Physicists Pin Down Nuclear Reaction from Moments After the Big Bang | Hacker News Reader

Physicists Pin Down Nuclear Reaction from Moments After the Big Bang

quantamagazine.org

71 pointsjorgesborges57 comments

How can one have a few grams of deuterium in their body from the time of Big Bang? And how does it make its way in one's body?

The protons that make up more than half of your mass are also mostly from the big bang.

Some of the hydrogen in the world around us is deuterium, again mostly from the big bang. When your body was assembled, and when you drink water/breathe, some of the generally-hydrogen sites in our bodies happen, instead, to be deuterated.

Aren't 100% of the protons that make up your body from the big bang?

They're from the big bang indirectly, but they haven't all been protons the whole time.

For example: https://en.wikipedia.org/wiki/Beta_decay

well neutrons decay in about 10min (avg) into fresh new protons (plus an electron) so perhaps not

Half-life ~10 minutes, which has a mean of ~15 minutes. Per https://en.wikipedia.org/wiki/Free_neutron_decay

*free neutrons

Is the free electron immediately captured by the H+ particle, or is the energy high enough that this can produce an acidic gas of H+?

The energy release in neutron beta-decay is ~780 keV. This should be compared with the hydrogen binding energy of ~13 eV.

Hence, there is far too much energy released for the electron to stay bound in essentially any case. If the neutrino carries away all the energy (with momentum conservation soaked up mostly by the recoiling proton), it could re-bind, but that process should be exceedingly rare. In that case, the result would see an (excited, probably) hydrogen atom recoiling away from the invisible neutrino.

Conservation of momentum dictates that proton and electron will move away from each other. The decay energy might be absorbed by a nearby nucleus in an medium to a large part, leaving only a small momentum for both particles so they can form an atom, but usually it won't. In the average low-density case, the proton and electron will find other partners only after quite some cooldown.

In very rare cases most of the energy might also be carried away by the neutrino that is produced as well, so decay to hydrogen is not impossible in a vacuum.

Wow, I didn't know that. Do you have a reference?

https://en.wikipedia.org/wiki/Free_neutron_decay

https://phys.org/news/2020-09-mystery-neutron-lifetime.html

TIL. Thanks so much for sharing, this is wild...Reminds me of pair-production. Nuclear physics is so fascinating.

Just being a bit pedantic ... more than half? Atoms typically have more neutrons than protons, and neutrons also weigh slightly more. Surely I'm >50% neutrons by mass?

Agreed -- I should have gone with "roughly half" or "almost half". As I wrote last night, I incorrectly recalled that n/p is often below 1 -- if I had thought for a moment about high-Z nuclei, the error would have been obvious.

A great place for a reader to begin to reach their own conclusions is here: https://en.wikipedia.org/wiki/Neutron%E2%80%93proton_ratio

Thank you for the correction.

Hydrogen mostly does not have neutrons. You have a lot of those.

It’s probably close to 50%. 80% of your mass is water and carbon. Hydrogen makes up 10% taken by itself but generally didn’t have a neutron which balances out the comparatives rare bigger elements with more neutrons.

> How can one have a few grams of deuterium in their body.

> ...

> And how does it make its way in one's body?

Just like any other element – it’s around, in parts-per-million, and it finds its way in to your cells as a result. It’s just a non-harmful form of hydrogen.

> from the time of Big Bang?

Because that’s where the majority of the deuterium in existence was made. Crazy, but true. Unlike most of the heavier elements, it isn’t a by-product of stellar evolution.

Extraordinary to think that the Universe is almost fourteen thousand million years old, and most of our deuterium was made in the first twenty seconds. After that, it cooled down and just wasn’t hot enough for fusion to do its work.

Physics is mental.

Reminds me of the idea that all the water you drink has already been drank and cycled through dinosaurs n times.

https://www.techtimes.com/articles/56727/20150530/you-are-dr...

This isn't really totally accurate. A lot of the water in the world is process water, it's created when things combust.

And much of the water consumed by plants is turned into carbohydrates which then gets processed back into water after combusting or being oxidized in a animal.

I actually highly doubt that any water remains that has never been broken back into its constituent atoms and then combined with other atoms.

I suspect the true answer is actually 0% of the water remaining today has ever been in a dinosaur. (I.e the atoms might have been, but not the molecule as a molecule.)

All elements come from somewhere. With the minor exception of heavy element decay, there is no nuclei synthesis on Earth. So elements planet and we consist of were made before, some of them in stars and then spread out though supernovae explosions, and some of them in the Big Bang.

Since we have so much heavy elements, our elements are not even from the first generation of stars. AFAIR it’s a third generation of supernovae explosions.

There‘s even a recent health fad that claims deuterium is bad for health, and those people also happen to sell deuterium-depleted water.

I suppose that’s one way to lose a little weight.

It does have health effects (in huge quantities).

https://www.thoughtco.com/can-you-drink-heavy-water-607731

Think about it, it’s a great business: they process water, sell deuterium, and then also sell by-products! :)

The mass of the observable universe is often quoted as 10^50 tonnes or 10^53 kg

So was all of this mass somehow condensed inside a space smaller than a bowling ball? Seems theoretically impossible?

Are there any examples of black holes/stars exploding after reaching some critical mass that might resemble the big bang?

Is the big bang effectively just a black hole that creates a massive nuclear bomb that starts a new universe?

The Big Bang was not an explosion in the conventional sense. When a bomb explodes, even a nuclear bomb, the energy created causes the atoms in the explosive to move violently in all directions. But in the Big Bang, instead suddenly all of those particles got farther apart without necessarily moving. The only real parallel in our current universe is the way the space between galaxies is expanding; nothing on our planet or even within our galaxy (afaik) behaves this way.

Ok thank you, so I guess 'big bang' is the wrong imagery. Really its someone stretching the taffy out that all mass sits on, a 'great expansion'.

However and wherever mass came from, the idea that mass exists at all is unfathomable. My only way of rationalizing it is that in some higher dimension things 'just exist' and its only because we perceive things in our three dimensions that we believe there must be a start and end to things.

There are a couple of details that make the early universe different then a black hole. The first is that, even though the matter/energy density was higher then the Chandrasekhar limit, space itself was expanding at faster than the speed of light so it could not collapse.

The second thing is that gravity has negative energy, and matter has positive energy, so the total energy of the universe was exactly zero. Zero total energy is the only amount compatible with our understanding of thermodynamics.

OK, but what about before that?

Space itself was expanding faster than the speed of light? There had to be a time when that was not true, because as you go back earlier in time, if that continued back far enough it would mean the diameter of the universe was negative. That's clearly absurd.

But that leaves you with all this mass, inside the Chandrasekhar limit, but the universe not expanding faster than the speed of light. And then it starts expanding.

All this presumes that time can be thought of as extending back before the big bang, which I don't know if that's actually a valid idea.

I have heard the claim that the laws of general relativity allow for a white hole as well - a singularity from which things go out, instead of things going in. That may be an accurate picture, or at least a decent model.

The big bang theory gets weirder the further back you go. I agree with you that at a certain point it starts to seem pretty hand-wavy. A popular explanation is that at very high energies, the laws of physics work differently, IE gravity, strong, weak and electromagnetic force were all combined into one and there was negative mass.

https://en.wikipedia.org/wiki/Chronology_of_the_universe#Pla...

>OK, but what about before that?

Well, the standard answer is that “time didn’t exist”. This is of course in unsatisfactory. The truth is that nobody knows, and so far it looks like we are unlikely to know.

If you think about it, it doesn’t make much sense at all: some huge amount of matter, came from somewhere, space expanded with it. Why? Where it came from? What is the reason? This is the area, where scientists that dwell on it too much, start to go crazy or become overly religious.

Since gravity is an illusion, did you mean to say that time dilation has negative energy? What does that mean?

If you have two apples floating in space, it takes positive energy to pull them apart against the force of gravity. Since energy must balance out, this means the gravitational field has negative energy.

https://www.livescience.com/33129-total-energy-universe-zero...

For energy balance it does not make a difference if you think of gravity as a real force or an illusion caused by curved space.

If you have a positive and a negative charge floating in space, it takes positive energy to pull them apart against the force of electromagnetism. And yet, the energy density of the electromagnetic field is strictly positive.

Thanks for the explanation.

There is energy in the curvature of spacetime, in the form of the gravitational stress-energy tensor.

> gravity has negative energy

Pretty sure that's wrong. Citation needed.

You can apply Noether's 2nd theorem to Friedmann cosmology with cosmological time as your generator. This yields the expected contributions to the energy density, with an additional negative term that is (if I remember correctly) proportional to H² and gets interpreted as energy of the gravitational field by some.

Where is all the lithium?

Big Bang fusion apparently produces mainly Li-7 which decays to He. There is still a perceived problem with the abundance of Li, Be, and B though when you go charting the observed abundance of all elements. But if the Lithium all decayed, why isn't there more Helium?

Helium in stars(current and past) doesn’t account for it?

That may be kind of hard to tell. Stars create helium, by fusing hydrogen. That makes it hard to tell how much helium the star started with. (Or so I suspect - I'm not an astronomer.)

It's kind of like, in a dynamite factory, trying to tell if there's any dynamite left over from when they blasted to make a hole for the foundations.

Usually, scientists are extremely careful with extrapolation, the big exception being the big bang, because it sells, I guess, and because it is hard, if not impossible, to verify.

Your comment reminds me of a study [1] published in 2019, demonstrating that people with the strongest views against genetically modified foods know the least about science but believe they know the most.

Implicating that the Big Bang Theory enjoys its popularity because it "sells" is, I think, showing complete arrogance to landmark discoveries made in the past century - such as the cosmic background radiation [2] (for which the Nobel Prize 1978 was awarded), supporting the thesis of the Big Bang.

[1] Extreme opponents of genetically modified foods know the least but think they know the most. https://www.nature.com/articles/s41562-018-0520-3 [2] https://en.wikipedia.org/wiki/Cosmic_microwave_background

What do you mean? There is an overwhelming amount of evidence for the Big Bang. Unless you mean something other than the scientific consensus by the expression "Big Bang". And what do you mean by sell? The Big Bang used to be a fringe interpretation of the equations until the evidence rolled, not because it sounded appealing. Einstein famously introduced the cosmological constant so the equations would allow another interpretation (a static universe), which he just as famously regretted as soon as the expansion was measured.

The term itself was coined in order to disparage the idea by Fred Hoyle, who was a proponent of the Steady State Theory. He never accepted that the Big Bang happened.

A prime example of the adage "Science progresses one funeral at a time"

But it is also true that Hoyle made many important contributions himself. For instance, he posited the idea of stellar nucleosynthesis.

We're all complex beings.

The square root of minus one obtained its name for a similar reason.

"Scientific consensus" does not equal "overwhelming evidence".

That is not at all what I was trying to say.

I was saying: Unless the OP meant something other than what every cosmologist means by the expression "Big Bang", there is overwhelming evidence for it.

I said this because contrary to what many people believe, the Big Bang Theory describes pretty much everything but the singularity at the beginning. The Big Bang Theory describes how the Universe evolved starting at some tiny fraction of a second after the "Big Bang". And this theory is confirmed without any reasonable doubt. You can literally see the expansion of the Universe right now, and you can see that remote regions of the Universe appear much younger than our region due to the finite speed of light.

What happened precisely in the very first tiny fraction of a second is still very much up to debate, mostly because we don't have a working theory of quantum gravity, because both gravity and quantum mechanics become important at the same time, similar to some aspects of black holes.

That's why I was phrasing it the way I did.

No, but overwhelming evidence, as we have for the Big Bang, generally does lead to scientific consensus. That’s how science works.

You definitely want people working against the scientific consensus.

They will mostly be wrong; but now and then that’s how we get a breakthrough.

Working against it, or holding its feet to the fire by testing its claims against new evidence, exactly as was done in this case. That’s the weird thing about science. If you’re trying your absolute level best to prove it wrong, you’re doing it right.

That’s an odd comment to make about a slow, painstaking, extremely careful long term experiment that’s all about verifying predictions about the Big Bang.

The entire concept of science is to extrapolate because things are impossible to verify. The Big Bang makes predictions that repeatedly are shown to be correct through experimentation.

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