Ask Ethan # 67: dark matter versus dark energy

The universe looks bizarre: an abundance of galaxies, a multitude of clusters, but there is nothing much larger in size. Why did she become just like that?

We incredibly casually treat the formation of our beliefs, but we find in ourselves an unjustified passion for them as soon as someone offers to deprive us of their company
- James Harvey Robinson

Of course, most of us have ideas about how our Universe has become what it is. But some details, no matter how scientifically grounded, are confusing. This week Tom Anderson asks me a question:

Dark matter attracts, dark energy repels. Dark energy continuously supports the expansion of the space between gravitationally bound galaxies / clusters, and the current scientific view is that it will expand forever, cool and everything will end in "big freezing." But gravitationally bound systems do not expand, and the joint force of dark and ordinary matter is equal to or greater than the force of repulsion of dark energy and ordinary energy. Why did the universe expand after the big bang? Why did dark matter not counteract dark energy at the beginning of the universe?

The question is saturated, so let's divide it into parts.



The principles of the Universe and the formation of structures, such as stars, galaxies and clusters, are outside our everyday experience. If it is extremely simplified, then the Universe consists of expanding space-time, and the expansion speed begins with some value determined by the physics of cosmic inflation and how the inflation period ends.



But the expansion rate does not remain constant at the end of inflation, since the Universe is filled with all sorts of other forms of energy: radiation, matter, antimatter, neutrino, dark matter, and a small amount of inherent energy of the space, known as dark energy. It is the combination of all this - changing with the expansion of the Universe - that determines the change in the rate of expansion in time.



On a global scale, if we take the entire Universe, it either recollapses completely, or expands indefinitely, or will be on the border of these two cases - depending on what exactly the ratios of all different forms of energy present in the Universe will turn out to be.

In the universe in which we live, everything looks like it will expand forever, and in the future dark energy will become the dominant force.



But this analysis does not work on all scales of the Universe: it only describes the scenario on a global scale. He says that in our case in the future all objects not connected with each other by gravity will fly apart from each other with acceleration.

But there are systems connected by gravity, existing on a small scale in huge quantities, on a medium scale - in moderate quantities, and on a large scale - rare, but occurring. And they are all part of the same cosmic story.



The universe was not born perfectly smooth, with absolutely identical amounts of matter, radiation, dark matter and dark energy in all places. If this were so, the Universe would be a terribly boring, perfectly homogeneous sea, where everything would be the same everywhere. There would be no stars, galaxies, planets, voids, people, animals, life, clusters, threads.

Instead, from the very beginning of time in the Universe there were areas with high and low density on all scales: small, medium and large.



Dark Matter helps high density areas grow, and they can do it quickly enough to collapse due to gravity in just tens of millions of years. Everything goes as if in the Universe there appeared small areas with such a large density that if the whole Universe were so dense, it would quickly recollapse!



Of course, in many other areas the density is below average, and they distribute their matter to high density areas. If the whole Universe were the same as these areas, we would have very, very few stars, galaxies and clusters.

But it is precisely this variety of initial conditions in the Universe that allowed us to obtain a variety of everything that we see now. In the cosmic battle between dark matter and dark energy, gravity with expansion, in the battle of cosmic "thrust", formative structure, and cosmic "repulsion" that impede this, there are winners and losers.



We notice winners more easily because they emit and absorb visible light and light from other parts of the electromagnetic spectrum, lead to gravitational lensing, and because the presence of something is easier to detect than the absence.



But there are also empty sections that also matter, and which are much more than “complete” ones! The total force of attraction of dark and ordinary matter can overcome both the initial expansion and the additional accelerating force of dark energy, but only on a small scale and in rather early times.

Increasing the scale, we see that the repulsion is becoming more and more victories, and on the largest scale, it always wins.



At first, the universe expanded due to the conditions set by inflation, and the possibility of recollapse - due to the gravitational pull of matter, dark matter, radiation and neutrinos - was enough for victories only in selected places. Attraction won not everywhere, but only in a minority of cases, and lost on average.

That is why, observing our Universe, we see a lot of galaxies scattered around it, many of which gathered in groups and large clusters, and on large scales they lined up along the lines. But these groups, made up of thousands of galaxies and stretching hundreds of millions of light years, are likely to be the largest structures associated with gravity. On a larger scale, all structures are temporary, and the presence of dark energy will eventually take them away.



If there were a bit more dark matter in the Universe - 1/10 ²⁴ more - it would recollapse billions of years ago. It was precisely balanced for a very long time - with gravity, somewhere winning locally, somewhere losing - but now, when dark energy began to prevail, we see that its effect will win. It will win at the end, on the largest scale, and for everything that was not connected by gravity in the first seven, or so, billions of years of life of the Universe.

Thank you for the wonderful question, and I hope that the explanation has been made clear for you and for the rest. Send me your questions and suggestions for the following articles.