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Ch. 6: Dark Energy

Posted: Thu Oct 12, 2017 2:37 pm
by Chris OConnor
Astrophysics for People in a Hurry
by Neil deGrasse Tyson

Ch. 6: Dark Energy
Please use this thread to discuss this chapter.

Re: Ch. 6: Dark Energy

Posted: Sat Dec 02, 2017 2:45 pm
by LanDroid
As if you didn’t have enough to worry about, the universe in recent decades was discovered to wield a mysterious pressure that issues forth from the vacuum of space and that acts opposite cosmic gravity. Not only that, this “negative gravity” will ultimately win the tug-of-war, as it forces the cosmic expansion to accelerate exponentially into the future.
In the previous chapter NDT discussed the opposing forces of coagulation and dilution. Up until very recently, everyone believed the universe is a steady-state, not expanding or contracting. In the 1920s and 30s Edwin Hubble observed the expansion of the universe. Next came a raging question of whether it expands forever or contracts periodically. Now we have that answer due to measurements of the effects of dark energy.
Since both mass and energy cause space-time to warp, or curve, omega tells us the shape of the cosmos. If omega is less than one, the actual mass-energy falls below the critical value, and the universe expands forever in every direction for all of time, taking on the shape of a saddle, in which initially parallel lines diverge. If omega equals one, the universe expands forever, but only barely so. In that case the shape is flat, preserving all the geometric rules we learned in high school about parallel lines. If omega exceeds one, parallel lines converge, and the universe curves back on itself, ultimately recollapsing into the fireball whence it came.
We know omega does equal one, but only after considering the huge amount of dark energy in the universe. This was understood only after very detailed measurements showed the expansion of the universe is accelerating, a paradox that lead towards dark energy.
In a trillion or so years, anyone alive in our own galaxy may know nothing of other galaxies. Our observable universe will merely comprise a system of nearby, long-lived stars within the Milky Way. And beyond this starry night will lie an endless void—darkness in the face of the deep.
A potentially depressing thought, but they won't know any better. :? 8)

Re: Ch. 6: Dark Energy

Posted: Sat Dec 09, 2017 1:12 pm
by Harry Marks
Hard to get excited about anything this new, this tenuously connected to observed mechanisms, and this unknown. "Dark energy" is likely to be in a category of outdated terms like "phlogiston" within 30 years, and could very well turn out to be a kludge made up to solve a number of problems which are actually not related as closely as the current thinking implies.

The idea that the universe somehow attains "flatness" (i.e. not negatively curved and not positively curved) by the same mechanism which explains the observed acceleration of the expansion of the universe seems like a cool coincidence. But it could be that Guth's "inflationary universe" is not as currently conceived, and so the flatness has some artificiality to it, or at least something that does not make it the most likely outcome of the "inflationary" mechanism (which has some observational confirmation in the evenness of the Microwave Background Radiation). It could also be that other factors are interacting of which we are not yet aware, leading our parsimonious approach to neglect some further factor. After all, 20 years ago we did not know the expansion was speeding up.