We know it is there but we don't know what it is: the fate of the Universe is in the hands of this energy

milky way

Approximately 25 years ago there was a revolution in our knowledge of the universe. Through detailed analysis of certain types of supernovas, gigantic explosions of stars that reach the end of their life cycle, two scientific teams, (who won the Nobel Prize in Physics in 2011), determined that space expands rapidly.

It was one of the most surprising discoveries in the history of science. What was expected was that the expansion of the universe would slow down as a consequence of the attraction between galaxies, due to the force of gravity. The accelerated expansion revealed that something else was happening.

There is an exotic entity, unnoticed until then, that produces repulsive gravity and pushes galaxies, causing them to move away faster and faster. It is dark energy.

LCDM vs SCDM Virgo simulation
 
Distribution of matter in a universe with dark energy and dark matter (left) and in a universe without dark energy (right). The simulations shown were carried out by the Virgo Supercomputing Consortium, using computers at the Max Planck Society Computing Center in Graching, Germany, and the Parallel Computing Center in Edinburgh, United Kingdom. The data is publicly available at the link: https://www.mpa-garching.mpg.de/galform/virgo/int_sims. Image credit: Virgo Supercomputing Consortium

The discovery of dark energy launched a powerful international scientific research program, which is still active, whose objective is to determine the composition of the universe. The results it has produced are astonishing.

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An image of the Dark Energy Survey (DES) deep field. Almost all of the objects seen in the image are distant galaxies. Image credit: DES Collaboration/NOIRLab/NSF/AURA/M. Zamani, CC BY-NC

We know it is there, but we don't know what it is.

The current successful theories of matter, formulated in the well-known standard model of particle physics, they only explain 5 % of the content of the universe. The remaining 95 % is made up of two exotic entities whose physical nature remains mysterious and which we have not been able to produce in the laboratory. These are dark matter (25 % of the content of the universe) and dark energy (70 %).

In the last quarter of a century, the standard model of cosmology, the “modern version” of big Bang Theory, known as ΛCDM. This theory explains all the observations that have been made and considers that dark energy is the cosmological constant (called Λ by physicists).

The name of the theory refers to those two dominant components of the content of the universe: dark matter (CDM, for example). Cold Dark Matter) and dark energy in the form of the cosmological constant (Λ).

A tiny amount and one of the biggest mysteries

The cosmological constant is the intrinsic energy of empty space. The current results indicate that this energy it is very small, a quantity that is equivalent to the mass of 3 protons for each cubic meter of space. That's why it couldn't be detected until 1998. Previous instruments were simply not sensitive to such a small value. Despite being tiny, it is one of the great mysteries of science.

The standard model of particle physics cannot explain its magnitude, and no one knows the deep reason for this discrepancy. It is known as cosmological constant problem. To solve it it will be necessary to develop new physical theories. It is even possible that dark energy is something even more exotic.

To clarify this, enormous cosmological observation projects are underway, such as DESI, LSST either Euclid, which will provide new and better data in the coming years.

The destiny of the universe

Whatever the physical nature of dark energy, it dominates the fate of the universe. The reason is its exotic behavior. All other components They decrease in density as the universe expands. This is expected, since there is the same amount of matter in a larger volume.

However, dark energy is such a strange entity that its density remains constant (if it is the cosmological constant) or almost constant (if it were something else) with expansion. So dark energy does not decrease (or decreases very little) with time. Therefore, it ends up dominating the density of the universe, and will increasingly dominate it in the future.

thermal death

At this point, it is important to note that predictions for the long-term future of the universe must be taken with extreme caution. It is very possible that new discoveries will be made that will alter them in an important way. But with current data, which is compatible with dark energy being the cosmological constant, the future that awaits the universe is what is known as thermal death.

It will expand forever, ever faster, making it emptier and emptier. The accelerated expansion will cause a flight of galaxies beyond the cosmological horizon, until we can only observe our cosmic neighborhood and nothing else.

The step by step of the Apocalypse

In about 5 billion years the Sun will have consumed all the fuel in its core and will become a red giant star. A few hundred million years later, it will collapse into a white dwarf, the remnant of a medium star like the Sun. In this process, it will evaporate the inner planets of the Solar System, including Earth.

At approximately the same time that the Sun becomes a red giant, our galaxy will collide with Andromeda, forming a single giant elliptical galaxy. No other galaxies will be visible in the sky. As a consequence of accelerated expansion they will have disappeared behind the cosmological horizon.

The giant galaxy formed after the merger of the Milky Way and Andromeda will also disappear little by little, as its stars die.

After an inconceivably long time, all objects will disappear, leaving only a dilute gas of photons with very small energy. It is the end, the thermal death we were talking about.

Both dark matter and dark energy are entities that we do not know well yet. We have a possible explanation for dark energy as the cosmological constant (although it is still possible that it is something more exotic), but the nature of dark matter is completely unknown. Therefore, this description of the future of the universe may undergo substantial changes as we elucidate its physical properties. It is a fascinating path, which promises revolutionary discoveries. How can we not be attentive!The Conversation

Eusebio Sánchez Álvaro, cosmology and particle physics, Center for Energy, Environmental and Technological Research (CIEMAT)

This article was originally published in The Conversation. read the original.