Far out cosmology: Big Bang neutrinos (Introduction)

by David Turell , Saturday, October 15, 2022, 17:53 (768 days ago) @ David Turell

Final proof of Big Bang theoryL

https://bigthink.com/starts-with-a-bang/big-bangs-final-prediction/?utm_source=mailchim...

"As time went forward, the Universe would cool, expand, and gravitate all together. First atomic nuclei would form from protons and neutrons, then neutral atoms would form, and then gravitation would lead to stars, galaxies, and the grand structures of the cosmic web. These leftover relics — the light elements formed in the Big Bang, the relic photons from the primordial plasma, and the large-scale structure of the Universe — would, along with the cosmic expansion of the Universe, form the four cornerstones of the Big Bang.

"But from an even earlier epoch, a fifth cornerstone should exist as well. There would be an early signal left over from when the Universe was just one second old: a bath of neutrinos and antineutrinos. Known as the cosmic neutrino background (CNB), it was theorized generations ago but was dismissed as undetectable. But no longer. Two very clever teams of scientists found a way to detect it. The data is in, and the results are incontrovertible: the cosmic neutrino background is real, and agrees with the Big Bang. Here’s how the Big Bang’s last great prediction was confirmed.

"Neutrinos are some of the most surprising and elusive particles in the Universe. They were predicted in 1930 to explain radioactive decays, as otherwise, energy and momentum would not be conserved.

***
"...neutrinos are real, and they’re fundamental particles, just like electrons or quarks are. They come in three generations: electron neutrino, muon neutrino, and tau neutrino, just like all of the other Standard Model fermions. They interact only through the weak and gravitational forces, so they neither absorb or emit light. But at high energies, like those achieved in the earliest stages of the hot Big Bang, the weak interactions were much stronger. Under those conditions, the early Universe spontaneously created enormous amounts of both neutrinos and their antimatter counterparts, antineutrinos.

***

"The neutrinos and antineutrinos, which stop interacting with the primordial plasma when the Universe is just one second old, should remain until the present day.

***

"This cosmic neutrino background (CNB) has been theorized to exist for practically as long as the Big Bang has been around, but has never been directly detected. Because neutrinos have such a tiny cross-section with other particles, we generally need them to be at very high energies in order to see them...No proposed experiments are theoretically capable of seeing them unless some novel, exotic physics is at play.

***

"If neutrinos weren’t present, the radiation content would be described by the photons alone; if neutrinos were present, however, the radiation content would need to be described by both photons and neutrinos combined. In other words, these neutrinos, if the cosmic neutrino background (CNB) is real, will create imprints in the CMB, and those imprints will persist all the way to the present day, where they should show up in the Universe’s large-scale structure as well.

"The effects on the CMB will be subtle, but measurable. The pattern of peaks and valleys will be stretched out and moved to larger scales — albeit extremely slightly — by the presence of neutrinos. In terms of what can be observed, the peaks and valleys will have their phases shifted by a measurable amount that depends on both the number of neutrinos that exist and the temperature (or energy) of those neutrinos at early times. This phase shift, if detectable, would provide not only strong evidence of the existence of the cosmic neutrino background, but would allow us to measure its temperature, putting the Big Bang to the test in a brand new way.

"Meanwhile, the downstream consequences of the cosmic neutrino background’s existence will show up by imprinting their effects on the present-day large-scale structure of the Universe. This imprint will also be subtle, but with enough precision in how we measure the various correlations between galaxies across cosmic distances, it should be theoretically measurable as well.

***

"The cosmic neutrino background may not be directly detectable today, but its indirect effects on two observables — the CMB and the large-scale structure of the Universe — should remain detectable, even 13.8 billion years after the hot Big Bang.

***

"In 2015, using the novel data from the ESA’s Planck satellite, a quartet of scientists published the first detection of the imprint of the cosmic neutrino background on the relic light from the Big Bang: the CMB. The data were consistent with there being three and only three species of light neutrino, consistent with the electron, muon, and tau species we’ve directly detected through particle physics experiments.

***

"Leveraging this large-scale structure data, we’ve now measured the phase shifts in the galaxy correlation data well enough to robustly announce that the presence of cosmic neutrinos have been detected."

***

Comment: the measurements fit exactly with the theory of the Standard Model. That makes the Big Bang really real.