Everyone knows that the universe is inhomogeneous. To idealize such a complex structure with a homogeneous solution is a bold idealization. ~ German cosmologist Thomas Buchert
Einstein’s theory of general relativity underpins modern cosmology. Matter curves spacetime around it by the force of gravity.
The spatial aspect of gravity is apparent with our feet on the ground and things not flying about willy-nilly. That gravity loosens its tug with distance celebrates celestial orbits without stretching the imagination.
It is harder to envision that clocks are inherently inconstant depending upon the gravitational environment: that time is zippy in galactic cores and grows sluggish in the voids between galactic clusters. But it is a proven fact.
When Prussian astronomer Nicolaus Copernicus laid out the Copernican principle in the 16th century, it scandalously cast Earth as not the center of the universe. Modern cosmology extended this into the cosmological principle: that Earth is nowhere special at all, nor is anywhere else.
The premise of the cosmological principle is that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large-enough scale. Homogeneity implies that the cosmos is much the same everywhere. Isotropy presupposes that the universe looks roughly the same from any viewpoint.
Both assumptions are false. The cosmos today is made of clusters of galaxies, strung along filaments of matter distributed around the boundaries of massive bulbous voids, which account for 10% of matter density, but cover over 60% of cosmic volume.
The cosmological principle supposes that the voids and galaxy clusters average out into uniformity. If the cosmological principle holds, no galactic structure should exceed 370 megaparsecs. The Milky Way galaxy is 0.1 Mpc. Typical galactic clusters are 2–3 Mpc. Large quasar groups are typically 200 Mpc or more across.
The cosmological principle has been unhinged by several observations. A large quasar group has been found that is 1,240 Mpc (4 billion light-years). One spherical void in the cosmos is 1.8 billion light-years in diameter. A cosmic construction 5.6 billion light-years across has been found: far larger than the theoretical limit that could support the cosmological principle.
This structure contradicts the current models of the universe. We don’t understand at all how it came to exist. ~ Hungarian astronomer Lajos Balazs in 2015
Finally, the Hercules-Corona Borealis Great Wall is a multiple galactic superstructure that is 3,000 Mpc (10 billion light-years) across.
There is no evidence of cosmic homogeneity; to the contrary. The very idea of universal uniformity is an affront to Nature’s demonstrated fondness for diversity at every scale.
Cosmic microwave background (CMB) radiation has been expanding and cooling with the universe since it formed. The CMB is the only indicator of directional continuity. It belies isotropy, as the cosmos looks decidedly hotter in the one direction than the other from where we sit. This dipole anisotropy is conventionally explained by Earth’s movement through space via the Doppler shift, which explains relative motion. Approaching objects appear warmer, receding objects cooler.
Take account of all the celestial motions through the lens of Doppler shift, and the hot and cold patches of the CMB supposedly melt away for the most part. But Earth’s place within the galaxy skews the view, as the Milky Way heads toward a greater density of matter while moving away from a void. This gives the impression of slowing cosmic expansion ahead, while the void behind produces the opposite effect. These polar discrepancies account for nearly all the dipole anisotropy, and so color our view of the entire universe. Earth is in a special cosmic position, as is most everywhere else.
Via gravity, matter causes spacetime to warp, creating a closed or open curvature to the cosmos, depending upon total mass. How much matter is in the universe determines its fate. Too much matter and spacetime will eventually ball up into a crunch. Too little and spacetime curves outwards, creating an open geometry of eternal cosmic expansion.
Based upon appearances coupled to the cosmological principle, the universe was long presumed flat: a sweet-spot matter density that avoided much curvature. Einstein’s equations predicted a slightly open universe, with eternal expansion gradually slowed by gravity. Cosmological observations tentatively confirm this.
General relativity conjoins space and time. If space expands at different rates in different places based upon gobs of matter, so too time flows at different rates depending upon matter density. Relativity theory argues that the cosmological principle is unsound.
Without the cosmological principle, the age of the universe is complete conjecture. Further, the large-scale size and shape of the cosmos becomes unknowable without a point of view that cannot be had.
Until recently no one doubted that the universe was homogeneous, so everyone just used the simplest models. Once you say that the universe can be very different in other parts of space, then you open up a can of worms. It would just be incredibly more complex to do cosmology. ~ American physicist Paul Halpern
The foundation of modern cosmology is flawed. At the most basic level, human comprehension of the physical universe is slight.