# Is the temperature of the universe rising?

I know the temperature of the universe is decreasing due to it's expansion after the big bang but after I came up with this article in AOP(please note I don't have the access of the journal,so I have just read the abstract) after reading this I am quite confused.

A news media states that:

The study by the Ohio State University Center for Cosmology and AstroParticle Physics shows that the "universe is getting hotter". This big revelation came amid the scientists' restless examinations on the thermal history of the universe over the last 10 billion years.

It has been also stated that:

The study also explained how, with the evolution of the universe, gravity pulls dark matter and gas in space together into galaxies and clusters of galaxies. The pull is so violent that more and more gas is shocked and heated up. Scientists used a new method to measure the temperature of gas farther away from Earth. The scientists, during the research, then compared those measurements to gases closer to Earth and near the present time. They said the "universe is getting hotter over time due to the gravitational collapse of cosmic structure, and the heating will likely continue". Data from the Planck and the Sloan Digital Sky Survey was used to observe how the universe's temperature has gone up.The universe is warming because of the natural process of galaxy and structure formation.

## So my question is:

•Is it indirectly violating the principle that the universe is cooling down due to expansion? Or is it just a additional factor in a small region of our space? Or is it happening as a regular phenomenon for a long time?

•Can someone explain to me the whole phenomenon with more science/ or any scientific explanation more than what I found?

•If the findings are true then what are the probable effects?

I am hoping to have clarity on the paper, maybe, being naive I haven't understood it completely, besides answer further suggestions are welcomed.

[Edit: check this]

• Let me introduce you to the arxiv site where research scientists submit their papers and very often if you search with the closed paper's title and the term "arxiv" you get the full pdf for free , as happened here arxiv.org/abs/2006.14650 Nov 14 '20 at 7:06

There is a difference between the "temperature of the universe" and the temperature of the cosmic microwave background radiation (CMBR).

The former can be changed by physical processes going on in the universe and for example, the conversion of gravitational potential energy, or the release of nuclear binding energy, into the thermal energy of particles. The CMBR temperature on the other hand is fixed when it is formed and modified just by the expansion history of the universe; it represents the temperature of a blackbody radiator with the same spectrum as the CMBR.

In the study you refer to, the "temperature of the universe" is the density-weighted mean electron temperature, and is of order $$10^6$$ K. These electrons have been heated via a variety of physical processes, ultimately linked to the formation of clusters of galaxies, galaxies and stars (for example supernovae, or collisionless shock heating in gravitationally accelerated flows - Kravtsov & Yepes 2000; Bykov et al. 2008), and have cooling times that are long compared with the age of the universe.

In contrast, the CMBR spectrum was formed about 400,000 years after the big bang, was essentially fixed at that point (at around 3000 K), and is only modified subsequently by the universe's expansion history, which stretches the wavelengths leading to a cooling temperature (currently 2.7 K).

The two temperatures would have been similar around the epoch when the CMBR was formed but have diverged since then because the matter became effectively transparent to the CMBR and decoupled. According to the paper that is referred to in the question, the density-weighted mean electron temperature has increased by about a factor of 3 between $$z=1$$ and the present day; from $$7\times 10^5$$ K to $$2\times 10^6$$ K. Over the same period, the CMBR would have cooled from 5.4 K to 2.7 K.

• So is the universe cooling or heating?
– Lost
Nov 14 '20 at 12:01
• @Lost according to the paper referred to in the question, the weighted mean electron temperature has increased between $z=1$ and the present day by about a factor of 3. Nov 14 '20 at 13:09
• @Lost the density-weighted mean electron temperature. i.e. The temperature you would assign to electrons based on their speed distribution. Nov 14 '20 at 14:07
• @nick012000 What do you mean by "the temperature of space as a whole"? Temperature is a property of matter. Most of the gas that exists in and around galaxies is at temperatures of $10^5$-$10^6$ K. Most matter in the universe is not inside stars. Nov 15 '20 at 12:02
• @nick012000 Vacuum doesn't have a (straightforward) temperature. Matter and (some) radiation can be assigned temperatures. The temperature of plasma in the interplanetary medium is of order $10^6$ K. The mass distribution in the solar system isn't representative of the universe. Nov 15 '20 at 13:34

A new study by an international team of researchers, including members of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), suggests that the mean temperature of gas in large structures of the Universe has increased about 3 times in the last 8 billion years, to reach about two million Kelvin today.

italics mine

....

The study determined that about eight billion years ago (at a redshift z=1), the mean electron temperature was some 700,000 Kelvin, rising to about two million Kelvin today. Furthermore, the scientists determined that its evolution is almost entirely driven by the growth of structures, as gas is shock heated in collapsing large-scale structures.

Since it is a fact that the cosmic microwave radiation is a few kelvins , it must be obvious that one is talking of the gas around large structures. The usual temperature taken in timelines of the Big bang is the cosmic microwave radiation temperature.

So it is two different "temperatures of the universe", as far as I can understand, the electron gas temperature not directly connected to the temperatures used for describing the Big Bang expansion , and the cooling of the cosmic microwave radiation with time.

I hope that an astrophysicist answers to make it clear.

• I read a while ago (and I now cannot remember the source) the total photon energy other than CMB has an energy about 10% of the CMB energy. I am guessing that the electron energy is about the same as the photon energy with which it is in equilibrium. Is this is correct, it should be possible to calculate a reasonably accurate mass-energy density of these electrons, and possibly also for the nuclei ions (mostly H and He) that have become ionized as a result.
– Buzz
Nov 14 '20 at 22:40
• @Buzz I do not think that " it is in equilibrium" is a correct hypothesis. As Rob says they were in equilibrium in the total universe back when the photons decoupled. Now the universe is separated in different regions , not in equilibrium with each other, as far as I understand. Nov 16 '20 at 5:29
• I apologize for being careless with my language. @anna v The electron energy (per electron) being discussed is much greater and more recent that the CMB energy (per photon) at decoupling. I am guessing that when a batch of new electron energy was created it interacted with photons, and for a while there was a local exchange of energy towards being locally in equilibrium. It this is correct, then the total new high energy photons, having approximately 10% of the total CMB energy, provides a basis for calculating the mass-energy of these electrons.
– Buzz
Nov 16 '20 at 16:06

You can define quite a few "universe temperatures":

1. Cosmic microwave background (2.7K as of now)
2. Cosmic neutrino background (theoretical, but lower than CMB, probably like 2K)
3. Cosmic gravitational waves background (theoretical, but more or less obvious, lower than the above)
4. Temperature of the baryon matter in some region of the universe (way higher, generally 10^2..10^7K)

All these things are in one way or another "decoupled" from each other - i.e. they cannot exchange significant amount of energy over observable timespan, so they retain their different temperatures. (2) and (4) can even host more than one particle populations, decoupled from each other and having different temperatures.

The article in question deals with the temperature of the electrons around the large cosmic structures.

Keep in mind that the temperature of the other more or less ordinary things in the same region may be quite different. If you have some kind of dust in the same place, it will be quite possibly near the CMB temperature. The gas is simply not dense enough to allow for an efficient heat exchange.

• "They could need hours to equalize." @fraxinus Can you make an estimate of how much time the collection of electrons and photons would have to exchange energy towards equilibrium?
– Buzz
Nov 16 '20 at 16:12
• OK, OK, the word is quite bad for the purpose intended. I'll delete the whole sentence until I find a beter formulation. Nov 16 '20 at 17:48