“Cosmic String” Gravitational Waves Could Solve Antimatter Mystery | Hacker News Reader

“Cosmic String” Gravitational Waves Could Solve Antimatter Mystery

scientificamerican.com

37 pointstazedsoul26 comments

It feels weird that there where nothing, then something exploded, and now I sit here on a rock, thinking about it.

I just read that in Kermit's voice. (Morgan Freeman was having a RDO.)

For those not on 9/80... RDO=Regular Day Off.

Anyone who says that there was nothing before is spouting nonsense. Any honest scientist will tell you that we don't know what happened at the Big Bag, much less before it. The math of the strong, weak, and electromagnetic forces workout to a unified path as you reach Big Bang energies, but except for a split second after the event. That's because gravity has to be accounted for and we have zero idea what gravity is. Worse, we have two theories: one that says that gravity is transmitted by a particle (that we can never hope to test ) and one that says that it bends spacetime (that has no mechanism in the Standard Model). Until this is resolved, you can't make any statement about at or before the Big Bang.

On a side note: quantum physics seems to suggest that the universe abhors the idea of nothingness. Emptiness has a vacuum energy that generates particles from "nothing." It seems happy to let "something" happen as long as it happens in a way conserving opposite effects (spin, charge, etc). One idea is that a mirror anti-matter Big Bang "exploded in the opposite direction," which is the balance to this one. But, it will all be speculation until we can explain the Big Bang's behavior, itself.

Can any physicist tell me if there is even necessarily a mystery involved? If matter and antimatter are made from the same building blocks, couldn't a prevalence of one over the other be down to chance?

Yeah, that's a valid idea; we considered that about 40 years ago and concluded it doesn't work. Yes, even if the amounts were perfectly equal, you would expect some regions with net matter or antimatter content. But when you run the numbers, you expect the sign to flip from place to place (which would produce matter-antimatter annihilations at the boundaries, which are not observed) and for the magnitude of the local imbalances to be far lower (by orders of magnitude of orders of magnitude) versus what is observed.

Weird question but when did antimatter start existing, ie when would the matter-antimatter annihilation happen? Couldn't it just be that it happened early enough that the universe was still densely packed enough for essentially a 1-1 annihilation to happen? Then any tiny imbalance towards either side would result in a universe entirely composed of one type of matter.

I'm sure someone's already thought of that and shot it full of holes though.

Weird side idea could this huge burst of energy that would be released from that be the Big Bang?

For that matter, could such an annihilation provide some of the energy of expansion?

(I'm asking totally in ignorance. This stuff fascinates me but I don't know nearly as much about it as I wish I did.)

> could such an annihilation provide some of the energy of expansion?

No. The universe was already rapidly expanding before the annihilation period. The expansion was slowing down all during that period.

Ah, got it. Thanks!

That actually IS the basic idea---there was almost a 1:1 annihilation. But it's not 1:1 exactly (look around you, matter everywhere). We quantify the baryon asymmetry as

η = (n_baryons - n_antibaryons) / n_photons

where the n_species are the number densities of those species. Since baryon number is additively conserved, the asymmetry now reflects the asymmetry in the past.

From WMAP we estimate η ~ (6±0.25)E-10. The question is: why is this SO damn small? There was alllllllmost a perfect cancellation. But not quite. What gives?

On the other hand, it's not QUITE as early as you hope---the anti/baryon annihilation is to light particles and radiation, not the negative pressure needed to cause inflationary Big Bang cosmology.

Doesn't an /almost/ perfect cancellation actually make the question "Why is it so close?" not "Why is there an asymmetry at all?". If I tossed a million coins and ended up 499,999 heads and 500,001 tails I wouldn't be surprised by the difference, but by it's small size.

Indeed, that's exactly how we used to talk about it: if you can just postulate some initial asymmetry, the question to answer is why it's small.

We don't think this way anymore because we're fairly sure inflation occurred, and its exponential expansion rapidly dilutes away any initial asymmetry. So we need a way to generate asymmetry after inflation ends.

> when did antimatter start existing

The same time matter started existing. According to our best current models, that was at the end of inflation, when the energy that had been stored in the inflaton field (the field that was driving inflation) was transferred to the Standard Model fields (leptons, quarks, and gauge bosons). This process is called "reheating" (somewhat of a misnomer since it was really the first "heating" of the SM fields).

The most natural expectation is that reheating would have populated matter and antimatter fields (i.e., quarks-antiquarks, leptons-antileptons) equally. Then, as the universe expanded and cooled, the matter and antimatter would gradually annihilate, producing radiation (mainly photons). If matter and antimatter fields had really been populated equally at reheating, this process would have left nothing but photons within a fairly short time (a fraction of a second) after the Big Bang. "Local imbalances" might have made that time a somewhat larger fraction of a second, but it is not at all plausible that they could have lasted for billions of years and allowed a region the size of our observable universe to have essentially no antimatter at all but billions of galaxies' worth of matter.

Since our observable universe does have essentially no antimatter and billions of galaxies' worth of matter, one of two things must have been the case: (1) reheating did not populate the matter and antimatter fields equally, or (2) there was some asymmetry in the interactions involved that created a slight excess of matter over antimatter. As I understand it, most physicists believe (2) to be the case, although it is not known exactly what the asymmetry was.

> could this huge burst of energy that would be released from that be the Big Bang?

No. The energy had to be put in to the matter and antimatter fields first. See above.

What interest me in this context is that when you say in fraction of a second then do you mean the second how we sense it or is this second somehow longer because of the huge gravity? Or was there gravity at all?

> do you mean the second how we sense it

I mean it in the sense of an SI second, which is defined in terms of a certain number of cycles of light of a certain frequency that comes from a certain energy level transition in cesium atoms.

> is this second somehow longer because of the huge gravity?

No. The concept of "gravitational time dilation" is not applicable in the circumstances we are talking about.

> was there gravity at all?

"Gravity" is an ambiguous term. On average, the universe is homogeneous and isotropic (and was even more so in the very early universe), which means that on average the matter in the universe exerts no net gravitational force on any particular piece of matter, such as us here on Earth or a tiny parcel of matter in the early universe. In that sense there is "no gravity" in a homogeneous, isotropic universe.

However, the presence of matter and energy certainly affects the spacetime geometry. In that sense, there is "gravity" present.

If they're from the same building blocks, chance puts it at 50-50. What we see is way skewed obviously since we exist and can observe galaxies and galaxies worth of matter.

While we can assume that our galaxy is made out of matter the same cannot be said about galaxies further away.

If matter and antimatter split into clusters it’s possible that some galactic clusters formed out of anti matter and some out of matter.

This for a time was the prevailing theory however for the most part we assume now that the universe is indeed made out of matter because we do not see annihilation events throughout the observable timeline of the observable universe.

Also if matter and antimatter are still at 1:1 ratios one would expect to detect annihilation events across the galactic void since space between galaxies isn’t completely empty and the current temperatures are too cold for annihilation to occur at any rate that would indicate that symmetry wasn’t broken during the early universe.

No, chance puts it at a /distribution centered/ on 50-50.

I've often wondered about this idea.

However, it seems that the most probable things come to pass in nature -- at least more frequently than less probable events. So, what makes matter more probable than antimatter (assuming this statement is even true)?

>But Dror and his team, through theoretical models and calculations, figured out a way we might be able to see this phase transition. They proposed that the change would have created extremely long and extremely thin threads of energy called “cosmic strings” that still pervade the universe.

Why does this remind me of

https://en.m.wikipedia.org/wiki/Aether_theories

And

https://en.m.wikipedia.org/wiki/Humorism

If there's one thing i've noticed about history and scientific discoveries, whenever we invent things that should exist to fill models we're usually not correct.

The obvious counter point would be the neutrino.

Good question. Why does it remind you of the aether and humors and not, say, the positron [1] and quarks? [2]

Sometimes experiment leads theory, but sometimes theory leads experiment. It's only when theory proceeds untethered to any possible experimental confirmation or rejection that it gets in trouble. In this case, they're making actual predictions that should be testable in the near future.

[1] https://en.wikipedia.org/wiki/Positron [2] https://en.wikipedia.org/wiki/Quark

Yeah, but isn't that just a necessary step in learning? It takes knowledge off-course for a while, but then who knows at the time what the right course will be?

Personally, I'm happy to see more theories and results around gravity and gravitational waves. When I read about physics in school and later, it was all about particles and interactions. They were talking about the theoretical Grand Unified Theory, but they only had 2 then 3 forces in the model. Gravity was always this big outlier, yet it makes the world go round (quite literally).

This article talks about the ability to detect gravity waves from neutron stars (LIGO, which is quite recent), and that made me think they are like spark gap transmitters: huge bursts of energy across the spectrum. Think of the history of radio: at first they used spark gap transmitters for wireless telegraph, but in time, they refined the equipment and now we have 5G. There was another recent article about detecting the fainter gravity waves caused by earthquakes. Maybe we'll get there in our lifetimes.

Almost every successful discovery ever was an example of us "inventing things that should exist". The Higgs boson, the W boson, the Z boson, the top quark, the bottom quark, the tau, each of the three neutrinos, the light quarks, the gluons...

You can never do anything without tentatively assuming something and computing the consequences. That opens the possibility that you may be wrong. That's not a condemnation of science, that's fundamental to how science works.

The neutrino itself was postulated in just this way, to balance mass-energy equations of beta decay. It looked like energy was being lost, but if some was being carried away by neutrinos, problem solved. The alternative was to reformulate the conservation laws.

Turns out it was neutrinos.

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