Cosmologic philosophy: no big crunch (Introduction)

by David Turell @, Saturday, May 28, 2022, 16:29 (32 days ago) @ David Turell

Ethan Siegel says universe continuing expansion is the future:

"What we learn is that, despite speculative reports to the contrary, a “Big Crunch” simply doesn’t add up. There’s no evidence that our Universe will turn around and start contracting, but instead will expand forever, owing to dark energy. Here’s why.


"By plotting the curve of how the Universe has expanded as a function of time and comparing that with the different theoretical predictions for how a Universe with various amounts of various types of matter-and-energy evolves, one clear picture emerges as the front-runner.

"This straightforward method of measuring the Universe is remarkably precise, given just how many objects we’ve been able to accurately measure over the expanse of space accessible to our instruments. Because different forms of energy evolve at different rates, simply measuring the relationship between redshift, or how much the wavelength of the observed light must differ from the light as it was when it was emitted, and distance, or how far away the object in question is, allows us to determine what makes up the Universe.


"There’s a set of equations — the Friedmann equations — that relates what’s in the Universe to how the Universe expands throughout all of cosmic history. Given that we can measure the expansion rate, how the expansion rate has changed, and that we can determine what’s actually in the Universe, it’s simply a matter of using these equations to calculate how the Universe will continue to expand (or not) into the far future.

"What we find is the following:

"...the Universe will continue to expand, as it does, the energy densities of photons, neutrinos, normal matter, and dark matter will all drop, while the energy density of dark energy will remain constant, which means that the Universe’s expansion rate will continue to drop, but not to 0; instead, it will approach a finite, positive value that’s about 80% of its value today, and will continue to expand, at that rate, for all eternity, even as the matter and radiation densities asymptote to zero.

"In other words, the Universe will expand forever, will never see the expansion rate drop to zero, will never see the expansion reverse, and will never end in a Big Crunch.


"...we’ve assumed, based on what we’ve observed, that dark energy has the following properties:

" was irrelevant to the Universe’s expansion rate for the first ~6 billion years after the Big Bang, then, as matter sufficiently diluted, it became important, it came to dominate the expansion rate over the next few billion years, and right around the time that planet Earth was forming, it became the dominant form of energy in the Universe. Everything we observe is consistent with dark energy having a constant density, meaning that even as the Universe expands, the energy density neither increases nor dilutes. It truly appears to be consistent with a cosmological constant.


"However, measurements of the cosmic microwave background’s polarization from WMAP, improved measurements by Planck, and measuring how galaxies are correlated throughout space and time through surveys like the two-degree field, WiggleZ, and the Sloan Digital Sky Survey gradually reduced those errors. By the early 2010s, dark energy still looked like a cosmological constant, but the uncertainties were down to ±12%. By the late 2010s, they were down to ±8%. Today, they sit at around ±7%, with NASA’s upcoming Nancy Roman Telescope poised to reduce that uncertainty down to just ±1%.

"Both theoretically and observationally, we have every indication that dark energy is a cosmological constant. We know its pressure is equal to -1 multiplied by its energy density, and not -⅔ or -1⅓. In fact, the only wiggle-room we have is that there’s some tiny variation, across either space or time, that lies below the limits of what we’ve been able to detect. Both theoretically and observationally, there’s no reason to believe that such a variation exists.


"According to all the evidence, dark energy is here to stay, and a Big Crunch, while possible, just doesn’t describe the future fate of the Universe we happen to live in."

Comment: if the universe started in a Big Bang and ends up by simply stretching out until no energy exists, what does that mean? Can it be one and done? Or did a series of universes appear serially over eternity? We cannot know. However, just one and done does not seem reasonable. Anything that can happen, can repeat, is my view. Especially if a first cause designer, who must exist, source unknown remains active.

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