratio of normal matter and dark matter, and how the rate of expansion has changed over time.
- And the most distant, luminous standard candles in the Universe, type Ia supernovae, tell us about the rate of expansion and dark energy, how they changed over time.
These chains of evidence, all together, give us a consistent 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 recently 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 the space itself: dark energy.
All this gives us a flat universe (with curvature0%), a universe without topological defects (magnetic monopoles, cosmic strings, domain walls or space textures), a universe with a known history of expansion.
The equations of general relativity are verydeterministic in this sense: if we know what the Universe consists of today and the laws of gravity, we know for sure how important each component was in each individual period of the past. At first, radiation and neutrinos dominated. Billions of years, the most important components were dark matter and normal matter. Over the past few billion years - and this will worsen over time - dark energy has become the dominant factor in the expansion of the universe. This makes the universe accelerate, and from that moment, many people cease to understand what is happening.
There are two things we can measure whenwe are talking about the expansion of the Universe: the speed of expansion and the speed with which individual galaxies, from our point of view, go into perspective. They are connected, but remain different. The rate of expansion, on the one hand, indicates how the fabric of space itself stretches over time. It is always defined as speed per unit distance, usually set in kilometers per second (speed) per megaparsec (distance), where a megaparsec is about 3.26 million light-years.
If there wasn’t dark energy, speedexpansion would decrease 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 density of dark energy. If dark energy, for example, were a cosmological constant, the expansion rate would equalize to a constant value. But at the same time, individual galaxies moving away from us would be accelerated.
Imagine the expansion rate of a certainvalues: 50 km / s / Mpc. If the galaxy is located at a distance of 20 Mpc from us, it apparently recedes from us at a speed of 1000 km / s. But give her time, and as the fabric of space expands, this galaxy will ultimately be further from us. Over time, it will be twice as far: 40 Mpc, and the removal speed will be 2000 km / s. Another time will pass, and it will be 10 times further: 200 Mpc, and a 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 goes, the faster the galaxy will leave us. This is why the Universe is “accelerating”: the rate of expansion is falling, but the speed of recession of individual galaxies is only growing from us.
All 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 with “Great Freezing”, when everything that is not connected together by gravity (like our local group, galaxy, Solar system) will diverge and diverge. If the dark energy is indeed a cosmological constant, this expansion will continue indefinitely until the Universe becomes cold and empty.
But if dark energy is dynamic - what is possibletheoretically, but remains without observational evidence - everything can end with a Big Compression or a Big Break. In Big Compression, dark energy will weaken and will gradually reverse the expansion process of the Universe so that it begins to contract. A cyclic Universe may even arise, where “contraction” gives rise to a new Big Bang. If dark energy will be strengthened, another 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 Freezing awaits us, when the Universe will expand forever.
The main scientific goals of future observatories likeEuclid ESA or NASA's WFIRST include measuring whether dark energy is a cosmological constant. Although the leading theory speaks in favor of constant dark energy, it is important to understand that there may be possibilities not ruled out by measurements and observations. Roughly speaking, the Universe can still collapse, and this is not excluded. Need more data.