Far out cosmology: Big Bang theory survives (Introduction)

by David Turell , Friday, May 07, 2021, 15:48 (1082 days ago) @ David Turell

All others fall by the wayside:

https://www.forbes.com/sites/startswithabang/2021/05/06/why-isnt-anyone-seriously-chall...

"As recently as 20 years ago, the Big Bang was one of many ideas that scientists continued to entertain: quasi-steady state theory, plasma cosmology, and quantized redshifts remained mainstays in the scientific literature. But today, it’s largely crackpots and a few fringe contrarians who muster even the flimsiest of challenges to the consensus position: that the Universe began with a hot Big Bang. Is the field of cosmology succumbing to groupthink, as its detractors often claim, or is the lack of alternatives justified? Let’s dive in and find out.

***

"But in Einstein’s General Relativity, only a few spacetimes are exactly solvable, and they’re all relatively simple cases. For instance:

"We can solve an empty Universe: that’s Minkowski space.
We can solve for a Universe with one uncharged, non-rotating mass: the Schwarzschild solution.
We can write down the equations for a Universe containing one massive, rotating object: the Kerr solution.
And we can solve the equations governing spacetime for a Universe that’s uniformly filled with matter and radiation: we get the Friedmann equations.
This last option, as was recognized almost immediately, could represent our Universe. If our Universe is homogeneous (the same in all location) and isotropic (the same in all directions), even on average, even only on the largest of cosmic scales, the Friedmann equations will tell us how the Universe evolves over time.

***

"What the Big Bang hypothesized was that the volume which the objects within our Universe occupied increased over time, and hence the Universe got less dense as time went on, as well as cooler, as light within it became shifted to longer wavelengths and lower temperatures.

"But in addition to extrapolating forwards, we could extrapolate backwards in time as well: to a hotter, denser state. In fact, there was no limit to this, in principle. We could go back to arbitrarily high temperatures and arbitrarily large densities, and if the Big Bang were correct, the act of expanding and cooling during the evolution of the cosmos would lead to three major predictions, in addition to the expanding Universe.

***

"This leftover glow — originally called the primeval fireball and today known as the cosmic microwave background — was discovered in the mid-1960s, and has been verified to be blackbody in spectrum and to have imperfections in it at the 1-part-in-30,000 level.

***

"But because of how quickly the Universe expands and cools, these reactions can only take place briefly. After the dust settles, the Universe becomes about 75% hydrogen, 25% helium-4, 0.01% each helium-3 and deuterium, and about 0.0000001% lithium-7. The science of Big Bang Nucleosynthesis — the process by which these elements are formed — is now standard fare for graduate students, and has been observationally validated for galaxies, quasars, gas clouds, and from the cosmic microwave background as well.

"The overwhelming agreement between the Big Bang’s predictions and these observations — including in greater and greater detail — was what led to its widespread acceptance. Initial alternatives fell by the wayside as:

"non-relativistic ideas, like the Milne Universe, failed to account for the subsequently verified tests of General Relativity, like the Pound-Rebka experiments,
the idea of tired light cosmology, where redshift was due to light losing energy as it traveled through space, was discredited by the observed sharpness of distant galaxies,
and the idea of the early Steady State Theory, which predicted a low-energy, background glow of reflected starlight, failed to match the observed spectrum of the cosmic microwave background.

***

"Today, the only serious challenges to the standard Big Bang picture come in the form of add-ons: Universes where exotic forms of matter or energy (including dark matter and dark energy) are present, Universes that depart significantly (but within the observational limits) from isotropy or homogeneity, Universes with a different theory of gravity than General Relativity (but that don’t conflict with any of General Relativity’s already-observed successes). All of the modern alternatives still possess a hot, dense, uniform, and rapidly expanding early state, which expands, cools, and gravitates to form the Universe we see today.

***

"If any scientifically viable alternatives to the Big Bang ever arise, almost every modern cosmologist would thoroughly welcome it, and then immediately put it to the test. The problem is that every such alternative is already ruled out by the evidence in hand. Until an idea arises that meets those necessary criteria, the Big Bang will stand alone as the only idea compatible with the full suite of data we now possess."

Comment: The Big Bang still survives over all others