Far out cosmology: we see only 5% of our universe (Introduction)

by David Turell , Saturday, June 10, 2023, 19:09 (322 days ago) @ David Turell

Ethan Siegel discusses:

https://bigthink.com/starts-with-a-bang/heavy-hydrogen-dark-universe/?utm_campaign=swab...

"The 20th century brought with it the realization that the majority of what we know and experience in this physical Universe — atoms, light, and the known subatomic particles along with everything they combine to form — makes up only 5% of the total amount of stuff in the Universe. The remaining 95% is a mixture of things that are completely “dark” to us: dark matter, which gravitates, and dark energy, which drives the expanding Universe ever farther apart.

***

"They indicate, combined, that our Universe is only 5% normal matter, with 27% being dark matter and 68% being dark energy. While it’s easy to attempt to poke holes in this consensus cosmology picture, it’s been incredibly resilient.

"Remarkably, there’s an easy and straightforward way to show that only ~5% of the Universe, total, can be made of normal matter: by looking at hydrogen and its rare, heavy isotope, deuterium. Here’s how measuring this “heavy hydrogen” reveals the existence of the dark Universe.

***

" Given the Standard Model of particle physics, and how nuclear processes are known to work, there should be a particular ratio of the light elements that survive today that depends on one property of the Universe alone: the ratio of baryons (protons and neutrons combined) to photons.

"This is such a powerful technique because the number density of photons from the Big Bang is so precisely known: it’s been measured to be 411 photons per cubic centimeter at present, and so long as we understand how the Universe has expanded over its lifetime, we can infer just how many photons were present in a given volume of space at any point in the cosmic past.

***

"In a Universe without pristine stars, how could you possibly try and reconstruct how much deuterium was present immediately following the Big Bang?

"One method you might consider is to measure the ratios of elements in a variety of stellar populations. If you measure, say, the oxygen-to-hydrogen or iron-to-hydrogen ratios, and also measure the deuterium-to-hydrogen ratio, you could graph them together, and use that information to extrapolate backward: to a zero oxygen or iron abundance. This is a pretty solid method, and gives us an estimate for how much deuterium would be present at a time before heavy elements, like oxygen or iron, had formed.

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"By looking at the best quasar data we have in the Universe, and finding the closest-to-unpolluted molecular clouds that exist along their lines-of-sight, we can reconstruct the primordial deuterium abundance to extreme precision. The latest results tell us that the amount of deuterium in the Universe, by mass, was 0.00253% of the initial hydrogen abundance, with an uncertainty of only ±0.00004%.

"This corresponds to a Universe that’s made up of about 4.9% normal matter: consistent to within ~0.1% of the value inferred from the imperfections in the cosmic microwave background radiation, but in an entirely independent way.

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"In November of 2020, at an underground laboratory in Italy, a plasma physics experiment published a paper detailing what they saw when they went and recreated the high temperatures and densities that were present during the hot Big Bang. They observed the reactions between deuterium and protons directly in their experiment, which ran from 2017-2020. It took three years to measure enough different conditions to high-enough precisions to recreate the necessary temperature ranges, but when all was said and done, they had the best measurement of this particular reaction rate ever: with an uncertainty of just 1.6%.

"Most importantly, though, it confirmed our expectations. Although the uncertainties had been larger, previously, the central value didn’t shift by very much at all, meaning that our estimates for how the deuterium abundance corresponds to and translates into an overall matter density was actually extremely good. The Universe, as best as we can tell, really is made of about 5% normal matter, and can’t be more than that. Even if we don’t know what dark matter’s nature is, this experiment shows us that practically none of it is allowed to be “normal matter that, for some reason, is just dark.”

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"That’s why we demand multiple, independent lines of evidence for a conclusion before we confidently accept it. The science of Big Bang Nucleosynthesis is one of those incredibly important cross-checks. It’s an independent test not only of the Big Bang model of the early Universe, but of our concordance cosmological model. It tells us, all on its own, what the total amount of normal matter in the Universe is. Since the other lines of evidence, like colliding galaxy clusters or the large-scale structure of the Universe, require far more matter than the deuterium (or heavy hydrogen) abundance tells us can exist, we can be much more confident that dark matter is real. From the latest observations and experiments, only 4.7-5.0% of the entire energy budget of the Universe can be composed of normal matter. The remaining 95%, whatever it is, must truly be dark."

Comment: Siegel presents clear proof. Which tells me life can only appear in lighted areas. Why all the dark is needed is an unsolved mystery.