Ask Ethan # 64: What happens to matter as the universe expands?

With the expansion of the universe, the radiation is stretched, and the wavelengths increase - but what happens to matter?

Slow growing tree brings the best fruits
- Moliere

Today is the end of the week, and therefore, after reviewing your questions, we chose one that will receive an answer in our column. Andrei Novak asks:

The Big Bang Theory claims that as space-time expands, light shifts to the area of ​​increasing wavelengths. Does this expansion affect particles of matter? After all, their size is finite.
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Amazing question, if you think about it.



On the one hand, the amazing story that began in our Universe about 13.8 billion years ago continues to evolve. All matter in the Universe, in all its forms, was in a hot and dense state, and expanded. It did not expand as it does in fragments of an explosion, but as a dough suitable in the oven.

If you imagine all the particles of matter in the form of atoms of bread, you will begin to understand how the expansion of the Universe works.



From the point of view of each of the atoms, all the other atoms escape from it, and those that were farther away, run away faster than those that were closer. This is not due to the fact that atoms move, or distant atoms move faster than neighboring ones - but because the space itself expands, where they live.

And if space expands, the Universe does an amazing thing over everything that is in it.



It cools everything! It is easy to see why this works for radiation. He has a certain wavelength, and this length determines his energy.

What happens with increasing distances in the universe? Waves are stretching and energy is falling. This is what allowed the formation of atoms in the sea of ​​ionized plasma: spontaneously formed electrons and nuclei were broken by photons, but as the Universe cooled, the photon energy decreased, and it ceased to be enough.



As a result, we have neutral atoms, and after 10-100 million years after that they collapsed into stars and galaxies. As the universe expands, the radiation cools as the waves expand. We have already explained this effect in detail .

What about matter? After all, matter first moved very quickly, and something had to cool it too — or it could not collapse into stars and galaxies. Remember, for the molecular cloud to clump together and form stars, the gas must be cold. Otherwise, nothing happens.



And in general, in order for a galaxy to form, and matter remained gravitationally bound in the form of a spiral or elliptical structure, the velocity of the particles must be lower than the escape velocity for this galaxy. For most galaxies, this speed is only a few hundred kilometers per second. And although this is quite fast, remember that initially the majority of atoms moved at speeds of hundreds of thousands of kilometers per second!

And yet today in the universe enough stars and galaxies.



So what happened to matter? Think not only about how waves behave in an expanding universe, but also what it means for particles moving at certain speeds. Speed ​​is the distance a particle travels in a given time, just like a wavelength is the distance between two adjacent wave crests. The velocity of the particle plays the same role as the wavelength of the radiation: it is a measure of the kinetic energy inherent in the system.

Radiation of higher energies (with shorter wavelengths) behaves more like gamma rays, and less like radio waves, while particles with higher velocities also have more energy. That is why the hotter particles — with a higher temperature — and the speeds are higher, so they can do more physical work under the right conditions.



With the expansion of the Universe and increasing distances between objects, not only the wavelengths increase, and not only the radiation energy decreases. The speeds also fall - and this means that the energy of the particles also falls! Think for yourself: let's say you are moving at a speed of 100 km / s relative to a certain point, and the Universe is expanding at a speed of 10 km / s per kiloparsec (the speed of expansion is calculated in speed per unit distance). This is 1000 times faster than today's expansion, but it is a good example of past speeds. A kiloparsec is a distance of just over 3,000 light years.

What happens if you travel ten million years - the time required for a body moving at a speed of 100 km / s to overcome one kiloparseek?



You are still moving at a speed of 100 km / s relative to your original position, but it is already in kiloparseque from you. It moves away from you at a speed of 100 km / s, but part of this speed - 10 km / s - relates to the expansion of the Universe. Therefore, your speed with respect to the expanding universe has slowed, and now you are only moving at a speed of 90 km / s. And as your universe expands, your speed continues to fall.

Therefore, in an expanding universe, radiation loses energy due to redshift, but matter also loses kinetic energy due to the expansion of the universe!



What is even more interesting is the ability to interpret everything that moves at near-light speed, like radiation. And everything that moves much slower - like matter. Therefore, initially even particles and protons behaved like radiation, and later (now, for example) even neutrinos begin to behave like matter. There are models that assign small, but nonzero, rest masses to particles like photons and gravitons. If the Universe continues to expand and cool further, and these particles really find their mass, they will begin to behave like matter and cool, and - if dark energy does not spread all matter away from each other - they will also begin to pile up!



Therefore, Andrei, the particles of matter are also affected by the expansion of the Universe: they cool and lose energy. For particles that do not move with relativistic velocities, the energy is proportional to the square of the velocity, therefore, when due to the expansion of the Universe the particle energy decreases twice, its velocity decreases by 29% (in 1 / √2). Protons and neutrons cease to be relativistic, and begin to behave like matter when the Universe is executed a microsecond; electrons - at the age of a second; neutrinos - tens of thousands of years; photons and gravitons, if they really have a mass, will behave no earlier than the universe will be a quintillion years!



For the formation of the molecules we observe today, stars, galaxies and planets, it took not only the radiation to lose energy, but also the kinetic energy of individual particles to fall. We are very fortunate that the expansion of the Universe works exactly this way - because of it, that Universe has turned out that we are now observing!

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.