How do we know if a distant galaxy is matter or anti-matter?

[fizzy]

There is a lot of discussion about the "imbalance" of matter/anitmatter in the universe.

It is said that the expectation would be symmetric creation of both which would eventually annihilate each other leaving just energy.

The remaining matter in the universe is thus anomalous.

Since anti-matter is an exact mirror of normal matter how can we determine whether a distance galaxy made of matter or anti-matter, or is this just assumed ?

TIA

 

[Ibix]

Empty space isn't so empty that you wouldn't be able to see radiation from annihilation reactions at the boundary between a region of matter and a region of antimatter, basically.

 

[phinds]

Well, it is pretty much assumed since a whole galaxy of antimatter would be ENORMOUSLY anomalous. A small amount of antimatter that had not yet interacted with anything would be possible but even that would be unlikely. The reason being that in the early stages the universe was unbelievably dense and antimatter particles had an almost statistical certainty of colliding with a matter particle and annihilating, so pretty much all antimatter was obliterated very early in the universe.

EDIT: also, what @Ibix just said

 

[Ibix]

Also, think about tossing a million coins. About half of them will be heads and half tails. What are the odds that the half that land on your left are all tails and the half that land on your right all heads?

Now remember that there are around a million million million million atoms in a few grams of a substance, about a thousand million million million million million grams in a star, and a few tens of billions of stars in a galaxy. You need about that many heads-or-tails to come up the same way in a region to make a galaxy, and we see it has happened billions of times.

The odds of this happening are so far beyond astronomical there are no words. If we were a lone star system in a universe otherwise empty except for the glow of matter/antimatter reactions we might buy that we were a lucky region. But we either see a universe full of impossibly lucky regions, or everything's matter due to a matter/antimatter imbalance we don't entirely understand.

 

[PeroK]

I wonder whether in some of the multiverse theories, some universes will be antimatter dominated?

It's hard to see how mixed regions could work in our universe, given the hot dense state at the beginning of the big bang theory.

That said, I wonder whether it would be possible to determine by observation alone whether a galaxy was composed of antimatter?

 

[Hornbein]

I wonder whether in some of the multiverse theories, some universes will be antimatter dominated?

It's hard to see how mixed regions could work in our universe, given the hot dense state at the beginning of the big bang theory.

That said, I wonder whether it would be possible to determine by observation alone whether a galaxy was composed of antimatter?

I don't know, but I been told....

https://www.iflscience.com/what-is-...cted-on-the-international-space-station-75428

 

[DaveC426913]

Also, think about tossing a million coins. About half of them will be heads and half tails. What are the odds that the half that land on your left are all tails and the half that land on your right all heads?

Are we certain that matter and anti-matter were produced homogenously (well-mixed) in the early universe?

 

[Hornbein]

Also, think about tossing a million coins. About half of them will be heads and half tails. What are the odds that the half that land on your left are all tails and the half that land on your right all heads?

Actually it's more like there is a one in a million(or so) bias toward heads on your right and a similar bias towards tails on your left. But I expect the odds against are still huge so it doesn't make any difference in the end..

 

[ohwilleke]

I wonder whether in some of the multiverse theories, some universes will be antimatter dominated?

One special class of such theories are mirror universe theories with an anti-universe that comes before the Big Bang where time flows in the opposite direction that is anti-matter dominated. In these theories the radiation spewing boundary IS the Big Bang.

See, for example:

If we look from a quantum perspective, the most natural way in which the universe can be created is in entangled pairs whose time flow is oppositely related. This suggests the idea of the creation of a universe-antiuniverse pair.

Assuming the validity of this hypothesis, in this paper, we show that the universe expands in an accelerated manner. The same reasoning holds for the anti-universe as well. This idea does not require any form of dark energy as used in the standard cosmological model ΛCDM or in modified theories of gravity.

Naman Kumar, "On the Accelerated Expansion of the Universe" 30 Gravitation and Cosmology 85-88 (April 4, 2024).

The author cites four papers by two other groups of scientists as a source for the anti-universe model:

* L. Boyle, K. Finn and, N. Turok, “CPT-symmetric universe,” 121 Phys. Rev. Lett. 251301 (2018).

* L. Boyle, K. Finn, and N. Turok, “The Big Bang, CPT, and neutrino dark matter,” 438 Annals of Phys. 168767 (2022).

* S. J. Robles-Pürez, “Time reversal symmetry in cosmology and the creation of a universe–antiuniverse pair,” 5 Universe 150 (2019).

* S. J. Robles-Pürez, “Quantum creation of a universe-antiuniverse pair,” arXiv: 2002.09863.

 

[Orodruin]

Going back to read a few semi-recent threads and noting that the premise

There is a lot of discussion about the "imbalance" of matter/anitmatter in the universe.

It is said that the expectation would be symmetric creation of both which would eventually annihilate each other leaving just energy.

The remaining matter in the universe is thus anomalous.

has not really been addressed.

This is not completely accurate. We know what is required from a universe in order to predictably result in an overdensity of matter over anti-matter. The required conditions, known as the Sakharov conditions were proposed by Sakharov in 1967:

1. Baryon number violation
2. C and CP violation
3. Out-of-equilibrium processes

In fact, all of these conditions are satisfied in the standard model, the only problem being that the resulting asymmetry predicted by the standard model becomes orders of magnitude too small. Most physicists would expect that some parts of the theory that is yet to be fully understood will provide further insight rather than that the asymmetry is random (for which the probability is extremely small).

 

[ohwilleke]

Going back to read a few semi-recent threads and noting that the premise



has not really been addressed.

This is not completely accurate. We know what is required from a universe in order to predictably result in an overdensity of matter over anti-matter. The required conditions, known as the Sakharov conditions were proposed by Sakharov in 1967:

1. Baryon number violation
2. C and CP violation
3. Out-of-equilibrium processes

In fact, all of these conditions are satisfied in the standard model, the only problem being that the resulting asymmetry predicted by the standard model becomes orders of magnitude too small. Most physicists would expect that some parts of the theory that is yet to be fully understood will provide further insight rather than that the asymmetry is random (for which the probability is extremely small).

We have also greatly narrowed the energy-scale and time frame in which this can happen, because, for example, the success of Big Bang Nucleosynthesis, implies that the new physics have to occur before that point, which is about 15 minutes after the Big Bang or earlier.

Likewise, the absence of new physics of the Large Hadron Collider implies that any new physics have to happen at a time when the prevailing temperature in the universe was higher than the highest temperature/energy scales of the LHC, which is an even shorter time after the Big Bang, on the order of a fraction of a second.

 

[Orodruin]

Considering some theories put the scale way higher (e.g., vanilla seesaw leptogenesis), I'm not sure I would say that the energy-scale has been greatly narrowed. It is still pretty unbound from above.