Cosmologic philosophy: is dark energy necessary (Introduction)

by David Turell , Friday, July 07, 2017, 15:24 (2696 days ago) @ David Turell

Dark energy is not proven, but is 'necessary' to fit the current interpretation of general relativity:

https://cosmosmagazine.com/physics/can-we-ditch-dark-energy-by-better-understanding-gen...

"Dark energy and dark matter are theoretical inventions that explain observations we cannot otherwise understand.

"On the scale of galaxies, gravity appears to be stronger than we can account for using only particles that are able to emit light. So we add dark matter particles as 25% of the mass-energy of the Universe. Such particles have never been directly detected.

"On the larger scales on which the Universe is expanding, gravity appears weaker than expected in a universe containing only particles – whether ordinary or dark matter. So we add “dark energy”: a weak anti-gravity force that acts independently of matter.

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"Since the late 1990s many independent observations have seemed to demand such accelerating expansion, in a Universe with 70% dark energy. But this conclusion is based on the old model of expansion that has not changed since the 1920s.

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"Unfortunately, Einstein left some basic questions unanswered. These include – on what scales does matter tell space how to curve? What is the largest object that moves as an individual particle in response? And what is the correct picture on other scales?

"These issues are conveniently avoided by the 100-year old approximation — introduced by Einstein and Friedmann — that, on average, the Universe expands uniformly. Just as if all cosmic structures could be put through a blender to make a featureless soup.

"This homogenising approximation was justified early in cosmic history. We know from the cosmic microwave background — the relic radiation of the Big Bang — that variations in matter density were tiny when the Universe was less than a million years old.

"But the universe is not homogeneous today. Gravitational instability led to the growth of stars, galaxies, clusters of galaxies, and eventually a vast “cosmic web”, dominated in volume by voids surrounded by sheets of galaxies and threaded by wispy filaments.

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"Further, standard cosmology also fixes the curvature of space to be uniform everywhere, and decoupled from matter. But that’s at odds with Einstein’s basic idea that matter tells space how to curve.

"We are not using all of general relativity! The standard model is better summarised as: Friedmann tells space how to curve, and Newton tells matter how to move.

Since the early 2000s, some cosmologists have been exploring the idea that while Einstein’s equations link matter and curvature on small scales, their large-scale average might give rise to backreaction – average expansion that’s not exactly homogeneous.

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"Intriguingly, the resulting expansion law fit to Planck satellite data tracks very close to that of a ten-year-old general relativity-based backreaction model, known as the timescape cosmology. It posits that we have to calibrate clocks and rulers differently when considering variations of curvature between galaxies and voids. For one thing, this means that the Universe no longer has a single age.

"In the next decade, experiments such as the Euclid satellite and the CODEX experiment, will have the power to test whether cosmic expansion follows the homogeneous law of Friedmann, or an alternative backreaction model.

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"Since Einstein’s equations can in principle make space expand in extremely complicated ways, some simplifying principle is required for their large-scale average. This is the approach of the timescape cosmology.

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"While successful in some aspects, many models of inflation are now ruled out by Planck satellite data. Those that survive give tantalising hints of deeper physical principles.

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"Whatever the final theory, it will likely embody the key innovation of general relativity, namely the dynamical coupling of matter and geometry, at the quantum level."

Comment: Is dark energy a fudge factor? Back to quantum gravity solutions.