Cosmology: Inflation theory under attack part 3 (Introduction)

by David Turell , Friday, October 27, 2017, 18:13 (2583 days ago) @ David Turell

The universe is expanding and the Hubble constant is narrowed to a small range now measured by three different methods, each of which has to make some estimates:

https://www.quantamagazine.org/colliding-neutron-stars-could-settle-cosmologys-biggest-...

"Scientists have hotly debated the cosmic expansion rate ever since 1929, when the American astronomer Edwin Hubble first established that the universe is expanding — and that it therefore had a beginning. How fast it expands reflects what’s in it (since matter, dark energy and radiation push and pull in different ways) and how old it is, making the value of the Hubble constant crucial for understanding the rest of cosmology.

"And yet the two most precise ways of measuring it result in different answers, with a curious 8 percent discrepancy that “is currently the biggest tension in cosmology,” said Dan Scolnic of the University of Chicago’s Kavli Institute for Cosmological Physics. The mismatch could be a clue that cosmologists aren’t taking into account important details that have affected the universe’s evolution. But to see if that’s the case, they need an independent check on the measurements.

"Neutron-star collisions — newly detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors — seem to be just the thing.

***

:In an expanding universe, the farther away an astronomical object is, the faster it recedes. The Hubble constant says how much faster. Edwin Hubble himself estimated that galaxies move away from us 500 kilometers per second faster for each additional megaparsec of distance between us and them (a megaparsec is about 3.3 million light-years). This was a gross overestimate; by the 1970s, astrophysicists favored values for the Hubble constant around either 50 or 100 kilometers per second per megaparsec, depending on their methods. As errors were eliminated, these camps met near the middle. However, in the past year and a half, the Hubble trouble has reheated. This time, 67 stands off against 73.

***

"Each jump from one rung to the next risks miscalculation. And yet, in 2016, a team known as SH0ES used the cosmic distance ladder approach to peg the Hubble constant at 73.2 with an accuracy of 2.4 percent.

"However, in a paper published the same year, a team used the Planck telescope’s observations of the early universe to obtain a value of 67.8 for the current expansion rate — supposedly with 1 percent accuracy.

***

"The similarity of the two Hubble measurements “is amazing” considering the vastly different approaches used to determine them, said Wendy Freedman, an astrophysicist at the University of Chicago and a pioneer of the cosmic distance ladder approach. And yet their margins of error don’t overlap. “The universe looks like it’s expanding about eight percent faster than you would have expected based on how it looked in its youth and how we expect it to evolve,” Adam Riess of Johns Hopkins University, who led the SH0ES team, told Scientific American last year. “We have to take this pretty darn seriously.”

"The 67-versus-73 discrepancy could come down to an unknown error on one side or both. Or it might be real and significant — an indication that the Planck team’s extrapolation from the early universe to the present is missing a cosmic ingredient, one that changed the course of history and led to a faster expansion rate than otherwise expected.

***

"From the first neutron-star collision alone, Holz and hundreds of coauthors calculated the Hubble constant to be 70 kilometers per second per megaparsec, give or take 10. (The major source of uncertainty is the unknown angular orientation of the merging neutron stars relative to the LIGO detectors, which affects the measured amplitude of the signal.) Holz said, “I think it’s just pure luck that we’re smack in the middle,” between the cosmic-distance-ladder and cosmic-microwave-background Hubble estimates. “We could easily shift to one side or the other.”

"The measurement’s accuracy will steadily improve as more standard sirens are heard over the next few years, especially as LIGO continues to ramp up in sensitivity. "

Comment: Reblak take notice. We now have three ways to measure expansion, and they generally agree. The universe is expanding and therefore had a beginning. I condensed a very long article which explains the problems with each type of measurement.