Far out cosmology: universe smooth or not (Introduction)

by David Turell , Tuesday, December 14, 2021, 19:43 (856 days ago) @ David Turell

Still not settled but favors uniform distribution of galaxies:

https://www.realclearscience.com/2021/12/14/cosmologists_parry_attacks_on_cosmological_...

"Lopez’s “Giant Arc” seemed to clash with an idea that has guided astronomy for centuries: that the universe has no conspicuous features. From a zoomed-out perspective, no matter where you are or which way you look, you should see roughly the same number of galaxies pinwheeling around.

"This assumption, enshrined as the “cosmological principle,” has let researchers draw sweeping conclusions about the whole universe based only on what we see from our corner of it.

“'If that turns out to be wrong, then we have to redo many of our measurements or reinterpret many of our measurements,” said Ruth Durrer, a cosmologist at the University of Geneva.

"As a load-bearing strut of modern cosmology, the cosmological principle has increasingly become a target. Some challengers, like Lopez and her colleagues, are astrophysicists puzzled by striking celestial conglomerations. Others are maverick cosmologists unsettled by the consensus view that most of the stuff in the cosmos hides from our instruments in the form of “dark matter” and “dark energy”; they wonder whether theorists may have conjured phantasms to patch up an overly simplistic theory of cosmology.

"Most everyone agrees that the cosmological principle is worth scrutinizing. So far, though, each new claim of a too-big structure or other anomaly has failed to make a dent. We are “trying to poke as many holes as we can,” said Seshadri Nadathur, a cosmologist at University College London, “while being very skeptical of someone else saying they’ve poked a hole.”

"Not only is Earth not special, but nothing anywhere is special. Over the past century, astronomical surveys solidified what’s become known as the cosmological principle in two ways. As powerful telescopes peered deeper into the darkness, they saw more-distant galaxies appearing in similar numbers. This suggests that the cosmos is homogeneous, with matter sprinkled smoothly throughout. (Think of the expanding universe as a rising fruitcake with galaxies evenly spread like fruit pieces, each one flying away from its neighbors as the batter between them expands.)

"Moreover, telescopes pointed in different directions have all seen similar scenes. Matter is evenly distributed along every line of sight, indicating that the universe is “isotropic.”

***

"The universe is clearly not homogeneous on the human scale. Teleport a person one light-year from here and you’ll ruin their day. But drop the Hubble Space Telescope halfway across the universe, and it will return familiar-looking galaxy-filled images. In this way, the cosmological principle treats the cosmos like the air in an inflating balloon. Up close, molecules mingle in complicated ways. But from far away, a bland gas expands with bulk properties like pressure and temperature changing steadily.

"Galaxy surveys have found that any patch of space larger than hundreds of millions of light-years across includes roughly the same amount of matter. So structures like the Giant Arc, which spans billions of light years, are as unexpected as a thick clot of air in an otherwise ordinary balloon.

***

"Strong evidence against any sort of cosmic flow comes from the afterglow of the Big Bang. Astronomers have determined that this “cosmic microwave background” (CMB) has an essentially identical average temperature of 2.725 degrees above absolute zero in every direction.

***

"Making grand inferences about the entire cosmos is hard because we are, after all, in a unique location: the here and now. Telescopes can only see so far, making it look as if galaxies peter out toward the limits of their vision. And as astronomers peer farther away and deeper into the past, they see galaxies in the early universe acting differently than they do today. Artifacts of our singular perspective are all too easily mistaken for failures of the cosmological principle itself.

“'Almost every effect that can screw you up works in that way,” Howell said. “It’s all because we only have one vantage point to look from in the universe.'”

Comment: I omitted much contrary 'evidence' which has convinced no one it is lumpy.