What Happens To All the Universe's Hydrogen?

StartsWithABang (3485481) writes "Just a second after the Big Bang, the Universe was a hot bath of radiation, with a small fraction of protons and neutrons in about equal numbers left over. By time it was four minutes old, it was 92% hydrogen (by number of atoms) and 8% helium. Yet the Universe has aged nearly 14 billion years since then, and have formed many generations of stars, all of which burn hydrogen into heavier elements. So how much hydrogen is left, and how much will be left far into the future? A lot more than you might think."

Comments

And what about dark matter?

[1 a bee]

I'm a bit skeptical of such cosmological estimates. If there is more dark matter in the universe than ordinary matter (by a factor of 4:1 they say), wouldn't you expect it to somehow figure in the "calculations" going back to the big bang? I saw no mention of it in the article. In fact, come to think of it, you seldom hear much about that big elephant dark matter in the room in the first minutes after the bing bag.

Love reading about cosmology, but I think readers should be warned this is a very speculative field of study. Ideas and models in vogue today will likely not be in a few decades. I'm reminded of my physics professor of many years ago who claimed "Cosmology is as mature as botany was before Darwin."

Re:And what about dark matter?

[boristhespider]

Eh?

"If there is more dark matter in the universe than ordinary matter (by a factor of 4:1 they say), wouldn't you expect it to somehow figure in the "calculations" going back to the big bang?"

Yes. And yes, it does, it "figures" right from the start.

"I saw no mention of it in the article."

Who died and made this article God?

"In fact, come to think of it, you seldom hear much about that big elephant dark matter in the room in the first minutes after the bing bag."

That's chiefly becase in the first few minutes after the big bang the universe was radiation dominated, meaning that the density of photons (and neutrinos) was vastly greater than that of dark and normal matter. The transition between radiation and matter domination is governed by the density of dark matter just as much as baryons. Where on Earth are you getting this idea that dark matter is an "elephant in the room"? Here's an interesting fact for you - you know there are waves imprinted both on the CMB and on the large-scale structure of galaxies, right? If you "love reading about cosmology" you must, right? Those waves are the result of oscillations while the universe was radiation-dominated, caused by baryons tending to cluster together under gravity, and a restoring force introduced by radiation pressure, which set up ringing oscillations across the universe. Without dark matter to provide extra clustering under gravity those waves are at totally the wrong wavelength. From the CMB *alone* you can find how much dark matter there has to be relative to normal matter. How's that for an "elephant in the room"?

"I think readers should be warned this is a very speculative field of study."

As is all theory. However, I think readers should be warned that the fundamentals of cosmology are very far from speculative - even if the results might in principle be phenomenological, they will not change. Cosmology, particularly in the early universe but after the first microsecond, say, is based on well-understood science and is anything *but* speculative, and questions about whether dark matter or dark energy are physical quantities or are emergent in one way or another are not unique to cosmology but also arise on astrophysical scales. (And in some ways are irrelevant, since whatever dark matter and dark energy actually are, they have to work as cosmology describes them anyway. Small changes to the Lambda CDM model cause large disagreements with the data.)

"I'm reminded of my physics professor of many years ago who claimed "Cosmology is as mature as botany was before Darwin." "

Err, yeah. How many years ago? If he held the same opinion now I'd be surprised. If he held that opinion after the late 90s then he was ignorant of the field. That's OK, my Masters supervisor, in the early 2000s, is a brilliant physicist and held a similar opinion (although not stated quite so... badly, with a lousy analogy that could never work), and he was wrong too, increasingly so as the datasets grow ever huger and the tools with which to analyse them evermore sophisticated. This kind of view is untenable, and I say that as a man who has gone on record repeatedly with statements such as "cosmology is wrong. It is demonstrably wrong, it is wrong in its fundamentals and it is wrong in its principles" - because that's a statement I also surround with caveats. Cosmology is "wrong" in the same way that thermodynamics is "wrong", or that much of chemistry is "wrong", or much of biology is "wrong", in that it's at heart a descriptive, phenomenological theory. (Before chemists or biologists come to me to scream, those fields are typically phenomenological, although both contain subfields that are avowedly not so; but ultimately if you're not mapping up from the behaviour of the individual atoms you're dealing with phenomenology, and I'm well aware of how brutally difficult it is to do chemistry directly from the Schroedinger equation, which is what that implies. 'Phenomenology' is not a criticism, unless it's taken as so by people who mistakenly think they're dealing with the underlying science directly.)

Re:And what about dark matter?

[boristhespider]

Time, yes. Not sure what you're referring to with thermodynamics - it's just a statistical theory that emerges when you deal with vast numbers of particles. (And if I did want to treat thermodynamics as inviolate, which it basically is for large enough systems of particles, there is no issue with conservation of energy with the loss of energy due to cosmological expansion. I'm not totally sure why you'd think there is: energy conservation is inherent in the system. There's nothing controversial about the idea that if you work in an expanding spacetime then photons that are not being pumped by an external source of energy will be stretched. Similarly if you work in a collapsing spacetime then photons not being drained with be blue-shifted. The Friedmann equation can ultimately be interpreted as an energy conservation equation if one is so minded, it just comes from the Hamiltonian constraint.)

Re:Survivalist

[Thanshin]

Well yeah. You know, in case he needs water, RIGHT NOW.